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PoE - TPS23756 - 7.5V out design

Newton
Expert 1125 points
Other Parts Discussed in Thread: TPS23756, TPS23754

Does TI has any ref design that uses TPS23756 & gives out  +7.5V out of the DC-DC rather than 5V or 12V, which are much more common.  

Is it possible to use one of the standard transformers, say for 5V, but modify the feedback circuit to give us +7.5V out?

Referencing the EVM schematic, it appears that the opto-isolated feedback circuit can be easily altered by changing resistors to give  +7.5V out.   This should result in an increase in the bias winding voltage to approximately 15V from around 10V unmodified.   Since this is still within the allowable range for VC, we are thinking that this might be a viable solution.  Would this work?    

  • 0
    TI__Mastermind 42795 points

    There may be some options, but we would really need to know more of the system level requirements. Considerations are:

    - Auxiliary adapter support? What adapter voltage range is required? TPS23756 can support a wider range of adapter input voltages, but the design becomes much more unique including specific transformer, switching FETs, compensation, etc. If a standard input voltage range is required, then there may be another design available using TPS23754.

    -Output voltage range, power level, efficiency requirements, safety, EMC ....

  • 0 in reply to Eric Wright
    Expert 1125 points

    Yes, we would like to use auxiliary power as a back-up when POE is not available.  The preferred auxiliary voltage wpuld be 24VDC, but 48VDC could be used if necessary.  The output is +7.5V +/- 5% at 3A.  Desired efficiency is greater than 80%.  We will need to meet all US, EU, Japanese electromagnetic compatibility requirements. 

    If you can send me your email address - I can send more info.

    regards

  • 0 in reply to Newton
    TI__Mastermind 42795 points

    TPS23756 would normally be used when a wide input range dc/dc converter design is required (usually, the 12V adapter requirement necessitates the use of TPS23756). For 24 and 48V adapters, the TPS23754 can be used.

    I found an existing reference design using TPS23754 which can support POE or a 48V adapter. The dc/dc input range is 33V-57V and the output is 8.5V at 3A (this output could most likely be reduced by changing the sense resistor values slightly. The efficiency is ~92.5% from the adapter input connector and ~89% from the POE input connector (with 48V input and full load). In order to support a 24V adapter input, a custom transformer would be required and the efficiency might go down slightly.

    I also found an existing reference design using TPS23756 which can support POE or a 12V, 24V, and 48V adapter.s The dc/dc input range is 10.5V-57V and the output is 8.0V at 3A (this output could most likely be reduced by changing the sense resistor values slightly. The efficiency is ~88.5% from the adapter input connector (with 12V input and full load) and ~86% from the POE input connector (with 48V input and full load).

    If neither of these can fit your entire requirements, you can request design services support through your local TI Sales Office.

  • 0 in reply to Eric Wright
    Expert 1125 points

    Eric,

    The preference is to use  24V adapters.  You mention that the TPS23754 design accepts 33-57V, so it would seem that the TPS23756 reference design for 8V output would be most suitable.  Can you get  me that design?

    Let me know.

    regards

  • 0 in reply to Newton
    TI__Mastermind 42795 points

    The schematic and BOM are attached.

    TPS23756_8V-3A.zip