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LM5022: Oscillation concern/question on SW node

Part Number: LM5022
Other Parts Discussed in Thread: PMP7338, PMP20676, PMP20468, PMP20127

My customer has a Flyback design based on the LM5022 and is concerned about the oscillation he sees on the SW node after the controller turns off the primary side power FET:

This is an isolated supply.

 

I’m seeing some oscillation on the switching node on both the primary and secondary side of the transformer.

 

I’ve attached a .pdf of the power supply schematic.

 

I’ve also attached a .pdf of the schematic, annotated with scope shots of the relevant signals.

 

All of the probed signals look fine, except the switching nodes.

 

The oscillation occurs when I raise the input voltage to 48V DC. As I ramp the input voltage down from 48V to 20V, the oscillation gets smaller, and eventually goes away.

 

Is there a way for me to get rid of the oscillation?

I don't believe this is an issue and have pointed my customer to the PMP7338 reference design 'Test Report' to show that the 'oscillation' is normal. (SNVU139)

Though the PMP20468, PMP20127 and PMP20676 would be better examples, as they are true Flyback designs, while the PMP7338 is a SEPIC.

Feedback on the attached schematics and scope shots would be great. Any App Notes / Articles that explain the LM5022 Flyback operation would also be great and would be of help to me as well!

Thanks!

LM5022_probed.pdf

LM5022_schematic.pdf

  • If you mean oscillation at the tail of the switch node voltage, marked "UNEXPECTED OSCILLATION", you should be assured that it is both normal and expected. It is because the flyback runs in discontinuous conduction (DCM) mode, and the oscillation basically is the natural resonance between the leakage inductance and the MOSFET Coss. This occurs in all flyback if the rectifier is just a diode but not a synchronous rectifier (FET).

    In short, nothing wrong. Such behavior is well documented in literature and textbooks.
  • Yes, that is what I was referring to and this answers the question.