UMZCH power supply diagram. Switching power supply for amplifiers. Switching power supply circuit

Other articles dedicated to the construction of this ULF.

Schematic diagram of the power supply.

The power supply is assembled according to one of the standard schemes. Selected to power the final amplifiers bipolar power supply. This allows the use of inexpensive, high-quality integrated amplifiers and eliminates a number of problems associated with supply voltage ripple and transition processes occurring when turned on. https://site/

The power supply must provide power to three microcircuits and one LED. Two TDA2030 microcircuits are used as final power amplifiers, and one TDA1524A microcircuit is used as a volume control, network base and tone.

Electrical diagram of the power supply.

A bipolar, full-wave rectifier with a midpoint is assembled using diodes VD3...VD6. This connection circuit reduces the voltage drop across the rectifier diodes by half compared to a conventional bridge rectifier, since in each half-cycle the current flows through only one diode.

Used as a rectified voltage filter electrolytic capacitors C3...C6.

On the IC1 chip a voltage stabilizer is assembled to power the circuit electronic regulator volume, stereo and timbre. The stabilizer is assembled according to a standard design.

The use of the LM317 chip is due only to the fact that it was available. Here you can use any integral stabilizer.

The protective diode VD2, indicated by a dotted line, is not necessary to use when the output voltage on the LM317 chip is below 25 Volts. But, if the input voltage of the microcircuit is 25 Volts or higher, and the resistor R3 is trimmer, then better diode install it anyway.

The value of resistor R3 determines the output voltage of the stabilizer. During prototyping, I soldered a trimmer resistor in its place, used it to set the voltage to about 9 Volts at the output of the stabilizer, and then measured the resistance of this trimmer so that I could install a constant resistor instead.

The rectifier feeding the stabilizer is made according to a simplified half-wave circuit, which is dictated by purely economic considerations. Four diodes and one capacitor are more expensive than one diode and one slightly larger capacitor.

The current consumed by the TDA1524A microcircuit is only 35mA, so this circuit is quite justified.

LED HL1 – amplifier power-on indicator. A ballast resistor for this indicator is installed on the power supply board - R1 with a nominal resistance of 500 Ohms. The LED current depends on the resistance of this resistor. I used a green LED rated at 20mA. When using a red LED type AL307 with a current of 5mA, the resistance of the resistor can be increased by 3-4 times.

PCB.

The printed circuit board (PCB) is designed based on the design of a specific amplifier and available electrical components. The board has only one hole for mounting, located in the very center of the PCB, which is due to its unusual design.

To increase the cross-section of copper tracks and save ferric chloride, the areas free from tracks on the PP were filled using the “Polygon” tool.

Increasing the width of the tracks also prevents peeling of the foil from the fiberglass laminate in the event of a violation of the thermal regime or during repeated re-soldering of radio components.

According to the drawing given above, a printed circuit board was made from foil fiberglass with a cross section of 1 mm.

To connect the wires to the printed circuit board, copper pins (soldiers) were riveted into the holes of the board.

And this is the already assembled printed circuit board of the power supply.

To see all six views, drag the picture with the cursor or use the arrow buttons located at the bottom of the picture.

The mesh on the PP copper tracks is the result of using this technology.

When the board is assembled, it is advisable to test it before connecting the final amplifiers and the regulator unit. To test the power supply, you need to connect an equivalent load to its outputs, as in the diagram above.

Resistors of the PEV-10 type at 10-15 Ohms are suitable as a load for the +12.8 and -12.8 Volt rectifiers.

It’s a good idea to look at the voltage at the output of a stabilizer loaded onto a resistor with a resistance of 100-150 Ohms with an oscilloscope to ensure there is no ripple when the alternating input voltage decreases from 14.3 to 10 Volts.

P.S. Refinement of the printed circuit board.

During commissioning, the printed circuit board of the power supply was damaged.

During modification, we had to cut one track, item 1, and add one contact, item 2, to connect the transformer winding that powers the voltage stabilizer.

