Buck Converter MOSFET + Driver Selection

Discussion in 'The Projects Forum' started by Schniz2, Aug 25, 2011.

  1. Schniz2

    Thread Starter New Member

    Apr 15, 2010
    I am building a Buck converter with a max output current of 3A @ 45V. Supply is 50V. I want the build to be relatively simple, im not concerned with efficiency or component cost (within reason). I am switching at 20kHz with a 3.3V microcontroller output.
    My first question is whether i would be better off using an NMOS and high side driver or a PMOS? Would i need a high side driver for a PMOS anyway?

    As far as i can tell the important parameters for the MOSFET woudl be (i want overrated components to be safe):
    Drain current = 8A+
    Vds = 100V+

    For the MOSFET Driver:
    I am looking at this one, not sure if i totally understand the parameters given:

    Does the peak output current relate to how quickly it can charge the capacitance of the MOSFET?
    Also, the following parameters are given... the way i understand it is, i need at least 10V above my 50V supply to drive an N Channel MOSFET. This module would allow me to have a 20V pk-pk square wave sitting on top of up to 200V DC (or in my case 50VDC)

    • Output Voltage:220V
    • Output Voltage Max:20V
    • Output Voltage Min:10V
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
    I'd use the same MOSFET for high and low side, so I don't have to think about different parameters. If you use the driver you suggested it already has high and low side driver in one chip.

    That's ok.

    Yes, the higher the output current the faster you'll charge the gate capacitance, depending on the value of the gate resistor of course.

    ??? No. You drive a MOSFET with a gate signal within it's voltage ratings, which is usually between 10V and 20V, most commonly used is 15V.

    You provide one supply voltage for the driver IC. It charges the high side driver capacitor when the lower power MOSFET is conducting. Then the lower MOSFET turns of and the energy stored in the upper driver capacitor is used to charge the gate of the upper power MOSFET.

    If you use this driver IC you will not be able to separate logic and power GND. Proper layout is therefore extremely important.
    There are isolated drivers too like the HCPL3180, but you'd need one for each MOSFET.
    Schniz2 likes this.
  3. Schniz2

    Thread Starter New Member

    Apr 15, 2010
    Thanks for your help, i ended up going with the IR2011, but i am only using one mosfet and one diode (ie im not implementing a synchronous buck).
    I am a little bit confused how to choose the capacitors and resistor for the following diagram ("Typical Connection" diagram from the datasheet) http://www.irf.com/product-info/datasheets/data/ir2011.pdf
    Im going to use a Vcc of 15V for the gate driver.

    I assume i would leave the LIN and LO pins unconnected.
    The resistor value would let me choose how fast to charge the gate capacitor... my MOSFET has a Typical gate charge of 27.4nC... i could use the equation Q=IT to select my gate turn-on... but what is the output current of the gate driver? The datasheet has IO+/- (Output High/Low Short circuit pulsed current <10us).. is this the current i would calculate it on? What do i use to calculate a gate turn-on time? The MOSFET dV/dt ruggedness? This is my MOSFET datasheet http://www.fairchildsemi.com/ds/FC/FCPF11N60NT.pdf

    And im not sure what the capacitors are for and how to choose them from Vcc to COM, Drain to COM and Vb to Vs (i guess this last one is to hold the 15V voltage for turning the gate on)