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Review of modern power supply circuits. Remaking a computer power supply

Linear and switching power supplies

Let's start with the basics. The power supply in a computer performs three functions. First, alternating current from the household power supply must be converted to direct current. The second task of the power supply is to reduce the voltage of 110-230 V, which is excessive for computer electronics, to the standard values ​​​​required by power converters of individual PC components - 12 V, 5 V and 3.3 V (as well as negative voltages, which we will talk about a little later) . Finally, the power supply plays the role of a voltage stabilizer.

There are two main types of power supplies that perform the above functions - linear and switching. The simplest linear power supply is based on a transformer, on which the alternating current voltage is reduced to the required value, and then the current is rectified by a diode bridge.

However, the power supply is also required to stabilize the output voltage, which is caused by both voltage instability in the household network and a voltage drop in response to an increase in current in the load.

To compensate for the voltage drop, in a linear power supply the transformer parameters are calculated to provide excess power. Then, at high current, the required voltage will be observed in the load. However, the increased voltage that will occur without any means of compensation at low current in the payload is also unacceptable. Excess voltage is eliminated by including a non-useful load in the circuit. In the simplest case, this is a resistor or transistor connected through a Zener diode. In a more advanced version, the transistor is controlled by a microcircuit with a comparator. Be that as it may, excess power is simply dissipated as heat, which negatively affects the efficiency of the device.

In the switching power supply circuit, one more variable appears, on which the output voltage depends, in addition to the two already existing: input voltage and load resistance. There is a switch in series with the load (which in the case we are interested in is a transistor), controlled by a microcontroller in pulse width modulation (PWM) mode. The higher the duration of the open states of the transistor in relation to their period (this parameter is called duty cycle, in Russian terminology the inverse value is used - duty cycle), the higher the output voltage. Due to the presence of a switch, a switching power supply is also called Switched-Mode Power Supply (SMPS).

No current flows through a closed transistor, and the resistance of an open transistor is ideally negligible. In reality, an open transistor has resistance and dissipates some of the power as heat. In addition, the transition between transistor states is not perfectly discrete. And yet, the efficiency of a pulsed current source can exceed 90%, while the efficiency of a linear power supply with a stabilizer reaches 50% at best.

Another advantage of switching power supplies is the radical reduction in the size and weight of the transformer compared to linear power supplies of the same power. It is known that the higher the frequency of alternating current in the primary winding of a transformer, the smaller the required core size and the number of winding turns. Therefore, the key transistor in the circuit is placed not after, but before the transformer and, in addition to voltage stabilization, is used to produce high-frequency alternating current (for computer power supplies this is from 30 to 100 kHz and higher, and as a rule - about 60 kHz). A transformer operating at a power supply frequency of 50-60 Hz would be tens of times more massive for the power required by a standard computer.

Linear power supplies today are used mainly in the case of low-power applications, where the relatively complex electronics required for a switching power supply constitute a more sensitive cost item compared to a transformer. These are, for example, 9 V power supplies, which are used for guitar effects pedals, and once for game consoles, etc. But chargers for smartphones are already entirely pulsed - here the costs are justified. Due to the significantly lower amplitude of voltage ripple at the output, linear power supplies are also used in those areas where this quality is in demand.

⇡ General diagram of an ATX power supply

A desktop computer's power supply is a switching power supply, the input of which is supplied with household voltage with parameters of 110/230 V, 50-60 Hz, and the output has a number of DC lines, the main ones of which are rated 12, 5 and 3.3 V In addition, the power supply provides a voltage of -12 V, and sometimes also a voltage of -5 V, required for the ISA bus. But the latter was at some point excluded from the ATX standard due to the end of support for the ISA itself.

In the simplified diagram of a standard switching power supply presented above, four main stages can be distinguished. In the same order, we consider the components of power supplies in the reviews, namely:

  1. EMI filter - electromagnetic interference (RFI filter);
  2. primary circuit - input rectifier (rectifier), key transistors (switcher), creating high-frequency alternating current on the primary winding of the transformer;
  3. main transformer;
  4. secondary circuit - current rectifiers from the secondary winding of the transformer (rectifiers), smoothing filters at the output (filtering).

⇡ EMI filter

The filter at the power supply input is used to suppress two types of electromagnetic interference: differential (differential-mode) - when the interference current flows in different directions in the power lines, and common-mode (common-mode) - when the current flows in one direction.

Differential noise is suppressed by capacitor CX (the large yellow film capacitor in the photo above) connected in parallel with the load. Sometimes a choke is additionally attached to each wire, which performs the same function (not on the diagram).

The common mode filter is formed by CY capacitors (blue drop-shaped ceramic capacitors in the photo), connecting the power lines to ground at a common point, etc. a common-mode choke (LF1 in the diagram), the current in the two windings of which flows in the same direction, which creates resistance for common-mode interference.

In cheap models, a minimum set of filter parts is installed; in more expensive ones, the described circuits form repeating (in whole or in part) links. In the past, it was not uncommon to see power supplies without any EMI filter at all. Now this is rather a curious exception, although if you buy a very cheap power supply, you can still run into such a surprise. As a result, not only and not so much the computer itself will suffer, but other equipment connected to the household network - switching power supplies are a powerful source of interference.

In the filter area of ​​a good power supply, you can find several parts that protect the device itself or its owner from damage. There is almost always a simple fuse for short circuit protection (F1 in the diagram). Note that when the fuse trips, the protected object is no longer the power supply. If a short circuit occurs, it means that the key transistors have already broken through, and it is important to at least prevent the electrical wiring from catching fire. If a fuse in the power supply suddenly burns out, then replacing it with a new one is most likely pointless.

Separate protection is provided against short-term surges using a varistor (MOV - Metal Oxide Varistor). But there are no means of protection against prolonged voltage increases in computer power supplies. This function is performed by external stabilizers with their own transformer inside.

The capacitor in the PFC circuit after the rectifier can retain a significant charge after being disconnected from power. To prevent a careless person who sticks his finger into the power connector from receiving an electric shock, a high-value discharge resistor (bleeder resistor) is installed between the wires. In a more sophisticated version - together with a control circuit that prevents charge from leaking when the device is operating.

By the way, the presence of a filter in the PC power supply (and the power supply of a monitor and almost any computer equipment also has one) means that buying a separate “surge filter” instead of a regular extension cord is, in general, pointless. Everything is the same inside him. The only condition in any case is normal three-pin wiring with grounding. Otherwise, the CY capacitors connected to ground simply will not be able to perform their function.

⇡ Input rectifier

After the filter, the alternating current is converted into direct current using a diode bridge - usually in the form of an assembly in a common housing. A separate radiator for cooling the bridge is highly welcome. A bridge assembled from four discrete diodes is an attribute of cheap power supplies. You can also ask what current the bridge is designed for to determine whether it matches the power of the power supply itself. Although, as a rule, there is a good margin for this parameter.

⇡ Active PFC block

In an AC circuit with a linear load (such as an incandescent light bulb or an electric stove), the current flow follows the same sine wave as the voltage. But this is not the case with devices that have an input rectifier, such as switching power supplies. The power supply passes current in short pulses, approximately coinciding in time with the peaks of the voltage sine wave (that is, the maximum instantaneous voltage) when the smoothing capacitor of the rectifier is recharged.

The distorted current signal is decomposed into several harmonic oscillations in the sum of a sinusoid of a given amplitude (the ideal signal that would occur with a linear load).

The power used to perform useful work (which, in fact, is heating the PC components) is indicated in the characteristics of the power supply and is called active. The remaining power generated by harmonic oscillations of the current is called reactive. It does not produce useful work, but heats the wires and creates a load on transformers and other power equipment.

The vector sum of reactive and active power is called apparent power. And the ratio of active power to total power is called power factor - not to be confused with efficiency!

