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

UC3845
PRINCIPLE OF OPERATION

Frankly speaking, it was not possible to defeat the UC3845 the first time - self-confidence played a cruel joke. However, wise with experience, I decided to finally figure it out - the chip is not that big - only 8 legs. I would like to express special gratitude to my subscribers, who did not stand aside and gave some explanations; they even sent a rather detailed article by email and a piece of the model in Microcap. THANK YOU VERY MUCH .
Using the links and materials sent, I sat for an evening or two and, in general, all the puzzles fit together, although some cells turned out to be empty. But first things first...
It was not possible to assemble an analogue of the UC3845 using logic elements in Microcap 8 and 9 - the logic elements are strictly connected to a five-volt power supply, and these simulators have chronic difficulties with self-oscillation. Microcap 11 showed the same results:

There was only one option left - Multisim. Version 12 was even found with a localization. I haven't used Multisim for a VERY long time, so I had to tinker. The first thing that pleased me was that Multisim has a separate library for five-volt logic and a separate library for fifteen-volt logic. In general, with grief in half, it turned out to be a more or less workable option, showing signs of life, but it didn’t want to work exactly the way a real microcircuit behaves, no matter how much I tried to persuade it. Firstly, the models do not measure the level relative to real zero, so an additional source of negative bias voltage would have to be introduced. But in this case they would have to explain in some detail what it is and why, but I wanted to be as close as possible to the real microcircuit.

Having rummaged through the Internet, I found a ready-made scheme, but for Multisim 13. I downloaded option 14, opened the model and it even worked, but the joy did not last long. Despite the presence in the libraries themselves of both the twelfth and fourteenth Multisim of the UC3845 microcircuit itself and its analogues, it quickly became clear that the model of the microcircuit does not allow working out ALL options for switching on this microcircuit. In particular, limiting the current and adjusting the output voltage work quite reliably (though it often falls out of the simulation), but the microcircuit refused to accept the use of applying a ground error to the output of the amplifier.

In general, although the cart moved, it did not travel far. There was only one option left - printing out the datasheet on the UC3845 and a board with wiring. In order not to get carried away with simulating the load and simulating current limiting, I decided to build a microbooster and use it to check what actually happens to the microcircuit under one or another variant of inclusion and use.
First, a little explanation:
The UC3845 microcircuit really deserves the attention of designers of power supplies of various powers and purposes; it has a number of almost analogues. Almost because when replacing a chip on a board, you don’t need to change anything else, but changes in ambient temperature can cause problems. And some sub-options cannot be used as a direct replacement at all.

VOLTAGE TURN ON - 16 V, OFF - 10 V	VOLTAGE ON - 8.4 V, OFF - 7.6 V	WORKING TEMPERATURE	COF FILLING
UC1842	UC1843	-55°С... +125°С	up to 100%
UC2842	UC2843	-40°С... +85°С
UC3842	UC3843	0°С... +70°С
UC1844	UC1845	-55°С... +125°С	up to 50%
UC2844	UC2845	-40°С... +85°С
UC3844	UC3845	0°С... +70°С

Based on the table above, it is clear that the UC3845 is far from the best version of this microcircuit, since its lower temperature limit is limited to zero degrees. The reason is quite simple - not everyone stores a welding machine in a heated room, and a situation is possible when you need to weld something in the off-season, but the welder either does not turn on or simply explodes. no, not to shreds, even pieces of power transistors are unlikely to fly out, but there will be no welding in any case, and the welder also needs repairs. Having skimmed through Ali, I came to the conclusion that the problem is completely solvable. Of course, UC3845 is more popular and there are more of them on sale, but UC2845 is also on sale:

UC2845 is of course somewhat more expensive, but in any case it is cheaper than ONE power transistor, so I personally ordered a dozen UC2845 despite the fact that there are still 8 pieces of UC3845 in stock. Well, as you wish.
Now we can talk about the microcircuit itself, or more precisely about the principle of its operation. The figure below shows the block diagram of UC3845, i.e. with an internal trigger that does not allow the duration of the control pulse to be more than 50% of the period:

