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UCC28740: UCC28740EVM-525 TVS too hot

user4370495
Expert 1155 points
Part Number: UCC28740

Dear :
        Custom 's  products  is wall switch ,  the Schematic  same as ucc28740EVM-525, 
But when load up to 5V 2A , The Surface of TVS  D3 is too hot, the temperture up to 110°C.
When check the  mosfet Vds waveform as follow  and the leakage inductance of transfomer is 15uh
     I suggest to increase the resistance of R12,  and add a 10nf capacitor beteween TVS tube. but it doesn't work.
i want to know the temperture of TVS tube is normal or not? if not , how to reduce the temperture of TVS tube.

thanks  a lot.

  • 0
    TI__Genius 9130 points
    Can you share your transformer spec, how does it compare to the EVM transformer?

    Can you measure the transformer leakage inductance and post the result?

    For the scope plot above, what is the AC input voltage?


    If the TVS is getting too hot, then it indicates that the leakage inductance is high, or the clamp voltage is too low, or both.

    Depending on the max VDS stress on Q1 at high line, you could try using a higher voltage TVS. Or use 2 lower voltage TVS devices in series to spread the power and reduce the temperature.

    Otherwise, you could investigate improving the transformer to reduce the leakage inductance.

    Thanks,
    Bernard
  • 0 in reply to Bernard Keogh
    Expert 1155 points
    Dear Bernard:
    1. transformer spec as fellow:
    Inductance value is 560uh, leakage inductance is 20uh ( test condition: 100khz 0.1v 25c)
    rurns ratio: 82/21/6
    2.AC input voltage: 220VAC 50HZ
    3. the leakage inductance is about 3.6% to the primary inductance. it is large or normal ?
    4. The Vds of MOSFET is 600V, The Vbr of TVS is 120V. does it enough or not?
    thanks for your support.
  • 0 in reply to user4370495
    TI__Genius 9130 points
    Is the transformer interleaved (i.e. the secondary is sandwiched between 2 half-primaries)? If so, then the leakage inductance is very high. Even for non-interleaved, it is somewhat high.

    How was the leakage inductance measured? It should be done with ONLY the main secondary winding short-circuit, leave the aux/bias winding open. This will often give a higher but more realistic result.

    If leakage inductance is measured with all other windings short-circuit, then this can give a misleading result, lower than the actual, because all the leakage inductances will be in parallel, giving a lower overall result.


    I checked the EVM-525, it uses Wurth transformer P/N 75081111:
    katalog.we-online.de/.../7508111111.pdf

    This has leakage inductance of 9 uH typ for Lmag 560 uH, or 1.6%. This is also measured with all windings shorted, but it shows less than half the leakage of your transformer.


    For 600-V VDS rating for the FET, the TVS clamp at 120 V seems ok. A higher TVS voltage might not clamp the voltage low enough at max Vin.

    So if the TVS is still too hot, you need to try to reduce the transformer leakage inductance, this will also improve the power supply efficiency a lot.

    If the leakage inductance can't be reduced, you will have to use more TVS devices in series (each with lower voltage, to achieve the same 120 V rating) to spread the power between them.

    If this answers your question, please click the "verify answer" button.

    Thanks,
    Bernard
  • 0 in reply to Bernard Keogh
    Expert 1155 points

    Dear  Bernard:

    thank you for your  strong support.  i will try to add more TVS Tube to spreed the power.