coupling of ac and dc

[moezbaba]

Hi,

I am designing a single phase 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 2110 gate driver ICs. I am using PWM generated by AVR atmega16 which gives the PWM signal to the 2110 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 2110 from a stepdown transformer à rectifier à 7815.

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



The output of the 2110 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 80V 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.



Thanks

[Tomi Engdahl]

Can you post a block diagram or preferably the schematic to some web site where I can see that?

The text description of the circuit leaves too much details for quessing.
If you can post schematic (or partial schematic where you might hide some details you don't want to reveal)
then it would be easier to answer to your question. In addition some pictures of your prototype would be nice.