# LT8316 Flyback Bode Plot

#### mehmet.kl

Joined Apr 29, 2021
2
Hi everyone

I am tryig to obtain the gain and frequency plots of a flyback converter controlled with LT8316. LTspice does not provide any frequency response feature but I use middlebrook's method to obtain frequency response. However, result is too weird

I simply insert a small ac signal thru the FB loop of the controller and sweep that source. Controller tries to detect DCM thru FB pin and operate in bounday conduction mode. I think inserting this feature creates problem for me
So, Does anyone know how can I obtain the true bode plot? I also include the simulation model

#### crutschow

Joined Mar 14, 2008
29,511
The AC small-signal source does not work in a transient simulation.
You would need to use the large-signal Sine source.
How did you "sweep the source"?

#### mehmet.kl

Joined Apr 29, 2021
2
The AC small-signal source does not work in a transient simulation.
You would need to use the large-signal Sine source.
How did you "sweep the source"?
Actually, I've forgot posting the model that I use to obtain bode plot.

In this model, I inject a 10mV sine wave through the feedback loop. Internal voltage that is to be compared is 1.2 V in this pin. So I think 10mV should not affect the proper opration. There are some .meas commands at the right. In these commands, I calculate the gain and phase of the Va and Vb and compare them. That is the ac signal before and afterbeing exposed to the closed loop tf. In second code block, I define the required time for the system to reach steady state and simulation time as 25 cycles. In last block, I sweep the frequency of the small signal source. That was what I meant while saying "sweeping the source". Sorry for misunderstanding. For more detail, you can check this link: https://www.analog.com/en/technical...-steps-in-generating-a-bode-plot-of-smps.html

The fact that controller operates with variable frequency to operate in BCM and TC and DCM pins are affected this small signal source creates problem I think. But I don't know how to obtain the true bode plot