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UCC2897A: doubt regarding selection of IC for forward topology

Eldho M P
Prodigy 10 points
Part Number: UCC2897A
Other Parts Discussed in Thread: UCC28180, , UCC2894

Tool/software:

for my 110w/18v  project i was thinking of using this UCC2897A IC. after pfc i am going to plce pwm so i want to know can i use this ic for my design my pfc IC is UCC28180 its output will be around 390vdc, and i am going with forward topology so  please let me know.

  • 0
    TI__Guru 73535 points

    Hi,

    We usually recommend UCC2894 for about 400V dc input for active clamp forward converter although UCC2897A is ok to use but as P-channel to sustain > 400Vdc may not be available, and when using N-channel MOSFET for clamping, you would need to make AUX signal inversed. But UCC2894 is for N-channel clamping so better to use.

  • 0 in reply to Hong Huang
    Prodigy 10 points

    Thank you for your response Sir. I just want to know  TI calculator is there for Ic ucc2894, because I am only able to  found for ucc2897. if it is there could you please share the link Sir

  • 0 in reply to Eldho M P
    TI__Guru 73535 points

    Hi,

    We do not have a calculator for UCC2894. You can use UCC2897A calculator to UCC2894 they are on the same core design. After your design you can send the design to me for review. You can also check based on the UCC2894 datasheet.

  • 0 in reply to Hong Huang
    Prodigy 10 points

    Thank you for your response Sir. Sir I Found one Reference Design for IC UCC2894 in which the output is 24V/7A ok, for my design i wanted 24V/4.6A  so only few values change like sense resistor and all, as per datasheet i calculated the value of sense resistor which comes around 5mOHM but they have choosen 2mOHM, in order to confirm i have verified with power stage designer its also showing 5mOHM, i just want to know how they choose that value. for your reference i am attachinfg the file 

  • 0 in reply to Eldho M P
    TI__Guru 73535 points

    Hi,

    Do you mean R19? One possible reason is to help reduce the spikes caused turn off. Another reason is to tolerate the parameters variations from IC, transformer and output inductor. You can start with either 0.5-ohm or 0.2-ohm and turn up to settle down when do the bench debugging.

  • 0 in reply to Hong Huang
    Prodigy 10 points

    Sir Thank you for your response. Sir one more doubt is there, My design input is 380v and output is 24V/4.6A. for that i found an reference design PMP7931.  it is also using same ic with 380v input 24V/7A output ok. where it's transformer turns ratio used was 5:1, But when I used TI's Power Designer tool it suggested me to use 7:1 ratio which one should i consider for my design Sir. for your reference I am attaching the photos

  • 0 in reply to Eldho M P
    TI__Guru 73535 points

    Hi,

    I think the reference design is based on bench debugging then adjusted to that turns ratio from test. Also you can try to change the design current to fill 7A to see any difference. But no matter what 5:1 is a tested result.

  • 0 in reply to Hong Huang
    Prodigy 10 points

    Thank you Sir. in ucc2894 reference design PMP7931 transformer T2 is used, it is for reset function or for any other purpose please let me know sir.ucc2894 pwm (1).pdf

  • 0 in reply to Eldho M P
    TI__Guru 73535 points

    Hi,

    T2 is a pulse transformer to help drive high side MOSFET Q1 which is on high side so need to use T2 to make level shift to drive.

  • 0 in reply to Hong Huang
    Prodigy 10 points

    Hi Sir, regarding designing of an transformer for the above project, 

     forward converter design,  have the following duty cycle values:

    • Worst-case Dmax = 0.33

    • Controller set Dmax = 0.51

    When calculating the transformer volt-time product (V-t) and primary inductance (Lp), which duty cycle should I use as Dmax— the  worst-case, or the controller-limited value? i have took the calculation part of transformer fromhttps://www.scribd.com/document/630575587/12-Steps-for-Designing-SMPS-Transformers

  • 0 in reply to Eldho M P
    TI__Guru 73535 points

    Hi,

    We suggest to use Vin(max) x Dmax x Tsw for transformer (V x t) design.

    Tsw and Dmax are based on the datasheet equation 2, 3, 4 and 5.

    This way is ok to cover all possible operations without transformer saturation.

  • 0 in reply to Hong Huang
    Prodigy 10 points

    Sir actually using RTON and RTOFF PIN OF UCC2894 I SET DMAX AS 0.51, BUT In my Design  WORST CASE  IS VDC(min) =360V AT THIS voltage DMAX Reach up to 0.34 ONLY. for my nominal condition that is VDC(NOM)=390V AT THIS CONDITION my DMAx will be 0.31 ok, so my doubt is for calculating 
    V-T=(1*10^6/100KHz)*DMAX*VDC(min), HERE DMAX=0.34 OR 0.51 SHOULD I CONSIDER?

  • 0 in reply to Eldho M P
    TI__Guru 73535 points

    Hi,

    These are for steady state. You need to estimate the transient time and depends on your transient response the duty cycle can become > 0.34, but won't be reach > 0.51 since ton setup. If you can estimate your transient resulted duty cycle, then you can use that. Otherwise 0.51 is a number can be used so your transformer should be designed with margin. So it is up to you how to do. Our recommendation is for the worst case.