Design of a switching power supply with active PFC. Episode I. How to make a switching power supply with your own hands

Unlike traditional linear power supplies, which involve extinguishing excess unstabilized voltage on a pass-through linear element, pulse power supplies use other methods and physical phenomena to generate a stabilized voltage, namely: the effect of energy accumulation in inductors, as well as the possibility of high-frequency transformation and conversion of accumulated energy into constant pressure. There are three typical circuits for constructing pulsed power supplies (see Fig. 3.4-1): step-up (output voltage is higher than input voltage), step-down (output voltage is lower than input voltage) and inverting (output voltage has the opposite polarity with respect to the input). As can be seen from the figure, they differ only in the way they connect the inductance; otherwise, the principle of operation remains unchanged, namely.

The key element (usually bipolar or MOS transistors are used), operating with a frequency of the order of 20-100 kHz, is periodically applied for a short time (no more than 50% of the time)

gives the full input unstabilized voltage to the inductor. Pulse current. flowing through the coil ensures the accumulation of energy reserves in its magnetic field of 1/2LI^2 at each pulse. The energy stored in this way from the coil is transferred to the load (either directly, using a rectifying diode, or through the secondary winding with subsequent rectification), the output smoothing filter capacitor ensures a constant output voltage and current. Stabilization of the output voltage is ensured by automatic adjustment of the pulse width or frequency on the key element (a feedback circuit is designed to monitor the output voltage).

This, although quite complex, scheme can significantly increase the efficiency of the entire device. The fact is that, in this case, in addition to the load itself, there are no power elements in the circuit that dissipate significant power. Key transistors operate in the saturated switch mode (i.e., the voltage drop across them is small) and dissipate power only in fairly short time intervals (pulse time). In addition, by increasing the conversion frequency, it is possible to significantly increase power and improve weight and size characteristics.

An important technological advantage of pulse power supplies is the ability to build on their basis small-sized network power supplies with galvanic isolation from the network to power a wide variety of equipment. Such power supplies are built without the use of a bulky low-frequency power transformer using a high-frequency converter circuit. This is, in fact, a typical switching power supply circuit with voltage reduction, where rectified mains voltage is used as the input voltage, and a high-frequency transformer (small-sized and with high efficiency) is used as a storage element, from the secondary winding of which the output stabilized voltage is removed (this transformer also provides galvanic isolation from the network).

The disadvantages of pulsed power supplies include: the presence of a high level of pulsed noise at the output, high complexity and low reliability (especially in handicraft production), the need to use expensive high-voltage high-frequency components, which in the event of the slightest malfunction easily fail “en masse” (with In this case, as a rule, impressive pyrotechnic effects can be observed). Those who like to delve into the insides of devices with a screwdriver and a soldering iron will have to be extremely careful when designing network switching power supplies, since many elements of such circuits are under high voltage.

3.4.1 Efficient low-complexity switching regulator

On an element base similar to that used in the linear stabilizer described above (Fig. 3.3-3), it is possible to build a pulse voltage stabilizer. With the same characteristics, it will have significantly smaller dimensions and better thermal conditions. A schematic diagram of such a stabilizer is shown in Fig. 3.4-2. The stabilizer is assembled according to a standard voltage reduction circuit (Fig. 3.4-1a).

When first turned on, when capacitor C4 is discharged and a sufficiently powerful load is connected to the output, current flows through the linear regulator IC DA1. The voltage drop across R1 caused by this current unlocks the key transistor VT1, which immediately enters saturation mode, since the inductive reactance of L1 is large and a sufficiently large current flows through the transistor. The voltage drop across R5 opens the main key element - transistor VT2. Current. increasing in L1, charges C4, while through feedback on R8 the recording occurs

Damage to the stabilizer and key transistor. The energy stored in the coil powers the load. When the voltage at C4 drops below the stabilization voltage, DA1 and the key transistor open. The cycle is repeated with a frequency of 20-30 kHz.

Circuit R3. R4, C2 will set the output voltage level. It can be smoothly adjusted within small limits, from Uct DA1 to Uin. However, if Uout is raised close to Uin, some instability appears at maximum load and an increased level of ripple. To suppress high-frequency ripples, filter L2, C5 is included at the output of the stabilizer.

