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PMP8740: How Shim inductor core loss is calculated in PMP8740

Aneesh TS
Expert 1735 points
Part Number: PMP8740
Other Parts Discussed in Thread: UCC28950

Dear Roberto,

I have two doubts regarding the PMP8740  2 kW parallelable power supply module

1. You have used PQ20/20 core as your shim inductor  with 10 Turns and  0.118T flux and you got 0.63W as core loss. How you arrived at 0.63W core loss by calculation?

2.   You have used IPP65R190CFD  as your PSFB Mosfets. What is the value of oss (a, vg) Used in your design?

Regards

Aneesh

  • 0
    TI__Expert 5365 points

    Dear Aneesh,

    Regarding the core losses of the shim inductor, I made the calculation by hand on the TDK/Epcos datasheet.

    In the attached presentation, I explain hot to do that (starting from slide 18).

    Coss of the MOSFET: I don´t remember which exact number I used, but, typically, you need to consider the "effective output capacitance, energy related", here 70pF to calculate the switching losses, while you should use the  "effective output capacitance, time related", which is here 336pF, for the calculation of the ZVS times.

    Please let me knwo if that answers your question.Magnetics in SMPS Basics by Roberto Scibilia.pdf

    Best regards,

    Roberto

  • 0 in reply to Roberto S.
    Expert 1735 points

    Dear Robert,

    1. Ve of PQ20/20 core is 2.85Cm3 and you got core loss as 0.63W so revere calculating the core loss density which comes 221mW/cm3 which is almost matching the EPCOS core loss curve at B=0.118T. 

    2. I have just calculated the shim inductor value as per your equation, i got some mismatch with your calculated value. My calculation as follows

    Coss(Qa,Avg)=336pF, Vin(Fb,nom)=400V, Load(min)=0.15, IPP=7.526A,Ilout=12A, Nps(fb)=9.5 , Llk=3.5uH

    As per the calculation i am getting Ls= ((2*336pF*400*400)/(0.15*(7.536-(12/9.5))*(0.15*(7.536-(12/9.5)))-3.5uH =118.43uH 

    and assume that if there is no square  term for Load(min) in your formula then i am getting Ls  =((2*336pF*400*400)/0.15*(7.536-(12/9.5))*(7.536-(12/9.5))-3.5uH= 14.79uH

    So as a summary core loss for shim inductor is OK for me

    But the shim inductor value calculation shows mismatch, Kindly tell me where is my calculation went wrong?

    I also appreciate your effort for making such a wonderful product.

    Regards

    aneesh

  • 0 in reply to Aneesh TS
    TI__Expert 5365 points

    Dear Aneesh,

    Thanks for your reply.

    The formulas I used are taken from a previously published application note of the UCC28950, see formula (39) on page 8 of:

    By using Coss = 336pF you are right: if we want ZVS down to 15%, you need 118.43uH.

    The reason why I have got 11.3uH, is because a first prototype I developed was using 650V, 125 milli-Ohm, standard FETs, which have Coss = 53pF (see page 13 of the "2kW parallelable power supply module" white paper, available also in the documentation as support of UCC28950 product page).

    So, if you want to use the latest FET I employed in the final prototype, with Coss = 336pF, and want to have ZVS down to 15% of the load, then you need ~ Ls = 120uH. I know that this inductor is too big, therefore I suggest to set the minimum load current for ZVS operation in the range 30%...50%.

    Best regards,

    Roberto