Last I saw, he was using N-channel mosfets. How do the diodes tied to the upper rail (20 VDC) help turn them off?
The usual design has them draining toward the driver, which hastens turn-off.
John
In Gary's original, non-pwm design, the mosfets were turned on by pullup resistors to +12V, and turned off by the schottkey diodes from the gate of one mosfet, to the other drain of the other mosfet. Basically, as one mosfet turns on, it turns the other mosfet off. The LC resonant timing controls the switching frequency.
In a design using PWM, the mosfet gates must be exclusively driven by the PWM controller. The pullup resistors, and the diodes to the opposite drain, need to go away. In PWM systems, diodes can by used to provide independent control of turn on and turn off transition times.