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UCC2897A: Design suggestion for active clamp 250W...

Peter Tseng
Mastermind 6210 points
Part Number: UCC2897A

Dear Sirs,

My Customer wants to use UCC2897A for 250W design.

The excel calculation result as attached.

The design requirement are: 

AC19.2V ~31.2V ， Vo=12V/21A (250W)

For transformer and secondary side inductor, customer will use PQ3220.

Here are the design calculation and some question need help.

 

Transformer:

Np=Vin(min)*ton/(ΔB*Ae) = (27*0.6*(1/250K))/(0.25*170) = 1.524Ts (N1=2)

Dmax = ( Vout * N ) / Vin_min = 0.6 = (12 * N )/27 = ratio = 1.33 (N2=1)

L = N1^2*AL = 2^2 * 7310 = 29uH

=============================

Secondary side Inductor:

L = Vout*T*(1-Dmin) / Id = ( 12V*(1/250K)*(1-0.35) )/21A = 1.49uH

N1^2*AL = N^2 * 7310 = 1.49uH………L is too small and cannot make it.

May I have a better suggestion for the transformer and inductor about customer design requirements?

Thanks. 

UCC2897A_SLUC662_12V_21A.XLSX

  • 0
    TI__Guru 74115 points

    Hi,

    It looks your Np = 2, and Ns =1, i.e., Np/Ns = 2, is it right? Where did you get the transformer design formulas?

    What is your converter topology, forward, or flyback?

    What is your switching frequency, 250kHz?

  • 0 in reply to Hong Huang
    Mastermind 6210 points

    Hi Sir,

    The reply as below.

    It looks your Np = 2, and Ns =1, i.e., Np/Ns = 2, is it right? [A] Yes, the best turns ratio is 1.33

    Where did you get the transformer design formulas? [A] The transformer calculation formula comes from the traditional forward calculation method.

    What is your converter topology, forward, or flyback? [A] Forward Design Architecture.

    What is your switching frequency, 250kHz? [A] Yes, it is a 250KHz switch design.

  • 0 in reply to Peter Tseng
    TI__Guru 74115 points

    Let me have a review and will reply to you.

  • 0 in reply to Hong Huang
    TI__Guru 74115 points

    Is PQ3220 with material 3C95? If not what is the core material?

  • 0 in reply to Hong Huang
    TI__Guru 74115 points

    I checked your calculation and found the below changes needed:

    (1) Nps = 1.33, if pick up Np =2 and Ns = 1, then Nps = 2 is too much far away, so when input = 27, secondary winding = 27V/Nps = 13.5V, then the Dmax = 12V/13.5V = 0.89, then your 0.6 is not correct. So you need to keep Nps = 1.33

    To achieve Nps = 1.33, you can pick up Np = 4 and Ns = 3, i.e., Nps = Np/Ns = 4/3 = 1.333

    (2) Your Lo formula is missing ripple Io, delta_Io, in percentage, usually 10% to 20%, if take 15%

    L = Vout*T*(1-Dmin) / (Io*fsw*delta_Io) = (12V*(1-0.35) )/(21A*250k*15%) = 9.9uH, so you can pick up 10uH