Recommended architecture for mains-powered design:
For a DC-input (24–36V) non-isolated buck converter: a20112 power supply schematic
Here is a troubleshooting table directly derived from the A20112 schematic logic. Recommended architecture for mains-powered design:
| Symptom | Likely Fault (per schematic) | Diagnostic Step |
|---------|-------------------------------|-----------------|
| No output, fuse blown | Shorted MOSFET (Q1) or bridge rectifier | Check diode mode across drain-source of MOSFET. Replace both MOSFET and current sense resistor. |
| Output cycles on/off (ticking sound) | Overcurrent protection active | Check secondary Schottky diode (short) or output capacitor (high ESR). Measure CS pin voltage – should be <1V. |
| Output present but drops under load | Failed bulk capacitor (C5, 400V) | The capacitance drops below 50µF. Replace with 105°C rated Lelon or Nichicon. |
| Very low output (e.g., 3V) | TL431 or optocoupler failure | Temporarily disconnect optocoupler; if output jumps high, FB loop is the issue. |
| No standby power, but fuse OK | Startup resistor (R2, 1MΩ) open or VCC capacitor (C13, 47µF 50V) dried out | Check R2 resistance; replace C13. | For a DC-input (24–36V) non-isolated buck converter: Here
Yes. The flyback transformer provides 3kV isolation between primary and secondary. The schematic clearly shows a physical gap between optocoupler pins and transformer windings.