Switching power supply 180W

The power of the power supply is about 180 W, the output voltage is 2x25 V at a load current of 3.5 A. The ripple range at a load current of 3.5 A does not exceed 10% for a conversion frequency of 100 Hz and 2% for a frequency of 27 kHz. The output impedance does not exceed 0.6 Ohm. Block dimensions - 170x80x35 mm; weight - 450 g.

After rectification by the diode bridge VD1, the mains voltage is filtered by capacitors C1-C4 (see diagram). Resistor R1 limits the charging current of the filter capacitors flowing through the rectifier diodes when the unit is turned on. The filtered voltage is supplied to a voltage converter built according to a half-bridge inverter circuit using transistors VT1, VT2. The converter is loaded with the primary winding of transformer T1, which converts the voltage and galvanically isolates the output of the unit from the AC mains. Capacitors C3 and C4 prevent RF interference from the power supply from entering the network. A half-bridge inverter converts direct voltage into rectangular alternating voltage with a frequency of 27 kHz. Transformer T1 is designed so that its magnetic circuit is not saturated. The self-oscillating mode of operation is ensured by a feedback circuit, the voltage of which is removed from winding III of transformer T1 and supplied to winding I of auxiliary transformer T2. Resistor R4 limits the voltage on winding I of transformer T2. The conversion frequency depends within certain limits on the resistance of this resistor (see note at the end of the page). You can read in detail about the operation of converters with a non-saturable transformer in.

To ensure reliable startup of the converter and its stable operation, a startup unit is used, which is a relaxation generator based on a VTZ transistor operating in avalanche mode. When the power is turned on, capacitor C5 begins to charge through resistor R5 and when the voltage across it reaches 50...70 V, transistor VTZ opens like an avalanche and the capacitor is discharged. The current pulse opens transistor VT2 and starts the converter.

Transistors VT1 and VT2 are installed on heat sinks with an area of ​​50 cm2 each. Diodes VD2-VD5 are also equipped with plate heat sinks. The diodes are sandwiched between five duralumin plates measuring 40x30 mm each (the three middle plates are 2 mm thick, the two outer ones are 3 mm thick). The entire package is tightened with two M3x30 screws passed through the holes in the plates. To prevent the plates from being locked with screws, pieces of polyvinyl chloride tube are placed on them.

The winding characteristics of transformers are summarized in the table.

Transformer		Number of turns		Magnetic core
				Ferrite 2000NN, two rings K31x18.5x7 glued together
				Ferrite 2000NN, ring K10x6x5

Winding wire - PEV-2. Winding I is placed evenly along the length of the ring. To facilitate starting the converter, winding III of transformer T1 should be located in a place not occupied by winding II (see figure). Inter-winding insulation in transformers is made with varnished fabric tape. Between windings I and II of transformer T1 there is three-layer insulation, between the remaining windings of transformers it is single-layer.
Capacitors C3, C4 in the block - K73P-3; C1, C2 - K50-12; C5 - K73-11; S8,S9 - KM-5; C6, C7 -- K52-2. Transistors KT812A can be replaced with KT812B, KT809A, KT704A-KT704V, diodes KD213A with KD213B.

A correctly assembled power supply usually does not need adjustment, but in some cases it may be necessary to select a VT3 transistor. To check its functionality, temporarily disconnect the emitter output and connect it to the negative terminal of the network rectifier. The voltage on capacitor C5 is observed on the oscilloscope screen - a sawtooth signal with a swing of 20...50 V and a frequency of several hertz. If there is no ramp voltage, the transistor must be replaced.

The use of this power source does not eliminate the need to block the output power circuits of the amplifier with large capacitors. Connecting such capacitors further reduces the ripple level.

Literature

1. V. Tsibulsky Economical power supply. Radio, 1981, No. 10, p. 56.
2. Romash E. M. Sources of secondary power supply for radio-electronic equipment. - M.: Radio and Communications, 1981.
3. Biryukov S. Digital frequency meter power supply, - Radio. 1981. No. 12, p. 54, 55.

