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UCC2894: About soft-stop function

Kaji@PAN
Mastermind 7215 points
Part Number: UCC2894

Hi,

I would like to ask you about the soft-stop feature of the UCC2894D.
Even if the same off-sequence is taken in the actual machine, an overvoltage may or may not occur between the drain and source of the primary side main switch.
This is probably because the soft-stop function may not be working.
(question 1)
Please tell me how the overvoltage is generated between the drain and source of the primary main switch if it does not soft stop.
Also, is there any material that explains this mechanism?
(Question 2)
What should be the soft-stop time when designing?
I assume the following.
Output capacitor, active clamp capacitor capacity
Timing when off (Which timing of the switch on the primary side)
Load side capacitor
Cable inductance to load

Best regards,

  • 0
    TI__Guru 53385 points

    Hi,

    Question 1: Please tell me how the overvoltage is generated between the drain and source of the primary main switch if it does not soft stop. Also, is there any material that explains this mechanism?

    [Hong Huang] this is particularly for secondary-side with self-driven SR (synchronous rectifier). When LineUV off is triggered, the secondary-side self-driven SR behaves like a boost converter, while not in control manner, to regenerate energy stored in output capacitors back to the primary side to charge up the primary side switch-node then causing high Vds of the MOSFETs. As the primary side MOSFETs are off, the energy charged to switch-node can become very high and exceeding MOSFET Vds then causing damage

    Soft stop is to help to keep switching during soft stop time, the primary side MOSFETs still being controlled so do SR MOSFETs, this way can help dissipate energy in controlled manner during soft stop time, to help reduce the primary side switch node voltage increase.


    (Question 2)
    What should be the soft-stop time when designing?

    [Hong Huang] The soft stop time is related to the output capacitor energy level. You need to decide what is your output voltage, and your output capacitor value, in order to find a proper soft stop time. Also, your primary MOSFET Vds rating should be considered as well, and so does the transformer turns ratio Nps.

    [Hong Huang] Soft stop helps to control the SR regeneration but the regeneration is still over there. The primary side MOSFET Vds will still increase but just with a possible less increase and in controlled manner. So proper MOSFETs selection is still needed.