PWM UC3842AN

The UC3842 is a PWM controller circuit with current and voltage feedback for controlling the key stage of an n-channel MOSFET, providing the discharge of its input capacitance with a forced current of up to 0.7A. The SMPS controller chip consists of a series of UC384X (UC3843, UC3844, UC3845) PWM controller chips. The UC3842 core is specifically designed for long-term operation with a minimum number of external discrete components. The UC3842 PWM controller features precise duty cycle control, temperature compensation, and is low cost. A special feature of the UC3842 is its ability to operate within 100% duty cycle (for example, the UC3844 operates with a duty cycle of up to 50%.). The domestic analogue of UC3842 is 1114EU7. Power supplies made on the UC3842 chip are characterized by increased reliability and ease of implementation.

Rice. Table of standard ratings.

This table gives a complete picture of the differences between the UC3842, UC3843, UC3844, UC3845 microcircuits.

General description.

For those wishing to become more deeply acquainted with the UC384X series PWM controllers, the following material is recommended.

• Datasheet UC3842B (download)
• Datasheet 1114EU7 domestic analogue of the UC3842A microcircuit (download).
• Article "Flyback converter", Dmitry Makashev (download).
• Description of the operation of UCX84X series PWM controllers (download).
• Article "Evolution of flyback switching power supplies", S. Kosenko (download). The article was published in the magazine "Radio" No. 7-9 for 2002.

A document from STC SIT, the most successful description in Russian for PWM UC3845 (K1033EU16), is highly recommended for review. (Download).

The difference between the UC3842A and UC3842B chips is that A consumes less current until startup.

UC3842 has two housing options: 8pin and 14pin, the pinouts of these versions are significantly different. In what follows, only the 8pin housing option will be considered.

A simplified block diagram is necessary to understand the operating principle of a PWM controller.

Rice. Block diagram of UC3842

A block diagram in a more detailed version is necessary for diagnosing and checking the performance of the microcircuit. Since we are considering the 8pin design, Vc is 7pin, PGND is 5pin.

Rice. Block diagram of UC3842 (detailed version)

Rice. UC3842 pinout

There should be material on pin assignments here, but it is much more convenient to read and look at the practical circuit diagram for connecting the UC3842 PWM controller. The diagram is drawn so well that it makes it much easier to understand the purpose of the microcircuit pins.

Rice. Connection diagram of UC3842 using the example of a power supply for TV

1. Comp:(Russian Correction) error amplifier output. For normal operation of the PWM controller, it is necessary to compensate for the frequency response of the error amplifier; for this purpose, a capacitor with a capacity of about 100 pF is usually connected to the specified pin, the second pin of which is connected to pin 2 of the IC. If the voltage at this pin is lowered below 1 volt, then the pulse duration at output 6 of the microcircuit will decrease, thereby reducing the power of this PWM controller.
2. Vfb: (Russian) Feedback voltage) feedback input. The voltage at this pin is compared with a reference voltage generated inside the UC3842 PWM controller. The comparison result modulates the duty cycle of the output pulses, as a result the output voltage of the power supply is stabilized. Formally, the second pin serves to reduce the duration of the output pulses; if it is applied above +2.5 volts, the pulses will be shortened and the microcircuit will reduce the output power.
3. C/S: (second designation I sense) (Russian) Current feedback) current limit signal. This pin must be connected to a resistor in the source circuit of the switching transistor. When the MOS transistor is overloaded, the voltage across the resistance increases and when a certain threshold is reached, the UC3842A stops operating, closing the output transistor. Simply put, the pin serves to turn off the pulse at the output when a voltage above 1 volt is applied to it.
4. Rt/Ct: (Russian) Frequency setting) connection of a timing RC circuit necessary to set the frequency of the internal oscillator. R is connected to Vref - the reference voltage, and C is connected to the common wire (usually several tens of nF are selected). This frequency can be changed within a fairly wide range; from above it is limited by the speed of the key transistor, and from below by the power of the pulse transformer, which decreases with decreasing frequency. In practice, the frequency is selected in the range of 35...85 kHz, but sometimes the power supply works quite normally at a much higher or much lower frequency.
For a timing RC circuit, it is better to abandon ceramic capacitors.
5.Gnd: (Russian) General) general conclusion. The common terminal should not be connected to the circuit body. This “hot” ground is connected to the device body through a pair of capacitors.
6.Out: (Russian) Exit) the output of the PWM controller is connected to the gate of the key transistor through a resistor or a resistor and diode connected in parallel (anode to the gate).
7.Vcc: (Russian) Nutrition) power input of the PWM controller, this pin of the microcircuit is supplied with a supply voltage in the range from 16 volts to 34, please note that this microcircuit has a built-in Schmidt trigger (UVLO), which turns on the microcircuit if the supply voltage exceeds 16 volts, if the same voltage for some reason it drops below 10 volts (for other microcircuits of the UC384X series, the ON/OFF values ​​may differ, see the Table of Type Ratings), it will be disconnected from the supply voltage. The microcircuit also has overvoltage protection: if the supply voltage on it exceeds 34 volts, the microcircuit will turn off.
8. Vref: output of the internal reference voltage source, its output current is up to 50 mA, voltage 5 V. Connected to one of the divider arms, it is used to quickly adjust the U output of the entire power supply.

