UCC28180: Audible noise under light loads

Hello Pravin,

The UCC28180 was designed as a pfc boost type controller. I am not familiar with a pfc LLC converter.
Do you have a schematic you can post ?

Regards

John

HI John,

Sorry, what  I meant was I am using the UCC28180 PFC controller + a separate IC driving the LLC. Please see the attached schematic

Thanks

Pravin

Hi Pravin,
If you are sure that the noise is coming from the pfc stage and not the LCC then I would investigate the boost inductor.
First test the pfc on a resistive load so that you can be sure that the LLC is not going into a burst mode of operation and causing audio noise.
Check the inductor current. It should be operating in continuous conduction mode with a switching frequency of 43kHz at this load condition.
Actually 1kW in a 3.5kW power supply is not really light load.
Does the noise get worse as the line voltage is dropped?
Or is it worse as the load is decreased?
I recommend that you check the transformer construction and make sure that it is rated for your design requirements.
Please use the calculator tool for the UCC28180 to help you with this.

Regards
John

Hi John,

Thanks for your input. I shall look into your suggestions.

Meanwhile, what I meant by light load was, the controller has been compensated to work with a wide voltage range and to be able to deliver 3.5kW at 85Vac for a short period of time. The sense resistor has been chosen to have a soft over current limit only at 64A.

1kW at 85Vac would draw more current than at 240Vac. At the moment I am drawing 1kW at 240Vac and there is audible noise. It reduces as I increase the load.

Any further thoughts?

Thanks

Pravin

Hi Pravin,
Please look at some of the switching nodes. Your 43kHz switching frequency is above the audible frequency range so you must have some lower frequency component at 1kW load.
The noise is most likely the inductor core responding to this.
Do you have the construction details for the inductor?

Regards
John