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UC2843A-Q1: Switching frequency at light load

Shinji Tsuji
Genius 14189 points
Part Number: UC2843A-Q1


Tool/software:

Hi team,

I designed isolated flyback converter with opt coupler feedback. VIn=500V, Vout=8V, Po=16W at Fsw=100kHz.

When I reduce the load current to near 0A, then the switching frequency drops to 10kHz. 

Is this normal operation? I thought this is constant frequency device. 

Could you please advise the mechanism of the switching frequency drop?

regards,

  • 0
    TI__Guru 74115 points

    Hi,

    At light load when Vout reaches to to high end regulation COMP can become so low so no PWM pulses for sometime then it looks fsw reduced. You need to capture RT/CT pin to see this pin waveform to decide if the IC frequency still the same at light load as you designed.

  • 0 in reply to Hong Huang
    TI__Genius 14189 points

    Hi Huang-san,

    Is there a description about this mode in the datasheet? 

    Is there a threshold value in COMP where the PWM pulse stops? 

    I would like to know what is the lowest load current the device starts to stop continuous switching frequency,

    and also would like to make sure there is no risk to be over voltage at light load.

    regards,

  • 0 in reply to Shinji Tsuji
    TI__Guru 74115 points

    Hi,

    Yes, see this block diagram: when COMP voltage < 2x diode VF, there will be no pulses due to current sense comparator threshold = 0.