Driving a Power MOSFET

Discussion in 'General Electronics Chat' started by Merc300d, Jul 6, 2009.

  1. Merc300d

    Thread Starter New Member

    Jul 3, 2009
    2
    0
    Hello, I have programmed a BX24 chip to output logic highs and lows and PWM signals that are currently controlling 12 volt circuits, pumps, valves and relays via MOSFETS.
    I wish to recterfy 240 VAC to DC and use a power MOSFET (500V 53 Amp) to control a 110Volt DC motor.
    With 240 VDC across the MOSFET is it possible to drive the gate with a 5V logic high from a BX24 chip? I have tried it with 12 Volts and that is ok but not sure what to expect with the higher voltage.
    Thanks for taking the time to consider my question.
    Graeme
     
  2. millwood

    Guest

    a couple issues for you to consider:

    1) you most likely need to switch substantially amount of current. As such, you will need to remove lots of charges during a very short period of time, or vice versa. I don't know of the chip but it may not have enough current capabilities to drive a big mosfet. if so, you should consider using some kind of driver circuitry.

    2) most mosfets need Vgs>10v to be fully on, in order to minimize conduction losses. your chip can only provide 5v on a good day. so you cannot use 90% of the mosfets out there.

    now, there are "logic level" mosfets that starts to open at Vgs=2v (typical) and gets fully open at Vgs=5v. They can be driven by your chip. However, those logic level mosfets are more expensive and in my experience more fragile than your typical mosfets.

    so if you do go with a logic level mosfet as the switcher, you will need to be careful about which mosfets to use and make sure heat dissipation isn't an issue. That may save you some complexity in the driver circuitry.

    if you do go with a conventional mosfet, you will have to consider a driver circuitry but it opens the door to a lot of choices and likely better performance.
     
  3. Merc300d

    Thread Starter New Member

    Jul 3, 2009
    2
    0
    Thank you very much for your reply, it has assisted greatly in my understanding. I did not understand that the MOSFET had capasitance and that when the gate was turned on there was an inrush of current while the voltage built. The max my BX24 can provide is 20 mA at 5 V, so this is why I would need a driver chip?.
     
  4. millwood

    Guest

    whether 20ma is sufficient depends on a lot of factors, like

    1) gate capacitance / c harge you have to fill / remove: the more charge you have to deal with, the higher the drive current requirement is.
    2) frequency at which you are switching: the more frequently you have to remove the charge, the higher the drive current requirement is.
    3) the efficiency requirement: the faster you can fill / remove the charge (dv/dt), the higher the drive curren trequirement is.

    ...

    to switch on around 50amp of current, you are likely dealing with a large to3p-class devices, with gate capacitance over 3000pf (likely in the 5000pf range). at 20khz switching frequency and a reasonably fast dv/dt, I would guess you need about 500ma (peak) current to drive the switcher, and I would go with a 1amp / 1.5amp or higher driver just to minimize switching and conduction losses.

    most mosfet gate driver and pwm controller datasheets provide far more detailed calculations on driver current requirement so you may want to check a few out.
     
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