piclist 2002\09\26\134739a >
Thread: 2-tran SMPS , NOW: Transient suppressors
www.piclist.com/techref/index.htm?key=transient+suppressors
picon face BY : Peter L. Peres email (remove spam text)



On Wed, 25 Sep 2002, Morgan Olsson wrote:

*>I still think that for outputs SCR is best when affordable. One problem
*>is that when the SCR fires it consumes all the capacitor stored energy
*>and that is a very high peak load i the SCR chip.  I seldom see series
*>resistors, but series resistors of only few mohm (PCB track) will easy
*>the life of the SCR.

You can use an inexpensive large VFET (like BUKxxxx series 50A 50V) as
crowbar, with a pair of resistors setting the threshold. It is *much*
faster than any thyristor scheme and protects against reverse pulses too
(slower).

The point with miliohms is that amperes * miliohms = volts and at 100+A
short currents from modern caps the protective effect could be nulled. So
the crowbar should be at least between the load and the caps so the
voltage develops in front of it, not across the load.

Peter

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