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TIDA-00779: Two sets of Bleed Resistor

Part Number: TIDA-00779

Hello,

I see there are two sets of bleed resistor in the design guide. One on the input side, one on the output side. Why is that? Why can't one set of bleed resistor be sufficient? Why is the input side resistor value so small when C needed to be discharged is lesser on input side?

I have attached an image of the design given in TIDA-00779 too. The question relates to R34, R35, R36, R37, R29, R30, R32, R33.

  • Hi Shyama,

    Q1:Why we need the input bleed resistor and whose resistance value is smaller.

    There are some X-capacitors in EMI stage, such as C1,C2 and C22, once un-plug the power cord, we need to discharge the X-capacitor within limited time(such as 1~2s), which is required by safety standard, so the bleed resistor is depend on the total capacitance of X-capacitor and required discharge time.

    Q2:Why we need the output blead resistor.

    It is used to discharge the output capacitor when power off. We cannot use input bleed resistor to discharge the output stage, because it is blocked by rectifier bridge.

    Felix

  • Hello Felix,

    Thank you for your precise reply. Can you please tell us which safety standard you're referring to?

  • Hi Shyama,

    IEC 60950-1 or GB4943 FYI.

    Felix