D. BARABOSHKIN
Radio, 6/85

NOTE

When turning on the power supply, measure the conversion frequency (at the terminals of winding II) - it may be significantly lower than 27 kHz (for example, 9 - 12 kHz). And although the device will work, the power transistors will fail due to overheating. Frequency adjustment is carried out by resistor R4. Moreover, the rating may differ from that indicated on the diagram by tens of ohms.
A properly configured power supply works great; at a load of 50 - 70%, the power transistors remain cold.

The sound quality depends almost as much on the parameters of the power source as on the amplifier itself, and you should not be negligent in its manufacture. There are more than enough descriptions of calculation methods for standard transformers. Therefore, here is a description of a switching power supply that can be used not only with amplifiers based on the TDA7293 (TDA7294), but also with any other 3H power amplifier.

The basis of this power supply unit (PSU) is a half-bridge driver with an internal oscillator IR2153 (IR2155), designed to control MOSFET and IGBT technology transistors in switching power supplies. The functional diagram of the microcircuits is shown in Figure 1, the dependence of the output frequency on the ratings of the RC-drive chain in Figure 2. The microcircuit provides a pause between the pulses of the “upper” and “lower” switches for 10% of the pulse duration, which allows you not to worry about “through” currents in the power part of the converter.

Rice. 1

Rice. 2

The practical implementation of the power supply is shown in Figure 3. Using this circuit, you can make a power supply with a power from 100 to 500 W, you only need to proportionally increase the capacitance of the primary power filter capacitor C2 and use the corresponding power transformer TV2.

Rice. 1

The capacitance of capacitor C2 is selected at the rate of 1...1.5 µF per 1 W of output power, for example, when manufacturing a 150 W power supply, a capacitor of 150...220 µF should be used. The VD primary power supply diode bridge can be used in accordance with the installed primary power supply filter capacitor; with capacitances up to 330 μF, 4...6 A diode bridges can be used, for example RS407 or RS607. With a capacitor capacity of 470... 680 µF, more powerful diode bridges are needed, for example RS807, RS1007.
We can talk about the manufacture of a transformer for a long time, but not everyone needs to delve into the deep theory of calculations for too long. Therefore, calculations according to Eranosyan’s book for the most popular standard sizes of ferrite rings M2000NM1 are simply summarized in Table 1.
As can be seen from the table, the overall power of a transformer depends not only on the dimensions of the core, but also on the conversion frequency. It is not very logical to make a transformer for frequencies below 40 kHz - harmonics can create insurmountable interference in the audio range. The manufacture of transformers for frequencies above 100 kHz is no longer permissible due to self-heating of the M2000NM1 ferrite by eddy currents. The table shows data on the primary windings, from which the turns/volt ratios can be easily calculated, and then it will not be difficult to calculate how many turns are needed for a particular output voltage. It should be noted that the voltage supplied to the primary winding is 155 V - the mains voltage of 220 V after the rectifier and smoothing filter will be 310 V DC, the circuit is semi-bridged, therefore half of this value will be applied to the primary winding. It should also be remembered that the shape of the output voltage will be rectangular, therefore, after the rectifier and smoothing filter, the voltage value will not differ significantly from the calculated value.
The diameters of the required wires are calculated from a ratio of 5 A per 1 sq mm of wire cross-section. Moreover, it is better to use several wires of smaller diameter than one, thicker wire. This requirement applies to all voltage converters with a conversion frequency above 10 kHz, since the skin effect - losses inside the conductor - is already beginning to affect, since at high frequencies the current no longer flows across the entire cross-section, but along the surface of the conductor, and the higher the frequency, the stronger the effect losses in thick conductors. Therefore, it is not recommended to use conductors thicker than 1 mm in converters with conversion frequencies above 30 kHz. You should also pay attention to the phasing of the windings - incorrectly phasing windings can either damage the power switches or reduce the efficiency of the converter. But let’s return to the power supply shown in Figure 3. The minimum power of this power supply is practically unlimited, so you can make a power supply with 50 W or less. The upper power limit is limited by certain features of the element base.
To obtain higher powers, more powerful MOSFET transistors are required, and the more powerful the transistor, the greater the capacitance of its gate. If the gate capacitance of a power transistor is quite high, then a significant current is required to charge and discharge it. The current of the IR2153 control transistors is quite small (200 mA), therefore, this microcircuit cannot control too powerful power transistors at high conversion frequencies.
Based on the above, it becomes clear that the maximum output power of a converter based on IR2153 cannot be more than 500...600 W at a conversion frequency of 50...70 kHz, since the use of more powerful power transistors at these frequencies quite seriously reduces the reliability of the device. The list of recommended transistors for power switches VT1, VT2 with brief characteristics is summarized in Table 2.
Rectifier diodes of secondary power circuits must have the shortest recovery time and at least two times the voltage reserve and three times the current. The latest requirements are justified by the fact that the self-induction voltage surges of a power transformer amount to 20...50% of the output voltage amplitude. For example, with a secondary power supply of 100 V, the amplitude of self-induction pulses can be 120... 150 V and despite the fact that the duration of the pulses is extremely short, it is enough to cause a breakdown in the diodes, when using diodes with a reverse voltage of 150 V. Threefold reserve current is necessary so that the diodes do not fail at the moment of switching on, since the capacitance of the secondary power filter capacitors is quite high, and quite a small current will be required to charge them. The most suitable diodes VD4-VD11 are summarized in Table 3.

