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TPS23753A: Secondary-side rectifier diode

Pjs
Intellectual 880 points
Part Number: TPS23753A
Other Parts Discussed in Thread: TPS23754, UCC24630, TPS23757, TPS23754EVM-420, , TPS23753

Hi, 

I'm using the TPS23753A with the Coilcraft POE13P-33L transformer. It looks like the current output will be too high to use a secondary-side rectifier diode and so I've been looking at some of the synchronous arrangements in the various TPS23753A evaluation boards. 

It looks like most the approaches here require another coil on the transformer which I don't have available. Is there a way of doing this without the extra coil - would the method used in the TPS23754 eval board be suitable for example?

Alternatively, are there any synchronous rectifier controller ICs such as the UCC24630 that could be used? I'm finding most of this type of IC needs a voltage higher than the 3.3V supply I have. 

Thanks

  • 0
    TI__Mastermind 21320 points

    Hello Pjs,

    Thank you very much for your question. Based on the input given it seems like a driven synchronous flyback topology would be an optimal choice for this design, since it does not require the secondary winding.

    I would encourage you to look at the TPS23757  EVM:  

    This design has an output option of 3.3V and 3.3A. It has a synchronous output and no additional coil.

    The TPS23754EVM-420 is a self driven synchronous flyback design. It too uses a pulse transformer, and has 5V 5A output. What power level are you designing for in this project?

    Please review and let me know if you have any questions. Thanks!

    Regards,

    Michael Pahl 

  • 0 in reply to Michael Pahl
    Intellectual 880 points

    Hi Michael, 

    Thanks for your quick reply. The design is for a 802.3af Type 1 design (so approx. 13W).

    The TPS23757 looks interesting thanks. The project I'm considering this for is a reasonably space constrained (mostly in height) and so I'm slightly concerned by the need of the larger 1uH+330uF output filter which appears to be much smaller in say the TPS23753AEVM-001 design. Is the larger filter needed when you have a synchronous design?

    What are the advantages/disadvantages of the self driven synchronous design in the TPS23754 vs. the driven TPS23757?

    With both these designs, are there WEBENCH or Mathcad calculation sheets available to assist with design?

    Many thanks

  • 0 in reply to Pjs
    TI__Mastermind 21320 points

    Hello,

    The advantages and disadvantages of the different designs is best explained in this article: https://e2e.ti.com/blogs_/b/powerhouse/archive/2015/11/10/maximizing-poe-pd-efficiency-on-a-budget 

    The flyback does not require the output inductor. We have some designs that do not use inductors at all. However, it does help reduce output ripple, so often it is needed to meet the ripple spec. So you can change it to something that fits your space requirement, but the trade off is less output filtering. 

    We do not have a WEBENCH or Mathcad calculation sheet at this time. Our datasheets do not include information on how to design the power stage, since many of our parts can be used in multiple topologies. 

    Below is a paper that details how to design a flyback with the TPS23753. This is a helpful tool as it walks through an example of design.

    "Designing with the TPS23753 Powered Device and Power Supply Controller (Rev. C)":  

    Please let me know if you have any further questions, or if this solved your issue please indicate on the forum. Thank you!

    Regards,

    Michael Pahl 

  • 0 in reply to Michael Pahl
    TI__Mastermind 21320 points

    Hello Pjs,

    We have not heard from you in almost a week. Do you have any update on this post? 
    I want to make sure you have everything you need to be successful on your project, but since we have not heard from you we will close this thread in a few days. Please let us know the status of your project and we can continue from there.

    Thank you!

    Regards,

    Michael Pahl 

  • 0 in reply to Michael Pahl
    Intellectual 880 points

    Hi Michael, 

    Thank you for your reply + apologies for the delay in responding. 

    Thanks for the article that provides some information on the why you might want self synchronous vs. gate driven secondary rectification - it sounds like the driven approach will provide the most efficiency. 

    Understood regarding the design notes but in this case, is it possible to continue this conversation offline so that we can discuss schematic review and more specific details? Is there an email I can use?

    Many thanks

  • 0 in reply to Pjs
    TI__Mastermind 21320 points

    Hello Pjs,

    Yes we can continue this conversation in the private messenger on E2E. There you can send me your schematic and we can talk further.

    Additionally, there is some great training on PoE online: http://www.ti.com/power-management/power-over-ethernet-poe/support-training.html

    And for the new standard specifically:

     https://training.ti.com/power-over-ethernet-getting-started-new-standard 

    These are great for anyone new to PoE in general to learn. 

    Regards,

    Michael Pahl