Uc3842 description of operating principle. Repair of TV power supplies. Composition: UC3842AN PWM controller and key field-effect transistor. Scheme of a switching power supply based on the UC3842 PWM controller

The combination of low cost and interesting circuit design allows you to use them at every opportunity. By the way, about the cost. UC 2843 costs about 19 rubles retail, UC 2844 - 26, UC 3843 - 14–16, UC 3845 - 16–20.

What are these “little animals”? The UC1842/3/4/5, UC2842/3/4/5, UC3842/3/4/5 family of microcircuits are essentially the same crystal, adapted to operate in different temperature ranges, at different on-off thresholds, with separate or shared output stage and power outputs and different maximum duty cycles.

Each device has several versions. The design option is indicated by the letter after the numbers (Fig. 1–3). Options N, J and D8 (Fig. 1) use an 8-pin DIP or SOIC package. In this case, the collector of the upper transistor of the output totem n-p-n-stage is connected to the positive power supply terminal, and the emitter of the lower transistor of the output stage is connected to the “ground” terminal. In options D, W (SOIC-14, CFP-14 packages) (Fig. 2) and Q (PLCC-20D package) (Fig. 3), the collector of the upper transistor of the output totem stage and the emitter of the lower transistor of the output stage have their own terminals. This, in principle, is the entire range of differences between devices within the family.

Table 1. Family of chips UC1842/3/4/5, UC2842/3/4/5, UC3842/3/4/5

Let's look at the block diagrams of the base crystals shown in Fig. 4 and 5. Fig. 4 is for an 8-pin package, and Fig. 5 is for a 14- and 20-pin package.

The microcircuits contain a protective shutdown unit when the supply voltage drops. The block consists of a Schmitt trigger with differential inputs and a reference voltage source. Using an RS flip-flop, this block controls a common 5 V reference. This source has its own pin and provides up to 50 mA current. In short circuit mode, it is capable of delivering a current of up to 100 mA. The operating thresholds of the protective shutdown unit are given in Table 1. By the way, these PWM controllers received the definition of “current” precisely because of this very protective shutdown unit (Fig. 6). The microcircuits begin to operate at a current consumption of about 1 mA and allow power from a high voltage source through a chain of resistors, the main thing is to ensure a range of operating currents and voltages across the power pins. For this purpose, a zener diode with a breakdown voltage of 34 V is connected between the positive and ground legs. In addition to the protective shutdown unit, the crystal contains an internal bias circuit and a logic power circuit. And of course, an indispensable part of such devices is a pulse generator. It has one output for connecting a timing RC circuit (Fig. 7). There are no restrictions on the minimum frequency. The maximum frequency of the generator is set at 500 kHz. The generator frequency is approximately calculated by the formula:

The expression is valid for R T >5 kOhm. The connection of the timing RC circuit and the graphs of the dependence of the duty cycle on the capacitor capacitance and frequency on the resistor resistance and capacitor capacitance are shown in Fig. 7. The chip has an error amplifier (Fig. 8), the non-inverting input of which “sits” on an internal voltage source of 2.5 V, and the inverting input has its own output, which serves as a feedback input. The output of this amplifier is connected to pin 1 and, through a level shift network, to the inverting input of the current limiting comparator. The non-inverting input of the current limiting comparator is output to a separate pin and is used for connection to an external current-measuring resistor (Fig. 9), through which the load current flows. The value of this resistor and, accordingly, the voltage drop across it determines the maximum current flowing through a powerful external switch controlled by the controller. Other on-chip devices are the RS latch and boom. Together they provide pulse width modulation depending on the error amplifier voltage and the current comparator signal. In addition, the UC X844/5 microcircuits have a T-trigger, which provides a maximum duty cycle of 50%.

And the last thing is the totem output stage. It consists of two npn transistors. The maximum current of the output stage is ±1 A. Such an output stage can ensure normal operation of a powerful MOS transistor at a decent frequency if the microcircuits are used in a voltage converter or directly switch the load. If these transistors are made in an 8-pin package, then they are connected to the power terminals, and if in a 14-20-pin package, then, as mentioned above, the power supply of the cascade has separate terminals. This connection provides greater flexibility in application.

Microcircuits can be turned on and off remotely. There are no special pins for this, but given the internal structure of the microcircuit, this function can be implemented in two ways (Fig. 10). The first method is to apply a voltage higher than 1 V to pin 3 (8-pin package), 5 (14-pin package), or 7 (20-pin package). The second method is to lower the voltage to 1 (8- and 14-pin package ) or 2 (20-pin package) output to ground level by the voltage value of a pair of diodes or transistor. These methods turn the current comparator off, which causes the output latch to reset. The signal from the latch is dominant on the logic element and passes to the output stage, opening the lower transistor and closing the upper one. Thus, a low level voltage appears at the output of the microcircuit. The output state will not change until the voltage at these inputs in the first case drops below the operating threshold of the current comparator, and in the second case it stops shunting the output of the error amplifier. The first method shown in Fig. 6 is suitable for turning the power source on and off. After all, the thyristor will be open until the voltage drops to zero, and when turned on again, everything will work as before. It should be noted that both methods are proposed by the manufacturer.