The capacity of the secondary power filters (C11, C12) should not be increased too much, since the conversion is carried out at fairly high frequencies. To reduce ripple, it is much more important to use large capacitance in the primary power circuits and correctly calculate the power of the power transformer. In the secondary circuits, capacitors of 1000 μF per arm are quite sufficient for amplifiers up to 100 W (the power supply capacitors installed on the UMZCH boards themselves must be at least 470 μF) and 4700 μF for a 500 W amplifier. The circuit diagram shows a version of the secondary power supply rectifiers, made on Schottky diodes, and a printed circuit board is installed under them (Figure 4). Diodes VD12, VD13 are used as a rectifier for the forced cooling fan of heat sinks; diodes VD14-VD17 are used as a rectifier for low-voltage power supply (pre-amplifiers, active tone controls, etc.). The same figure shows a drawing of the location of parts and a connection diagram. The converter has overload protection made on the current transformer TV1, consisting of a K20x12x6 ring of M2000 ferrite and containing 3 turns of the primary winding (the cross-section is the same as the primary winding of the power transformer and 3 turns of the secondary winding, wound with a double wire with a diameter of 0.2.. .0.3 mm. If there is an overload, the voltage on the secondary winding of transformer TV1 will become sufficient to open the thyristor VS1 and it will open, closing the power supply to the IR2153 chip, thereby stopping its operation. The protection threshold is adjusted by resistor R8. The adjustment is made without load, starting with maximum sensitivity. and achieving stable startup of the converter. The principle of adjustment is based on the fact that at the moment of starting the converter it is loaded to the maximum, since it is necessary to charge the capacitance of the secondary power filters and the load on the power part of the converter is maximum.

About the remaining details: capacitor C5 - film capacitor 0.33... 1 µF 400V; capacitors C9, C10 - film capacitors 0.47...2.2 µF at least 250V; inductances L1...L3 are made on K20x12x6 M2000 ferrite rings and are wound with 0.8...1.0 mm wire until they are filled turn to turn in one layer; C14, C15 - film 0.33...2.2 µF for a voltage of at least 100 V with an output voltage of up to 80 V; capacitors C1, C4, C6, C8 can be ceramic, type K10-73 or K10-17; C7 can also be ceramic, but film, such as K73-17, is better.

Many radio amateurs are interested in how a switching power supply works and on what mechanisms it is based. Let's take a closer look at the example of a block from the BBK DV811X DVD player. This block was chosen because all the components of the circuit are free, clear, and not filled with glue. This will greatly help beginners understand how they work. For comparison, a typical laptop power supply. It is difficult to immediately understand what is here and where.
To clearly explain all the points, we will build a schematic diagram. We will tell you as simply as possible about each element, why it is there and what function it performs.

