LM51551-Q1: Cross regulation of flyback

song.lin

Part Number: LM51551-Q1
Other Parts Discussed in Thread: LM5155

For the application of flyback, the primary side output(VPRI_5V0) current load will influence the second side output voltage VISO_P18V,VISO_N5V,VISO_12V0,VISO_5V0_REF.

The output voltage of second side will rise a lot when the primary side load rise.

The second side voltage rise bring the problem that the reference supply and zenor diode(in red circle) on the second side's temperature rise a lot.

Is this phenomenon the cross regulation? How can we handle this issue?

over 1 year ago

0 Stefan Schauer over 1 year ago

TI__Mastermind 46391 points

Hi Song,

thank you for using the E2E forum.

Yes, i think this is a problem of the cross regulation.
Cross regulation is heavily impacted by the Transformer design

You can check these reports, it gives some info about that:

Under the Hood of Flyback SMPS Designs Article

https://www.ti.com/lit/an/snva562/snva562.pdf

Best regards,

Stefan

0 song.lin over 1 year ago in reply to Stefan Schauer

Prodigy 150 points

Hi Stefan

We have optimized the transformer, the secondary side output has been lower than before.

Now there is a problem. When the flyback's load is light, the transformer whistling.

Then I measure the SW of LM51551-Q1. How can I handle this issue? Thanks!

0 Niklas Schwarz over 1 year ago in reply to song.lin

TI__Mastermind 20940 points

Hi Song,

Sorry for the long delay.
To find the source of this error, I would need to look closer into the design.
Would it be possible to fill in the quickstart calculator for the LM5155 desing?
https://www.ti.com/tool/download/SNVC240

This way I also get the missing data on input voltage range, output voltage rails and inductance.

Thanks and best regards,
Niklas

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LM51551-Q1: Cross regulation of flyback