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TPS23757 no power issue

Jacky Chen29
Genius 4735 points
Other Parts Discussed in Thread: TPS23757

Hola,

We designed TPS23757 in new project and production, but our customer return some product due to TPS23757 has no power.

after analysis and we found the D15 was burn down：

Capri_sku3_POE.pdf

Our customer wants to clarify the problem of this issue so can someone help to answer this question？

1. D15's part number is SS110, VRM =100V , Io=1A.

2. Transformer's spec is N=1/0.25=4

3. D15 transient analysis are all in normal range：

1. When MOSFET is turn on and the diode works at reverse voltage, VBR = VI = 56V、Diode's VRM is 100V > 56V.

2. When MOSFET is turn off and the Diode is short, VD = 0.85V、ID = ( NVo - 0.85 ) / 39K = (20 - 0.85) / 39K = 0.49mA

3. To confirm the MOSFET's VDS whether is normal？(when MOSFET is trun off)：VDS = 56V + NVo = 56 + 4X5 = 76V

4. To confirm R285 & C293 whether is shorted to causes the D15 burned down：Confirm R285's impedance is normal.

  • 0
    TI__Mastermind 41480 points
    Hi Jacky,

    For this design with the TPS23757 in this topology, I have seen the DFLU1200 diode used. It has a higher RMS voltage rating and fast reverse recovery time. Is it possible to replace the diode to one similar to this?
  • 0 in reply to Darwin Fernandez
    Genius 4735 points

    Hola Darwin,

    Thank you for your reply and I summarize waveform as below：

    1. Circuit transient analysis：

    D15_VD = High = 76 ~ 77V、LOW = 0 ~ 2V

    Is this correct？Can help confirm？

    2. The diode works at reverse voltage when MOSFET is turned on and its VBR = 56V + 20V = 76V and the diode's VRM is 100V > 76V.

    3. When MOSFET turned off, transformer (3-4) = -20V、VDS = 56V + NVo = 56 + 4*5 = 76V、VD = 0.85V

    4. To confirm the MOSFET's VDS whether is normal？(when MOSFET is truned off)：

    VDS = 56V + NVo = 56 + 4*5 = 76V

    5. To confirm R285 & C293 whether is shorted to causes D15 burned down：

    Yes, confirmed R285 impedance is normal.

    D15_VD：

    Transformer_(3-4)：

    XFM3：

    XFM4：