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PTH08T241W: Ceramic capacitor voltage rating

Jonathan Navor
Genius 9475 points
Part Number: PTH08T241W

Hi Team,

Good day!

We would like to know the voltage rating for the ceramic capacitor within the DC-DC converter for PTH08T241W. The datasheet did specify that the voltage rating and derating of this capacitor only allows it to be used for voltages that are equal or less than 5.1 V.

The voltage rating of this capacitor only allows it to be used for output voltage that is equal to or less than 80% of the working voltage. Is this for ceramic capacitor?

Thank you in advance. 

Best regards,

Jonathan

  • +1
    TI__Mastermind 18370 points

    Hi Jonathan,

    Yes. The ceramic capacitors are the ones that de-rate (capacitance decreases with voltage).
    Some of the big output caps in this module of of 6.3V rating.
    5.1V/6.3V= 0.8, meaning 80%, and that is why the voltage should be equal or less than 5.1V.

    Hope this helps.

    Regards,

    Yitzhak Bolurian

  • 0 in reply to Yitzhak Bolurian
    TI__Genius 9475 points

    Hi Yitzhak,

    Good day! We have a follow up question from the customer. 

    The datasheet indicates 14V for Vi. Our application needs it at 12V, which is currently working for the non-ceramic options. Since you mentioned about the derating for the ceramic options, it seems that 12/0.8=15V which would have exceeded the 14V recommended limit.

    Am I correct? or the 14V limit has already taken into account the derating for ceramic cap?
    Best regards,
    Jonathan
  • +1 in reply to Jonathan Navor
    TI__Mastermind 18370 points

    Hi Jonathan,

    When the data sheet mentions minimum requirement for the input capacitor, it "talks" in terms of Capacitance, not the written value on the capacitor.

    In other words the 200 uF should be the derated capacitance of the input capacitor.
    Meaning, the written value on the input capacitor should actually be more than 200 uF.

    Hope this makes sense.

    Regards,

    Yitzhak

  • 0 in reply to Yitzhak Bolurian
    TI__Genius 9475 points

    Hi Yitzhak,

    We have a follow up question. Kindly confirm that as long as the minimum input "effective" capacitance of 200uF is achieved, we would not need to bother if the operating voltage is close to the voltage rating of the capacitor?
    Best regards,
    Jonathan
  • +1 in reply to Jonathan Navor
    TI__Mastermind 18370 points

    Hi Jonathan,

    Please do not assume that all capacitors follow our 80% of rated voltage formula.
    You should read the capacitor's manufacturer data-sheet for the exact derating of the specific capacitor that you have used.

    Please also note that capacitors have initial tolerances and that some capacitors derate at cold temperatures.
    So if your customer power supply works at colder temperatures, that needs to be taken into account.

    Also, being too close to the rate voltage is not recommended. And that is not necessarily related to the derating concerns.
    Just operating at voltages too close to the rating voltage means that the cap may be breeched and may shorten the capacitor's life.

    Hope this makes sense.

    Regards,

    Yitzhak

  • 0 in reply to Yitzhak Bolurian
    TI__Genius 9475 points

    Hi Yitzhak,

    We have a follow up question one again. Thanks & Noted your remarks.

    Is the part PTH08T241W going to be compatible with electrolytic/tantalum capacitor?

    May we know what is the risk if PTH08T241W is used but electrolytic/tantalum (instead of ceramic capacitor is used)? What will be the impact?

    Thank you! 

    Best regards,

    Jonathan

  • +1 in reply to Jonathan Navor
    TI__Mastermind 18370 points

    Hi Jonathan,

    PTH08T241W is designed for use with ceramic capacitors.

    As you know the electrolytic/tantalum capacitors have higher ESR than the ceramic capacitors.

    And PTH08T241W will not work well if the (C0 * ESR) product is more than 3000 uFx mOhms (without TT utilization), or more than 1000 uFx mOhms (with TT utilization) .

    The impact of exceeding the above (C0 * ESR) product limits will be at least poor transient response, and often even poor stability.

    If this addresses your question, let's close this post, as the title of this post may not cover the whole discussion.

    Please open a new post for further questions.

    Thanks and regards,

    Yitzhak Bolurian

  • 0 in reply to Yitzhak Bolurian
    TI__Genius 9475 points

    Hi Yitzhak,

    Thank you so much for your support. Yes we'll close this case. 

    Best regards,

    Jonathan