The scheme is quite simple and most effective for this level of complexity. All power elements VT1, VT2, VD1, DA1 are equipped with small radiators. The input voltage must not exceed 30 V, which is the maximum for KR142EN8 stabilizers. Use rectifier diodes for a current of at least 3 A.

3.4.2 Uninterruptible power supply device based on a switching stabilizer

In Fig. 3.4-3 we propose for consideration a device for uninterruptible power supply of security and video surveillance systems based on a pulse stabilizer combined with a charger. The stabilizer includes protection systems against overload, overheating, output voltage surges, and short circuits.

The stabilizer has the following parameters:

Input voltage, Uvx - 20-30 V:

Output stabilized voltage, Uvyx-12V:

Rated load current, Iload nom -5A;

Trip current of the overload protection system, Iprotect - 7A;.

Operation voltage of the overvoltage protection system, Uout protection - 13 V;

Maximum battery charging current, Icharge battery max - 0.7 A;

Ripple level. Upulse - 100 mV,

Temperature of operation of the overheating protection system, Tzasch - 120 C;

Switching speed to power from the battery, tswitch - 10ms (relay RES-b RFO.452.112).

The operating principle of the pulse stabilizer in the described device is the same as that of the stabilizer presented above.

The device is supplemented with a charger made on elements DA2, R7, R8, R9, R10, VD2, C7. Voltage stabilizer IC DA2 with current divider on R7. R8 limits the maximum initial charge current, the divider R9, R10 sets the output charge voltage, diode VD2 protects the battery from self-discharge in the absence of supply voltage.

Overheat protection uses thermistor R16 as a temperature sensor. When the protection is triggered, the sound alarm, assembled on the DD 1 IC, turns on and, at the same time, the load is disconnected from the stabilizer, switching to power from the battery. The thermistor is mounted on the radiator of transistor VT1. Fine adjustment of the temperature protection response level is carried out by resistance R18.

The voltage sensor is assembled on the divider R13, R15. resistance R15 sets the exact level of overvoltage protection (13 V). If the voltage at the output of the stabilizer exceeds (if the latter fails), relay S1 disconnects the load from the stabilizer and connects it to the battery. If the supply voltage is turned off, relay S1 goes into the “default” state - i.e. connects the load to the battery.

The circuit shown here does not have electronic short circuit protection for the battery. This role is performed by a fuse in the load power supply circuit, designed for the maximum current consumption.

3.4.3 Power supplies based on high-frequency pulse converter

Quite often, when designing devices, there are strict requirements for the size of the power source. In this case, the only solution is to use a power supply based on high-voltage, high-frequency pulse converters. which are connected to a ~220 V network without the use of a large low-frequency step-down transformer and can provide high power with small size and heat dissipation.

The block diagram of a typical pulse converter powered from an industrial network is shown in Figure 34-4.

The input filter is designed to prevent impulse noise from entering the network. Power switches provide high voltage pulses to the primary winding of a high-frequency transformer (single- and

push-pull circuits). The frequency and duration of the pulses are set by a controlled generator (control of the pulse width is usually used, less often - frequency). Unlike low-frequency sinusoidal signal transformers, pulsed power supplies use broadband devices that provide efficient power transfer on signals with fast edges. This imposes significant requirements on the type of magnetic circuit used and the design of the transformer. On the other hand, with increasing frequency, the required dimensions of the transformer (while maintaining the transmitted power) decrease (modern materials make it possible to build powerful transformers with acceptable efficiency at frequencies up to 100-400 kHz). A special feature of the output rectifier is the use of high-speed Schottky diodes rather than conventional power diodes, which is due to the high frequency of the rectified voltage. The output filter smoothes out output voltage ripple. The feedback voltage is compared with a reference voltage and then controls the oscillator. Please note the presence of galvanic isolation in the feedback circuit, which is necessary if we want to ensure isolation of the output voltage from the network.

In the manufacture of such IP, serious requirements arise for the components used (which increases their cost compared to traditional ones). Firstly, this concerns the operating voltage of the rectifier diodes, filter capacitors and key transistors, which should not be less than 350 V to avoid breakdowns. Secondly, high-frequency key transistors (operating frequency 20-100 kHz) and special ceramic capacitors should be used (conventional oxide electrolytes will overheat at high frequencies due to their high inductance

activity). And thirdly, the saturation frequency of the high-frequency transformer, determined by the type of magnetic core used (as a rule, toroidal cores are used) must be significantly higher than the operating frequency of the converter.