By the way, if you click on the picture, it will open in a new tab. It’s not entirely convenient to jump between tabs, but in any case it’s more convenient than turning the mouse wheel back and forth, returning to the picture that went to the top.
The chip provides dual control of the supply voltage. COMP1 monitors the supply voltage as such and if it is less than the set value, it issues a command that turns the internal five-volt regulator off. If the supply voltage exceeds the switching threshold, the internal stabilizer is unlocked and the microcircuit starts. The second element supervising the power supply is element DD1, which, in cases where the reference voltage differs from the norm, produces a logical zero at its output. This zero goes to inverter DD3 and, transformed into a logical one, goes to logical OR DD4. In almost all block diagrams, this one simply has an inverse input, but I took the inverter outside of this logical element - it’s easier to understand the principle of operation.
The OR logic element works on the principle of determining the presence of a logical one at any of its inputs. That is why it is called OR - if there is a logical one at input 1, OR at input 2, OR at input 3, OR at input 4, then the output of the element will be a logical one.
When a logical one appears at the first input of this adder of all control signals, a logical one will appear at its direct output, and a logical zero will appear at its inverse output. Accordingly, the upper driver transistor will be closed, and the lower one will open, thereby closing the power transistor.
The microcircuit will be in this state until the reference power analyzer gives permission to operate and a logical unit appears at its output, which, after the inverter DD3, unlocks the output element DD4.
Let's say our power supply is normal and the microcircuit starts working. The master oscillator begins to generate control pulses. The frequency of these pulses depends on the values ​​of the frequency-setting resistor and capacitor. There is a slight discrepancy here. The difference doesn’t seem to be big, but nevertheless it exists and there is a possibility of getting something that is not exactly what you wanted, namely a very hot device when a “faster” microcircuit from one manufacturer is replaced with a slower one. The most beautiful picture of the dependence of frequency on the resistance of the resistor and capacitance of the capacitor is from Texas Instruments:

Things are a little different for other manufacturers:

Dependence of frequency on RC ratings of a Fairchild microcircuit

Dependence of frequency on RC ratings of a microcircuit from STMicroelectronics

Dependence of frequency on RC ratings of a microcircuit from UNISONIC TECHNOLOGIES CO

The clock generator produces fairly short pulses in the form of a logical unit. These impulses are divided into three blocks:
1. The same final adder DD4
2. D-trigger DD2
3. RS trigger on DD5
The DD2 trigger is available only in microcircuits of the 44 and 45 subseries. It is this that prevents the duration of the control pulse from becoming longer than 50% of the period, since with each arriving edge of a logical unit from the clock generator it changes its state to the opposite. By doing this, it divides the frequency into two, forming zeros and ones of equal duration.
This happens in a rather primitive way - with each edge arriving at clock input C, the trigger writes to itself the information located at the information input D, and input D is connected to the inverse output of the microcircuit. Due to the internal delay, the inverted information is recorded. For example, the inverting output has a logical zero level. When the edge of the pulse arrives at input C, the trigger manages to record this zero before zero appears at its direct output. Well, if the direct output is zero, then the inverse output will be a logical one. With the arrival of the next edge of the clock pulse, the trigger already writes a logical unit into itself, which will appear at the output after some nanoseconds. Writing a logical one leads to the appearance of a logical zero at the inverse output of the trigger and the process will begin to repeat from the next edge of the clock pulse.

