LTSpice initial conditions help, please?

Hi all,
can someone familiar with LTSpice please figure this out? I'm trying to work out a simple "soft start" circuit for a simple transformer-rectifier-smoothing cap power source, that will be connected to a 60V high current transformer, to avoid any possibility of damaging a 1A bridge if it's turned on near a voltage peak.

For the sim I'm just using a fixed 90V DC input, on the basis that's going to be a higher surge that on rectified AC.

The problem is, the bypass FET is conducting at the instant the sim starts?? I've tried setting the initial condition on the caps, but to no effect. Any ideas appreciated! The component values are not final, just approximations until I work out the dissipation.

ps. I did figure out once how to plot power dissipation in a component - but now I cannot remember; it is two years since I last ran the simulator, from what it said about updates!

Circuit & plot - note the initial spike on the blue trace, which is current through P channel FET U1.
The green trace is the voltage on the main smoothing cap.



Simulator file attached.
The FET model is from the an Infineon model pack on github, here:

I think I know what your asking.
What you do is in the parameters for the cap, in the field for the voltage you type "ic=v".
v is your initial voltage.

ctrl<mouse click> with the cursor over the trace title gets you the power.
ctrl alt with the cursor over the component gets you the power.

rjenkinsgb said:

The problem is, the bypass FET is conducting at the instant the sim starts?

Click to expand...

When I run the sim (albeit using a different PFET) the FET switches on at about 80-90mS.

alec_t said:

When I run the sim (albeit using a different PFET) the FET switches on at about 80-90mS.

Click to expand...

When I run it, the wanted conduction starts at somewhere around 150mS; presumably different gate thresholds.

The problem on mine is that there is a high current pulse through the FET in the first couple of milliseconds, causing the capacitor voltage to rise abnormally - then it turns off until the gate cap actually charges to a suitable voltage.

Your run shows a smooth charge curve without that initial kick.

What FET are you using & what version of LTSpice? I wonder if the FET model is bad, or incompatible?

Lightium said:

What you do is in the parameters for the cap, in the field for the voltage you type "ic=v".
v is your initial voltage.

Click to expand...

That's what I though it should be? I have ic=0 to start the cap at zero volts, but the FET is still turned on in the first couple of milliseconds. As it works OK for Alec when using a different FET, it looks like a bad or incompatible model.

Also thanks for the tip about the power.

rjenkinsgb said:

What FET are you using & what version of LTSpice?

Click to expand...

FET = RSJ250P10. LTspice version x64: 24.0.12. TS's settings for the sim.

My sim:
I think you may have a faulty MOSFET model.