Power supply with active PFC. How to choose a power supply for your computer

What is an Active PFC Power Supply Power Factor Correction module?

1. PFC (Power Factor Correction)

   The usual, classic, 220V AC voltage rectification circuit consists of diode bridge and a smoothing capacitor. The problem is that the capacitor charge current is pulsed in nature (duration about 3mS) and, as a consequence of this, very high current. For example, for a power supply with a load of 200W, the average current from a 220V network will be 1A, and the pulse current will be 4 times more. What if there are many such power supplies and (or) are they more powerful? ..then the currents will be simply crazy - the wiring and sockets will not withstand, and you will have to pay more for electricity, because the quality of the current consumption is very much taken into account. For example, large factories have special capacitor units for cosine compensation. In modern computer technology We faced the same problems, but no one would install multi-story structures, and we went the other way - they installed a special element in the power supplies to reduce the “pulse” of the consumed current - PFC. It is built between the rectifier and the capacitor, limits the current in amplitude and extends it in time. PFCs are either passive or active, which is determined by the damping element.

2. I don’t know exactly, but this is a built-in noise filter in the electrical network. That is, such a computer does not need network filter.
3. PFC (Power Factor Correction) is translated as Power Factor Correction, also called reactive power compensation.
4. A conventional switching power supply is powered by a sine wave (the same one that is 220V) through a rectifier (bridge) with a capacitive load. Therefore, the current consumed is far from being sinusoidal; it has the form of short peaks located at the tops of the sinusoid. That is, from the point of view of circuit theory, it is a nonlinear element and causes strong interference (50Hz harmonics) to be emitted into the network. At large quantities such loads are also disrupted normal operation transformer substation - losses increase, efficiency decreases. PFC is an additional converter powered by a rectifier without capacitive load (pulsating voltage with a frequency of 100 Hz) and outputting constant pressure, from which the main converter is already powered. The advantage of such a scheme is that the current consumed is close to a sinusoid, the level of interference is reduced and the transformer operates in normal mode. The disadvantage is complexity and price. Typically such schemes are found in power supplies high power, starting from hundreds of watts, including the now popular converters for asynchronous motors.
5. PFC (Power Factor Correction) is translated as Power Factor Correction, also called reactive power compensation. The simplest and therefore most common is the so-called passive PFC, which is a conventional inductor of relatively high inductance, connected to the network in series with the power supply.
   The active PFC is another pulse source supply, and increasing the voltage.
   active PFC, in contrast to passive one, improves the operation of the power supply - it additionally stabilizes the input voltage of the main stabilizer of the unit; the unit becomes noticeably less sensitive to low mains voltage; also, when using active PFC, units with a universal power supply of 110...230V are quite easily developed, not requiring manual switching of the mains voltage. (Such PSUs have a specific feature: they are used in conjunction with cheap UPSs (uninterruptible power supply) that provide step signal when operating on batteries, it can lead to computer malfunctions, so manufacturers recommend using a Smart class UPS in such cases)
   Also, the use of active PFC improves the response of the power supply during short-term (fractions of a second) dips in the mains voltage; at such moments, the unit operates using the energy of high-voltage rectifier capacitors, the efficiency of which more than doubles. Another advantage of using an active PFC is that it low level high-frequency interference on the output lines, i.e. such power supplies are recommended for use in PCs with peripherals designed to work with analog audio/video material.

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 voltage alternating current is reduced to the required value, and then the current is rectified by the 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 increased voltage which would occur without any means of compensation at low payload current 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 is best case scenario reaches 50%.

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 alternating current high frequency(for computer power supplies this is from 30 to 100 kHz and above, and as a rule - about 60 kHz). Transformer operating at mains frequency 50-60 Hz for the power required standard computer, would be tens of times more massive.

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

BP desktop computer is a switching power supply, the input of which is supplied with household voltage with parameters 110/230 V, 50-60 Hz, and the output has a number of lines direct current, the main ones 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).

⇡ EMF filter

The filter at the power supply input serves to suppress two types of electromagnetic interference: differential (differential-mode) - when the interference current flows into different sides in power lines, and 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 they install minimum set filter parts, 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 powerful source 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 to protect against short circuit(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 harmonic vibrations 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 supports on-capacitor constant charge voltage of about 400 V. In this case, the current from the supply network is consumed in short pulses, the width of which is selected so that the signal is approximated by a sinusoid - which is what 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 benefit- no 110/230 V network switch and corresponding voltage doubler are required 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.

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.

⇡ 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 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 capacitors large capacity. 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 power supply components 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 used in the circuit feedback 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 at different meanings Both variables determine the voltage deviation from the nominal value on a particular bus. 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.

