Originally posted by machinealive
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watching your drawing I need to ask if you really have the cap connected exatly across the coil?
If yes:
- You have - initially a dead short circuit up to charging the cap )Battery, cap, FET, GND, battery). When pulse is off it will try to discharge throgh coil.
- Once your FET survived initial charge you enter at certain frequency in a tank circuit (cap / motor coils). This setup will pulse severe voltages onto your FETs. Kind of "asymmetric rotoverter"
- (speculation) Your pulse frequency and tank frequency produce bounce effects. Once adjusted well you have a synchronous resonnace setup. Your 300V FET will suffer on breakdown and avalanche effects -> it pulses along your tank circuit. Thus it might build an active relaxation oscillator being controlled by gate voltage - oscillating with genuine resonance frequency of your cap/coil setup. I suppose you will not be avle to repeat that effect if you use FETs rated with higher voltage.
- Scope shot of this condition will be enlighting. I expect pulse packets of higher grade frequency instead solid pulse packets.
JohnS
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