Electronic – How to self start a MOSFET multivibrator circuit

I think you may be exceeding what is known as the volt-microsecond limit for the transformer. Basically this figure tells you how long you can apply a pulse of a certain voltage before the voltage reverses. Real transformers will saturate and the V.\$\mu s\$ limit tells you, in practical terms how far you can push the transformer.

The waveform initially looks good on the rising edge - it attains a peak of 4 or 5 volts (it looks that way from your pictures) but after about 10 us it goes wrong - this means your transformer is probably rated as having a volt-microsecond product of about 50.

Other problems with the circuit are: -

• You need more gate drive voltage to adequately and efficiently turn the mosfet on
• As you vary mark-space ratio, the peak voltage seen by the gate (from the transformer) will vary considerably - you need to have a clamp circuit on the secondary that keeps the gate drive at a decent level for varying mark-space.
• I don't think that transformer is man-enough to provide the current needed by the gate to efficiently switch the mosfet without incurring heavy losses.
• The transformer drive circuitry also isn't powerful enough to drive a transformer should you have a transformer (and clamp) that would fit the bill efficiency-wise.

Here is a helpful (in parts) article on the subject. And here is a helpful guide on designing transformer circuits for driving mosfets: -

You can probably see in one of the diagrams (fig 3) the problem you are getting.

The drain appears to be ringing at about 600KHz.. Probably due to inductance in the drain circuit. The simple circuit you are using to drive the MOSFET is very asymmetrical.. the turn-off will be much faster than turn-on because your pull-up is much higher value than the ~50 ohm resistance of the MOSFET in the Si8710AC, so you are seeing the ringing.

I suggest a complimentary BJT emitter follower after the Si8710AC with a series resistor to the gate selected to trade off ringing vs efficiency. You can also minimize inductance in the drain circuit by shortening or twisting the wires arc., add a snubber or even a flyback diode to the supply.