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UCD3138FW-PSFB: comparing PSFB WITH LLC

robin gangopadhya18
Mastermind 6050 points
Part Number: UCD3138FW-PSFB

Hello:

The LLC expert refers me to ask the following question to PSFB experts. So here you go.

Fortunately for us, the modified LLC breadboard by putting in our values just to ensure we get a better understanding of the circuit, its limits etc.- WORKED FINE THE FIRST TIME.

I read some TI lit & viewed some videos- comparing LLC with PSFB & others. In those documents, no one seems to bother about the most annoying aspect of  PSFB: the need for a substantial DC inductor. Trust me, it is very big, and lossy and if you have a charger to build, you do not want to use an inductor for 400V output where load current will change from 200AMP to negligible all the while controlling output voltage.

IF we were to compare, we ought to consider what happens to LLC with smaller values of AC inductor to "filter" out the capacitor ripple current.....is that an idea that can work given that freq variation of the ripple is confined within a narrow range?

If we reduce the output capacitor ripple substantially, LLC in my mind will take a front seat.

What is your take on this?

Appreciate any reviews of this thinking.

r

  • 0
    TI__Genius 13570 points

    Hi,

    Thanks for reaching out.  You are correct. The LLC ripple current  is big concern. you need lots of cap in parallel to handle the current ripple. Also, the input voltage need smaller variation for LLC. PSFB's advantages over LLC are also appear clear. It can handle large input voltage, smaller ripple although the output inductor is bulky.  This is a tradeoff.

    Regards,

  • 0 in reply to Sean Xu
    Mastermind 6050 points

    Hello Sean:

    Thanks for the review.

    Since LLC ripple only needs "filtering" out higher harmonics, it can be a much smaller inductor demand. While PSFB inductor has to integrate to get DC with low ripple- and it is big, lossy especially if you have to cover broad output current as in a charger.

    So the question is : how large an ac( as opposed to DC integrating)  ripple attenuating inductor can LLC handle for comparatively similar ripple output to PSFB?

    I think this is where LLC will be a better topology: in terms of overall size, loss and ability to handle 0 to full load- especially if we use Interleaved Delfino-based LLC.

    What would you think about this observation?

    -r

  • +1 in reply to robin gangopadhya18
    TI__Genius 13570 points

    Hi, Robin,

    I agree LLC provides higher power density. Output choke of PSFB is too bulky. Regarding to the first question, I don't have good answer for you.

    Regards,