Diagnosis of abnormal switching power supply voltage output
The fault diagnosis method and steps
The primary task of troubleshooting such faults is to determine whether the fault is located at the collector, base, or switching transistor. The specific method is to measure the collector and base voltages of the switching transistor. There may be the following situations:
(1) The collector voltage of the switching transistor is 0V, which is 1.4 times lower than the mains voltage. The switch tube does not have a normal operating voltage. If the voltage is 1.4 times, it indicates that the collector voltage of the switch tube is working normally, and the AC220V and rectifier filter circuit are working properly.
(2) The base voltage of the switching transistor is 0V (including the start-up time). This indicates that the starting circuit did not provide starting (conductive) voltage to the base of the switch tube, or that the related components between the base and emitter were damaged. The starting circuit, switch tube emitter, and related components should be inspected. If the voltage is between 0.6 and 0.7 (including the moment of startup), it indicates that the starting circuit, emitter, and components of the switch are normal. When the voltage is higher than 0.7V, it indicates that the starting circuit is normal, but the emitter junction or its components of the switch tube are open or the resistance value increases.
(3) The switching transistor has conductivity conditions: the base voltage of the switching transistor is 0.6-0.7V, and the collector voltage is greater than 250V, indicating that the switching transistor has working conditions. The fault occurs in the positive feedback circuit, including the positive feedback resistor, capacitor, freewheeling diode, and the positive feedback winding of the switching transformer, as well as the connection board between them.
