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Original question:

UC3879: CYCLE-BY-CYCLE CURRENT LIMIT - delay time

UC3879: Voltage Control Mode - Series Capacitor

Charles Alicea
Prodigy 20 points
Part Number: UC3879

Cannot identify best method to correctly value/size of the series capacitor for the primary winding in voltage control mode. We are seeing failures when load is close to short with a series film capacitor rated at 470nF but see significant improvement with a higher value of 2.2uF. The issue became more noticeable when MOSFETS with lower input/output capacitance where implemented. Is there a white paper available describing the relationship of the below series capacitor (see attached schematic) to MOSFET parasitic and T10 inductance?

much thanks

  • 0
    TI__Guru 52875 points

    Hi,

    That capacitor is used for dc flux blocking. That capacitor value is usually determined by its voltage drop (I / (2 x pi x fsw x C) is about 5% to 10% of the input voltage. I is the current at its max, i.e., full load, or before over load.

    You can search the internet to find many discussions, or go to college text book about full bridge voltage mode control, where you can find the materials related to this.