Inefficient use of electricity, interference in the power grid caused by connected devices are problems that are being addressed all over the world.

In December 2010, Europe began to work according to the new standard EN 61000-3-12. New standard defines standards for harmonic current components generated by equipment connected to low-voltage systems common use, with a rated current of more than 16 A and less than 75 A in one phase. The purpose of this standard is to prevent the connection to the electrical network of devices that can create interference on the line, i.e. provide electromagnetic compatibility devices at the point public access and increase the efficiency of electricity consumption.

The use of PFC (Power Factor Corrector) blocks or source power factor correctors KM is one of effective solutions problems of efficient energy consumption and complies with the EN 61000-3-12 standard.

Let's consider the operation of electricity consumers with and without a PFC block.

The power factor Km of a source is defined as the ratio of active power to apparent power. The maximum power factor value is 1, this is an ideal option when all the power consumed is used to produce useful work without loss. Power factor is used to evaluate the efficiency of electricity consumption (the higher the factor, the higher the efficiency), to calculate the power network (wiring) and to calculate power devices.

During operation, all sources without a PFC power factor correction unit consume current from the power supply in short pulses (Fig. 1).

As a consequence, the current consumption values ​​are quite high. For example, power supplies with a load of 200 W, the average current will be 1 A, and impulse current will be 4 times higher. If the source has heavy load, then the consumption currents will be very large, accordingly the load on the supply network will increase significantly and you will have to pay more for electricity.

The power factor Km of sources without a PFC block is approximately 0.7, i.e., only 70% of the energy is consumed by the power supply to produce useful work, and 30% creates only additional load on the network.

The shape of the current consumed by the source with the PFC block is practically no different from the shape of the supply voltage (Fig. 2), power factor Km in in this case will reach values ​​of 0.95-0.98, i.e., as much electricity will be used from the network as is necessary for useful work.

Current consumption values ​​with and without PFC blocks are shown in Fig. 3.

As we can see, the efficiency of using power factor correctors (PFC) is very high. Correctors stabilize the consumed current, reduce its value, accordingly reduce power consumption and increase the efficiency of sources. Correctors are widely used in electronic devices, especially in computer technology. The use of these devices in welding equipment, as one of the main consumers of electricity in production, allows significantly reduce costs associated with electricity consumption and increase production efficiency.

In order to increase the operating efficiency of the equipment produced and the compliance of welding equipment with the European standard EN 61000-3-12, the company

Conversion technology

Introduction

In recent decades, the number of electronics used in homes, offices and factories has increased dramatically, and most devices use switching power supplies. Such sources generate harmonic and nonlinear current distortions, which negatively affect the electrical wiring and electrical appliances connected to it. This influence is expressed not only in various types interference, affecting the operation of sensitive devices, but also in overheating of the neutral line. When currents flow in loads with significant harmonic components that are out of phase with the voltage, the current in the neutral wire (which, with a symmetrical load, is practically equal to zero) may increase to a critical value.

The International Electrotechnical Commission (IEC) and the European Organization for Electrotechnical Standardization (CENELEC) have adopted standards IEC555 and EN60555, which set limits on harmonic content in the input current of secondary power supplies, electronic loads fluorescent lamps, DC motor drivers and similar devices.

One of effective ways The solution to this problem is the use of PFC (Power Factor Correction) power factor correctors. In practice, this means that the input circuit of almost any electronic device With pulse converters it is necessary to include a special PFC circuit that provides reduction or complete suppression of current harmonics.

Power factor correction

A typical switching power supply consists of a mains rectifier, a smoothing capacitor and a voltage converter. Such a source consumes power only at those moments when the voltage supplied from the rectifier to the smoothing capacitor is higher than the voltage across it (the capacitor), which occurs for about a quarter of the period. The rest of the time, the source does not consume power from the network, since the load is powered by a capacitor. This leads to the fact that power is taken by the load only at the voltage peak, the consumed current has the form of a short pulse and contains a set of harmonic components (see Fig. 1).

A secondary power source with power factor correction consumes current with low harmonic distortion, takes power from the network more evenly, and has a crest factor (the ratio of the amplitude value of the current to its rms value) lower than that of an uncorrected source. Power factor correction reduces the RMS current consumption, allowing more power to be connected to one outlet. different devices without creating overcurrents in it (see Fig. 2).

