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OUTPUT TRANSISTOR SATURATES ON INCREASING PWM--BOOST CONVERTER

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Achu:
Hi,

I've been playing with boost converters based on TL494 and also arduino.
But when I increase the pwm over 40% the output Mosfet stops switching and saturates drawing heavy current and I burned three of them already.I have also tried this on bjt with same effect.
I used irf540,irf840 and also mje13007 bjt.
Same thing happens on arduino based circuit.
I hooked the pwm directly to the gave via a resistance.The gate is of course pulled down by a resistance.

I'm no expert on switching power supplies.

help me out guys,its driving me crazy.

Buriedcode:
With a boost converter, the on time (low side switch) determines the peak current in the inductor.  The longer the on time, the higher the peak current.  Inductors saturate at a certain current, and depending on the type of core, this can be gradual or sudden - this means the current will ramp up linearly, then suddenly increase.   

Saturation of the inductor isn't desired, so you must either 1) increase the inductance - which slows down the current ramp so the peak current is lower for a given period. 2) Increase the frequency - reducing the switching period which will reduce the on time - or  3) use a large core where the saturation current is higher for a given inductance.

Put a scope on a low value/current sense resistor in series with the inductor and slowly increase the duty cycle - you will see the current rise linearly, increasing as you increase the duty.

Altohugh I probably shouldn't link to other forums, this post has some valuable images that demonstrate this: https://electronics.stackexchange.com/questions/83072/calculating-the-saturation-current-of-an-inductor

Achu:
Could it be because of the lack of a gate drive circuit.
Could the persisting gate charge keep transistor turned on?,beacause when we increase pwm width the gate does not have enough time to remove the charge.
Also I'm using an inductor from SMPS.

Siwastaja:
You need a current sensing circuit and pulse-by-pulse limiting. Usually a shunt resistor between the MOSFET source and GND is used.

Without it, it's hard to get working reliably.

DannyTheGhost:
Either way, your inductor enters into saturation mode
If you could bring more details about your design: output power, what exactly inductor you are using etc., maybe we will have more chances to help you
If you have small-power design, the probability to get Miller effect (MOSFET latching 'on' during closing period) is close to none

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