Fast turn on/off Mosfet Circuit

Discussion in 'The Projects Forum' started by mleka, Aug 17, 2010.

  1. mleka

    Thread Starter Member

    May 27, 2008
    15
    0
    Would this circuit work? I am using a 555 timer to generate the frequency of need and then use a mosfet driver max4420 to drive the Mosfet.
    What would be an issue with this circuit.
    Do I have to protect Mosfet against high voltage if I have an inductor on drain side or source side (between Rd and Mosfet) or against parasitic inductance?
    I would like to keep IRFP running below 40C (may need heat sink). Any suggested max current/voltage limitation? Varaible resistor should accomplish the desired current for maximum efficiency.
    What transistor would be adequate for this circuit?
    How do we choose which Diode is best suited (I beleive this is dependent on driving current and 1N4148 can be used up to 150MA and BAS40 up to 300MA)
    I am newbie in electronics (Mechanical Engineer) and am trying to learn so bare with me. Any advice is appreciated.
     
  2. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    First of all unless Rd is the load it looks like you're trying to make a high side switch which means all the asociated driving circuitry needs to be at a voltage even higher than the voltage you're trying to control or just slightly below if it's a P type MOSFET. I also see a few parts that could be considered as extra, especially if you're looking for speed.

    Perhaps if you started off by telling us what it is you're trying to control.
     
  3. mleka

    Thread Starter Member

    May 27, 2008
    15
    0
    I was hoping to use this circuit as a frequency generator for various voltages/currents and wanted it to be fast turn on/off times hence the mosfet. IRFP450 is an N-Channel type mosfet. You mean the voltage at the gate has to be near 400V? IRFP450 data sheet suggests only a max 20V as the gate-source. I am not quite understanding it correctly?
    Does this make any sense?
     
  4. mleka

    Thread Starter Member

    May 27, 2008
    15
    0
    In addition - Rd is there only to control current and is not the load, that is the intention. Consider it as part of the power supply circuit if you will.
     
  5. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Ask yourself a question, what is the voltage at the MOSFET Source pin, during no output and full output?

    If you said 400V at full output, then you will need to have 410V on the gate, giving a voltage difference of 10V between gate and source so that the MOSFET will fully turn ON.

    Your present circuit doesn't seem to be able to provide that value of gate voltage.
     
  6. mleka

    Thread Starter Member

    May 27, 2008
    15
    0
    Voltage at source pin will have to be 0V on both cases - now I get it - my output will have to be on the drain side. Will have to revise the schematic...a bit later...
     
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