Mosfet Limitations in class D amp
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 21st July 2019, 09:18 AM #1 badboy6120   diyAudio Member   Join Date: Feb 2011 Location: Malaysia Mosfet Limitations in class D amp Hello all I'm working on my 1000W rms half bridge class D amp and want to choose the best mosfet suited to this much power I used IRFP4227 with more than 1000W rms for half an hour with everything ok the rail voltages are +-110 V as I calculated from the below equation I need a mosfet with at least 214V reverse breakdown voltage(250V mosfet) 2*root((Output power *Load ohms * 2)/0.85)+10% 2*root((1000*4*2)/0.85)= 194V 194V + 10% = 214 V Am I using the right calculations?? __________________ **** ONE KIND WORD can change someones's entire day ****
MorbidFractal
diyAudio Member

Join Date: Nov 2017

In an ideal world that is the required Vds rating of your mosfets.

Then you apply your fudge factors.

Major is the maximum duty cycle your modulator can achieve. [Your 0.85?] Next would likely be RDSon and other losses. [Your 10%?]

As to why your mosfet survives. It would help if you read the data sheet...

https://www.infineon.com/dgdl/irfp42...53562927642004

Repetitive Avalanche Capability for Robustness
and Reliability

In particular look for the word Avalanche and also refer to the transient thermal impedance curves.

The main numbers are quoted as being energies so the application will generally involve inductive turn off where that energy will be defined by the inductor being turned off and the current through it at turn off.

So... yes you can exceed the VdsMAX rating of 200V but that depends on your application, whether you can do the sums and how lucky you feel.

Also note that the 240V figure is given as being typical and as such you cannot rely on it. In the case of a half bridge if you avalanche one mosfet whilst the other one is switched on things will blow up very quickly.
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 22nd July 2019, 03:34 AM #3 badboy6120   diyAudio Member   Join Date: Feb 2011 Location: Malaysia Hello and thanks for your reply I'm using IR2110 as mosfet driver and the duty cycle should be around 85% about those numbers in my calculation I found them in the below article (top of page 3) https://www.google.com/url?sa=t&sour...=1563765523452 yesterday I ordered 2 different mosfets with higher BVDS IRFP4768 IXFK64N50P the second one has much higher voltage rating (500V) RDSon 85 mOhms totall gate charge 150nC T on = 30ns T r = 25ns T off = 85ns T f = 22 ns and high power rating __________________ **** ONE KIND WORD can change someones's entire day ****
manojtm
diyAudio Member

Join Date: Aug 2008
Location: KERALA
HI,

Quote:
 yesterday I ordered 2 different mosfets with higher BVDS IRFP4768 IXFK64N50P
Both one huge Ciss (need a totem pole to drive)

Regards
MANOJ

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