Mosfet gate discharge resistor problem

Discussion in 'General Electronics Chat' started by yasinji, Dec 29, 2010.

  1. yasinji

    Thread Starter Member

    Mar 25, 2009
    I am driving two mosfet working in push pull mode(Electronic CFL ballast) directly from microchip microcontroller I/O output with 25mA source current. I have used a gate resistor of 22ohm and gate discharge resistor of 1K. The mosfet I have used is IRFZ44N the datasheet attached.
    g Total Gate Charge ––– ––– 63 nc
    Qgs Gate-to-Source Charge ––– ––– 14 nC
    Qgd Gate-to-Drain ("Miller") Charge ––– ––– 23 nc
    td(on) Turn-On Delay Time ––– 12ns
    tr Rise Time ––– 60 ns
    td(off) Turn-Off Delay Time ––– 44ns
    tf Fall Time ––– 45 ns.

    The problem is the CFL is not turning on with 1K gate discharge resistor.
    When I increase the gate discharge resistor to 10K the CFL is working ok.
    The switching frequency is 38Khz. Can please anyone tell me why this is happening.
  2. marshallf3

    Well-Known Member

    Jul 26, 2010
    Too tired to research it but why not just use the values that work?

    I'm sure an expert in the matters of gate charge will come along and provide an answer before tomorrow is over here in the US.
  3. timrobbins

    Senior Member

    Aug 29, 2009
    You must be in a marginal gate voltage drive level situation. You probably have competing criteria for turn-on and turn-off. Faster turn-off requires lower value discharge resistance. Lower Rds-on requires higher value discharge resistance. Tricky to optimise unless you want to do an efficiency test, and then change the resistor value and compare efficiencies.
  4. edgetrigger


    Dec 19, 2010
    with schematic it would be easy to analyse. 1k may be bleeding the gate driver so that gate of the mosfet is not getting required voltage to saturate the mosfet. You could check the voltage at gate with 1k / 10k and see it for yourself if in 1k the Vgs is falling short to switch on the Mosfet.
  5. jpanhalt


    Jan 18, 2008
    The IRFZ44 is not logic level. Its specifications are given at Vgs = 10V. You need the IRLZ44 or similar. Why do you need a turn-off resistor? If it is to ensure the mosfet is off when power is first applied to the circuit, you can use a larger value, if needed. That still doesn't change the fact that the IRFZ is not designed for operation at 5 volts for the gate.