A switching power supply initially has a rather low power factor - about 0.7. For a private consumer, reactive power is not a problem (fortunately, it is not taken into account by electricity meters), unless he uses a UPS. The uninterruptible power supply is responsible for the full power of the load. At the scale of an office or city network, excess reactive power created by switching power supplies already significantly reduces the quality of power supply and causes costs, so it is being actively combated.

In particular, the vast majority of computer power supplies are equipped with active power factor correction (Active PFC) circuits. A unit with an active PFC is easily identified by a single large capacitor and inductor installed after the rectifier. In essence, Active PFC is another pulse converter that maintains a constant charge on the capacitor with a voltage of about 400 V. In this case, current from the supply network is consumed in short pulses, the width of which is selected so that the signal is approximated by a sine wave - which is required to simulate a linear load . To synchronize the current consumption signal with the voltage sinusoid, the PFC controller has special logic.

The active PFC circuit contains one or two key transistors and a powerful diode, which are placed on the same heatsink with the key transistors of the main power supply converter. As a rule, the PWM controller of the main converter key and the Active PFC key are one chip (PWM/PFC Combo).

The power factor of switching power supplies with active PFC reaches 0.95 and higher. In addition, they have one additional advantage - they do not require a 110/230 V mains switch and a corresponding voltage doubler inside the power supply. Most PFC circuits handle voltages from 85 to 265 V. In addition, the sensitivity of the power supply to short-term voltage dips is reduced.

By the way, in addition to active PFC correction, there is also a passive one, which involves installing a high-inductance inductor in series with the load. Its efficiency is low, and you are unlikely to find this in a modern power supply.

⇡ Main converter

The general principle of operation for all pulse power supplies of an isolated topology (with a transformer) is the same: a key transistor (or transistors) creates alternating current on the primary winding of the transformer, and the PWM controller controls the duty cycle of their switching. Specific circuits, however, differ both in the number of key transistors and other elements, and in qualitative characteristics: efficiency, signal shape, noise, etc. But here too much depends on the specific implementation for this to be worth focusing on. For those interested, we provide a set of diagrams and a table that will allow you to identify them in specific devices based on the composition of the parts.

Transistors Diodes Capacitors Transformer primary legs
Single-Transistor Forward 1 1 1 4
2 2 0 2
2 0 2 2
4 0 0 2
2 0 0 3

In addition to the listed topologies, in expensive power supplies there are resonant versions of Half Bridge, which are easily identified by an additional large inductor (or two) and a capacitor forming an oscillatory circuit.

Single-Transistor Forward

⇡ Secondary circuit

The secondary circuit is everything that comes after the secondary winding of the transformer. In most modern power supplies, the transformer has two windings: 12 V is removed from one of them, and 5 V from the other. The current is first rectified using an assembly of two Schottky diodes - one or more per bus (on the highest loaded bus - 12 V - in powerful power supplies there are four assemblies). More efficient in terms of efficiency are synchronous rectifiers, which use field-effect transistors instead of diodes. But this is the prerogative of truly advanced and expensive power supplies that claim the 80 PLUS Platinum certificate.

The 3.3V rail is typically driven from the same winding as the 5V rail, only the voltage is stepped down using a saturable inductor (Mag Amp). A special winding on a transformer for a voltage of 3.3 V is an exotic option. Of the negative voltages in the current ATX standard, only -12 V remains, which is removed from the secondary winding under the 12 V bus through separate low-current diodes.

PWM control of the converter key changes the voltage on the primary winding of the transformer, and therefore on all secondary windings at once. At the same time, the computer's current consumption is by no means evenly distributed between the power supply buses. In modern hardware, the most loaded bus is 12-V.

To separately stabilize voltages on different buses, additional measures are required. The classic method involves using a group stabilization choke. Three main buses are passed through its windings, and as a result, if the current increases on one bus, the voltage drops on the others. Let's say the current on the 12 V bus has increased, and in order to prevent a voltage drop, the PWM controller has reduced the duty cycle of the key transistors. As a result, the voltage on the 5 V bus could go beyond the permissible limits, but was suppressed by the group stabilization choke.

The voltage on the 3.3 V bus is additionally regulated by another saturable inductor.

A more advanced version provides separate stabilization of the 5 and 12 V buses due to saturable chokes, but now this design has given way to DC-DC converters in expensive high-quality power supplies. In the latter case, the transformer has a single secondary winding with a voltage of 12 V, and the voltages of 5 V and 3.3 V are obtained thanks to DC-DC converters. This method is most favorable for voltage stability.

Output filter

The final stage on each bus is a filter that smoothes out voltage ripple caused by the key transistors. In addition, the pulsations of the input rectifier, whose frequency is equal to twice the frequency of the supply network, penetrate to one degree or another into the secondary circuit of the power supply.

The ripple filter includes a choke and large capacitors. High-quality power supplies are characterized by a capacitance of at least 2,000 uF, but manufacturers of cheap models have reserves for savings when they install capacitors, for example, of half the nominal value, which inevitably affects the ripple amplitude.

⇡ Standby power +5VSB

A description of the components of the power supply would be incomplete without mentioning the 5 V standby voltage source, which makes the PC sleep mode possible and ensures the operation of all devices that must be turned on at all times. The “duty room” is powered by a separate pulse converter with a low-power transformer. In some power supplies, there is also a third transformer, which is used in the feedback circuit to isolate the PWM controller from the primary circuit of the main converter. In other cases, this function is performed by optocouplers (an LED and a phototransistor in one package).

⇡ Methodology for testing power supplies

One of the main parameters of the power supply is voltage stability, which is reflected in the so-called. cross-load characteristic. KNH is a diagram in which the current or power on the 12 V bus is plotted on one axis, and the total current or power on the 3.3 and 5 V buses is plotted on the other. At the intersection points for different values ​​of both variables, the voltage deviation from the nominal value is determined one tire or another. Accordingly, we publish two different KNHs - for the 12 V bus and for the 5/3.3 V bus.

The color of the dot indicates the percentage of deviation:

  • green: ≤ 1%;
  • light green: ≤ 2%;
  • yellow: ≤ 3%;
  • orange: ≤ 4%;
  • red: ≤ 5%.
  • white: > 5% (not allowed by ATX standard).

To obtain KNH, a custom-made power supply test bench is used, which creates a load by dissipating heat on powerful field-effect transistors.

Another equally important test is determining the ripple amplitude at the power supply output. The ATX standard allows ripple within 120 mV for a 12 V bus and 50 mV for a 5 V bus. A distinction is made between high-frequency ripple (at double the frequency of the main converter switch) and low-frequency (at double the frequency of the supply network).

We measure this parameter using a Hantek DSO-6022BE USB oscilloscope at the maximum load on the power supply specified by the specifications. In the oscillogram below, the green graph corresponds to the 12 V bus, the yellow graph corresponds to 5 V. It can be seen that the ripples are within normal limits, and even with a margin.

For comparison, we present a picture of ripples at the output of the power supply of an old computer. This block wasn't great to begin with, but it certainly hasn't improved over time. Judging by the magnitude of the low-frequency ripple (note that the voltage sweep division is increased to 50 mV to fit the oscillations on the screen), the smoothing capacitor at the input has already become unusable. High-frequency ripple on the 5 V bus is on the verge of permissible 50 mV.

The following test determines the efficiency of the unit at a load from 10 to 100% of rated power (by comparing the output power with the input power measured using a household wattmeter). For comparison, the graph shows the criteria for the various 80 PLUS categories. However, this does not cause much interest these days. The graph shows the results of the top-end Corsair PSU in comparison with the very cheap Antec, and the difference is not that great.

A more pressing issue for the user is the noise from the built-in fan. It is impossible to directly measure it close to the roaring power supply testing stand, so we measure the rotation speed of the impeller with a laser tachometer - also at power from 10 to 100%. The graph below shows that when the load on this power supply is low, the 135mm fan remains at low speed and is hardly audible at all. At maximum load the noise can already be discerned, but the level is still quite acceptable.

