strange effect in mosfet (h-bridge)

alfsch · 2nd October 2006, 01:59 PM

i built a simple half bridge with 2x irf730, v+ 60 volt.
for testing, no load, switching at 80khz (with about 1us deadtime).
so mosfets only switching...but they heat up! (just a little, about 10° or so).
(no rf seen on scope)
when coil (load) connected, mosfet stay cold !!

can anybody explain, why "free" switching mosfet, with lots of deadtime, can have power losses ??

Pafi · 2nd October 2006, 06:40 PM

2 reasons:

- There is output capacitance.

- High dv/dt of hard switching can induce turn on impulse on opposit device.

"hard switching": high Vds when MOSFET starts to turn on.

"Soft switching", or "resonant switching": current of inductance makes voltage of drain-source alternate before MOSFET turns on.

alfsch · 2nd October 2006, 10:49 PM

a: outp. cap. is also here with load, so this should make no heat up

b: High dv/dt of hard switching can induce turn on impulse , great idea, indeed there is a 5v spike at the "wrong" gate

thanks to your help, i can sleep good now :-)

IVX · 3rd October 2006, 10:23 AM

alfsch, some pics:

Pafi · 3rd October 2006, 06:23 PM

Quote:

outp. cap. is also here with load, so this should make no heat up

Cap is there, but with load it is charged and discharged by current of choke!

You're welcome!

alfsch · 3rd October 2006, 07:35 PM

ok, the caps make a (small) part of additional loss, your right.

#ivx: the spike is in the gate signal , similar to your pics :-(

well, ill try at 300v with coil ... seems to work, without heating up.

switching-spike at gate is still there, but only 2v peak, so no unintended turn-on should happen

2nd October 2006, 01:59 PM	#1
alfsch diyAudio Member Join Date: Jan 2002 Location: augsburg	strange effect in mosfet (h-bridge) i built a simple half bridge with 2x irf730, v+ 60 volt. for testing, no load, switching at 80khz (with about 1us deadtime). so mosfets only switching...but they heat up! (just a little, about 10° or so). (no rf seen on scope) when coil (load) connected, mosfet stay cold !! can anybody explain, why "free" switching mosfet, with lots of deadtime, can have power losses ??

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2nd October 2006, 06:40 PM	#2
Pafi diyAudio Member Join Date: May 2004 Location: Budapest	2 reasons: - There is output capacitance. - High dv/dt of hard switching can induce turn on impulse on opposit device. "hard switching": high Vds when MOSFET starts to turn on. "Soft switching", or "resonant switching": current of inductance makes voltage of drain-source alternate before MOSFET turns on.

2nd October 2006, 10:49 PM	#3
alfsch diyAudio Member Join Date: Jan 2002 Location: augsburg	a: outp. cap. is also here with load, so this should make no heat up b: High dv/dt of hard switching can induce turn on impulse , great idea, indeed there is a 5v spike at the "wrong" gate thanks to your help, i can sleep good now :-)

3rd October 2006, 07:35 PM	#6
alfsch diyAudio Member Join Date: Jan 2002 Location: augsburg	ok, the caps make a (small) part of additional loss, your right. #ivx: the spike is in the gate signal , similar to your pics :-( well, ill try at 300v with coil ... seems to work, without heating up. switching-spike at gate is still there, but only 2v peak, so no unintended turn-on should happen

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