MOSFET breakdown prevention in a boost converter

I suggest that you change your design so that the drain never sees a voltage anywhere near its maximum.

You seem to think that the MOSFET turns off instantly and that there is no capacitance. The capacitance is likely to be very significant.

Ok, I am thinking about it. I guess I would like to know whether the rated maximum is for *any* duration, however short, or there is some "uprating" as the duration decreases, maybe some some bound on the integral of the voltage-voltage_max over the duration.

No, I don't think so. There *is* a capacitance, and the turn-off time is finite, but the spike - as determined via a simulation - is still rather high.

Put a tap in the inductor not far from the output. This will reduce the voltage swing on the MOSFET, but increase the current slightly. Bear in mind that the breakdown voltage often has a negative temperature coefficient.

Graham H

The datasheet may give you that information. An avalanche rated MOSFET will withstand some over voltage and act like a zener. A non avalanche rated part may be destroyed instantly.

What kind of part is this and what is the circuit topology? In a normal booster, the output diode clamps the inductor's swing.

Actually Vds limits for discrete mosfets increase with higher temperature but there are very few app environments that can benefit from this feature. Discrete mosfet voltage breakdown is not catastrophic, only energy-limited (as per TVS-characterized zeners) and moisy. The self-heating characteristic of this breakdown limit can even be self-regulating at really low temperatures.

Integrated mos structures may show the limit characteristics of the host structure, however, where breakdown behavior is not so predictable. By reducing layout loop area, adding current-snubbing and using rectifiers with low forward overvoltage (schottkys), you should be able to approach paper limits in boost converters, if transient response (start-up and transient output voltage overshoot) is well-controlled. Larger than nominal output capacitance may help.

Some power integrated circuits display a odd behavior in the presence of a range of dv/dt or di/dt values - you should make enquiries with the device mfr about any unexplained behavior. There may be a simple empirical solution, whether or not the mechanism is fully understood, or whether an explanation is forthcoming.

RL

Hello, I am *very* sorry - I've mixed up the things ... The switch is not a mosfet but a bjt :-| The rest of the problem applies.. The part is a L T 1 2 6 8. This is a normal booster, and yes the diode forwards the inductor's swing to the output (capacitors). The spikes I am talking about are due to the (several tens of nH) parasitic inductances of the paths across the switch and across the output diode and capacitors.