Maximum permissible parameters:

Table 2. Electrical parameters of controllers UC1842/3/4/5, UC2842/3/4/5, UC3842/3/4/5

• Power supply voltage (low impedance source) - 30 V;
• Power supply voltage (a source capable of delivering no more than 30 mA) - internal limiter;
• Output current - ±1 A;
• Maximum input voltage at analog inputs (pins 2,3; 8-pin case) - –0.3 to +6.3 V;
• The maximum sink current of the error amplifier is 10 mA;
• Maximum power dissipation at tcorp 25 °C: DIL-8 - 1 W SOIC-14 - 725 mW;
• Soldering temperature (no more than 10 s) - 300 °C.

More detailed information is on the manufacturer's website.

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 ()
• Datasheet 1114EU7 is a domestic analogue of the UC3842A chip ().
• Article "Flyback converter", Dmitry Makashev ().
• Description of the operation of UCX84X series PWM controllers ().
• Article "Evolution of flyback switching power supplies", S. Kosenko (). 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. ().

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 feel) (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 voltage for some reason it drops below 10 volts (for other UC384X series microcircuits the ON/OFF values ​​may differ, see Table), 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.

Circuits and printed circuit boards of power supplies based on UC3842 and UC3843 chips

Microcircuits for building switching power supplies of the UC384x series are comparable in popularity to the famous TL494. They are produced in eight-pin packages, and the printed circuit boards for such power supplies are very compact and single-sided. The circuitry for them has been debugged for a long time, all the features are known. Therefore, these microcircuits, along with TOPSwitch, can be recommended for use.

So, the first scheme is an 80W power supply. Source:

Actually, the diagram is practically from the datasheet.


click to enlarge
The printed circuit board is quite compact.

PCB file: uc3842_pcb.lay6

In this circuit, the author decided not to use the input of the error amplifier due to its high input impedance in order to avoid interference. Instead, the feedback signal is connected to a comparator. The Schottky diode on the 6th pin of the microcircuit prevents possible voltage surges of negative polarity, which may be due to the characteristics of the microcircuit itself. To reduce inductive emissions in the transformer, its primary winding is sectioned and consists of two halves separated by a secondary one. The closest attention should be paid to inter-winding insulation. When using a core with a gap in the center core, external interference should be minimal. A current shunt with a resistance of 0.5 Ohm with the 4N60 transistor indicated in the diagram limits the power to around 75W. The snubber uses SMD resistors, which are connected in parallel and in series, because They generate noticeable power in the form of heat. This snubber can be replaced with a diode and a 200-volt zener diode (suppressor), but they say that this will increase the amount of impulse noise from the power supply. A space for an LED has been added on the printed circuit board, which is not reflected in the diagram. You should also add a load resistor parallel to the output, because At idle, the power supply can behave unpredictably. Most of the output elements on the board are installed vertically. The power supply to the microcircuit is removed during the reverse stroke, so when converting the unit into an adjustable one, you should change the phasing of the microcircuit's power winding and recalculate the number of its turns, as for a forward one.

The following schematic and PCB are from this source:

The dimensions of the board are a little larger, but there is room for a slightly larger mains electrolyte.

The scheme is almost similar to the previous one:

click to enlarge
A trim resistor is installed on the board to adjust the output voltage. Likewise, the chip is powered from the power winding in reverse, which can lead to problems with a wide range of power supply output voltage adjustments. To avoid this, you should also change the phasing of this winding and power the microcircuit in forward motion.

PCB file: uc3843_pcb.dip

The UC384x series microcircuits are interchangeable, but before replacing you need to check how the frequency is calculated for a specific microcircuit (the formulas are different) and what the maximum duty cycle is - they differ by half.

To calculate the transformer windings, you can use the Flyback 8.1 program. The number of turns of the microcircuit power winding in forward motion can be determined by the ratio of turns to volts.

The article is devoted to the design, repair and modification of power supplies for a wide range of equipment based on the UC3842 microcircuit. Some of the information provided was obtained by the author as a result of personal experience and will help you not only avoid mistakes and save time during repairs, but also increase the reliability of the power source. Since the second half of the 90s, a huge number of televisions, video monitors, faxes and other devices have been produced, the power supplies of which use the UC3842 integrated circuit (hereinafter - IC). Apparently, this is explained by its low cost, the small number of discrete elements needed for its “body kit” and, finally, the fairly stable characteristics of the IC, which is also important. Variants of this IC produced by different manufacturers may differ in prefixes, but always contain a 3842 core.

As can be seen from the circuit diagram, the power supply is designed for a network voltage of 115 V. The undoubted advantage of this type of power supply is that with minimal modifications it can be used in a network with a voltage of 220 V, you just need to:

• replace the diode bridge connected at the input of the power supply with a similar one, but with a reverse voltage of 400 V;
• replace the electrolytic capacitor of the power filter, connected after the diode bridge, with one of equal capacity, but with an operating voltage of 400 V;
• increase the value of resistor R2 to 75...80 kOhm;
• check the CT for the permissible drain-source voltage, which must be at least 600 V. As a rule, even in power supplies designed to operate on a 115 V network, CTs capable of operating on a 220 V network are used, but, of course, exceptions are possible. If the CT needs to be replaced, the author recommends the BUZ90.

