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Working with Mosfets

Discussion in 'General Electronics Chat' started by azgreenb, Sep 19, 2008.

  1. azgreenb

    Thread Starter New Member

    Sep 18, 2008
    I've been tasked to upgrade a board to provide higher current MOSFETs to control a foil type heater and a door solenoid. I've had little experience with MOSFETs, but understand how they work. Here are my questions:

    Will this driver (IXYS IXDN404SI) be able to activate either of these FETs (Fairchild FDT55AN06LA0 or ON-Semi NIF5003N)? Not sure of the cycle time for the Heater but the door solenoid will open for 1sec then close, open for another 1 sec then shut down the system.

    2nd question more basic. How do you select the series resistor on the gate side of the MOSFET? What are the criteria that you look at?
  2. azgreenb

    Thread Starter New Member

    Sep 18, 2008
    Found an answer to my 2nd question:

    "On page two in application note AN6048 we discuss the addition of a series gate resistor to reduce the tendency of the MOSFET to oscillate at high frequency. As a general rule of thumb, the RC time constant of the gate-resistor and input capacitance of the MOSFET should be 3 to 5 times the combined "turn-on" and "turn-off" times. For example a nameless MOSFET data sheet has typical rise-time of 50ns and typical fall-time of 75ns. The input capacitance (Ciss) for that MOSFET is from 7000pF. Thus an appropriate gate resistance would result in a time constant of 5 * 125 ns or 625ns. Doing the math results in 89-ohms, thus a 100-ohm resistor would be fine. Remember that the HVOUT charge pumps provide a constant current of less than 50uA, so 100-ohms would only drop 5mV and work just fine to reduce RF-oscillation."
    Last edited: Sep 19, 2008
  3. scubasteve_911

    AAC Fanatic!

    Dec 27, 2007
    Gate resistor selection is probably one of the last things you should choose. Usually the current doesn't guarantee operation, it's more about power dissipation. Then, you need to make the tradeoff between switching losses and conduction losses. Only then, you can decide on a driver for it. After that, you can figure out a rough estimate for a gate resistor. Most of the time, they recommend you experimentally finalize a gate resistor's value. Sometimes, you need to use a separate charge resistance from discharge, using a diode.