What is wrong when the Unit Under Test is off

Thread Starter


Joined Oct 19, 2009

It's very rare when I see this problem and I still don't know why.
During my power supply unit testing, there is no failure and no problem.
But when the Unit finished testing and powered down, my inverter board inside of unit blew up and smoked. Does anyone know why?



Joined Dec 26, 2010
You have given very little information to help anyone comment, but I could make a few general observations.

  1. Is the output load removed before the input power is disconnected? Can the converter handle this, for instance in terms of duty cycle and output voltage rise?
  2. How are the switching devices controlled during and immediately after shut-down? Is there any chance of mishandling e.g. shoot-through?
  3. Is energy stored in any capacitor or inductor that could be released in a destructive manner during or after shut-down?


Joined Apr 24, 2011
Some power supplies have a failure mode that happens when they are turned off with no load: the output capacitor holds it's charge longer then the input capacitor and the regulation stage gets back biased and fails catastrophically.

So on these rare occasions, is there no load connected? Does it ever happen when turned off with a large load still connected?


Joined Dec 26, 2010
Similar horrors can arise in a SMPS if the switching devices cannot be held "parked" in a suitable state for long enough, particularly if the input power is lost suddenly. For instance, output switching devices can be a real menace:

  1. If they come on accidentally, or worse half-on in the dying moments while the output filter is still charged. A lot of the energy gets dumped say in some poor MOSFET, which promptly blows up.
  2. If a low-side device is supposed to stay on to dump the charge, but the bias keeping it that way does not last long enough. It slowly turns off, starts to dissipate, and here again the sparks fly


Joined Dec 26, 2010
That is good policy, provided that the user requirement does not dictate otherwise. Some applications may require hot swapping, load dumps, output or input open / shorted...