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UCC28700 datasheet. VS pin function

Peter Han1
Intellectual 1020 points
Other Parts Discussed in Thread: UCC28700

ucc28700_datasheet.pdfpage 10 

8.3.1.3

The VS pin also senses the bulk capacitor voltage to provide for AC-input run and stop thresholds, and to compensate the current-sense threshold across the AC-input range.

i need help about the above sentence.

  • 0
    TI__Mastermind 31580 points
    Hi Peter

    When the primary side mosfet turns on, the voltage (Vaux) on the aux winding goes negative with respect to the ground pin of the UCC28700. The reverse voltage on the Vaux winding is the Vblk voltage divided by the turns ratio of the aux winding to the primary winding of the transformer.

    The reverse voltage (Vaux) on the aux winidng will draw a current from the Vs pin of the UCC28700 through Rs1, when that current exceeds 220uA the UCC28700 will start to operate normally. Once the UCC28700 is operating normally, the Vblk voltage must drop to a level such that the current drawn for the Vs pin during the on time of the primary mosfet drops below 80uA for the UCC28700 to turn off.

    Regards

    Peter
  • 0 in reply to Peter Meaney
    TI__Intellectual 1020 points

    Hi Peter,

    thank you for your reply. and i have another two questions.

    as you said, " ... Once the UCC28700 is operating normally, the Vblk voltage must drop to a level.. "

    1. how the Vblk drop ? i mean how is going on ?

     

    2. i found  " the current also compensate the current-sense threshold across the AC-input range". why we need this compensate?

  • 0 in reply to Peter Han1
    TI__Mastermind 31580 points
    Hi Peter,

    1.
    The Vblk voltage will follow the rectified AC line voltage. If we increase the AC line voltage from 0 to 100Vac using an AC source, the Vblk will also increase and the UCC28700 will turn-on when the current drawn from the Vs pin exceeds 220uA.

    If the AC line voltage decreases or if the AC line cord is unplugged then the Vblk voltage will drop and when the current from the Vs pin drops below 80uA the UCC28700 will stop switching.

    2.
    At higher input voltages the di/dt (rate of change of current) in the primary swicth increases and the peak current can overshoot the current limit value. This causes the current limit on the output in constant current (CC) mode to increase with increasing line voltage.
    The UCC28700 compensates the current limit with increasing AC line voltage to reduce the variation in the CC output value for different AC line voltages.

    Regards

    Peter