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Start-up circuit requirement for UCC2897A device

kirankumar bg
Prodigy 50 points
Other Parts Discussed in Thread: UCC2897A

Hi,

In TI UCC2897A EVM design, there is a start up circuit for UCC2897A device but i observed in many of TI UCC2897A reference designs doesn't have start up circuit. Here they have taken supply voltage from transformer auxillary winding for VDD and PVDD pin.

Please clarify me on start up circuit requirement for UCC2897A PWM controller device.

Thanks and Regards

KiranKumar BG

  • 0
    TI__Genius 9130 points
    Just to clarify, by "start-up circuit", I assume that you are referring to Q8, D10, D7 etc in the EVM schematic? I believe that this is not a startup circuit, since the level at the cathode of D7 would not be high enough to overcome the VDD start threshold of 13.2 V max.
    Startup is provided by the internal linear regulator on the IC VIN pin, which will charge the VDD capacitor to the VDD start threshold, and then turn off. Once PWM activity has started, the bias to the IC PVDD and VDD pins is provided by the bias winding on the output inductor and diode D8, which peak rectifies to a level that will be approx. proportional to Vout.
    I believe that the circuit using Q8, D10, D7, etc is added to provide a bias supply from the main DC-input rail under output overload or short-circuit conditions, where there would be insufficient bias voltage from the output inductor bias winding, since is approx. proportional to Vout.
    Other designs using the UCC2897A use a bias winding on the transformer, with the bias winding full-wave rectified and then LC-filtered, so this maintains a regulated bias rail even if the main output collapes due to overload or short-circuit.
    I hope this answers your query?Thansk,Bernard
  • 0 in reply to Bernard Keogh
    Prodigy 50 points
    Bernard,
    Thanks for your clarification. Your answer is solved my query.
    In brief Q8,D10,D7 components are added to provide a bias supply from the main DC-input rail under output overload or short-circuit conditions. So even with out the Q8,D10,D7 components UCC2897A circuit should work with output inductor bias winding or transformer bias winding.

    Thanks and Regards
    KiranKumar BG
  • 0 in reply to kirankumar bg
    Prodigy 50 points
    Bernard,

    I just wanted to make sure EVM design will work with out Q8,D10,D7 components, l removed D7 component in UCC2897A EVM . I observed only 1V output voltage and expected was 3.3V output voltage. So Q8,D10,D7 components are required only in EVM board or other designs also.
    Please clarify and help me.

    Thanks and Regards
    KiranKumar BG