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TIDA-01494: resonant capacitor RMS voltage

venkata subash73
Prodigy 60 points
Part Number: TIDA-01494
Other Parts Discussed in Thread: UCC25600

in TIDA-0194  http://www.ti.com/tool/TIDA-01494 

split capacitor LLC resonant converter is used ,  how to select the RMS voltage in this case.

Is it the same way what is mentioned in SLUP263 for this case also?

if it is same then in TIDA-0194  , for minimum frequency of 50kHZ the RMs voltage required is 238V but where as the selected capacitor(B32652A0473J000) is is rated for only 70V around. 

  • 0
    TI__Mastermind 26770 points
    Hello Venkata,

    Thank you for your interest in the TIDA-01494 reference design which features the UCC25600 LLC controller.

    To answer your questions, I'll need to check with the designer of this TIDA about his choice of capacitors and their ratings.
    I'll get back to you as soon as I can.

    Regards,
    Ulrich
  • 0 in reply to Ulrich Goerke
    TI__Intellectual 2815 points
    Hi Venkata,

    For the AC RMS voltage, at 50Khz switching frequency, the RMS Voltage component at 50KHz is about 122V. This leads to about 61V per cap as we use split cpapcitor instead of single resonant cap.

    Even this extreme case only occurs for a versy short period under input AC turn-off condition, where the LLC could operate at max gain point. Ideally the RMS voltage can be estimated at around 80KHz for this design.

    Best Regards,
    Ram
  • 0 in reply to Ramkumar S
    Prodigy 60 points

    Hi Ram,

    Thanks for the reply.

    In Designing an LLC Resonant Half-Bridge Power Converter (  SLUP263)   the RMS voltage  calculated based on the Dc voltage and Vcr.

    Just want to understand for AC rms voltage whether we need to take this DC component or not?

    Can you share any  reference document for Split capacitor resonant Cap selection.

    Thanks and regards,

    Venkat.

  • 0 in reply to venkata subash73
    Prodigy 60 points

    HI Ram , 

    Can you update on this.

  • 0 in reply to venkata subash73
    TI__Mastermind 26770 points

    Hello Venkata,

    I'm not sure if Ram will be able to have a timely reply or not.
    I, and another expert I consulted with, believe that you do have to include the DC component of the voltage as well as the AC component in the total rms calculation.

    In the TIDA-0194 design, the resonant caps C2 and C3 (B32652A0473J000) have a rating of 1000Vdc and 250Vac. I see that the AC rating is derated for frequencies higher than 25kHz.  This suggests that this part may not be suitable for long-term reliability even if it can operate without failure for a limited time.  Perhaps several lower-value higher rated parts in parallel can be used, or a single part with suitable high-frequency rating can be found.

    To your point, the total rms = SQRT(Vdc^2 + Vac^2), and each cap sees the full ac voltage component (122Vrms, not 61Vrms) despite this being a split-capacitor design.

    Unfortunately, I do not have any reference literature on split-cap selection, however, the Excel UCC25630x Design Calculator Tool will generate the correct total capacitance for Cr and the split-cap design simply divides Cr by 2 for each cap.
    I hope this answers your questions.

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    Prodigy 60 points

    HI Ulrcih,

    Thanks for the reply, Can I have your mail id so that I can discuss with you over the mail.

  • 0 in reply to venkata subash73
    TI__Mastermind 26770 points
    Hi Venkata,

    Please contact me by ulrich_goerke@ti.com for further discussion.

    Regards,
    Ulrich