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Original question:

UCC28780: About setting conditions of UCC28780

UCC28780: Output Filter Calculation

Tak
Intellectual 480 points
Part Number: UCC28780

I am now reading the UCC28780 data sheet. I have a question about LC filter design. You are using ceramic capacitors 33uF / 25V in parallel.
It also states that the ESR should be very low.
However, the actual capacitance value is several uF. ‥
Since I design Vout at 32V, I need a ceramic capacitor of 35V or more. If the requirements are only a few uF and low ESR, how about using the MU series (35V, 4.7uF) of POLYMER MULTI-LAYER CAPACITORS (RUBICON) as an alternative to ceramic capacitors? (MU35 / 4.7 ≈ Ceracon 33uF × 2)
Of course, it is not as low ESR as Ceracon, but it is also stated in the manufacturer's HP that it is quite small.
We believe that alternatives will contribute to downsizing.

Ceramic capacitors also care about destruction due to mechanical (shock) cracks.
Please give us your opinion.

B.R. Tak

  • 0
    TI__Mastermind 26840 points

    Hello Tak-san,

    Thank you for your interest in the UCC28780 ACF controller.

    In my opinion, I see no reason to not consider any type of capacitor for use in this application as long as it can reliably withstand the operating conditions for the expected life of the product.  In other words, you can certainly evaluate the MU series as a resonance capacitor, provided that the component ratings are not violated.

    In the MU datasheet there are graphs of "Permissible Current for Frequency" which peak near 2.2Arms at 200kHz for 10uF, 35-V rating.  In the notes for "Caution for Use" file, Rubycon warns about non-sinusoidal currents and internal self-heating limits (as well as limits on hand-soldering, etc).  They recommend to contact the factory for non-sinusoidal ratings.  Provided that these limits are not violated, I think these capacitors can work in this application. 

    But consider that for most power levels where ACF is appropriate (25W+) the resonance current rms value may be several times the average output current, so I expect that more than one capacitor may need to be paralleled to meet the Arms/Hz and temperature rise limits.  These parts may be more applicable to higher voltage, lower current designs, but can be evaluated (at least on paper) for any design.

    Thank you for bringing this option to our attention.

    Regards,
    Ulrich 

  • 0 in reply to Ulrich Goerke
    Intellectual 480 points

    Thank you for useful tips . I will evaluate carefully.