click on the picture to enlarge

The control controller in this power supply is TL494. After the controller there is an IR2110 half-bridge driver, which actually controls the gates of the power transistors. The use of a driver made it possible to abandon the matching transformer, which is widely used in computer units nutrition. The IR2110 driver is loaded onto the gates through the R24-VD4 and R25-VD5 chains that accelerate the closing of the field gates.
Power switches VT2 and VT3 operate at primary winding power transformer. The midpoint required to obtain alternating voltage in the primary winding of the transformer is formed by elements R30-C26 and R31-C27.
Current transformer TV1 is connected in series with the primary winding of the power transformer, allowing you to control the current flowing through the power switches and build on this current protection. In addition, using the output voltage from the current transformer, you can control the fan speed forced cooling(VT4).
Power voltages are stabilized using group stabilization choke L1.
The capacitance of the primary power supply filters is calculated from the ratio of 1 µF per 1 W of output power, and the power transistors must have maximum current at least 30% more than the current flowing through the primary winding of the power transformer at maximum power.
A few words about the operating algorithm of this power supply:
At the moment of supplying a mains voltage of 220 V, the capacitances of the primary power filters C15 and C16 are infected through resistors R8 and R11, which does not allow the VD bridge to be overloaded by the short circuit current of completely discharged C15 and C16. At the same time, capacitors C1, C3, C6, C19 are charged through a line of resistors R16, R18, R20 and R22, stabilizer 7815 and resistor R21.
As soon as the voltage on capacitor C6 reaches 12 V, the zener diode VD1 “breaks through” and current begins to flow through it, charging capacitor C18, and as soon as the positive terminal of this capacitor reaches a value sufficient to open the thyristor VS2, it will open. This will turn on relay K1, which will bypass the current-limiting resistors R8 and R11 with its contacts. In addition, the opened thyristor VS2 will open the transistor VT1 to both the TL494 controller and the IR2110 half-bridge driver. The controller will begin a soft start mode, the duration of which depends on the ratings of R7 and C13.
During a soft start, the duration of the pulses that open the power transistors increases gradually, thereby gradually charging the secondary capacitors and limiting the current through the rectifier diodes. Stabilization of the output voltage occurs by changing the duration of control pulses of power transistors at a constant frequency. This is only possible if the value of the secondary voltage of the power transformer is higher than that required at the output of the stabilizer by at least 30%, but not more than 60%. As the load increases, the output voltage begins to decrease, the optocoupler LED begins to glow less, the optocoupler transistors close, thereby increasing the duration of the control pulses until the effective voltage reaches the stabilization value. As the load decreases, the voltage will begin to increase, the LED of optocoupler IC1 will begin to glow brighter, thereby opening the transistor and reducing the duration of the control pulses until the effective value of the output voltage decreases to a stabilized value. The amount of stabilized voltage is regulated by trimming resistor R26.
It should be noted that the TL494 controller does not regulate the duration of each pulse depending on the output voltage, but only the average value, i.e. the measuring part has some inertia. However, even with capacitors installed in the secondary power supply with a capacity of 2200 μF, power failures at peak short-term loads do not exceed 5%, which is quite acceptable for the equipment HI-FI class. We usually install capacitors in the secondary supply of 4700 uF, which gives a confident margin for peak values, and the use of group stabilization choke L1 allows us to control all output voltages.
This switching power supply is equipped with overload protection, the measuring element of which is the current transformer TV1. As soon as the current reaches a critical value, thyristor VS1 opens and bypasses the power supply to the final stage of the controller. The control pulses disappear, and the power supply goes into standby mode, which it can remain in for quite a long time, since the thyristor VS2 continues to remain open - the current flowing through resistors R16, R18, R20 and R22 is enough to keep it in the open state.
To exit the power supply from standby mode, you must press the SA3 button, which will bypass the thyristor VS2 with its contacts, the current will stop flowing through it and it will close. As soon as the contacts SA3 open, transistor VT1 closes, thereby removing power from the controller and driver. Thus, the control circuit will switch to minimum consumption mode - thyristor VS2 is closed, therefore relay K1 is turned off, transistor VT1 is closed, therefore the controller and driver are de-energized. Capacitors C1, C3, C6 and C19 begin to charge and as soon as the voltage reaches 12 V, the thyristor VS2 opens and starts pulse block nutrition.
If you need to put the power supply into standby mode, you can use the SA2 button, when pressed, the base and emitter of transistor VT1 will be connected. The transistor will close and de-energize the controller and driver. The control pulses will disappear, and the secondary voltages will disappear. However, power will not be removed from relay K1 and restart the converter will not happen.
A little about the details:
Power transformer We manufacture on cores from horizontal TV transformers. However, similar parameters can be obtained with ferrite rings, although the conversion frequency should not be raised above 70 kHz, since even at this frequency ferrite 2000 begins to heat up due to internal losses. We use a TPI core as a group stabilization choke. The windings are arranged oppositely, as shown in schematic diagram. The conductor cross-section is calculated from a ratio of 3-4 A per mm square. The windings are wound until the window is filled. If a ferrite ring is used as a core for a group stabilization choke, it is better to use a K40x25x11 ring. The windings are wound until the hole inside is reduced to 14...16 mm. As additional filtering inductances, we use cores from TV mains power filters, but these filters can also be wound on rings with a diameter of 20...25 mm. The winding is wound until it is filled with the same wire as the group stabilization choke.
To adjust as a load, all power voltages should be loaded with resistors with a power of 2 W and a resistance of 4.7 k...6.8 k. With an output voltage of 60...90 V, this will simulate the quiescent current of power amplifiers. For lower output voltages, the resistance should be reduced slightly.

