Isolated ground planes on a MCU controlled high voltage IGBT

Discussion in 'General Electronics Chat' started by stube40, Feb 23, 2010.

  1. stube40

    Thread Starter Member

    Feb 3, 2010
    42
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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?
     
  2. S_lannan

    Active Member

    Jun 20, 2007
    247
    2
    you can completely isolate the circuits with an opto-coupler...
     
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  3. stube40

    Thread Starter Member

    Feb 3, 2010
    42
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    Yeah, I think you're right - it would be good to drive the TC427 mosfet driver with an optocoupler to totally de-couple the circuits.

    Now all I need is to work out an alternative solution for the resistor-divider voltage measurement feed from the high-power source to the CPU. Any suggestions?
     
  4. stube40

    Thread Starter Member

    Feb 3, 2010
    42
    0
    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?
     
  5. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    A schematic would help us visualize.
     
  6. stube40

    Thread Starter Member

    Feb 3, 2010
    42
    0
    PDF of the circuit attached.

    I've included the Vishay VO3120 MOSFET driver which has opto-isolation built in.

    The 15V power supply can be created from the CPU side via a DC-DC converter with galvanic isolation. My main concern is getting the IGBT Gate-Emitter drive voltage to be negative rails of both the 150V PSU and 15V DC-DC are not directly connected together.
     
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