The switching power supply circuit does not oscillate
Due to work reasons, the equipment requires a lot of switching power supplies, so I have had the opportunity to contact and repair switching power supplies, accumulating some maintenance experience. This time, I will talk about some maintenance experience of switching power supplies that do not vibrate, hoping to provide some reference for everyone during maintenance.
If the switch power supply does not vibrate during maintenance, the optocoupler can be used as the dividing point to first determine whether the fault is in front of or behind the optocoupler. Then, step by step, the scope of investigation can be narrowed down. The specific operation is as follows: first, short-circuit the optocoupler directly. It is very simple to solder a piece of tin to reduce the internal resistance of the optocoupler. After short circuiting the optocoupler, we measure the voltage. If the main voltage remains unchanged, the fault is in the circuit behind the optocoupler, otherwise, in the circuit before the optocoupler. The switch power supply here needs to carry a light load, and some switch power supplies cannot vibrate without load.
If the fault is in front of the optocoupler, the main check is whether there is a problem with the AC input, whether there is an open circuit in the capacitor circuit, and if there is a management chip, check whether the chip power supply and the ground voltage of each pin of the chip are correct. Although we short circuit the optocoupler, we also need to rule out whether there is a fault in the optocoupler itself.
If there is a turn to turn short circuit or open circuit in the switch transformer after the optocoupler, it is necessary to suspect whether it is caused by a short circuit in the peak absorption circuit to prevent misjudgment, because the probability of the switch transformer breaking is very small, and a short circuit in the pulse width modulation transistor can also cause the circuit to not vibrate.
If the repair experience is insufficient and it is not possible to determine whether the original parts are damaged online, it is more troublesome to disassemble and measure them, which is more accurate. If the disassembled components are fine, then you can investigate the related small components associated with their faults, gradually narrow down the scope of investigation, determine the problem, and scratch the beard and eyebrows together, which can easily lead to confusion when facing faults and not knowing where to start.
As for the so-called oscillation, I would like to give a simple explanation here: in the process of converting AC into DC, the AC mains power is first rectified and filtered into DC for electronic circuit operation. This circuit includes a high-frequency oscillation circuit, which converts DC into pulses with variable frequency or pulse width. This part is very important in the switching circuit. When the input voltage changes or the load increases and decreases, the oscillation circuit will adjust the frequency or pulse width to maintain output stability. This is its function. To remember this function, it is necessary to oscillate. If it does not oscillate, it means that the switching power supply has malfunctioned, which is equivalent to overshoot or no adjustment.
