Hi

I am designing a variable frequency drive for an ac induction motor. It consists of a 220VAC to 312VDC bridge rectifier connected to the pull up transistors of an H bridge inverter implemented by IRFP450. The gates of the 4 NMOSFET of the H bridge inverter are driven by two IR2110 gate driver ICs. I am using PWM generated by AVR (atmega16) which gives the PWM signal to the IR2110 gate driver IC through optocouplers of PC817. My circuit has a 220VAC 50 HZ supply input and contains the following voltage levels:

1. 312VDC for the drains of H bridge pull-up NMOSFET.

2. 15VDC supply for optocouplers and IR2110 from a stepdown transformer à rectifier à 7815.

3. 5VDC supply for AVR from a same stepdown transformer à rectifier à 7815 à 7805.



The output of the IR2110 IC is all ok when the bridge rectifier for 312VDC is not plugged in. As soon as it is plugged in, the microcontroller does not produce the output. I tried to change the polarity of the ac input of the 312 VDC (phase in place of neutral and vice versa) but the problem remains. The ground of all the power supplies, i.e. 312VDC, 5VDC, 15 VDC are connected together. When I checked the potential difference between the neutral (the ground of 312VDC) and the ground of 5VDC to the microcontroller, there is a sinusoidal signal of 20V present. To my understanding, the neutral of the 220 VAC is not at zero potential due to unbalanced load and it disturbs the ground potential of the circuit.



The question is how should I handle the ac and dc grounds? Any practical idea or suggested reading is gratefully welcomed.

I checked the circuit by separating the grounds of both power(315VDC) and control(15VDC,5VDC) unit but it didnt work also.

Thanks