It is for this reason that the UC3844 and UC3845 microcircuits have an output frequency that is 2 times less than that of the UC3842 and UC3843 - it is shared by the trigger.
When the first pulse enters the unit setting input of the RS trigger DD5, it switches the trigger to a state where its direct output is logical one, and its inverse output is zero. And until one appears at input R, trigger DD5 will be in this state.
Suppose we do not have any control signals from the outside, then at the output of the error amplifier OP1 a voltage will appear close to the reference voltage - there is no feedback, the inverting input is in the air, and the non-inverting input is supplied with a reference voltage of 2.5 volts.
Here I’ll make a reservation right away - I personally was somewhat confused by this error amplifier, but after studying the datasheet more carefully and thanks to poking the noses of subscribers, it turned out that the output of this amplifier is not entirely traditional. In the output stage OP1 there is only one transistor connecting the output to the common wire. A positive voltage is generated by a current generator when this transistor is slightly open or completely closed.
From the output of OP1, the voltage passes through a kind of limiter and voltage divider 2R-R. In addition, this same bus has a voltage limit of 1 volt, so that under any conditions more than one volt does not reach the inverting input OP2.
OP2 is essentially a comparator that compares the voltages at its inputs, but the comparator is also tricky - a conventional operational amplifier cannot compare such low voltages - from actual zero to one volt. A conventional op-amp needs either a higher input voltage or a negative side of the supply voltage, i.e. bipolar voltage. The same comparator quite easily copes with the analysis of these voltages, it is possible that there are some biasing elements inside, but we don’t really care about the circuit diagram.
In general, OP2 compares the voltage coming from the output of the error amplifier, or more precisely, the remaining voltage that is obtained after passing through the divider with the voltage at the third pin of the microcircuit (DIP-8 package is meant).
But at this moment in time, we have nothing at all on the third pin, and a positive voltage is applied to the inverting input. Naturally, the comparator will invert it and form a clear logical zero at its output, which will not affect the state of the RS trigger DD5 in any way.
As a result of what is happening, we have a logical zero at the first input from the top, DD4, since our power supply is normal, at the second input we have short pulses from the clock generator, at the third input we have pulses from the D-flip-flop DD2, which have the same duration of zero and one . At and at the fourth input we have a logical zero from the RS trigger DD5. As a result, the output of the logic element will completely repeat the pulses generated by the D-trigger DD2. Therefore, as soon as a logical one appears at the direct output of DD4, transistor VT2 will open. At the same time, the inverse output will have a logical zero and transistor VT1 will be closed. As soon as a logical zero appears at the DD4 output, VT2 closes, and the inverse output of DD4 opens VT1, which will be the reason for opening the power transistor.
The current that VT1 and VT2 can withstand is one ampere, therefore this microcircuit can successfully control relatively powerful MOSFET transistors without additional drivers.
In order to understand exactly how the processes occurring in the power supply are regulated, the simplest booster was assembled, since it requires the least number of winding parts. The first GREEN ring that came to hand was taken and 30 turns were wound on it. The quantity was not calculated at all, just one layer of winding was wound and nothing more. I wasn’t worried about consumption - the microcircuit operates in a wide range of frequencies and if you start with frequencies under 100 kHz, then this will be quite enough to prevent the core from entering saturation.

The result was the following booster circuit:

All external elements have the prefix out, meaning that they are OUTSIDE microcircuit details.
I’ll immediately describe what’s on this diagram and why.
VT1 - the base is essentially in the air, the ends are soldered on the board for putting on jumpers, i.e. the base is connected either to ground or to a saw generated by the chip itself. There is no resistor Rout 9 on the board - I even missed its necessity.
Optocoupler Uout 1 uses the error amplifier OP1 to adjust the output voltage, the degree of influence is regulated by resistor Rout 2. Optocoupler Uout 2 controls the output voltage bypassing the error amplifier, the degree of influence is regulated by resistor Rout 4. Rout 14 is a current measuring resistor, specially taken at 2 Ohms so as not to remove the power transistor. Rout 13 - adjusting the current limit threshold. Well, Rout 8 - adjusting the clock frequency of the controller itself.

