I'm researching a good (and simple) driver for a HV transformer, and found this very simple and elegant circuit on the web:
It uses a feedback winding to turn transistors on in an alternating pattern. I assembled it and it works very well, but later I tried to replace the bipolar transistors with mosfets, to experiment.
The result was awful, the nice sinusoidal output I get with bipolar transistors turns into a very erratic and deformed waveform when using mosfets. Current draw increased a lot, and the secondary voltage was much smaller, the efficiency dropped a lot (by 10x of times).
My question is: is it possible to use mosfets in this configuration? Is the distortion the result of badly chosen mosfets? May I need to change R1, R2 or C1 to different values to make it work with mosfets?
Or are mosfets just incompatible with this self-oscillating configuration?
UPDATE: I forgot to add that the erratic oscillation with mosfets happened only after I changed R1 and R2 placement: I connected them between the gates and GND, instead of gates to Vcc, as in the schematics. Merely replacing the bipolar transitors with mosfets in the circuit above resulted in no oscillation at all, since the gates got tied directly to Vcc, making both mosfets conducting all the time.
So, the description of erratic mosfet oscillation in the original question applies to the circuit with this modification: resistors between gates and GND.
UPDATE 2: Thanks to Andy's reply, I understood what may be obvious for the more experienced: the resistor just bias the base to conduction state, so the feedback winding can make the transistor cut on negative pulses. I have applied the same logic to the mosfet version, biasing the gate to Vgs(th) (Gate Voltage Threshold), and it works.
However, this circuit is not as efficient as one using an active oscillator and a driver, which will ensure rise and fall times are short, and the transistor (BJT or FET) will not be speding time in it's linear region.