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WEBENCH® Tools/UCC28630: WEBENCH and multiple outputs

Manu Abraham
Genius 3510 points
Part Number: UCC28630
Other Parts Discussed in Thread: PMP4444

Tool/software: WEBENCH® Design Tools

Hi,

Used WEBENCH to design a flyback controller with a 12V/6A output. The result a transformer with 30turns primary and 4 turns in the secondary.

At this point, I am interested in adding a 5V output to the design. A quick thought was to add a tap at 6V and add a voltage regulator.

My questions are two fold.

#1. Adding a tap after 2 turns is the right/best thing to do ? Or should a complete 5V separate turn should be added instead ?

#2. I wonder what's the maximum current, that I can draw at the 5V tap ?

Can someone please clarify ?

Thanks,

Manu

  • 0
    TI__Mastermind 25330 points

    Hi Manu,

    The default WEBENCH design only provides single output. I have contacted the product experts to provide further guidance on feasibility/implementation of an additional 5V output. Thanks for your patience.

    Regards,

    Amod

  • 0 in reply to Amod Vaze
    TI__Mastermind 31580 points

    Hi Manu

    PMP4444 is a 24V / 12V design that should help with the schematic changes.

    The difficulty is getting the correct turns ratio to get the correct ratio between the two output voltages 12V and 5V. The 12V and 6V is a good compromise but it may not leave you much head room for the voltage regulator.

    If you made the secondary turns 36 in total then tap off the 5V winding at 16 turns you would have 5.38V, which with a Schottky rectifier would give you a 5V output.

    The combined loading of the two outputs must within the designed input power of the power stage. The max current on the 5V should be the same as the max current on the 12V as its related to the wire diameter and copper loss of the secondary winding.

    I hope this helps.

    Regards

    Peter

  • 0 in reply to Peter Meaney
    Genius 3510 points

    Hi Peter,

    I get what you are saying .. but I get nowhere since I have 4 turns in the secondary and that leads nowhere.

    Looking at PMP4444:

    I see that it is tapped at a 12V turn. In this case, how is the current at the tap estimated ? (Or is it assumed that power is constant ? 12V/3A (36W) . 24V/1.3A (31.2W))

    Thanks,

    Manu

  • 0 in reply to Manu Abraham
    TI__Mastermind 31580 points

    Hi Manu,

    Sorry I misunderstood the turns on the secondary,

    If you only have 4 turns on the secondary for a 12V output then you can tap midway and get a 6V output. If you rectify this rail with a Schottky you can get a 5.5V rail or you can adjust down the 12V out to 11.5 and get 5.2V.

    Yes the power is limited by the primary side design and the choice of Current sense resistor. Also if you draw twice the current from the 5V rail than the 12V rail then the I squared by resistance loss in the secondary side winding will be four times higher.

    Also you will have cross regulation to consider if you have a mismatch in the power been taken from each winding.

    TI has a number of useful papers on Power Supply Design which cover some of issues with multiple output designs,

    http://www.ti.com/ww/en/power-training/login.shtml?DCMP=pwr-psds-archive&HQS=pwr-psds-archive-psds

    Regards

    Peter

  • 0 in reply to Peter Meaney
    Genius 3510 points

    Hi Peter,

    Thanks for the explanation. can you please provide a link to the document covering multiple outputs ?

    Thanks,

    Manu

  • 0 in reply to Manu Abraham
    Genius 3510 points

    Hi Peter,

    Saw the link just now. The link did not appear as expected, reload of the page after the reply, helped to show the link (weird behaviour)

    Thanks for the link anyways. My bad that I missed the links initially.

    Thanks,

    Manu