The power transistor is something that was soldered out of a car converter that was once being repaired - one arm flared up, I changed all the transistors (why ALL the answer is HERE), and this is, so to speak, a surrender. So I don’t know what it is - the inscription is very worn, in general it’s something like 40-50 amperes.
Rout 15 type load - 2 W at 150 Ohm, but 2 W turned out to be not enough. You need to either increase the resistance or increase the power of the resistor - it starts to stink if it works for 5-10 minutes.
VDout 1 - to exclude the influence of the main power on the operation of the controller (HER104 seems to have been a hit), VDout 2 - HER308, well, so that it doesn’t immediately go off if something goes wrong.
I realized the need for resistor R9 when the board was already soldered. In principle, this resistor will still need to be selected, but this is purely optional for those who REALLY want to get rid of the relay method of stabilization at idle. More on this a little later, but for now I stuck this resistor on the side of the tracks:

First start - engines ALL interlinear connectors must be connected to ground, i.e. they do not affect the circuit. The Rout 8 engine is installed so that the resistance of this resistor is 2-3 kOhm, since the capacitor is 2.2 nF, the frequency should be about 300-odd kHz, therefore at the output of the UC3845 we will get somewhere around 150 kHz.

We check the frequency at the output of the microcircuit itself - this is more accurate, since the signal is not cluttered by shock processes from the inductor. To confirm the differences between the generation frequency and the conversion frequency, we turn the yellow ray to pin 4 and see that the frequency is 2 times higher. The operating frequency itself turned out to be 146 kHz:

Now we increase the voltage on the optocoupler LED Uout 1 in order to control the change in stabilization modes. Here it should be recalled that the resistor Rout 13 slider is in the lower position in the diagram. A common wire is also supplied to the VT1 base, i.e. Absolutely nothing happens at pin 3 and comparator OP2 does not respond to the non-inverting input.
By gradually increasing the voltage on the optocoupler LED, it becomes obvious that control pulses simply begin to disappear. By changing the scan this becomes most clear. This happens because OP2 only monitors what is happening at its inverting input and as soon as the output voltage of OP1 drops below the threshold value, OP2 forms a logical one at its output, which sets trigger DD5 to zero. Naturally, but a logical one appears at the inverse output of the trigger, which blocks the final adder DD4. Thus the microcircuit stops completely.

But the booster is loaded, therefore the output voltage begins to decrease, the Uout 1 LED begins to decrease brightness, the Uout 1 transistor closes and OP1 begins to increase its output voltage and as soon as it passes the OP2 response threshold, the microcircuit starts again.
In this way, the output voltage is stabilized in relay mode, i.e. the microcircuit generates control pulses in batches.
By applying voltage to the LED of the optocoupler Uout 2, the transistor of this optocoupler opens slightly, entailing a decrease in the voltage supplied to the comparator OP2, i.e. the adjustment processes are repeated, but OP1 no longer takes part in them, i.e. the circuit is less sensitive to changes in output voltage. Thanks to this, the control pulse packets have a more stable duration and the picture seems more pleasant (even the oscilloscope is synchronized):

We remove the voltage from the Uout 2 LED and, just in case, check for the presence of a saw on the upper terminal of R15 (yellow beam):

The amplitude is slightly more than a volt and this amplitude may not be enough, because there are voltage dividers on the circuit. Just in case, we unscrew the slider of the tuning resistor R13 to the upper position and control what is happening at the third pin of the microcircuit. In principle, hopes were fully justified - the amplitude is not enough to start limiting the current (yellow ray):

Well, if there is not enough current through the inductor, it means either many turns or a high frequency. Rewinding is too lazy, because the board has a trimming resistor Rout8 to adjust the frequency. We rotate its regulator until the required voltage amplitude is obtained at pin 3 of the controller.
In theory, as soon as the threshold is reached, that is, as soon as the voltage amplitude at pin 3 becomes not much more than one volt, the duration of the control pulse will begin to be limited, since the controller is already beginning to think that the current is too high and it will turn off the power transistor.
Actually, this begins to happen at a frequency of about 47 kHz, and further decreases in frequency had virtually no effect on the duration of the control pulse.