In Fig. 3.4-5 shows a schematic diagram of a classic power supply based on a high-frequency converter. The filter, consisting of capacitors C1, C2, SZ and chokes L1, L2, serves to protect the supply network from high-frequency interference from the converter. The generator is built according to a self-oscillating circuit and combined with a key stage. Key transistors VT1 and VT2 operate in antiphase, opening and closing in turn. Starting the generator and reliable operation is ensured by transistor VT3, operating in avalanche breakdown mode. When the voltage on C6 increases through R3, the transistor opens and the capacitor is discharged to the base of VT2, starting the generator. The feedback voltage is removed from the additional (III) winding of the power transformer Tpl.

Transistors VT1. VT2 is installed on plate radiators of at least 100 cm^2. Diodes VD2-VD5 with a Schottky barrier are placed on a small radiator 5 cm^2. Data of chokes and transformers: L1-1. L2 is wound on ferrite rings 2000NM K12x8x3 into two wires using PELSHO wire 0.25: 20 turns. TP1 - on two rings folded together, ferrite 2000NN KZ 1x18.5x7;

winding 1 - 82 turns with PEV-2 0.5 wire: winding II - 25+25 turns with PEV-2 1.0 wire: winding III - 2 turns with PEV-2 0.3 wire. TP2 is wound on a ferrite ring 2000NN K10x6x5. all windings are made with PEV-2 0.3 wire: winding 1 - 10 turns:

windings II and III - 6 turns each, both windings (II and III) are wound so that they occupy 50% of the area on the ring without touching or overlapping each other, winding I is wound evenly throughout the ring and insulated with a layer of varnished cloth. Rectifier filter coils L3, L4 are wound on ferrite 2000NM K 12x8x3 with PEV-2 1.0 wire, number of turns - 30. KT809A can be used as key transistors VT1, VT2. KT812, KT841.

The element ratings and winding data of the transformers are given for an output voltage of 35 V. In the case when other operating parameter values are required, the number of turns in winding 2 Tr1 should be changed accordingly.

The described circuit has significant drawbacks due to the desire to extremely reduce the number of components used. These include a low level of output voltage stabilization, unstable unreliable operation, and low output current. However, it is quite suitable for powering the simplest designs of different power (if appropriate components are used), such like: calculators. Caller IDs. lighting devices, etc.

Another power supply circuit based on a high-frequency pulse converter is shown in Fig. 3.4-6. The main difference between this scheme and the standard structure presented in Fig. 3 .4-4 is the absence of a feedback circuit. In this regard, the voltage stability on the output windings of the HF transformer Tr2 is quite low and the use of secondary stabilizers is required (the circuit uses universal integrated stabilizers based on the KR142 series IC).

3.4.4 Switching stabilizer with a key MIS transistor with current reading.

Miniaturization and increased efficiency in the development and construction of switching power supplies is facilitated by the use of a new class of semiconductor inverters - MOS transistors, as well as: high-power diodes with fast reverse recovery, Schottky diodes, ultra-high-speed diodes, field-effect transistors with an insulated gate, integrated circuits for controlling key elements. All these elements are available on the domestic market and can be used in the design of highly efficient power supplies, converters, ignition systems for internal combustion engines (ICE), and starting systems for fluorescent lamps (LDL). A class of power devices called HEXSense - MOS transistors with current sensing - may also be of great interest to developers. They are ideal switching elements for ready-to-control switching power supplies. The ability to read switch transistor current can be used in switching power supplies to provide the current feedback required by a pulse width modulation controller. This achieves simplification of the design of the power source - the exclusion of current resistors and transformers from it.

In Fig. Figure 3.4-7 shows a diagram of a 230 W switching power supply. Its main performance characteristics are as follows:

Input voltage: -110V 60Hz:

Output voltage: 48 V DC:

Load current: 4.8 A:

Switching frequency: 110 kHz:

Efficiency at full load : 78%;

Efficiency at 1/3 load: 83%.

