How does this gate drive circuit work?

Question

This circuit is taken from page 6 of this paper by Bruno Putzeys. It is used as a gate drive circuit in a self-oscillating class D amplifier.

I am having a hard time wrapping my head around how it is supposed to work. All my attempts at simulating it in LTSpice or KiCad(ngspice) were unsuccessful. I tried many different resistor values, different diodes and transistors but had no luck.

My questions are the following:

1. How does the circuit work in theory?
2. How do I get the circuit to work in my simulation and in practice?

Answer 1

The circuit expects a current to be sunk from I_in, which turns Q1 on and charges the MOSFET gate via Rg and D2. When no current is sunk from I_in, R2 pulls the base of Q2 low, which then discharges the gate. D1 prevents saturation of Q1 and thereby enhances the turn-off speed.

The following component values should work:

• R1 = 1k Ohm
• Rg = 22 Ohm
• R2 = 1k Ohm

Sinking 5mA from I_in will then turn the MOSFET on. When you stop sinking current from that node, the MOSFET will turn back off.

Applying a voltage (instead of a current) to I_in will make the circuit malfunction.

Note that these component values are just a rough guesstimate; while they should work, they're probably not optimal. Tuning them further will likely improve the circuit's switching speed.