Power factor

Power Factor PF is a parameter characterizing the distortion created by the load (in our case, the secondary power source) in the AC network. There are two types of distortions - harmonic and nonlinear. Harmonic distortion is caused by a reactive load and represents a phase shift between current and voltage. Nonlinear distortions are introduced into the network by “nonlinear” loads. These distortions are expressed in the deviation of the current or voltage waveform from a sinusoid. When harmonic distortion The power factor is the cosine of the phase difference between current and voltage or the ratio of active power to total power consumed from the network. For nonlinear distortion The power factor is equal to the share of the power of the first harmonic current component in the total power consumed by the device. It can be considered an indicator of how uniformly the device consumes power from the mains.

In general power factor is the product of the cosine of the phase difference angle between voltage and current and the cosine of the angle between the fundamental harmonic vector and the vector apparent current. The reasoning given below leads to this definition. The effective current flowing in the active load has the form:

I 2 eff =I 2 0 +I 2 1eff +SI 2 neff,

where I 2 neff is the constant component (in the case of sinusoidal voltage it is zero), I 2 1eff is the main harmonic, and under the sum sign are the lower harmonics. When working with a reactive load, a reactive component appears in this expression, and it takes the form:

I 2 eff =I 2 0 +(I 2 1eff(P) +I 2 1eff(Q))+SI 2 neff. Active power- this is the average value of the power allocated to the active load over the period.

It can be represented as a product effective voltage to the active component of the current P=U eff Ch I 1eff(P). Physically, this is energy released in the form of heat per unit time through active resistance. Reactive power is understood as the product of the effective voltage and the reactive component of the current: Q = U eff H I 1 eff (Q). The physical meaning is energy that is pumped twice per period from the generator to the load and twice from the load to the generator. Total power is the product of the effective voltage and the total effective current: S = U eff H I eff (total). On the complex plane, it can be represented as the sum of vectors P and Q, from which the dependence I 2 =I 1eff(total) cos j is visible, where j is the angle between vectors P and Q, which also characterizes the phase difference between the current and voltage in the circuit.

Based on the above, we derive the definition for power factor:

PF=P/S=(I 1eff cos j)/(Ieff(total)).

It is worth noting that the ratio (I 1eff)/(Ieff(total)) is the cosine of the angle between the vectors corresponding to the effective value total current and the effective value of its first harmonic. If we denote this angle as q, then the expression for the power factor takes the form: PF=cos j Х cos q. The task of power factor correction is to bring the phase difference angle j between voltage and current, as well as the harmonic distortion angle q of the consumed current, closer to zero (or, in other words, to bring the shape of the current curve as close as possible to a sinusoid and compensate for the phase shift as much as possible).

The power factor is expressed as decimal, the value of which lies in the range from 0 to 1. Its ideal value is unity (for comparison, a typical switching power supply without correction has a power factor value of about 0.65), 0.95 is a good value; 0.9 - satisfactory; 0.8 - unsatisfactory. Applying power factor correction can increase a device's power factor from 0.65 to 0.95. Values ​​in the range of 0.97...0.99 are also quite realistic. In the ideal case, when the power factor is unity, the device draws sinusoidal current from the network with zero phase shift relative to the voltage (corresponding to a fully resistive load with a linear current-voltage characteristic).

Passive power factor correction

The passive correction method is most often used in inexpensive low-power devices (where there are no strict requirements for the intensity of lower current harmonics). Passive correction allows you to achieve a power factor of about 0.9. This is convenient in the case when the power source has already been designed, all that remains is to create a suitable filter and include it in the circuit at the input.

Passive power factor correction consists of filtering the current consumption using an LC bandpass filter. This method has several limitations. An LC filter can be effective as a power factor corrector only if the voltage, frequency and load vary within a narrow range of values. Since the filter must operate in the low frequency region (50/60 Hz), its components are large in size, weight and low quality factor(which is not always acceptable). Firstly, the number of components with a passive approach is much smaller and, therefore, the time between failures is longer, and secondly, with passive correction, less electromagnetic and contact interference is created than with active one.

Active power factor correction

An active power factor correction must satisfy three conditions:

1) The shape of the consumed current should be as close to sinusoidal as possible and “in phase” with the voltage. The instantaneous value of the current consumed from the source must be proportional to the instantaneous network voltage.

2) The power taken from the source must remain constant even if the network voltage changes. This means that when the network voltage decreases, the load current must be increased, and vice versa.

3) The voltage at the output of the PFC corrector should not depend on the load size. As the voltage across the load decreases, the current through it must increase, and vice versa.

There are several schemes that can be used to implement active power factor correction. The most popular currently is the “boost converter circuit”. This circuit satisfies all the requirements for modern power supplies. Firstly, it allows you to work in networks with different meanings supply voltage (from 85 to 270 V) without restrictions and any additional adjustments. Secondly, she is less susceptible to deviations electrical parameters networks (voltage surges or short-term power outages). Another advantage of this scheme is that it is more simple implementation surge protection. A simplified diagram of a “boost converter” is shown in Fig. 3.