The circuit is built on the basis of a pulse-width modulator (PWM) with a high-frequency converter at the output. The operating principle is as follows.

The control signal for the key transistor comes from output 6 of the PWM controller DA1, the duty cycle is limited to 50% by resistor R4, R4 and SZ are the timing elements of the generator. Power supply for DA1 is provided by the chain VD5, C5, C6, R6. Resistor R6 is designed to supply supply voltage during generator startup; subsequently, voltage feedback through LI, VD5 is activated. This feedback is obtained from the additional winding of the output choke, which operates in reverse mode. In addition to powering the generator, the feedback voltage through the chain VD4, Cl, Rl, R2 is supplied to the voltage feedback input DA1 (pin 2). Through R3 and C2 compensation is provided, which guarantees the stability of the feedback loop.

Based on this circuit, it is possible to build pulse stabilizers with other output parameters.

Most modern electronic devices practically do not use analog (transformer) power supplies; they are replaced by pulsed voltage converters. To understand why this happened, it is necessary to consider the design features, as well as the strengths and weaknesses of these devices. We will also talk about the purpose of the main components of pulsed sources and provide a simple example of an implementation that can be assembled with your own hands.

Design features and operating principle

Of the several methods of converting voltage to power electronic components, two that are most widespread can be identified:

Analogue, the main element of which is a step-down transformer, in addition to its main function, it also provides galvanic isolation.
Impulse principle.

Let's look at how these two options differ.

PSU based on a power transformer

Let's consider a simplified block diagram of this device. As can be seen from the figure, a step-down transformer is installed at the input, with its help the amplitude of the supply voltage is converted, for example, from 220 V we get 15 V. The next block is a rectifier, its task is to convert the sinusoidal current into a pulsed one (the harmonic is shown above the symbolic image). For this purpose, rectifying semiconductor elements (diodes) connected via a bridge circuit are used. Their operating principle can be found on our website.

The next block performs two functions: it smoothes the voltage (a capacitor of appropriate capacity is used for this purpose) and stabilizes it. The latter is necessary so that the voltage does not “drop” when the load increases.

The given block diagram is greatly simplified; as a rule, a source of this type has an input filter and protective circuits, but this is not important for explaining the operation of the device.

All the disadvantages of the above option are directly or indirectly related to the main design element - the transformer. Firstly, its weight and dimensions limit miniaturization. In order not to be unfounded, we will use as an example a step-down transformer 220/12 V with a rated power of 250 W. The weight of such a unit is about 4 kilograms, dimensions 125x124x89 mm. You can imagine how much a laptop charger based on it would weigh.

Secondly, the price of such devices is sometimes many times higher than the total cost of the other components.

Pulse devices

As can be seen from the block diagram shown in Figure 3, the operating principle of these devices differs significantly from analog converters, primarily in the absence of an input step-down transformer.

Figure 3. Block diagram of a switching power supply

Let's consider the operating algorithm of such a source:

Power is supplied to the network filter; its task is to minimize network noise, both incoming and outgoing, that arises as a result of operation.
Next, the unit for converting sinusoidal voltage into pulsed constant voltage and a smoothing filter come into operation.
At the next stage, an inverter is connected to the process; its task is related to the formation of rectangular high-frequency signals. Feedback to the inverter is carried out through the control unit.
The next block is IT, it is necessary for automatic generator mode, supplying voltage to the circuit, protection, controller control, as well as the load. In addition, the IT task includes ensuring galvanic isolation between high and low voltage circuits.

Unlike a step-down transformer, the core of this device is made of ferrimagnetic materials, this contributes to the reliable transmission of RF signals, which can be in the range of 20-100 kHz. A characteristic feature of IT is that when connecting it, the inclusion of the beginning and end of the windings is critical. The small dimensions of this device make it possible to produce miniature devices; an example is the electronic harness (ballast) of an LED or energy-saving lamp.

Next, the output rectifier comes into operation, since it works with high-frequency voltage; the process requires high-speed semiconductor elements, so Schottky diodes are used for this purpose.
At the final phase, smoothing is performed on an advantageous filter, after which voltage is applied to the load.

Now, as promised, let’s look at the operating principle of the main element of this device – the inverter.

How does an inverter work?

