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Ideas how to handle the ac and dc grounds needed

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Jun 30, 2009
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coupling of ac and dc


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.


coupling of ac and dc


In AC there is no talk about ground. Ground available in DC only.

If you want universal zero potential, then go for Earth.

If you are ready to post your circuit and input signal profile in graphical manner, i will try to give the simulation result for that.

give me the expected output signal profile for tunning your design.


make sure there are no noise injected into circuit. there is the possiblitlies of high frequency injection from powerside.check the ground and VCC signal at controller . u can reduce noise by adding pi filters and common mode chock at power supply. add capacitors 0.1uf (and/or 10uf )nearer to vdd pins of all ICs. in PCB reduce ground trace length as much as possible.some even have physical seperation from power side by slits( :D i forget the name ) below the opto isolatiors .

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