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UCC28780EVM-021: can I short the L3/L4 on EVM?

Part Number: UCC28780EVM-021

Hello Experts,

My customer is now evaluating UCC28780EVM-021 and he has question as follows. Would you please answer it?
1. If customer short L3/L4 on EVM, the EVM will still work? or the EVM will not work(it will be broken the EVM)?


2. Would you please tell how to design Cclamp(C6, C24 and C33) and C8/C9?

Best Regards,
Fujiwara

  • Hi Yasushi,

    If L3/L4 be shorted , the EVM board still can working but the full load efficiency will drop around 0.8% . if L3/L4 is existing , we called it secondary resonance, the resonance period is dominated by  Lk and C8//C9 . after shorted , it will change to primary resonance, the resonance period is dominated by Lk and C6//C24.  secondary resonance has lower RMS current both on primary side and secondary side, so conduction loss is a little lower than primary resonance. 

    We have a design calculation tool that you can download it from TI.COM . in this calculator , you can get the Clamp value and secondary side MLCC C8/C9 value . in theory , the designed 3/4 resonance period equal or a little longer than high side FET turn on time is preferred .  this proposed value generated by the calculator is just a good start point . you can also adjust during the debug by monitor the resonance period . the design tool also shows how to adjust it in section "Secondary Resonance and ARC".

    https://www.ti.com/lit/zip/sluc664.

    Thanks.

  • Hi Jaden,

    Thank you for answer. I have question as follows. Would you please answer them?
    1. According to below thread, the datasheet has an error "(Co1 < CCLAMP/NPS 2)", p41, section "8.2.2.3 Clamp Capacitor Calculation". It should be "(Co1 < CCLAMP * NPS^2). Is this correct? If so, when you can modify the datasheet to correct one?
    https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/711713/ucc28780-data-sheet-equation-error?tisearch=e2e-sitesearch&keymatch=UCC28780%25252520cclamp#

    2. Regarding the UCC28780EVM-021, Co1, Cclamp and NPS are a following values.
    Co1: 33uF*2pcs = 66uF
    Cclamp: 0.33uF*3pcs = 0.99uF
    NPS: 1/0.19=5.26
    According to the above equation(Co1 < CCLAMP*NPS^2), Co1 should be less than 27.4uF. But EVM components(C8//C9) is 66uF. Why have it chosen? I think that the EVM designed by secondary resonance ACF...

    Best Regards,
    Fujiwara

  • Hi Yasushi,

    Thanks for your reminder on the datasheet error. We will correct it in the next version , but I am not sure when we will release it . anyway , please give us sometimes .

    About your second question. Actually , the secondary side resonance is a multiple components resonant circuits . The calculated results of Co1 and Cclamp is just a start point . we will adjust it based on the actual board debug to optimize the efficiency or SR FET voltage stress. we also shown the Co1 adjustment guideline in the calculator section "Secondary Resonance and ARC", please look at it if you are interesting in .

    Thanks.