Simulating inductor with double transistor switch in LT Spice, getting unexpected voltage output. Problem with model?

Thread Starter

avarner

Joined Sep 5, 2021
2
Hello!

I'm using a SIMFET model as switches and ideal diode models in my circuit. My specifications are t0=40ms and T=100ms. I expect the voltage across the inductor to be Vcc=75V from 0 < t < t0, and for it to be -Vcc when the transistors are off between t0 < t < T (This is the theory presented in my textbook for this circuit, Daniel Hart's Power Electronics)

Instead, the voltage goes to a weird roughly -1mV at t0, and it shoots up to Vcc/2 at 2t1. Does anyone have any insight on this? Maybe there is something wrong with my model or my understanding? Thank you!photo.PNG
 

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Papabravo

Joined Feb 24, 2006
18,408
Von and Voff are not defined for the voltage controlled switch. You should read the help page for the valid parameters for a switch. the behavior you are seeing is probably due to the actual default values for the switch component.
 

ronsimpson

Joined Oct 7, 2019
2,062
For t=0 to t=40mS there is 75V across the inductor.
For T=40mS to 80mS the top of the inductor goes to 0V. (Very close to 0V) AND The bottom end goes to +75V. (or very colse)
You have +75V then -75V then at 80mS the voltage should be 0V. This is what it should do.
 

Thread Starter

avarner

Joined Sep 5, 2021
2
This is solved now. I was measuring the node-ground reference, but I just wanted to see the voltage across the inductor. I added node references to see what I wanted to see. Also I did define Von and Voff in my .model directive for the SIMFET. Thank you!
 

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Papabravo

Joined Feb 24, 2006
18,408
What is the purpose of continuing to use Von and Voff in the switch definition. what was your intention in picking the values that you did?
 

MrAl

Joined Jun 17, 2014
8,984
This is solved now. I was measuring the node-ground reference, but I just wanted to see the voltage across the inductor. I added node references to see what I wanted to see. Also I did define Von and Voff in my .model directive for the SIMFET. Thank you!
Hello,

You really should show your switch switching pattern with the sim waveforms.
That way you can get a good feel for how this works.

Since you solved this already, the inductor current response is:
iL(t)=-(t*E2)/L+(t*E1)/L+iL(0)

where iL(0) is the initial current in the inductor.
Note you set E1 and E2 as required based on the switch modes and the diode switching modes. For each part of the cycle you get a new iL(t).
This assumes continuous operation.

If you add a series resistor it gets a little more complicated but not much.
 
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