Charger on uc3842 diagram. Switching power supplies based on the UC3842 chip
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
Any designer may face the challenge of creating a simple and reliable power source for the device he is designing. Currently, there are quite simple circuit solutions and their corresponding element base, which allow creating switching power supplies with a minimum number of elements.
Your attention is invited to a description of one of the options for a simple network switching power supply. The power supply is based on the UC3842 chip. This chip has been widely used since the second half of the 90s. It implements many different power sources for TVs, fax machines, VCRs and other equipment. The UC3842 gained such popularity due to its low cost, high reliability, simplicity of circuitry and the minimum required piping.
At the input of the power supply (Fig. 5.34), there is a mains voltage rectifier, including a fuse FU1 for a current of 5 A, a varistor P1 of 275 V to protect the power supply from overvoltage in the network, a capacitor C1, a thermistor R1 of 4.7 Ohm, diode bridge VD1 ... VD4 on FR157 diodes (2 A, 600 V) and filter capacitor C2 (220 uF at 400 V). Thermistor R1 in the 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. Further, the resistor is heated due to the current passing through it, and its resistance drops to tenths of an ohm. However, it has virtually no effect on further work scheme.
Resistor R7 provides power to the IC during the startup 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 the IC power supply filter. Elements R4, C5 constitute a timing chain for the internal IC pulse generator.
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 room 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 thoroughness of the insulation of each layer of windings with the help of varnished cloth, and several layers of varnished cloth should be laid between the windings I, II and the rest of the windings, ensuring reliable isolation of the output part of the circuit from the mains. The windings should be wound in a "coil-to-coil" 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 stabilizer for 5 V (5 A) - is made on the IC of the linear stabilizer A2 SD1083 / 84 (DV, LT). This microcircuit has a switching circuit, package and parameters similar to the MC KR142EN12, however, the operating current is 7.5 A for SD1083 and 5 A for SD1084.
The second block - the stabilizer +12/15 V (1 A) - is made on the IC of the linear stabilizer A3 7812 (12 V) or 7815 (15 V). Domestic analogues of these ICs are KR142EN8 with the corresponding letters (B, C), as well as K1157EN12 / 15. The third block - stabilizer -12/15 V (1 A) - is made on the IC of a linear stabilizer. A4 7912 (12V) or 7915 (15V). Domestic analogues of these ICs are K1162EN12D5.
Resistors R14, R17, R18 are needed to dampen excess voltage at idle. Capacitors C12, C20, C25 are selected with a voltage margin 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 as intended. The fan is connected to a +12/15V stabilizer, although it is possible to make an additional +12V rectifier or regulator 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 strap clips into a separate bundle and equipped with a connector of the same type that is used in a personal computer for connecting various peripheral devices. The stabilization parameters are determined by the parameters of the IC stabilizers. Ripple voltages are determined by the parameters of the converter itself and are approximately 0.05% for each stabilizer.
PWM UC3842AN
The UC3842 is a PWM controller circuit with current and voltage feedback for driving a key stage on an n-channel MOS transistor, discharging its input capacitance with a forced current of up to 0.7A. The SMPS controller chip consists of the UC384X (UC3843, UC3844, UC3845) series of PWM controller chips. The UC3842 core is specially designed for long-term operation with a minimum number of external discrete components. The UC3842 PWM controller features precise duty cycle control, temperature compensation and low cost. A feature of the UC3842 is the ability to operate within a 100% duty cycle (for example, the UC3844 operates with a duty cycle of up to 50%.). The domestic analogue of UC3842 is 1114EU7. Power supplies made on the UC3842 chip are distinguished by increased reliability and ease of execution.
Rice. Type table.
This table gives a complete picture of the differences between the UC3842, UC3843, UC3844, UC3845 microcircuits.
General description.
For those who want to get more deeply acquainted with the UC384X series PWM controllers, the following material is recommended.
- Datasheet UC3842B (download)
- Datasheet 1114EU7 domestic analogue of the UC3842A chip (download).
- Article "Flyback converter", Dmitry Makashev (download).
- Description of the operation of PWM controllers of the UCX84X series (download).
- Article "Evolution of flyback switching power supplies", S. Kosenko (download). The article was published in the magazine "Radio" No. 7-9 for 2002.
The document from STC SIT, the most successful description in Russian for PWM UC3845 (K1033EU16), is highly recommended for review. (Download).
