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Original question:

TPS23756: Short RTN and VSS

TPS23756: How to improve transient response

Hector Chen
Intellectual 410 points
Part Number: TPS23756
Other Parts Discussed in Thread: UCC24612, ,

Tool/software:

Hi 

We heard an audible noise from the flyback transformer when the output load transient was heavy.

We didn't hear the noise when the load current was stable even if the output loading was much heavier. 

UCC24612 was used for the secondary switching control. 

The switching frequency was stable but I noticed a pattern on MOS VDS, some successive VDS waveforms showed DCM operation and some behaved more like CCM.

The frequency of the pattern is about KHz. I guess the pattern caused the audible noise.

I want to improve the transient response of the flyback power supply using TPS23756 and UCC24612 and hear whether the noise is still there or not.

Any suggestions?

Thanks.

  • 0
    TI__Mastermind 37071 points

    Hi Hector,

    Thanks for reaching out!

    We will look into your questions and get back to you in 3-7 business days.

    Best regards,

    Diang

  • 0 in reply to Diang Xing
    TI__Mastermind 37071 points

    Hi Hector,

    The audible noise is normally less than 20kHz caused by LC resonance. Are your switching frequency 250kHz?  Transient response at this frequency is normally not audible. Normally you need try with a smaller dc-link capacitance or considering using a different flyback transformer. 

    But if you are testing a load transient at  <20kHz, you may still have the source of noise.

    UCC24612 may be an ideal diode controller and theoretically it still has DCM at light load. 

    Best regards,

    Diang

  • 0 in reply to Diang Xing
    Intellectual 410 points

    Hi Diang,

    Yes, the switching frequency is 250KHz.

    Normally you need try with a smaller dc-link capacitance
    => what is smaller dc-link capacitance?

    Please see the output load current below, the obvious load transient frequency is about 25Hz.

    Thanks.

  • 0 in reply to Hector Chen
    TI__Mastermind 37071 points

    Hi Hector,

    Current ripple could be load related. And 25Hz could be related to the AC grid isolation is not good. Do you have same 25Hz ripple on the Vout?

    DC-link cap normally means the non-ceramic on the Vin and Vout bus, they have high capacitance and do not have voltage bias. Such as the two cycled cap on below TPS23756EVM (https://www.ti.com/tool/TPS23756EVM)

    Best regards,

    Diang