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.


Oh, then no matter what inductor i use, no matter
what the time scale i use, that is the excuse?



No excuses, but you might apologize so getting so excited
about having the errors in hour schematic pointed out.
Anyway, the time scale is hard to vary because the 1619
wants to switch at 300KHz.  But if you remove the
capacitance, then you get want you expect, like the
other schematic I posted.


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. 


Eh, just click on the inductor to plot its current.  There's
nothing wrong there.  Drag the mouse across the inductor to
plot it's waveform.  Corresponds to the dI/dt.  Duh.


BTW, i originally used a FET from those available in the software package, and not an ideal one - same result. 


OK, the FET from the software package has capacitance modeled and
that that keeps your design as posted from working.  It's just
telling you your design doesn't work.  The ideal switch does
make the corrected version of your design convert power and
regulate.


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. 


Only if there's a square wave across it.  With the 300KHz 1619,
freq, you're a bit pressed to see the ramp in a milliHenery
for the current spike of the highly non-linear Miller cap
of the FET in your posted schematic.


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! 


It's a mistake to confuse flybacks with boost converters.


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...



I never said inductor energy gets swapped.  I said the circuit
was swapped with capacitance as in AC shorted out.  If you use
the capacitance-free switch you get square wave across the
inductor, a ramp in current through the sense resistor that
the 1619 can servo to regulate the output.  You haven't run
the corrected version of your circuit that I posted.  But with
the circuit as you posted, with a 1nF MOSFET and 100pF diode,
you won't see the ramp in the sense resistor because the current
is swamped by displacement currents.

The simulation is correct, your design as posted is the problem.

I can only tell what the problem is, you have to do a bit of effort,
say, running the corrected circuit as I posted.

--Mike


I have run every circuit you gave.
As i mentioned, i also used a simple test fixture with a 10uH inductor, 3.3V supply, and "slow" FET and saw a ramp and a good-sized flyback pulse - not the square wave stuff in the "1619" circuit.
A faster FET made for some speed improvements, but the basic ramp and flyback pulse was still there.
And would still be there even with a mercury wetted relay as a switch.

The simple FET + inductor circuit needed to have the "generic" FET specified; the "generic" does not work.
Used:

..model IRFBG20 VDMOS(Rg=3 Vto=3.8 Rd=3.6m Rs=2.7m Rb=5m Kp=120 Cgdmax=700p Cgdmin=30p Cgs=900p Cjo=200p Is=68p mfg=International_Rectifier Vds=1000 Ron=6 Qg=25n)

  One sub sank.
.

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