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coupling of ac and dc

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moezbaba

New Member
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 IRF ???. 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 ???? which gives the PWM signal to the 2110 gate driver IC through optocouplers of ????. 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 2110 from a stepdown transformer à rectifier à 7815.

3. 5VDC supply for AVR 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 ????? 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
 

smanches

New Member
Do not connect the power path ground (315V through the H-bridge) to the control circuitry ground. It causes you to need much much more filtering.

I'm assuming you have a transformer to make your 15vAC then rectifying it and regulating it with the 7805 and 7815? Make sure you have both input and output caps to both regulators. Use a rather large output cap for the 7815, as the current draw from the mosfet drivers is going cause a lot of ripple.

Also make sure you put 0.1uF decoupling caps at every IC VDD pin, as close to it as possible, and preferrably inline with the trace to the pin. This will help reduce the high frequency noises that can get injected.
 

mneary

New Member
The (-) output of the bridge rectifier off the AC mains is NOT neutral. At best each (+) and (-) is a DC potential with half of the mains AC riding on it. You're lucky it didn't literally blow up.

The only way for the 312VDC power supply to have a 'ground side' as you describe is to include a transformer in it like the 5VDC and 15VDC supplies have.

[edit] The 5V power supply is the logic supply and its only connection to the output stage is through the optos. The 5V side may ONLY connect to the opto LEDs and the output side may ONLY connect to the opto transistor side. The 15V and 5V can have a potential difference of 312V or more and cannot share a transformer! You should be able to draw a box around the logic which passes ONLY through the optos and no other components.

Consider your -312V as the common (NOT GROUND) for the output stage. The 15V (-) side is connected to the -312V common. The output stage must NEVER be touched with grounded test equipment and you must NEVER touch it, unless it is properly isolated with a transformer (and then only if you know what it does and doesn't do for your safety).
 
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