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.
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 UC3862 wiring diagram
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"
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:
The 7th pin of the microcircuit is supplied with a supply voltage in the range 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
Chip UC3842(UC3843)- is a PWM controller circuit with current and voltage feedback for controlling a key stage on an n-channel MOS transistor, ensuring the discharge of its input capacitance with a forced current of up to 0.7A. Chip SMPS the controller consists of a series of microcircuits UC384X (UC3843, UC3844, UC3845) PWM controllers. Core UC3842 specifically designed for long-term operation with a minimum number of external discrete components. PWM controller UC3842 It features precise duty cycle control, temperature compensation and is low cost. Feature UC3842 is the ability to operate within 100% duty cycle (for example UC3844 works with a fill factor of up to 50%.). Domestic analogue UC3842 is 1114EU7. Power supplies made on a microcircuit UC3842 are characterized by increased reliability and ease of execution.
Differences in supply voltage between UC3842 and UC3843:
UC3842_________| 16 Volt / 10 Volt
UC3843_________| 8.4 Volt / 7.6 Volt
Differences in pulse duty cycle:
UC3842, UC3843__| 0% / 98%
Tsokolevka UC3842(UC3843) shown in Fig. 1
The simplest connection diagram is shown in Fig. 2 
The 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.
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 circuit is a classic flyback power supply based on the UC3842 PWM. Since the circuit is basic, the output parameters of the power supply can be easily converted to the required ones. As an example for consideration, we selected a power supply for a laptop with a power supply of 20V 3A. If necessary, you can obtain several voltages, independent or related.
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 100 W in a given core size. The operating frequency of the unit is 29 kHz and can be adjusted by capacitor C1. The power supply is designed for a constant or slightly changing load, hence the lack of output voltage stabilization, although it is stable when the network fluctuates 190...240 volts. The power supply operates without load, there is adjustable short-circuit protection. The unit efficiency is 87%. There is no external control, but it can be entered using an optocoupler or relay.
The power transformer (frame with core), output choke and network choke are borrowed from a computer power supply. The primary winding of the power transformer contains 60 turns, the winding for powering the microcircuit contains 10 turns. Both windings are wound turn to turn with 0.5 mm wire with single interlayer insulation made of fluoroplastic tape. The primary and secondary windings are separated by several layers of insulation. The secondary winding is calculated 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 high, at low output voltages precise adjustment with resistor R3 will be required within the range of 15...30 kOhm.
Settings
If you need to obtain several voltages, you can use schemes (1), (2) or (3). The number of turns is counted separately for each winding in (1), (3), and (2) differently. Since the second winding is a continuation of the first, the number of turns of the second winding is determined 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 power supply unit, as well as the maximum drain current of the power transistor. It is recommended to select a maximum drain current of no more than 1/3 of the rating current for a given transistor. The current can be calculated using the formula I(Ampere)=1/R7(Ohm).
Assembly
The power transistor and rectifier diode in the secondary circuit are installed on radiators. Their area is not given, because for each design option (in a housing, without a housing, high output voltage, low, etc.) the area will be different. The required radiator area can be determined experimentally, based on the temperature of the radiator during operation. The flanges of the parts should not heat above 70 degrees. The power transistor is installed through an insulating gasket, the diode - without it.
ATTENTION!
Observe the specified values of capacitor voltages and resistor powers, as well as the phasing of the transformer windings. If the phasing is incorrect, the power supply will start, but will not provide power.
Do not touch the drain (flange) of the power transistor while the power supply is running! There is a voltage surge of up to 500 volts at the drain.
Replacing elements
Instead of 3N80, you can use BUZ90, IRFBC40 and others. 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 supplying mains voltage. To protect against burnout of elements due to incorrect installation, the first start of the power supply is carried out through a powerful 100 Ohm 50 W 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 µF 400V) before the first start-up. The unit is turned on for a few seconds, then turned off and the heating of the power elements is assessed. Next, the operating time is gradually increased, and in case of successful starts, the unit is switched on directly without a resistor with a standard capacitor.
Well, one last thing.
The described power supply 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. If the power supply fails, Q1, R7, 3842, R6 will fail, and C3 and R5 may burn out.
List of radioelements
| Designation | Type | Denomination | Quantity | Note | Shop | My notepad |
|---|---|---|---|---|---|---|
| PWM controller | UC3842 | 1 | To notepad | |||
| Q1 | MOSFET transistor | BUZ90 | 1 | 3N80, IRFBC40 | To notepad | |
| D1, D2 | Rectifier diode | FR207 | 2 | To notepad | ||
| D3 | Diode | KD2994 | 1 | KD636, KD213, BYV28 | To notepad | |
| C1 | Capacitor | 22 nF | 1 | To notepad | ||
| Diode bridge | 1 | To notepad | ||||
| C2 | Capacitor | 100 pF | 1 | To notepad | ||
| C3 | Capacitor | 470 pF | 1 | To notepad | ||
| C4 | Capacitor | 1 nF / 1 kV | 1 | To notepad | ||
| C5 | 100 µF 25V | 1 | To notepad | |||
| C6, C7 | Electrolytic capacitor | 2200 uF 35V | 2 | To notepad | ||
| C8 | Electrolytic capacitor | 100 µF 400V | 1 | To notepad | ||
| C9, C10 | Capacitor | 0.1 µF 400V | 2 | To notepad | ||
| C11 | Capacitor | 0.33 µF 400V | 1 | To notepad | ||
| C12 | Capacitor | 10 nF | 1 | To notepad | ||
| R1 | Resistor | 680 Ohm | 1 | To notepad | ||
| R2 | Resistor | 150 kOhm | 1 | To notepad | ||
| R3 | Resistor | 20 kOhm | 1 | To notepad | ||
| R4 | Resistor | 4.7 kOhm | 1 | To notepad | ||
| R5 | Resistor | 1 kOhm | 1 | To notepad | ||
| R6 | Resistor | 22 Ohm | 1 | To notepad | ||
| R7 | Resistor | 1 ohm | 1 |