A little theory.

Shutdown circuit when input voltage drops.

Rice. Shutdown circuit when input voltage drops.

The Under-Voltage LockOut circuit, or UVLO circuit, ensures that Vcc is equal to the voltage that makes the UC384x fully operational to turn on the output stage. In Fig. It is shown that the UVLO circuit has turn-on and turn-off threshold voltages of 16 and 10, respectively. Hysteresis of 6V prevents random switching on and off of voltage during power supply.

Generator.

Rice. Generator UC3842.

The frequency-setting capacitor Ct is charged from Vref (5V) through the frequency-setting resistor Rt, and discharged by an internal current source.

The UC3844 and UC3845 chips have a built-in counting trigger, which serves to obtain a maximum generator duty cycle of 50%. Therefore, the generators of these microcircuits must be set to a switching frequency twice as high as desired. The UC3842 and UC3843 chip generators are set to the desired switching frequency. The maximum operating frequency of the UC3842/3/4/5 family of generators can reach 500 kHz.

Reading and limiting current.

Rice. Organization of current feedback.

The current-voltage conversion is performed on an external resistor Rs connected to ground. RC filter to suppress output switch emissions. The inverting input of the UC3842 current-sensing comparator is internally biased by 1V. Current limiting occurs if the voltage at pin 3 reaches this threshold.

Error signal amplifier.

Rice. Block diagram of an error signal amplifier.

The non-inverting error input does not have a separate output and is internally biased by 2.5 volts. The output of the error amplifier is connected to pin 1 to connect an external compensating circuit, allowing the user to control the frequency response of the converter's closed feedback loop.

Rice. Compensating circuit diagram.

A compensating circuit suitable for stabilizing any converter circuit with additional current feedback, except flyback and boost converters operating with inductor current.

Blocking methods.

There are two possible ways to block the UC3842 chip:
increasing the voltage at pin 3 above the level of 1 volt,
or raising the voltage at pin 1 to a level not exceeding the voltage drop across the two diodes relative to ground potential.
Each of these methods results in setting a HIGH logic voltage level at the output of the PWM coparator (block diagram). Since the main (default) state of the PWM latch is the reset state, the output of the PWM comparator will be held LOW until the state of pins 1 and/or 3 changes in the next clock period (the period that follows the one in question). clock period when a situation arose that required blocking the microcircuit).

Connection diagram.

The simplest connection diagram for the UC3842 PWM controller is purely academic in nature. The circuit is the simplest generator. Despite its simplicity, this scheme works.

Rice. The simplest connection diagram 384x

As can be seen from the diagram, for the UC3842 PWM controller to operate, only an RC circuit and power are required.

Connection diagram for the PWM controller of the UC3842A PWM controller, using the example of a TV power supply.

Rice. Power supply diagram for UC3842A.

The diagram gives a clear and simple representation of the use of the UC3842A in a simple power supply. The diagram has been slightly modified to make it easier to read. The full version of the circuit can be found in the PDF document "Power supplies 106 circuits" Tovarnitsky N.I.

Connection diagram of the PWM controller of the UC3843 PWM controller, using the example of the power supply of the D-Link router, JTA0302E-E.

Rice. Power supply diagram for UC3843.

Although the circuit is made according to the standard connection for the UC384X, however, R4 (300k) and R5 (150) are taken out of the standards. However, successfully, and most importantly, logically allocated circuits help to understand the principle of operation of the power supply.

Power supply based on the UC3842 PWM controller. The diagram is not intended to be repeated, but is for informational purposes only.

Rice. Standard connection diagram from the datasheet (the diagram has been slightly modified for easier understanding).

Repair of PWM-based power supply UC384X.

Checking using an external power supply.

