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Need help on allowable ripple input from a 24VAC half wave rectifier for power supply input

Joel Capehart
Prodigy 70 points
Other Parts Discussed in Thread: TPS54560, TPS54260, UCC2809-2

I am working on a design that require outputs of 15VDC@3A and 5VDC@3A from an input of 24VAC which requires a half wave rectifier circuit only.  I am concerned about how much of a smoothing capacitor I will need to get the TI power supplies circuits to work properly.  

I roughly need 4700UF to smooth out the half wave rectified voltage when I may have as much as 3A of current required on the input.  Even this may not be enough.

A 4700UF 63V capacitor is a lot bigger then I really want to use.

Does anyone have a example circuit or feedback on what the best approach would be for this?

How much ripple can I allow on the input of the power supplies before the TI switching supply circuits can keep up?

  • 0
    TI__Expert 7395 points
    Joel,
    thanks for your interest in TI here.
    It really depends on the UVLO of the controller you are looking at. typical UVLO might be around 10-12 V or similar but some are lower. A few questions to help clarify:
    1) does this need to be isolated?
    2) do you have any controllers in mind already?

    i couldn't find any reference designs close to those requirements.

    Regards,
    John
  • 0 in reply to John Stevens
    Prodigy 70 points
    Isolation is not required. I am looking at using the TPS54560 IC for the 2 on board power supplies.
  • 0 in reply to Joel Capehart
    TI__Guru**** 191445 points

    I found a reference design for 18 - 32 VAC input to 12 V @ 3 A and 5 V @ 2.5 A output.  Pretty similar to your requirements.  It is isolated however and does use quite a bit of filter capacitance (3 x 1000 uF).  It uses UCC2809-2 controller.  I also found another (PMP10908) that is a single output.  24 VAC nominal input to 5 V@ 2.5 A using TPS54260.  TPS54260 is very similar to TPS54560.  This design is only using 470 uF on the input.  There is no test report for it, so who knows if it was ever built and tested. There are probably others as well.  Those were just two that I found.