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UCC28950: Abnormal Oscillation During Burst Mode Transition

Conor
Guru 15050 points

Part Number: UCC28950

Tool/software:

Hi,

I would like to consult with you regarding an issue observed in our full-bridge ZVS converter design using the UCC28950. We are experiencing abnormal oscillations when transitioning from burst mode to normal operation.

[Operating Conditions]

  • Input Voltage: 300V
  • Output Voltage: 120V
  • Output Current: 6.7A

We have observed abnormal oscillations occurring in the load current range of 1A to 3A, specifically during the transition from burst mode to normal operation.

Could you provide advice on how the burst mode characteristics and loop compensation settings might affect this abnormal oscillation?
Additionally, any recommendations on debugging approaches or information on whether similar cases have been reported in the past would be highly appreciated.

Thanks,

Conor

  • 0
    TI__Guru* 86286 points

    Hello,

     

    Your inquiry has been received and will be answered in the order that it was received.

     

    Regards,

  • 0 in reply to Mike O'
    TI__Guru* 86286 points

    Hello,

     

    Burst mode should only be engaged if the duty cycle is less than ton_min*fsw, which is programmed by Rtmin.    You had mentioned that light load burst mode occurs between 1 – 3 A which is roughly 14.9% to 44.8% load.  This should not trigger light load burst mode.

    If your output capacitor bank (Cout) is too small it could trigger burst mode when it should not.  The following information can be used to check the Cout capacitor size.

    Light load burst mode is best used to improve standby power.  I would recommend setting Rtmin to 10 k to the lowest value and see if this resolves the issue if the other two recommendations don’t work.

    If you want to improve light load efficiency I would use the DCM comparator to turn-off the SR FETs.  There is a point where driving the SR FETs Ids^2*Rdson will not improve efficiency and driving the FETs will dissipate more power than it is worth. 

    If (Qg*fsw +Ids^2*Rdson) is greater than Ids*Vbody_diode you should turnoff the SRs.

    Thankyou for your interested in power control devices from Texas Instruments.  If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

     

    Regards,