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Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
If everything is tuned correctly, there should be a half-cycle of LC oscillation when the MOSFET turns off. The drain voltage should spike up in a half cycle of a sine wave, and, the important bit: It should oscillate back down to zero before the MOSFET turns back on! Otherwise, that capacitor would indeed discharge through the MOSFET on every cycle and cause carnage.
If you think this reminds you of a Class-E SSTC, you are right, it's exactly the same principle
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Well, the resistor trick didn't work, it started smoking in 20 seconds and performance was poor, it was better with smaller resistors but I was afraid to go too low. I came up with a simple trick, a diode paralelled to resistor is connected in series with the gate, that allows slow turn-on so maximum peak current of the fet isn't exceeded, and fast turn-off to minimize turn-off switching losses. Performance seems the same, the fet runs probably just a little bit hotter, and still doesn't need heatsink for normal operation. It seems that it's turn-off time that's critical for performance and efficiency in this topology.
Edit> it seems that I'm exceeding the gate voltage of the fet a little to about 24V, is this ok?
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Edit> it seems that I'm exceeding the gate voltage of the fet a little to about 24V, is this ok?
No, it is not OK. This is now the top possible reason why is your mosfet dying.
Simple problems always result from simple causes.
If your mosfet dies cold just like that there is something cardinally wrong witht he circuit. Avalanching just can't be cause because mosfet is heavily resistant to it, most snubbers willa ctually burn out much before the mosfet. Other thing I can think off is that you pushed extreme duty cycle and saturated the core. Your frequency may be too low, your core may be too small.
But it's all speculative until you show the schematic and all the specs.
Why in the world did you feed 24V into the SG3525? Are you maybe using a GDT of some sort?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
I'm now sure that the cause was overcurrent of the fet as the cap discharged through it, now with the fix no more problems. Yes, I'm feeding 24V to the sg3525 to avoid any voltage regulators, as I want the circuit simple. But if it matters, it's no problem to include one. I know that the fet can take 35V to the gate, as once accidentally happened to me and the fet survived it no problem. It's just that I'm afraid it could die after some time by prolonged exceeding of the gate voltage (I think I read about this somewhere)
Registered Member #63
Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425
Just because the FET took 35V to the gate "accidentally, once" and "survived" doesn't make it OK. You were just lucky :P
If you look up your FET datasheet, you'll find that the gate is only rated to take probably +/-15V. If it's an ST MOSFET with built-in zener diodes, it can probably take up to +/-30V transients.
Most MOSFETs are pretty well "ON" at 8V, so 12V is all you need in this application. It's worth buying that regulator.
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Mosfet failure mechanism
