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Registered Member #33
Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
The gate capacitance of a typical IRFP460 is around 3nF (not counting the miller capacitance), so a half bridge of IRPF460s should represent about 6nF of capacitive load to the gate driver. You used a 50nF dummy load and this is way too much. From the scope shot, it looks like you're running around 300kHz, and I'm not surprised the gate driver is having trouble driving 50nF at this frequency. Replace the dummy cap by something around 6nF, or even better, the actual MOSFET gates, and the waveforms should get a lot nicer.
Registered Member #2115
Joined: Fri May 08 2009, 01:17PM
Location: Singapore
Posts: 46
Hi Anders
The IRFP460 datasheet states a 2980pF input capacitance, and a 12.8nF 'switching on' gate capacitance. So, which value is taken? Anyways, i'm going to do some tests tomorrow with a 6.8 nf cap and a 27nF cap. I will also try it with the mosfet gates.
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
Effective gate capacitance is best approximated using Total Gate Charge value in the datasheet.
For the IRFP460, this is 105nC.
Using q=CV equation, solve for C. V = gate voltage swing which i believe in your case is 12V.
So 105nC/12V = C = 8.75nF.
This capacitance is actually quite difficult to determine mathmatically, especially since its non-linear, but as a first order of approximation, this works out just fine.
BTW - VERY STUPID question -
Why are you using capacitors as loads?
If you have IRFP460's, then just use them. Typically, you'd only calcualte gate capacitance if you had to simulate your gate driver. If you're testing and measuring - just use the real device.
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Strange GDT waveforms
