UCC256303: Unstable switching frequency, audible noise

Niclas Samuelsson

Part Number: UCC256303

Hello. I'm making a 150W converter using the UCC256303. One problem I'm having right now is that the switching frequency doesn't seem to be stable. It's "jittering" when I look at the oscilloscope, and the transformer is making a lot of noise because of it, I think.

Do you know any reasons why the switching would be unstable? I'm including a short video of the "jittering" going on, measured on the secondary side of the transformer. Hope its possible to see it

over 2 years ago

0 Manikanta Pallantla over 2 years ago

TI__Genius 13245 points

Hi Niclas,

We strongly recommend using our latest generation LLC controller UCC25640x with much improved features which is a pin to pin replacement to UCC25630x.

Could you share the design calculator and schematics with me.

Regards

Manikanta P

0 Niclas Samuelsson over 2 years ago in reply to Manikanta Pallantla

Prodigy 70 points

Ok, I will try to replace the controller. Thank you!

I haven't used the design calculator. My schematic is the same as the evaluation board,

https://www.ti.com/lit/ug/sluubq8b/sluubq8b.pdf?ts=1659638691558&ref_url=https%253A%252F%252Fwww.google.com%252F

0 Niclas Samuelsson over 2 years ago in reply to Niclas Samuelsson

Prodigy 70 points

Adding C19 at the optocoupler helps a lot. Its marked as DNP in the eval schematic so I didnt include it first. Still some noise left but 80% gone

0 Niclas Samuelsson over 2 years ago in reply to Niclas Samuelsson

Prodigy 70 points

As I lower the input voltage the switching frequency decreases and the audible noise lowers a lot. Somehow the controller gets unstable when the switching frequency is high (80kHz, which is where my resonance frequency is also). At 50kHz switching frequency the noise is gone.

+1 Manikanta Pallantla over 2 years ago in reply to Niclas Samuelsson

TI__Genius 13245 points

Hi Niclas,

I just realized that EVM is designed for 120W. But you are using it for 150W? Did you change the resonant component values?

Regards

Manikanta P

0 Niclas Samuelsson over 2 years ago in reply to Manikanta Pallantla

Prodigy 70 points

I noticed that as well. I'm having the same problem at all power levels.

0 Niclas Samuelsson over 2 years ago in reply to Niclas Samuelsson

Prodigy 70 points

I accidentally pressed issue resolved, it's not resolved yet :)

+1 Manikanta Pallantla over 2 years ago in reply to Niclas Samuelsson

TI__Genius 13245 points

Hi Niclas,

The resonant frequency of the EVM is around 100kHz where as your resonant frequency is around 80kHz. Which means there is a change in the resonant circuit.

So could you send me updated schematics and design calculator.

Regarding the frequency jittering, I think that you need to further tune the compensator values on the secondary side.

Regards

Manikanta P

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UCC256303: Unstable switching frequency, audible noise