Let's consider the general principles of operation of power supplies.
For starters, linear.

In it, the mains voltage is supplied to a transformer, which lowers it, after which there is a rectifier, filter and stabilizer. Transformers in such blocks are large in size and most often find their use in laboratory power supplies and audio amplifiers.

Now switching power supplies. 220 volts is rectified, after which the direct voltage is converted into pulses with a higher frequency, which are fed to a high-frequency transformer. The voltage is removed from the output windings and straightened. It is then fed through a feedback circuit to the pulse shaper to maintain a stable output voltage by adjusting the duration or duty cycle of the pulses. The rectified one is filtered to obtain a stable value.
Explanation of the circuit
The terminals are powered from a 220 volt network and the power button, and we see the fuse. When the current passing through the fuse exceeds its rated threshold, it burns out, disconnecting the power supply from the network. Next we see the surge protector.

It consists of two capacitors and an electromagnetic interference suppression choke.
Let's look at a typical circuit of this filter. Most modern devices are equipped with such a filter. It consists of 2 X capacitors and an EMI choke. These are capacitors that have been specially designed for the use of surge protectors. They can withstand voltage surges of up to several kilovolts and are made of non-flammable materials. For anti-phase interference that occurs between phase and neutral, this is the shortest path to follow, which means they prevent network interference from entering the power supply and, accordingly, power supply noise from entering the network.
When it comes to EMI suppression chokes, there are many types, but in general, they are coils wound on a ferrite core. Interference induces current of different signs, compensating each other. It is worth adding something else about common-mode interference - between the phase and the housing or between the neutral and the housing. To compensate for such interference, so-called Y-capacitors are often used. In case of burnout, they will definitely be open. They also withstand voltage surges. A pair of such capacitors is connected between the network wires and the housing. And the housing, in turn, is connected to ground.

If there is no grounding in your outlet, then the device body will draw about 110 Volts with a very small current. This power supply provides seats for these capacitors.

But the manufacturer supplied the network wire without grounding. Therefore, there is no point in these capacitors in this case. After the mains filter there is a diode bridge made of 4 1n 4007 diodes. The rectified voltage is supplied to the capacitor. It smoothes its shape. The capacitor in this case is 22 microfarads, 400 volts. The voltage across the capacitor should be about 290-300 volts. Now we need to convert it into a high frequency pulse train. First, let's see what kind of microcircuit this is. Marking dh321. Let's look at how such converters are structured in general.

Online calculator: http://cxem.net/calc/divider_calc.php

Questions about switching power supplies: http://forum.cxem.net/index.php?showtopic=1480

Or create a winding, you can assemble a switching type power supply with your own hands, which requires a transformer with only a few turns.

In this case, a small number of parts are required, and the work can be completed in 1 hour. In this case, the IR2151 chip is used as the basis for the power supply.

For work you will need the following materials and parts:

1. PTC thermistor any type.
2. Pair of capacitors, which are selected with the calculation of 1 μF. at 1 W. When creating the design, we select capacitors so that they draw 220 W.
3. Diode assembly"vertical" type.
4. Drivers type IR2152, IR2153, IR2153D.
5. Field effect transistors type IRF740, IRF840. You can choose others if they have a good resistance indicator.
6. Transformer can be taken from old computer system units.
7. Diodes, installed at the outlet, it is recommended to take from the HER family.

In addition, you will need the following tools:

1. Soldering iron and consumables.
2. Screwdriver and pliers.
3. Tweezers.

Also, do not forget about the need for good lighting at the work site.

Step by step instructions

circuit diagram
block diagram

Assembly is carried out according to the drawn circuit diagram. The microcircuit was selected according to the characteristics of the circuit.

