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Class D Switching Power Amplifiers and Power D/A conversion

Mosfet Limitations in class D amp
Mosfet Limitations in class D amp
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Old 21st July 2019, 09:18 AM   #1
badboy6120 is offline badboy6120  Malaysia
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Default 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??
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Old 21st July 2019, 02:19 PM   #2
MorbidFractal is offline MorbidFractal
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VLoadRMS = Root(Power*R)
VLoadRMS = 63.24V

VLoadPeak = Root(2)*Vrms
VLoadPeak = 89.44V

VLoadPeakToPeak = 2*VloadPeak
VLoadPeakToPeak = 178.88V

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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Old 22nd July 2019, 03:34 AM   #3
badboy6120 is offline badboy6120  Malaysia
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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
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Old 24th July 2019, 08:28 AM   #4
manojtm is offline manojtm  India
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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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