Re: Another discrepancy (LTspice/switchcad3)

Mike Engelhardt wrote:

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.

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.

I had thought that i made it clear that i have 


That wasn't clear to me because the circuit you
posted clearly does not work.


The models show *square waves* !!
The models show *AMPS* of current !!
The model does not show anything near reality!



The difference is that you are using a different
MOSFET in simulation then the bench.  The simulation
results would agree with using a the MOSFET you have
on that schematic you posted.  Now you're here talking
about different MOSFETs.

LTspice is showing what reality would look like for the
parts you have in the schematic.  Your bench circuit is
entirely different.  The schematic is totally shorted
out with capacitance, your bench circuit apparently
not.  See the corrected schematic I posted with
capacitance-free switches.


The classic definition of an inductor is a (linear)
component that opposes a change of current.



And the simulation you posted does show that.  But you
have to look at the current in the inductor and
the voltage across it.  Neither the sense resistor
nor the switch current will follow that ramp, because
capacitive displacement currents swamp the inductor
current ramp.

--Mike


Oh, then no matter what inductor i use, no matter what the time scale i use, that is the excuse?
Put a resistor in series with the inductor and see what that current is; you will get the *same* square wave as every where else.
BTW, i originally used a FET from those available in the software package, and not an ideal one - same result.
An ideal switch VS a slow, high capacitance FET like the IRFBG20 makes little difference on the workbench; current thru an inductor will be a ramp - pure and simple.
My tests show the "slow" turnoff, wider flyback pulse with no "flat" top when using the IRFBG20.
The same test fixture, with everything else the same - but using an IRLZ24N shows a reasonably fast turnoff, a reasonably narrow flyback pulse, and the classic flat top.
The model shows only square waves.
No ramp, no flyback pulse!
With an ideal switch, one cannot say that the inductor energy "got swampped" to explain away the loss of a flyback pulse.
Be at least a little realistic...


.

Quantcast