• State TI Thinks Resolved
  • Locked Locked
  • Replies 1 reply
  • Answers 1 answer
  • Subscribers 63 subscribers
  • Views 142 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

UCC28742: Question about VdsMax of switching NMOS FET in the attached Webench design file.

Kazuya Nakai59
Guru 20150 points
Part Number: UCC28742

Hello guys,

One of my customers run Webench to design a flyback power supply as the following conditions using UCC28742.

Vin = 1200V, (1.2kV)
Vo = 24V,
Io = 6A

The result is attached.
In the result, transformer drive NMOS FET (M1) is ideal FET because suitable FET could not be found in the current database.  
The result says that the NMOS FET VdsMax is 1.516kV.

The customer has the following question at this moment.
Could you please give me your reply?

Q. Does this 1.516kV have any margin?
     Or do they need to add any margin to 1.516kV?

Your reply would be much appreciated.

Best regards,
Kazuya.

WBDesignVin1200V_Vo24V_Io6A.pdf

  • 0
    TI__Expert 5486 points

    Hi Kazuya,

    The voltage rating of your MOSFET can be estimated with the following equation during the demagnetizing time (MOSFET OFF):

    Vswitch=Vin+(Vout)*n

    Where:

    Vswitch is voltage across the FET.

    Vin is input voltage.

    Vout is output voltage.

    n is primary to secondary turns ratio

    With you values we can estimate a voltage rating of Vswitch=1458V

    To this number, the peak voltage due to the leakage inductance (right after MOSFET switch off) needs to be added. This peak voltage is going to be applied during a very short time of the switching period. Saying that, the WEBENCH results for Vdsmax make sense for me. Is always good to have a margin of the results (higher voltage), but having an idea of the voltage rating of your MOSFET is always good.

    If you have any further questions, let me know replying to this thread.