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I wanted to show some waveforms of my circuit driving a 1 ohm semi-inductive load (12 volt car battery) @ 12 amps. I'm using a 1 ohm, 3000W resistive coil. I apologize for the blurry pictures - I'll try to edit the post with better pics later. O-scope set to 20v/div.
The first picture shows drain voltage without a flyback diode. Note the initial voltage spike (when the mosfet opens) reaching about 180 volts and significant ringing afterwards due to inductive feedback.
Second picture shows drain voltage WITH flyback diode in circuit. Note a reduction of peak voltage to 80 volts and reduction of ringing. When I attach the diode to the positive terminal, it sparks considerably, so it's drawing lots of current.
Third picture shows drain voltage with a flyback diode and 20 ohm resistor in series. Note peak voltage hitting 110V but less ringing after resistor added.
Fourth picture shows drain voltage using NO flyback diode but instead using 2200uF cap and 50ohm series resistor from drain to ground (B-). Note peak voltage hitting about 110V (similar to flyback diode/resistor combination) but with less ringing.
Last picture shows combination of flyback diode/resistor attached to positive battery terminal AND ALSO capacitor/resistor hooked to ground terminal. Note peak feedback voltage is now back to about 80V (similar to flyback diode alone) but ringing is almost completely gone.
What I have learned is that a flyback diode alone can be very ineffective at dissipating inductive feedback voltage. A flyback diode is best used in series with a resistor to reduce ringing, otherwise the power in the inductive field has no way of dissipating effectively. Also, a combination of capacitor/resistor and flyback diode/resistor seems most effective at dissipating inductive feedback and thus protecting the mosfets.
I don't intend any offense to those that have contributed extensively to the current circuit design. I have been impressed at the work and degree of sophistication that has been put into the design but I also believe it can be improved upon and made more cost-effective.
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