[08] Why Do Power Supplies Fail?

Switching power supplies remain a weak link of most electronic systems and fail more often than they should. The reasons for failure are discussed.

Introduction

Modern switching power supplies have been with us for three decades now. From full-bridge circuits operating at power levels up to 10 kW to flyback circuits operating at less than 1 W, a enormous amount of research and development has transpired over the years. Any yet, despite 30 years of design maturity, switching power supplies remain a weak link of most electronic systems. Most of them are noisy, hot, and fail more often than they should.

The Heart of the Problem

Let’s get right to the problem—it is the power FET. Don’t misunderstand this statement. Modern FETs from reputable manufacturers are rugged and reliable when operated properly. The problem is that it is easy for them to be improperly operated.

There is a huge disparity in the power capability of a FET, and the thermal rating of the package. For example, the common IRF640 device has an on-resistance of 0.15 ohms, and a voltage rating of 200 V. Electrically, the power rating is 26.67 kW! Thermally, however, the power rating is about 10 W for a TO220 package with appropriate heatsink. If the components around the FET that make up the power supply do not behave properly, a huge amount of stress can be placed on the device, leading to failure.

It is almost always the FET that eventually fails, but it is not always the direct cause of the problem. The reason for failure can often be very difficult to observe or prove. There are hundreds of parts in some power supplies, and numerous mechanisms that can cause destructive waveforms. Improper operation of the FET can lead to failures in less that 1 µs, and unless you just happen to be probing the exact waveform at the right time, you cannot see the failure occurring.

As I started planning this article, I envisioned giving detailed waveforms showing the failure mechanisms. However, there are so many potential causes, all there is time for is a list of types of failure that can be encountered. Each one of these could merit a full length conference paper in itself. 

MOSFETs

As mentioned above, the FET is usually the ultimate victim of erroneous operation in a power supply. In my experience, it is not often the major cause of a failure, but just the ultimate symptom that lets you know something is not working properly. Since so much destructive energy can be unleashed in the FET, it is the most dramatic sign of failure.

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