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TPS23751EVM-104: Output oscillation

Daan van der Pas
Prodigy 45 points
Part Number: TPS23751EVM-104
Other Parts Discussed in Thread: TPS23751, , TIDA-00617

Hello,

I am using the TPS23751 to power my design. I already produced a couple of 100 boards and sometimes I see weird behaviour of the output voltage. During startup some of the boards starts oscillating a bit. The oscillation is between +-200mV around the original voltage. I also see this kind of oscillation when I attach a heavy load(2Ohm) and then just disconnect it. Is this reliable to my first problem? 

P.S. I don't see this behaviour at all the boards.

  • 0
    TI__Genius 9230 points
    Hi Daan,

    It sounds to me like are encountering some instability.

    Could you give me more details about your design? (output voltage/current, input voltage range, topology)

    Additionally, you can post your schematic or you can send it via direct message. I can give feedback.

    Thanks,
    Tom Amlee
    PoE PD Apps Engineer
  • 0 in reply to Thomas Amlee
    Prodigy 45 points
    Hi Tom,

    I send you the schematic. I will give you some more info through this message. This circuit is based on the TPS23751EVM-104 to power the PF3000. The disadvantage of the PF3000 is that it cannot be powered with 5V. Therefore we changed the feedback resistor so there is 3.8V at the output. Is there a reference design of the TPS23751 with 3.8V/4.5A at the output?
  • 0 in reply to Daan van der Pas
    TI__Genius 9230 points
    Hi Daan,

    It sounds like you are encountering some light load regulation issues. Is VFO a requirement for your design? The change in the output voltage would require additional changes to the VFO circuit.

    We typically recommend active clamp forward for low voltage class 4 designs, but we also have TIDA-00617. It is a 5V/5A TPS23751 design without VFO. It would need to be modified for 3.8V.

    Regards,
    Tom Amlee
    PoE PD Apps Engineer
  • 0 in reply to Thomas Amlee
    Prodigy 45 points

    Hi Tom,

    Yes we need VFO.

    Can you tell me something about the function of R33(49.9Ohm). In our schematic we removed that resistor. Is it possible that we have some issues with the soft start. We also changed the octocoupler in that part.

    Regards,

    Daan van der Pas

  • 0 in reply to Daan van der Pas
    TI__Genius 9230 points
    Hi Daan,

    Apologies for the delay. I'll follow up with you this week.

    Regards,
    Tom Amlee
    PoE PD Apps Engineer
  • 0 in reply to Thomas Amlee
    TI__Genius 9230 points

    Hi Daan,

    To answer your question about R33 on TPS23751EVM-104: it can be shorted. It is used for loop injection for measuring frequency response.

    Additionally, be sure that the optocoupler you have used is for the feedback loop (U4 on TPS23751EVM-104) has a CTR of 80% to 160%. It is important for the stability of the feedback loop. The optocoupler (U3 on TPS23751EVM-104) used for the SRD signal can have a high CTR: 300-600%. I am unable to determine the CTR of your chosen optocouplers (U26/U27), but be sure the CTR follows the advice above.


    On your schematic, I noticed that the R173 & R164 resistors are slightly different from the TPS23751EVM-104. This is fine since you are using a different output voltage, but be sure that those resistors are sized according to the guidance provided in section 2.8 of SLVA305C.

    Regards,
    Tom Amlee
    PoE PD Apps Engineer

  • 0 in reply to Thomas Amlee
    Prodigy 45 points

    Hi Tom,

    Thank you for your response. I found the solution of my problem. I added R33 in my schematic and then it is working. So I guess it can't be shorted haha. Now all of my PCBs are stable.

    Regards,

    Daan van der Pas