The power supply provides electricity to all PC components. We will tell you how this device works.

Even though your computer plugs into a standard electrical outlet, its components cannot draw power directly from the electrical outlet for two reasons.

First, the network uses alternating current, while computer components require direct current. Therefore, one of the tasks of the power supply is to “rectify” the current.

Secondly, different computer components require different supply voltages to operate, and some require several lines with different voltages at once. The power supply provides each device with current with the necessary parameters. For this purpose, it has several power lines. For example, the power connectors for hard drives and optical drives supply 5 V for electronics and 12 V for the motor.

Power supply characteristics

The power supply is the only source of electricity for all PC components, so the stability of the entire system directly depends on the characteristics of the current it produces. The main characteristic of a power supply is power. It should be at least equal to the total power that the PC components consume at maximum computing load, and even better if it exceeds this figure by 100 W or more. Otherwise, the computer will turn off at times of peak load or, what is much worse, the power supply will burn out, taking other system components with it to the next world.

For most office computers, 300 watts of power is sufficient. The power supply of a gaming machine must have a power of at least 400 W - high-performance processors and fast video cards, as well as the additional cooling systems they require, consume a lot of energy.

If the computer has several video cards, then 500- and 650-watt power supplies will be required to power it. There are already models on sale with a power of more than 1000 W, but buying them is almost pointless.

Often, power supply manufacturers shamelessly inflate the rated power value; this is most often encountered by buyers of cheap models. We advise you to choose a power supply based on testing data. In addition, the power of a power supply is most easily determined by its weight: the larger it is, the higher the likelihood that the actual power of the power supply matches the declared one.

In addition to the total power of the power supply, its other characteristics are also important: Maximum current on individual lines.

The total power of the power supply consists of the powers that it can provide on individual power lines. If the load on one of them exceeds the permissible limit, the system will lose stability even if the total power consumption is far from the power supply rating. The load on lines in modern systems is usually uneven. The 12-volt channel has the hardest time, especially in configurations with powerful video cards. When specifying the dimensions of a power supply, manufacturers, as a rule, limit themselves to the designation of the form factor (modern ATX, outdated AT or exotic BTX). But manufacturers of computer cases and power supplies do not always strictly adhere to the norm. Therefore, when purchasing a new power supply, we recommend comparing its dimensions with the dimensions of the “seat” in your PC case.

Connectors and cable lengths. The power supply must have at least six Molex connectors. A computer with two hard drives and a pair of optical drives (for example, a DVD-RW writer and a DVD reader) already uses four such connectors, and other devices can also be connected to Molex - for example, case fans and video cards with an AGP interface.

The power cables must be long enough to reach all required connectors. Some manufacturers offer power supplies whose cables are not soldered into the board, but are connected to connectors on the case. This reduces the number of wires dangling in the case, and therefore reduces the clutter in the system unit and promotes better ventilation of its interior, since it does not interfere with the air flow circulating inside the computer.

Noise. During operation, the components of the power supply become very hot and require increased cooling. For this purpose, fans built into the PSU case and radiators are used. Most power supplies use one 80 or 120 mm fan, and the fans are quite noisy. Moreover, the higher the power of the power supply, the more intense the air flow is required in order to cool it. To reduce noise levels, high-quality power supplies use circuits to control fan speed in accordance with the temperature inside the power supply.

Some power supplies allow the user to determine the fan speed using a regulator on the back of the power supply.

There are power supply models that continue to ventilate the system unit for some time after the computer is turned off. This allows PC components to cool down faster after use.

Presence of a toggle switch. The switch on the back of the power supply allows you to completely de-energize the system if you need to open the computer case, so its presence is welcome.


Additional power supply characteristics

High power supply power alone does not guarantee high-quality performance. In addition to this, other electrical parameters are also important.

Efficiency factor (efficiency).

This indicator indicates what share of the energy consumed by the power supply from the electrical network goes to the computer components. The lower the efficiency, the more energy is wasted on wasteful heat. For example, if the efficiency is 60%, then 40% of the energy from the outlet is lost. This increases power consumption and leads to strong heating of the power supply components, and therefore to the need for increased cooling using a noisy fan.

Good power supplies have an efficiency of 80% or higher. They can be recognized by the “80 Plus” sign. Recently, three new, more stringent standards have been in effect: 80 Plus Bronze (efficiency of at least 82%), 80 Plus Silver (from 85%) and 80 Plus Gold (from 88%).

The PFC (Power Factor Correction) module allows you to significantly increase the efficiency of the power supply. It comes in two types: passive and active. The latter is much more efficient and allows you to achieve an efficiency level of up to 98%; a power supply with passive PFC is characterized by an efficiency of 75%.

Voltage stability. The voltage on the lines of the power supply fluctuates depending on the load, but it should not go beyond certain limits. Otherwise, system malfunctions or even failure of individual components may occur. The first thing you can rely on for voltage stability is the power of the power supply.

Safety. High-quality power supplies are equipped with various systems to protect against power surges, overloads, overheating and short circuits. These features protect not only the power supply, but also other components of the computer. Note that the presence of such systems in the power supply does not eliminate the need to use uninterruptible power supplies and network filters.

Main characteristics of the power supply

Each power supply has a sticker indicating its technical characteristics. The main parameter is the so-called Combined Power or Combined Wattage. This is the maximum total power for all existing power lines. In addition, the maximum power for individual lines also matters. If there is not enough power on a certain line to “feed” the devices connected to it, then these components may operate unstably, even if the total power of the power supply is sufficient. As a rule, not all power supplies indicate the maximum power for individual lines, but all of them indicate the current strength. Using this parameter, it is easy to calculate the power: to do this, you need to multiply the current by the voltage in the corresponding line. 12 volts are supplied primarily to powerful consumers of electricity - the video card and the central processor. The power supply must provide as much power as possible on this line. For example, a 12-volt power supply line is designed for a current of 20 A. At a voltage of 12 V, this corresponds to a power of 240 W. High-end graphics cards can deliver up to 200W or more. They are powered via two 12-volt lines.

5 V. The 5V lines supply power to the motherboard, hard drives, and optical drives of the PC.

3.3 V. The 3.3V lines go only to the motherboard and provide power to the RAM.

Motherboard power. Initially ATX power supplies had a 20-pin motherboard power connector. It has one contact +12 V through which it is possible to supply current up to 6 A (when using standard Molex contacts. There are also Molex HCS contacts - 9 A and Molex Plus HCS - 11 A. I couldn’t find any information about them other than the name. What contacts are are not yet known to be used in modern components). This was quite enough before the advent of PCI-E slots. In this regard, the main power supply was increased to 24 contacts. We added one more line +3.3 V, +5 V, +12 V and ground.

The last 4 pins 11,12,23 and 24 are made removable and are not used when connecting to the 20-pin socket of the motherboard. This is done for compatibility. You can also connect the 20-pin connector of the power supply to the 24-pin connector on the motherboard in the case of a new board and an old unit. In this case, it is better to make do with the video built into the processor, because When using a discrete graphics adapter, there may be a lack of power for the PCI-E slot with all the ensuing consequences, including the possibility of buying a new computer.

3.3 V Sense (Brown) - contact intended for feedback. With his help power supply regulates voltage+3.3 V.

5 V (White) - not used in modern power supplies and excluded from the 24-pin connector. Used for backward compatibility of the ISA bus.

Power ON (Green) - a contact that allows modern operating systems to control the power supply. When you turn off your computer through the Start menu, a system with Power ON will turn off the power supply. Systems without a Power ON contact can only display a message that the computer can be turned off.

