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Registered Member #2140
Joined: Tue May 26 2009, 09:16PM
Location:
Posts: 53
Ok, so i started making a small bipolar SGTC. Before doing anything else, i put together some capacitors in seris, to test the sparkgap. The final capacitor string was 12 1.6kV .33uf capacitors in seris. I used a flyback as the highvoltage source.
The flyback driver was simple, a Function Generator fed in a 80% duty cycle square wave into a MOSFET, which switched a rewound mot, rectified for ~26v, through the primary.
The flyback worked fine, albeit a bit of heating, the Mosfet reaching 100 degrees fairly fast (have a thermocouple on the heat sink). I hooked up the flyback to the capacitors and had a sparkgap at around 10mm. It did a few pulses, but the MOSFET quickly died, and started smoking. The temperature was 70 degrees.
What could have caused it to burn? Could it have been the EM fields?
Registered Member #1533
Joined: Wed Jun 11 2008, 02:13PM
Location: ReykjavÃk, Iceland
Posts: 46
This is either a problem of heat dissipation (or rather lack thereof) or breakthrough due to a transient. The latter is less likely. Look in your datasheet for P_D (that's P subscript D). The power dissipated in your MOSFET should not go above that number (that's roughly speaking P = I_{D(rms)}^2 * R_{DS(on)} assuming insignificant switching losses).
Registered Member #1643
Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039
Just wondering, What fets do you have? I have IRFP460s, with my flyback driver (555), on a SGTC, and they don't get hot at all. I snubber it for protection, but even without that they just get warm is all. Maybe your using more amps though, I'm just using a multi-output 200W stereo transformer.
Registered Member #2140
Joined: Tue May 26 2009, 09:16PM
Location:
Posts: 53
I get ZVS - like arcs, so i am pumping alot of power. there is massive heating, without the heatsink and fan, the MOSFET gets hot to the touch in about 2 seconds.
I dont think it is a heat problem, as it was running for a very short time, less than a second, and read only 70 degrees Fahrenheit. It is mounted on a large CPU Heatsink with a fan connected to a 12v reg and MOT output. The fan could have been running very low due to the current draw from flyback.
Registered Member #2140
Joined: Tue May 26 2009, 09:16PM
Location:
Posts: 53
I only have 1 left, so im hoping i dont burn it out ;(.
I was looking again, and yes, the PD is 600 watts. With 22 volts, that is just above 27 amps. Im sure my PSU could supply that, but i doubt the wiring would outlast a power mosfet.
Is EM fields ruled out?
EDIT: I dont know if this helps any, but i took out the MOSFET, and on the side, from a hairline crack, some black thick jelly-like stuff was smeared. It smells like silicon.
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Blown MOSFET
