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MOSFET going to continuous conduction

ravi kondeti

Hello Guys,

Following is a circuit i am working on. input is from a isolated DC-DC converter (flyback configuration) set at 12v. using bk precision electronic load i am testing at 500mA. i wanted to observe the dynamic behaviour of flyback converter while switching the load at higher rate so i kept a mosfet rated for 60V,2A. while switching observed that current is peaking and FET is gone. later changed FET to a higher rating one (TK4R1A10PL). even then MOSFET after sometime goes into continuous conduction even when i am removing gate pulses (using a function generator to give pulses), this is happening when i am increasing the frequency of the gate pulses (5-10kHz). using function generator a wrong approach? or any corrections i need make. please suggest

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  • 0

    Hi,

    Continuous conduction means the MOSFET is damaged, one or more parameters was exceeded. To troubleshoot, you'd need a 'scope probe to see the gate voltage and the drain with respect to the source. It could be as simple as needing a 50 ohm resistor wired between the gate and source (otherwise the gate sees a signal which is different from what you have configured on the function generator, but you won't know that without looking at it with a 'scope). Also it will be noticeable at high frequencies that the function generator impedance might not charge/discharge the MOSFET gate fast enough. Also, long wires will act as inductors, causing voltage spikes when the MOSFET switches off. There are techniques to eliminate that, you could try a ferrite beads.. but really you're flying blind without looking at it. Before the MOSFET blows, start at a lower voltage or frequency and record the signals from your 'scope, and actually see what is causing it.

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  • 0

    Hi,

    Continuous conduction means the MOSFET is damaged, one or more parameters was exceeded. To troubleshoot, you'd need a 'scope probe to see the gate voltage and the drain with respect to the source. It could be as simple as needing a 50 ohm resistor wired between the gate and source (otherwise the gate sees a signal which is different from what you have configured on the function generator, but you won't know that without looking at it with a 'scope). Also it will be noticeable at high frequencies that the function generator impedance might not charge/discharge the MOSFET gate fast enough. Also, long wires will act as inductors, causing voltage spikes when the MOSFET switches off. There are techniques to eliminate that, you could try a ferrite beads.. but really you're flying blind without looking at it. Before the MOSFET blows, start at a lower voltage or frequency and record the signals from your 'scope, and actually see what is causing it.

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  • 0 in reply to shabaz

    hello shabaz,

    i was observing the current through the MOSFET and voltage output of the flyback converter. there is a current spike of 8 times the set load current. unfortunately i was not able to save those plots as i was not sure what was happening in the moment. i will repeat the test with a plain R load and shorter leads and try to check on the parameters properly. i will try to replace the function generator with a proper gate driver and check again.