Power good (Gray) - has a voltage of +5 V and can fluctuate within acceptable limits from +2.4 V to +6 V. When you press the POWER button (turn on the computer), the power supply turns on and performs self-testing and stabilizes the voltage at the output +3.3 V, +5 V and +12 V. This process takes 0.1-0.5 s. After which the power supply sends a Power good signal to the motherboard. This signal is received by the processor's power management chip and starts the latter. If there is a surge or loss of voltage at the input of the power supply, the motherboard does not receive the Power good signal and stops the processor. When power is restored at the input, the Power good signal is also restored and the system starts. Thus, thanks to the Power good signal, the computer is guaranteed to receive only high-quality power, which in turn increases the reliability and performance of the entire system.

CPU power. The processor is powered through a device called the Voltage Regulator Module (VRM). The module converts voltage from +12 V to that required by the processor and has an efficiency factor of about 80%. Initially, when processors consumed minimal energy and were powered from +5 V, power supply through the motherboard was sufficient. There were only 12 contacts (2 to 6). As productivity increases, power consumption also increases. Modern processors consume up to 130 W and this is without overclocking. The task was the following: to provide power to the processor without melting the contacts on the motherboard. To do this, we switched from +5 V to +12 V, because this made it possible to reduce the current by more than 50% while maintaining power. Through one +12 V contact on the motherboard it was possible to transmit up to 6 A (the 2nd +12 V line powers the PCI-E slots). The solution was borrowed, as usual, from the server segment. A separate connector was made for the processor directly from the power supply.

The connector consisted of 4 contacts, 2 +12 V and 2 - ground. According to the specification, it was possible to supply up to 8 A per contact.

For top processors, several VRM modules were used. To better distribute the load between them, it was decided to use two 4-pin connectors physically combined into one 8-pin

As you can see from the figure above, the connector contains 4 +12 V lines, which provides stable power to the most powerful processors. The connector can be divided into 2 to 4 pins.

It is also worth noting that especially powerful power supplies(I came across ones from 1000 W and above) have two 8-pin connectors. Probably for powering systems with two processors

Graphics adapter power. The motherboard's 24-pin power connector provides 75W for the PCI-E slot. This is enough only for entry-level graphics adapters. For more advanced solutions, an additional 6-pin connector is used

This connector supplies an additional 75 Watts, resulting in 150 Watts for the graphics adapter.

In 2008, an 8-pin video card power connector was introduced

This provides an additional 150 W, for a total of 225 W. Both connectors are backward compatible. This means that the 6-pin power connector can be connected to the 8-pin power connector on the graphics adapter by sliding it to the side. Conversely, the 8-pin connector of the computer power supply can be connected to the 6-pin connector on the graphics adapter. The design of the connector eliminates incorrect connection.

In addition to the +12 V lines and ground, both connectors have Sense contacts. The graphics adapter uses them to determine which (6 or 8-pin) connector is connected to the video adapter and whether the connector is connected at all. If the connector is not connected, the system will not start. If instead of an 8-pin connector a 6-pin connector is connected, depending on the graphics card firmware, the system may not start at all or may start with limited functionality

The 8-pin graphics adapter power connector and the 8-pin processor power connector have different keys (foolproof), so you cannot connect the connectors incorrectly. These connectors are also divided in different ways: for powering the graphics adapter 6+2, for powering the processor 4+4 or 8 pins together.

In some power supplies, PCI-E connectors are marked with a sticker saying “PCI-Express” for better identification.

Important! All power supply connectors connect without much effort!

Graphics adapters in the middle and high price segments have two connectors at once. Depending on the power: 2x6, 1x6 and 1x8, 2x8.

There are times when the power supply does not have enough PCI-E power connectors. In such situations, use Y-shaped adapters

The adapter uses two Molexes to connect peripherals, because two +12 V lines are required for one 6-pin connector.

When connecting a graphics adapter via an adapter, make sure that the +12 V line can withstand it. That is, find information on the power consumption of the video card in reviews or on the official website. After look at the power supply specifications(on the power supply sticker or on the manufacturer’s website) along the +12 V line

Add up the maximum power of the graphics adapters and the TDP of the processor, I multiply the resulting amount by 1.5 and compare it with the figure in the power supply specification. If the resulting power value is greater than that given in the characteristics, then problems are possible; if it is less, you can try. If you have modern power supply and the figure turns out to be close or even slightly less than in the specification, then you can try the video card in your applications. It is unlikely that you will load it 100%. If you have old power supply, it's better not to take risks.

Peripheral Power. Almost all peripheral devices are powered from the following connectors:

  • power supply for peripheral devices
  • floppy drive power supply
  • Serial ATA power supply

Power supply for peripheral devices. Usually called Molex as it is manufactured by the company of the same name

Has 4 contacts: +5 V, +12 V and 2 ground. Rated for a current of 11 A per contact. Used to connect old hard drives, optical drives, fans and other devices using +5 V or +12 V power supply

The design of the plug includes keys (cut corners) that prevent incorrect connection of peripheral devices. Some manufacturers (Sirtec in particular) make this connector with special semi-circular devices for easier disconnection from devices.

Floppy drive power. Powering less powerful peripherals. It also has 4 contacts. The distance between the contacts, compared to the previous connector, has been reduced by 2 times and is 2.5 mm

Each contact is designed for a current of 2 A, which will determine the maximum power of the connector at 34 W

Unlike the plug for powering peripheral devices, the +5 V and +12 V contacts in this one are inverted. The floppy drive can be connected on the go. To do this, you must first connect the data cable and then the power cable. Disabling occurs in reverse order. Make sure you are not using an FDD drive, turn off the power, then turn off the data cable. The floppy drive plug contains a key for correct connection, but you need to be careful when connecting (especially on the go), you can easily move the contacts when connecting.

Serial ATA power supply. All modern drives, both HDD and SSD, are connected to this connector.

This is a 15-pin plug for connecting peripherals with 3 pins for each power line

Provides the same power as a standard peripheral connector. There is also a key on one side that prevents incorrect connection. For legacy power supplies adapters of the following type are used, allowing you to connect one or two SATA devices

The adapters do not have a +3.3 V power line, since modern HDDs and SSDs do not use it.

Efficiency - efficiency of power supplies

Any device powered by an alternating current network has its own coefficient of performance (efficiency). Computer power supplies not an exception. Efficiency is the amount of energy that performs a useful function (powering a computer). Everything else is converted to heat. Currently there are efficiency levels presented in the table below

Advantages of high efficiency power supply:

  • lower energy consumption compared to a non-certified power supply. For example, a 500 W power supply with 80 Plus Gold certification (90% efficiency) and without certification (about 75% efficiency). At a load of 50% (250 W), a certified power supply will consume 277 W from the network, and a non-certified one will consume 333 W.
  • Less heating since significantly less heat needs to be dissipated
  • longer life of the power supply due to lower temperatures
  • less noise, since a fan operating at lower speeds is required to remove a small amount of heat
  • better power supply for components, hence more reliable and stable operation of the entire computer
  • minimal distortion of power supply characteristics. Each device powered by AC introduces its own interference. Certified power supplies use a special APFC (Active Power Factor Correction) device that increases efficiency and virtually eliminates interference from the computer power supply.

There is only one drawback - the price, which is more than compensated by the advantages.

Internal structure and principle of operation of power supplies for a computer

Let us briefly describe the operating principle of a computer power supply.

The input is supplied with 220 V / 50 Hz power (ideally). Otherwise, a filter (1) works, which removes ripples and network interference. Afterwards, the power is supplied to the mains voltage inverter (2), which increases the frequency from 50 Hz to 100 KHz and higher. Thanks to this, it is possible to use cheap transformers (3) of small dimensions. This transformer, due to its high frequency, can transmit enormous power when converting high voltage to low voltage. Next to the main transformer there is also a standby voltage transformer. The latter is always present when power is supplied to the unit. Next, diode assemblies (5) come into operation, which, together with capacitors and chokes, smooth out high-frequency ripples and produce constant voltages that are supplied directly to the computer components.

