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UCC27524: What is the difference between using a PNP and a Pmos as dual-polarity, symmetrical drive-gate transformer turn off SW?

ye matt
Prodigy 40 points
Part Number: UCC27524

What is the difference between using a PNP and a Pmos as dual-polarity, symmetrical drive-gate transformer turn off SW Q1/Q2? Because when using PNP, the base resister will consume energy when power Mosfet is turned off. However when using Pmos, the gate resister cosumes no energy and can even be NC. Why not always use a Pmos instead of a PNP? Are there any drawbacks when using a Pmos?

Reference article:https://e2e.ti.com/blogs_/b/powerhouse/archive/2015/11/04/gate-drive-transformer-vs-high-low-side-driver-a-close-look-of-detailed-implementation?tisearch=e2e-sitesearch&keymatch=%2520user%253A228507

  • 0
    TI__Genius 13445 points

    Hi Matt,

    I assume you're referring to the local turn-off circuit q1 and q1 to turn-off power transistor Q1 and Q2.

    If so, sections 3.4.3 and 3.4.4 of the attached application note discuss differences between pnp and Pmos in detail.

    Regards,

    -Mamadou

  • 0 in reply to Mamadou Diallo
    Prodigy 40 points

    Hi Mamadou:

    Thank you for your reply.

    However in this article 3.4.3 is NPN turn-off and 3.4.4 is NMOS turn-off.

    What I want to know is what if in section 3.4.2 PNP turn-off circuit, the PNP is replaced by a PMOS, will it be better, or will it be worse?

    Because like i said, the gate resister of a Pmos does not consume energy like the base resister of PNP, and the turn-off current seems larger using a PMOS, and both seems that a PMOS is better then a PNP.

    Could you help me look into this question? Thank you very much!

  • 0 in reply to ye matt
    TI__Genius 13445 points

    Hi Matt, 

    Whether it is worse or better will depend on your application's requirements including thermal, switching speed, parasitic, cost, board size, etc...

    BJT's are better in terms of cost while PMOS may not be cost optimized but will offer better size area, thermal performance due to better thermal coefficient.

    Regards,

    -Mamadou

  • 0 in reply to Mamadou Diallo
    Prodigy 40 points

    Hi Mamadou,

    Thanks for your support.

    I find one different thing between using a PNP and a PMOS, is that the power Mosfet Vgs voltage when the transformer winding is shorted(happens during the dead time).

    Because PNP's Vbe is lower than the VGSth of Pmos,  power Mos Vgs would be higher using a Pmos during dead time. So a PNP would be better to prevent power Mos to turn-on during the dead time.

    Do you think it may be the main reason PNP is most used in this type of circuit?

    Matt

  • 0 in reply to ye matt
    TI__Genius 13445 points

    Hi Matt,

    You're correct, that is certainly a valid point. you certainly want to ensure that during deadtime, Vgs of power transistor is pulled to GND to obviously avoid shoot-through.

    Regards,

    -Mamadou