IGBT circuit analysis

I would like to analyze this circuit I found on line (at Electronics Hub) for educational purposes. The stated purpose of the circuit is to control power to a resistive load using PWM. I understand the concept of PWM input from the microprocessor and also the bridge. I would like to know how to determine the current and voltages through the control part of the circuit, specifically the voltage at the gate when the IGBT is conducting and not. I have not built the circuit and do not really intend to. I just wish to gain an understanding of the operation of the circuit as the IGBT sees it. I think I understand that R1 and C1 are a high pass filter to help smooth out the output from the bridge. My attempt at analysis gave me about 2.5ma through R4 so .0025X22k Ohms =55 volts at the gate when the opto coupler is on. That doesn’t seem right. Any help steering me in the right direction is appreciated. If needed for the analysis we could assume a resistive load of 22 ohms or about 10 amps. There is of course no guarantee that the person that designed the circuit did so properly or that the circuit will work as specified. I guess that is what I am trying to determine.

D1 1N4007
T1 IRGP4063
BR1 GBPC 610
R1 220 ohms
R2,R3 33k
R4 22k
C1 220nF 250v
C2 2.2uF 50v
Z1 not specified
IC1 CNY65