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slua669-Reason of diode in gate drive transformer solution

Catalin Muntean
Intellectual 280 points

Hello,

I would like to know how will Q1 and Q3 NMOSFETs from figure 5 from http://www.ti.com/lit/an/slua669/slua669.pdf will get discharged. 

In my view, D1 and D3 are preventing their discharge yielding in always ON.

If I am mistaken, what are the purpose of those diodes ?

Thank you for your time.

  • 0
    TI__Mastermind 21495 points

    slua669_fig5 edit.pdfHello Catalin,

    Thank you for your interest in gate driver products from TI. I am an application engineer in the High Power Driver product line with TI and will work to answer your questions. I reviewed Figure 5 in the SLUA669 app note regarding your concern. And I see the reason for your question, as drawn in the simplified diagram D1 and D3 would prevent the gate drive turn off path as you mention.

    I am attaching a modified version of Figure 5 showing a popular configuration for the Mosfet gate drive turn off. The diodes D1 and D3 can be used with addition of a PNP transistor as shown, where the turn on current path is thru D1 and D3, and the turn off path is thru the PNP pull down transistors. If the PNP transistors are not used, D1 and D3 should not be in the circuit.

    Regards,

    Richard Herring

  • 0 in reply to Richard Herring
    Intellectual 280 points
    Thanks Richard, that solved my question.
    Now that we are at it, can you tell me popular choices for D1 and PNP pull down transistor ?
  • 0 in reply to Catalin Muntean
    TI__Mastermind 21495 points
    Hello Catalin,
    I am glad I was able to help with the question. For common parts I see in these applications, the 2N2907 (or many similar devices) and 2N3906 are PNP transistors used often in gate drive applications. For the diode, since the average current is low I have seen 1N4148 or 1N914 diodes in many cases, or a small schottky diode such as the MSS1P3L 1A 30V diode is more capable to drive larger devices.

    Regards,
    Richard Herring