RF modulation can be done in three ways:

pulse-frequency;
phase-pulse;
pulse width.

In practice, the last option is used. This is due both to the simplicity of implementation and to the fact that PWM has a constant communication frequency, unlike the other two modulation methods. A block diagram describing the operation of the controller is shown below.

The operating algorithm of the device is as follows:

The reference frequency generator generates a series of rectangular signals, the frequency of which corresponds to the reference one. Based on this signal, a sawtooth U P is formed, which is supplied to the input of the comparator K PWM. The UUS signal coming from the control amplifier is supplied to the second input of this device. The signal generated by this amplifier corresponds to the proportional difference between U P (reference voltage) and U RS (control signal from the feedback circuit). That is, the control signal UUS is, in fact, a mismatch voltage with a level that depends on both the current on the load and the voltage on it (U OUT).

This implementation method allows you to organize a closed circuit that allows you to control the output voltage, that is, in fact, we are talking about a linear-discrete functional unit. Pulses are generated at its output, with a duration depending on the difference between the reference and control signals. Based on it, a voltage is created to control the key transistor of the inverter.

The process of stabilizing the output voltage is carried out by monitoring its level; when it changes, the voltage of the control signal U PC changes proportionally, which leads to an increase or decrease in the duration between pulses.

As a result, the power of the secondary circuits changes, which ensures stabilization of the output voltage.

To ensure safety, galvanic isolation between the power supply and feedback is required. As a rule, optocouplers are used for this purpose.

Strengths and weaknesses of pulsed sources

If we compare analog and pulse devices of the same power, the latter will have the following advantages:

Small size and weight due to the absence of a low-frequency step-down transformer and control elements that require heat removal using large radiators. Thanks to the use of high-frequency signal conversion technology, it is possible to reduce the capacitance of the capacitors used in the filters, which allows the installation of smaller elements.
Higher efficiency, since the main losses are caused only by transient processes, while in analog circuits a lot of energy is constantly lost during electromagnetic conversion. The result speaks for itself, increasing efficiency to 95-98%.
Lower cost due to the use of less powerful semiconductor elements.
Wider input voltage range. This type of equipment is not demanding in terms of frequency and amplitude; therefore, connection to networks of various standards is allowed.
Availability of reliable protection against short circuits, overload and other emergency situations.

The disadvantages of pulse technology include:

The presence of RF interference is a consequence of the operation of the high-frequency converter. This factor requires the installation of a filter that suppresses interference. Unfortunately, its operation is not always effective, which imposes some restrictions on the use of devices of this type in high-precision equipment.

Special requirements for the load, it should not be reduced or increased. As soon as the current level exceeds the upper or lower threshold, the output voltage characteristics will begin to differ significantly from the standard ones. As a rule, manufacturers (even recently Chinese ones) provide for such situations and install appropriate protection in their products.

Scope of application

Almost all modern electronics are powered from blocks of this type, as an example:

Assembling a switching power supply with your own hands

Let's consider the circuit of a simple power supply, where the above-described principle of operation is applied.

Designations:

Resistors: R1 – 100 Ohm, R2 – from 150 kOhm to 300 kOhm (selectable), R3 – 1 kOhm.
Capacities: C1 and C2 – 0.01 µF x 630 V, C3 -22 µF x 450 V, C4 – 0.22 µF x 400 V, C5 – 6800-15000 pF (selectable), 012 µF, C6 – 10 µF x 50 V, C7 – 220 µF x 25 V, C8 – 22 µF x 25 V.
Diodes: VD1-4 - KD258V, VD5 and VD7 - KD510A, VD6 - KS156A, VD8-11 - KD258A.
Transistor VT1 – KT872A.
Voltage stabilizer D1 - microcircuit KR142 with index EH5 - EH8 (depending on the required output voltage).
Transformer T1 - a w-shaped ferrite core with dimensions 5x5 is used. The primary winding is wound with 600 turns of wire Ø 0.1 mm, the secondary (pins 3-4) contains 44 turns Ø 0.25 mm, and the last winding contains 5 turns Ø 0.1 mm.
Fuse FU1 – 0.25A.

The setup comes down to selecting the values of R2 and C5, which ensure excitation of the generator at an input voltage of 185-240 V.

