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TPS23754: 8.5V on TPS23754

Chip Brown
Prodigy 20 points
Part Number: TPS23754
Other Parts Discussed in Thread: TPS2373, TPS2372

We have a proto build for testing of a supply using TPS23754, based on the -383 EVM (SLVU304C ref design), with FET and diode bridge options to see whether they matter at 25W.  It works fine, but we are now looking at a design that will need 8.5V rather than 12V. 

We need to push this design to get as much power as possible out of a standard 802.3at switch output, so efficiency matters.  Should we consider a custom transformer with different turns ratio, and if so, are there any tools to help design it?  Or would it make so little difference that it's not worth the hassle and cost?

  • 0
    TI__Mastermind 21320 points

    Hello, 

    The transformer is one of the most important parts of a design that determines the system's performance. 8.5V is a non-standard output voltage for PoE, so you probably won't find many reference designs with it. I'll do my best to layout some options for you:

    Option 1:  

    Have you considered the PMP6584? It is 8V/24W design. 

    http://www.ti.com/tool/PMP6584

    Option 2:

    You design a 8.5V output with the TPS23754. In this case, if efficiency matters you will probably have to define your own transformer. I have gotten to work with Wurth, Linkcom and Semitel with defining new transformers. They are all great and if you tell them your output voltage and that efficiency is important (and therefore a low leakage inductance) they can create samples for you. Then you will have to optimize your design.

    Option 3: 

    You could use one of our standalone PD's (like the TPS2373 or TPS2372) that do not have the DC-DC integrated. Then you could look through other DC-DC designs that are also standalone (like the UCC or LMS families) that has 8.5V and stick the PoE PD on the front end. This might be easier if you find the right DC-DC stage. 

    Option 4: 

    We have some Class 4 efficiency optimized reference designs you could consider using instead. Both are 5V output, and one is an active clamp forward topology. You can either try to make these 8.5V by changing the transformer and then other components as needed, that might be a good starting place. Or if you can change the requirement to 5V then you could use the designs as is:

    Lastly, we have a post on how to approach efficiency on a budget:

    If this post answers your question, please indicate so by marking this thread as resolved. Thank you.

     

    Regards, 

     

    Michael P.

    Applications Engineer

    Texas Instruments