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TPS23734: 15W af standard Circuit

Part Number: TPS23734
Other Parts Discussed in Thread: TPS23731

Hello Team,

1.secondary side of PoE circuit used in our design, MOSFET based complex circuit is suggested in Evaluation module schematic. Can we change that to a diode based circuit? Please also let us know the need/working of this MOSFET based circuit for "at" and "af" standard. We are facing placement related issues so we want to optimize space and cost.


2  We are looking for a low cost solution which should support "af" and "at" POE standards without changing schematics and layout design. The only BOM related change for Transformer or any other passive components are allowable. If no solutions are available other than TPS23734, then please share the reference schematic of TPS23734 for 15W output(af standard).

please find attached schematic for review 

  • Hello, 

    1. Replacing the FETs on the secondary side of an ACF would dramatically reduce the efficiency. ACF are more complex than flybacks but have the best performance. So changing the FETs out would basically ruin the only reason to use an ACF.. Flybacks are optimal for space and cost. Please consider the TPS23731EVM for reference. Please also note that the .bt/.at/.af standard are backwards compatible, so if it is .at compliant then it is .af compliant.

    2. Please work with your local FAE for more details on this inquiry. 

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

     

    Regards, 

     

    Michael P.

    Applications Engineer

    Texas Instruments 

  • Hello Michael,

    1  Can you please provide us efficiency testing results for both ACF and flyback?
    2 As of now we have designed the circuit by using TPS23734 and not with the TPS23731 as per our earlier communication. We have verified that both parts are pin to pin compatible however there are certain differences in the circuit used. so, what's the cost effective solution among both please suggest. Also note that our current design using TPS23734 is with 5V/25W output.
    3 Regarding bridge rectification at the primary supply side, in one of the reference circuits four high side diodes and four low side MOSFETs have been used instead of 8 diodes as per the below reference image. However, in our design we have used 8 diodes to save the cost and on board space.Our design supports 5V@5A output so what TI suggests here to go ahead with 8 diodes option or 4 diode- 4 MOSFET option. please share your inputs.

  • Parag, 

    1. This is found in the EVM's of the TPS23734 and the TPS23731. 

    2. The TPS23731 is the cost effective solution. The TPS23734 is essentially the same IC but with an additional gate drive. If you are going to use a topology that does not need the additional gate drive, then it is more cost effective to use the TPS23731. 

    3. I apologize, I thought you were talking about the secondary side MOSFETs in the ACF. 

    For the primary input, you can use any rectification you want. Discrete diodes will be less efficient than FETs, but as you pointed out, they are less expensive and save on space. You could also look into an integrated diode or FET bridge to save on space, but they cost more. 

    The 4 diodes + 4 FETs is somewhere between 8 diodes and 8 FETs on efficiency. It is basically the middle option between cost and efficiency. 

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

     

    Regards, 

     

    Michael P.

    Applications Engineer

    Texas Instruments