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Isolated ground planes on a MCU controlled high voltage IGBT


stube40

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I am putting together a PCB that has a CPU to control the on-off state of a high power IGBT. There are two power supplies associated with this:

1) A 24 DC lab PSU that is regulated on the PCB to both 5V (CPU) and 15V (TC427 MOSFET driver)

2) A meaty 150V / 40A DC PSU that flows through the IGBT and into a massive inductor

The trouble is, I'm nervous about connecting the grounds of both PSUs together due to my suspicion that in certain conditions in my application I'll get large negative currents on the ground plane and/or other nasty stuff that the CPU and related electronics will hate.

However, if I dont I have a concern regarding getting the correct 15V gate voltage to turn the IGBT on and also another circuit where I use a 2-resistor voltage divider to downscale the 150V to a meagre 5V so that it can be fed into one of the CPU's ADC inputs to measure the incoming voltage.

Can anyone suggest a way forward?

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What's the switching frequnecy?

You can buy ready made solid state relays if it's low frequency.

If it's high frequency a pulse transformer can be used to transfer the gate drive across the tow grounds.


What if I used an opto-coupler instead?? The switching frequency is only around 10Hz, but reaction time has to be within 1ms - I think a good opto-coupler could achieve this.

If I used an opto-coupler, the only remaining problem is what to do with the ground for the resistor-divider voltage measurement feed that allows the CPU to measure the amplitude of the high-power voltage source. Maybe using the CPU's differential ADC inputs?
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OK, I've been pointed to the existence of linear optocouplers to solve this problem! You learn something new everyday....

Having decided to go down the opto-isolator route, I'm struggling with the logistics of powering the IGBT-side of the opto-isolator.

To elaborate, there are actually 4x IGBTs and drivers in an H-bridge formation. They are routing the 150V / 40A power source through are large superconducting coil. We have found from previous experiments that all sorts of strange things happen when the coil is being charged and switched including zero voltage and negative voltages. Hence, this cannot be the source of the 15V for the IGBT side of the opto-coupler and the TC247 driver. Yet, to turn the IGBT on I need Vge to be 15V, but I'm worried about connecting the ground of the 150V PSU to the 15V PSU.

Does this make any sense at all, or should I upload a diagram?

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Upload a diagram for us to work on.


OK, the design has changed a little since I last posted - I've gone for the Vishay VO3120 opto-isolated driver powered by dedicated isolated DC/DC converters. I've attached 2 JPGs - one which shows the general layout and one which zooms in showing how the VO3120s are powered and connected.

I'd be intrigued to get some feedback on what people think of the circuit.

Also, I can't decide on whether to use a bi-polar DC/DC to provide the Vcc / Vee pins of the VO3120 with +/- 15V, ot whether to use a uni-polar DC/DC and provide 0V/15V - any thoughts?

post-49942-14279144106839_thumb.jpg

post-49942-14279144106999_thumb.jpg

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  • 2 weeks later...

Unipolar 15V should be sufficient assuming the totem pole drivers at the output of the isolators conduct good enough. Quantitative calculations will reveal that.

The schematic of the system will need to show a reference of the supply to the isolators to the emitter of the igbts like it is shown in the black&white schematic for pins 5.

You dont need an isolated supply for the low-side igbts.

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What about using something like this?

http://www.electro-tech-online.com/attachments/electronic-projects-design-ideas-reviews/39571d1267245370-series-connection-battery-cells-fets-.pdf

You need to use two transistors, as it's DC, not AC, one will do.

Of course you could replace the logic gates with an MCU output or use a voltage multiplier if the voltage is too low.

It should also work with an IBGT if you don't want to use a MOSFET.

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