MOSFET driver questions

Discussion in 'The Projects Forum' started by iulian28ti, Dec 4, 2009.

  1. iulian28ti

    Thread Starter Member

    Dec 4, 2009
    Hi guys, i have a couple of questions.

    But first, what i understood until now about drivers.
    What is to be understood ? That a mosfet driver will send a higher voltage pulse(?) to charge the parasitic gate capacitor quicker so the mosfet will turn on in a shorter time, in consequence minimizing switching losses.
    I'm pretty sure that's correct.

    Now the questions.

    1. What is high-side/low-side ? High side is on the positive lead of the load, and low side on the negative one ?

    2. How do i interpret datasheet info ? What do all the voltages mean ?

    3. How exactly do they work ? Do they first send a higher voltage pulse, cut it off when gate reached desired voltage, and immediately connect a continuous stable voltage source ?

    4. Do they work for both turn on and turn off ?

    Thank you very much in advance. :)
    Last edited: Dec 4, 2009
  2. hgmjr

    Retired Moderator

    Jan 28, 2005
    I suggest you track down the manufacturer's datasheet for the high-side or low-side driver you have chosen and read it. You may want to check out the manufacturer's website for any application notes specific to your chosen mosfet driver.

  3. SgtWookie


    Jul 17, 2007
    That's sort of the general idea.
    MOSFET gates have a "gate charge" that is given as nC's or nano coulombs. There's a total gate charge specification, and it's further broken down into Miller charge, etc. See a datasheet for a MOSFET of interest to get their individual specifications. Very generally, the gate charge is a function of maximum Vdss, Rds(on) and Id rating. The higher the Vdss, the higher the gate charge will be. The lower the Rds(on), the higher the gate charge will be. The higher the Id rating, the higher the gate charge will be. This is not a "hard and fast" rule, but just in general.

    To put it as simply as possible, the gates have capacitance. In order to charge and discharge the gates as quickly as possible, high current of short duration is required. The higher the charge/discharge current, the more quickly the gates can be turned on/off.

    High-side drivers came about because it was discovered that N-channel MOSFETs generally have a lower Rds(on) than P-channel MOSFETs do. But in order to get an N-channel MOSFET to turn on, Vgs needs to be +5v (logic level) or +10v (for standard MOSFETs). It was easier to use P-channel MOSFETs because you didn't have to use a positive voltage to turn them on; Vgs in a P-channel is negative or zero. But in the interest of efficiency, high-side drivers came into common use.


    It takes a lot of reading through to make sense out of them.
    But, look at Rds(on) in respect to Vgs, the Vdss rating, the Id rating,
    and the total gate charge. Usually, you will want to use a MOSFET that has a Vdss of at least 1-1/2 times the voltage you'll be using them with, and twice the current rating. Both of these affect the gate charge.

    They either connect a comparatively large capacitor to the gate to charge it up, or connect the gate to the source terminal to discharge it.


    However, high-side drivers won't keep the MOSFET gate turned on indefinitely. You'll need to turn the high-side gate off and back on at intervals to re-charge the capacitor.