A switching power supply, providing bipolar voltage +/-50V with a power of up to 300 W, is intended for use, or high-power laboratory power supplies (). This one is relatively simple circuit The pulse power supply is assembled mainly from radio elements taken from old AT/ATX power supplies.

Schematic diagram of the converter 220/2x50V

The inverter transformer was wound on an ETD39 ferrite core. The winding data is practically the same, only the output windings are slightly wound to accommodate the increase in voltage. The key transistors are powerful IRFP450. The driver is the popular TL494 chip. Power is supplied through a special stabilizer. In it, the starting resistor with the rectified mains voltage charges the power capacitor, on which, when the voltage reaches the threshold, the stabilizer turns on, starting the driver. It will be powered only when energy is accumulated on the capacitor, and after the converter starts, the additional winding of the transformer will take over the driver power. The operating principle of this launch option has been known for a long time and is used in the popular m/s UC384x.

Power cascade

Another feature of the power supply circuit design is control field effect transistors. Here the lower IRFP450 circuit is controlled directly from the driver output, and the upper one is controlled using a small transformer.

In addition, the system was equipped with current protection, monitoring the current of the lower field worker using its resistance Rdson.

PSU test results

In practice, it was possible to obtain about 100-150 output power from 4 ohm speakers. The voltage +/-50V is set by resistor P1 10k. Of course, it can take any value, depending on the application ULF circuits. The system currently operates as a .

An audio frequency amplifier (AFA), or a low frequency amplifier (LF) is one of the most common electronic devices. We all receive sound information using one or another type of ULF. Not everyone knows, but low-frequency amplifiers are also used in measurement technology, flaw detection, automation, telemechanics, analog computing and other areas of electronics.

Although, of course, the main use of ULF is to bring a sound signal to our ears using acoustic systems that convert electrical vibrations into acoustic ones. And the amplifier must do this as accurately as possible. Only in this case do we receive the pleasure that our favorite music, sounds and speech give us.

From the advent of Thomas Edison's phonograph in 1877 to the present day, scientists and engineers have struggled to improve the basic parameters of the ULF: first of all, the reliability of transmission sound signals, as well as for consumer characteristics such as power consumption, size, ease of manufacture, configuration and use.

Since the 1920s, a letter classification of classes has been formed electronic amplifiers, which is still in use today. Classes of amplifiers differ in the operating modes of the active electronic devices used in them - vacuum tubes, transistors, etc. The main “single-letter” classes are A, B, C, D, E, F, G, H. Class designation letters can be combined in case of combining some modes. The classification is not a standard, so developers and manufacturers can use letters quite arbitrarily.

Class D occupies a special place in the classification. The active elements of the ULF output stage of class D operate in a switching (pulse) mode, unlike other classes, where the linear mode of operation of the active elements is mostly used.

One of the main advantages of Class D amplifiers is the coefficient of performance (efficiency) approaching 100%. This, in particular, leads to a reduction in the power dissipated by the active elements of the amplifier, and, as a consequence, to a reduction in the size of the amplifier due to the reduction in the size of the radiator. Such amplifiers place significantly lower demands on the quality of the power supply, which can be unipolar and pulsed. Another advantage can be considered the possibility of using digital signal processing methods and digital control of their functions in class D amplifiers - after all, it is digital technologies that prevail in modern electronics.

Taking into account all these trends, the Master Kit company offers wide selection of class amplifiersD, assembled on the same TPA3116D2 chip, but having different purposes and power. And so that buyers do not waste time searching for a suitable power source, we have prepared amplifier + power supply kits, optimally suited to each other.

In this review we will look at three such kits:

1. (D-class LF amplifier 2x50W + power supply 24V / 100W / 4.5A);
2. (D-class LF amplifier 2x100W + power supply 24V / 200W / 8.8A);
3. (D-class LF amplifier 1x150W + power supply 24V / 200W / 8.8A).

First set is intended primarily for those who need minimal dimensions, stereo sound and a classic control scheme simultaneously in two channels: volume, low and high frequencies. It includes and.

Myself two channel amplifier has unprecedentedly small dimensions: only 60 x 31 x 13 mm, not including control knobs. Dimensions of the power supply are 129 x 97 x 30 mm, weight – about 340 g.

Despite its small size, the amplifier delivers an honest 50 watts per channel into a 4-ohm load at a supply voltage of 21 volts!