Main group stabilization choke (6). It is used in mid-price power supplies and is responsible for stabilizing all output voltages. If the load on one of the channels increases sharply, the voltage sags. With this scheme, the power supply increases the voltage on all lines at once. High-quality, expensive power supplies have completely independent power lines, so this effect does not occur.

Fan speed control circuit (7). Allows you to regulate the speed of the Carlson. There is also a board for monitoring voltage and current consumption. It is responsible for protecting the unit from short circuits and overload.

High level power supplies They are mainly manufactured with modular cable connections. In this case, there is a board with power connectors (8) where the wires are directly connected

  • Motherboard - 50W
  • Processor from 35 to 130 and above. It is necessary to look at the TDP level in the specifications
  • Memory module - 5 W
  • HDD and optical drive - 15 - 20 W
  • SSD - less than 10 W
  • fan - from 0.5 to 5 W
  • graphics adapter - must be looked at in the specifications

For systems with video built into the processor, a 400-500 W power supply is sufficient. More precisely, 250 W is enough, but it’s better to take it with a reserve.

How and where to look at the approximate processor power consumption. We go to the official website of the manufacturer, find your product and look at the characteristics. We are interested in the Max field. TDP. I take this figure as the processor power consumption when calculating.

It's easier with graphics adapters. We also go to the official website of the graphics chip manufacturer and look for your product. Open the specifications tab and if it is an nvidia video card, then in the “Power and Temperature” section we find the card’s consumption indicators and recommendations for the power of the power supply. I didn’t find the consumption of the card from a competitor, you need to read the review, but there are also recommendations on the required power of the power supply.

When assembling systems with several graphics adapters, you should know exactly how much maximum consumption a given model can consume. Multiply this figure by the number of graphics adapters in the system, add the consumption of the processor and other devices. Multiply the resulting amount by 2 and you get the power of the recommended power supply with a decent margin. Why is it recommended to choose a power supply with a reserve? Because if there are several computers in the same room with the same components, but with different power supplies, the power parameters will leave much to be desired. In this situation Systems with more powerful power supplies will be more stable.

Conclusion

In this article, we looked at the characteristics of the computer power supply. We examined in detail the connectors that power all components of the system. The connectors have certain “foolproof” keys and without applying too many “Newtons” during assembly, you will assemble the system correctly. We also superficially walked through the internal structure and operating principle of a computer power supply. We learned that by increasing the frequency from 50 Hz to 100 KHz and higher, it is possible to place all the components of the unit in modest dimensions, without loss of power. The certification of the power supply and the efficiency factor were discussed. We looked at the positive and negative aspects of high efficiency. This is not only lower electricity bills, which will reduce the difference in cost to zero in 3-4 years, but also more stable and reliable operation of your computer.

P.S. Choose a power supply for your computer with a power reserve of 1.5 - 2 times and the highest possible certification standard. This guarantees your personal computer high-quality and stable power supply.

I will be happy to answer questions in the comments. Thank you for sharing the article on social networks. All the best!

Circuitry of computer power supplies

Circuits for computers

R. ALEXANDROV, Maloyaroslavets, Kaluga region.
Radio, 2002, No. 5, 6, 8

UPSs of household computers are designed to operate from a single-phase alternating current network (110/230 V, 60 Hz ≈ imported, 127/220 V, 50 Hz ≈ domestic production). Since the 220 V, 50 Hz network is generally accepted in Russia, the problem of choosing a unit for the required mains voltage does not exist. You just need to make sure that the mains voltage switch on the unit (if there is one) is set to 220 or 230 V. The absence of a switch indicates that the unit is capable of operating in the mains voltage range indicated on its label without any switching. UPSs designed for 60 Hz operate flawlessly on a 50 Hz network.

The UPS is connected to AT format motherboards with two wire harnesses with sockets P8 and P9, shown in Fig. 1 (view from the nests). The wire colors indicated in brackets are standard, although not all UPS manufacturers strictly adhere to them. To properly orient the sockets when connecting to the motherboard plugs, there is a simple rule: the four black wires (GND circuit) going to both sockets must be located next to each other.

The main power circuits of ATX format motherboards are concentrated in the connector shown in Fig. 2. As in the previous case, view from the side of the socket sockets. UPSs of this format have a remote control input (PS-ON circuit), when connected to a common wire (COM ≈ "common" circuit, equivalent to GND), the unit connected to the network begins to operate. If the PS-ON≈COM circuit is open, there is no voltage at the UPS outputs, with the exception of the “standby” +5 V in the +5VSB circuit. In this mode, the power consumed from the network is very low.

ATX format UPSs are equipped with an additional output socket, shown in Fig. 3. The purpose of its circuits is as follows:

FanM ≈ output of the fan speed sensor cooling the UPS (two pulses per revolution);
FanC ≈ analog (0...12 V) input for controlling the rotation speed of this fan. If this input is disconnected from external circuits or a constant voltage of more than 10 V is applied to it, the fan performance is maximum;
3.3V Sense ≈ feedback signal input of the voltage stabilizer +3.3 V. It is connected with a separate wire directly to the power pins of the microcircuits on the system board, which allows you to compensate for the voltage drop on the supply wires. If there is no additional socket, this circuit can be routed to socket 11 of the main socket (see Fig. 2);
1394R ≈ minus of an 8...48 V voltage source isolated from the common wire to power the IEEE-1394 interface circuits;
1394V ≈ plus of the same source.

A UPS of any format must be equipped with several sockets to power disk drives and some other computer peripherals.

Each “computer” UPS produces a logical signal called R G. (Power Good) in AT blocks or PW-OK (Power OK) in ATX blocks, the high level of which indicates that all output voltages are within acceptable limits. On the “motherboard” of the computer, this signal is involved in generating the system reset signal. After turning on the UPS, the RG signal level. (PW-OK) remains low for some time, prohibiting the processor from operating until the transient processes in the power circuits are completed.

When the mains voltage is turned off or the UPS suddenly malfunctions, the logical level of the P.G. signal (PW-OK) changes before the unit’s output voltages drop below permissible values. This causes the processor to stop, prevents corruption of data stored in memory and other irreversible operations.

The interchangeability of a UPS can be assessed using the following criteria.

Number of output voltages to power an IBM PC AT format there must be at least four (+12 V, +5 V, -5 V and -12 V). The maximum and minimum output currents are regulated separately for each channel. Their usual values ​​for sources of various powers are given in table. 1 . ATX computers additionally require +3.3 V and some other voltages (they were mentioned above).

Please note that normal operation of the unit at a load less than the minimum is not guaranteed, and sometimes this mode is simply dangerous. Therefore, it is not recommended to connect the UPS without load to the network (for example, for testing).

The power of the power supply (total for all output voltages) in a household PC fully equipped with peripheral devices must be at least 200 W. In practice, it is necessary to have 230...250 W, and when installing additional hard drives and CD-ROM drives, more may be required. PC malfunctions, especially those that occur when the electric motors of the mentioned devices are turned on, are often associated with an overload of the power supply. Computers used as information network servers consume up to 350 W. Low-power UPSs (40...160 W) are used in specialized, for example, control computers with a limited set of peripherals.

The volume occupied by a UPS usually increases due to an increase in its length towards the front panel of the PC. The installation dimensions and mounting points of the unit in the computer case remain unchanged. Therefore, any (with rare exceptions) block can be installed in the place of the failed one.

