Sinrace Power Supply Technology (Shenzhen) Co., Ltd.

The following sections provide a set of guidelines for attacking SMPS problems. Those in the next 5 paragraphs are common to SMPS using both discrete and integrated controllers:

First, determine that it is not something trivial like a blown fuse due to a legitimate overload (that has since been removed). I have a SCSI peripheral box that will blow its SMPS fuse if the SCSI cable is inserted live.

Categorize the problem into: startup problem, catastrophic failure, incorrect outputs, or excessive ripple or noise.

Determine what the proper output voltages should be. Identify the main (regulated) output.

Disconnect the supply from the equipment it is powering if possible. This will prevent the possibility of expensive damage should the output voltages soar to stratospheric levels for some reason. If this is not possible, you will need to be extra careful - always use a Variac to bring up the input slowly and monitor the main output at all times.
Note: Some SMPS designs require power to be applied instantly to provide the startup voltage to the controller. If this is the case with yours, it won't be possible to bring up the voltage slowly (unless you power that chip separately). However, it should still be possible to run the unit somewhat reduced line voltage.

CAUTION: Running any SMPS at greatly reduced line voltage will be stressful for it, especially if the output load is a significant fraction of its full load ratings. In addition, at some range of line voltage, the output regulation may not work properly and the output(s) may go much higher than expected. Use dummy loads in place of the valuable equipment if possible when doing such testing!

Determine an appropriate load for the outputs (if not connected to the equipment). A typical SMPS will want a minimum of 5% to 20% of full load current at least on the main output to regulate properly. Others may not need any load - it depends on the design or they may have an internal load. Here are some typical load currents:

VCR - 0.2 A on +5 V and +12 V outputs.
PC - 2 A on +5 V, 1 A on +12 V. A dual beam auto head light works well.
Monitor - 0.2 A on +60 V to +120 V output.
Typical 40 W switcher = 1 A on +5 V and +12 V.