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Old 07-01-2011, 08:24 PM
valerun valerun is offline
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Default Re: Another DIY DC controller mock

added a 5 Ohm turn-on resistor parallel with the 20Ohm I already have. Turn on now takes 700-800ns as it should. no additional ringing so I am happy.

took it to 800A today. a bit scary.

turn-off spike is ~80V at 800A. It was fun to put one end of the scope on one end of the DC+ bar and slide the second end along the bar - the further apart I took it, the more voltage spike I saw ;-) ~30V along the 5" of a bar. Power electronics is fun ;-)

Measured voltage spikes across DC link and across freewheel diode separately to understand what drives voltage stress. DC link spike only ~1/4-1/3 of the overall C-E spike on the switch so not really a problem. So if I want to reduce the spike, I have to deal with the diode part. How's this for an idea: using a smaller u-fast diode (say, 60A 1200V part) in parallel with the big one? It will turn back on much faster and will conduct for the first 200ns while the large diode ramps up. Yes, the initial current will be close to the IGBT current (say, 500A if I push 1000A through the controller) but this will go on for just 200ns and the remaining 200 micro-seconds it will be chilling. So I would definitely not exceed the power dissipation ratings. Is this a crazy idea?

EDIT: also, any ideas how to confirm whether my IGBTs share the current evenly or not?

V

Last edited by valerun; 07-01-2011 at 10:23 PM.
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