DRV8350: what it means that the GLX is shorted to vgls

Hi Shengzhong,

If GLx is shorted to VGLS, then damage likely occurred at GLx in the internal pre-driver FET. 

Please ensure your IDRIVE value is selected carefully to ensure you are not overstressing the MOSFETs or switching motor current too quickly: https://e2e.ti.com/support/motor-drivers-group/motor-drivers/f/motor-drivers-forum/796378/faq-selecting-the-best-idrive-setting-and-why-this-is-essential

Please ensure there is no negative ground bouncing at SLx or negative gate voltages at GLx, or else you can blow up the internal body diode of the HS or LS predriver FET internally and cause GLx damage. Ensure good layout removes parasitics from the gate driver and motor current paths. You can use gate-to-GND diode to remove negative transients, or add capacitance from VDRAIN-SLx to suppress negative transients at low-side source. 

Follow this app note more ways to reduce pin damage for higher power designs: www.ti.com/.../slvaf66

Thanks,
Aaron

Hello Lining,

If excessive capacitance is present at the gate drive outputs, then too little IDRIVE current can also cause potential issues as well. First, you want to ensure enough gate drive current turns the MOSFETs on and off to avoid shoot through current conditions. Ideal timing diagram is below. 

Too slow of a gate drive current may cause VGS rise/fall time to be too long with potential shoot through if VGS > Vth for high side and low side MOSFETs. This is due to RC time constant of gate rise and fall times. The higher RC value can be caused by:

• Gate resistors limiting gate current
• Capacitors from gate to source
• Small IDRIVE currents
• Higher MOSFET Qgd values

In reviewing the schematic, I see there are gate to source capacitors on the LS FETs. Can you remove these and see if gate driver performance improves?

The device has selectable IDRIVE source and sink current settings, which removes the need for external sink diodes. The gate resistors are okay, but we recommend starting with 0-ohm resistors so gate current can be selected via the IDRIVE registers in SPI. 

The MOSFETs used are 3nC Qgd, which is a very small value. We recommend the VDS rise/fall time to be no faster than 100ns, so we recommend choosing an IDRIVE setting no more than 3nC/100ns = 30mA. You do not need much gate drive current to turn on these FETs, 100mA may be slewing VDS too fast and causing damage at the GLx gates via negative transients. 

Choose an IDRIVE setting based on this: https://e2e.ti.com/support/motor-drivers-group/motor-drivers/f/motor-drivers-forum/796378/faq-selecting-the-best-idrive-setting-and-why-this-is-essential

Hope this helps,
Aaron

Hi Lining,

I am referring to the external MOSFETs. Device uses an internal state machine to ensure GHx and GLx are not turned on at the same time via dead time. 

Please monitor GHx/SHx/GLx waveforms and ensure that VDS slew rate of HS MOSFET (VDRAIN-SHx) and LS MOSFET (SHx-GND) is 100ns or slower to reduce inductive spiking at the powerstage. Spiking too quickly can cause damage as shown in dV/dt coupling below. 

Please read this app note on power stage design for higher power motors: www.ti.com/.../slvaf66

Thanks,
Aaron

Hi Shengzong,

Yes, high side only includes VGS gate-to-ground diode. This is a 15-V rated Zener clamp diode to protect GHx from rising to 15V over SHx. VCP needs to be more protected than the low side VGS because VCP-VM ESD diode can be damaged if VCP increases to 15-V over SHx (or VGSH = 15V). 

VGSL (low side VGS) can tolerate higher gate voltage because VGLS regulator is operational to 20-V max. Hence gate-to-GND diode is not in device design. You can implement an external gate-to-GND diode on the low side VGS to protect against negative GLx voltages, or mitigate inductances in the power stage. 

Thanks,
Aaron

Hi Shengzhong,

Your Qgd of your MOSFETs are very small (3nC). 3nC / 100mA = 30ns VDS rise/fall time. Please set to min setting  (15mA source / 30mA sink). It should lengthen your VDS rise/fall time. 

Thanks,
Aaron

Hello,

If GLx-SLx = -2.1V for a transient amount of time, it may cause damage over time based on these abs max specs. There is generally an inverse relationship (1/t) on the length of the transient time and negative voltage tolerance the GLx pin can handle between these two specs. 

The device should be okay if it hasn't damaged yet so far over continuous motor operation, but it is best to try and mitigate these transients as best as possible during evaluation to reduce potential damage to the DRV or MOSFETs. You can use a GLx-GND diode to mitigate these transients or reduce inductance in the gate drive path by using a thicker trace (such as a wire). 

Thanks,
Aaron

Hi Shengzong,

Leaded components such as MOSFETs add parasitic inductance to the gate drive path and motor paths. Adding parasitic inductance at the gate drive path means it will take more time to change the current path (source vs sink current), so a smaller IDRIVE value, gate series resistor, or gate-to-source capacitance (Cgs) is needed to lengthen the rise and fall time. 

We recommended using surface mount package MOSFETs to reduce parasitic inductance in the motor and gate current paths. 

Thanks,
Aaron

Hi Shengzong, 

SMAJ15CA or an alternative looks to be good choice. 

Thanks,
Aaron