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DRV8302: Ringing noise in supply because of the MOSFET switching.

Mohammad Asif
Prodigy 70 points
Part Number: DRV8302

Hi everyone,

I am working on a BLDC motor controller design using DRV8302 IC for 24V 20A application. I am following the design recommended in datasheet as shown below.

When i turn on the 24V battery supply, i get ringing noise at SH_A, SH_B, and SH_C, when i use 4.7E gate resistor. This ringing noise gets coupled with the DRV8302 SO1 and SO2 signals and also disturbs the Micro-controller ADC.

There is no ringing noise when i increase the gate resistor to 100E. But i want to know what's the reason behind that ringing noise and what are the other ways to remove that ringing noise.

My PWM signal frequency is 25KHz and I am using two 470uF capacitor in parallel configuration at the 24V input power rail.

I am attaching the images of PCB layout design here for the reference.

Thanks.

  • 0
    TI__Mastermind 26046 points

    Hi Mohammad,

    I would recommend a few options:

    1) Reducing the gate current by increasing the gate resistance for each GH_x/GL_x output. If you are using 10 ohms, then increase to a higher value like 30 ohms to see if that reduces the ringing at the SH_x switch nodes.

    2) Using an RC snubber if possible. An RC snubber directly helps dampen oscillations of the ringing formed from the LC tank of the power stage by placing a resistor-capacitor network in parallel with each FET as close as possible. Use this E2E FAQ to help out in choosing the right values: e2e.ti.com/.../faq-proper-rc-snubber-design-for-motor-drivers

    3) Increasing the thickness/width of the gate drive trace outputs. If the gate drive output paths are too thin, not thick enough in copper, or has too many vias, then this can limit the gate drive current. Ensuring at least 10 mil per 1 ounce of copper for 1A of gate current is a good rule to follow. The DRV8302 supports up to 1.75A gate drive current so higher gate currents may not work with the thinner traces used on the PCB. 

    Thanks,
    Aaron

  • 0 in reply to Aaron Barrera
    Prodigy 70 points

    Thanks for the suggestions.

    I have changed gate resistor from 4.7 ohms to 100 ohms and observed that the ringing noise is gone. As increasing the value of gate resistor increases the turn ON time of MOSFET.

    i will try out the other solutions as well, but i am more what interested in knowing why this ringing noise occur, and what is the reason behind it?

  • 0 in reply to Mohammad Asif
    TI__Mastermind 26046 points

    Hi Mohammad,

    I forgot to link the E2E FAQ on RC Snubbers on the previous post. Please review this and it will describe more on phase oscillations occurring in switch nodes of motor driver powerstages. 

    The main reason this ringing occurs is because of slewing voltages across MOSFETs very quickly, i.e. <100ns. When a large voltage is slewed, or switched very quickly, this can create overshoot and ringing due to the LC parasitics of the power stage. The LC parasitics create a "tank" which creates self-resonance if the inductance and capacitance is high enough. 

    Specifically, the inductances (L) are from the MOSFET leads, thin PCB traces, and motor inductance. The capacitances (C) are from long PCB traces and unneeded capacitors in the motor current path. 

    The best way to remove ringing is via lowering the gate drive current, which can be done by adding gate resistance or lowering the gate drive current setting if the device has configurability to do it through SPI or hardware. However, lowering the gate drive current increases the "turn on time" of the MOSFET gates because the gate paths are essentially RC time constants, so more resistance means a longer RC time to turn on the gate.

    Watch out for these longer turn on/off times to overlap between the high-side and low-side FETs, or else both FETs can turn on simultaneously causing a large amount of shoot through current to go from your motor voltage to ground. The best way to avoid both FETs being "on" at the same is to add dead time through the DRV8302 DTC pin or by adding deadband from the MCU's PWM inputs.

    Hope this helps!

    Aaron

  • 0 in reply to Aaron Barrera
    Prodigy 70 points

    Thanks for this wonderful and informative FAQ on RC Snubbers.

    By the way, using 100 ohms of gate resistor solved my ringing problem. But now i have to ensure that the dead  time is high enough to avoid the overshoot problem.

    Thanks.