Technical progress does not stand still, and today transformer-type power supplies have been replaced by switching units. There are many reasons for this, but the most important are:

Simplicity and low cost of production;
Ease of use;
Compact and significantly comfortable overall dimensions.

Read the guide on how to choose a hidden wiring detector and how to use it.

From a technical point of view, a switching power supply is a device that rectifies the mains voltage and then forms a pulse from it with a frequency response of 10 kHz. It is worth noting that the efficiency of this technical device reaches 80%.

Principle of operation

In fact, the entire principle of operation of a switching power supply boils down to the fact that a device of this type is aimed at rectifying the voltage that is supplied to it when connected to the network and then forming a working pulse, due to which this electrical unit can function.

Many people wonder what are the main differences between a pulse device and a regular one? It all comes down to the fact that it has improved technical characteristics and smaller overall dimensions. Also, the pulse unit provides more energy than its standard version.

Kinds

At the moment, on the territory of the Russian Federation, if necessary, you can find switching power supplies of the following varieties and categories:

Downtime on IR2153 - this modification is the most popular among domestic consumers;
On TL494
On UC3842
From an energy-saving lamp - it is something like a modified technical device of a hybrid type;
For an amplifier – it has high technical characteristics;
From electronic ballast - it is clear from the name that the device is based on the operation of an electronic type balance. Read the review of what types of LED lamps there are for the home and how to choose.
Adjustable - this type of mechanical unit can be configured and adjusted on its own;
For UMZCH - has a narrow specific application;
Powerful – has high power characteristics;
200 volts - this type of device is designed for a maximum voltage of 220V;
Network 150 W – works only from the network, maximum power – 150 W;
12 V – a technical device that can function normally at a voltage of 12 V;
24 V – normal operation of the device is possible only at 24 V
Bridge – during assembly, a bridge connection scheme was used;
For a tube amplifier - all technical specifications are designed to work with a tube amplifier;
For LEDs – has high sensitivity, used for working with LEDs;
Bipolar has double polarity, the device meets high quality standards;
Flyback - focused on reverse operation, has high power and voltage ratings.

Scheme

All switching power supplies, depending on the scope of operation and technical features, have different circuits:

12 V - is the standard option for assembling a system of this type;
2000 W - this circuit is intended only for high-power technical devices;
For an 18 V screwdriver, the circuit is specific and requires special knowledge from the master during assembly;
For a tube amplifier - in this case we are talking about a simple schematic design, which, among other things, takes into account the output to the tube amplifier;
For laptops – requires the presence of a special system of protection against voltage surges;
On the Top 200 - the technical characteristics of the device will be 40 V and 3 A. Read about the design of the alternator.
On TL494 the circuit takes into account current limitation and input voltage regulation;
On UC3845, assembling a switching power supply according to this scheme is not difficult;
switching power supply based on ir2153 circuit - applicable for low-frequency amplifiers;
On the LNK364PN chip – implemented on the basis of the microcircuit design of UC 3842;
On a field-effect transistor, it is already clear from the name that this circuit is applicable to a field-effect transistor;
The circuit of a forward-mode switching power supply is simple in design and does not require special skills during assembly.

Repair

The principle of realizing secondary power through the use of additional devices that provide energy to circuits has been used for quite a long time in most electrical appliances. These devices are power supplies. They serve to convert voltage to the required level. PSUs can be either built-in or separate elements. There are two principles for converting electricity. The first is based on the use of analog transformers, and the second is based on the use of switching power supplies. The difference between these principles is quite big, but, unfortunately, not everyone understands it. In this article we will figure out how a switching power supply works and how it differs so much from an analog one. Let's get started. Go!

Transformer power supplies were the first to appear. Their operating principle is that they change the voltage structure using a power transformer, which is connected to a 220 V network. There, the amplitude of the sinusoidal harmonic is reduced, which is sent further to the rectifier device. Then the voltage is smoothed by a parallel connected capacitor, which is selected according to the permissible power. Voltage regulation at the output terminals is ensured by changing the position of trimming resistors.

Now let's move on to pulse power supplies. They appeared a little later, however, they immediately gained considerable popularity due to a number of positive features, namely:

Availability of packaging;
Reliability;
Possibility to expand the operating range for output voltages.