A distinctive feature of the UC3845 is that it controls the flow through the power transistor at almost every cycle of operation, and not the average value, as for example the TL494 does, and if the power supply is designed correctly, then it will never be possible to damage the power transistor...
Now we raise the frequency until the current limitation ceases to have an effect, however, we will make a reserve - we set it to exactly 100 kHz. The blue ray still shows control pulses, but we put the yellow one on the LED of the optocoupler Uout 1 and begin to rotate the trimmer resistor knob. For some time, the oscillogram looks the same as during the first experiment, however, a difference also appears; after passing the control threshold, the duration of the pulses begins to decrease, i.e., real regulation occurs through pulse-width modulation. And this is just one of the tricks of this microcircuit - as a reference saw for comparison, it uses a saw that is formed on the current-limiting resistor R14 and thus creates a stabilized voltage at the output:

The same thing happens when the voltage on the optocoupler Uout 2 increases, although in my version it was not possible to get the same short pulses as the first time - the brightness of the optocoupler LED was not enough, and I was too lazy to reduce the resistor Rout 3.
In any case, PWM stabilization occurs and is quite stable, but only in the presence of a load, i.e. the appearance of a saw, even of no great significance, at pin 3 of the controller. Without this saw, stabilization will be carried out in relay mode.
Now we switch the base of the transistor to pin 4, thereby forcibly feeding the saw to pin 3. There is not a big stumble here - for this feint you will have to select a Rout 9 resistor, since the amplitude of the dust and the level of the constant component turned out to be somewhat too large for me.

However, now the principle of operation itself is more interesting, so we check it by lowering the Rout 13 trimmer engine to the ground and begin to rotate Rout 1.
There are changes in the duration of the control pulse, but they are not as significant as we would like - the large constant component has a strong effect. If you want to use this inclusion option, you need to think more carefully about how to organize it correctly. Well, the picture on the oscilloscope is as follows:

With a further increase in voltage on the optocoupler LED, a breakdown occurs in the relay mode of operation.
Now you can check the load capacity of the booster. To do this, we introduce a limitation on the output voltage, i.e. Apply a small voltage to the Uout 1 LED and reduce the operating frequency. The sociogram clearly shows that the yellow ray does not reach the level of one volt, i.e. There is no current limit. The limitation is provided only by adjusting the output voltage.
In parallel with the load resistor Rour 15, we install another 100 Ohm resistor and the oscillogram clearly shows an increase in the duration of the control pulse, which leads to an increase in the time of energy accumulation in the inductor and its subsequent release to the load:

It is also not difficult to notice that by increasing the load, the voltage amplitude at pin 3 also increases, since the current flowing through the power transistor increases.
It remains to see what happens at the drain in stabilization mode and in its complete absence. We turn a blue beam onto the drain of the transistor and remove the feedback voltage from the LED. The oscillogram is very unstable, since the oscilloscope cannot determine which edge it should synchronize with - after the pulse there is quite a decent “chatter” of self-induction. The result is the following picture.

The voltage on the load resistor also changes, but I won’t make a GIF - the page is already quite “heavy” in terms of traffic, so I declare with full responsibility that the voltage on the load is equal to the voltage of the maximum value in the picture above minus 0.5 volts.

LET'S SUM IT UP

UC3845 is a universal self-clocking driver for single-ended voltage converters, can work in both flyback and forward converters.
Can operate in relay mode, can operate in full-fledged PWM voltage stabilizer mode with current limitation. It is precisely a limitation, since during an overload the microcircuit goes into current stabilization mode, the value of which is determined by the circuit designer. Just in case, a small sign showing the dependence of the maximum current on the value of the current-limiting resistor:

I, A	1	1,2	1,3	1,6	1,9	3	4,5	6	10	20	30	40	50
R, Ohm	1	0,82	0,75	0,62	0,51	0,33	0,22	0,16	0,1	0,05	0,033	0,025	0,02
								2 x 0.33		2 x 0.1	3 x 0.1	4 x 0.1	5 x 0.1
P,W	0,5	1	1	1	1	2	2	5	5	10	15	20	25

