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UCC28951-Q1: Need help with the calculation when shim inductor is not needed in the design

Natthapol Vanasrivilai
Prodigy 240 points
Part Number: UCC28951-Q1

Hello TI,

i stuck with the calculation, when the shim inductor is not needed in the design.

In the design review for 600W PSFB http://www.ti.com/lit/an/slua560c/slua560c.pdf

the equation 120 onwards require me to have a value of shim inductor. If i select the shim inductance to be 0 H, then i get division-by-zero in the equation.

  • How should i select the appropriate delay time, to achieve ZVS down to 50% load just like in the design review?
  • Should i program the delay time t_ABSET to the minimum value of 30ns?

Thanks

  • 0
    Prodigy 240 points
    Additional info to the question.

    I know what are the CS voltage level on different loads (100%, 50%, 20%). And the voltage divider setup on the ADEL pin (K ratio). The question is related on the 50% load current. The datasheet suggests "large" delay, but how large should i aim for exactly?
  • 0
    TI__Mastermind 25570 points

    Hi Natthapol

    It's not uncommon for the shim inductor calculation to return a 0 value. The energy for the Zero Voltage Switching (ZVS) transition comes from the  Inductance in series with the transformer primary (to a first approximation which is accurate for this discussion). If the transformer leakage inductance is not high enough then a separate shim inductor is added in series with the primary so that the sum of the transformer leakage and shim inductance is enough to drive ZVS.

    The formulae in the Application Note uses Ls to mean the sum of transformer leakage + shim inductance (although I'd admit that this isn't clear from the document)

    I'd suggest that you use the transformer leakage inductance for Ls and recalculate.

    Please re-post you have any further questions.

    Regards

    Colin

  • 0 in reply to Colin Gillmor
    Prodigy 240 points
    Hello Colin,

    is it possible to anyway add a small shim inductor of a few nH (even though the calculation suggested it is not needed) into the circuit to make sure that, at very light load, the converter will still able to achieve ZVS all the way down to 5-10% load?

    The target application is a battery charger which at the end of charging cycle, the current drawn into the battery will be extremely small.

    Thanks.
  • 0 in reply to Natthapol Vanasrivilai
    TI__Mastermind 25570 points
    Hello Natthanapol

    The short answer to your question is 'yes' it is possible.
    The only downside is that the added inductance makes the current transitions longer and during this time there is no energy being delivered to the secondary - this is sometimes called 'duty cycle loss' but in practice the loss is usually 2% or 3%.

    The UCC28951-Q1 drops into burst mode at light loads. If you set it up right the energy during the bursts is enough to ensure ZVS. Even if you achieve only partial ZVS - eg Vds drops to only 50% instead of 0% then the losses are still significantly lower because they are proportional to V^2

    Regards
    Colin