The basis of most UPSs is a push-pull half-bridge inverter operating at a frequency of several tens of kilohertz. The inverter supply voltage (approximately 300 V) is rectified and smoothed mains voltage. The inverter itself consists of a control unit (pulse generator with an intermediate power amplification stage) and a powerful output stage. The latter is loaded onto a high-frequency power transformer. The output voltages are obtained using rectifiers connected to the secondary windings of this transformer. Voltage stabilization is carried out using pulse width modulation (PWM) of pulses generated by the inverter. Typically, only one output channel is covered by the stabilizing OS, usually +5 or +3.3 V. As a result, the voltages at other outputs do not depend on the network voltage, but remain subject to the influence of the load. Sometimes they are additionally stabilized using conventional stabilizer chips.

MAINS RECTIFIER


In most cases, this unit is performed according to a scheme similar to that shown in Fig. 4, the differences are only in the type of rectifier bridge VD1 and a greater or lesser number of protective and safety elements. Sometimes the bridge is assembled from individual diodes. When switch S1 is open, which corresponds to the unit being powered from a 220...230 V network, the rectifier is a bridge, the voltage at its output (capacitors C4, C5 connected in series) is close to the amplitude of the network. When powered from a network of 110... 127 V, by closing the contacts of the switch, they turn the device into a rectifier with doubling the voltage and obtain at its output a constant voltage that is twice the amplitude of the network voltage. Such switching is provided in UPSs, the stabilizers of which keep the output voltages within acceptable limits only when the mains voltage deviates by 20%. Units with more effective stabilization are able to operate at any mains voltage (usually from 90 to 260 V) without switching.

Resistors R1, R4 and R5 are designed to discharge the rectifier capacitors after it is disconnected from the network, and C4 and C5, in addition, equalize the voltages on capacitors C4 and C5. Thermistor R2 with a negative temperature coefficient limits the amplitude of the inrush current charging capacitors C4, C5 at the moment the unit is turned on. Then, as a result of self-heating, its resistance drops, and it practically does not affect the operation of the rectifier. Varistor R3 with a classification voltage greater than the maximum amplitude of the network protects against surges of the latter. Unfortunately, this varistor is useless if a unit with a closed switch S1 is accidentally turned on in a 220 V network. The serious consequences of this can be avoided by replacing resistors R4, R5 with varistors with a classification voltage of 180...220 V, the breakdown of which entails the combustion of the fuse-link FU1. Sometimes varistors are connected in parallel with the specified resistors or only one of them.

Capacitors C1 ≈ SZ and two-winding inductor L1 form a filter that protects the computer from interference from the network, and the network from interference created by the computer. Through capacitors C1 and SZ, the computer case is connected via alternating current to the network wires. Therefore, the voltage of touching an ungrounded computer can reach half the network voltage. This is not life-threatening, since the reactance of the capacitors is quite high, but it often leads to failure of the interface circuits when peripheral devices are connected to the computer.

POWERFUL INVERTER CASCADE

On rice. 5 shows part of the circuit diagram of the common GT-150W UPS. The pulses generated by the control unit are sent through transformer T1 to the bases of transistors VT1 and VT2, opening them alternately. Diodes VD4, VD5 protect transistors from reverse polarity voltage. Capacitors C6 and C7 correspond to C4 and C5 in the rectifier (see Fig. 4). The voltages of the secondary windings of transformer T2 are rectified to obtain output. One of the rectifiers (VD6, VD7 with filter L1C5) is shown in the diagram.

Most powerful UPS cascades differ from those considered only in the types of transistors, which can be, for example, field-effect ones or contain built-in protective diodes. There are several options for the design of basic circuits (for bipolar) or gate circuits (for field-effect transistors) with different numbers, ratings and circuits for connecting elements. For example, resistors R4, R6 can be connected directly to the bases of the corresponding transistors.

In steady state, the inverter control unit is supplied with the output voltage of the UPS, but at the moment of switching on it is absent. There are two main ways to obtain the supply voltage necessary to start the inverter. The first of them is implemented in the scheme under consideration (Fig. 5). Immediately after turning on the unit, the rectified mains voltage flows through the resistive divider R3 ≈ R6 into the base circuits of transistors VT1 and/T2, opening them slightly, and diodes VD1 and VD2 prevent the base-emitter sections of the transistors from being shunted by windings II and III of transformer T1. At the same time, capacitors C4, C6 and C7 are charged, and the charging current of capacitor C4, flowing through winding I of transformer T2 and through part of winding II of transformer T1, induces a voltage in windings II and III of the latter that opens one of the transistors and closes the other. Which transistor will close and which will open depends on the asymmetry of the characteristics of the cascade elements.

As a result of the action of positive feedback, the process proceeds like an avalanche, and a pulse induced in winding II of transformer T2 through one of the diodes VD6, VD7, resistor R9 and diode VD3 charges the capacitor SZ to a voltage sufficient to start operation of the control unit. Subsequently, it is powered by the same circuit, and the voltage rectified by diodes VD6, VD7, after smoothing by the L1C5 filter, is supplied to the +12 V output of the UPS.

The version of the initial startup circuits used in the LPS-02-150XT UPS differs only in that the voltage to the divider, similar to R3 ≈ R6 (Fig. 5), is supplied from a separate half-wave rectifier of the mains voltage with a small-capacity filter capacitor. As a result, the inverter transistors open slightly before the main rectifier filter capacitors (C6, C7, see Fig. 5) are charged, which ensures a more reliable start.

The second method of powering the control unit during startup involves the presence of a special low-power step-down transformer with a rectifier, as shown in the diagram in Fig. 6 used in the PS-200B UPS.

The number of turns of the secondary winding of the transformer is selected so that the rectified voltage is slightly less than the output in the +12 V channel of the unit, but sufficient for the operation of the control unit. When the output voltage of the UPS reaches its nominal value, the diode VD5 opens, the diodes of the bridge VD1 ≈ VD4 remain closed during the entire period of alternating voltage and the control unit switches to power supply with the output voltage of the inverter, without consuming more energy from the “starting” transformer.

In high-power inverter stages triggered in this way, there is no need for an initial bias at the bases of the transistors and positive feedback. Therefore, resistors R3, R5 are not required, diodes VD1, VD2 are replaced with jumpers, and winding II of transformer T1 is made without a tap (see Fig. 5).

OUTPUT RECTIFIERS

In Fig. Figure 7 shows a typical diagram of a four-channel UPS rectifier unit. In order not to violate the symmetry of magnetization reversal of the magnetic circuit of a power transformer, rectifiers are built only using full-wave circuits, and bridge rectifiers, which are characterized by increased losses, are almost never used. The main feature of rectifiers in UPSs is smoothing filters, starting with inductance (choke). The voltage at the output of a rectifier with such a filter depends not only on the amplitude, but also on the duty cycle (the ratio of the duration to the repetition period) of the pulses arriving at the input. This makes it possible to stabilize the output voltage by changing the duty cycle of the input. Rectifiers with filters starting with a capacitor, used in many other cases, do not have this property. The process of changing the duty cycle of pulses is usually called PWM ≈ pulse width modulation (English PWM ≈ Pulse Width Modulation).

Since the amplitude of the pulses, proportional to the voltage in the supply network, at the inputs of all rectifiers in the block changes according to the same law, stabilizing one of the output voltages using PWM stabilizes all the others. To enhance this effect, filter chokes L1.1 ≈ L1.4 of all rectifiers are wound on a common magnetic core. The magnetic connection between them additionally synchronizes the processes occurring in the rectifiers.

For proper operation of a rectifier with an L-filter, it is necessary that its load current exceed a certain minimum value, depending on the inductance of the filter choke and the pulse frequency. This initial load is created by resistors R4 ≈ R7, connected in parallel with the output capacitors C5 ≈ C8. They also serve to speed up the discharge of capacitors after the UPS is turned off.

Sometimes a voltage of -5 V is obtained without a separate rectifier from a voltage of -12 V using an integrated stabilizer of the 7905 series. Domestic analogues are microcircuits KR1162EN5A, KR1179EN05. The current consumed by computer nodes along this circuit usually does not exceed several hundred milliamps.

