PWM Switched FET - Oscillations at 50% Duty Cycle

[IanW 0]

Hi, I am using a TECO SG2 PLR to produce a PWM output that switches a MOSFET smart switch (VNB35N07). I have tried PWM periods of between 200mS and 1000mS. The MOSFET is switching an inductive dump load across the output of a small generator. The switching works well when the duty cycle is up about 40% but when the duty cycle approaches 50% the MOSFET starts to oscillate at 500Hz causing incorrect dump load operation and also excessive heat in the device. I wonder if anyone has experienced similar problems when using PWM FET switches. The drive signal looks quite clean the gate but the oscillations appear across the load resistance when monitored with a scope. Does anyone know the reason for this behaviour and possibly a cure ? Any help would be greatly appreciated Kind Regards Ian

[Shrek 0]

i am familiar with pwm control but your case is unique in my studies

[Ice 1]

hi there I dont know if this will help, but i once had a prolem an inductive coil saturating, try maybe a pull down resistor that when the output is off you get a proper off condition.

[panic mode 246]

VNB35N07 is "autoprotected" (it is not just transistor, it has built in circuitry which is activated in certain conditions). i'll put my money in that it's the overvoltage clamp that gets active every time transistor turns off. a) study circuit and design proper supression for used load b) replace VNB35N07 with compatible transistor without the "autoprotect" logic c) use properly rated transistor