I uploaded my "TSS Inrush Suppressor Relay Driver Rev 4.asc" file to the group temp folder, so that interested parties can follow my inquiry. I would like to be able to model a DN3545 n-channel depletion-mode Vertical DMOS FET for use in the circuit described below:
The circuit uses a time-delayed relay to bypass a couple of parallel 27 Ohm current limit resistors in series with the AC mains to limit the inrush surge to an audio power amplifier.? (The Resistors are not shown in the simulation, just the relay coil (R_Relay_Coil). When AC Mains stops, the relay is immediately released to prepare for the next surge.
The circuit uses a MID400 ac line monitor logic level opto-coupler, to control Q4 which prevents the timing capacitor C4 from charging until AC mains is detected, and also discharges C4 immediately when AC mains disappears.? Q1 + Q2 form a discrete Schmitt trigger to provide a fast turn-on and turn-off to the Relay Coil.
I simulated the effect of the MID400 opto-coupler using a pulsed voltage source since no model was readily available.? The actual MID400 in my circuit will run on the +5V6 supply generated by the bottom Zener Diode D9. J1 is used during start-up to supply current to D9 so it will follow the charging of the main filter cap C2. Without the Pull-Up circuitry, +5V6 rail does not function until C2 has reached +24V2, when the entire string of zener diodes begins to conduct almost 1 full second after mains is applied.
J1 conducts only during turn-on and turn-off, then pinches off when the series zeners begin to conduct.
The 2N5486 JFET is only a 25V part used in a 24V circuit.? Typically ist has Vds of about 18V.
I would like to use a DN3545 n-MOS, Depletion-Mode Mosfet for this application instead of the JFET, but I don't know if LT spice even supports depletion mode mosfets.
Alternatively, I would be happy if someone sees a more clever way to pull up the D9 5V6 Zener during power-up and power-down to immediately give the MID400 power. It must only be active during turn-on and turn-off of the AC mains.
Thanks for your kind help.? (BTW I do not have the math or LTSpice chops to translate the DN3545 datasheet numbers into a model on my own.? I could likely draw a schematic symbol for the depletion mode mosfet.)
Steve Hogan

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Steve,

I think depletion mode FETs have been a part of SPICE since day 1 (early 1970s). LTspice inherits SPICE's long heritage and continues to support them.

Don't confuse LTspice's lack of a depletion-mode MOSFET symbol as an indication that it can't use depletion-mode MOSFETs. The appearance of the symbol has no real meaning. It's only an icon. I guess Mike Engelhardt gave us only that symbol because the vast majority of MOSFETs are enhancement-mode. You are welcome to create a MOSFET symbol that looks like a proper depletion-mode transistor to make your schematics look better. (That's been on my low priority wish-list for a long time now.)

If all you have is a datasheet (no SPICE model), you can PROBABLY create a model by using Hendrik's VDMOS tool, which is here:

Files > Util > Model Tools > Board Level MOSFET (VDmos)


I do not know if that tool handles depletion-mode MOSFETs. But I think it's one of the two best ways to come up with a MOSFET SPICE model if you have none. The other is to find a SPICE model for a closely similar MOSFET. And then, there is Mr. Bordodynov, who seems to have models for almost everything, and a skill for creating them.

Regards,
Andy