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Gradually switching a FET ON

Discussion in 'General Electronics Discussion' started by eem2am, Mar 23, 2013.

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  1. eem2am


    Aug 3, 2009
    Hello, i periodically (every 10 minutes) need to switch a fet on (ON for 1 minute) which delivers 400mA current from the secondary of a current transformer.

    The primary contains a 50KHz sinusoidal current (2A9 Average), which is produced by a highly resonant LC tank circuit.

    Because of the resonant nature, i cannot simply and suddenly switch the switch from OFF to ON (because it would excite resonances in the resonant system)......but instead i must PWM the switch ON/OFF/ON...etc until its ON all the i must gradually increase the dutcy cycle from 0 to 100%, until its ON all the time.

    I need the gradual switching interval to be 5ms.

    Do you know of a way of doing this?
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Can you place an inductor in series with the mosfet to limit di/dt to a reasonable value?

    Of course this will also affect how you switch it off. You can either place a diode across the inductor to dump the energy when the mosfet is switched off, or you can place a diode across the entire load and the inductor to allow current to flow until it decays.

    Re-reading this again, it sounds like the mosfet is the load. Presumably the output from the current transformer is rectified (bridge rectified?). In this case you're still going to bees a diode (across the inductor) but the secondary current will fall to zero quickly, and that's going to generate its own problems.

    A zener from drain to (suitably protected) gate can be used to turn on the mosfet sufficiently to clamp the voltage across it.

    I'm not sure if this is any assistance.
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