Assembly is carried out as follows:

1. At the entrance install a PTC thermistor and diode bridges.
2. Then, a pair of capacitors is installed.
3. Drivers necessary to regulate the operation of the gates of field-effect transistors. If drivers have a D index at the end of the marking, there is no need to install FR107.
4. Field effect transistors installed without shorting the flanges. When attaching to the radiator, use special insulating gaskets and washers.
5. Transformers installed with shorted leads.
6. The output is diodes.

All elements are installed in the designated places on the board and soldered on the reverse side.

Examination

In order to correctly assemble the power supply, you need to be careful about installing the polar elements, and you should also be careful when working with mains voltage. After disconnecting the unit from the power source, there should be no dangerous voltage remaining in the circuit. If assembled correctly, no further adjustment is required.

You can check the correct operation of the power supply as follows:

1. We include in the circuit, at the output of the light bulb, for example, 12 volts. At the first short-term start, the light should be on. In addition, you should pay attention to the fact that all elements should not heat up. If something gets hot, it means the circuit is assembled incorrectly.
2. On the second start We measure the current value using a tester. Let the unit operate for a sufficient amount of time to ensure that there are no heating elements.

In addition, it would be useful to check all elements using a tester for the presence of high current after turning off the power.

1. As previously noted, the operation of a switching power supply is based on feedback. The considered circuit does not require a special organization of feedback and various power filters.
2. Particular attention should be paid to the selection of field-effect transistors. In this case, IR FETs are recommended because they are renowned for their thermal resolution. According to the manufacturer, they can operate stably up to 150 degrees Celsius. However, in this circuit they do not heat up very much, which can be called a very important feature.
3. If the transistors heat up constantly, active cooling should be installed. As a rule, it is represented by a fan.

Advantages and disadvantages

The pulse converter has the following advantages:

1. High rate stabilization coefficient allows you to provide power conditions that will not harm sensitive electronics.
2. Designs considered have a high efficiency rate. Modern versions have this figure at 98%. This is due to the fact that losses are reduced to a minimum, as evidenced by the low heating of the block.
3. Large input voltage range- one of the qualities due to which such a design has spread. At the same time, the efficiency does not depend on the input current indicators. It is the immunity to the current voltage indicator that allows you to extend the service life of electronics, since jumps in the voltage indicator are a common occurrence in the domestic power supply network.
4. Input frequency affects the operation of only the input elements of the structure.
5. Small dimensions and weight, are also responsible for their popularity due to the proliferation of portable and portable equipment. After all, when using a linear block, the weight and dimensions increase several times.
6. Organization of remote control.
7. Lower cost.

There are also disadvantages:

1. Availability pulse interference.
2. Necessity inclusion in the circuit of power factor compensators.
3. Complexity self-regulation.
4. Less reliability due to the complexity of the chain.
5. Dire consequences when one or more circuit elements fail.

When creating such a design yourself, you should take into account that mistakes made can lead to failure of the electrical consumer. Therefore, it is necessary to provide protection in the system.

Design and operating features

When considering the operating features of the pulse unit, the following can be noted:

1. At first The input voltage is rectified.
2. Rectified voltage depending on the purpose and features of the entire structure, it is redirected in the form of a high-frequency rectangular pulse and fed to an installed transformer or filter operating at low frequencies.
3. Transformers They are small in size and weight when using a pulse unit due to the fact that increasing the frequency makes it possible to increase the efficiency of their operation, as well as reduce the thickness of the core. In addition, ferromagnetic material can be used in the manufacture of the core. At low frequency, only electrical steel can be used.
4. Voltage stabilization occurs through negative feedback. Thanks to the use of this method, the voltage supplied to the consumer remains unchanged, despite fluctuations in the incoming voltage and the generated load.

Feedback can be organized as follows:

1. With galvanic isolation, an optocoupler or transformer winding output is used.
2. If you don't need to create a junction, a resistor voltage divider is used.

Using similar methods, the output voltage is maintained with the required parameters.

