Uc3842 description of working principle. UC3842 description, principle of operation, wiring diagram. Switching power supplies based on the UC3842 chip

UC3845
PRINCIPLE OF OPERATION

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

There was only one option - Multisim. Version 12 was found even with crack. I have not used Multisim for a VERY long time, so I had to tinker. The first thing that made me happy was that in Multisim there is a separate library for five-volt logic and a separate library for fifteen-volt logic. In general, with grief in half, a more or less workable version turned out, showing signs of life, but he didn’t want to work exactly the way a real microcircuit behaves, no matter how much I persuaded him. Firstly, the models do not measure the level relative to the real zero, so an additional source of negative bias voltage would have to be introduced. But in this case, I would have to explain in some detail what it is and why, but I wanted to get as close as possible to a 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 was not long. Despite the presence in the libraries themselves of the twelfth and fourteenth Multisim of the UC3845 chip itself and its analogues, it quickly became clear that the microcircuit model does not allow working out ALL options for including this microcircuit. In particular, current limiting and output voltage adjustment work quite confidently (although 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 from its place, it did not travel far. There was only one option left - a printout of the datasheet on the UC3845 and a board with a strapping. In order not to look at the simulation of the load and the simulation of the current limit, I decided to build a microbooster and already check on it what actually happens to the microcircuit with one or another option of switching on and using it.
First, a little explanation:
The UC3845 chip really deserves the attention of designers of power supplies of various capacities and purposes, it has a number of almost analogues. Almost because when replacing a microcircuit in the board, nothing else needs to be changed, however, changes in ambient temperature can cause problems. And some sub-options cannot be used for direct replacement at all.

Based on the above table, 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 keeps the welding machine in a heated room, and a situation is possible when you need to weld something in the off-season, and the welder either does not turn on or corny explodes. no, not to shreds, even pieces of power transistors are unlikely to fly out, but there will be no welding in any, and even the welder needs repairs. Having slipped through Ali, I came to the conclusion that the problem is completely solvable. Of course, the UC3845 is more popular and there are more of them on sale, but the UC2845 is also on sale:

The UC2845 is of course a little more expensive, but in any case it is cheaper than ONE power transistor, so I personally ordered a dozen UC2845s despite the fact that there are still 8 UC3845s in stock. Well, as you wish.
Now we can talk about the microcircuit itself, or rather about the principle of its operation. The figure below shows the block diagram of the 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 is not very convenient to jump between tabs, but in any case it is more convenient than turning the mouse wheel back and forth, returning to the picture that has gone to the top.
The microcircuit provides double control of the supply voltage. COMP1 monitors the supply voltage as such and if it is less than the set value, it gives a command that causes the internal five-volt regulator to turn off. If the supply voltage exceeds the turn-on threshold, the internal stabilizer is unlocked and the microcircuit starts. The second power-supervising element is the DD1 element, which, in cases where the reference voltage differs from the norm, outputs a logical zero at its output. This zero falls on the inverter DD3 and converted to a logical unit falls on the logical OR DD4. In almost all block diagrams, this one simply has an inverse input, but I brought the inverter outside of this logical element - it's easier to understand the principle of operation.
The logical OR element works on the principle of determining the presence of a logical unit on any of its inputs. That is why it is called OR - if at the input 1, OR at the input 2, OR at the input 3, OR at the input 4 there is a logical unit, then the output of the element will be a logical unit.
When a logical unit appears at the first input of this adder of all control signals, a logical unit will appear at its direct output, and a logical zero at the inverse one. Accordingly, the upper transistor of the driver will be closed, and the lower one will open, thereby closing the power transistor.
In this state, the microcircuit will be until the reference power analyzer gives permission to work and a logical unit appears at its output, which, after the inverter DD3, does not unlock the output element DD4.
Let's say we have normal power 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. Here there is a little inconsistency. The difference does not seem to be big, but nevertheless it is there and there is a chance to get not quite what you wanted, namely a very hot device, when a more "fast" microcircuit from one manufacturer will be replaced by a slower one. The most beautiful picture of the dependence of the frequency on the resistance of the resistor and the capacitance of the capacitor from Texas Instruments:

Other manufacturers do things a little differently:

The dependence of the frequency on the RC ratings of the chip from Fairchild

The dependence of the frequency on the RC values ​​\u200b\u200bof the chip from STMicroelectronics

The dependence of the frequency on the RC ratings of the microcircuit from UNISONIC TECHNOLOGIES CO

From the clock generator, rather short pulses are obtained in the form of a logical unit. These impulses are divided into three blocks:
1. All the same final adder DD4
2. D-trigger DD2
3. RS flip-flop on DD5
The DD2 trigger is available only in microcircuits of the subseries 44 and 45. It is he who does not allow the duration of the control pulse to become longer than 50% of the period, since it changes its state to the opposite with each incoming edge of a logical unit from the clock generator. By this, he divides the frequency by two, forming zeros and ones of the same duration.
This happens in a rather primitive way - with each incoming front to the clock input C, the trigger writes to itself the information located at the information input D, and the 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 is a logic zero level. With the arrival of the front of the pulse at the input C, the trigger has time to write this zero before the zero appears at its direct output. Well, if we have a zero direct output, then there will be a logical unit on the inverse. With the arrival of the next edge of the clock pulse, the trigger already writes a logical unit to itself, which will appear at the output after some nanoseconds. Writing a logical unit 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 output frequency of the UC3844 and UC3845 microcircuits is 2 times less than that of the UC3842 and UC3843 - it is divided by the trigger.
Getting to the input of the RS unit setting of the DD5 trigger, the first pulse puts the trigger into a state where its direct output is a logical one, and the inverse output is zero. And until a unit appears at the input R, the trigger DD5 will be in this state.
Suppose we don’t have any control signals from the outside, then a voltage close to the reference voltage will appear at the output of the error amplifier OP1 - there is no feedback, the inverting input is in the air, and a reference voltage of 2.5 volts is applied to the non-inverting input.
Here I’ll immediately make a reservation - I personally was somewhat embarrassed by this error amplifier, but after studying the datasheet more carefully and thanks to the subscribers poking their noses, it turned out that the output of this amplifier is not quite traditional. There is only one transistor in the OP1 output stage, connecting the output to a common wire. A positive voltage is generated by the current generator when this transistor is ajar or completely closed.
From the OP1 output, the voltage passes through a kind of limiter and voltage divider 2R-R. In addition, the same bus has a voltage limit of 1 volt, so under any conditions, more than one volt does not fall on the inverting input of OP2 under any circumstances.
OP2 is essentially a comparator that compares the voltages at its inputs, but the comparator is also cunning - 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 arm 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 kind of biasing elements inside, but we don’t seem to care much about the circuit diagram.
In general, OP2 compares the voltage coming from the output of the error amplifier, more precisely, those voltage residues that are obtained after passing through the divider with the voltage at the third output of the microcircuit (the DIP-8 package is meant).
But at this point in time, we don’t have anything at all on the third output, 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 on the first input from above, since the power supply is normal, on the second input we have short pulses from the clock generator, on the third input we have pulses from the D-trigger DD2, which have the same duration of zero and one . On and on the fourth input we have a logical zero from the DD5 RS flip-flop. As a result, the output of the logic element will completely repeat the pulses that generates the D-flip-flop DD2. Therefore, as soon as a logical unit appears on the direct output of DD4, the transistor VT2 will open. At the same time, a logical zero will be located at the inverse output and the transistor VT1 will be closed. As soon as a logical zero appears at the output of DD4, VT2 closes, and the inverse output of DD4 opens VT1, which will serve as a reason for opening the power transistor.
The current that VT1 and VT2 can withstand is one ampere, therefore this microcircuit can successfully drive relatively powerful MOSFET transistors without additional drivers.
In order to understand exactly how the processes occurring in the power supply are adjusted, 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 didn’t worry about consumption - the microcircuit operates in a wide frequency range, and if you start from frequencies under 100 kHz, then this will already be quite enough to prevent the core from entering saturation.

As a result, the following booster scheme was obtained:

All external elements are prefixed with out meaning that they are OUTSIDE detail microcircuits.
I’ll immediately sign what is on this diagram and why.
VT1 - the base is essentially in the air; the base is connected either to ground or to a saw generated by the microcircuit itself. There is no Rout 9 resistor on the board - I even missed the need for it.
Optocoupler Uout 1 uses 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. knock out the power transistor. Rout 13 - adjustment of the current limit operation threshold. Well, Rout 8 - adjusting the clock frequency of the controller itself.

