BQ78350-R1: ESD Test : BMS goes into shutdown

Hi Adam,

I'm sorry to hear you are experiencing this issue. The change to Shutdown Voltage showing a better result is interesting, but it seems unlikely that the minimum cell voltage was below 1.75V for 10 seconds. I suppose this is possible with multiple consecutive ESD pulses. If the BQ78350 detects this condition, it sends a command to the BQ76930 to enter Ship mode which will turn off the REGOUT pin. As a result, the BQ78350 will lose its VCC which will cause it to power down.
Another possibility of what might be occurring is that the BQ76930 is going into Ship mode as a result of one of its critical voltages going low causing it to go into POR. If this device resets, it will be in Ship mode until the boot voltage is applied to TS1. From the thread below, WM describes some of the pins that can be involved if the event is caused by a POR to the BQ76930:

e2e.ti.com/.../770574

I'm not sure if you have a way to differentiate which cause is more likely in your pack. If you are able to monitor REGOUT and CAP1 it might help narrow down the possibilities.

Best regards,
Matt

Hi Matt,

Many thanks for the quick response. You are very professional and your answer is always very useful.
I was able to take a few measurements, the results are next:

ESD Hits always to PACK- Connector and CAP1, REGOUT, REGSRC, ALERT were measured. (Oscilloskop)

Results:
CAP1 and REGOUT have the same behavior.The supplies have disruption by ESD. Probaly it can cause reset or shutdown.

REGOUT:

 

CAP1:

REGSRC
It is disturbed by ESD too, but not so hard as CAP1,REGOUT. I think its OK.

ALERT
It does not see directly the ESD hit but sends something to BQ78350 if I generate an ESD pulse.This signal are going to REGOUT for a longer time. (DSG will be off and again on)

This is an ESD hit to pack- without shutdown:

After this ESD hit turn off the BMS

Probably this "command" is the shutdown but I dont see always this signal on Alert if the ESD cause shutdown.

If you have any idea how I could filter the supplies and how could I solve this problem please don't hasitate and contact me :)

Kindly Regards

Adam

Hi Adam,

This is great information - thank you for capturing this. I will consult with my colleagues here how these waveforms might explain the Shutdown and hopefully we can offer some suggestions. I think this might take a couple of days. I hope this is okay.

Best regards,
Matt

Hi Matt,

Thank you for your help. I am patiently waiting for your suggestions.

Regards,

Adam

Hi Adam,

I noticed that the FAQ document for the BQ76930 (www.ti.com/.../sluub41.pdf) states that a transient on one of the CAPx pins can cause a POR while a short on REGOUT will not reset the device (see FAQ 10.13 and 10.14). I experimented a little on the EVM today to test the effects of quickly shorting the CAP2, CAP1, and REGOUT pins.

Shorting the CAP1 pin to ground immediately causes the device to go into POR and a boot signal to TS1 is needed to wake up the device. I could not duplicate the very fast timing of your ESD waveform since I was doing this manually, but I do think this is the most likely cause. The ALERT pin behavior also looked like your 2nd ALERT pin scope capture.

Shorting the CAP2 pin low (I shorted to VC5X) does not cause the device to go into POR. The CAP2 pin is used by the upper cell group of the device, so I expect it will cause an XREADY fault if shorted. So I monitored the ALERT pin and it has a long pulse - it looks like your 1st ALERT pin waveform. When you see this long waveform without POR, I suspect it is indicating CAP2 is going low due to the ESD pulse.

Shorting REGOUT to GND did not cause the BQ76930 to go into POR and I did not notice anything unusual on the ALERT pin.

I wonder if it might help to place a 2nd smaller capacitor on CAP1 and CAP2 to filter the high frequency transient.

Best regards,
Matt

Hi Matt,

I have tested our system with different 2nd capacitors on line CAP1 and CAP2 yesterday to filter the high frequency transients. (4000pF,1000pF,27pF)

The system does not have bigger immunity to ESD hit. Could help an LC or "PI" Low-Pass-Filter ?

Best regards,

Adam

Hi Adam,

I'm not sure how to best address this. We will try to set up an experiment in our lab by early next week to look into this.

Matt

Hi Matt,

I have some new experiences about to filter CAP1 and CAP2.

CAP1 and CAP2 are protected directly on the pin with SZESD7351 ESD Protection Diode. It looks to have a bigger immunity for ESD but it does not solve fully the issue.

There is an other question about current measurement:

should I have to add 2nd protection component to SRN,SRP to protect them from power glitch? Can it cause  POR voltage on CAPx internally?

Best Regards,

Adam

Hi Adam,

Sorry for the delay - I am checking with the design team and awaiting their answer. Hopefully I will have an answer tomorrow.

Regards,
Matt

Hi Adam,

I talked with the designer. He asked if a TVS is being used (www.ti.com/.../slyt492.pdf). He does not think it is likely the SRP/SRN pins are passing the signal to the CAP pins.

His suggestion to further debug the issue is to capture waveforms on the pins marked below. I know you have already captured several of these. It might be very interesting to see what is happening on VC5x and VSS.

Best regards,

Matt

Hi Matt,

Thank you that you are investing so many time to help us.

I would like to confirm your experiences on our system:

Shorting CAP1 pin to ground immediately causes shutdown and a boot signal to TS1 is needed to wake it up.

Shorting REGOUT pin to GND causes immediately the same behavior like with ESD pulse without shutting it down:

DSG is going to turn off for 2-4sec (the voltage of gate will be zero for this time)

Output voltage is going to zero. (In our design the charge FET connects directly to the sense resistor)

After releasing the short circuit the system continues to work.

Probably BQ78350-R1 is reset until REGOUT is shorted.

 

There is an unidirectional TVS on PACK output:

I have captured some signals please see attached. You see always the biggest impulses that I could shot. The signals were attacked by ESD.

TI_BQ76930_78350.pdf

Best Regards,

Adam

Thanks for capturing all of these waveforms Adam. I've sent these to the design team to see if they can understand what might be causing the Shutdown. I'll let you know once I get feedback from them.

Sorry for the delay,
Matt

Hi Matt,

In our design the charge FET connects directly to the sense resistor.  We could solve the ESD problem with a bigger (1206(3216M)) MLC capacitor on DSG FETs (D-G).This capacitor is showing in application report (Avoiding ESD and EMI Problems in bq20zxx Battery PackElectronics ) , but we did not know exactly what is the expected value, size and  type of this capacitor.

Could you approve that the ESD strike on PACK can shutdown BQ76930 and it can be solved by a relativ big capacitor?  Could you please write me if you have more knowledge about this capacitor.

To avoid high frequency problems in bq76930 should we place a 2nd smaller capacitor on DSG to filter the transients?

Best Regards,

Adam

Hi Matt,

I would have a short question regarding on ESD:

on the eval board there is two series capacitor parallel with the MOSFETs. Could you please clearify me why?

Best regards,

Adam

Hi Adam,

The caps are for ESD protection. The reason there are 2 is to protect against a single point fault. If there is only one cap and it is shorted, it would cause a short across the FETs. With 2 series caps it takes 2 faults to bypass the FETs.

Best regards,

Matt