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Design the PFC choke for UCC28070 interleaved system.

sagaert
Intellectual 310 points
Other Parts Discussed in Thread: UCC28070

Hi

If i need 3200W , can i approach this as 2  boost converters of each 1600W and use the Ti Powerstage designer tool to calculate the choke ?

The boost designer tool gives me a choke of around 430µH ( vin 150-320 ,100Khz,output 4A@400V)

If i use the UCC28070 excel calculation tool , then the excel suggests Lminimum 28µH , quite a big difference.

I don't know what i am missing,

I looked at the reference design http://www.ti.com/lit/ml/slur217/slur217.pdf , and the PFC choke is just 200µH for a switching freq of 37.5Khz

I just want to determine the core i need (a first guess is ETD49 ETD54)

So, all these differences confuse me a lot on the correct formulas to use.

Johan

  • 0
    TI__Genius 9790 points

    Hi, Johan

    It is right to design bosst choke with 1600W. But inductance of boost choke is related to a lot of things. You should design inductor under 150Vac input and 1600W output. what is the switch frequency, current ripple, Bmax, and current density of wire of inductor? If there factores are selected, the inductance is got.

    thanks.

  • 0 in reply to Yunsheng Qu
    Intellectual 310 points

    Hi

    Thanks, frequency will be 100Khz ,Vin min 150V vmax 320 Vout 400V , this gives me 400µH , Ipeak 12A , so far it is not difficult.

    I know how to calculate the number of turns, but i am stuck on how to select a core that will not saturate , and how to calculate the gap... So knowing Ipeak, the Turns and the Bsat , Ae of a given core, how do i go further from here ? 

    Johan

  • 0 in reply to sagaert
    TI__Genius 9790 points

    hi, Johan

    for select a core: you have L=400uH, and Ipeak=12A, you should select window factor(Kw) of core, usually Kw=0.6, and Bmax of Core, Bmax=0.5 for dust core and Bmax=0.3 for ferrite core. and have to select current density of winding of inductor Jmax, Jmax=8A/mm^2. then you can get AwAe of core, AwAe=(L*Ipeak^2)/(Kw*Bmax*Jmax*1.414). then go to select core based on AwAe calculation result.

     

  • 0 in reply to Yunsheng Qu
    Intellectual 310 points

    Hi ,thanks for the hint,

    I found this formula for calculating Bmax :

    BMAX= L * Ipeak / N * Ae²

    If i have L  and AL(no gap)  then can calculate the number of turns, If the space i need for the turns is much less then
    the available space i need to re-calculate the Turns and apply a gap :, gap=  0..4*PI* Ae * ( 1/ALneeded - 1/ALnogap )

    Then i need to evaluate again for Bmax.

    I found these formulas in a fairchild APP Note on page 8 , i assume they are correct

     2313.Power_Stage_Design_for_a_200W_Off_Line_Power_Supply.pdf

  • 0 in reply to Yunsheng Qu
    Intellectual 310 points

    Hi

    8A/mm² is that not a bit too much ? I mostly see values of 5A/mm² , or is this safe since i will be using litz wire, and its physical diameter is much more then the raw diameter of the copper area. (more surface area then a solid wire with the same copper area)

    Kind Regards ,Johan

  • 0 in reply to sagaert
    TI__Genius 9790 points

    hi, Johan

    8A/mm^2 is OK with Fan cool, if there is no Fan for the application, should be lower.

    For inductor design, you can use the reference document form www.ti.com for UCC28070. Because the gap depends on the core you selected, dust core or ferrite. the gap is constant in dust core, but the gap can be adjuset in Ferrite core.