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Original question:

TPS23730: EMI compliance for isolated, non-synchronous flyback configuration with 12V output

TPS23753A: 5V@2A Flyback with TL431 (Ref=2.5V)?

Janis Sarikas
Prodigy 175 points
Part Number: TPS23753A
Other Parts Discussed in Thread: TL431, TLV431

Hello,

We used your reference design slvu160 and slvrb20 to make a prototype of a PoE converter with 12V or 5V output via assembly variation. The 12V output variant is working fine.

Unfortunately we used the TL431ACDBZ (SOT-23-3) in the feedback part for the 12V output variant and failed to see that it isn't pin-compatible TLV431ACDBZ (SOT-23-3). Is it possible to use the  TL431ACDBZ for the 5V output too?

We tried to changed R24 to 44.2K in slvrb20 to get 5V output. But than the output is not stable with low load (~100mA) and goes from 5.3V to 2V every 1s and with higher load (~400mA) the output stays at 5.3V.

Best regards

Janis Sarikas

  • 0
    TI__Mastermind 21320 points

    Hello Janis,

    I would advise against using the TL431 in a 5V output design. The reference voltage is ~2.5V, which is about 1/2 the output voltage. So the TL431 part will not turn on until the voltage is at 50%. That is why we almost always use the TLV431 part in 5V designs, but it is fine to use the TL431 in 12V designs. 

    Technically, it would work, but I don't recommend it. 

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

     

    Regards, 

     

    Michael P.

    Applications Engineer

    Texas Instruments 

  • 0 in reply to Michael Pahl
    Prodigy 175 points

    Hello,

    okay, thanks for your fast reply.

    If I would use TL431 for the 5V design just for now, could you give me a hint why the output is 5.3V and unstable at low load when I setup the voltage divider to 1/2 V_out with R24=44.2kΩ? Is it because of the "turning-on-problem" of the TL431 you mentioned? 

    Do I have to adjust the loop compensation when I use the TL431? I assembled the components recommended in slvrb20 (5V) and not  slvu160 (12V) because I thought that the PI-output filter in the 5V design has the highest effect on the transfer function. Furthermore I assembled R100 of slvu160 (12V) with a value of 2.2kOhm to still provide a bias current of >1mA to the cathode of TL431. 

    Regards,

    Janis S. 

  • 0 in reply to Janis Sarikas
    TI__Mastermind 21320 points

    Hello,

    If the output voltage is off, adjust the voltage divider. 

    You should not, but it might affect the current through the opto which would affect the gain margin. If you changed the resistor that also affects the gain margin. 

    I would take a bode plot to check stability.

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

     

    Regards, 

     

    Michael P.

    Applications Engineer

    Texas Instruments  

  • 0 in reply to Michael Pahl
    TI__Mastermind 21320 points

    Hello,

     Hello, I have not heard from you in a few days. Do you have any update on this post? I want to make sure you are setup for success for your project, but if I do not hear from you I will close this thread. Feel free to post here or on a new E2E post to continue the conversation. Thank you 

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

     

    Regards, 

     

    Michael P.

    Applications Engineer

    Texas Instruments 

  • 0 in reply to Michael Pahl
    Prodigy 175 points

    Hello Michael,

    sorry, I totally forgot to reply. Thanks for your answer.

    Unfortunately we are not ably to take a bode plot. But now we decided to use the 12V version anyway. 

    Regards,

    Janis S.