Re: Another discrepancy (LTspice/switchcad3)

Robert,

>>>>>The results are way off.
>>>>
>>>>The simulation is probably right around correct.
>>>>That FET and diode have capacitance.
>>>
>>> Adding a small capacitance has no effect.
>>
>>
>> But that doesn't remove the capacitance that's already
>> swamping the circuit. The problem is the circuit design,
>> not the models.
>
> There is no problem with the design. Both theory and
> practice show that when a voltage is presented across
> an inductor, that the current starts at zero and increases in the
> standard L/R exponential "decay" to maximum, the
> assumption is a linear inductor. Classical definition of
> an inductor! The problem, is that the model does *not*
> show that.

The current does ramp in the inductor when there's voltage
across it in the simulation. But the capacitance(and other
problems) prevents your circuit "design" from working.

Anyway, the "design" you have will not work. The simulation
is correct. If you build it, you will see the same waveforms
as in the simulation until the MOSFET dies due to being
over-voltaged. MOSFET catastrophic failure is not modeled
in LTspice in hope that this allows you to separately debug
the design and then find parts that don't die in the
circuit's operation.

> And a pF or two cannot "swamp" anything here.

Not a pF or two. A 1.5nF for the MOSFET and 126pF
for the diode. It's hard to run a power SMPS down to
the tiny currents you're trying to do.

Regards,

--Mike


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