In some cases, integrated stabilizers are installed in other UPS channels. This solution eliminates the influence of a changing load on the output voltages, but reduces the efficiency of the unit and for this reason is used only in relatively low-power channels. An example is the diagram of the PS-6220C UPS rectifier assembly shown in rice. 8. Diodes VD7 ≈ VD10 ≈ protective.

As in most other units, the +5 V voltage rectifier here contains Schottky barrier diodes (VD6 assembly), which are characterized by a lower forward voltage drop and reverse resistance recovery time than conventional diodes. Both of these factors are favorable for increasing efficiency. Unfortunately, the relatively low permissible reverse voltage does not allow the use of Schottky diodes in the +12 V channel. However, in the unit under consideration, this problem is solved by connecting two rectifiers in series: the missing 7 V is added to the 5 V by a rectifier on the Schottky diode assembly VD5.

To eliminate voltage surges that are dangerous for diodes and occur in the transformer windings at pulse fronts, damping circuits R1C1, R2C2, R3C3 and R4C4 are provided.

CONTROL UNIT

In most “computer” UPSs, this unit is built on the basis of the TL494CN PWM controller chip (domestic equivalent ≈ KR1114EU4) or its modifications. The main part of the diagram of such a node is shown in Fig. 9, it also shows the elements of the internal structure of the mentioned microcircuit.

The sawtooth voltage generator G1 serves as a master. Its frequency depends on the ratings of external elements R8 and SZ. The generated voltage is supplied to two comparators (A3 and A4), the output pulses of which are summed by the OR element D1. Next, the pulses through the NOR elements D5 and D6 are supplied to the output transistors of the microcircuit (V3, V4). Pulses from the output of element D1 also arrive at the counting input of trigger D2, and each of them changes the state of the trigger. Thus, if a log is applied to pin 13 of the microcircuit. 1 or, as in the case under consideration, it is left free, the pulses at the outputs of elements D5 and D6 alternate, which is necessary to control a push-pull inverter. If the TL494 chip is used in a single-ended voltage converter, pin 13 is connected to the common wire, as a result, trigger D2 is no longer involved in the operation, and pulses appear at all outputs simultaneously.

Element A1 is an error signal amplifier in the UPS output voltage stabilization circuit. This voltage (in this case ≈ +5 V) is supplied to one of the amplifier inputs through a resistive divider R1R2. At its second input ≈ the reference voltage obtained from the stabilizer A5 built into the chip using a resistive divider R3 ≈ R5. The voltage at output A1, proportional to the difference between the input ones, sets the operating threshold of comparator A4 and, consequently, the duty cycle of the pulses at its output. Since the output voltage of the UPS depends on the duty cycle (see above), in a closed system it is automatically maintained equal to the exemplary voltage, taking into account the division coefficient R1R2. The R7C2 chain is necessary for the stability of the stabilizer. The second amplifier (A2), in this case, from the switches by supplying the appropriate voltages to its inputs, does not participate in the operation.

The function of comparator A3 is to guarantee the presence of a pause between pulses at the output of element D1, even if the output voltage of amplifier A1 is outside the permissible limits. The minimum response threshold A3 (when connecting pin 4 to common) is set by the internal voltage source GV1. As the voltage at pin 4 increases, the minimum pause duration increases, therefore, the maximum output voltage of the UPS drops.

This property is used for smooth startup of the UPS. The fact is that at the initial moment of operation of the unit, the filter capacitors of its rectifiers are completely discharged, which is equivalent to shorting the outputs to the common wire. Starting the inverter immediately “at full power” will lead to a huge overload of the transistors of the powerful cascade and their possible failure. Circuit C1R6 ensures a smooth, overload-free start of the inverter.

At the first moment after switching on, capacitor C1 is discharged, and the voltage at pin 4 of DA1 is close to +5 V received from stabilizer A5. This guarantees a pause of the maximum possible duration, up to the complete absence of pulses at the output of the microcircuit. As capacitor C1 is charged through resistor R6, the voltage at pin 4 decreases, and with it the duration of the pause. At the same time, the output voltage of the UPS increases. This continues until it approaches the exemplary one and stabilizing feedback comes into effect. Further charging of capacitor C1 does not affect the processes in the UPS. Since capacitor C1 must be completely discharged before each UPS is turned on, in many cases circuits for its forced discharge are provided (not shown in Fig. 9).

INTERMEDIATE CASCADE

The task of this cascade is to amplify the pulses before feeding them to powerful transistors. Sometimes the intermediate stage is missing as an independent unit, being part of the master oscillator microcircuit. The diagram of such a cascade used in the PS-200B UPS is shown in Fig. 10 . Matching transformer T1 here corresponds to the one of the same name in Fig. 5.

The APPIS UPS uses an intermediate stage according to the circuit shown in Fig. 11, which differs from the one discussed above by the presence of two matching transformers T1 and T2 ≈ separately for each power transistor. The polarity of the transformer windings is such that the intermediate stage transistor and the power transistor associated with it are in the open state at the same time. If special measures are not taken, after a few cycles of inverter operation, the accumulation of energy in the magnetic circuits of the transformers will lead to saturation of the latter and a significant decrease in the inductance of the windings.

Let's consider how this problem is solved, using the example of one of the “halves” of the intermediate stage with transformer T1. When the transistor of the microcircuit is open, winding Ia is connected to the power source and the common wire. A linearly increasing current flows through it. A positive voltage is induced in winding II, which enters the base circuit of the powerful transistor and opens it. When the transistor in the microcircuit is closed, the current in winding Ia will be interrupted. But the magnetic flux in the magnetic core of the transformer cannot change instantly, so a linearly decreasing current will appear in winding Ib, flowing through the opened diode VD1 from the common wire to the plus of the power source. Thus, the energy accumulated in the magnetic field during the pulse returns to the source during the pause. The voltage on winding II during the pause is negative, and the powerful transistor is closed. The second “half” of the cascade with transformer T2 operates in a similar way, but in antiphase.

The presence of pulsating magnetic fluxes with a constant component in magnetic circuits leads to the need to increase the mass and volume of transformers T1 and T2. In general, an intermediate stage with two transformers is not very successful, although it has become quite widespread.

If the power of the transistors of the TL494CN microcircuit is not enough to directly control the output stage of the inverter, use a circuit similar to that shown in Fig. 12, which shows the intermediate stage of the KYP-150W UPS. The halves of winding I of transformer T1 serve as collector loads of transistors VT1 and VT2, alternately opened by pulses coming from the DA1 microcircuit. Resistor R5 limits the collector current of the transistors to approximately 20 mA. Using diodes VD1, VD2 and capacitor C1 on the emitters of transistors VT1 and VT2, the voltage required for their reliable closing is +1.6 V. Diodes VD4 and VD5 dampen the oscillations that occur when switching transistors in the circuit formed by the inductance of winding I of transformer T1 and its own capacity. Diode VD3 closes if the voltage surge at the middle terminal of winding I exceeds the cascade supply voltage.

Another version of the intermediate stage circuit (UPS ESP-1003R) is shown in Fig. 13. In this case, the output transistors of the DA1 microcircuit are connected according to a circuit with a common collector. Capacitors C1 and C2 are boosting. Winding I of transformer T1 does not have a middle terminal. Depending on which of the transistors VT1, VT2 is currently open, the winding circuit is closed to the power source through resistor R7 or R8 connected to the collector of the closed transistor.

TROUBLESHOOTING

Before repairing the UPS, it must be removed from the computer system unit. To do this, disconnect the computer from the network by removing the plug from the outlet. Having opened the computer case, release all the UPS connectors and, by unscrewing the four screws on the back wall of the system unit, remove the UPS. Then remove the U-shaped cover of the UPS case by unscrewing the screws securing it. The printed circuit board can be removed by unscrewing the three self-tapping screws that secure it. A feature of many UPS boards is that the printed conductor of the common wire is divided into two parts, which are connected to each other only through the metal body of the unit. On the board removed from the case, these parts must be connected with an overhead conductor.