The RC4508 microcircuit is used as a pre-amplifier - dual specialized operational amplifier for audio signals. It allows the amplifier input to be perfectly matched to the signal source, and has extremely low nonlinear distortion and noise levels.

The input signal is supplied to a three-pin connector with a pin pitch of 2.54 mm, supply voltage and speaker systems Connect using convenient screw connectors.

A small heatsink is installed on the TPA3116 chip using heat-conducting glue, the dissipation area of ​​which is quite sufficient even at maximum power.

Please note that in order to save space and reduce the size of the amplifier, there is no protection against reverse polarity of the power supply connection (reversal), so be careful when supplying power to the amplifier.

Taking into account its small size and efficiency, the scope of application of the kit is very wide - from replacing an outdated or broken old amplifier to a very mobile sound reinforcement kit for dubbing an event or party.

An example of using such an amplifier is given.

There are no mounting holes on the board, but for this you can successfully use potentiometers that have fastenings for a nut.

Second set includes two TPA3116D2 chips, each of which is enabled in bridged mode and provides up to 100 watts of output power per channel, as well as with an output voltage of 24 volts and a power of 200 watts.

With the help of such a kit and two 100-watt speaker systems, you can sound a major event even outdoors!

The amplifier is equipped with a volume control with a switch. A powerful Schottky diode is installed on the board to protect against polarity reversal of the power supply.

The amplifier is equipped with effective low-pass filters, installed according to the recommendations of the manufacturer of the TPA3116 chip, and together with it, ensuring high quality of the output signal.

The supply voltage and speaker systems are connected using screw connectors.

The input signal can be supplied either to a three-pin connector with a pitch of 2.54 mm, or using a standard 3.5 mm Jack audio connector.

The radiator provides sufficient cooling of both microcircuits and is pressed against their thermal pads with a screw located at the bottom printed circuit board.

For ease of use, the board also has a green LED indicating when the power is turned on.

The dimensions of the board, including capacitors and excluding the potentiometer knob, are 105 x 65 x 24 mm, the distances between the mounting holes are 98.6 and 58.8 mm. Dimensions of the power supply are 215 x 115 x 30 mm, weight about 660 g.

Third set represents l and with an output voltage of 24 volts and a power of 200 watts.

The amplifier provides up to 150 watts of output power into a 4 ohm load. The main application of this amplifier is to build a high-quality and energy-efficient subwoofer.

Compared to many other dedicated subwoofer amplifiers, the MP3116btl excels at driving large-diameter woofers. This is confirmed by customer reviews of the ULF in question. The sound is rich and bright.

Radiator occupying most of PCB area ensures efficient cooling of the TPA3116.

To match the input signal at the amplifier input, the NE5532 microcircuit is used - a two-channel low-noise specialized operational amplifier. It has minimal nonlinear distortion and wide bandwidth.

The input signal amplitude regulator with a slot for a screwdriver is also installed at the input. With its help, you can adjust the volume of the subwoofer to the volume of the main channels.

To protect against supply voltage reversal, a Schottky diode is installed on the board.

Power and speaker systems are connected using screw connectors.

The dimensions of the amplifier board are 73 x 77 x 16 mm, the distances between the mounting holes are 69.4 and 57.2 mm. Dimensions of the power supply are 215 x 115 x 30 mm, weight about 660 g.

All kits include MEAN WELL switching power supplies.

Founded in 1982, the company is the world's leading manufacturer of switching power supplies. Currently, MEAN WELL Corporation consists of five financially independent partner companies in Taiwan, China, the USA and Europe.

MEAN WELL products are characterized by high quality, low failure rate and long service life.

Switching power supplies, developed on a modern element base, meet the highest requirements for output quality DC voltage and differ from conventional linear sources in their low weight and high efficiency, as well as the presence of protection against overload and short circuit at the output.

Power supplies LRS-100-24 and LRS-200-24, used in the presented kits, have LED indicator switches and a potentiometer for precise adjustment of the output voltage. Before connecting the amplifier, check the output voltage and, if necessary, set its level to 24 volts using a potentiometer.

The sources used use passive cooling, so they are completely silent.

It should be noted that all the amplifiers considered can be successfully used to design sound reproduction systems for cars, motorcycles and even bicycles. When powering amplifiers with a voltage of 12 volts, the output power will be slightly less, but the sound quality will not suffer, and the high efficiency allows you to effectively power the ULF from autonomous power sources.

We also draw your attention to the fact that all the devices discussed in this review can be purchased individually and as part of other kits on the website.

