DRV8711: Regarding predriver fault

Hi Higa,

I think it may be possible that this noise is the cause of the PDF fault. Recently there was a thread with links describing this problem very well

https://e2e.ti.com/support/motor-drivers-group/motor-drivers/f/motor-drivers-forum/1129643/drv8711-predriver-fault-in-indexer-mode

In my case 56 Ohm LS gate resistors has solved this problem for good.

Best Regards,

Grzegorz

Hi Pablo-san

Thank you for your advice.

I proposed your advice to my customer and got the feedback.

They evaluated varying gate resistances between 30Ω and 120Ω. However, it did not improve.

Additionally, they made the following checks:

- Increasing tDRIVE: 2us -> no improve 

-  IDRIVEEP=50mA,IDRIVEN=100mA -> no improve

- Check Q_G -> OK

TBLANK : 2us

TOFF: 50.5us

DTIME : 450ns

PWM frequency : 18.7kHz

Q<267nF

FET : CSD18543Q3A

- PDF errors tend to occur when the load torque increases.

Could you give me your advice？

Best regards,

Higa

Hi Higa-san,

There are settings below that I use for my drivers, there is little chance that they would help but it might be worth of trying.

TOFF = 31us

TBLANK = 3,56 us

Auto mixed decay at all times

DTIME = 850ns

TDRIVE = 500ns

IDRIVE = 300/150 mA -  I use CSD18531Q5A (the same as in DRV8711EVM) which have 3.5x higher Qgd than CSD18543Q3A. I think "IDRIVEP=50mA, IDRIVEN=100mA" would be optimum or even slightly to high for CSD18543Q3A (unfortunately DRV8711 does not have lower settings).

If I had a problem like this I would:

1. Try to find any anomalies in voltages (LS and HS gates), H-bridge outputs, VM, VCP during increased load torque. I would check which H-bridge is causing the fault, set nFault signal as a trigger and check voltages in that bridge some 10us back. I would also check phases currents for any anomalies.

2. Try to slow Mosfets even more by:

- placing series resistors also for HS gates

- increasing their value (HS or/and LS) even more (no more than lets say 220 Ohm) - that and point above may cause damage to Mosfets and DRV8711 (remember about DTIME = 850ns)

- replacing Mosfets with ones that have higher Qgd.

3. Review PCB layout.

I you could tell at what current level (phase) the fault happens (rms/peak), VM voltage, motor parameters like inductance, resistance, size and speed before the fault? 

Regards,

Grzegorz

Daisuke,

Daisuke Higa said:

- PDF errors tend to occur when the load torque increases.

Could you give me your advice？

It may be possible that noise from the motor is getting coupled to the gate of the FET through the internal capacitance. The schematic below is from the DRV8770 gate driver. In this circuit, a 100kΩ resistor is placed between the gate and source of each FET to prevent accidental turn-on of the FET during switching. Try adding this resistor along with Grzegorz's recommendations.

Regards,

Pablo Armet

Hi Pelikan-san

Thank you for your advice.

I will propose your advice to my customer.

Thanks!

Higa

Hi Pablo-san

Thank you for your reply.

I will proposed you advice.

I have a question about the motor noise that you mentioned.

Is it BEMF? or ringing noise that caused by L(cable or layout)?

If yes,  in case of the motor drives with constant current and rotation is constant speed at heavy load torque, I think motor noise is no change.

Is my understanding correct?

It would be greatly appreciated if you could provide a little more detail about the motor noise you mentioned.

Best regards,

Higa

Hi pable-san

Thank you for answering

I will let you know as soon as I get feedback from my customer. 

Thanks,

Regards,

Higa

Higa-san,

Let me know if they require further assistance.

Regards,

Pablo A.

Hi Pablo-san

I got feedback from my customer.

They confirmed no improvement at 100k and improvement at 220k. On the other hand, going to 51k made it worse.

Do you know anything about the reason for this?

The high side series resistance is 1Ω and the low side series resistance is 47Ω.

Best regards,

Higa

Higa-san,

Please see this section in the datasheet:

8.1.2 Optional Series Gate Resistor
In high current or high voltage applications, the low side predriver fault may assert due to noise in the system. In
this application, TI recommends placing a 47 to 120-Ω resistor in series with the low side output and the gate of
the low side FET. TI also recommends setting the dead time to 850 ns when adding a series resistor.

Have they tried a larger series resistor on the low-side?  Also, they need to increase the dead time when doing this.

Regards,

Ryan