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Original question:

TPS23755: Design/schematic review

TPS23755: Schematic review and troubleshooting assistance

Donald Posterick
Prodigy 30 points
Part Number: TPS23755

Tool/software:

Hello,

I'm evaluating the TPS23755 IC in the flyback configuration, and was looking to get some assistance troubleshooting the design.

I have made a few minor changes from the EVM, like removing the auxiliary supply related components (only need power from POE), AUX_V disconnected based on 8.2.2.7 from the manual regarding leaving the R_AUX unpopulated when not using AUX_D functionality.

Minor value changes to current sense, and added a 68.1 ohm resistor for class 2 POE. I have adjustments added to the converter feedback to control the output voltage, but I am only able to get a very low (.2 - 2.5V with 50 ohm load), or 18-26V with a 5k ohm load when adjusting the pot RV4. 

I have tried transplanting the HR051142 ( similar to the SM13117EL) flyback transformer onto the original EVM with jumper wires, and have confirmed it operates with the new transformer, although slightly different voltage which is why I wanted to adjust the values and get the layout figured out before using it in our actual product. 

Am I missing something related to the feedback with the AUX_V when not using AUX_D? Because it seems like a feedback issue considering the lack of stability under differing loads.

Please advise,

Don

Images attached below

https://ibb.co/PZQjpRcP
https://ibb.co/R4TYLrDt
https://ibb.co/ZpBV22Bt
https://ibb.co/TDXtPmqG

  • 0
    TI__Mastermind 37121 points

    Hi Don,

    Thanks for reaching out! We will look into your question and get back in 5 business days. Thanks for your patience!

    Best regards,

    Diang

  • 0 in reply to Diang Xing
    TI__Mastermind 37121 points

    Hi Don,

    The AUX_D and AUX_V are used to adjust the resistor divider in the error amplifier. The purpose is when you have an adapter on the secondary side, AUX_D will be pulled down and AUX_V to RTN FET will be set open. In this case the PoE Vout will be lower than an adapter, which makes PoE as an redundancy power source. 

    For example, in TPS23755EVM. When AUX_D is high, 1.75V / (6.49k || 8.66k) x (24.9k + 6.49k || 8.66k) = 13.5V , 13.5V= Vout x (Naux/Nsec), => Vout = 12V. When AUX_D is low, 1.75V / 6.49k x (24.9k + 6.49k) = 8.46V, 8.46V= Vout x (Naux/Nsec), => Vout = 7.5V.

    So if you plan to disable the AUX_D function, you can DNP the 8.66k AUX_V resistor, and change the 6.49k resistor to 3.71k resistor. 

    Besides, sorry that the links you shared are blocked. Could you share the PoE schematic in pdf. version? 

    Best regards,

    Diang

  • 0 in reply to Diang Xing
    Prodigy 30 points

    Here's images of the relevant pages on the schematic, I have the AUX_V, and AUX_D disconnected as I'm not using the AUX_D function. 

  • 0 in reply to Donald Posterick
    TI__Mastermind 37121 points

    Hi Don,

    Thank you for sharing the schematic. 

    Could you share your transformer datasheet too? I need to know the AUX : SEC turns ratio.

    For the error amplifier resistor divider, you can make (1.75V / R15) x (R15 + R5 + RV4) = 12V / (SEC : AUX) 

    Best regards,

    Diang

  • 0 in reply to Diang Xing
    Prodigy 30 points

    Here's the datasheet, it has a 2:1 from primary to secondary, and 2:1 from primary to aux. aux to secondary is 1:1

    I changed the R15 to a 3.5k (with RV4 = 10k), as with the formula you provided I was getting 8.5 calculated (actual was 2.5V)

    with a 3.5k the calculated should be 14.3V, but I'm only getting 4.7 with a 50 ohm load (500 mW approx across load resistor, but circuit is consuming 2W)

    2409302331_HANRUN-Zhongshan-HanRun-Elec-HR051142_C55680.pdf

  • +1 in reply to Donald Posterick
    TI__Mastermind 37121 points

    Hi Don,

    It looks like the flyback transformer bias could be incorrect. The current stored in Lm could be revised bias to D7. 

    You may need to reverse transformer winding 3&4 and winding 1&2

    Best regards,

    Diang

  • 0 in reply to Diang Xing
    Prodigy 30 points

    This resolved my problem, reversing the windings. I overlooked that previously in the design, and when I soldered the transformer with jumper wires onto the EVM, I had put it in the correct orientation which is why it worked on the EVM, but not on my board.