Have a good time everyone. Let me introduce a power inverter for powering a powerful audio amplifier. Unfortunately, they are especially repeatable. Therefore, it was decided to make such a power source from scratch. It took a lot of time to design, build and test this UPS. And now, having carried out the last tests (all tests were successful), we can say that the project is finished and can be presented to the respected amateur radio audience of the site. 2 Schemes.ru

The project of this inverter is perfect for, in fact, it was developed for it. The converter is not complicated and should be successfully assembled by not very advanced electronics engineers. You don't even need an oscilloscope to run it, but of course it would be useful. The basis of the power supply circuit is m/s TL494.

It is short circuit protected and should provide 250W of continuous power. The converter also has an additional output voltage of +/- 9..12 V, which will be used to power the preamplifier, fans, etc.

Switching power supply for an amplifier - circuit diagram

The converter is made in accordance with this scheme. Board dimensions 150×100 mm.

The inverter consists of several basic modules, present in most similar power supplies, such as the ATX power supply. The fuse, thermistor and line filter consisting of C21, R21 and L5 go to the 220V AC power supply. Then the rectifier bridge D26-D29, inverter input capacitors C18 and C19 and power transistors Q8 and Q9 to switch the voltage at the transformer. Power transistors are controlled using an additional transformer T2 by one of the most popular PWM controllers - TL494 (KA7500). Current transformer T3 for measuring output power is connected in series with the primary winding. Transformer T1 has two separated secondary windings. One of them generates a voltage of 2×35 V, and the other 2×12 V. On each of the windings there are fast diodes D14-D17 and D22-D25, which total form 2 rectifier bridges.

After loading the +/- 34 V line with a 14 ohm resistor, the voltage drops to +/- 31 V. This is quite good result for such a small ferrite core. After 5 minutes, diodes D22-D25, the main transformer and MOSFET heated up to a temperature of about 50C, which is quite safe. After connecting two TDA7294 channels, the voltage dropped to +/- 30 V. The inverter elements heated up like resistive load. After experiments, the output circuit is equipped with 2200uF capacitors and 22uH/14A chokes. The voltage drop is slightly higher than in the case of 6.8uH, but their use clearly reduces the heating of the MOSFETs.

Output voltage under load of both outputs with 20 W bulbs:

Operating principle of a switching power supply

The 220 V voltage is rectified by a bridge with diodes D26-D29. The input capacitors C18 and C19 charge to a total voltage of 320V, and since the inverter operates in a half-bridge system, they divide them in half, resulting in 160V per capacitor. This voltage is further balanced by resistors R16 and R17. Thanks to this separation, it is possible to connect transformer T1 to one channel. The potential between the capacitors is then treated as ground, one end of the primary winding is connected to +160 V, the other to -160 V. The switching voltage of the primary winding of transformer T1 is carried out using a variable N-MOSFET transistor Q8 and Q9.

Capacitor C10 and the primary winding of current transformer T3 are located in series with the primary winding. The coupling capacitor is not needed for the operation of the circuit, but it plays a very important role - it protects against unbalanced energy consumption from the input capacitors and, therefore, before charging one of them to more than 200 V. The current transformer T3, also located in series with the primary winding, acts as short circuit protection. The current transformer provides galvanic isolation and allows you to measure the current value, reduced to the accuracy of its transmission. Its task is to inform the controller about the amount of current flowing through the primary winding T1.

In parallel with the primary winding of the main transformer there is a so-called pulse suppression circuit, which is formed by C13 and R18. It suppresses voltage surges generated when switching power transistors. They are not harmful to MOSFETs because their built-in diodes effectively protect against drain overvoltage. However, voltage surges can negatively affect the efficiency of the inverter, so it is important to eliminate them.

The power MOSFETs cannot be driven directly by the controller due to the change in the potential of the upper transistor source. The transistors are controlled using a special transformer T2. This is an ordinary pulse transformer operating in push-pull mode, opening power transistors. The control transformer T2 has at the input a set of voltage control elements on the windings, which, in addition to generating the voltage dictated by the controller, protect against the occurrence of core demagnetizing voltage. An uncontrolled demagnetization voltage would keep the transistor open. The elements directly responsible for eliminating the demagnetization voltage are diodes D7 and D9, as well as transistors Q3 and Q5. During idle, when both MOSFETs are off, current flows through D7 and Q5 (or D9 and Q3) and maintains the demagnetization voltage around 1.4 V. This voltage is safe and cannot open the power transistor.

Voltage oscillogram at MOSFET inputs:

On the oscillogram you can clearly see the moment when the core stops being demagnetized by diodes D7 and D8 (D6 and D9) and begins to be magnetized in the opposite direction by transistors Q3 and Q4 (Q2 and Q5). During the core demagnetization phase, the voltage at the T2 gate reaches 18 V, and during the magnetization phase it drops to approximately 14 V.
Why is one of the IR type drivers not used? First of all, the control transformer is more reliable, more predictable. IR drivers are very capricious and error prone.

On the secondary winding of the main transformer T1 is generated alternating voltage, so it needs to be straightened. The role of the rectifier is played by rectifying fast diodes, generating a symmetrical voltage. The output chokes are located behind the diodes - their presence affects the efficiency of the inverter, suppressing the surges that charge the output capacitors when one of the power transistors is turned on. Next are the output capacitors with preload resistors, which prevent the voltage from rising to too high values.

Pulse IP controller

The controller is the basis of the inverter, so we will describe it in more detail. The inverter uses a TL494 controller with a set operating frequency the same as in ATX power supplies, that is, 30 kHz. The inverter does not have output voltage stabilization, so the controller operates with a maximum duty cycle of 85%. The controller is equipped with a system soft start, consisting of elements C5 and R7. After starting the inverter, the circuit provides a smooth increase in the duty cycle starting from 0%, which eliminates the charging surge of the output capacitors. The TL494 can operate from 7 V, and this voltage supplying the buffer of the control transformer T2 causes the generation of a voltage at the gates of the order of 3 V. Such not fully open transistors will output tens of volts, which will lead to huge power losses and there is a high probability of exceeding the dangerous limit. To prevent this, protection is provided against too high a voltage drop. It consists of a resistor divider R4 - R5 and transistor Q1. After the voltage drops to 14.1 V, Q1 discharges the soft start capacitor, thereby reducing the charge to 0%.

Another function of the controller is to protect the inverter from short circuit. Information about the primary winding current is obtained by the controller through the current transformer T3. The current in the secondary winding T3 flows through resistor R9, across which a small voltage drops. Information about the voltage on R9 is sent via potentiometer PR1 to the error amplifier TL494 and compared with the voltage of the resistor divider R1 and R2. If the controller detects a voltage higher than 1.6 V on potentiometer PR1, it turns off the transistors before they cross the dangerous limit and is latched through D1 and R3. The power transistors remain off until the inverter is restarted. Unfortunately, this protection only works correctly on the +/- 35 V line. The +/- 12 V line is much weaker and in the event of a short circuit there may not be enough current for the protection to operate.

The controller's power supply is transformerless using capacitor resistance. The two capacitors C20 and C24 consume reactive energy from the mains and hence by causing current to flow, they charge the filter capacitor C1 through the rectifier D10-D13. Zener diode DZ1 protects against too high voltage on C1 and stabilizes them at 18 V.

Pulse transformers in power supply

The quality and performance of a pulse transformer affects the efficiency of the entire converter and the output voltage. However, the transformer not only performs the function of converting electricity, but also provides galvanic isolation from the 220 V network and thus has a great impact on safety.

Here's how to make such a transformer correctly. First of all there must be a ferrite core. It cannot have an air gap; its halves must fit perfectly together. Theoretically, a toroidal core can be used here, but making good insulation and winding will be quite difficult.

We recommend taking the main ETD34, ETD29 as a last resort, but then the maximum continuous power will be no more than 180 W. They don't cost much, so the best solution will receive a damaged ATX power supply. Burnt PC power supplies, in addition to all the necessary transformers, contain many more useful elements, including a surge protector, capacitors, diodes, and sometimes TL494 (KA7500).

Transformers must be carefully desoldered from the ATX power supply board, preferably using a hot air gun. After desoldering, do not try to disassemble the transformer because it will break. The transformer should be placed in water and boiled. After 5 minutes, you need to carefully grab the core halves through the fabric and separate them. If they don't want to separate, don't pull too hard - you'll break them! Place back and cook for another 5 minutes.

The process of winding the main transformer should begin with counting the amount of wire that will be wound. Due to the constant operating frequency and the specified maximum induction, the number of primary windings depends only on the cross-sectional area of ​​the main ferrite core column. The maximum induction is limited to 250 mT due to half-bridge operation - here the magnetization asymmetry is simple.

Formula for calculating the number of turns:

n = 53 / Qr,

• Qr is the cross-sectional area of ​​the main core rod, given in cm2.

Thus, for a core with a cross-section of 0.5 cm2, you need to wind 106 turns, and for a core with a cross-section of 1.5 cm2, you will only need 35. Remember, do not wind half a turn - always round up to one plus. Calculating the number of secondary windings is the same as for any other transformer - the ratio of the output voltage to the input voltage is exactly equal to the ratio of the number of secondary windings to the number of primary windings.