A power transistor is something that was soldered out of a car converter that was being repaired - one shoulder flared up, changed all the transistors (why ALL the answer is HERE), and this is change, so to speak. So I don’t know what it is - the inscription is very shabby, in general it’s something like 40-50 amperes.
Rout 15 type load - 2 W into 150 ohms, but 2 W was not enough. It is necessary either to increase the resistance, or 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 supply on the operation of the controller (HER104 seems to have fallen into the hands), VDout 2 - HER308, well, this is so that it does not immediately bang if something goes wrong.
I realized the need for the R9 resistor when the board was already soldered. In principle, this resistor will still need to be selected, but this is already purely optional, who VERY wants to get rid of the relay method of idling stabilization. More on this later, but for now I put this resistor on the side of the tracks:

The first inclusion - engines ALL interlineators must be connected to ground, i.e. they do not affect the circuit. The Rout 8 engine is set so that the resistance of this resistor is 2-3 kOhm, since the capacitor is 2.2 nF, then the frequency should be about 300 kHz with a tail, 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 - more precisely, since the signal is not littered with shock processes from the throttle. To confirm the differences between the generation frequency and the conversion frequency, we stand on pin 4 with a yellow ray and see that the frequency is 2 times higher. The very same operating frequency turned out to be equal to 146 kHz:

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

But the booster is loaded, so the output voltage starts to decrease, the Uout 1 LED starts to decrease in brightness, the Uout 1 transistor closes and OP1 starts to increase its output voltage, and as soon as it passes the OP2 threshold, the microcircuit starts up again.
Thus, the output voltage is stabilized in the 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, resulting in a decrease in the voltage supplied to the comparator OP2, i.e. the adjustment processes are repeated, but OP1 no longer participates in them, i.e. the circuit is less sensitive to changes in the 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 the presence of a saw on the upper output 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 engine of the tuning resistor R13 to the upper position and control what is happening on the third output of the microcircuit. In principle, the hopes were fully justified - the amplitude is not enough to start the current limit (yellow ray):

Well, since there is not enough current through the inductor, it means either a lot of turns or a high frequency. Rewinding is too lazy, because the Rout8 tuning resistor is provided on the board to adjust the frequency. We rotate its regulator until the required voltage amplitude is obtained at terminal 3 of the controller.
In theory, as soon as the threshold is reached, i.e. as soon as the voltage amplitude at pin 3 becomes a little more than one volt, the control pulse duration will be limited, since the controller is already starting to think that the current is too high and it will close the power transistor.
Actually, this begins to happen at a frequency of about 47 kHz, and a further decrease in frequency had practically 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 the power transistor will never be able to stagger ...
Now we raise the frequency until the current limitation ceases to make an impact, however, we will make a margin - we set exactly 100 kHz. The blue ray still shows control pulses, but we put the yellow one on the LED of the Uout 1 optocoupler and begin to rotate the tuning resistor knob. For some time, the oscillogram looks the same as in the first experiment, but there is also a difference, having passed the control threshold, the pulse duration begins to decrease, i.e., real adjustment 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 Uout 2 rebate is increased, 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 Rout 3 resistor.
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, not even of great importance, at the output of 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. This is not a big stumble - for this feint, you will have to pick up the Rout 9 resistor, since the amplitude of the dust and the level of the constant component turned out to be somewhat large.

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 - a 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 turned out as follows:

With a further increase in the voltage on the LED of the optocoupler, a breakdown into the relay mode of operation occurs.
Now you can check the load capacity of the booster. To do this, we introduce a limitation on the output voltage, i.e. we apply a small voltage to the LED Uout 1 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 gives only the adjustment of 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 return 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 on the drain in the stabilization mode and in its complete absence. We become a blue beam on the drain of the transistor and remove the feedback voltage from the LED. The oscillogram is highly unstable, since the oscilloscope cannot determine which edge to synchronize on - after the pulse, there is a pretty decent "talk" of self-induction. The result is the following picture.

