Low-Line Overstress of Constant-Power Loads(4 of 4)

I learned this the hard way, as have many others, by burning out power transistors by slowing bringing up the input voltage on a switching-mode power supply breadboard. It took me a while to realize what was happening and to get into the habit of applying voltage as a step function until all protection circuitry was in place. Learning this used to be a common initiation rite for those designing switching-mode power supplies for the first time. Now PWM controller IC's and cycle-by-cycle protection of the power transistors in some forms of current-mode control have masked the problem so some designers are barely aware of the problem. However, if you are interested in field reliability, you should carefully check the turn-on / turn-off transient stress and add additional protection circuitry if needed.

I have not found any information yet about this problem on the web.

A 1978 PESC paper by Aron Levy provides additional information. Other topics in this hypertext provide additional insight into the problem and its solution, including Latchup of Constant Power Load With Current Limited Source and Solar-Array Power System Battery Lockup.

Editor: Jerrold Foutz