Standard switching power supplies, which can be used, for example, to regulate the output voltage during power supply , consists of the following elements:

1. Input part, high voltage. It is usually represented by a pulse generator. Pulse width is the main indicator that affects the output current: the wider the indicator, the greater the voltage, and vice versa. The pulse transformer stands at the section between the input and output parts and separates the pulse.
2. There is a PTC thermistor at the output part. It is made of a semiconductor and has a positive temperature coefficient. This feature means that when the temperature of the element increases above a certain value, the resistance indicator increases significantly. Used as a key security mechanism.
3. Low voltage part. The pulse is removed from the low-voltage winding, rectification occurs using a diode, and the capacitor acts as a filter element. The diode assembly can rectify current up to 10A. It should be taken into account that capacitors can be designed for different loads. The capacitor removes the remaining pulse peaks.
4. Drivers carry out quenching of the resistance that arises in the power circuit. During operation, drivers alternately open the gates of installed transistors. Work occurs at a certain frequency
5. Field effect transistors selected taking into account resistance indicators and maximum voltage when open. At a minimum value, the resistance significantly increases efficiency and reduces heating during operation.
6. Transformer standard for downgrade.

Taking into account the chosen circuit, you can begin to create a power supply of the type in question.

A switching power supply is an electronic circuit where the input voltage is rectified, filtered, and cut into bursts of high-frequency pulses for transmission through a small-sized transformer. The block becomes manageable, with flexibly adjustable parameters. The mass of the heaviest part of the source, the transformer, is reduced. In English literature, such devices are called Switching-Mode Power Supply (SMPS).

SMPS (Switching Mode Power Supply) device

The emergence of switching power supplies

The dimensions of transformers also worried Tesla. The scientist, repeating experiment after experiment, established: high current frequencies are safe for humans and provoke large losses in transformer cores. The result of the debate was the adoption of a frequency of 60 Hz for the construction of the Niagara Hydroelectric Power Station. We started with Nikola Tesla, because he was the first person who realized that fast vibrations cannot be achieved mechanically. Consequently, it is necessary to use oscillatory circuits. This is how the Tesla transformer appeared (September 22, 1896), with the help of which the scientist decided to transmit messages and energy over a distance.

The essence of the invention is described in the section about, we provide brief information. The transformer is formed by two parts connected in series. The primary winding of the first was connected to an alternating voltage source of relatively low frequency. Due to the low transformation ratio, the capacitor connected to the secondary winding was charged to a high potential. The voltage reached the threshold, the spark gap connected in parallel with the capacitor broke through. The oscillatory process of discharge began through the primary winding of the second transformer into the external circuit. Tesla received radio voltages with an amplitude of millions of volts.

The first step in creating switching power supplies, where relatively low-frequency voltage is converted into pulses. A similar design was created in 1910 by Charles Kettering, equipping car ignition systems. Switching power supplies appeared in the 60s. The idea of ​​minimizing the size of transformers (after Nikola Tesla) was put forward by General Electric in 1959 in the person of Joseph Murphy and Francis Starchetz (U.S. Patent 3,040,271). The idea did not immediately find a warm response (there was no suitable element base); in 1970, Tektronics released a line of oscilloscopes with a new power source.

Two years later, inverters are used in electronics (Patent US3697854 A), the main thing is that the first domestic models appear! Patents refer to each other, it is impossible to understand who first proposed using the idea in personal computers. In the USSR, development began in 1970, associated with the appearance on sale of the high-frequency powerful germanium transistor 2T809A. As stated in the literature, the first to achieve success in 1972 was a Muscovite, candidate of technical sciences L. N. Sharov. Later, a 400 W switching power supply appeared, authored by A. I. Ginzburg, S. A. Eranosyan. EC computers were equipped with a new product in 1976 by a team led by Zh. A. Mkrtchyan.

The first switching power supplies, known to domestic consumers from digital TVs and VCRs, often broke down; modern products have no drawback - they work continuously for years. The moment of the early 90s provides the following information:

1. Specific power: 35 - 120 W per cubic decimeter.
2. Inverter operating frequency: 30 - 150 kHz.
3. Efficiency: 75 - 85%.
4. MTBF: 50 - 200 thousand hours (6250 working days).