The difference between the UC3842A and UC3842B chips, A consumes less current until the start.
UC3842 has two versions of the package 8pin and 14pin, the pinout of these versions is significantly different. Further, only the variant of the 8pin package will be considered.
A simplified block diagram is necessary to understand the principle of operation of a PWM controller.
Rice. Block diagram of UC3842
A structural diagram in a more detailed version is necessary for diagnosing and testing the performance of the microcircuit. Since we are considering the 8pin version, Vc is 7pin, PGND is 5pin.
Rice. UC3842 block diagram (detailed version)
Rice. Pinout UC3842
There should be material on the purpose of the conclusions, but it is much more convenient to read and look at the practical circuit for switching on the UC3842 PWM controller. The circuit is drawn so well that it makes it much easier to understand the purpose of the pins of the microcircuit.
Rice. Wiring diagram UC3842 on the example of a power supply for TV
1. Comp:(rus. Correction) error amplifier output. For normal operation of the PWM controller, it is necessary to compensate for the frequency response of the error amplifier; for this purpose, a capacitor with a capacity of about 100 pF is usually connected to the indicated output, the second output of which is connected to output 2 of the IC. If the voltage at this pin is lowered below 1 volt, then the pulse duration will decrease at output 6 of the microcircuit, thereby reducing the power of this PWM controller.
2. Vfb: (rus. Feedback voltage) feedback input. The voltage at this pin is compared with the reference voltage generated inside the UC3842 PWM controller. The result of the comparison modulates the duty cycle of the output pulses, as a result, the output voltage of the power supply stabilizes. Formally, the second output serves to reduce the duration of the output pulses, if you apply more than +2.5 volts to it, then the pulses will be reduced and the microcircuit will reduce the output power.
3.C/S: (second designation I feel) (rus. Current feedback) current limit signal. This pin must be connected to a resistor in the source circuit of the switching transistor. At the moment of overloading the MOS transistor, the voltage across the resistance increases and, when a certain threshold is reached, the UC3842A stops its operation, closing the output transistor. Simply put, the output serves to turn off the pulse at the output when a voltage above 1 volt is applied to it.
4.Rt/Ct: (rus. Frequency reference) connection of the timing RC circuit required to set the frequency of the internal generator. R is connected to Vref - the reference voltage, and C to the common wire (usually several tens of nF are selected). This frequency can be changed within a fairly wide range, from above it is limited by the speed of the key transistor, and from below by the power of the pulse transformer, which decreases with decreasing frequency. In practice, the frequency is selected in the range of 35 ... 85 kHz, but sometimes the power supply works quite normally even at a much higher or much lower frequency.
For a timing RC circuit, it is better to abandon ceramic capacitors.
5.Gnd: (rus. General) general conclusion. The common terminal must not be connected to the body of the circuit. This "hot" ground is connected to the body of the device through a pair of capacitors.
6.Out: (rus. Exit) the output of the PWM controller is connected to the gate of the key transistor through a resistor or a resistor and a diode connected in parallel (with the anode to the gate).
7. Vcc: (rus. Nutrition) power input of the PWM controller, this output of the microcircuit is supplied with a supply voltage in the range from 16 volts to 34 volts, please note that this microcircuit has a built-in Schmidt trigger (UVLO), which turns on the microcircuit if the supply voltage exceeds 16 volts, if the voltage for some reason it will become lower than 10 volts (for other microcircuits of the UC384X series, the ON / OFF values may differ, see the Table of Ratings), it will be disconnected from the supply voltage. The microcircuit also has overvoltage protection: if the supply voltage on it exceeds 34 volts, the microcircuit will turn off.
8. Vref: output of an internal reference voltage source, its output current is up to 50 mA, voltage is 5 V. It is connected to one of the divider arms and is used to quickly adjust the U output of the entire power supply.
A bit of theory.
Shutdown circuit when the input voltage drops.
Rice. Shutdown circuit when the input voltage drops.
The Under-Voltage LockOut or UVLO circuit ensures that Vcc is equal to the voltage that makes the UC384x fully operational to turn on the output stage. On Fig. it is shown that the UVLO circuit has on and off threshold voltages, the values of which are 16 and 10, respectively. A hysteresis of 6V prevents erratic switching on and off during power-up.
Generator.
Rice. Generator UC3842.
The frequency-setting capacitor Ct is charged from Vref(5V) through the frequency-setting resistor Rt, and is discharged by the internal current source.
