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Power management chip UC3842, troubleshooting

baijiahao | 28/05/2021

UC3842 chip is used as a PWM pulse width modulation chip for low-power switching power supply. During the maintenance of switching power supply, it is often encountered that uc3842/uc3844 can not work normally due to faults. Now the power supply cannot start or slightly start to vibrate (measurement output The terminal voltage is low), but it does not work normally (indicated by 8Pin without 5V), the possible reasons are summarized as follows:

1. First check the electrolytic capacitor connected to the 7Pin (or the electrolytic capacitor connected to the feedback coil) to see if its capacity meets the requirements. If the capacity of the capacitor is significantly reduced, the fault that should not vibrate after replacement can be restored; The capacitance is normal, proceed to the next check.

2. Separately add 16V to the 7Pin of uc3842/uc3844 on the circuit board, and measure whether the 8Pin has 5V. If the measured 8Pin has 5V voltage, it means that there is no problem with the chip; if there is no 5V voltage, the uc3842/uc3844 must be removed Next, power on 16V to 7Pin separately, and measure whether 8Pin has 5V. If there is still no 5V, it can prove that the chip has been damaged. If measuring 8Pin has 5V, there should be a short circuit between the peripheral components connected to the 8Pin and the ground. This step is mainly to detect whether the c3842/uc3844 chip itself is damaged. If the chip is not damaged, the basic part of the fault can be eliminated, and the next step can be checked.

Attachment: Another way to detect whether the uc3842/uc3844 chip is damaged or not is: after detecting the peripheral components of the chip (or replacing the damaged peripheral components), first do not install the power switch tube, and add the input power to test uc3842/uc3844 If the voltage of 7Pin fluctuates between 10-17V, and the other pins also have voltage fluctuations, it means that the circuit has started oscillating, and the uc3842 is basically normal. If the voltage of pin 7 is low, and the remaining pins have no voltage or voltage fluctuations, then uc3842 /uc3844 is damaged.

3. Check the secondary side. It is speculated that the output overload or short circuit of the secondary side causes the current to increase, which is reflected to the primary side to protect the 3Pin of the uc3842/uc3844 chip, which requires the implementation of overcurrent protection on the secondary side. Measure the electronic components one by one until the fault is found.

The main pin voltages of the uc3842/uc3844 chip are listed below when it is working normally:

1Pin: 1.5V; 2Pin: 2.5V; 3Pin: 0.005V; 6Pin: 1.05V; 7Pin: 14.1V; 8Pin: 5V;

Here we take a 75KW driver board power supply from Siemens as an example. The main complaint is that the power supply screams, the switch tube is hot, and the secondary voltage is "normal". The circuit boards have almost been "swept" by their peers. After taking over, there is no major problem in the whole circuit. After power-on, a scream is heard, and the heat sink of the switch tube is hot in less than 1 minute.

There are generally two situations for the screaming of the switching power supply: one is the low switching frequency, and the other is that there is a short circuit in the secondary pole. Power on again and measure C3844 "VCC" "Vref" and other voltages are normal. After power off, touch the transformer without any temperature rise!

Since the transformer has no heat generation, the secondary pole short circuit is excluded. However, a low switching frequency will generally not cause the switch tube to heat up so fast or even not overheat at all. Then it must be the abnormality of the switching tube and its peripheral drive circuit that causes the loss of the switching tube to increase. Change the switch tube and test the machine, the situation remains the same. When measuring the UC3844 drive pin to the G pole circuit of the switch tube, it is found that the 22Ω resistance changes. Replaced with a new patch resistance test machine, the switching power supply works normally.

Going back to measure the original resistance and found that the resistance has become 8.45KΩ. When its value changes with the 27KΩ resistance of the switching tube GS pole, the actual driving voltage of the switching tube decreases, and the front and rear edges of the driving waveform are deformed. This is what the field effect tube cannot tolerate, so a strong scream is found. .

The power board takes less than ten minutes from taking over to troubleshooting. A total of "how many axes" are used. The maintenance process of the switching power supply UC3842 is similar to the peripherals, and the maintenance methods are basically the same. The prerequisites for the following process are: no short circuit in the switch tube, and the switch tube There is no open circuit for the current-limiting protection resistor to the ground, and the switch tube will not break down immediately when it is energized. Remember: test UC3842 first.

Whether the 7-pin 15V power supply is normal: if there is no voltage, check the starting resistance, or the starting circuit (some models use a separate diode to rectify the 7-pin power supply), or the 7-pin short-circuit to the ground regulator tube; there is voltage but high, change 7-pin to ground filter capacitor, 100UF/50V; there is voltage but the voltage is low and fluctuating, the adjustment circuit of 3842 is faulty. Pin 7 voltage is normal; turn off and measure 300V voltage disappearing speed: if it disappears quickly, then the power will start to vibrate, check that the 1K resistance of pin 3 to ground and the voltage of the voltage regulator tube to ground do not disappear, the fault point is 3842 not vibrated, check 3842 1, 2, the external resistance of the foot, the potentiometer and the replacement of UIC3842 itself. 3. The voltage of pin 7 is low and fluctuating: focus on checking the diode of the FBT synchronous feedback circuit; for models with optocouplers, check the input end of the back stage optocoupler, and focus on the periphery of the IC (LM431).

UC3842 pin introduction and good or bad judgment 1 pin error signal amplification output 2 pin feedback input 3 pin switch tube overcurrent detection 4 pin oscillation circuit time constant 5 pin ground 6 pin switch tube drive pulse output 7 pin power supply 8 pin 5V reference voltage For a simple judgment of good or bad, use the 47-type multimeter Rx1 block.



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