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Transformer selection in LM5017 Fly-Buck circuit

Bell Ye
Intellectual 1135 points
Other Parts Discussed in Thread: LM5017

I encountered a problem when I design the Fly-Buck circuit with LM5017. 

 

The below schematic 1 and 2 use two kinds of transfer.

In schematic 2: the 3 isolated output use separated coils.

In schematic 1: the coil of 19V output and 6.5V output is shared.

 My question:

  1. As to the current calculation on secondary side, is there any differences in scheme 1 compared to scheme 2?
  2. Is there any disadvantages(performance, ripple, …) in shared transformers, like in scheme 1?
  3. Which usage is better, scheme 1 or scheme 2?

 

Many thanks and look forward to your reply!

 -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Schematic1:

 

 

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Schematic2:

 

 Best Regards,

Bell Ye 

  • 0
    TI__Expert 4015 points

    Hi Bell,

    1. As to the current calculation on secondary side, is there any differences in scheme 1 compared to scheme 2?

    In scheme 1, all the load current in both the 19.5V and 6.5V outputs flows in the lower portion of the winding. This increase in RMS current will increase the conduction loss in this winding and will probably require extra strands of wire to carry the current. 

    1. Is there any disadvantages(performance, ripple, …) in shared transformers, like in scheme 1?

    Scheme 1 is sometimes more complicated to wind, although this depends on the detail of turns and layer count for the particular transformer

    1. Which usage is better, scheme 1 or scheme 2?

    Scheme 1 is better for cross regulation and is generally recommended. 

  • 0 in reply to Billy Long
    TI__Intellectual 1135 points
    Hi Billy,

    Thanks a lot for your answer, it's much more clear to me now!