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N-Channel MOSFET

Discussion in 'General Electronics Discussion' started by michan, Jul 11, 2016.

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  1. michan


    Feb 21, 2015
    Hi guys,

    Im struggling to understand how an N channel mosfet will work in my situation. Im charging a bank of capacitors and the mosfet is being used as the switch to start charging. Only problem is I keep reading that the gate voltage has to be higher than the source voltage which is why they are usually placed after the load to tie the source to ground. So if i charge the bank of capacitors therefore increasing their voltage, and hence the source voltage, will the mosfet switch off eventually when the source is higher than the gate voltage?

    If so, what can i use to switch this charging circuit on that can also handle 0.15A at 450V? Would a P channel mosfet work?

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  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    can you not place the N channel mosfet in the -ve (possibly gnd) side of the capacitors?

    Another option is a floating power supply or a charge pump. There are gate drivers which include a charge pump.

    otherwise, yes you can use a p Ch mosfet as a high side switch. In this case the gate is pulled low to turn it on.
    michan likes this.
  3. michan


    Feb 21, 2015
    Ok yeah i did end up placing it on the other side of the caps haha, i definitely over thought that. Also am i able to power the gate of the mosfet with a seperate power source without having a common ground? eg: have the gate just connected to the positive of a 9V battery and the source and drain of the mosfet passing the 450V 0.15A current through it?
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    With common ground: o.k. You need to oebserve the threshold voltage of the MOSFET to ensure that the gate-source voltage is able to fully drive the MOSFET open.

    When placing the MOSFET in the ground leg of the capacitor bank, make sure there is no ground loop between the capacitor bank and the power source which would short circuit the MOSFET making all your efforts to control the current into and out of the capacitor bank moot.

    With that high voltage, take extra care to ensure all necessary safety precautions! Have the circuit checked by someone experoenced with this voltaeg level. We don't want any casualties.
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