Rice. Simulation of PWM controller operation.

The operation is checked without unsoldering the microcircuit from the power supply. Before carrying out diagnostics, the power supply must be disconnected from the 220V network!

From an external stabilized power supply, apply a voltage to pin 7 (Vcc) of the microcircuit with a voltage greater than the UVLO turn-on voltage, in general more than 17V. In this case, the UC384X PWM controller should work. If the supply voltage is less than the UVLO turn-on voltage (16V/8.4V), the microcircuit will not start. You can read more about UVLO here.

Checking the internal voltage reference.

ExaminationUVLO

If the external power supply allows you to regulate the voltage, then it is advisable to check the operation of the UVLO. By changing the voltage on pin 7(Vcc) within the UVLO voltage range, the reference voltage on pin 8(Vref) = +5V should not change.

It is not recommended to supply a voltage of 34V or higher to pin 7 (Vcc). It is possible that there is a protective zener diode in the power supply circuit of the UC384X PWM controller, then it is not recommended to supply this zener diode above the operating voltage.

Checking the operation of the generator and external circuits of the generator.

You will need an oscilloscope to check. There should be a stable “saw” at pin 4(Rt/Ct).

Checking the output control signal.

You will need an oscilloscope to check. Ideally, pin 6(Out) should have rectangular pulses. However, the circuit under study may differ from the one shown, and then it will be necessary to turn off the external feedback circuits. The general principle is shown in Fig. – with this activation, the UC384X PWM controller is guaranteed to start.

Rice. Operation of UC384x with feedback circuits disabled.

Rice. An example of real signals when simulating the operation of a PWM controller.

If a power supply with a control PWM controller such as UC384x does not turn on or turns on with a long delay, then check by replacing the electrolytic capacitor that filters the power supply (pin 7) of this m/s. It is also necessary to check the elements of the initial start circuit (usually two 33-100kOhm resistors connected in series).

When replacing a power (field-effect) transistor in a power supply unit with a control m/s 384x, be sure to check the resistor that serves as a current sensor (located at the source of the field-effect switch). A change in its resistance at a nominal fraction of an ohm is very difficult to detect with a conventional tester! An increase in the resistance of this resistor leads to false operation of the current protection of the power supply unit. In this case, you can look for a very long time for the reasons for the power supply overload in the secondary circuits, although they are not there at all.

Chip UC3842(UC3843)- is a PWM controller circuit with current and voltage feedback for controlling a key stage on an n-channel MOS transistor, ensuring the discharge of its input capacitance with a forced current of up to 0.7A. Chip SMPS the controller consists of a series of microcircuits UC384X (UC3843, UC3844, UC3845) PWM controllers. Core UC3842 specifically designed for long-term operation with a minimum number of external discrete components. PWM controller UC3842 It features precise duty cycle control, temperature compensation and is low cost. Feature UC3842 is the ability to operate within 100% duty cycle (for example UC3844 works with a fill factor of up to 50%.). Domestic analogue UC3842 is 1114EU7. Power supplies made on a microcircuit UC3842 are characterized by increased reliability and ease of execution.

Differences in supply voltage between UC3842 and UC3843:

UC3842_________| 16 Volt / 10 Volt
UC3843_________| 8.4 Volt / 7.6 Volt

Differences in pulse duty cycle:

UC3842, UC3843__| 0% / 98%

Tsokolevka UC3842(UC3843) shown in Fig. 1

The simplest connection diagram is shown in Fig. 2

The UC3842 PWM controller chip is the most common in the construction of monitor power supplies. In addition, these microcircuits are used to build switching voltage regulators in horizontal scanning units of monitors, which are both high-voltage stabilizers and raster correction circuits. The UC3842 chip is often used to control the key transistor in system power supplies (single-cycle) and in power supplies for printing devices. In a word, this article will be of interest to absolutely all specialists in one way or another connected with power supplies.

Failure of the UC 3842 microcircuit occurs quite often in practice. Moreover, as the statistics of such failures show, the cause of a microcircuit malfunction is a breakdown of a powerful field-effect transistor, which is controlled by this microcircuit. Therefore, when replacing the power transistor of the power supply in the event of a malfunction, it is strongly recommended to check the UC 3842 control chip.

There are several methods for testing and diagnosing a microcircuit, but the most effective and simplest for practical use in a poorly equipped workshop are checking the output resistance and simulating the operation of the microcircuit using an external power source.

For this work you will need the following equipment:

There are two main ways to check the health of the microcircuit:

The functional diagram is shown in Fig. 1, and the location and purpose of the contacts in Fig. 2.