The voltage on the load resistor also changes, but I will not make a GIF - the page turned out to be quite “heavy” in terms of traffic, so I declare with all responsibility that the voltage at the load is equal to the voltage of the maximum value in the picture above minus 0.5 volts.

SUMMING UP

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

For full PWM voltage regulation, the IC needs a load because it uses a sawtooth 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, therefore, two methods can be considered basic:
Using the integrated error amplifier. In this case, the feedback optocoupler transistor is connected by a 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, as a high error amplifier gain can cause it to energize.
Without using the 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 a common wire.
The principle of operation of PWM is based on the control of the average value of the output voltage and the maximum value of the current. In other words, if we reduce the load, the output voltage increases, and the amplitude of the saw on the current-measuring resistor drops and the pulse duration decreases until the lost balance between voltage and current is restored. When 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 the overload of the power stage with the right choice of a power transistor and a current-limiting, or rather measuring resistor, installed on the source of the field-effect transistor. It is this fact that has made the UC3845 the most popular in the design of household welding machines.
UC3845 has a rather serious "rake" - the manufacturer does not recommend using the microcircuit at temperatures below zero, so it would be more logical to use UC2845 or UC1845 in the manufacture of welding machines, but the latter are in some shortage. UC2845 is somewhat more expensive than UC3845, not as catastrophically as indicated by domestic sellers (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 coff filling cannot exceed 50%, then it is most favorable for converters with a transformer. But XX42 and XX43 microcircuits are best suited for PWM stabilizers, since the duration of the control pulse can reach 100%.

Now, having understood the principle of operation of this PWM controller, you can return to the design of a welding machine based on it ...

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

The principle of operation of microcircuits ka3842, UC3842, UC2842

Chip 3842 or 2842 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.

Consider the block diagram of the 3842 and 2842 series microcircuits:
The 7th output of the microcircuit is supplied with a supply voltage in the range from 16 Volts to 34 Volts. 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 drops below 10 Volts. Microcircuits 3842 and 2842 series also have overvoltage protection: if the supply voltage exceeds 34 Volts, the microcircuit will turn off. To stabilize the pulse generation frequency, the microcircuit has its own 5 volt voltage regulator inside, the output of which is connected to pin 8 of the microcircuit. Pin 5 ground (ground). Pin 4 sets the pulse frequency. This is achieved by a resistor R T and a capacitor C T connected to 4 pins. - see the typical wiring diagram below.

6 output - output of PWM pulses. 1 pin of the 3842 chip is used for feedback, if 1 pin. the voltage is lowered 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. 2 output of the microcircuit, like the first one, serves to reduce the duration of the output pulses, if the voltage at pin 2 is higher than +2.5 Volts, then the duration of the pulses will decrease, which in turn will reduce the output power.

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

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

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

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

The UC3842 PWM controller chip is the most common when building monitor power supplies. In addition, these microcircuits are used to build switching voltage regulators in horizontal scanners 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 printer power supplies. In a word, this article will be of interest to absolutely all specialists, one way or another connected with power sources.

The failure of the UC 3842 chip in practice occurs quite often. Moreover, as the statistics of such failures show, the breakdown of a powerful field-effect transistor, which is controlled by this microcircuit, becomes the cause of the microcircuit malfunction. 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 easiest to put into practice in a poorly equipped workshop are checking the output resistance and simulating the operation of a microcircuit using an external power source.

For this work you will need the following devices:

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

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

Checking the output resistance of the microcircuit

Very accurate information about the health of the microcircuit is given by its output impedance, 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. Such a measurement is best done with a soldered microcircuit. In the event of a breakdown of the microcircuit, this resistance becomes equal to several ohms.

If you measure the output resistance without soldering 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 usually the circuit has a terminating resistor connected between the output of the microcircuit and the "case". Therefore, a serviceable microcircuit may have an output impedance during testing. Although, it usually does not happen 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.

Modeling the operation of the microcircuit

Such a check is carried out without soldering the microcircuit from the power supply. The power supply must be turned off before carrying out 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 a voltmeter.

The workflow includes the following steps:

1) Unplug the monitor from the AC power (disconnect the power cable).

2) From an external stabilized current source, apply a supply voltage of more than 16V to pin 7 of the microcircuit (for example, 17-18 V). In this case, the microcircuit should start. If the supply voltage is less than 16 V, then 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 V DC.

