We repair the computer power supply. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Computers

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After disassembling the PSU, ring the key transistors for a short circuit (typically BUT11A), 1..3 Ohm resistors in their base for an open circuit, a bridge for a short / open circuit, pre-output transistors for a short circuit / open circuit, diodes in the secondary circuits for a breakdown. As a pre-outlet when replacing, you can set our KT315, weekend or our KT872, KT8114 (but then for self-starting it may be necessary to reduce the value of the resistors between the base and collector to 200k ... 150k), or imported ones like these: 2SC3447, 2SC3451, 2SC3457, 2SC3460 (61), 2SC3866, 2SC4706 2, 4744SC11, BUT12A, BUT18A, BUT46A, BUV13005, MJE494 It is advisable to remove the 220/120v switch for prevention. When connected to the network for testing, it is necessary to turn on an incandescent lamp of the 100W 220V type instead of a fuse, and a 5 ... 2 Ohm 5 W resistor into the + 20V output circuit.

Checking the TL494 chip and its analogues (M1114EU4, mPC494C, IR3M02). This IC includes: a driving sawtooth voltage generator A1, the generator frequency is set by an external resistor R1 and a capacitor C1 and can be approximately determined by the formula f=1/(C1*R1). R1 is connected between pins 6 and 7, and C1 between pins 5 and 7. The amplitude of the saw does not depend on the ratings of R1 and C1 and is approximately equal to 4V; feedback circuit amplifier DA2; pulse-width modulator, made on the comparator DA4; converter protection amplifier against current overload or short circuit on the load DA1; frequency divider by two, made on the counting tiger DD2; coincidence cascades on elements DD1, DD5, DD6; cascade on the DA3 comparator, allowing you to build:

- scheme for eliminating overvoltage at the output of the converter in transient modes;

- a scheme for limiting the range of variation of the fill factor within the required limits;

- a scheme for ensuring a smooth transition of the converter to the mode;

- logic elements DD3, DD4 designed to set the control mode for either single-cycle or push-pull converters;

- output transistors Q1 and Q2;

- built-in continuous voltage regulator DA5 and voltage relay (threshold device) DA6;

- decoupling diodes D1, D2 to provide the "OR" function for the output signals of the microcircuits DA1, DA2.

The control chip works as follows. A continuous voltage regulator provides power to all functional units of the IC and sets the reference voltage to + 5V (pin 14) relative to common pin 7. The voltage relay DA6 allows the passage of control signals to the bases of transistors Q1 and Q2 only if DA5 has entered the mode. The sawtooth voltage (pin 5) generated by generator A1 is fed to the input of comparators DA3, DA4. The other input of the PWM comparator DA4, through the decoupling diode D2, receives a mismatch signal from the error amplifier DA2. A reference voltage source from pin 2 is connected to one of the DA14 inputs, directly or through a divider, and the feedback circuit voltage is supplied to the other input, i.e. output of any channel (usually from the + 5V channel). Between pins 3 and 3, as a rule, a corrective RC circuit is connected to ensure stable operation of the stabilizing converter. From the output of the PWM comparator, rectangular pulses are fed to one input of the coincidence circuit DD1, from its output the pulses pass to the counting trigger DD2 and to the coincidence circuits DD5, DD6.

If a logical unit is applied to the control input of the elements DD3, DD4 (pin 13), then the microcircuit provides control of push-pull converters with pauses at zero, and if a logical zero is applied to pin 13 (pin 13 is connected to pin 7), then DD2 does not affect operation of keys DD3, DD4 and in this case the microcircuit can be used for PWM or PWM control of single-cycle converters. To build current protection, as noted earlier, DA1 can be used, while one of its inputs is supplied with a reference voltage that determines the level of operation of the current protection, and a signal from the current sensor is applied to the second input. Nodes using DA1 and DA3 schemes can be very diverse. Some examples are discussed in the description of the following power supply circuits. To increase the output power of the microcircuit when driving single-cycle transistors Q1 and Q2 can be paralleled, since in this mode they operate synchronously and in phase. The conclusions of M1114EU4 fully correspond to the above listed foreign analogues, and the correspondence between the conclusions of M1114EU3 and M1114EU4 is presented below.

M1114EU4 -- 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
M1114EU3 -- 4 5 6 7 8 9 15 10 11 12 13 14 16 1 2 3

Main parameters M1114EU3, M1114EU4
Upit.microcircuits (pin 12) - Upit.min=9V; Upp.max=40V
Permissible voltage at the input DA1, DA2 no more than Upit / 2
Permissible parameters of output transistors Q1, Q2:
Us less than 1.3V;
Uke less than 40V;
Ik.max less than 250mA
The residual voltage of the collector-emitter of the output transistors is not more than 1.3V.
I consumed by the microcircuit - 10-12mA
Permissible power dissipation:
0.8W at ambient temperature +25C;
0.3W at ambient temperature +70C.
The frequency of the built-in reference oscillator is not more than 100 kHz.
 

Troubleshooting algorithm M1114EU3, M1114EU4 Checking the performance of the microcircuit is carried out with the power supply turned off and when the IC is powered from an external IVEP with a voltage of + 9V ... + 15V applied to the 12th output relative to the 7th. All measurements are also carried out relative to the 7th conclusion. In addition, it is better to connect to the IC by soldering wires, and not using alligator clips, this will provide increased contact reliability and eliminate the possibility of false contacts.

1. When an external voltage is applied, we oscilloscope the voltage at the 14th output, it should be + 5V (+/-5%) and remain stable when the voltage at the 12th output changes from + 9V to + 15V. If this does not happen, then the internal voltage regulator DA5 has failed.
2. Using an oscilloscope, we observe the presence of a sawtooth voltage at pin 5 (see Fig. 1.1a UinDA4) if it is absent or has a distorted shape, then it is necessary to check the serviceability of the timing elements C1 and R1 connected to the 5th and 6th pins, respectively, if these elements are working, then the built-in generator is faulty and replacement is necessary.
3. We check the presence of rectangular pulses on pins 8 and 11. They should correspond to the diagram 5 and 5' in fig. 1.1a. If there are no pulses, then the IC is faulty, and if there are, then we check the performance of other IC nodes.
4. By connecting the 4th pin to the 7th with a conductor, we should see that the width of the pulses on the 8th and 11th pins has increased; by connecting the 4th output to the 14th, the pulses should disappear, if this is not observed, then it is necessary to change the IS. By lowering the external voltage to 5V, we should see that the pulses have disappeared (this means that the DA6 voltage relay has worked), and by raising the voltage to + 9V ... + 15V, the pulses should reappear, if this did not happen and the pulses (which can be arbitrary) are present at 8 and 11, then the voltage relay in the IC is faulty and the microcircuit needs to be replaced.
5. Checking the performance of DA2. We remove the previously installed jumper between the 4th and 7th pins, apply a supply voltage of + 12V ... + 9V to the 15th pin, and connecting the 1st pin with the 14th pin, we should see that on the 8th and 11th pins, the pulse width has become zero, if this does not happen, then DA2 is faulty and the IC needs to be changed.
6. In the BP in Fig. 2, fig. 3, fig. 3.4, DA1 is used in current protection nodes and if previous tests have shown that all other IC nodes are functioning normally, then the DA1 health check is carried out as follows: we apply + 12V ... + 9V to the 15th output and observe rectangular pulses on 8 and 11. From another power source, we apply a negative voltage to the 15th output (relative to the 7th), while the pulses on 8 and 11 should disappear. If this does not happen, then the protection node on DA1 does not work.

Publication: cxem.net

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