Principle of operation

The standard power factor corrector is an AD/DC converter with pulse width modulation (PWM). The modulator controls a high-power (usually MOSFET) switch that converts DC or rectified mains voltage into a sequence of pulses, after rectification of which a constant voltage is obtained at the output.

Timing diagrams of the corrector's operation are shown in Fig. 4. When the MOSFET switch is turned on, the current in the inductor increases linearly - while the diode is locked, and capacitor C2 is discharged to the load. Then, when the transistor is turned off, the voltage across the inductor “opens” the diode and the energy stored in the inductor charges capacitor C2 (and simultaneously powers the load). In the above circuit (unlike a source without correction), capacitor C1 has a small capacitance and serves to filter high-frequency interference. The conversion frequency is 50...100 kHz. In the simplest case, the circuit operates with a constant duty cycle. There are ways to increase the efficiency of correction by dynamically changing the duty cycle (matching the cycle with the voltage envelope from the mains rectifier).

The "boost converter" circuit can operate in three modes: continuous , discrete and the so-called " critical conductivity mode" IN discrete mode, during each period the inductor current manages to “fall” to zero and after some time begins to increase again, and in continuous- the current, not having time to reach zero, begins to increase again. Mode critical conductivity used less frequently than the previous two. It is more difficult to implement. Its meaning is that the MOSFET opens at the moment when the inductor current reaches zero value. When operating in this mode, adjusting the output voltage is simplified.

The choice of mode depends on the required output power of the power supply. Devices with a power of more than 400 W use continuous mode, while low-power devices use discrete mode. Active power factor correction allows you to achieve values ​​of 0.97...0.99 with a THD (Total Harmonic Distortion) coefficient of 0.04...0.08.

Hello friends! Delving into specifications components, you can see the PFC option in the power supply, what it is, why it is needed and how it works, I will tell you in today’s publication. Go.

Let's remember the school physics course

Those who studied physics well at school remember that power can be active or reactive. Active power is the power that performs useful work– causes an iron to heat up, an incandescent lamp to glow, or powers PC components.

IN reactive circuits The current strength can lag behind the voltage or lead it, which is determined by the cos φ parameter (cosine Phi). With an inductive load, the current lags behind the voltage (inductive load) or leads it (capacitive load).

The latter is often found in complex electrical diagrams where capacitors are used, including in computer power supplies.

Reactive power does not perform any payload, “wandering” through electrical circuits and heating them. It is for this reason that a reserve cross-section of wires is provided. The greater the cos φ, the more energy will be dissipated in the circuit in the form of heat.

Reactive power of computer power supply

Since computer power supplies usually use high-capacity capacitors, the reactive component in such a circuit is noticeable. Fortunately, it is not taken into account by the household electricity meter, so the user will not have to overpay for electricity.

The cos φ value for such devices usually reaches 0.7. This means that the wiring power reserve must be at least 30%. But, since the current flows through the power supply circuit in short pulses with variable amplitude, this reduces the service life of capacitors and diodes.

If the latter do not have a reserve in terms of current strength and are selected “back to back” (as is often the case in cheap power supplies), the service life of such a device is reduced.

To combat these reactive phenomena, a power factor corrector, i.e. PFC, is used.

What is PFC type

There are two types of devices with a Power Factor Correction module:

• With passive - a choke included in the circuit between the capacitors and the rectifier;
• With active - an additional switching power supply to increase the voltage.

The inductor is a device with a complex resistance, the nature of which is symmetrically opposite to the reactivity of the capacitors. This to some extent makes it possible to compensate for negative factors, but cos φ increases slightly.

In addition, the input voltage of the main block of stabilizers is partially stabilized.

Active PFC, that is, an active circuit (APFC), can increase this parameter to 0.95, that is, make it close to ideal. Such a power supply is less susceptible to short-term “dips” of current, allowing it to operate on the charge of capacitors, which is an undeniable advantage.

It should be taken into account that such design features affect the price of the device.

Today on sale you can find power supplies in the ATX form factor, both with power factor correction and without PFC. Whether PFC is needed or not should be decided based on the specific use of the computer. For example, on a gaming computer its presence is desirable, but not at all necessary.

I would like to draw your attention to the following point. Among other things, PFC reduces the level of high-frequency noise on the output lines. Such a power supply is recommended for use in conjunction with peripheral devices for processing analog video and audio signals - for example, in a recording studio.

But even if you are an ordinary amateur who connects an electric guitar to a computer with a Guitar Rig installed, it is recommended to use a power supply with a power factor correction.

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