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Old 10th August 2009, 05:17 PM   #1
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Default Dual Rail mosfet Matching??

I am working on a class H amp that uses 2 power supply rails at 80 and 160V and Bipolars for the first amp and Mosfets for the rail switch.

I need to match a batch of mosfets for the amp and I'm wondering what voltage and current to match at??

In a class H amp, as the upper tier transistor bank is connected between the power supply rails. In this case, between 160V and 80 volts. at idle they would only see the difference between the rails correct? 80v in this case. So then should i match at 80V?

And how do I determine the quiescent current through the second tier devices? at a guess I would imagine it would have to be the idle current of the entire first tier divided by the number of second tier devices??

I have several of these amps to repair and so far it appears as if the mosfets for the 2nd tier are what fail on this amp. I have yet to find one of the bipolars dead thus far. previous repair techs did not match the 2nd tier mosfet devices and presumably this is why they continue to fail. in fact they didn't even replace the entire bank of mosfets, just replaced the fail devices with mix n match bands. So i want to make sure i get a matched bank in there this time.
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