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theAnonymous1 13th December 2008 01:42 PM

150v Mosfet @ 146v?
I have an amp with dead 200V 38A STW38NB20 FETs that has +/-73V rails. The only FETs I have on hand are 150V 35A FDP42AN15A0. Would it be stupid to use these? There are spaces on the PCB to add another set of FETs, but that would only help with the current rating and the voltage.

sawreyrw 13th December 2008 03:47 PM

200 volt parts are marginal; 150 volts is a nonstarter.

Cristi 13th December 2008 04:42 PM

the 4V voltage margin is far too small. need to be at least 40 V to be sure it won't blow up. there are few possible causes. first is the variation of mains voltage wich can increase the voltage rail. second will be Bus Pumping phenomenon, which is common at Half-Bridge topology. 150V is maximum guaranteed by the manufacturer, but u have some chance that the device will breakdown at 170-180V. i wouldn't count on that.
also try to replace with the original, there are many other factors than DS voltage, there is switching times, gate charge, rDS on, etc. and the performances will be different.

theAnonymous1 13th December 2008 06:17 PM

It's a better FET as far as the other specs, the voltage is just too low. The amp works fine with this replacement FET, but I haven't turned it up enough for bus pumping to stress it.

Maybe it would be OK if I used it for the highs? Worst thing that could happen is they blow. The amp is fully protected so the load would be immediately disconnected.

I would just use the original part, but I can't find them for sale anywhere. If anyone knows where to I can get a pair of STW38NB20 please let me know.

Cristi 13th December 2008 06:40 PM

better FET dosen't mean better sound. THD are greatly dependent on switching timing. a faster transistor can lead to higher crossover distorsion if was just replaced without decreasing deadtime. anyway the challenge is to keep the balance between distorsion levels and shoot-through currents wich will affect efficiency and eventually blow up transistor.

if u insist to use those transistors, u need to try to decrease supply voltage to a maximum of 120V.

i compared the datasheet of STW38NB20 with STW34NB20 which i use for my TA3020 boards and they are very similar. send me an e-mail and i will try to help you to get them.

theAnonymous1 13th December 2008 08:17 PM

Thanks for the offer, but if I can't find the STW38NB20 I will just do some research for a better 200V >38A part, probably something from IR.

I had a bunch of the STW34NB20 and gave them away sometime last year. I didn't think I would ever need them. :o

MOER 14th December 2008 06:23 AM

200v Mosfets
I have plent of these 34NB20s in stock

I also have many other 200v parts as well.

Email me at or call me at the factory at 805 526 5315 PST.

Many thousands of MOSFETS, and other parts in stock as we are an OEM manufacturer.

Stephen Mantz
Zed Audio Corporation
Los Angeles

darkfenriz 15th December 2008 03:02 PM

How much rail pumping should one typically expect?
Is it totally non-existing in full bridge topology?

theAnonymous1 15th December 2008 03:22 PM


Originally posted by darkfenriz
How much rail pumping should one typically expect?
Is it totally non-existing in full bridge topology?

From my understanding it is more or less non-existant with full- bridge.

I would like to know how rail-pumping effects an unregulated supply like mine. For example; if the amp is putting out enough low frequency power to suffer rail-pumping, then how does this add to the fact the unregulated supply will have a voltage drop with a heavy load?

Maybe I should try this with one of my good amps and see for myself.

theAnonymous1 15th December 2008 03:41 PM

OK, I guess it isn't such an issue with an unregulated supply. At idle the rails are 73.3V but drop to 67V @ 20Hz right before the over-current trips.

NOTE TO SELF: Never leave meter positive lead in the "current" position when measuring rail voltage. That woke me up. :eek:

EDIT: @ 50Hz the rails drop all the way down to 60V. Tests are with only one channel driven.

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