All devices that use the principle of pulsed power supply are practically no different from each other.

The elements of a pulse power supply are:

Linear power supply;
Standby power supply;
Generator (ZPI, control);
Key transistor;
Optocoupler;
Control circuits.

To select a power supply with a specific set of parameters, use the ChipHunt website.

Let's finally figure out how a switching power supply works. It uses the principles of interaction between the elements of the inverter circuit and it is thanks to this that a stabilized voltage is achieved.

First, the rectifier receives a normal voltage of 220 V, then the amplitude is smoothed using capacitive filter capacitors. After this, the passing sinusoids are rectified by the output diode bridge. Then the sinusoids are converted into high-frequency pulses. The conversion can be performed either with galvanic separation of the power supply network from the output circuits, or without such isolation.

If the power supply is galvanically isolated, then the high-frequency signals are sent to a transformer, which performs galvanic isolation. To increase the efficiency of the transformer, the frequency is increased.

The operation of a pulse power supply is based on the interaction of three chains:

PWM controller (controls pulse width modulation conversion);
A cascade of power switches (consists of transistors that are switched on according to one of three circuits: bridge, half-bridge, with a midpoint);
Pulse transformer (has primary and secondary windings, which are mounted around the magnetic core).

If the power supply is without decoupling, then the high-frequency isolation transformer is not used, and the signal is fed directly to the low-pass filter.

Comparing switching power supplies with analog ones, you can see the obvious advantages of the former. UPSs have less weight, while their efficiency is significantly higher. They have a wider supply voltage range and built-in protection. The cost of such power supplies is usually lower.

Disadvantages include the presence of high-frequency interference and power limitations (both at high and low loads).

You can check the UPS using a regular incandescent lamp. Please note that you should not connect the lamp into the gap of the remote transistor, since the primary winding is not designed to pass direct current, so under no circumstances should it be allowed to pass.

If the lamp lights up, then the power supply is working normally, but if it doesn’t light up, then the power supply is not working. A short flash indicates that the UPS is locked immediately after startup. A very bright glow indicates a lack of stabilization of the output voltage.

Now you will know what the operating principle of switching and conventional analog power supplies is based on. Each of them has its own structural and operating features that should be understood. You can also check the performance of the UPS using a regular incandescent lamp. Write in the comments if this article was useful to you and ask any questions you have about the topic discussed.

Switching power supplies (SMPS) are the most widely used today and are successfully used in all modern radio-electronic devices.

Figure 3 shows a block diagram of a switching power supply made according to a traditional circuit. The secondary rectifiers are made according to a half-wave circuit. The names of these nodes reveal their purpose and do not need explanation. The main components of the primary circuit are: input filter, mains voltage rectifier and HF rectified supply voltage converter with transformer.

Line rectifier filter

Transformer

RF converter

Secondary rectifiers

Input filter

Figure 3 - Block diagram of a pulse power supply

The basic principle underlying the operation of the SMPS is the conversion of an alternating mains voltage of 220 volts and a frequency of 50 Hz into an alternating high-frequency rectangular voltage, which is transformed to the required values, rectified and filtered.

The conversion is carried out using a powerful transistor operating in switch mode and a pulse transformer, together forming an RF converter circuit. As for the circuit design, there are two possible converter options: the first is made according to a pulse self-oscillator circuit (for example, this was used in the UPS of TVs) and the second with external control (used in most modern radio-electronic devices).

Since the frequency of the converter is usually selected from 18 to 50 kHz, the dimensions of the pulse transformer, and, consequently, the entire power supply, are quite compact, which is an important parameter for modern equipment. A simplified diagram of a pulse converter with external control is shown in Figure 4.

Figure 4 - Schematic diagram of a pulse power supply with a power supply unit.

