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need help for Flyback transformer design

ramanathan muthusamy
Prodigy 130 points
Other Parts Discussed in Thread: UC3844, UCC28600

hi budies,

              in flyback transformer design, wat the major role in primary winding inductance. i consider winding ratio only.can anybody tell the reason plz...?....

  • 0
    TI__Guru 73825 points

    In flyback converter topology, one function of the transformer primary winding is designed as an inductor to store energy in its magnetic field (most of the energy will be stored in its air gap) during the time when the primary switch turns on. The stored energy then delivers to the load through its sedondary winding with associated circuit when the primary switch turns off (the energy in the magnetic field is discharged/delivered to the load).

    The transformer in typical flyback topology is sometimes also called coupled inductor as well. The transformer in flyback topoplogy provides three main functions: (a) set up proper turns-ratio to help the voltage conversion; (b) set up proper isolation/insulation between primary and secondary (between different sides if more than two isolation is needed); and (c) store energy in the magnetic field through its energized winding, and discharging the stored energy to outside circuit/load through alternative winding(s) when the energized winding is off energized.   

  • 0 in reply to Hong Huang
    Prodigy 20 points

    Thanks for the primer.  Can you please assist: Looking for assistance with a universal input, 28VDC output at 1.-1.3A.  Came up with the UC3844 as a candidate controller.  Any suggestions on: 1. and updated controller with potentially higher efficiency/other features, and 2. power transformer design tools and supplier sources.

    Thanks much.

    Stewart Hough

  • 0 in reply to Stewart Hough
    Prodigy 20 points

    3 weeks and no response, this forum is worthless, TI is wasting my time with this crap.

  • 0 in reply to Stewart Hough
    TI__Prodigy 270 points

    This may be too late for you, but I'll give it a whirl.

    1.  Most flyback controllers offer similar efficiency through the mid and upper portions of their power range.  For general purposes, the UC3844 has been around for a while and is pretty straightforward.  There are also CMOS versions and a TLV3844 (a bit lower cost) version as well. 

    Light load is typically where efficiency gains are to be found, especially with regard to Energy Star requirements.  For that purpose, the UCC28600 is one of our "Green Mode" controllers and has excellent performance across the load range.  Our online product folder is below.  Hopefully the link works but you may need to copy and paste.

    http://focus.ti.com/docs/prod/folders/print/ucc28600.html

    Compared to the UC3844, the UCC28600 uses less power for internal functions, but also incorporates a number of medium and light load efficiency enhancements.  It is more complex to use so you would need to consider your particular requirements to see if it is necessary.

    2.  Flyback transformer design can be fun but also challenging.  If you've done some magnetics design or have some time on your hands, we have a variety of training materials on line at the following link:

    http://focus.ti.com/analog/docs/training.tsp?familyId=64

    Of particular interest would be our 2001 Magnetics Design Handbook, which has a section on flyback design.

    http://focus.ti.com/docs/training/catalog/events/event.jhtml?sku=SEM401014

    For most people, it is usually best to engage a magnetics house to design and build a transformer to your specification.  This is especially true when product safety standards must be met, which have very specific requirements for construction, spacing, materials, and dielectric withstand.  Who you choose depends to a large degree on your estimated annual usage and target market.  For commercial and consumer designs, if the volume is higher (10K+) an offshore house may be an option, or a local house with offshore connections.  For low volume and specialty applications (mil/aerospace/medical) a local vendor is generally preferred.  Many of these are regional, so it is difficult to recommend one.  Perhaps other engineers in your area might know of or have a preferred magnetics house.