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) Use an oscilloscope to check the signal on pin 4 (CR). In the case of a working microcircuit and its external circuits, there will be a linearly changing voltage (sawtooth) on this contact.

6) By changing the output voltage of the external current source, make sure that the amplitude and frequency of the sawtooth voltage on pin 4 are 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 these signals are present and behave in accordance with the above rules, then we can conclude that the microcircuit is in good condition and that it 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 a current protection signal. These elements often turn out to be faulty during breakdowns.

The circuit is a classic flyback power supply based on PWM UC3842. Since the circuit is basic, the output parameters of the PSU can be easily recalculated to the required ones. As an example, a power supply unit for a laptop with a power supply of 20V 3A was chosen for consideration. If necessary, you can get several voltages, independent or coupled.

Outdoor power output 60W (continuous). Depends mainly on the parameters of the power transformer. By changing them, you can get an output power of up to 100W in this core size. The operating frequency of the block is 29kHz and can be tuned by capacitor C1. The power supply is designed for an unchanging or slightly changing load, hence the lack of stabilization of the output voltage, although it is stable with network fluctuations of 190 ... 240 volts. The PSU works without load, there is a configurable short circuit protection. Block efficiency - 87%. There is no external control, but can be entered using an optocoupler or relay.

The power transformer (core frame), output inductor and network inductor are borrowed from a computer PSU. The primary winding of the power transformer contains 60 turns, the winding for powering the microcircuit - 10 turns. Both windings are wound turn to turn with a 0.5 mm wire with a single interlayer insulation of fluoroplastic tape. The primary and secondary windings are separated by several layers of insulation. The secondary winding is recalculated at the rate of 1.5 volts per turn. For example, a 15-volt winding will have 10 turns, a 30-volt winding will have 20, etc. Since the voltage of one turn is quite large, at low output voltages, fine tuning of the resistor R3 within 15 ... 30 kOhm will be required.

Setting
If you need to get several voltages, you can use the schemes (1), (2) or (3). The number of turns is calculated separately for each winding in (1), (3), and (2) otherwise. Since the second winding is a continuation of the first, the number of turns of the second winding is defined as W2=(U2-U1)/1.5, where 1.5 is the voltage of one turn. Resistor R7 determines the threshold for limiting the output current of the PSU, as well as the maximum drain current of the power transistor. It is recommended to choose the maximum drain current no more than 1/3 of the nameplate for this transistor. The current can be calculated using the formula I (Amps) \u003d 1 / R7 (Ohm).

Assembly
The power transistor and rectifier diode in the secondary circuit are mounted on radiators. Their area is not given, because for each version (with case, without case, high output voltage, low voltage, etc.) the area will be different. The required area of ​​the radiator can be set experimentally, according to the temperature of the radiator during operation. Flanges of parts should not be heated above 70 degrees. The power transistor is installed through an insulating gasket, the diode - without it.

ATTENTION!
Observe the specified voltages of capacitors and powers of resistors, as well as the phasing of the transformer windings. If the phasing is incorrect, the power supply will start, but will not give power.
Do not touch the drain (flange) of the power transistor while the PSU is running! There is a surge of voltage up to 500 volts on the drain.

Replacing elements
Instead of 3N80, BUZ90, IRFBC40 and others can be used. Diode D3 - KD636, KD213, BYV28 for a voltage of at least 3Uout and for the corresponding current.

launch
The unit starts up 2-3 seconds after the mains voltage is applied. To protect against burnout of elements in case of incorrect installation, the first start-up of the power supply unit is carried out through a powerful 100 Ohm 50W resistor connected in front of the mains rectifier. It is also advisable to replace the smoothing capacitor after the bridge with a smaller capacitance (about 10 ... 22 uF 400V) before the first start. The unit is turned on for a few seconds, then turned off and the heating of the power elements is evaluated. Further, the operating time is gradually increased, and in case of successful launches, the unit is switched on directly without a resistor with a standard capacitor.

Well, the last.
The described PSU is assembled in the MasterKit BOX G-010 case. It holds a load of 40W, at higher power it is necessary to take care of additional cooling. In the event of a PSU failure, Q1, R7, 3842, R6 crashes, C3 and R5 may burn out.

List of radio elements