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UCC256403: Input Capacitor design

Gautham Venkat Venkateswaran
Prodigy 180 points
Part Number: UCC256403


Hello Experts,

I am looking to use the IC for a LLC design of a 600W DC-DC application. 

Input = 80-120VDC

Output = 12-13.5VDC 600W

How would I go about designing the Input capacitor for the LLC? 

Many thanks.

Regards,

Gautham Venkat

  • 0
    TI__Genius 16155 points

    Hi Gautham,

    Thanks for reaching to us.

    Usually, LLCs are powered from the PFCs. In this case the LLC input cap value is usually based on the holdup time requirements. In some cases, it also depends on the voltage ripple requirements. You can refer page 40 of the datasheet for these two formulas: UCC28064A Natural InterleavingTm Transition-Mode PFC Controller with High Light-Load Efficiency datasheet (Rev. B)

    In case, if LLC is directly powered from the DC input, the input cap is used to allow the high frequency ripple component. 

    This will depend on the whether you use single resonant cap or split resonant cap.

    You can see here that based on the resonant cap arrangement, the input current waveform changes (Iin in above pictures). Input RMS current would be low (~30%) in case of split resonant capacitor case compared to single resonant capacitor.

    You can follow the following steps to find out the rms current:

    1. In simulation you can find out the input current waveform at the low input voltage and at the peak power which would be worst case scenario. 

    2. Assume all the input current ripple will go through the input cap. Once you have the current waveform from simulation, from i=Cdv/dt, you can find out the cap value for a given ripple voltage requirement.

    Since there is no closed loop form for the LLCs, you have to use the simulation to find out the input current waveform.

    Regards

    Manikanta P

  • 0 in reply to Manikanta Pallantla
    Prodigy 180 points

    Hi Manikanta,

    Thank you for your reply. I do not have access to SIMPLIS but I do have access to LTSPICE. Would it be possible to get a spice model?

    Or is there a theoretical assumption/formula that I can use to overestimate the ripple at the input and pick a cap as such?

    Regards,

    Gautham Venkat

  • +1 in reply to Gautham Venkat Venkateswaran
    TI__Genius 16155 points

    Hi Gautham,

    We do not have a LTspice model for this family of controllers.

    You can pick any 60uF-100uF ,150-200V DC rated capacitor.

    Regards

    Manikanta P