Advantages of switching power supplies

Linear power supplies are bulky, and efficiency is poor. Efficiency rarely exceeds 30%. For switching power supplies, the average figures are in the range of 70 - 80%; there are products that stand out greatly from the range. For the better, of course. The following information is provided: The efficiency of the switching power supply reaches 98%. At the same time, the required filtering capacitance of the capacitors is reduced. The energy stored per period decreases greatly with increasing frequency. Depends directly proportionally on the capacitance of the capacitor, quadratically on the voltage amplitude.

An increase to a frequency of 20 kHz (compared to 50/60) reduces the linear dimensions of the elements by 4 times. Flowers compared to expectations in the radio range. Explains the reason for equipping receivers with small capacitors.

Switching power supply design

The input voltage is rectified. The process is carried out by a diode bridge, or less often by a single diode. Then the voltage is cut into pulses; here the literature quickly moves on to a description of the transformer. Readers are probably wondering how a chopper (a device that generates pulses) works. Based on a microcircuit powered directly by a mains voltage of 230 volts. Less often, a zener diode (parallel type stabilizer) is specially installed.

The microcircuit generates pulses (20 - 200 kHz), of relatively small amplitude, that control a thyristor or other semiconductor power switch. The thyristor cuts high voltage into pulses, according to a flexible program generated by the generator microcircuit. Since there is high voltage at the input, protection is needed. The generator is protected by a varistor, the resistance of which drops sharply when the threshold is exceeded, shorting the harmful surge to ground. From the power switch, packets of pulses are sent to a small-sized high-frequency transformer. Linear dimensions are relatively low. For a 500W computer power supply, it fits in a child's palm.

The resulting voltage is rectified again. Schottky diodes are used due to the low voltage drop of the metal-semiconductor junction. The rectified voltage is filtered and supplied to consumers. Due to the presence of many secondary windings, it is quite easy to obtain values ​​of different polarity and amplitude. The story is incomplete without mentioning the feedback loop. The output voltages are compared with a standard (for example, a zener diode), and the pulse generator mode is adjusted: the transmitted power (amplitude) depends on the frequency and duty cycle. The products are considered relatively unpretentious and can operate in a wide range of supply voltages.

Case power supply

The technology is called inverter and is used by welders, microwave ovens, induction cooktops, cell phone adapters, and iPads. A computer power supply works in a similar way.

Circuitry of switching power supplies

Nature has provided 14 basic topologies for implementing switching power supplies. With inherent advantages and unique characteristics. Some are suitable for creating low-power power supplies (below 200 W), while others show their best qualities when powered by a mains voltage of 230 volts (50/60 Hz). And to choose the right topology, be able to imagine the properties of each. Historically, the first three are called:

• Buck - buck, deer, dollar.
• Boost – acceleration.
• Polarity inverter – polarity inverter.

Three topologies are related to linear regulators. The type of device is considered the predecessor of switching power supplies, without including advantages. The voltage is supplied through a transformer, straightened, and cut into a power switch. The operation of the regulator is controlled by feedback, the task of which is to generate an error signal. This type of device accounted for a multi-billion-dollar turnover in the 60s; it could only reduce the voltage, and the common wire of the consumer was connected to the power supply network.

Buck topology

This is how the “deer” appeared. Initially intended for constant voltage, the input signal was cut into pulses, then the packets were straightened and filtered to obtain average power. The feedback controlled the duty cycle and frequency (pulse width modulation). A similar thing is done today with computer power supplies. Almost immediately, power densities of 1 - 4 W per cubic inch were achieved (later up to 50 W per cubic inch). It’s great that it’s now possible to get multiple output voltages decoupled from the input.

The disadvantage is the loss at the moment the transistor switches; the voltage changes polarity and remains below zero until the next pulse. The specified part of the signal, bypassing the diode, is shorted to ground, not reaching the filter. The existence of optimal switching frequencies at which costs are minimized has been discovered. Range 25 - 50 kHz.

