# Stability issue: Compensation changes using Bode plot in LTspice

I am encountering issues with a LT8390 design, particularly in higher load current conditions, which leads me to suspect a stability problem. The design parameters are as follows: vin 18-36 (28v nominal), vout=20v, fsw=150kHz, maximum current=20A. I conducted simulations in LTspice and utilized .fra to generate a bode plot.

Here is the simulation:

I understand that for stability, the design should have phase margin>60, frequency bandwidth<1/5 switching frequency. Examining the bode plot reinforces my suspicion of instability. The phase margin is almost zero!

Bode plot:

Despite my awareness of the impact of output capacitors on pole and zero positions and the presence of a compensation circuit to address the output zero and pole, I am unable to achieve a phase margin close to 60 degrees, regardless of the capacitor values I select, with Rth=R9, Cth=c2, and RTp=C1.

I seek guidance on how to stabilize this circuit. Is there any rationale behind modifying the compensation (Vc circuitry) that could assist in achieving stability? I used the method explained here: Generate a Bode Plot for an SMPS in LTspice 17.1

I utilized LTPOWER CAD as recommended by @Voltage Spike, and now it appears that I am experiencing transient issues:

• You need to increase the amount of points on the AC plot. Commented Feb 21 at 20:38

I have designed several circuits with the LT8390, including a variable voltage output converter for peltiers that could source and sink current.

An ac analysis can't be run in LT spice for a DC/DC, you can do a transient simulation. The AC response won't make sense because a solver needs to do both, it needs to run the switching which changes the AC response.

So how do you overcome this? Use LT power cad or use SIMetrix simplis (Chris Basso has a lot of free models for this, check his webpage (I am not affiliated) )

LT Powercad will have models for most Linear power converters and will give AC response calculations and help with design.

SIMetrix is very useful because it will simulate both AC and switching.

• @ Voltage Spike♦I employed LT POWER CAD for LT8390. Even though I can adjust the phase margin and bandwidth within the desired range, the transient response depicted in the plot appears to be significantly high. This led me to consider using the method explained here: Generate a Bode Plot for an SMPS in LTspice: ez.analog.com/design-tools-and-calculators/ltspice/w/faqs-docs/… Commented Feb 21 at 21:10
• .fra analysis was added in LTspice 17.1 and is what is shown in the question. Is it not sufficient for solving this type of problem, or are you not yet familiar with the new .fra feature? I’m asking because I know nothing about doing this kind of SMPS analysis, and would like to know where .fra wouldn't work. Commented Feb 21 at 22:17
• It's clearly not working. Its a new feature, I don't know if I'd expect it to work. It may be having trouble finding the circuit parameters for the LT8390. Try configuring it again. Commented Feb 21 at 22:30