Help with high voltage pulse generator and selecting gate drivers

Discussion in 'The Projects Forum' started by k2n2a2, Jul 16, 2012.

  1. k2n2a2

    Thread Starter Member

    Jul 16, 2012
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    Hello,

    I need help completing a design for a high voltage pulse generator that is driven by a TTL PWM signal (100kHz). The output of the pulse generator needs to swing from +10V (when PWM is high) to -1500V (when PWM is low), 20A max. I have two power supplies to switch between: 10V and -1500V
    I need to use N channel MOSFETs for switching both the negative and the positive supplies.
    Can any one help me finding the right resources please?
    I already completed the logic part that controls each MOSFET based on the PWM signal, only the power switching part that I have trouble with..
    I need help with selecting the gate driver for each MOSFET (the one that switches the negative supply and the one that switches the positive supply).
    I am not really looking for a complete schematic, but I need help with selecting the right gate driver for each FET, and what way to connect each FET with the load and supply (high side vs. low side).

    thanks
     
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Is that 20A at -1500V ?

    What is the load?
     
  3. k2n2a2

    Thread Starter Member

    Jul 16, 2012
    37
    1
    The load is purely resistive load, 4ohms.
    The 20A max current is for the low positive voltage, for the -1500 the current draw is much less..
    I already picked the power mosfets that I need to use, they are n channel rated at 1500v and 25A max, I just need help connecting them to the right driver. I read a lot about high side n mos gate drivers, bootstrapping, charge pumps, but I am not sure how to use then for my application.
    Thank you
     
  4. k2n2a2

    Thread Starter Member

    Jul 16, 2012
    37
    1
    The load is purely resistive load, 4ohms.
    The 20A max current is for the low positive voltage, for the -1500 the current draw is much less..
    I already picked the power mosfets that I need to use, they are n channel rated at 1500v and 25A max, I just need help connecting them to the right driver. I read a lot about high side n mos gate drivers, bootstrapping, charge pumps, but I am not sure how to use then for my application.
    Thank you
     
  5. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Ok, if your MOSFET is rated 1500V, you cannot apply 1500V to it. That's its maximum rating.

    What 1500V model did you choose?
    What is the current for the 1500V?
     
  6. k2n2a2

    Thread Starter Member

    Jul 16, 2012
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    The supply is actually 1000V DC, I said 1500 just to be safe..
    this is the part # for the MOSFET I picked: STW9N150 by STMicroelectronics
    I plan to use three of them in parallel to get the 20A current...
    I just need help finding a high side driver that I can use with this mosfet that can handle the 1000VDC supply. it needs to be high side switch because I need a ground connection for the load.
    If I know how to drive a single mosfet with this supply, I can figure out the rest.

    Thanks
     
    Last edited: Jul 17, 2012
  7. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Sorry, something isn't right here. If for both power supplies the load is purely resistive AND 4 Ohms then there will be a current much higher than 20A for the 1000V. You say it's much less.

    First we need to know your exact requirements, then we can elaborate a circuit.

    Are these power supplies (10V and -1000V) somehow related to another or are they completely isolated? You will probably need one or two more power supplies for the gate drive circuits.
     
  8. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    I drew up a quick schematic. I assumed the 10V and 1000V are separated power supplies and you can connect them however you want.

    The gate driver's maximum bootstrap voltage I found is 1200V, that's too near to 1000V. So I would prefer using isolated gate drivers for both upper and lower MOSFET. You will need 2 additional power supplies (15V) for these. It may be possible to make something bootstrap like with a high voltage diode, especially because you have a fixed duty cycle but I didn't think about this yet.

    There are several things to consider when you work with such high voltages/currents.

    1. You are at 100kHz, that means you will need to switch them fast, you only have 5us available for the load. Switching fast will cause voltage peaks due to the inductances. These peaks can exceed the maximum rating of your MOSFETs. I recommend starting with a lower voltage. Somehow I doubt that you can get a clean pulse that short at the load.
    2. You need a deadtime. This has to be created before the driver, i.e. the two TTL signals are not the same signal but two different signals. You need to make sure that NEVER both MOSFET switches are ON.
    3. The additional power supplies and isolated gate drivers must have a higher isolation voltage than the highest voltage present on the power side.
    4. Make high current paths short, make MOSFET - gate drive connections short, use bypass caps on the gate driver output side, don't pass the gate drive signals trough the same wires as the load current.

    The 4 Ohms still don't make sense. I strongly recommend to start with much less than 1000V.
    You will need an oscilloscope.

    Look here for some gate drivers:
    http://www.digikey.ca/product-search/en/isolators/optoisolators-logic-output/3736381

    Use one that can provide more than 2A.

    [​IMG]
     
    k2n2a2 likes this.
  9. k2n2a2

    Thread Starter Member

    Jul 16, 2012
    37
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    Thank you for the reply,
    i will try it and see if I get the results I am looking for..
    Also, what value of gate resistor do you recommend (Rg)?

    Thanks again.
     
  10. k2n2a2

    Thread Starter Member

    Jul 16, 2012
    37
    1
    Yes, the two power supplies are separate units, and the load will draw up to 20A max from the positive 10V supply, and draw about 2 to 3 Amps from the negative 1000V supply.
    I can set the maximum current for each supply individually, that is why I am saying the -1000V switch will draw less current.
    I also dont care if I have to use PMOS switches, if they will be simpler to drive and dont require more supplies to switch them. I have 24VDC supply to power the ICs.
     
  11. k2n2a2

    Thread Starter Member

    Jul 16, 2012
    37
    1
    what do I need to change in this schematic if the 1000V supply is actually negative?

    Thank you.
     
  12. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Nothing, it is negative with regard to the 10V.

    The positive of the 10V will be on the right side of the load resistor.
    The positive of the 1000V will be on the left side of the resistor.
    Therefore measuring with an oscilloscope over the load resistor you will see +10V to -1000V (or the inverse).

    You can use one MOSFET for the high side and 3 or 4 in parallel for the low-side.
     
    k2n2a2 likes this.
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