The UC3844 and UC3845 have a built-in counting flip-flop that is used to obtain a maximum oscillator duty cycle of 50%. Therefore, the generators of these microcircuits must be set to a switching frequency twice as high as desired. The UC3842 and UC3843 chip generators are set to the desired switching frequency. The maximum operating frequency of the UC3842/3/4/5 family generators can reach 500 kHz.
Reading and current limiting.
Rice. Organization of current feedback.
The current-to-voltage conversion is performed with an external resistor Rs connected to ground. RC filter to suppress output key spikes. The inverting input of the current sense comparator UC3842 is internally biased by 1V. Current limiting occurs if the voltage at pin 3 reaches this threshold.
Error signal amplifier.
Rice. Structural diagram of the error signal amplifier.
The non-inverting error input has no separate pin and is internally biased by 2.5 volts. The output of the error amplifier is connected to pin 1 to connect an external compensating circuit, allowing the user to control the frequency response of the converter's closed loop feedback.
Rice. Scheme of the compensating circuit.
A compensating circuit suitable for stabilizing any converter circuit with additional current feedback, except flyback and boost converters operating with inductor current.
Blocking methods.
There are two ways to block the UC3842 chip:
increasing the voltage at pin 3 above the level of 1 volt,
or pulling up the voltage at pin 1 to a level not exceeding the voltage drop across two diodes, relative to the ground potential.
Each of these methods results in a logic HIGH voltage level at the output of the PWM coparator (structural diagram). Since the main (default) state of the PWM latch is reset, the output of the PWM comparator will be held LOW until the state on pins 1 and/or 3 changes in the next clock period (the period following the one in question). clock period when a situation has arisen that requires blocking the microcircuit).
Connection diagram.
The simplest circuit connecting the UC3842 PWM controller is purely academic. The circuit is the simplest generator. Despite its simplicity, this scheme works.
Rice. The simplest switching scheme 384x
As can be seen from the diagram, the UC3842 PWM controller requires only an RC circuit and power to operate.
Scheme of switching on the PWM controller of the UC3842A PWM controller, using the example of a TV power supply.
Rice. Power supply diagram for UC3842A.
The diagram gives a visual and simple representation of the use of the UC3842A in a simple power supply. Scheme for easier reading, slightly modified. The full version of the circuit can be found in the PDF document "Power supplies 106 circuits" Tovarnitsky N.I.
Scheme of switching on the PWM controller of the UC3843 PWM controller, using the example of the power supply of the D-Link router, JTA0302E-E.
Rice. Schematic diagram of the power supply on the UC3843.
Although the circuit is made according to the standard inclusion for UC384X, however, R4 (300k) and R5 (150) are deduced from the standards. However, successfully, and most importantly, logically selected circuits help to understand the principle of operation of the power supply.
Power supply on a UC3842 PWM controller. The scheme is not intended to be repeated, but is for informational purposes only.
Rice. Standard scheme inclusions from datasheet-a (the scheme has been slightly changed, for easier understanding).
Repair of the Power Supply based on PWM UC384X.
Checking with an external power supply.
Rice. Simulation of the PWM controller.
Checking the operation is carried out without soldering the microcircuit from the power supply. The power supply must be disconnected from the 220V network before carrying out diagnostics!
From an external stabilized power supply, apply voltage to pin 7 (Vcc) of the microcircuit, a voltage greater than the UVLO turn-on voltage, in the general case, more than 17V. In this case, the UC384X PWM controller should work. If the supply voltage is less than the UVLO turn-on voltage (16V / 8.4V), then the microcircuit will not start. You can read more about UVLO here.
Checking the internal voltage reference.
ExaminationUVLO
If the external power supply allows voltage regulation, then it is advisable to check the operation of the UVLO. By changing the voltage on pin 7(Vcc) of the pin within the UVLO voltage range, the reference voltage on pin 8(Vref) = +5V should not change.
It is not recommended to apply voltage of 34V and higher to pin 7(Vcc). It is possible that there is a protective zener diode in the power supply circuit of the UC384X PWM controller, then it is not recommended to apply this zener diode above the operating voltage.
Checking the operation of the generator and external circuits of the generator.
You will need an oscilloscope to check. Pin 4(Rt/Ct) should have a stable "saw".
Checking the output control signal.