Boost topology

The topology is called a ring choke and is placed in front of the switch. It is possible to increase the input voltage to the desired rating. The scheme works as follows:

1. At the initial moment of time, the transistor is open, the inductor stores the energy of the voltage source through the collector, emitter p-n junctions, and ground.
2. Then the key is locked and the process of charging the capacitor starts. The throttle releases energy.
3. At some point the feedback amplifier operates and the load begins to be powered. The capacitor is unable to transfer energy towards the power switch; the diode interferes. The charge is taken by the payload.
4. The voltage drop will cause the feedback circuit to fire again, and the inductor will begin to accumulate energy.

Polarity Inverter topology

The topology of the polar inverter is similar to the previous circuit; the inductor is located behind the switch. It works like this:

In this case, we observe parallelism in the processes of storing/expending energy. All three schemes considered demonstrate the following disadvantages:

1. There is a DC connection between the input and output. In other words, there is no galvanic isolation.
2. It is not possible to obtain multiple voltage values ​​from one circuit.

The disadvantages are eliminated by push-pull and latter topologies. Both use a chopper with forward technology. In the first case, a differential pair of transistors is used. It becomes possible to use one key for half the period. To control, you need a special forming circuit that alternately swings this swing, improving the conditions for heat removal. The chopped voltage is bipolar, powers the primary winding of the transformer, there are many secondary windings - in accordance with the requirements of consumers.

In the retarded topology, one transistor is replaced by a diode. The circuit is often operated by low-power power supplies (up to 200 W) with a constant output voltage of 60 - 200 V.

In many electrical devices, the principle of realizing secondary power has long been used through the use of additional devices, which are entrusted with the functions of providing electricity to circuits that require power from certain types of voltage, frequency, current...

For this purpose, additional elements are created: converting voltage of one type to another. They may be:

built inside the consumer case, as on many microprocessor devices;

or made in separate modules with connecting wires similar to a conventional mobile phone charger.

In modern electrical engineering, two principles of energy conversion for electrical consumers, based on:

1. using analog transformer devices to transfer power to the secondary circuit;

2. switching power supplies.

They have fundamental differences in their design and operate using different technologies.

Transformer power supplies

Initially, only such designs were created. They change the voltage structure due to the operation of a power transformer, powered from a 220-volt household network, in which the amplitude of the sinusoidal harmonic decreases, which is then sent to a rectifier device consisting of power diodes, usually connected in a bridge circuit.

After this, the pulsating voltage is smoothed out by a parallel-connected capacitance, selected according to the permissible power, and stabilized by a semiconductor circuit with power transistors.

By changing the position of the trimming resistors in the stabilization circuit, it is possible to regulate the voltage at the output terminals.

Switching power supplies (UPS)

Similar design developments appeared en masse several decades ago and became increasingly popular in electrical devices due to:

availability of common components;

reliability in execution;

possibilities to expand the operating range of output voltages.

Almost all switching power supplies differ slightly in design and operate according to the same scheme, typical for other devices.

The main parts of power supplies include:

a network rectifier assembled from: input chokes, an electromechanical filter that provides noise rejection and static isolation from capacitors, a network fuse and a diode bridge;

storage filter tank;

key power transistor;

master oscillator;

feedback circuit made using transistors;

optocoupler;

a switching power supply, from the secondary winding of which voltage emanates to be converted into a power circuit;

rectifier diodes of the output circuit;

output voltage control circuits, for example, 12 volts with adjustment made using an optocoupler and transistors;

filter capacitors;

power chokes that perform the role of voltage correction and diagnostics in the network;

output connectors.

An example of an electronic board of such a switching power supply with a brief designation of the element base is shown in the picture.

How does a switching power supply work?

The switching power supply produces a stabilized supply voltage by using the principles of interaction between the elements of the inverter circuit.

The 220 volt network voltage is supplied through the connected wires to the rectifier. Its amplitude is smoothed by a capacitive filter through the use of capacitors that can withstand peaks of about 300 volts, and is separated by a noise filter.