Class E Amplifier Harmonics....

Discussion in 'General Electronics Chat' started by bob800, Jan 5, 2014.

  1. bob800

    Thread Starter Member

    Dec 10, 2011
    So I finally achieved "breakout" (continuous spark discharge into air) with my Class E tesla coil setup, based on this schematic:

    However the amplifier is still not working quite right and the IRFP450 MOSFET gets hot pretty quickly... Usually this is normal before the circuit is tuned such that the drain-source voltage is at 0V before the MOSFET switches on.

    However I'm having a hard time tuning the circuit as "harmonics" (correct me if that is not the correct term) keep appearing in both the gate-source and drain-source waveforms. This is evident in the attached waveform (drain-source 50V/div, gate-source 10V/div). This is actually the best waveform I have achieved, as in the past everything was plagued by extreme noise/harmonics (see second waveform).

    This doesn't seem to be a "normal problem" among various Class E tesla coil projects I've studied on the internet. In fact from the waveform it seems like the circuit is mostly tuned----the gate turns on when the drain is "off". But due to the harmonics/ringing, it looks like in certain places, the drain and gate do begin to overlap...

    What usually causes this sort of behavior? I've attached pictures of my setup in case physical layout might be a factor...

    Other information:
    --Supply voltage was rectified 50V from isolated variac, with ~1000uF of filter capacitors
    --I am using a 1kV 470pF drain-source cap instead of the prescribed 330pF
    --For the "RF choke" I am using this inductor. I tried using several other types, including chokes from power supplies and winding on a real "RF Choke" material, but the linked inductor seemed to work best.
    --Breakout begins almost exactly at 50V
  2. bob800

    Thread Starter Member

    Dec 10, 2011
    In case anyone else has this problem (doubtful...), I managed to solve it by:

    1. Adding more secondary turns
    2. Increasing the coupling by using a narrower form for the primary coil