I cut the high frequency cap out of circuit now so there is only the fet capacitance. The high voltage peak base looks to be about 800ns is this my high frequency tuned period 1/800ns = 1.25MHz half cycle so my resonant frequency is = 625KHz, if thats correct for my 121uH then that equates to a capacitance of 535pf, which means my fet is having a massive effect. The above was based on the fact that the inductance of the coil under test was 121uH but having thought about that the whole idea is that the inductance will change, so i cannot see how we can do any calculations to work out the fet capacitance because at high frequency we have an unknown fet capacitance and an unknown inductor inductance. EDIT Its quiet interesting the capacitance changes when the pulse goes from rising to falling, when rising it takes 260ns and when falling it takes 340ns which corresponds to a frequency of 961KHz and 735KHz respectively. So if the inductance was the same 121uH at 961KHz the we have a capacitance off 226pf when rising and 387pf when falling. I suppose really what i need to do is now try caps to see what cap value i need to use to get resonance of 961KHz and then calculate the inductance from that and then we would also know for sure the fets capacitance which should give us a more accurate estimation of the energy in the fets capacitance. So in the above experiment i have an initial voltage of 18v and a cap of 19.56nF = 3.17uJ and the output voltage of 178V and a estimated cap value of 226pf = 3.58uJ or a gain of 113% so if the theory that the inductance increases at resonance then we can say that the capacitance must be smaller than our above calculations, lets say that the inductance increases by 5% then we can assume we have a inductance of 127.05uH and therefore out fet's capacitance would calculate out to be 215.88pf and a total stored energy at 178v of 3.4199uJ so we are still > unity at 5% increase in inductance which is interesting. How about a 30% increase in inductance, 157.3uH giving a capacitance of 174.37pf and energy at 2.762uJ, so now we are under unity. So it looks like it really comes down to how much our inductance increases at resonance.
« Last Edit: 2016-10-17, 19:05:43 by Peterae »
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Topic: Magnetic Compression Study" by Cyril Smith (Read 32114 times)
