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UCC28704: Calculation for minimum demag time

TI-CSC
Genius 9600 points
Part Number: UCC28704

Dear TI Experts,

Refer datasheet: SLUSCA8A –FEBRUARY 2016–REVISED FEBRUARY 2016)

I need to calculate the shortest Tdemag (time when the UCC28704 will sample the Vs pin after turn-off of MOSFET), refer datasheet Fig 14. Of course, in an existing simple DCM flyback with known components, this can be easily calculated once we know the minimum MOSFET current at turn-off, which can be calculated from the minimum turn-off voltage at the CS pin, and value of resistor Rcs. Datasheet Table 6.5 states that minimum CS turn-off voltage is 170mV. However, what I do not understand at this time is how the resistor RLC modulates this turn-off voltage (refer Fig 24). Please advise how to calculate the minimum MOSFET turn-off current from the components of Figure 24, taking into account RLC

Regards,

Jonard Rico

  • 0
    TI__Guru 66135 points

    Hello,

    The RLC resistor adds offset to the CS signal so the gate drive will turn off earlier at high line.  This offset is developed with a current mirror attached to the VS pin and and the current sense resistor.  The current that is developed at the CS pin is based on the current coming out of the VS pin when the switch is on.  This is know as the IVSL current. 

    The current that is coming out of the CS pin is IVLS/KLC.  Refer to the data sheet for the KLC value and the block diagram for reference.  You should be able to calculated the IVLS current by Np/Ns and  RS1.  Please note this is discussed in more detail in section 7.3.1.3 in the data sheet.  The RS1 equation just needs to be algebraically manipulated to get IVSL.

    Regards,