1. When overhauling the switching power supply, first check the circuit with a relatively high failure rate
When overhauling the switching power supply, if it is judged that there may be problems in two or more parts at the same time, the part with a higher failure rate should be checked first. It is the overvoltage protection caused by the problem of the voltage regulator circuit that causes the output voltage to be too high, or it may be caused by a short circuit or overcurrent fault at a certain output end of the switching power supply. However, because the possibility of the latter is far greater than that of the former, when overhauling, it is necessary to first check whether there is a short circuit or overcurrent fault at the voltage output end of the switching power supply. Then press the overvoltage protection to check the voltage regulator circuit.
2. When overhauling the switching power supply, check the wearing parts first
In terms of the structure of switching power supply, each switching power supply has its own advantages and disadvantages, as well as relatively fixed wearing parts. In terms of the use time of electronic equipment, the wearing parts of switching power supply also have certain rules. For example, when the switching power supply is used for a long time, the electrolytic capacitor failure of the hot end of the switching power supply is relatively high. When overhauling the switching power supply, before overhauling according to the conventional method, the inspection of these wearing parts should be focused on, which can achieve a multiplier effect with half the effort.
3. When overhauling the switching power supply, be good at using the observation method
(1) Observe whether the fuse in the insurance is blown, and judge whether the switching power supply has a serious short-circuit fault
The direct cause of the fuse blowing in the AC 220V rectifier filter circuit is only in several components fixed at the hot end of the switching power supply, that is, the switch tube, the 300V large filter capacitor, and the breakdown of the AC 220V rectifier diode. Therefore, if you observe that the fuse in the insurance is blown, there must be a problem with one of the above components.
(2) Observe whether the top of the electrolytic capacitor in the switching power supply is bulging, and you can judge whether there is a problem with this capacitor
If the capacitor swells, it means that the capacitor is basically invalid and needs to be replaced. At the same time, it is also necessary to check the reason for the bulging of the capacitor, whether it is naturally damaged after working for a long time, or damaged by excessive voltage, so as to prescribe the right medicine.
4. When overhauling the switching power supply, it is necessary to learn the skills of using the voltage method
Using the voltage method to repair the switching power supply, it can be judged whether the fault is in the switching power supply, and which part of the switching power supply is located. The common key points of the voltage method to repair the switching power supply are: the AC input end of the switching power supply, the output end of the rectifier filter, the drain and gate of the switching tube, the starting end of the switching power supply control chip, the output end of the switching power supply, etc. By comparing the measured value with the normal value, the fault point can be identified.
5. When overhauling the switching power supply, it is necessary to learn the skills of using the resistance method
The resistance method can judge whether there is a breakdown short circuit and serious leakage fault at each voltage output terminal of the switching power supply, and confirm whether a certain component in the switching power supply is broken down, open circuit or leakage. The resistance method is most commonly used to check the open circuit of the fuse resistor in the switching tube and AC 220V rectifier and filter.
In the following cases, the resistance method can be used for inspection.
①When some output terminals of the switching power supply have a small voltage, and some output terminals have no voltage all the time, it is necessary to measure the terminal resistance without voltage output by the resistance method. If the resistance value is close to zero, it means that the rectifier diode or load at the output end has a breakdown fault.
②When the drain voltage of the switch tube is zero, it is necessary to check the AC 220V bridge rectifier filter circuit by the resistance method, and check whether the fuse circuit connected in series with the AC 220V bridge rectifier is open.
③When the fuse in the fuse is blown, it is necessary to use the resistance method to measure the resistance of the switch drain to ground. If the measurement result is close to zero, it means that the reason for the blown fuse is the breakdown of the switch or 300V large filter capacitor and bridge rectifier.
6. When overhauling the switching power supply, it is necessary to learn the skills of using the current method
The current method can judge whether the main load of the switching power supply is abnormal in the working voltage of the switching power supply caused by the overcurrent. The method is to connect the ammeter at the point where the main output end of the switching power supply is disconnected from the main load, as shown in Figure 4-3, the ammeter's gear is set to a larger gear (such as 500mA), and the current at the moment of startup is measured. If the current exceeds the normal range, it can be judged that the main load is normal. If the current exceeds the normal range, it can be judged that the main load has a short-circuit fault.
