Ask about IGBT and Power Mosfet. In dire need of help!!!

Discussion in 'General Electronics Chat' started by soljiang, Jul 24, 2009.

  1. soljiang

    Thread Starter Member

    Jul 24, 2009
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    've been stuck on an electronic problem for weeks. I would really appreciate if you could help me out. I need some mechanism to kill the kickback voltage of a coil that I drive with transistors. I plan to use two power supplies, one is low voltage, 12V and used most of the time during the cycle, and another high voltage 30V, which only comes into play when the kickback voltage exceeds +/-12V. I'm wonder if it's common in electronics to use IGBT or power MOSFET to switch the power supply. I've read that it's used in logic switch and in switch-mode power supply itself. However, I don't know if it's common to use it, or if it's proper to use in my case. I attached a picture of the circuit. The picture might be a bit confusing. The control pin is the gate, and the in and out pins are just drain and source. I just want that only one power supply is used at a time.

    PIC2 is another circuit I thought of and I found it nicer to use, or at least it's more straightforward to me. But then I realized that it doesn't work becuase when the 30V is turned on, it will be shortened by the protector diode of 15V. Control block is just some circuit to decide whether the voltage exceeds a certain limit decide which transistor pairs to switch on. I can't think of any ways to modify it to make it work.

    Any advice would be very much appreciated. It will be a great help to me. Thanks a lot.
     
  2. millwood

    Guest

    the bottom diode has its polarity reversed.

    you will likely have to figure out a way to redirect the kick-back current, because the igbt/mosfet/bjt you use is one directional - they allow the current to go from the power supply to the motor, not the other way around.

    the solution could be a tvs, or a solid state relay, for example.
     
  3. soljiang

    Thread Starter Member

    Jul 24, 2009
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    Thank, Millwood. The diodes are just protector diodes for transistors.
    The purpose of using a second higher voltage supply is to draw more current to kill the kickback so that I don't need to redirect the kickback current.
    So you think IGBT can be used in this configuration? Shall I use IGBT or Power Mosfet. I want more than tens of kHz of bandwidth.
    Thank you.
     
  4. millwood

    Guest

    mosfets will be good here. igbt, being foundamentally a bjt, has advantage at very high switching current.
     
  5. soljiang

    Thread Starter Member

    Jul 24, 2009
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    My concern is that I need to pass 2A current through the coil. I remember that Mosfet R(on) is typically tens of ohms. That would mean huge heat dissipation issue, wouldn't it? What kind of Mosfet should I use? Any recommendation? Thank you so much!
     
  6. millwood

    Guest

    you can easily switch mosfets hundreds of amps. 2A is no big deal.

    if it is just 2a, you may be able to stay with dpak, or to220. look for Vgs(th) that fits your design and try to minimize Rds(on). I use primarily irf540 from IRF and FQA from Fairchild, but mostly for linear applications.
     
  7. soljiang

    Thread Starter Member

    Jul 24, 2009
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    I find it hard to understand that with 0/15V gate control voltage, how can Mosfet be turned on and serve as a 30V power supply follower. Vgs is negative when turned on in this case, right? Also the Vgs break-voltage is +/-20V
     
  8. millwood

    Guest

    I am not sure if I follow your question.

    yes, typical Vgs breakdown voltage is +/- 20v.
     
  9. soljiang

    Thread Starter Member

    Jul 24, 2009
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    I mean, isn't it required that the gate voltage must be higher than source voltage? When using MOSFET as a switch the gate voltage is the output of logical circuit, say, no more than 15V at high state, however, the source voltage will in essence be close to drain voltage when the switch is on, which is, for example, 30V in my case as the output of a voltage power supply, higher than the gate voltage. If I use the enhancement N-Channel power mosfet, for example IR540, the mosfet can't be turn on.
     
  10. millwood

    Guest

    yes, that's true for n-channel mosfets.

    how much higher it needs to be is a function of the particular mosfets, and your desire to drive it hard or not.

    in general, conventional n-channel vertical mosfets require 10v to be fully saturated. you can drive it higher but you don't gain much, other than to increase the current demand on your driver exponetially.

    logic level mosfets typically open at 2v and get fully saturated at 5v Vgs. and they generally cannot switch high current as well as the conventional n-channels. and they are usually more expensive.

    lateral mosfets typically opens at slightly below 1v, and they are not generally used as switches so people don't talk about their Vgs for saturation.

    to drive a conventional n-channel from a mcu, you almost definitely need a driver.
     
  11. soljiang

    Thread Starter Member

    Jul 24, 2009
    12
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    Thank you, Millwood. So I figured that my intention to use it as a switch to switch on/off power supply won't work at all (as shown in PIC1 in the fist post), since the control voltage for gate would be 5V/15V depending on logic circuit, while the drain voltage is connected to power supplies of 12 and 30V. Am I right about it? Are there any way to get around it? Thank you.
     
  12. millwood

    Guest

    you CAN make it work.

    all it takes is a small npn transistor driving a p-mosfet through a resistor.

    n-channel will work as well.
     
  13. millwood

    Guest

    here is a conceptual drawing as to how that can be done.

    the green trace is the 12v output and the blue trace is a 3.3v input from the mcu.
     
  14. soljiang

    Thread Starter Member

    Jul 24, 2009
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    uh...I'm quite lost. Would you please give me some more information on that? I really appreciate it. It's very important for me. I reattached that part of the circuit. Thank you.
     
  15. soljiang

    Thread Starter Member

    Jul 24, 2009
    12
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    Thank you very much. I'm looking at your file!
     
  16. soljiang

    Thread Starter Member

    Jul 24, 2009
    12
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    Is the transistor used to bring the gate voltage above 12V? I guess it works for 30V as well? What program is it? I would like to try it out too. Thank you very much!
     
  17. soljiang

    Thread Starter Member

    Jul 24, 2009
    12
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    Hi, Millwood,
    I tried simulating the circuit in Multisim. The gate voltage swings between 0-12
    V, but the output of mosfet, that's the voltage of the resistor is between about 10-12V with pk-pk of 2V. I attached the screensnap. Is there anything wrong with the way I draw my circuit? What's the simulation software that you used? Thank you very much!
     
  18. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Nice circuit, Millwood. :rolleyes:
    Don't forget to tell the OP that the 110 Ohm resistor needs to be rated for 3 Watts, since power dissipation in it will be 1.31 Watts when the transistor is on.

    I don't suppose it occurred to you to inform the OP that using a simple inverter to drive an H-bridge WILL result in shoot-through (even disregarding the necessary dead-time) due to propagation delays?

    Sorry, soljiang.
    Millwood is very new to these forums, and the "old timers" don't know anything about him - except that he makes a lot of posts, which frequently have very dubious (questionable) value to these Forums.

    It will not help you to switch to a higher Vsupply when the MOSFETS/IGBT's/transistors turn off. Basically, all that you need are strategically placed diodes/rectifiers that will allow the current from the coil a path to recirculate.

    With that in mind, why don't you try to more clearly state what your objectives are?
     
  19. soljiang

    Thread Starter Member

    Jul 24, 2009
    12
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  20. soljiang

    Thread Starter Member

    Jul 24, 2009
    12
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    Oh, you are also right that "It will not help you to switch to a higher Vsupply when the MOSFETS/IGBT's/transistors turn off", because I only want to use it when the kickback voltage overwhelms the 12V, which would only be transient. I figured in this way I can avoid causing too much heat dissipation issue becuase I don't have much room for heat sinks and the lower heat problem the better.
     
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