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flyback transformer design for UCC28600

Siddartha koganti
Prodigy 70 points
Other Parts Discussed in Thread: UCC28600

hi,

We are designing the QR based flyback converter using the UCC28600 controller. The switching frequencies are varying from 40 to 130KHz in the QR mode of operation. But the transformer design has been done using a value of 80 KHz. We have designed it as per 80 KHz only. But facinf some heating issues. Can anyone please tell why the 80Khz has been used when the converter is going as low as 40KHz.

  • 0
    TI__Genius 9130 points
    Siddartha

    Can you explain your question and problem in more detail?

    Is this your transformer design? Or are you talking about one of TI's reference designs? I need to know more before I can comment about 40 kHz vs. 80 kHz question.


    For the UCC28600, the freq can vary from 40-130 kHz. The transformer and power stage should be designed to run a little higher than 40 kHz at full load and at minimum input voltage (allowing for bus cap ripple). If the load increases further, or if the input voltage falls further, the IC will start to power limit when the freq drops to the 40 kHz minimum.

    Thanks,
    Bernard
  • 0 in reply to Bernard Keogh
    Prodigy 70 points

    hi bernard,

    Actually i am using UCC28600 for designing my flyback converter. so i was going through the calculation example given for this specific IC and it was mentioned that 80KHz is to be taken as the switching frequency for the calculations to get an adequate margin in min Voltage input and max load. We know that the frequency will vary from 40 to 130 KHz. Why should I take a value of 80Khz it the range is 40 to 130 KHz . I have already designed it for 80KHz and i am facing heating issues. Please tell me which frequency is to be considered and how the frequency effects the heating

  • 0 in reply to Siddartha koganti
    TI__Genius 15910 points
    Hi Siddartha,
    The maximum switching frequency at max load, min line is intentionally limited to 80kHz in the design calculator because the on-time in QR-mode needs to be limited to less than approximately 6.8us. This is because there is an internal blanking time (called QR_DONE) that expires after approx. 7us that would interpret the turnoff of the gate drive as a valley and give an extra pulse. The controller will still switch up to 130kHz while in DCM; the calculator guides the user to design the magnetics so that in QR the switching frequency will go from 130kHz DCM down to 80kHz QR and not transition all the way down to 40kHz at full load.
    I hope this helps.