For full PWM voltage regulation, the IC requires a load because it uses a ramp voltage to compare with the controlled voltage.
Voltage stabilization can be organized in three ways, but one of them requires an additional transistor and several resistors, and this conflicts with the formula LESS PARTS - MORE RELIABILITY, so two methods can be considered basic:
Using an integrated error amplifier. In this case, the feedback optocoupler transistor is connected by the collector to a reference voltage of 5 volts (pin 8), and the emitter supplies voltage to the inverting input of this amplifier through the OS resistor. This method is recommended for more experienced designers, since if the gain of the error amplifier is high, it may become excited.
Without using an integrated error amplifier. In this case, the collector of the regulating optocoupler is connected directly to the output of the error amplifier (pin 1), and the emitter is connected to the common wire. The input of the error amplifier is also connected to the common wire.
The operating principle of PWM is based on monitoring the average output voltage and maximum current. In other words, if our load decreases, the output voltage increases, and the saw amplitude at the current-measuring resistor drops and the pulse duration decreases until the lost balance between voltage and current is restored. As the load increases, the controlled voltage decreases and the current increases, which leads to an increase in the duration of the control pulses.

It is quite easy to organize a current stabilizer on a microcircuit, and the control of the flowing current is controlled at each cycle, which completely eliminates overloading of the power stage with the correct choice of the power transistor and the current-limiting, or more precisely, measuring resistor installed at the source of the field-effect transistor. It is this fact that has made the UC3845 the most popular when designing household welding machines.
UC3845 has quite serious “rake” - the manufacturer does not recommend using the microcircuit at temperatures below zero, so in the manufacture of welding machines it would be more logical to use UC2845 or UC1845, but the latter are in some shortage. UC2845 is slightly more expensive than UC3845, not as catastrophically as domestic sellers indicated (prices in rubles as of March 1, 2017).

The frequency of the XX44 and XX45 microcircuits is 2 times less than the clock frequency, and the coefficient of filling cannot exceed 50%, then it is most favorable for converters with a transformer. But the XX42 and XX43 microcircuits are best suited for PWM stabilizers, since the duration of the control pulse can reach 100%.

Now, having understood the operating principle of this PWM controller, we can return to designing a welding machine based on it...

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:

1) multimeter (voltmeter and ohmmeter);

2) oscilloscope;

3) a stabilized power source (current source), preferably regulated with a voltage of up to 20-30 V.

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

checking the output resistance of the microcircuit;

modeling the operation 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

Switching power supplies based on the UC3842 chip

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 whose power supplies (PS) 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.

The UC3842 IC is available in SOIC-8 and SOIC-14 packages, but in the vast majority of cases it is modified in a DIP-8 package. In Fig. 1 shows the pinout, and Fig. 2 - its block diagram and typical IP diagram. Pin numbers are given for packages with eight pins; pin numbers for the SOIC-14 package are given in parentheses. It should be noted that there are minor differences between the two IC designs. Thus, the version in the SOIC-14 package has separate power and ground pins for the output stage.

The UC3842 microcircuit is intended for building on its basis stabilized pulse power supplies with pulse width modulation (PWM). Since the power of the output stage of the IC is relatively small, and the amplitude of the output signal can reach the supply voltage of the microcircuit, an n-channel MOS transistor is used as a switch together with this IC.

Rice. 1. Pinout of the UC3842 chip (top view)

Let's take a closer look at the assignment of IC pins for the most common eight-pin package.

