As power supplies are the very heart of every electronic equipment, special attention must be paid to their reliability. For other electronics a certain failure mode, at least for a part of the total system, can often be tolerated without serious (critical) aftereffects. However, for the power supply no such condition can be accepted, since very high demands on the reliability must be achieved. The MTBF value (Mean Time Between Failures) is a quantitative measure of a product’s reliability performance.