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Can anyone please explain how the 3vpp measured gate leak between the source and drain is capable of keeping the circuit running NOW at 30vdc, 55KHz with 50% duty cycle without using any current and maintaining 0.56vdc on the pickup coil with 49,850 Ohm load. Thank you for your help and time
Test 5 video: http://www.youtube.com/watch?v=RhrYzBld74w
Test 6 video: http://www.youtube.com/watch?v=UflGpzijWIA
Luc
Hi Luc,
I copied my answer here too from the other forums, so that others here also could read it.
The drain source channel of the MOSFET is practically a short circuit for the ON time (actually the RDS value of the IRF640, data sheet), the ON time now means half wave duration (earlier you used 18-20% or so duty cycle, this meant less ON time, more OFF time).
The drain source channel of the MOSFET is very high resistance for the OFF time, and both the output capacitance of the MOSFET and the body diode is in parallel with the drain source, the OFF time means the other half wave duration of the full pulse time. Now the output capacitance is about a few ten to a few hundred pF only, because the drain source voltage is much higher than earlier, up to 30V.
I still think the resonant tank LC circuit is formed by the output capacitance of the FET and from the 221mH (magnet-tuned) toroidal coil.
The FET as a switch pumps energy into the tank circuit from your generator and you have to consider not only the 3V peak to peak voltage but the current spikes shown in your earlier videos as flat lines between the spikes.
I think the input energy comes from these two: the 3Vpp and the spikes. The flat line between the spikes is explainable from the fact that the input is a square-wave: suddenly appears across the coil then its amplitude remains more or less constant, this means no or a very little flux change, then the square wave returns to zero, this also causes a flux change in the core again, current spike appears again.
So to estimate the real input power to the tank somehow those current spikes should be studied, I believe these maintain the voltage in the caps.
Resonant LC circuits have voltage 'amplification' properties, this depends on the loaded Q factor too. In you circuit this is modified a little, the normal Q times multiplier is not fully valid, due to the half wave rectification inside the tank.
If I can, I will address some unanswered questions tomorrow.
Thanks, Gyula