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

Startup is the most stressful time for a typical switchmode power supply. The output filter capacitors as well as the load must be driven while the input voltage is changing - possibly wildly. With careful design, these factors can be taken into consideration. Not all power supplies are designed carefully or thoroughly tested under all conditions. When power is restored, surges, dips, brownouts, and multiple on-off cycles are possible. This is why it is always recommended that electronic equipment be unplugged until power has been restored and is stable.

PC power supplies are often ideal for other purposes but the required loads represent wasted power. So, it would be nice to be able to eliminate them. Unfortunately, it probably isn't easy to modify a PC power supply so less/no load is needed for regulation. However, it is worth testing a supply to see how low you can actually go on the loads - many WILL regulate the +5 with no load on the +12 but probably not the reverse. While 20 percent load is often recommended, 5 percent or less may work just fine. And, some don't need any additional loads on either output (they will probably include a minimal load resistor internally).

The problem is that it is probably a flyback converter and these are pretty finicky about changes. In addition to the caps, and switching transistor, the transformer would probably saturate at the higher voltage unless the switching frequency were also doubled. Getting these things to work normally without blowing up is touchy enough. To change one without a thorough understanding of all the design parameters would be really risky.

Any time the switchmode transistor requires replacement, check all semiconductors for shorts and fusable resistors for opens. even if you locate what is thought to be the problem early on. Multiple parts often fail and just replacing the transistor may cause it to fail as a result of something else still being bad. In particular, check primary side electrolytic capacitors for reduced capacity or opens. These conditions can result in a blown switchmode transistor as it attempt to supply adequate current during the troughs of the rectified high voltage DC. It only takes a few more minutes. For other problems like an open startup resistor this excessive caution is unnecessary as these are usually isolated failures.

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:

Where an SMPS doesn't start and it isn't obvious why, it might help to drive the chopper from an external signal source to see what then works. The only time this is really practical is where a single transistor or MOSFET is used - generating a push-pull waveform probably isn't worth it.