The next step is to calculate the thickness of the winding wires. The most important thing to consider when calculating the thickness of the wires is the need to fill the entire core window with wire - this determines the magnetic connection of the transformer windings, and therefore the output voltage drop. The total cross-section of all wires passing through the core window should be about 40-50% of the cross-section of the main window (the main window is where the wire passes through the core). If this is your first time winding a transformer, you should get closer to this 40%. The calculations must also take into account the currents flowing through the cross-section of the windings. Typically the current density is 5 A/mm2 and this value should not be exceeded, the use of lower current densities is desirable. In the simulation, the primary side current is 220 W / 140 V = 1.6 A, so the wire cross-section should be 0.32 mm2, which means its thickness will be 0.6 mm. On the secondary side, a current of 220W/54V would be 4.1A, resulting in a cross-section of 0.82mm and an actual wire thickness of 1mm. In both cases, the maximum voltage drop during loading was taken into account. It should also be remembered that due to the skin effect of pulse transformers, the thickness of the wire is limited by the operating frequency - in our case at 30 kHz the maximum wire thickness is 0.9 mm. Instead of a 1mm thick wire, it is better to use two thinner wires. After calculating the number of coils and wires, check whether the calculated filling of the copper window corresponds to 40-50%.

The primary winding of the transformer must be placed in two parts. The first part of the primary (of 35 turns) is wound like the first, on an empty frame. It is necessary to maintain the direction of the winding towards the frame - the second part of the winding must be wound in the same direction. After winding the first part, you need to solder the other end to an adapter, shortened pin, which is not included in the board. Then put 4 layers of insulation tape on the winding and wind the entire secondary winding - this means the winding method. This improves the symmetry of the windings. The following +/- 12V secondary winding can be wound directly onto the +/- 35V winding in areas where a small amount of free space has been saved, and then completely insulated with 4 layers of electrical tape. Of course, it is also necessary to insulate the places where the ends of the windings are driven to the housing pins. As the last winding, wind the second part of the primary winding, always in the same direction as the previous one. After winding, you can insulate the last winding, but it is not necessary.

When the windings are ready, fold the core halves. The best and proven solution is to connect it with electrical tape and a drop of glue. We wrap the core with insulating tape several times.

The control transformer is made like any other pulse transformer. A small EE/EI obtained from ATX power supplies can be used as the core. You can also buy a TN-13 or TN-16 toroidal core. The number of windings depends, as usual, on the cross-section of the core.

In the case of toroids, the formula is:

n = 8 / Qr,

• where n is the number of windings of the primary winding,
• Qr is the cross-sectional area of ​​the core, given in cm2.

The secondary windings should be wound with the same number of turns as the primary windings, only minor deviations are allowed. Since the transformer will only drive one pair of MOSFETs, the thickness of the wire is not important; minimum thickness is less than 0.1 mm. In this case 0.3 mm. The first half of the primary winding must be wound in series - insulating layer - first secondary winding - insulating layer - second secondary winding - insulating layer - second half of the primary winding. The direction of the winding of the windings is very important, here the MOSFETs must be turned on one by one, and not simultaneously. After winding, we connect the core in the same way as in the previous transformer.

The current transformer is similar to the above. The number of coils here is arbitrary; in principle, the number of windings of the secondary winding is sufficient:

n = 4 / Qr,

• where n is the number of windings of the secondary winding,
• Qr is the cross-sectional area of ​​the core circumference, given in cm2.

But since the currents here are very small, it is always better to use a larger number of turns. On the other hand, it is more important to maintain the appropriate ratio of the number of turns of both windings. If you decide to change this ratio, you will have to adjust the value of resistor R9.

Here is the formula for calculating R9 depending on the number of turns:

R9 = (0.9Ω * n2) / n1,

• where n2 is the number of windings of the secondary winding,
• n1 is the number of windings of the primary winding.

With R9 changed, C7 also needs to be changed accordingly. It is easier to wind a current transformer on a toroidal core, we recommend TN-13 or TN-16. However, you can make a transformer with an Sh-core. If you wind a transformer on a toroidal core, first wind the secondary winding with a large number of turns. Then insulating tape and, finally, the primary winding with 0.8 mm thick wire.

Description of circuit elements

Almost all elements can be found in an ATX power supply. Diodes D26-D29 with a breakdown voltage of 400 V, but it is better to take a little higher, at least 600 V. The finished rectifier can be found in the ATX power supply. It is also advisable to use diode bridges for powering the controller at least 600 V. But they can be cheap and popular 1N4007 or similar.

The zener diode that limits the controller supply voltage must withstand 0.7 W of power, so its power rating must be 1 W or more.

Capacitors C18 and C19 can be used with a different capacitance, but not less than 220 µF. Capacitances of more than 470 uF should also not be used due to the excessively increased current when the inverter is connected to the network and the large size - they may simply not fit on the board. Capacitors C18 and C19 are also found in each ATX power supply.

