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Original question:

TPS23755: EVM schemtic

TPS23755: About CBULK

user5397266
Prodigy 230 points
Part Number: TPS23755

Hi team,

I forget where I found the document, but I think it says that the CBULK must be an electrolytic capacitor, and if a ceramic capacitor is used, it should be connected in series with a 1 ohm resistor.

What is the reason for this?

Also, in the related thread it says that ceramic capacitors can be used at low power levels, but if the application is 8W or less can ceramic capacitors be used and no series resistors are needed?

Thanks.

  • 0
    TI__Mastermind 37081 points

    Hi Sir/Madam,

    To consider a worst conditions that including 250 us no input power, 100 meter cable, and highest operation power, the recommended bulk cap values are shown below. A ceramic cap may have capacitance vs dc bias drop which causes it could not sustain the same energy compared to electrolytic cap at the same condition. 

    Power                  Bulk Cap

    13W                      10-22uF

    25W                      33-47uF

    50W                      68-100uF

    70W                      100uF-200uF

    Best regards,

    Diang

  • 0 in reply to Diang Xing
    Prodigy 230 points

    Hi Diang,

    Is the DC bias characteristic the only reason why electrolytic capacitors are recommended?

    If so, why is it recommended to connect 1Ω in series when using ceramic capacitors?

    Does connecting a resistor in series with a ceramic capacitor improve the DC bias characteristics?

    Thanks.

  • 0 in reply to user5397266
    TI__Mastermind 37081 points

    Hi Sir/Madam,

    It is not mandatory to have 1Ω in series when using ceramic capacitors. Normally ceramic cap has lower ESR and ESL. Adding series resistor could increases its damping to oscillation. In some cases when too high overshoot/undershoot happens during transient, the series resistor may help.

    The resistor does not help the DC bias characteristics.

    Best regards,

    Diang

  • 0 in reply to Diang Xing
    TI__Mastermind 37081 points

    Hi Sir/Madam,

    I will close this thread for now. Please reply or open a new thread if you have further questions.  

    Best regards,

    Diang

  • 0 in reply to Diang Xing
    Prodigy 230 points

    Hi Diang,

    I am not going to put in a resistor to improve the DC bias characteristics.

    I found a document stating that a 1 ohm resistor is required.

    PoE PD Schematic Review Guidelines

    It says "strongly recommended", but is it really necessary?

     

    Thanks.

  • 0 in reply to user5397266
    TI__Mastermind 37081 points

    Hi Sir/Madam,

    Adding 1 Ohm resistor help to damp the transient oscillation. Meanwhile it increases the primary side FET switching time and make the converter easier to pass some EMI tests.

    But as another side, the resistor added additional stray inductance to cause more overshoot on the FET Vds, more energy dissipate on clamping / snubber circuits. Switching loss also increases due to switching speed is slower.

    I would consider the 1 Ohm is optional. It depending on your DC/DC converter design from both schematic and layout aspects.

    Best regards,

    Diang