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TI Designs 30-W Ultra-Wide Range Power Supply for Protection Relay

Bogdan Nicola
Prodigy 140 points
Other Parts Discussed in Thread: TPS40210, LMS33460

Hi,

I have a question from a customer regarding TI Design TIDA-00127.
The customer's question is: 
"There is an BSS126(Q2) mosfet in page 33. When you look carefully, we can realize that source voltage is greater than gate voltage when MOSFET is on.(i.e. drain current exists on MOSFET, Vs>Vg)In this case, mosfet can not be on. Most respectfully, why do we put mosfet there? May you have an mistake while designing that project? I am looking forward to your answer."

Except for the phrase "The design uses Snubber circuits to minimize the transients across the DC-DC booster MOSFET, flyback converter MOSFET, and output diodes. These circuits also reduce EMI" I could not find information to explain the use of the BSS126 in the design. Can you please help?

Thank you in advance for your support

  • 0
    TI__Expert 7395 points
    Bogdan,
    As a whole, this circuit looks like a UVLO circuit with U2 to Enable/Disable the TPS40210 depending on bulk input voltage. But it seems Vin of U2 is floating so i'm not certain how that component works with Q2. I've emailed the team that did this design and expect them to respond here soon. let me know if they don't reply in a timely manner.
    Regards,
    John
  • 0 in reply to John Stevens
    Prodigy 140 points

    Hi John

    Thank you very much for helping with this and for the information provided so far.
    Kind regards,

    Bogdan

  • 0 in reply to Bogdan Nicola
    TI__Guru** 123998 points

    Hello All

    Depletion-mode devices are “normally on”. They conduct current at Vgs=0, and an applied gate voltage is required to stop the current flow and turn them “off”

    Please note that Q2 is a Depletion Mode MOSFET configured as  current source.

    The LMS33460 is an under voltage detector with a 3.0V threshold. This IC generates an active output whenever the input voltage drops below 3.0 Volts.

    Pin 3 of U1 is disable pin. Pulling this pin high, places the part into a shutdown mode. Shutdown mode is characterized by a very low quiescent current.

    Analysis of the circuit, Considering the above Point provides the required function explanation.

    Thanks

    Sreenivasa