Introduction
A power supply is present in every electrical system. Ideally, we desire for it to operate flawlessly. However, when it doesn’t that’s when it’s critical to debug the power supply. THAT is the purpose of this article.
Modern converter IC’s have lots of additional features such as soft start, current-limiting, pre-bias startup and boot capacitors. Additionally, power sequencing requirements must be followed.
The Checklist
- Firstly, perform a physical inspection of the PSU. Are the correct components loaded? Are the solder joints satisfactory?
- Verify the connections. Is the ammeter in series? Is the fuse not blown? Confirm the decoupling caps are receiving power and are not shorted. Then confirm that the device under test (DUT) is receiving power
- Ensure proper sequencing procedures are followed. Some controllers require a 3.3 V or 5 V bias rail before applying VIN or toggling EN. Consult the datasheet.
- Probe SW, VIN, VOUT and EN pin to capture the first milliseconds. The converter might be attempting to switch before failing. Probe SW node, VIN, VOUT, and EN. Use a normal trigger on SW around ~1 ms/div. Your goal is to see if the controller even attempts to switch because the controller might be disabled due to overvoltage (OVP), undervoltage (UVP) protection. Or something else is occurring that is overriding startup.
- If there is no attempt at switching, confirm the FET’s aren’t blown by measuring the gate-to-source and drain-to-source impedances using a multimeter. The body diode drop should be 0.3-1V.
- Confirm if the proper soft-start is implemented. A poor soft-start causes the regulators to fail regulation as UVP or current protection or another protection feature is triggered.
- Substitute another device or board to eliminate the possibility of defects.
A summary of troubleshooting steps is tabulated below:
