Current sense issue in MC33771C

ksaurabh7885 · ‎09-11-2022

Hi ,

We are using MC33771C IC for Our BMS solutions . I am facing issue that Current sense pin is getting damaged after connecting to battery pack after 2-3 charge discharge cycle. during these 2-3 charge cycle current reading is exactly matching with amount of charging current and discharge current. after checking all the zener diode found ok, after replacing with new BMS AFE , current sensing is again matching exactly with amount of charging current and discharge current. I am using low side Mosfet based Control for BMS charge and discharge circuit (attached the image). Need suggestion on two points 1. is there any problems in current sense circuits attached and MOSFET circuit attached.

2. is there any chance that due to Mosfet turnoff during charge & discharge causing any transient buildup.

Currently we are checking the solution for 30 A charge and discharge current only.

ksaurabh7885 · ‎09-12-2022

Hi @JozefKozon

Thanks for your clarifications , 2nd picture is not for cell balancing , it is for charge and discharge control, which is connected to shunt line in series .

for Cell balancing , I have made circuit similar to recommendations by NXP .

I have query that the connected Zener diode for current sense line is not working that's why it is failing. is there any specific circuit or component you can suggest to avoid the current failure scenario. However I am ensuring through pre charge circuit that it shall not allow any inrush current.

Just wanted to know is my failure is different types or this kind of issue reported from different user as well.

Yes , The shunt value is only 300u ohm , meanwhile I am also checking the transient waveform using oscilloscope on I sense+/- pin ,

However , I would like to know about the maximum rating of I sense pin , as per table 5, page 11 of data sheet it is showing from -0.3 V to 2.5 V (as per attached 3rd image ) , what is the significance of that ratings.

JozefKozon · ‎09-12-2022

Hi Saurabh,

thank you for the additional information. I better understand the circuit now. Yes, you are right. The values in the Table 5. are correct. I have missed the +2.5V for the ISENSE pins. The ISENSE pins can withstand the voltage up to +2.5V, but the negative voltage is only -300mV, so its the same as the voltage in the Table 8.

Regarding the charge and discharge control, unfortunately this is not tested and not recommended by NXP. Please refer to any MC3377xC evaluation board. No such a circuit is there. I believe this is the reason for the issue. Please follow the evaluation board schematics. These circuits were tested by NXP application engineers. You can take is as a reference.

https://www.nxp.com/products/power-management/battery-management/battery-cell-controllers/14-channel...

With Best Regards,

Jozef

JozefKozon · ‎09-11-2022

Hi Saurabh,

I have checked the schematics you provided. Regarding the first one, the ISENSE+ and ISENSE- protection is correct. Exactly according our recommendation in the section 13.2.4 in the MC33771C datasheet.

1. is there any problems in current sense circuits attached and MOSFET circuit attached.

[A] Regarding the second picture, I suppose you are using external transistors for Cell balancing. Is that correct? If yes, I have compared the second picture with our recommended schematic for external Cell balancing and I am missing some components in your schematic. Unfortunately I cannot share the schematic on this community, because is confidential. If you would like to receive the schematic, please create a new ticket here. In the Subject please mention this case number 00488657, so we know, that the new ticket is related to this case.

2. is there any chance that due to Mosfet turnoff during charge & discharge causing any transient buildup.

Currently we are checking the solution for 30 A charge and discharge current only.

[A] Yes, the transient voltages can be a reason for damaging the ISENSE pins. The differential voltage on the ISENSE+ against ISENSE- cannot exceed +/-150mV and voltage on the ISENSE+ and ISENSE- pins against AGND cannot exceed +/-300mV. Otherwise the pins will be damaged.

Please use an oscilloscope on the ISENSE+ and ISENSE- pins to measure if the voltage doesn't exceed the limits in the Table above. If I read the shunt value correctly, the differential voltage on the ISENSE+ against ISENSE- should be V=R*I=300uOhm*30A=9mV, which is way below the 150mV limit.

With Best Regards,

Jozef