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drv8305: HS and LS Gate Drive Control settings, driving mosfets correctly

Roman Simonyan
Intellectual 440 points
Part Number: DRV8305

Hi. I am driving mosfets with 80nC total gate charge using DRV8305 in FOC ESC. PWM frequency is 45kHz. I have several questions.

1.  As I understand I need 2-3 Amp peak gate drive current to switch my MOSFETs efficiently, but DRV8305 gives max 1 Amp. In my case, to what negative effect this may lead? Do I need to change to MOSFETs with lower gate charge?

2. Do I need to change HS and LS Gate Drive Control settings in registers to max? Now my ESC drives my motor very well with standart settings, which are 50-60 mA.

Some explanation would really help, there is not much about this topic in documentation.

Thank you,

Roman

  • 0
    TI__Expert 4650 points
    Hi Roman,

    1.) Using a lower gate drive current will mean that your MOSFETs are slewing slower than they would with 2-3 Amp peak gate drive.
    2.) This is dependent on the switching losses in your MOSFETs and the specific Qd, or Qmiller of the MOSFETs you've selected.

    We actually have an app note that outlines how to select the correct Source / Sink currents using IDRIVE based on your MOSFET parameters. I would advise reviewing the following document, but please let us know if you still have questions after reviewing this:

    www.ti.com/.../slva714a.pdf
  • 0 in reply to Phil Beard
    Intellectual 440 points
    Phil, thank you. This note explained many things to me. One question remained. Why would I choose one slew rate over other, what are benefits of different rates in different applications.

    Best regards,
    Roman
  • 0 in reply to Roman Simonyan
    TI__Expert 4650 points

    Hi Roman,

    Using different slew rates allows for fine tuning of the switching vs. conducted losses as well as ringing and EMI / EMC issues in your power-stage design.

    If you have to fast of a slew rate, the MOSFET gate will "ringing" (also referred to as oscillate) when you switch the gate on or off.  This can cause unwanted radiated / conducted emissions in your power-stage that can do things like couple into adjacent traces.

    If you slew the MOSFET gate to slowly you'll spend more time in the "Miller Region" charging up the gate and creating a larger channel for electrons to flow through (Linear Region -> Active Region).

    You can see in the figure below that during the Miller Region the Vds voltage is transitioning from fully blocked to conducting, but during this time the Ids is constant, meaning the device is dissipating a large amount of power (V * I) at the beginning of the miller region.  So, the more time you spend at this point (slow slew rate), the more power you will burn up during switching.