I am using this program for a simulation for a pre-lab and I can not figure out how I am supposed to change this to be able to print out a graph of the Vo. It keeps giving me an error of "Error in transient solver, illegal start or step input", but I have no idea what that means. I haven't used a MOSFET before and followed exact directions for the parameters that were given by my professor. Is the square voltage source wrong? It should be between 0V and 5V with 50% duty cycle and 100uS period, so I just inverted that to 10000Hz and assume 50% duty cycle is automatic.

by mgetman
October 24, 2020

I entered the following values for the Time Domain Simulation for your circuit:

Start Time: 0

Stop Time: 1m

Time Step: 1u

Expressions:V(Vin), V(Vo)

The graph showed two plots: +/-5v and (almost) 0v to 3v @ 50% duty cycle. Looks good to me! Please post back if this is not what you're expecting.

by EF82
October 24, 2020

Often the transient solver error you have indicates that the Start, Stop, and Time steps are not consistent. EF82's suggestion is consistent and brings you down to a time interval to see the circuit action, hence useful output.

If you do not mind a couple suggestions - First, I find it useful to specify in the sq. wave generator: Amplitude = 2.5, and DCoffset = 2.5. This gives a 0-5 volt step. With the +5 to -5, and no offset, as you have it specified, the 0 to -5 doesn't really help you, and will be different from a real-world circuit unless you are using a dual supply. Second, I notice that your mosfet SUM75N06 has almost no parameters in the spice model which will give you a very generic model performance. You have two options: has both their datasheet and a spice model they recommend. You can try adding appropriate parameters from their model to make a custom model in CircuitLab. Or, you can look for a mosfet among the selections offered in CircuitLab which is fairly close to the switch you desire to use. I would pick the STP55NF06L model which is populated save for the Lambda parameter. It should give you very similar results (albeit only 55A vs 75A) to a SUM75, especially when you start adding other circuit elements to you circuit. Hope this is helpful. John

by jaf2009
October 24, 2020

1 Answer

Answer by mgetman

I literally just fogot to enter teh stop and time steps. This is supposed to be pretty simplistic just to give us a rough idea, hopefully matching a switch with an Ron resistance of 500 ohms. I got it to work, though my Vo is less than I thought it would be, but still a low voltage. Also I fixed my square wave because it needed to be 0 to 5, so A=2.5, offset=2.5, just like you said. Thankyou for the feedback though.

+1 vote
by mgetman
October 24, 2020

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