Several maintenance steps of the switching power supply are as follows:
1. Check whether the rectifier circuit D1 – D4 breaks down or opens, whether the capacitor of the filter circuit is damaged, whether the balance resistors R1, R2 are normal, and whether the step-down resistor R3 is blown or the resistance value increases failure (test under power off conditions).
2. Check if there is a breakdown short circuit between the b-e junction and the c-e junction of the switch and measure whether each winding of the switch transformer has a short circuit to determine the quality of the switch tube and the switch transformer (test under power off conditions).
3. Inspect the rectifying filter components of the secondary output windings, focusing on whether the filter capacitor is bulged or damaged to eliminate the possibility of a secondary circuit short circuit.
4. Check whether the absorption circuit D5, R11, C9 is normal (test under power off condition).
5. Under the condition that the above components are normal, we can remove the switching power supply board from the inverter and perform the power-on test alone. Use a voltage regulator to slowly adjust the rated voltage of the switching power supply. At this time, you should be able to hear the hum of the transformer. If you do not hear a start-up sound, use a multimeter to check the positive and negative levels of the UC3844. Is there a DC voltage of about 12V to 16V?
6. After confirming that the voltage of the power supply terminal of UC3844 is normal, you can use an oscilloscope to check whether the PWM wave output to the trigger terminal of the switch is available from pin 6 of the UC3844 (according to the different circuit design, the frequency of the PWM wave is generally between 20KHZ and 100KHZ). .
7. If there is no PWM wave output, replace the timing element C5, R8, C6 or UC3844. After the elimination of the above several steps, the switching power supply should be able to work normally. In the inverter, there are many types of switching power supply, but the basic principles are the same. For example, each PWM management chip has a power supply terminal, a RC network for timing elements, a port for outputting PWM waves, etc., as long as we understand their work. Principle, according to a certain method steps can eliminate the fault.