Some MOSFET Questions


I am developing two circuits - a H-bridge inverter and a boost converter, both of which use a MOSFET(s) with a PWM drive and resistive load. I observe:

a. The rise time measured across the drain and source *reduces* as the output load (as in more amps taken at a given output voltage) on these circuits *increases*.

b. On the boost converter, for very light loads, the drain to source waveform does not remain completely rectangular. The rising/leading edge of the switching waveform is correct, but the later part (just before the falling/lagging edge) turns "wavy".

I would like to know if the two behaviours described above are expected. Anybody else can confirm/deny that they have observed something like this? Any explanation of what could be happening is welcome.

Regards, Anand

Reply to
Anand P. Paralkar
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If the gate drive is sufficiently strong that switching is not limited by miller effect (a typical case when using snubbers), then the output voltage swing (commutation) occurs as a result of the load current, at the moment of switching, charging node capacitances (Cdss/Cjo, dV/dt snubbers). When Isw < C * Vcc / DT (DT = dead time), the remaining node voltage is commutated in hard switching.


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Reply to
Tim Williams

Yes, transformers do that when the load (secondary side) increases, since they are mutually sharing the same magnetic field, the load is causing the effects of lowering your induction on the primary side which gives you a faster raise from your supply.

Most likely ringing. Just think of it as stray capacitance from your fets and coil resonating and bouncing back and forth decaying on each cycle until there is no more. The off cycle of the fets allows the field to collapse and the L will release energy that gets things started. and things like that,,,,,etc,,etc


Reply to
Maynard A. Philbrook Jr.

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