You will need an oscilloscope to check. Ideally, pin 6(Out) should have square wave pulses. However, the circuit under study may differ from the one shown, and then it will be necessary to turn off the external feedback circuits. The general principle is shown in fig. - with this inclusion, the UC384X PWM controller is guaranteed to start.
Rice. UC384x operation with feedback circuits disabled.
Rice. An example of real signals when simulating the operation of a PWM controller.
If the power supply unit with a UC384x PWM controller does not turn on or turns on with a long delay, then check by replacing the electrolytic capacitor that filters the power supply (pin 7) of this m / s. It is also necessary to check the elements of the initial start circuit (usually two 33-100kOhm resistors connected in series).
When replacing a power (field) transistor in a power supply unit with a control m / s 384x, it is imperative to check the resistor that acts as a current sensor (it is at the source of the field). A change in its resistance at a nominal value in fractions of an ohm is very difficult to detect with an ordinary tester! An increase in the resistance of this resistor leads to a false operation of the PSU current protection. At the same time, it is possible to look for the reasons for PSU overload in secondary circuits for a very long time, although they are not there at all.
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
| 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 | 100uF 25V | 1 | To notepad | |||
| C6, C7 | electrolytic capacitor | 2200uF 35V | 2 | To notepad | ||
| C8 | electrolytic capacitor | 100uF 400V | 1 | To notepad | ||
| C9, C10 | Capacitor | 0.1uF 400V | 2 | To notepad | ||
| C11 | Capacitor | 0.33uF 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 |
The article is devoted to the device, repair and refinement of power supplies for a wide range of equipment, made on the basis of the UC3842 chip. 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 supply. Since the second half of the 90s, a huge number of TVs, video monitors, faxes and other devices have been produced, in the power supplies (IP) of which the UC3842 integrated circuit (hereinafter referred to as the IC) is used. Apparently, this is due to its low cost, a small number of discrete elements needed for its "body kit" and, finally, fairly stable characteristics of the integrated circuit, which is also important. Variants of this IC, produced by different manufacturers, may differ in prefixes, but must contain the 3842 kernel.
The UC3842 is available in SOIC-8 and SOIC-14 packages, but in the vast majority of cases, its modification is found in a DIP-8 package. On fig. 1 shows the pinout, and in fig. 2 - its block diagram and a typical IP diagram. The pin numbering is for the 8-pin package, the pin numbers in parentheses are for the SOIC-14 package. It should be noted that there are minor differences between the two versions of the IS. So, the version in the SOIC-14 package has separate power and ground pins for the output stage.
The UC3842 chip is designed to build stabilized pulsed power supplies with pulse-width modulation (PWM) on its basis. 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 key in conjunction with this IC.
Rice. 1. UC3842 pinout (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 this output, the second output of which is connected to output 2 of the IC.
2. vfb: feedback input. The voltage at this pin is compared with the reference voltage generated inside the IC. The comparison result modulates the duty cycle of the output pulses, thus stabilizing the output voltage of the MT.
3. C/S: current limit signal. This output must be connected to a resistor in the source circuit of the key transistor (CT). With an increase in current through the CT (for example, in the event of an overload of the IP), the voltage across this resistor increases and, after reaching the threshold value, stops the IC and switches the CT to the closed state.
4. Rt/Ct: pin for connecting the timing RC circuit. The operating frequency of the internal oscillator is set by connecting a resistor R to the reference voltage Vref and a capacitor C (usually about 3000 pF) to ground. 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 works quite normally even at a much higher or much lower frequency. It should be noted that a capacitor with as high a resistance as possible should be used as a time-setting capacitor. direct current. In the author's practice, there were instances of ICs that generally refused to start when some types of ceramic capacitors were used as a timer.
5. Gnd: general conclusion. It should be noted that the common wire of the IP should in no case be connected to the common wire of the device in which it is used.
6. out: output of the IC, connected to the gate of the CT through a resistor or a resistor and a diode connected in parallel (anode to the gate).
7. Vcc: IC power input. The considered IC has some very significant power-related features, which will be explained when considering a typical IC power circuit.
8. Vref: Internal reference voltage output, its output current is up to 50mA, voltage is 5V.
The exemplary voltage source is used to connect one of the arms of a resistive divider to it, designed to quickly adjust the output voltage of the IP, as well as to connect a timing resistor.
Let us now consider a typical circuit for switching on the IS, shown in Fig. 2.