1. Comp: This pin is connected to the output of the compensation error amplifier. For normal operation of the IC, 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 terminal of which is connected to pin 2 of the IC.
2. Vfb: Feedback input. The voltage at this pin is compared with the reference voltage generated inside the IC. The result of the comparison modulates the duty cycle of the output pulses, thus stabilizing the output voltage of the IP.
3. C/S: Current limit signal. This pin must be connected to a resistor in the source circuit of the switch transistor (CT). When the current through the CT increases (for example, in the case of an overload of the IP), the voltage across this resistor increases and, after reaching a threshold value, stops the operation of the IC and transfers the CT to the closed state.
4. Rt/Ct: output intended for connecting a timing RC circuit. The operating frequency of the internal oscillator is set by connecting resistor R to the reference voltage Vref and capacitor C (typically about 3000 pF) to common. This frequency can be changed within a fairly wide range; from above it is limited by the speed of the CT, 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 IP operates quite normally at a much higher or much lower frequency. It should be noted that a capacitor with the highest possible resistance to direct current should be used as a timing capacitor. In the author’s practice, there were instances of ICs that generally refused to start when using certain types of ceramic capacitors as a timing device.
5. Gnd: general conclusion. It should be noted that the common wire of the power supply should in no case be connected to the common wire of the device in which it is used.
6. Out: IC output, connected to the CT gate through a resistor or parallel connected resistor and diode (anode to gate).
7. Vcc: IC power input. The IC in question has some very significant power-related features, which will be explained when considering a typical IC switching circuit.
8. Vref: Internal reference voltage output, its output current is up to 50mA, voltage is 5V.

The reference voltage source is used to connect to it one of the arms of a resistive divider, designed for rapid adjustment of the output voltage of the IP, as well as for connecting a timing resistor.

Let us now consider a typical IC connection circuit shown in Fig. 2.

Rice. 2. Typical connection diagram of UC3862

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.

Alexey Kalinin
"Electronic equipment repair"

Any developer may be faced with the problem of creating a simple and reliable power source for the device he is designing. Currently, there are quite simple circuit solutions and the corresponding element base that make it possible to create switching power supplies using a minimum number of elements.

We present to your attention a description of one of the options for a simple network switching power supply. The power supply is based on the UC3842 chip. This microcircuit has become widespread since the second half of the 90s. It implements many different power supplies for TVs, faxes, VCRs and other equipment. The UC3842 gained such popularity due to its low cost, high reliability, simplicity of circuit design and minimal required wiring.

At the input of the power supply (Fig. 5.34), there is a mains voltage rectifier, including a 5 A fuse FU1, a 275 V varistor P1 to protect the power supply from excess voltage in the network, a capacitor C1, a 4.7 Ohm thermistor R1, diode bridge VD1...VD4 on FR157 diodes (2 A, 600 V) and filter capacitor C2 (220 µF at 400 V). Thermistor R1 in a cold state has a resistance of 4.7 Ohms, and when the power is turned on, the charge current of capacitor C2 is limited by this resistance. Next, the resistor heats up due to the current passing through it, and its resistance drops to tenths of an ohm. However, it has virtually no effect on the further operation of the circuit.

Resistor R7 provides power to the IC during the startup period of the power supply. Winding II of transformer T1, diode VD6, capacitor C8, resistor R6 and diode VD5 form the so-called feedback loop (Loop Feedback), which provides power to the IC in operating mode, and due to which the output voltages are stabilized. Capacitor C7 is a power filter for the IC. Elements R4, C5 make up the timing chain for the internal pulse generator of the IC.

The converter transformer is wound on a ferrite core with an ETD39 frame from Siemens+Matsushita. This set features a round center ferrite core and plenty of space for thick wires. The plastic frame has leads for eight windings.

The transformer is assembled using special mounting springs. Particular attention should be paid to the thorough insulation of each layer of windings using varnished cloth, and several layers of varnished cloth should be laid between windings I, II and the remaining windings, ensuring reliable insulation of the output part of the circuit from the network. The windings should be wound in a “turn to turn” manner, without twisting the wires. Naturally, the wires of adjacent turns and loops should not be allowed to overlap. The winding data of the transformer are given in table. 5.5.

The output part of the power supply is shown in Fig. 5.35. It is galvanically isolated from the input part and includes three functionally identical blocks, consisting of a rectifier, an LC filter and a linear stabilizer. The first block - a 5 V (5 A) stabilizer - is made on the A2 SD1083/84 (DV, LT) linear stabilizer IC. This microcircuit has a switching circuit, housing and parameters similar to the MS KR142EN12, however, the operating current is 7.5 A for SD1083 and 5 A for SD1084.