As mentioned earlier, the IC has some features related to its power supply. Let's take a closer look at them. At the first moment after connecting the IP to the network, the internal generator of the IC is not yet working, and in this mode it consumes very little current from the power circuits. To power the IC in this mode, the voltage obtained from resistor R2 and accumulated on capacitor C2 is sufficient. When the voltage on these capacitors reaches 16...18 V, the IC generator starts and it begins to generate CT control pulses at the output. Voltage appears on the secondary windings of transformer T1, including windings 3-4. This voltage is rectified by pulse diode D3, filtered by capacitor C3, and supplied to the IC power circuit through diode D2. As a rule, a zener diode D1 is included in the power circuit, limiting the voltage to 18...22 V. After the IC has entered the operating mode, it begins to monitor changes in its supply voltage, which is fed through the divider R3, R4 to the feedback input Vfb. By stabilizing its own supply voltage, the IC actually stabilizes all other voltages removed from the secondary windings of the pulse transformer.

When there are short circuits in the circuits of the secondary windings, for example, as a result of breakdown of electrolytic capacitors or diodes, energy losses in the pulse transformer increase sharply. As a result, the voltage obtained from winding 3-4 is not enough to maintain normal operation of the IC. The internal oscillator turns off, a low level voltage appears at the output of the IC, which turns the CT into a closed state, and the microcircuit is again in low power consumption mode. After some time, its supply voltage increases to a level sufficient to start the internal generator, and the process repeats. In this case, characteristic clicks (clicking) are heard from the transformer, the repetition period of which is determined by the values ​​of capacitor C2 and resistor R2.

When repairing power supplies, situations sometimes arise when a characteristic clicking noise is heard from the transformer, but a thorough check of the secondary circuits shows that there is no short circuit in them. In this case, you need to check the power supply circuits of the IC itself. For example, in the author’s practice there were cases when capacitor C3 was broken. A common reason for this behavior of the power supply is a break in the rectifier diode D3 or the decoupling diode D2.

When a powerful CT breaks down, it usually has to be replaced along with the IC. The fact is that the CT gate is connected to the output of the IC through a resistor of a very small value, and when the CT breaks down, a high voltage from the primary winding of the transformer reaches the output of the IC. The author categorically recommends that if the CT malfunctions, replace it together with the IC; fortunately, its cost is low. Otherwise, there is a risk of “killing” the new CT, because if a high voltage level from the broken IC output is present at its gate for a long time, it will fail due to overheating.

Some other features of this IC were noticed. In particular, when a CT breaks down, resistor R10 in the source circuit very often burns out. When replacing this resistor, you should stick to a value of 0.33...0.5 Ohm. Overestimating the resistor value is especially dangerous. In this case, as practice has shown, the first time the power supply is connected to the network, both the microcircuit and the transistor fail.

In some cases, an IP failure occurs due to a breakdown of the zener diode D1 in the IC power circuit. In this case, the IC and CT, as a rule, remain serviceable; it is only necessary to replace the zener diode. If the zener diode breaks, both the IC itself and the CT often fail. For replacement, the author recommends using domestic KS522 zener diodes in a metal case. Having bitten out or removed the faulty standard zener diode, you can solder the KS522 with the anode to pin 5 of the IC and the cathode to pin 7 of the IC. As a rule, after such a replacement, similar malfunctions no longer occur.

You should pay attention to the serviceability of the potentiometer used to adjust the output voltage of the IP, if there is one in the circuit. It is not in the above diagram, but it is not difficult to introduce it by connecting resistors R3 and R4 into the gap. Pin 2 of the IC must be connected to the motor of this potentiometer. I note that in some cases such modification is simply necessary. Sometimes, after replacing the IC, the output voltages of the power supply turn out to be too high or too low, and there is no adjustment. In this case, you can either turn on the potentiometer, as mentioned above, or select the value of resistor R3.

According to the author’s observation, if high-quality components are used in the IP, and it is not operated under extreme conditions, its reliability is quite high. In some cases, the reliability of the power supply can be increased by using resistor R1 of a slightly larger value, for example, 10...15 Ohms. In this case, transient processes when the power is turned on proceed much more calmly. In video monitors and televisions, this must be done without affecting the demagnetization circuit of the kinescope, i.e., the resistor must under no circumstances be connected to the break in the general power circuit, but only to the connection circuit of the power supply itself.

Below are links to various microcircuits analogues of UC3842, which can be purchased from Dalincom UC3842AN dip-8, KA3842A dip-8, KA3842 sop-8, UC3842 sop-8, TL3842P, and others in the power supply microcircuits section.

Alexey Kalinin
"Electronic equipment repair"

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.