Power transistors Q8 and Q9 are very popular IRF840, available in most electronic stores for 30 rubles. In principle, you can use other 500V MOSFETs, but this will involve changing resistors R12 and R13. When set to 75 ohms, the gate open/close time is about 1 µs. Alternatively, they can be replaced with either 68 - 82 ohms.

Buffers in front of the MOSFET inputs and control transformer I, using BD135 / 136 transistors. Any other transistors with a breakdown voltage above 40 V can be used here, such as BC639 / BC640 or 2SC945 / 2SA1015. The latter can be torn out from ATX power supplies, monitors, etc. A very important element of the inverter is capacitor C10. This should be a polypropylene capacitor adapted to high pulse currents. This capacitor is found in ATX power supplies. Unfortunately, it is sometimes the cause of power supply failure, so you need to check it carefully before soldering it into the circuit.

The diodes D22-D25 that rectify +/- 35V are used UF5408 connected in parallel, but a better solution would be to use BY500/600 single diodes which have a lower drop voltage and higher current rating. If possible, these diodes should be soldered on long wires - this will improve their cooling.

Chokes L3 and L4 are wound on toroidal powder cores from ATX power supplies - they are characterized by a predominant yellow and white color. Cores with a diameter of 23 mm, 15-20 turns on each of them, are sufficient. However, tests have shown that they are not needed - the inverter works without them, reaches its power, but the transistors, diodes and capacitor C10 become hotter due to pulse currents. Reactors L3 and L4 improve inverter efficiency and reduce failure rates.

The D14-D17 +/- 12V rectifiers have a big impact on the efficiency of this line. If this line will power a preamp, additional fans, an additional headphone amplifier, and for example a level meter, diodes should be used at least 1 A. However, if the +/- 12 V line will only power the preamp, which pulls up to 80 mA , you can even use 1N4148 here. Chokes L1 and L2 are practically not needed, but their presence improves the filtering of interference from the power supply. As a last resort, you can use 4.7 Ohm resistors instead.

Voltage limiters R22 and R23 can consist of a series of power resistors connected in series or parallel to produce a single resistor with more high power and corresponding resistance.

Starting and setting up the inverter

After etching the boards, begin assembling the elements, starting from the smallest to the largest. It is necessary to solder all components except inductor L5. After completing assembly and checking the board, set the PR1 potentiometer to the leftmost position and connect the mains voltage to the 220 V INPUT connector. There should be a voltage of 18 V across the capacitor C1. If the voltage stops at approximately 14 V, this indicates a problem with the control of the transformer or power transistors, that is short circuit in the control circuit. Oscilloscope owners can check the voltage at the transistor gates. If the controller is working correctly, check that the MOSFET is switched correctly.

After turning on the 12 V power and the controller power supply, +/- 2 V should appear on the +/- 35 V line. This means that the transistors are controlled properly, one at a time. If the light on the 12V power supply was turned on and there was no voltage at the output, this would mean that both power transistors were opening at the same time. In this case, the control transformer must be disconnected, and the wires of one of the secondary windings of the transformer must be changed. Next, solder the transformer back and try again with a 12 V power supply and a lamp.
If the test will pass successfully and we get +/- 2 V at the output, you can turn off the lamp power supply and solder the inductance L5. From this moment on, the inverter must operate from a 220 V network through a 60 W lamp. After connecting to the network, the light should blink briefly and immediately turn off completely. +/- 35 and +/- 12 V should appear at the output (or other voltage depending on the transformer speed ratio).

Load them with a small amount of power (for example, from an electronic load) for testing and the light at the input will start to glow a little. After this test, you need to switch the inverter directly to the network, and connect a load with a resistance of about 20 Ohms to the +/- 35 V line to check the power. PR1 should be adjusted so that the inverter does not turn off after charging the heater. When the inverter starts to heat up, you can check the voltage drop on the +/- 35V line and calculate the power output. A 5-10 minute test is sufficient to check the power output of the inverter. During this time, all components of the inverter will be able to heat up to their rated temperature. It is worth measuring the temperature of the MOSFET heatsink, it should not exceed 60C at temperature environment 25C. Finally, you need to load the inverter with an amplifier and set the PR1 potentiometer as far to the left as possible, but so that the inverter does not turn off.

The inverter can be adapted to any power needs of various UMZCHs. When designing the plate, we tried to make it as universal as possible for installation various types elements. The location of the transformer and capacitors allows for a fairly large MOS transistor heatsink to be mounted along the entire length of the board. After proper bending of the leads of the diode bridges, they can be installed in metal case. Increasing the heat dissipation allows the converter power to be theoretically increased to 400 W. Then you need to use a transformer on the ETD39. This change requires 470uF capacitors C18 and C19, 1.5-2.2uF capacitors C10 and the use of 8 BY500 diodes.