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UCC28740: application issue

?? ?
Prodigy 30 points
Part Number: UCC28740
Other Parts Discussed in Thread: UCC28600, UCC28630

Tool/software:

Hello, when designing the QR quasi-resonant flayback in UCC28740, in section 7.4.2 of the datasheet, the maximum value of Dmagcc is limited to 0.425, I would like to ask a few questions:

1. Is the restriction of Dmagcc triggered only in CC mode? Is there such a restriction in CV mode?

2. Under what conditions will CC mode be entered? Whether the switching frequency reaches the maximum value of 100KHZ will enter CC mode

3. If Dmagcc has a maximum value limit in CV mode, then if we want to increase the power of flayback, due to the limitation of Dmagcc, in QR mode, only the turns ratio or primary side shutdown current can be increased, then the secondary side loss of the transformer in QR mode is greater than the loss in DCM mode, and the QR mode will not reflect the advantage. (Input voltage range: 500V~1000VDC, output power: 85W (double winding output 12V&24V, 12V winding: 35W, 24V winding: 50W); QR parameters: shutdown current 1.9A, transformer turn ratio 10:1:2, Lm=650uH; DCM parameters: transformer turn ratio: 27:2:4, excitation inductance 700uH. )

4. Is there any other QR quasi-resonant control chip, there is no Dmagcc limit or Dmagcc is limited to about 0.7?

  • 0
    TI__Guru 74115 points

    Hi,

    1.

    That 0.425 demag time is the max that the IC can do no matter in CC or not - which means when reach this number, CC is to happen if load further increase

    2.

    It is related to your design. The below equation specifies at what load current to reach 0.425 when you change tDM to tDMAgcc. The switching frequency is also designable based on a DCM flyback Pin = 0.5 x Lp x Ipp^2 x fsw. So fsw is a function of Pin, Lp and Ipp in your design. You can use the excel sheet on ti.com to help. After you specify Iou_cc then these will be designed.

    3.

    The below is fundamental for DCM flyback. so the power can be increased by Ipp or fsw along with Lp.

    Pin = 0.5 x Lp x Ipp^2 x fsw

    You can use the excel sheet on ti.com to help your design

    4.

    No, and not needed. Your understanding to need to have 0.7 does not seem correct to me. Why do you need this? Please use the excel sheet on ti.com to help your design.

    https://dr-download.ti.com/design-tools-simulation/calculation-tool/MD-tXtv2g4BkX/01.00.00.0B/sluc487b.zip

    On the other hand, 85W is a bit too high for this IC. You will need to add external driver. You may consider other devices such as UCC28600, or UCC28630.

  • 0 in reply to Hong Huang
    Prodigy 30 points

    In the UCC78740 application, the output current was too large, and the tdm/tsw was limited to 0.425, so I had to increase the Ipp and Np/Ns, which leads to high losses in the transformer windings

    I flipped through the data sheet of the UCC28600 and there is no limit of Dmagcc=0.425, I would like to ask if UCC28600 has this limit when designing.

  • +1 in reply to ?? ?
    TI__Guru 74115 points

    Hi,

    UCC28600 does not have this Dmagcc limit. Dmagcc is for constant current operation. UCC28600 does not have the constant current operation accordingly.

    You can use ucc28630 if constant current is required.