150v Mosfet @ 146v?

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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.
 
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.
 
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.
 
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.
 
darkfenriz said:
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.
 
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.
 
Whoa, I was wrong. :eek:

A 2Hz sine was able to drive the rail voltage to almost 90V without the over-current protection tripping.

It fried the 150V MOSFETs I had put in. They were working fine with the normal 73V tail voltage, even with a continuous 20Hz sine at almost full volume.

So this is a real problem, even with unregulated supplies. I don't think it is that big of a concern normally as no one in their right mind would let their woofers oscillate at ~2Hz, let alone at any significant volume.
 
Modify the amplifier, use the low frequency cutoff points from the various coupling capacitors to prevent excessive rail pumping.

Try to find a 200V replacement MOSFET with a fast body diode and low charges, even if it comes in another package, for example IRFB4227.

Paralleling more devices is likely to cause trouble due to the gate charge increase.
 
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