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Mosfet drive

  1. Dec 21, 2012 #1
    Hey I have a thing that confuses me.
    I have a mosfet that has to drive a transformer in pulsed regime.
    Now I have a multivibrator that gives out 100khz square wave signal and the amplitude of the signal is let's say 30 volts but the mosfet is 330v DC to the Drain and source goes to the primary winding can a 30v amplitude square wave drive this mosfet fully or will the mosfet open just as much as the signal given to it's gate will allow it to?

    Well basically what I'm asking is how strong in terms of voltage the average mosfet gate drive signal has to be ?
  2. jcsd
  3. Dec 21, 2012 #2
    You need to put some "," in the sentence, I really can't understand exactly what you are saying.

    Look at the data sheet!!! When I was designing MOSFET pulsing circuits, very few if any can take over +/-20V from gate to source voltage ( Vgs). You blow the gate if you put voltage above the limit.
    Most MOSFET drivers are designed to drive from say 0V to +15V and it's plenty good. It is irrelevant whether the Vds is spec for 330V. I don't even know where you get a multivibrator that drive 30V.......get another one, there are plenty.
  4. Dec 21, 2012 #3
    So basically for the mosfet to get fully open and let the drain voltage/current to run down to source it needs only like some 10/15 volts on it's gate respective to 0v?
    I guess that is enough for the electric field to be high enough for the FET to open.?

    Speaking of multivibrators I just built a 300volt one. It gives me a square wave with a frequency of about 150khz and the amplitude about 300v, But it is a crazy one and uses some huge soviet made wire wound resistors so not a handy thing at all but does it's job pretty well ofcourse with a "little heating" :D
  5. Dec 21, 2012 #4
    You need to be careful, there are two type of MOSFET, one is enhancement mode and the other is depletion mode. The most common type we use for switching application is the enhancement mode MOSFET.

    Enhancement mode N channel MOSFET is that you need a +ve Vgs to turn on the transistor. Read the data sheet of the particular FET you choose, that's the most important. In general, a FET start to conduct current from drain to source when Vgs is at or above 3 to 4 volt for a N channel FET. BUT for fast turn on and switching, you drive from 0V to +12V or over to ensure you get fast switching. But to keep it on and conducting, you only need a few volts. Also, the on channel resistance gets lower when the Vgs is higher. The on channel resistance is higher when you only have say 4 V than if you have 15V. These are details that are given in the data sheet. I am just pulling some numbers out from my head of a typical MOSFET.

    I can't comment on your multi vibrator, I don't know what you try to do with your circuit. But it is not suitable to drive a MOSFET. There are many cheap IC like 555 that can do the job. All you need is a MOS driver to buffer the 555 and you have a good solid circuit.
  6. Dec 21, 2012 #5
    Thanks yungman I got it now.
    Yes the gate voltage should be closer to the limit I guess as to keep the field stronger and the mosfet fully open to minimize the resistance hence the heat I guess that will be dissipated.

    The multivibrator was more of a test circuit I wanted to see how high both in voltage and frequency I could go with a comparably small set of parts working at extreme conditions.
    I could drive a ferrite transformer at high frequency from a npn multivibrator itself when the transistors are high voltage /amperage and the freuency close to 100khz the problem is the heat which is far more in bjt's than in mosfets , But the device itself is as easy and simple as it can get also works pretty well from different voltages in the range from 120v AC to 240
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