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TIDA-00443: Usage of the diode D2 in the schematics

sivakumar suseendran
Prodigy 85 points
Part Number: TIDA-00443

Currently am trying to do the PFC circuit and I have few queries here about the design (TIDA-00443)

  • Why diode D-2 is used across the inductor?
  • Resistor R-3 (1.05ohm ) is not available can I use 1.1 ohm instead of that?

Thanks in advance,

Sivakumar.S

  • 0
    TI__Guru**** 189399 points
    Hi Sivakumar,

    I have asked my colleague to help you with your questions.
  • 0 in reply to Don Dapkus
    Prodigy 85 points
    Hi Don Dapkus,

    Thanks! awaiting for the reply :)

    Regards,
    Sivakumar.S
  • 0 in reply to sivakumar suseendran
    TI__Guru**** 189399 points
    Hi Sivakumar,

    Thanks for your patience!

    R3 is the gate resistor used to adjust the slew rate of the FET. It is fine to use a slightly different value; in fact, depending on your layout you may need to adjust it anyway.

    As for D2, that is used to charge the output cap at start-up. Typically, the schottky diode isn't rated high enough for the inrush current, so we used D2 as a path to partially charge the output cap at start up to protect the schottky diode in the boost circuit. You may be able to adjust the value of RCL1 to limit the inrush current to a level the schottky diode (D1) can handle.

    Let me know if you have any other questions!
  • 0 in reply to Don Dapkus
    Prodigy 85 points
    Hi Don Dapkus,

    Really thanks for the support!

    I'll try out the circuit and come back if I have any more doubts.Am done with the board I'll test it and let u know.

    Thanks & regards,
    Sivakumar.S
  • 0 in reply to Don Dapkus
    Prodigy 85 points
    Hi Don Dapkus,

    We have tested the PFC board and I have faced the following issues and observations...

    1. We gave an external 15 V DC supply from an RPS for the bias voltage and PFC_ON
    2. we slowly increased the input AC voltage ( using an auto transformer )of the PFC converter.. we observed that there was abnormal inrush current (even @ around 40 V AC) . We also found that the duty cycle of the gate pulse at that point was more than 90 %.. Then we turned off the both the Vcc and mains AC.
    3. Then as a next step we set the input voltage to 200 V AC RMS ( expecting the duty cycle would be limited to a less value) and then turned ON the bias voltage and PFC_ON. It resulted in a flashover in the PCB near the rectifier. The fuse has blown. We later found that the MOSFET has failed.. which has continuously short circuited the boost inductor.

    We guess the MOSFET has failed due to the huge inrush current because of high duty cycle ( > 90 %).. Is it ok to have a max duty cycle of the controller upto 90 % ..?

    Also we observed that there is a peak current limit on Isense @ 0.48V ( which corresponds to around 20 A with a resistor of 0.02 ohms and 0.2 ohms connected in parallel).. Will it be ok to allow such a high current for a 20 A rated MOSFET..

    Kindly help resolve in proceeding further...

    Thanks,
    Sivakumar.S
  • 0 in reply to sivakumar suseendran
    TI__Prodigy 670 points
    Hi Sivakumar,

    The key issue seems to be the turn-on of the relay before the inrush current limit is active.
    If the PFC-ON and Bias are both present at once as per above description, the relay turns-on by-passing the in-rush current limit resistor.

    You should just give bias supply and NO signal should be given for PFC-ON. PFC-ON is just additional input provided for external micro-controller to turn-on relay once the voltage is boosted. Please keep PFC-ON pin open for relay to operate with AC voltage sense circuitry present on board.

    Thanks,

    With Best Regards,
    Latif
  • 0 in reply to Latif Ameer
    Prodigy 85 points
    Hi Latif,

    VCC to the controller will be turned ON only when the PFC_ON signal is given(PFC Shutdown circuit) and PFC_ON will also activate the relay ON.
    I don't think giving a delayed signal to the PFC_ON will resolve the issue.

    Regards,
    Sivakumar.S
  • 0 in reply to sivakumar suseendran
    TI__Prodigy 670 points
    Hi Sivakumar,
    Please de-mount D10 so that the PFC-ON signal to isolate the signal going to drive relay.
    Thank you,
    Regard,
    Latif Ameer