This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

PMP5711: Why Q7 and Q8 totem pole driver stage required in PMP5711

Part Number: PMP5711

Dear TI Experts,

What is the use of  Q7 and Q8 totem pole driver stage in PMP5711 ?. UCC2897A already having 2A source and sink capacity.

Regards

Aneesh

  • Hello Anesh,

    that's a single ended 175W forward; for effcy 94% you need a FAST and STRONG switching;
    the two main FETs could end up at 2x 15nC total gate charge - and it's important that you're able
    to charge and discharge the Miller capacitance Crss as fast as possible.
    So I boosted the internal driver of UCC2897A w/ a cost effective bipolar push pull stage.
    Furthermore you got more flexibility at layout; it's pretty easy to locate the PP driver stage
    directly to the FETs.

    Best regards, Bernd
  • Hi Aneesh,

    You are right, the UCC2897A has 2A sink/source drivers. The NPN/PNP are used for current amplification to drive the parallel NFETs. The designer of PMP5711 is traveling this week. He can provide a more detailed response when he returns.

    Thanks,
    Ryan

  • Please ignore my comment. Bernd addressed the question.
    Ryan
  • Dear Bernd,

    Thanks for your valuable comments.
    But i have some more doubt regarding this

    1. Both Transistors Q7 and Q8 having only 0.2A current capacity, while UCC2897A having 2A current capacity. so in effect the driving capacity drops from 2A to 0.2A. will this affect the performance?

    2. BJT Transistors can switch 250KHz effectively?

    Regards
    Aneesh
  • Hello Aneesh,

    we're not talking on continuous current here; the driver just provides peak currents around Miller plateau.
    Here you have to regard current gain beta of BJT. Just check test report page 9, figure 10/11,
    so Vds and Vgs of primary FETs - do you like the waveforms ?
    If there would be sth. wrong at the control of the FETs you won't meet the 94%.
    And YES, BJT could work at even higher frequencies; almost 40 years ago I built some SMPS w/ BJT's;
    there has been less FETs on the market, started w/ BUZ11 - but at much lower Fsw used as power switch...

    Best regards, Bernd

  • Dear Bernd,

    Thanks for the feedback.

    The switching waveforms are awesome. Now my doubt regarding the speed of switching of the Transistor driver is cleared.

    But peak current rating of Q7(MMBT3904LT1G) is 900mA and of Q8(MMBT3906LT1G) is 800mA from the datasheet. Is this peak current(less than 1A) is enough to drive the main MOSFETs?


    Regards
    Aneesh
  • Hello Aneesh,

    if you don't like the BJT PP just use i.e. UCC27517,
    a dedicated FET driver - powerful, but in a tiny package.
    Place this driver close to the gates.

    Best regards, Bernd