Checking the output resistance of the microcircuit

Very accurate information about the health of the microcircuit is provided by its output resistance, since during breakdowns of the power transistor, a high-voltage voltage pulse is applied precisely to the output stage of the microcircuit, which ultimately causes its failure.

The output impedance of the microcircuit must be infinitely large, since its output stage is a quasi-complementary amplifier.

You can check the output resistance with an ohmmeter between pins 5 (GND) and 6 (OUT) of the microcircuit (Fig. 3), and the polarity of connecting the measuring device does not matter. It is better to make such a measurement with the microcircuit soldered off. In the event of a breakdown of the microcircuit, this resistance becomes equal to several ohms.

If you measure the output resistance without unsoldering the microcircuit, then you must first unsolder the faulty transistor, since in this case its broken gate-source junction may “ring.” In addition, it should be taken into account that the circuit usually has a matching resistor connected between the output of the microcircuit and the “case”. Therefore, when tested, a working microcircuit may have an output resistance. Although, it is usually never less than 1 kOhm.

Thus, if the output resistance of the microcircuit is very small or has a value close to zero, then it can be considered faulty.

Simulation of microcircuit operation

This check is carried out without unsoldering the microcircuit from the power supply. The power supply must be turned off before performing diagnostics!

The essence of the test is to supply power to the microcircuit from an external source and analyze its characteristic signals (amplitude and shape) using an oscilloscope and voltmeter.

The operating procedure includes the following steps:

1) Disconnect the monitor from the AC power supply (disconnect the power cable).

2) From an external stabilized current source, apply a supply voltage of more than 16V (for example, 17-18V) to pin 7 of the microcircuit. In this case, the microcircuit should start. If the supply voltage is less than 16 V, the microcircuit will not start.

3) Using a voltmeter (or oscilloscope), measure the voltage at pin 8 (VREF) of the microcircuit. There should be a reference stabilized voltage of +5 VDC.

4) By changing the output voltage of the external current source, make sure that the voltage on pin 8 is stable. (The voltage of the current source can be changed from 11 V to 30 V; with a further decrease or increase in voltage, the microcircuit will turn off and the voltage on pin 8 will disappear).

5) Using an oscilloscope, check the signal at pin 4 (CR). In the case of a working microcircuit and its external circuits, there will be a linearly varying voltage (sawtooth-shaped) at this contact.

6) By changing the output voltage of the external current source, make sure that the amplitude and frequency of the sawtooth voltage at pin 4 is stable.

7) Using an oscilloscope, check for the presence of rectangular pulses on pin 6 (OUT) of the microcircuit (output control pulses).

If all of the indicated signals are present and behave in accordance with the above rules, then we can conclude that the chip is working properly and is functioning correctly.

In conclusion, I would like to note that in practice it is worth checking the serviceability of not only the microcircuit, but also the elements of its output circuits (Fig. 3). First of all, these are resistors R1 and R2, diode D1, zener diode ZD1, resistors R3 and R4, which form the current protection signal. These elements often turn out to be faulty during breakdowns

PWM controller chips ka3842 or UC3842 (uc2842) is the most common when constructing power supplies for household and computer equipment; it is often used to control a key transistor in switching power supplies.

Operating principle of ka3842, UC3842, UC2842 microcircuits

The 3842 or 2842 chip is a PWM - pulse-width modulation (PWM) converter, mainly used to operate in DC-DC mode (converts a constant voltage of one value to a constant voltage of another) converter.

Let's consider the block diagram of microcircuits 3842 and 2842 series:
Pin 7 of the microcircuit is supplied with a supply voltage ranging from 16 Volts to 34. The microcircuit has a built-in Schmidt trigger (UVLO), which turns on the microcircuit if the supply voltage exceeds 16 Volts, and turns it off if the supply voltage for some reason falls below 10 Volts. The 3842 and 2842 series microcircuits also have overvoltage protection: if the supply voltage exceeds 34 Volts, the microcircuit will turn off. To stabilize the frequency of pulse generation, the microcircuit has its own 5-volt voltage stabilizer inside, the output of which is connected to pin 8 of the microcircuit. Pin 5 mass (ground). Pin 4 sets the pulse frequency. This is achieved by resistor R T and capacitor C T connected to 4 pins. - see typical connection diagram below.

