LM51561-Q1: SEPIC 12Vnom input - 24V PSpice for TI output subharmonic oscillation

Mohammad Alnabhan

Part Number: LM51561-Q1
Other Parts Discussed in Thread: PMP22817

I'm designing a 12V-24V SEPIC using LM51561-Q1 controller following the SEPIC excel sheet calculator.

I understand since the D>50%, special care should be giver for current reference slope compensation.

I followed the values calculated in the excel sheet calculator but the results from simulation always show undamped oscillations in inductor currents in steady state.

I even simulated the SEPIC in the PMP22817 reference design, and the simulation result (pdf attached) also shows oscillatory average inductor currents.

Is this behaviour related to the simulation or the values calculated for COMP network?

pmp22817a.pdf

over 2 years ago

0 Stefan Schauer over 2 years ago

TI__Guru 50851 points

Hi Mohammad,

thank you for using the E2E forum.

Can you also share you Simulation project, so that I can simulate and check on that?

Best regards,

Stefan

0 Mohammad Alnabhan over 2 years ago in reply to Stefan Schauer

Prodigy 10 points

Hi Stefan,

Thanks for your prompt response. Please find the sim project attached - let me know if it's not compressed correctly.

Regards,

Mohammad

PMP22817.zip

0 Stefan Schauer over 2 years ago in reply to Mohammad Alnabhan

TI__Guru 50851 points

Hi Mohammad,

thanks the project is OK.

Please allow me 2-3 days to look into this - PSpice simulation take quite some time.

Best regards,

Stefan

0 Mohammad Alnabhan over 2 years ago in reply to Stefan Schauer

Prodigy 10 points

Hi Stefan,

Have you got any insight on the matter?

Many thanks,

Mohammad

0 Stefan Schauer over 2 years ago in reply to Mohammad Alnabhan

TI__Guru 50851 points

HI Mohammad,

sorry for the long delay but this simulation took very long time.

The oscillation you see here are the resonant frequency of the inductor and the coupling cap.

Best regards,

Stefan

0 Mohammad Alnabhan over 2 years ago in reply to Stefan Schauer

Prodigy 10 points

Thanks, Stefan.

Is this a normal behaviour? Or should it be mitigated in a certain way?

Regards,

Mohammad

0 Stefan Schauer over 2 years ago in reply to Mohammad Alnabhan

TI__Guru 50851 points

HI Mohammad,

i would like to double check this with one of our SEPIC experts.

Best regards,

Stefan

0 Mohammad Alnabhan over 2 years ago in reply to Stefan Schauer

Prodigy 10 points

Thanks, Stefan!

Of some circuit simulations I've run, this can be mitigated by using coupled inductors instead of discrete inductors.

My concern on PSpice for TI simulation is that those oscillations are undamped (even when I try to simulate designs from TI reference designs, designs from Webench or from TI Excel sheet calculator).

Regards,

Mohammad

0 Stefan Schauer over 2 years ago in reply to Mohammad Alnabhan

TI__Guru 50851 points

Hi Mohammad,

you are right - i also just got this info and tried with simulations.

You get rid of that if you are using coupled inductors.

This is really pushing energy from one side to the other and would also give very bad efficiency, but it is not an instability of the feedback loop.

Best regards,

Stefan

0 Stefan Schauer over 2 years ago in reply to Stefan Schauer

TI__Guru 50851 points

Hi Mohammad,

I have not seen an update for the last 14 days, so I assume the questions are/or answered and the issue is solved.
I close this thread now. If there is still something open reply and the thread will get open again.
If you have any other question or of the thread gets locked, please open a new one.

Clicking the Resolved Button also helps us to maintain this forum.

Best regards,
Stefan

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LM51561-Q1: SEPIC 12Vnom input - 24V PSpice for TI output subharmonic oscillation