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Multi output Flayback design

Other Parts Discussed in Thread: UCC28911, UCC28740, LM5023, UCC2813-2, UCC2813-0, UC3842, UCC2800, UCC2802, UCC28C42, UCC28C43

Dear all.

I think about the multi-output by flayback.
(DC3.3V/450mA, DC15V/10mA, DC-15V/10mA, DC24V/30mA)
Our customer made the following transformer.
inductance : 700uH
Turn ratio (24V output) : 2.69:1
Turn ratio (12V output) : 5.38:1
Turn ratio (3.3V output) : 17.5:1
Feedback is 3.3V.
I want to get good cross regulation of 12V and 24V, what kind of method is there?
I am understood that the output voltage changes to some extent because Duty changes.


  • Hello Pan-M,

    I believe this posting is a spin-off of your previous posting which was answered, in part, by Billy Long.
    Here are some additional resources to investigate about cross-regulation:
    - output cross-regulation&tisearch=Search-EN-Everything
    - (See section 10.2.3 on multiple output windings.)

    The key to good cross-regulation without LDO post-regulators is to balance the leakage inductance of all of the windings, which is not a trivial task. Extreme care is required in transformer winding structure design to obtain the desired leakage inductance targets and to keep them consistent from one transformer to the next. That will help minimize voltage variations as load changes.

    But even a perfectly balanced transformer cannot prevent voltage changes if the loads on each output vary widely and independently. So try to minimize the changes in load, or make them track in the same direction.

  • Yes, that's correct.
    We tried it by UCC28911, but do not work well.
    I thought about the change of the design and asked this question.


  • Can we design ucc28740 in the input range of 170V from 20V?
    What is the best device that we achieve this input and output condition?
    In addition, will there be the reference design?
  • Hello PAN-M,

    The turn-on voltage threshold at VDD of the UCC28740 is typically 21 V and can be as high as 23 V, so it cannot be used with a system input down to 20 Vdc. TI has several other controllers with lower turn-on thresholds that can work in your application. The LM5023 is similar to the UCC28740, with quasi-resonant operation and variable frequency switching.

    Because of your wide input range (8.5 : 1), some sort of switchable start-up current source needs to be used to limit the start current and disable it after start-up. This circuit is described in the LM5023 datasheet, page 11. If you desire programmable fixed-frequency operation, consider the LM5021-1 device which allows up to 80% duty cycle to accommodate the input range. Other devices are updated derivatives of the original UC3842 family, such as UCC28C42 or UCC28C43, UCC2800 or UCC2802, and UCC2813-0 or UCC2813-2. (The UCC2813-x series is a lower-cost version of the UCC280x series with a few relaxed specifications, as indicated in this document: ).

    The LM5023 operates strictly in DCM, whereas all the others are capable of operating in CCM. At your power level, designing for DCM would be simpler to avoid having to add slope compensation (automatic in LM5021-1). However, a short-circuit or overload condition may force the converter into CCM unless the controller is the LM5023. On the other hand, slope-compensation is used to prevent sub-harmonic oscillation which is undesirable during normal steady-state operation, but you may or may not consider it to be acceptable during an overload condition or a temporary load-step transient.

    There are a number of multi-output reference designs with input voltage ranges between 18 V and 70 V, and a few even to 110 Vdc, but I could not find any that span 20 to 170 V, and most of the power levels were much higher. Also, most of them used post-regulation on their outputs to avoid the tricky leakage and load balancing needed for tight cross-regulation. To inquire whether a special reference design can be done for your application, please contact the TI Designs team at .


  • Thank you for comment.
    I check contents of your comment.