Pin 6 – output of PWM pulses. 1 pin of the 3842 chip is used for feedback, if on 1 pin. lower the voltage below 1 Volt, then at the output (6 pins) of the microcircuit the pulse duration will decrease, thereby reducing the power of the PWM converter. Pin 2 of the microcircuit, like the first, serves to reduce the duration of the output pulses; if the voltage at pin 2 is higher than +2.5 Volts, then the pulse duration will decrease, which in turn will reduce the output power.

The microcircuit with the name UC3842, in addition to UNITRODE, is produced by ST and TEXAS INSTRUMENTS, analogues of this microcircuit are: DBL3842 by DAEWOO, SG3842 by MICROSEMI/LINFINITY, KIA3842 by KES, GL3842 by LG, as well as microcircuits from other companies with different letters (AS, MC, IP etc.) and digital index 3842.

Scheme of a switching power supply based on the UC3842 PWM controller

Schematic diagram of a 60-watt switching power supply based on a UC3842 PWM controller and a power switch based on a 3N80 field-effect transistor.

UC3842 PWM controller chip - full datasheet with the ability to download for free in pdf format or look in the online reference book on electronic components on the website

The article will provide a description, operating principle and connection diagram of the UC3842. This is a microcircuit that is a pulse width controller. Scope of application - in DC-DC converters. Using one microcircuit, you can create a high-quality voltage converter that can be used in power supplies for various equipment.

Pin assignment of the microcircuit (brief overview)

First you need to consider the purpose of all the pins of the microcircuit. The description of the UC3842 looks like this:

1. The voltage necessary for feedback is supplied to the first pin of the microcircuit. For example, if you lower the voltage on it to 1 V or lower, the pulse time at pin 6 will begin to decrease significantly.
2. The second output is also necessary to create feedback. However, unlike the first one, a voltage of more than 2.5 V must be applied to it in order to reduce the pulse duration. This also reduces power.
3. If a voltage of more than 1 V is applied to the third pin, then pulses will stop appearing at the output of the microcircuit.
4. A variable resistor is connected to the fourth pin - with its help you can set the pulse frequency. An electrolytic capacitor is connected between this terminal and ground.
5. The fifth conclusion is general.
6. PWM pulses are removed from the sixth pin.
7. The seventh pin is intended for connecting power in the range of 16..34 V. Built-in overvoltage protection. Please note that the microcircuit will not work at voltages below 16 V.
8. To stabilize the pulse frequency, a special device is used that supplies +5 V to the eighth pin.

Before considering practical designs, you need to carefully study the description, operating principle and connection diagrams of the UC3842.

How does the microcircuit work?

Now we need to briefly consider the operation of the element. When a DC voltage of +5 V appears on the eighth leg, the OSC generator starts. A positive pulse of short length is supplied to the trigger inputs RS and S. Then, after a pulse is given, the trigger switches and zero appears at the output. As soon as the OSC pulse begins to fall, the voltage at the direct inputs of the element will be zero. But a logical one will appear at the inverting output.

This logic unit allows the transistor to turn on, so that electric current will begin to flow from the power source through the collector-emitter circuit to the sixth pin of the microcircuit. This shows that there will be an open pulse at the output. And it will stop only when a voltage of 1 V or higher is applied to the third pin.

Why do you need to check the microcircuit?

Many radio amateurs who design and install electrical circuits purchase parts in bulk. And it’s no secret that the most popular shopping places are Chinese online stores. The cost of products there is several times lower than on radio markets. But there are also a lot of defective products there. Therefore, you need to know how to test the UC3842 before starting to build the circuit. This will avoid frequent unsoldering of the board.

Where is the chip used?

The chip is often used to assemble power supplies for modern monitors. They are used in line scan TVs and monitors. It is used to control transistors operating in switch mode. But elements fail quite often. And the most common reason is a breakdown of the field switch controlled by the microcircuit. Therefore, when independently designing a power supply or repairing, it is necessary to diagnose the element.

What you need to diagnose faults

It should be noted that the UC3842 was used exclusively in converter technology. And for normal operation of the power supply, you need to make sure that the element is working. You will need the following devices for diagnostics:

1. Ohmmeter and voltmeter (the simplest digital multimeter will do).
2. Oscilloscope.
3. Source of current and voltage stabilized power supply. It is recommended to use adjustable ones with a maximum output voltage of 20..30 V.

If you do not have any measuring equipment, then the easiest way to diagnose is to check the output resistance and simulate the operation of the microcircuit when operating from an external power source.

