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Power mosfet proper gate drive current

  1. Jan 11, 2013 #1
    Hey guys could you help me out , so I have built this simple smps project that i needed to the multivibrator part works great frequency is stable square wave but then I have these power mosfets IRFP460 , now the multivibrator has two output each 180 opposite to the other I use each of these outputs to drive two of these n channel mosfets the problem is when i switch the transformer and then try to switch the apparatus in it, the square wave in the multivibrator dies off and turns to a flat dc about zero volts line.

    Well I'm assuming either one of two things or both , one could be that the current in my multivibrator is too little and when it tried to charge the gate it just sags down and the multivibrator stops generating., the other is that maybe i need to use a small capacitor in the gate drive path rather than a resistor?
    I don't know what's your thoughts on the subject?

    P.S. I apologize for the blurred schematic it was just late and i had no better paper or camera around. :)

    Attached Files:

  2. jcsd
  3. Jan 11, 2013 #2
    Im on my phone so the schematic is difficult to read N style mosfets need a load on the drain leg usually done with pull up resistors.

    Try the following google mosfet circuits. Match the type you have with the example circuits then try to get one mosfet working in a standalone circuit. Then build up to yoir complete mosfet circuit.

    Like I stated being on my phone I may have missed the loads to bias the required gate
  4. Jan 11, 2013 #3

    jim hardy

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    1. it is unclear to me what that phrase means. Do you have a switch that's not on your schematic?

    2. I don't see a transformer, just two inductors.
    Are those two windings on the same core? If so, where are the polarity dots?
    If you didn't have opposing polarities, what would happen when mosfets switched on?

    3. What is frequency of your multivibrator? What kind of core are you using?
  5. Jan 12, 2013 #4
    I apologize for not clearing things up , was late i was tired, well
    @jim hardy
    1)there is no switch all I meant was that i tested the multivibrator with the mosfets in but without the transformer attached as no load on the mosfet the vibrator still worked then i put the transformer in for the two mosfets that are driven from one of the two multivibrator outputs and the square wave died and the mosfet wasnt even opened one time.
    2)As for the transformer again i was lazy excuse me please i forgot two draw the secondary windings.

    3)there are two separate transformers.Each transformer is driven by two mosfets connected in parallel and each of those two mosfets are then driven by one of the two outputs from my multivibrator.Well that's the thought basically.

    the frequency of the multivibrator ir about 100khz , ofcourse i can change that easily as I have potentiometers in the transistor bases.I am using the philips 3c85 core, to be honest not quite sure that it can handle up to 100khz but if i remember correctly from the datahseet it should.
    Well my guess is that because my multivibrator is powered from a resistor voltage divider the current is too low and when connected to a load it sags and stops the multivibrator.
    As the mosfet gate takes no current but it needs one to charge it up to a level when the layers start to conduct, and for these big power mosfts I guess that capacitance of tha gate is pretyt large. Ok waiting on your thoughts.

    Another thing I thought that from each of the multivibrator outputs I have a 47ohm resistor in series to the each of two parallel mosfet gates, maybe i should change that resitor to a something like a 47pF capacitor as that would not let the DC through just the square wave and would help top protect the multivibrator from current/voltage sagging.?

    Attached Files:

    Last edited: Jan 12, 2013
  6. Jan 12, 2013 #5

    jim hardy

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    Yse, always check your power supply voltage first thing.

    As mordred said, one usually expects to see the load at drain not source.

    Take a look at this old appnote from National (now TI), section 5.6

    then observe that your circuit can never turn your mosfets fully on for they are connected as source follower amplifiers not saturating switches..

    There exist mosfet driver IC's , try a quick search.

    TL494 datasheet and appnotes is where i got started on switchers.

    old jim
  7. Jan 12, 2013 #6
    Well I just tried the drain case rearranging the trafo so that it is before drain and the source is connected to ground.
    Well nothing happens still and then I just came to the conclusion that I maybe need a second stage something like a voltage amplifier to drive the big mosfets as i have the 15 volt amplitude in the square wave but very few miliamps I guess just too little to charge the gate capacitance sufficiently.

    Oh by the way ofcourse it would be the easy way around to use a IC bu the rule is i can't use any ic's in the making of this smps

    Can someone basically tell me how much current is needed to turn on these "irfp460" mosfets sufficiently fast and so that they would operate normally?
    I made my math but it doesn't seem legit to me.
    Last edited: Jan 12, 2013
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