The converter is made on transistor VT1 and transformer T1. The mains voltage is supplied through the mains filter (SF) to the mains rectifier (SV), where it is rectified, filtered by the filter capacitor (SF) and through the winding W1 of the transformer T1 is supplied to the collector of the transistor VT1. When a rectangular pulse is applied to the base circuit of the transistor, the transistor opens and an increasing current flows through it I j. The same current will flow through the winding W1 of transformer T1, which will lead to the magnetic flux in the transformer core increasing, while a self-induction emf is induced in the secondary winding W2 of the transformer. Ultimately, a positive voltage will appear at the output of the diode VD. Moreover, if we increase the duration of the pulse applied to the base of transistor VT1, the voltage in the secondary circuit will increase, because more energy will be released, and if the duration is reduced, the voltage will decrease accordingly. Thus, by changing the pulse duration in the base circuit of the transistor, we can change the output voltages of the secondary winding T1, and therefore stabilize the output voltages of the power supply. The only thing that is needed for this is a circuit that will generate trigger pulses and control their duration (latitude). A PWM controller is used as such a circuit. PWM – pulse width modulation.

To stabilize the output voltages of the UPS, the PWM controller circuit “must know” the magnitude of the output voltages. For these purposes, a tracking circuit (or feedback circuit) is used, made on optocoupler U1 and resistor R2. An increase in voltage in the secondary circuit of transformer T1 will lead to an increase in the intensity of the LED radiation, and therefore a decrease in the junction resistance of the phototransistor (part of the optocoupler U1). Which in turn will lead to an increase in the voltage drop across resistor R2, which is connected in series with the phototransistor and a decrease in the voltage at pin 1 of the PWM controller. A decrease in voltage causes the logic circuit included in the PWM controller to increase the pulse duration until the voltage at the 1st pin corresponds to the specified parameters. When the voltage decreases, the process is reversed.

The UPS uses two principles for implementing tracking circuits - “direct” and “indirect”. The method described above is called “direct”, since the feedback voltage is removed directly from the secondary rectifier. With “indirect” tracking, the feedback voltage is removed from the additional winding of the pulse transformer (Figure 5).

Figure 5 - Schematic diagram of a pulse power supply with a power supply unit.

A decrease or increase in the voltage on winding W2 will lead to a change in voltage on winding W3, which is also applied through resistor R2 to pin 1 of the PWM controller.

SMPS protection against short circuit.

Short circuit (SC) in the UPS load. In this case, all the energy supplied to the secondary circuit of the UPS will be lost and the output voltage will be almost zero. Accordingly, the PWM controller circuit will try to increase the pulse duration in order to raise the level of this voltage to the appropriate value. As a result, transistor VT1 will remain open longer and longer, and the current flowing through it will increase. Ultimately, this will lead to the failure of this transistor. The UPS provides protection for the converter transistor against current overloads in such emergency situations. It is based on a resistor Rprotection, connected in series to the circuit through which the collector current Ik flows. An increase in the current Ik flowing through transistor VT1 will lead to an increase in the voltage drop across this resistor, and, consequently, the voltage supplied to pin 2 of the PWM controller will also decrease. When this voltage drops to a certain level, which corresponds to the maximum permissible current of the transistor, the logic circuit of the PWM controller will stop generating pulses at pin 3 and the power supply will go into protection mode or, in other words, turn off.

In conclusion, it is necessary to dwell in detail on the advantages of the UPS. As already mentioned, the frequency of the pulse converter is quite high, and therefore the overall dimensions of the pulse transformer are reduced, which means, as paradoxical as it may sound, the cost of a UPS is less than a traditional power supply because less metal consumption for the magnetic core and copper for the windings, even though the number of parts in the UPS increases. Another advantage of the UPS is the small capacitance of the secondary rectifier filter capacitor compared to a conventional power supply. Reducing the capacitance was made possible by increasing the frequency. And finally, the efficiency of a switching power supply reaches 80%. This is due to the fact that the UPS consumes power from the electrical network only when the converter transistor is open; when it is closed, energy is transferred to the load due to the discharge of the secondary circuit filter capacitor.

Disadvantages include increased complexity of the UPS circuit and an increase in pulse noise emitted by the UPS. The increase in interference is due to the fact that the converter transistor operates in switch mode. In this mode, the transistor is a source of pulse noise that occurs during transient processes of the transistor. This is a disadvantage of any transistor operating in switching mode. But if the transistor operates with low voltages (for example, transistor logic with a voltage of 5V), this is not a problem; in our case, the voltage applied to the collector of the transistor is approximately 315 V. To combat this interference, the UPS uses more complex network circuits filters than in a conventional power supply.