Rice. 2. Typical wiring diagram UC3862
As seen from circuit diagram, IP is designed for a mains voltage of 115 V. The undoubted advantage of this type of IP is that it can be used with minimal modifications in a network with a voltage of 220 V, you just need:
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 an equal one in capacity, but with an operating voltage of 400 V;
- increase the value of the resistor R2 to 75 ... 80 kOhm;
- check the CT for the allowable drain-source voltage, which should be at least 600 V. As a rule, even in the IP designed to operate on a 115 V network, CTs that can operate 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, IP has some features related to its power supply. Let's consider them in more detail. At the first moment after turning on the IP in 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 the resistor R2 and accumulated on the capacitor C2 is sufficient. When the voltage on these capacitors reaches a value of 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 the transformer T1, including windings 3-4. This voltage is rectified by pulse diode D3, filtered by capacitor C3, and fed through diode D2 to the power supply circuit of the IC. As a rule, a zener diode D1 is included in the power circuit, limiting the voltage at the level of 18 ... 22 V. After the IC has entered the operating mode, it begins to track 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 taken from the secondary windings of the pulse transformer.
In case of short circuits in the circuits of the secondary windings, for example, as a result of a breakdown of electrolytic capacitors or diodes, the energy losses in the pulse transformer increase sharply. As a result, the voltage received from windings 3-4 is not enough to maintain the normal operation of the IC. The internal oscillator turns off, a low-level voltage appears at the output of the IC, turning the CT into a closed state, and the microcircuit is again in low power mode. After a while, its supply voltage rises to a level sufficient to start the internal generator, and the process repeats. In this case, characteristic clicks (clicks) are heard from the transformer, the repetition period of which is determined by the values of the capacitor C2 and resistor R2.
When repairing a power supply, situations sometimes arise when a characteristic ticking 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 circuits of the IC itself. For example, in the author's practice, there were cases when the capacitor C3 was broken. common cause such behavior of the IP is a break in the rectifier diode D3 or the decoupling diode D2.
When a powerful CT breaks down, as a rule, it has to be changed together with the IC. The fact is that the gate of the CT is connected to the output of the IC through a resistor of a very small value, and in the event of a breakdown of the CT, a high voltage from the primary winding of the transformer enters the output of the IC. The author categorically recommends that in case of a malfunction of the CT, it should be changed along with the IC, fortunately, its cost is low. Otherwise, there is a risk of “killing” a new CT, because if a high voltage level from a broken IC output is present on its gate for a long time, then it will fail due to overheating.
Some other features of this IP were noticed. In particular, during a breakdown of the CT, the resistor R10 in the source circuit very often burns out. When replacing this resistor, you should adhere to the nominal value of 0.33 ... 0.5 Ohm. It is especially dangerous to overvalue the resistor. In this case, as practice has shown, at the first inclusion of the IP in the network, both the microcircuit and the transistor fail.
In some cases, the failure of the IP occurs due to the breakdown of the zener diode D1 in the power supply circuit of the IC. In this case, the IC and CT, as a rule, remain serviceable, it is only necessary to replace the zener diode. In the event of a break in the zener diode, 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 or soldered a faulty standard zener diode, you can solder the KS522 with the anode to terminal 5 of the IC, the cathode to terminal 7 of the IC. As a rule, after such a replacement, similar malfunctions no longer occur.
You should pay attention to the health of the potentiometer used to adjust the output voltage of the IP, if any in the circuit. It is not in the above circuit, but it is not difficult to introduce it by including resistors R3 and R4 in the gap. Pin 2 of the IC must be connected to the slider of this potentiometer. I note that in some cases such refinement is simply necessary. Sometimes, after replacing the IC, the output voltages of the SP are too high or too low, and there is no adjustment. In this case, you can either turn on the potentiometer, as mentioned above, or choose the value of the resistor R3.
According to the author's observation, if high-quality components are used in the IP, and it is not operated in extreme conditions, its reliability is quite high. In some cases, the reliability of the IP can be improved by using a resistor R1 with a slightly larger rating, for example, 10 ... 15 ohms. In this case, power-on transients are much more relaxed. In video monitors and TVs, this must be done without affecting the kinescope demagnetization circuit, i.e., the resistor should in no case be included in the break in the common power circuit, but only in the connection circuit of the IP itself.
Alexey Kalinin
"Repair of electronic equipment"
