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UCC 28180 Startup

Dheemanth Prabhu Hejamady
Prodigy 310 points

Hi,

I had a couple of questions with regard to the UCC 28180 EVM. 

  1. Why “D Start” diode ( D2) is required and will it not create Inrush at startup ?
  2. What would be the other sources for inrush current?
  3. Is  NTC ( RT1) for limiting inrush?

  • 0
    TI__Mastermind 25570 points

    Hello

    The diode (D2) is used to divert the inrush current away from the PFC diode (D3). D2 is chosen to have a high forward surge capability so that it can safely carry the large inrush current that can flow when the line voltage is connected and the PFC output capacitance is discharged. Silicon Carbide diodes, such as the one used in the EVM (D3) have a relatively poor surge current handling capacity and the bypass diode is used to protect them from overstress during start-up.

    When the line is connected an inrush current will flow. The amplitude of the current depends on a number of things - the line source impedance, the resistance in the path on the PSU PCB, any current sensing resistors, the state of charge of the PFC output capacitor (normally discharged, but not necessarily so), the resistance presented by any inrush current limiting devices - NTCs etc.

    RT1 is used to limit the inrush current.

    Regards

    Colin