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TIDA-00364: Explanation of PNP turnoff device and associated diode

Lucas Schulte9
Expert 6955 points
Part Number: TIDA-00364

So I see and understand the value of the PNP transistor acting as a passive Miller clamp here but I would like clarification of the diode placed between the gate and emitter. I also have more questions about the PNP transistor selection and turnoff capability.

1. My guess is that this diode keeps the turnoff loop contained to the PNP transistor rather than including the gate driver's turnoff capability. Am I correct? Can the diode be excluded or does that compromise the effectiveness of this passive miller clamp?

2. How would one control the turnoff switching time with this passive Miller clamp? Often gate drive design includes a way to control both turnon and turnoff switching rates with diodes and resistors. I see turnon time in this design is limited by the 24ohm resistors but how to do this for turnoff here?

3. I see the chosen PNP device has a collector emitter breakdown of 80V and a max collector current of 8A. Why a value of 80V? Wouldn't the voltage difference never exceed Vdd between collector and emitter? 

Respectfully,

Lucas

  • 0
    TI__Mastermind 21915 points

    Hello Lucas,

    Thank you for the interest in the TIDA00364.

    The diodes you highlight provide the turn on current path for the drive to the MOSFET. If the diodes are not there, there is no path for the turn on current since the PNP will be off and there would be no conduction path for the gate drive positive voltage state.

    The PNP provides the ability to locate the PNP close to the MOSFET which improves the turn of capability since the trace length, and inductance can be low. Also if a higher current rated PNP, than the driver turn off rating, the turn off current can be increased.

    For the PNP device selected, you are correct that the 80V rating is not required, this device was likely chosen for the current rating.

    Confirm if this addresses your questions, or you can post additional questions on this thread.

    Regards,