Before repairing damaged equipment, especially for Switching Power Supplies, there are several steps that can help in the repair process, namely:
- Observe the symptoms of damage that occurs
- Analyze the damage or estimate which part/block is damaged due to the symptoms.
- Perform a test on the part you suspect or perform a systematic test if you are not sure which part is damaged.
In systematic damage tracking on Switching Power Supply, testing should start from the input grid to the primary part of the switch circuit because generally damage occurs in that part. If in the primary part all components have been tested well, as well as the voltage on each DC filter capacitor is normal ± 150 V, then the next step is to test the secondary part, namely the PWM driver and the PWM IC circuit, either testing the supply voltage or testing the components passively.
Testing on the output rectifier and error amplifier is the last because in this part rarely occurs damage unless the power supply is old can occur damage to the rectifier capacitors bad / broken solder to the components or connectors or damaged rectifier diodes. Symptoms of damage and their causes are given as follows:
1. Power Supply is Completely Dead
Possible causes:
a. On the RFI filter block:
There is a short circuit capacitor so the fuse is broken
b. In the rectifier block:
- Broken or short circuit diode
- Short circuit filter capacitor
- NTC (surge current limiter) is broken,
c. On the Switching Block:
- Switch transistor is damaged (short circuit or broken)
- Transistor base trigger resistor is open
- Diode open or short circuit.
d. In the Isolation block:
For isolating transformers this is rare
e. On the output rectifier block:
Open diode and short circuit capacitor
f. In the PWM block:
The IC is damaged or the supporting components are damaged.
2. Power Supply Voltage Drops
Cause: Only a portion of the switching pulse is processed. This is due to one of the switching transistors, either the main transistor or the driver, not working or possibly a broken pulse path.
3. Other symptoms of damage that could be caused are:
Shorted cables, dirty circuits with dust, dirty connectors, and bad switches.
Figure 6.29: One Form of Power Supply on a Computer