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TPS23753A: The VC and GATE are unstable in TPS23753A

David Hsu1
Prodigy 70 points
Part Number: TPS23753A
Other Parts Discussed in Thread: LM431

I built a PD board with this part and when I connect to PSE board, ths PSE board can indetify the PD board in class0. but the VC and GATE are unstable. the waveform are as below

do you have any comment abbut it?

thanks

David

 

  • 0
    TI__Mastermind 21320 points

    Hello David,

    Thank you so much for the screen shots. Can you please provide a schematic of your system for review? It would help me understand how you have your system set up as well as check component values. It is hard to say definitively what could cause this problem without it. 

    Additionally, can you capture the startup sequence on VDD and Vout? I am curious to see how those pins are behaving and if that can tell us more information. Thank you!

    Regards,

    Michael Pahl

  • 0 in reply to Michael Pahl
    Prodigy 70 points

    Hello Michael

    the design concept is 5V/7W output

    finally, I found there is something wrong in my schematic design(please see attached file, ) and I also found the TPS23753A is damage.

    the behavior is

    1. when BAV99 is in wrong design and I plug PoE injector, there is no 5V output and TPS23753A is damaged

    2. when I reworked the BAV99 first and then plug PoE injector, the 5V output is normal

    could you help me

    1. why it causes the IC damage

    2. what is the purpose for BAV99 circuit

    3. review the schematic

    thanks 

    CA-NF21A_PD.pdf

  • 0 in reply to David Hsu1
    TI__Mastermind 21320 points

    Hello David,

    Thank you for providing the schematic. I am sorry to hear a board got damaged. 

    The BAV99 is designed to have a small forward voltage, large breakdown voltage, and low capacitance. When hooked in the original configuration, it looks like the anode (pin 1) is connected to a higher voltage that is between the inductors. The cathode (pin 2) has a capacitor and the LM431 between it and ground. It could be possible that the voltage here was enough to discharge. 

    Whereas, when you flip the device so that the cathod is connected to the higher voltage, the BAV99 has the large breakdown voltage which would prevent any current flow.

    Commonly this device is used for ESD protection, so that a discharge can have a safer path to take. I found this article that discusses the operation of the BAV99 in regards to using it as an ESD protection device:

    http://www.ti.com/lit/an/slva898/slva898.pdf

     

     However, all of this is happening on the output side of the transformer, so there could be another reason the IC is damaged. Unless the photodiode is transmitting the wrong feedback and that could cause damage to the IC. I am going to do a deeper dive into your schematic and we can discuss more. Do you have any questions on this? 

    Regards,

    Michael Pahl

  • 0 in reply to Michael Pahl
    TI__Mastermind 21320 points

    Hello David,

    I reviewed your schematic, and I found something I wanted to share with you. The TVS diode (Schematic label: EKPD502) is rated for 64V. We generally say to design for 58V TVS diodes because if the circuit was subjected to a surge, a 58V TVS would clamp the voltage around 89 volts. The IC is rated for 100V max. So, using a TVS that has a higher rating will likely result in a higher clamping voltage that could be above the 100V max. This could damage the IC, which is what I believe is the source in this case. 

    Are you able to try a different TVS? If you would like a suggestion, the TPS23753A EVM-004 uses the SMAJ58A.

    Regards,

    Michael Pahl 

  • 0 in reply to Michael Pahl
    Prodigy 70 points

    Hello Michael

    thanks for your comment, I will check it

    Regards,

    David

  • 0 in reply to David Hsu1
    TI__Mastermind 21320 points

    Hello David,

    I wanted to check in on you with this question. Have you had the chance to check the TVS diode? I want to make sure you are setup for success with your project. 

    If I do not hear from you in a few days I will assume your question is answered and I will close this thread. Please let me know your progress and we can continue from there. Thank you!

    Regards,

    Michael Pahl

  • 0 in reply to Michael Pahl
    Prodigy 70 points

    Hello Michael

    sorry to reply late

    currently, I donlt find any IC damage till now with P4SMAJ64CA and I will change the TVS to P4SMAJ58CAG in my next production for safe

    thanks

    David