How to Use the fra Analysis in LTSpice for an Isolated Topology

Additional information: 50V input, 48V output @ 12.5A [600W]. The duty cycle is extremely high, which makes it challenging to stabilize the loop. It's running out of slope compensation.

Additional information: 50V input, 48V output @ 12.5A [600W]. The duty cycle is extremely high, which makes it challenging to stabilize the loop. It's running out of slope compensation.

Hmm...I'm wondering if this is more an issue for technical support directly?

Hi 20K ,

This appears to be a duplicate thread. I've moved both to LTspice. Let's continue in this thread. If it's OK with you, I'll delete the other one.

In the meantime, please upload your files. This design may require use of the fraprobe.

mike

LTC3722-VPX_2_1.asc

1172.tl431.zip

Here is the file. I also included the TL431, which will be needed. Take note that sometimes when opening the file for the first time, the "GND" and "PGND" nets can shift, leading to inaccurate results.

Hi 20K ,

Will take a look. In the meantime, check out this in-depth webinar on FRA:

 LTspice®︎ 24 Webinar: Faster, Better, Easier Circuit Simulation 

Skip the LTspice 24 stuff and go to the FRA section and Special Cases and FRAprobe near the end.

mike

Hi 20K ,

I've passed this on to our resident LTspice FRA guru.

A note about PGND and GND. You can use a separate PGND net and define it as connected to GND via a very high value resistor and a low value cap.

I also, put the Infineon library in a file and removed it from the schematic, adding a .lib statement pointing to the new file.

mike

I've reviewed the webinar and found something which appears applicable on the last slide of the presentation, as it is an isolated topology. I noticed that it isn't using the FRA probe as the other examples but instead is using behavioral voltage sources feeding into a behavioral sample & hold function block. I tried this setup and I am getting errors: 

C:\Users\...: Expected node name here.
Vhigh=100
^^^^
C:\Users\... Expected device instantiation or directive here.
Vhigh=100
^^^^^^^^^
C:\Users\...: Expected node name here.
ref=8m
^^^
C:\Users\...: Expected device instantiation or directive here.
ref=8m
^^^^^^

Obviously, something isn't correct about the setup. There wasn't much information to be gleaned from that slide in the presentation, so I had to make some educated guesses. I attached a screenshot of the setup in-circuit.

Any word?

Hi 20K ,

I'll check. Thanks.

mike