If the power supply was disconnected from the power supply less than half an hour ago, you need to find and discharge 220 or 470 uF x 250 V oxide capacitors on the board (these are the largest capacitors in the block). During the repair process, it is recommended to repeat this operation after each disconnection of the unit from the network, or to temporarily bypass the capacitors with 100...200 kOhm resistors with a power of at least 1 W.

First of all, they inspect the parts of the UPS and identify those that are clearly faulty, for example, those that are burnt or have cracks in the case. If the failure of the unit was caused by a fan malfunction, you should check the elements installed on the heat sinks: powerful transistors of the inverter and Schottky diode assemblies of the output rectifiers. When oxide capacitors “explode,” their electrolyte is sprayed throughout the unit. To avoid oxidation of metal live parts, it is necessary to wash off the electrolyte with a slightly alkaline solution (for example, diluting the “Fairy” product with water in a ratio of 1:50).

Having connected the unit to the network, first of all you should measure all its output voltages. If it turns out that in at least one of the output channels the voltage is close to the nominal value, the fault should be sought in the output circuits of the faulty channels. However, as practice shows, output circuits rarely fail.

In case of malfunction of all channels, the method for determining faults is as follows. Measure the voltage between the positive terminal of capacitor C4 and the negative terminal of C5 (see Fig. 4) or the collector of transistor VT1 and the emitter VT2 (see Fig. 5). If the measured value is significantly less than 310 V, you need to check and, if necessary, replace the diode bridge VD1 (see . Fig. 4) or its individual diodes. If the rectified voltage is normal, but the unit does not work, most likely, one or both transistors of the powerful inverter stage (VT1, VT2, see Fig. 5), which are subject to the greatest thermal overloads, have failed. If the transistors are working, all that remains is to check the TL494CN microcircuit and the associated circuits.

Failed transistors can be replaced with domestic or imported analogues that are suitable in terms of electrical parameters, overall and installation dimensions, guided by the data given in table. 2. Replacement diodes are selected according to the table. 3.

The rectifier diodes of the network rectifier (see Fig. 4) can be successfully replaced with domestic KD226G, KD226D. If the network rectifier has capacitors with a capacity of 220 μF, it is advisable to replace them with 470 μF; there is usually space for this on the board. To reduce interference, it is recommended to bypass each of the four rectifier diodes with a 1000 pF capacitor to a voltage of 400...450 V.

Transistors 2SC3039 can be replaced with domestic KT872A. But the PXPR1001 damping diode to replace the failed one is difficult to purchase even in big cities. In this situation, you can use three KD226G or KD226D diodes connected in series. It is possible to replace the failed diode and the powerful transistor protected by it by installing a transistor with a built-in damping diode, for example, 2SD2333, 2SD1876, 2SD1877 or 2SD1554. It should be noted that many UPSs released after 1998 have already undergone such a replacement.

To enlarge, click on the image (opens in a new window)

To increase the reliability of the IED, it is recommended to connect chokes with an inductance of 4 μH in parallel with resistors R7 and R8 (see Fig. 5). They can be wound with wire with a diameter of at least 0.15 mm in silk insulation on any ring magnetic cores. The number of turns is calculated using known formulas.

Many UPSs do not have a tuning resistor for adjusting the output voltage (R3, see Fig. 9); a constant one is installed instead. If adjustment is required, it can be done by temporarily installing a trim resistor, and then again replacing it with a constant of the found value.

To increase reliability, it is useful to replace the imported oxide capacitors installed in the filters of the most powerful + 12 V and +5 V rectifiers with K50-29 capacitors equivalent in capacity and voltage. It should be noted that on the boards of many UPSs, not all capacitors provided for by the circuit are installed (apparently, to save money), which negatively affects the characteristics of the unit. It is recommended to install the missing capacitors in their designated places.

When assembling the unit after repair, do not forget to remove the temporarily installed jumpers and resistors, and also connect the built-in fan to the corresponding connector.

LITERATURE
1. Kulichkov A. Switching power supplies for IBM PC. - M.: DMK, series "Repair and Service", 2000.
2. Guk M. IBM PC hardware. - St. Petersburg: Peter, 2000.
3. Kunevich A.. Sidorov I. Inductive elements on ferrites. - St. Petersburg: Lenizdat, 1997.
4. Nikulin S. Reliability of radio-electronic equipment elements. - M.: Energy, 1979.

In the modern world, the development and obsolescence of personal computer components occurs very quickly. At the same time, one of the main components of a PC - the ATX form factor - is practically has not changed its design for the last 15 years.

Consequently, the power supply of both an ultra-modern gaming computer and an old office PC work on the same principle and have common methods for diagnosing faults.

The material presented in this article can be applied to any personal computer power supply with a minimum of nuances.

A typical ATX power supply circuit is shown in the figure. Structurally, it is a classic pulse unit on a TL494 PWM controller, triggered by a PS-ON (Power Switch On) signal from the motherboard. The rest of the time, until the PS-ON pin is pulled to ground, only the Standby Supply with a voltage of +5 V at the output is active.

Let's take a closer look at the structure of the ATX power supply. Its first element is
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Its task is to convert alternating current from the mains to direct current to power the PWM controller and standby power supply. Structurally, it consists of the following elements:

  • Fuse F1 protects the wiring and the power supply itself from overload in the event of a power supply failure, leading to a sharp increase in current consumption and, as a consequence, to a critical increase in temperature that can lead to a fire.
  • A protective thermistor is installed in the neutral circuit, which reduces the current surge when the power supply is connected to the network.
  • Next, a noise filter is installed, consisting of several chokes ( L1, L2), capacitors ( C1, C2, C3, C4) and counter-wound choke Tr1. The need for such a filter is due to the significant level of interference that the pulse unit transmits to the power supply network - this interference is not only picked up by television and radio receivers, but in some cases can lead to the malfunction of sensitive equipment.
  • A diode bridge is installed behind the filter, converting alternating current into pulsating direct current. Ripple is smoothed out by a capacitive-inductive filter.

Standby power supply is a low-power independent pulse converter based on the T11 transistor, which generates pulses through an isolation transformer and a half-wave rectifier on the D24 diode, powering a low-power integrated voltage stabilizer on the 7805 chip. Although this circuit is, as they say, time-tested, its significant drawback is high voltage drop across the 7805 stabilizer, which leads to overheating under heavy load. For this reason, damage in the circuits powered from the standby source can lead to its failure and subsequent inability to turn on the computer.

The basis of the pulse converter is PWM controller. This abbreviation has already been mentioned several times, but has not been deciphered. PWM is pulse width modulation, that is, changing the duration of voltage pulses at their constant amplitude and frequency. The task of the PWM unit, based on a specialized TL494 microcircuit or its functional analogues, is to convert DC voltage into pulses of the appropriate frequency, which, after an isolation transformer, are smoothed by output filters. Voltage stabilization at the output of the pulse converter is carried out by adjusting the duration of the pulses generated by the PWM controller.

An important advantage of such a voltage conversion circuit is also the ability to work with frequencies significantly higher than 50 Hz of the power supply. The higher the frequency of the current, the smaller the dimensions of the transformer core and the number of turns of the windings are required. That is why switching power supplies are much more compact and lighter than classical circuits with an input step-down transformer.

A circuit based on transistor T9 and the stages following it are responsible for turning on the ATX power supply. At the moment the power supply is turned on to the network, a 5V voltage is supplied to the base of the transistor through the current-limiting resistor R58 from the output of the standby power supply; at the moment the PS-ON wire is shorted to ground, the circuit starts the PWM controller TL494. In this case, failure of the standby power supply will lead to uncertainty in the operation of the power supply startup circuit and a possible switching failure, as already mentioned.