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Original question:

UCC256302: Power management forum(Regarding the conditions under which Vgs is output)

UCC256302: Gate Drive Turn-on Timing

Kuramochi Tadahiko
Guru 28715 points
Part Number: UCC256302

Hello,

I saw the following thread.

https://e2e.ti.com/support/power-management/f/196/t/912152

Mike-san commented that "So when coming out of a burst packed it is possible for the gate drive to turn-on when the Isns pin has the long polarity. " on the related thread.

Is this "long polarity" a writing mistake for "wrong polarity"?

Best Regards,

Kuramochi 

  • 0
    TI__Mastermind 27145 points

    Hi Tadahiko-san,

    Under normal operating conditions, the gate turn-on is determined by the adaptive dead time function that looks at the switch node to determine when the switch node is done "slewing" up to Vin or down to gnd. If the switch node dV/dt is too slow to detect, the controller will use the change in ISNS polarity (ISNS voltage changing from positive to negative or negative to positive) to determine when to turn on the next gate. Figure 39 in the UCC256302 datasheet provides a decision tree for when the gate turn-on is determined:https://www.ti.com/lit/ds/symlink/ucc256302.pdf?ts=1597673006671&ref_url=https%253A%252F%252Fwww.google.com%252F

    Please let me know if this addresses your question.

    Best Regards,

    Ben Lough

  • 0 in reply to Benjamin Lough
    Guru 28715 points

    Hello Ben-san,

    Thank you for your quick response.

    I understand your explanation.

    And I would like to know means "long polarity" Mike-san commented at related thread.

    Do you know what is this mean?

    I thought that I can understand Mike-san's comment if the word is not "long" but "wrong".

    Best Regards,

    Kuramochi

  • 0 in reply to Kuramochi Tadahiko
    TI__Mastermind 27145 points

    Hi Tadahiko-san,

    Yes, Mike O'loughlin is referring to the "wrong" ISNS polarity detection of the ZCS protection function. 

    Best Regards,

    Ben Lough

  • 0 in reply to Benjamin Lough
    Guru 28715 points

    Hello Ben-san,

    I have an additional question.

    Mike-san commented that "when both FETs turn off during burst mode there is ringing in the primary and to stop the transformer from saturating the opposite gate turns on when coming out of the burst.".

    Is this turning-on behavior included the figure 39 on the datasheet?

    I would like to know the mechanism which HO turns on during ISNS polarity is positive as below.

    Best Regards,

    Kuramochi

  • 0 in reply to Kuramochi Tadahiko
    TI__Mastermind 27145 points

    Hi Tadahiko-san,

    I would say it's not explicitly captured by figure 39 in the UCC256302 datasheet but the controller will always deliver an equal amount of HO and LO pulses within each burst packet. This also helps to keep the resonant capacitor DC voltage at 1/2 the input voltage and keep the SR stresses fairly equal when in burst mode. If an unequal amount of pulses are delivered in the burst packet, the resonant capacitor DC offset voltage may not stay at 1/2 the input voltage and stresses may not be equal through each leg of the power stage.

    Best Regards,

    Ben Lough