The second block - stabilizer +12/15 V (1 A) - is made on the IC linear stabilizer A3 7812 (12 V) or 7815 (15 V). Domestic analogues of these ICs are KR142EN8 with the corresponding letters (B, V), as well as K1157EN12/15. The third block - stabilizer -12/15 V (1 A) - is made on a linear stabilizer IC. A4 7912 (12 V) or 7915 (15 V). Domestic analogues of these ICs are K1162EN12D5.

Resistors R14, R17, R18 are necessary to dampen excess voltage at idle. Capacitors C12, C20, C25 were selected with a voltage reserve due to a possible increase in voltage at idle. It is recommended to use capacitors C17, C18, C23, C28 type K53-1A or K53-4A. All ICs are installed on individual plate radiators with an area of ​​at least 5 cm2.

Structurally, the power supply is made in the form of one single-sided printed circuit board installed in the case from the power supply of a personal computer. The fan and network input connectors are used for their intended purpose. The fan is connected to a +12/15V stabilizer, although it is possible to make an additional +12V rectifier or stabilizer without much filtering.

All radiators are installed vertically, perpendicular to the air flow exiting through the fan. Four wires 30...45 mm long are connected to the outputs of the stabilizers; each set of output wires is crimped with special plastic clamps-straps into a separate bundle and is equipped with a connector of the same type that is used in a personal computer to connect various peripheral devices. Stabilization parameters are determined by the parameters of the stabilizer ICs. Ripple voltages are determined by the parameters of the converter itself and are approximately 0.05% for each stabilizer.

In power supplies (PS), PWM controllers, paired with a reference field-effect transistor, are widely used not only in televisions, but also in other electronic devices, including DVDs, receivers, and so on. They have the same operating principle, the repair method is also the same, only the diagrams are different.

The proposed technique is to check and repair the PWM generator itself. As a basis, I will take the HORIZONT 14A01 TV IP, ShchTsT-739M1 chassis, UC3842AN PWM controller.

The source can be roughly divided into three parts:
a) PWM generator
b) power part of the primary circuits of the IP
c) secondary power circuits

So, PWM UC3842AN.

The power supply circuit to the microcircuit is standard, but there are some subtleties here.

At the moment of switching on, 300 volts, through R808, are supplied to the 7th leg of the microcircuit. The microcircuit starts and sends a burst of pulses to the field-effect transistor. But the peculiarity of this microcircuit is that its starting voltage is higher, in our case by 2 volts, than the operating voltage. And the resistor R808 is designed in such a way that on the 7th leg of the microcircuit, in the absence of recharge from the TPI (in our case from the 3rd leg of the TPI via VD806), there is a working voltage, but not a starting voltage! That is, if the IP does not start or goes into protection, then there is no recharge from the VD806, and the microcircuit does not produce pulses.

So, if the IP is unstable or does not start, or produces low voltages, the first thing to do is measure the voltage on the 7th leg; if it is lower than the working one (12-12.5 volts), then the C816 should be replaced. If there is no voltage, then R808 is broken, or the microcircuit is faulty.

Further. In case of other malfunctions, in particular if the field-effect transistor fails or does not start.
To eliminate the influence of the power part on the PWM itself, it is enough to unsolder the reference transistor VT800 and you can check and repair the generator with the voltage turned on, without fear of failure of other elements of the power supply and the rest of the circuit.

Based on the results of measuring the supply voltage and the output to the field-effect transistor, one can almost 100% judge the serviceability of the microcircuit.
Using the device, we measure the voltage on the 7th leg. Everything is very clearly visible on the pointer instrument. The needle from 12 volts should jump to 14. If so, then the power supply is fine. If not, then again the C816 or R808, or the same chip, is faulty. As soon as the voltage on the 7th leg is normal, you should measure the voltage on the 6th leg, this is the output through R816 to the field-effect transistor. If the needle twitches at the limit of 1-2-2.5 volts, then the PWM generator is 99% working. The field-effect transistor is soldered back in and, if necessary, the IP is repaired further.