Flyback open loop transfer function in LTSpice

[steinar96]

Greetings.

I have a flyback with a pwm controller in LTSpice. I'd like to plot the open loop transfer function in order to aid me in designing the control loop compensation. How is this best performed in LTSpice. Can i somehow sweep the feedback input pin of the controller or do i measure the output stage somehow independently ?

Assistance greatly appreciated.

 

[crutschow]

Typically you open the loop and do an AC gain/phase (Bode plot) simulation from input to output.

 

[steinar96]

I have opened the loop. But if i sweep the input (the comp pin) i do not get the bode plot i'm expecting. It starts at -140dB

 

[LvW]

I have opened the loop. But if i sweep the input (the comp pin) i do not get the bode plot i'm expecting. It starts at -140dB

Yes, simply opening the loop will destroy the dc bias point. For such a task there are specific simulation procedures that have been discussed - also in this forum - several times.

 

[FvM]

I guess you are looking for the continuous time transfer function of the control loop, including modulator and power stage? It can't be easily "measured" in LTSpice, at least not in AC simulation.

It has to be derived from a transient simulation with suitable sine or step stimulation. You would be able to add an auxilary measurement circuit to your simulation setup that filters the response signals and determines it's magnitude and phase, similar to a network analyzer.

Usually, flyback transfer function is put in the calculation according to models derived in literature.

 

[crutschow]

Post your circuit and .asc file.