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TPS53679: Input capacitors selection for 3-phase Buck regulator

user4948849
Prodigy 180 points
Part Number: TPS53679

Hi,

I actually search some information about multiphase buck design and more particularly about input capacitors selection.
I find a report (SLTA055) which talk about input capacitors calculation. However, this article is only talking about monophase design.
So i found other reports such as slyt675, slyt 449 and more recenty slva882. However, Cin(min) formula for multiphase regulator are no more provide.
From normalized RMS input current calculation, i find total minimum capacitance needed for a 3-phase design. I think it's good but can you confirm  the formula of Cin(min) ? My design is the same as shown on the Figure 1 from SLVA882.

Here is the formula i use:

Cin(min)=(Iout/(Vin p-p ripple * Fsw))*(D-(m/N))*(((m+1)/N)-D)

where:

Iout is the output load current

Vin p-p is the maximum allowed peak to peak ripple for input volage

Fsw is the switching frequency

D is the duty cycle (Vout/(Vin *efficiency))

m= floor (N*D)

N, number of phase

Then, here is my other question: is this Cin(min) calculated is available for only 1-phase or for all 3-phase regulators? I mean, I have to add the same number of capacitor for each phase or is it available for all 3-phase (I think this is the good solution)?

Thank you for your support.

Best Regards,

Mickaël

  • 0
    TI__Expert 4060 points

    Hi Mickael,

    Happy to help you out here, it was confusing to me first as well. For SLVA882 the input capacitance formula, equation 4 on page 13, calculates the amount of per phase ceramic capacitance needed for a design. This number doesn't include any hold-up capacitance for transient events, it only takes into account the RMS ripple current and steady state switching ripple target.

    For bulk capacitance I recommend tjhe procedure from SLYT670, reference 10 from SLVA882. It's the most straightforward method I've found so far and it gives a great starting point for your design.

    If there's anything else you need help with feel free to ask.

    Cheers,

    Carmen

  • 0 in reply to Carmen Parisi
    Prodigy 180 points

    Hi Carmen,

    Thank you for your reply and sorry for my late response!

    For my 3-POL design, i understand that there have to be input capacitors to handle the RMS current/DC ripple in steady-state conditions + bulk capacitors to keep Vin within the tolerance during load transients.

    Actually, only input capacitors that handle the RMS current/DC ripple in steady-state conditions interested me.

    If i take account the example of the equation 4, page 13 of SLVA882 , it is calculated 22.3 uF (without derating) for 1-POL design. So for a 3-POL design, can i use the same equation? Do I have to put in front of each POL a 22uF (without derating) input capacitor (3x22uF in total for the 3-POL module)? I am confused with this concept. In fact, in the equation 4, I know the Iphasemax current value for the whole 3-POL design but not for 1-POL only.

    Best Regards,

    Mickaël

  • 0 in reply to user4948849
    TI__Expert 4060 points

    Hi Mickael,

    You can definitely use the same equation for your 3 phase design. The equation in the app note is using to calculate the per phase ceramic capacitance needed for a 6 phase design but it's a general equation you can use for any number of phases. To get the per phase current simply divide your max current by the number of phases.

    For the app note example the per phase max current was 240A/6 phases or 40A. In your case it would be Imax/3. The resulting number is the amount of capacitance you need to have on each phase after derating is taken into account to hit your ripple spec. You also have to factor in the RMS current rating too however.

    In the app note, the design example says you have to have 22µF per phase and those caps collectively need to handle 20A of RMS current. The selected caps were rated to 5A RMS at the switching frequency and so 6 of them (one per phase) can handle the current with margin. But, looking at the derating of the caps, although they could handle the current there wouldn't be enough effective capacitance with the 12V bias to meet our ripple spec and so 3-4 caps per phase would be needed depending on what package size you chose to go with.

    On your design once you calculate the RMS current and the capacitance per phase start looking at the datasheets to figure out exactly how many caps you'll need on each of the three phases to hit both specs.

    Let me know if you have any questions.

    Thanks,

    Carmen

  • 0 in reply to Carmen Parisi
    Prodigy 180 points
    Thanks for your answer Carmen. It help me a lot!
    I will contact you if I have any questions in mind.

    Mickaël