Checking the output resistance

One of the main diagnostic methods is to measure the resistance value at the output. We can say that this is the most accurate way to determine breakdowns. Please note that in the event of a breakdown of the power transistor, a high-voltage pulse will be applied to the output stage of the element. For this reason, the microcircuit fails. At the output, the resistance will be infinitely large if the element is working properly.

Resistance is measured between terminals 5 (ground) and 6 (output). The measuring device (ohmmeter) is connected without special requirements - polarity does not matter. It is recommended to unsolder the microcircuit before starting diagnostics. During breakdown, the resistance will be equal to several ohms. If you measure resistance without soldering the microcircuit, the gate-source circuit may ring. And do not forget that in the power supply circuit on the UC3842 there is a constant resistor, which is connected between ground and output. If it is present, the element will have an output resistance. Therefore, if the output resistance is very low or equal to 0, then the microcircuit is faulty.

How to simulate the operation of a microcircuit

When simulating operation, there is no need to solder the microcircuit. But be sure to turn off the device before starting work. Checking the circuit on the UC3842 consists of applying voltage to it from an external source and evaluating the operation. The work procedure looks like this:

1. The power supply is disconnected from the AC mains.
2. A voltage greater than 16 V is supplied from an external source to the seventh pin of the microcircuit. At this moment, the microcircuit should start. Please note that the chip will not start working until the voltage is above 16 V.
3. Using an oscilloscope or voltmeter, you need to measure the voltage at the eighth pin. It should be +5 V.
4. Make sure the voltage on pin 8 is stable. If you reduce the power supply voltage below 16 V, then the current will disappear at the eighth pin.
5. Using an oscilloscope, measure the voltage at the fourth pin. If the element is working properly, the graph will show sawtooth-shaped pulses.
6. Change the voltage of the power supply - the frequency and amplitude of the signal at the fourth pin will remain unchanged.
7. Check with an oscilloscope whether there are rectangular pulses on the sixth leg.

Only if all the signals described above are present and behave as they should, can we talk about the serviceability of the microcircuit. But it is recommended to check the serviceability of the output circuits - diode, resistors, zener diode. With the help of these elements, signals are generated for current protection. They fail when broken.

Switching power supplies on a chip

For clarity, you need to consider the description of the operation of the power supply on the UC3842. It first began to be used in household appliances in the second half of the 90s. It has a clear advantage over all competitors - low cost. Moreover, reliability and efficiency are not inferior. To build a complete one, practically no additional components are required. Everything is done by the “internal” elements of the microcircuit.

The element can be made in one of two types of housing - SOIC-14 or SOIC-8. But you can often find modifications made in DIP-8 packages. It should be noted that the last numbers (8 and 14) indicate the number of pins of the microcircuit. True, there are not very many differences - if the element has 14 pins, pins are simply added for connecting ground, power and the output stage. Stabilized pulse-type power supplies with PWM modulation are built on the microcircuit. A MOS transistor is required to amplify the signal.

Turning on the chip

Now we need to consider the description, operating principle and connection circuits of the UC3842. Power supplies usually do not indicate the parameters of the microcircuit, so you need to refer to special literature - datasheets. Very often you can find circuits that are designed to be powered from an alternating current network of 110-120 V. But with just a few modifications you can increase the supply voltage to 220 V.

To do this, the following changes are made to the power supply circuit on the UC3842:

1. The diode assembly, which is located at the input of the power source, is replaced. It is necessary that the new diode bridge operates at a reverse voltage of 400 V or more.
2. The electrolytic capacitor is replaced, which is located in the power circuit and serves as a filter. Installed after the diode bridge. It is necessary to install a similar one, but with an operating voltage of 400 V and higher.
3. The nominal value in the power supply circuit increases to 80 kOhm.
4. Check whether the power transistor can operate at a voltage between drain and source of 600 V. BUZ90 transistors can be used.

The article is shown on UC3842. has a number of features that must be taken into account when designing and repairing power supplies.

Features of the microcircuit

If there is a short circuit in the secondary winding circuit, then when diodes or capacitors break down, the loss of electricity in the pulse transformer begins to increase. It may also turn out that there is not enough voltage for the normal functioning of the microcircuit. During operation, a characteristic “clanking” sound is heard, which comes from the pulse transformer.

Considering the description, operating principle and connection diagram of the UC3842, it is difficult to ignore the repair features. It is quite possible that the reason for the behavior of the transformer is not a breakdown in its winding, but a malfunction of the capacitor. This happens as a result of the failure of one or more diodes that are included in the power circuit. But if a breakdown of the field-effect transistor occurs, it is necessary to completely change the microcircuit.