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Registered Member #54416
Joined: Wed Feb 04 2015, 01:30AM
Location:
Posts: 13
I have the diodes directly on the mosfets. I agree the leakage inductance back emf should cause problems. I no problems when I was running only one set of mosfets. I have a theory, with 3 mosfets paralled would have 800 to 900 pico farads and if the mosfets were connect fairly close say like 2nh ,would give a resonant frequency of 125 Mhz. that would be an impedance of 1.57 ohms. But if their 6-7 " away that would be 100nh and resonate at 18 Mhz with a resonate frequency of 11 ohms which is higher than my tank impedance. This is just a guess. All I know If I hook the mosfets directly together, they blow instantly. and if I use like superfet or any fet output capacitance higher than 1000 pf it won't work. I gone through A LOT of different mosfets, these are the only ones that have ever worked. But in theory I agree with everything you said.
Registered Member #33460
Joined: Tue Aug 27 2013, 06:23PM
Location: Seattle
Posts: 46
I agree with you on those values for impedance of the leakage inductance at the resonant frequency of the output capacitance and leakage inductance. However, the impedance of the tank at those frequencies is much smaller than that of the leakage inductance, if I did my calculations correctly.
Since I've been simulating with 150nH leakage inductance, I stuck with that, but used 2nH for the "close connection" case.
For a tank of 1µH and 50nF, at 14MHz I got 0.227 ohms, and at 122MHz I got 0.0261 ohms. The impedance of the capacitor dominates.
Registered Member #11363
Joined: Mon Mar 18 2013, 04:29PM
Location: Brazil
Posts: 19
some question about this: i have two server psu's 12v@90A. Can run 24v@90A or 12v@180A... what are better to start a coil with a ZVS driver? i run the ZVS direct on the primary of coil or the zvs drive a flyback thats run with a spark gap on the primary?
Registered Member #54416
Joined: Wed Feb 04 2015, 01:30AM
Location:
Posts: 13
forget the flyback, it will saturate . I would parallel the psu's ,so not to have separate drive voltage. you will need a tank impedance of 66 milliohms. with 250nh+250nh primary coil and a 57 uf tank capacitor @42 kilohertz. if your seconardy resonance is close enough , you should rock out!
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ZVS flyback driver SSTC
