Cell balancing issue in 2AFE

[thailivenkatesh]

Hi,

I am working on Passive Cell balancing feature on S32K144 28S CELL configuration EVK board. Having BCC software driver for MC33771C, To test CB as per the MC33771C document procedure for 2AFE applied cell balancing but for 1st AFE it is applying and for 2nd AFE it is not applying .when applied only for 2nd AFE it is applying when both applied it is not working for both only 1st AFE is applied.

Please support to solve the problem.
Please find the Below attached copy of software.

Thanks & Regards

venkatesh thaili.

[12345678]

Hi,

Please suggest buck converter design for MCU supply and others component using 2 AFE.

[JozefKozon]

Hi Venkatesh,

please share your schematic with voltage levels and part values. Please state the full part number of the MC33771C AFEs you are using.

With Best Regards,

Jozef

[thailivenkatesh]

Hi Jozef,

Thanks for the reply.

We have configured existing 28S BMS EVAL board to 20S (AFE1 - 13S ,  AFE2 - 7S).

Please find the attached schematic PDF & cell voltages of 2 AFE configuration. The part number of AFE is: MC33771CTP1AE & the Isolated network high-speed transceiver IC part number is: MC33664ATL1EG. Please let me know if you require any other information further.

Thanks & regards.

Venkatesh Thaili.

[JozefKozon]

Hi Venkatesh,

thank you for the schematic. I have counted 14Cells in the first AFE. Please check it again. Haven't you missed to remove the Cell 5 and short circuit the Cell5 to Cell4?

Please refer to the section 13.2.6.2, Figure 48. and Table 95. in the MC33771C datasheet. For capacitive isolation, a serial resistor is recommended on the RDTX lines. 

Please remove the 10k pull-down resistor on the SPI_COM_EN pin and please connect it directly to GND.

With Best Regards,

Jozef

[thailivenkatesh]

Dear Jozef,

Please go through the inline comments:

I have counted 14Cells in the first AFE. Please check it again. Haven't you missed to remove the Cell 5 and short circuit the Cell5 to Cell4?
We have shorted cell 4 & Cell 5 on the battery stack itself, as there was no design provision on the board.

Please refer to the section 13.2.6.2, Figure 48. and Table 95. in the MC33771C datasheet. For capacitive isolation, a serial resistor is recommended on the RDTX lines.
We are using Capacitive isolation.

Please remove the 10k pull-down resistor on the SPI_COM_EN pin and please connect it directly to GND.
Ok, we have replaced the 10K pull down resistor on SPI_COM_EN pin by a 0E resistor & tested. Still cell balance is not happening on both AFEs simultaneously.

May I know the difference between pulling down the pin directly to GND & through 10K resistor.

Thanks & regards,

Venkatesh Thaili

[JozefKozon]

Hi Venkatesh,

We have shorted cell 4 & Cell 5 on the battery stack itself, as there was no design provision on the board.

[A] If you have done it, then the configuration should be correct. Please refer to a Figure 4. in the AN12536 "Using MC33771 with less than 14 cells". There is a 7Cells configuration with full filter population for the MC33771. And for the 13Cells configuration please refer to the Table 89. second column in the MC33771C datasheet. Please confirm, that you have followed the configuration recommended. The AN12536 and the MC33771C can be downloaded from this link under the secure files. 

Please make sure, that you use very low impedance cable or bar to connect the gap between the unused pins. 

We are using Capacitive isolation.

[A] Please confirm, that you are using the serial resistor as recommended in the section 13.2.6.2, Figure 48. and Table 95. in the MC33771C datasheet.

Ok, we have replaced the 10K pull down resistor on SPI_COM_EN pin by a 0E resistor & tested. Still cell balance is not happening on both AFEs simultaneously.

May I know the difference between pulling down the pin directly to GND & through 10K resistor.

[A] Please refer to the RD33771CDSTEVB schematic. The direct connection of the SPI_COM_EN to the GND without a pull down resistor is recommended. I don't know reason behind it, but the EVB was tested by our application engineers and is guaranteed to work correctly. Please take the RD33771CDSTEVB schematic as a reference.

The schematic and other design materials to the RD33771CDSTEVB can be downloaded from this link under the secure files.

With Best Regards,

Jozef

[thailivenkatesh]

Dear Jozef,

Please go through below comments.

1. We have verified the existing AFE section schematic, it is meeting the "AN12536" document connections. 20S configuration(AFE1: 13 cells, AFE2: 7 cells) is as per the NXP documents only, please go through the schematic PDF (attached previously as : CELL bal issue-Copy.pdf) file for AFE connections.

Please make sure, that you use very low impedance cable or bar to connect the gap between the unused pins. 

Unused pins are shorted through 0E resistors on the board itself for AFE 2.

2. Please confirm, that you are using the serial resistor as recommended in the section 13.2.6.2, Figure 48. and Table 95. in the MC33771C datasheet.
Serial resistors are not present on the board, as we have followed the attached schematic(2 AFE.pdf) as reference for capacitive isolation between AFE to AFE from NXP. I'm worried that if we cut the trace & place the serial resistors, that may effect the trace impedance of the traces, which may effect the present communication between AFEs

3. Please refer to the RD33771CDSTEVB schematic. The direct connection of the SPI_COM_EN to the GND without a pull down resistor is recommended. I don't know reason behind it, but the EVB was tested by our application engineers and is guaranteed to work correctly. Please take the RD33771CDSTEVB schematic as a reference.
OK. One more clarification here I need that, the both CSB & RESET pins connected to GND through 10K resistor, shall I replace these 10K resistors also by 0E resistors?

Could you please go through the code also (attached earlier in the chain as: cb.PNG) & let me know any deviations observed in software point of view.

[JozefKozon]

Hi Venkatesh,

Unused pins are shorted through 0E resistors on the board itself for AFE 2.

[A] Please make sure, that the used 0E resistors have enough current capability. Please refer to this link. 

Serial resistors are not present on the board, as we have followed the attached schematic(2 AFE.pdf) as reference for capacitive isolation between AFE to AFE from NXP. I'm worried that if we cut the trace & place the serial resistors, that may effect the trace impedance of the traces, which may effect the present communication between AFEs.

[A] Yes, this could happened if you place the serial resistors, but you can easily remove them and replace them with 0E resistors if this doesn't help. The current is very low in the communication lines. Please test it and if this doesn't help, we can go further.

OK. One more clarification here I need that, the both CSB & RESET pins connected to GND through 10K resistor, shall I replace these 10K resistors also by 0E resistors?

[A] Yes please. In our MC33771C EVB schematics the CSB, RESET and SPI_COM_EN are connected directly to GND without any pull-down.

Could you please go through the code also (attached earlier in the chain as: cb.PNG) & let me know any deviations observed in software point of view.

[A] I have checked your code, and I have only one note. If the BMS_MAX_CELLS is 20, please add = in the if condition. Instead of "if (i>13)", please use "if (i>=13)". Otherwise I don't see anything wrong with it. We have a software driver for the MC33771C. Please download it and please check it. Please download it from this link under the secure files. 

Please also check and download the BCC SW Driver Package for MC33771C and MC33772C, and the UM11346 from this link. There is a software for the TPL communication.

With Best Regards,

Jozef

[thailivenkatesh]

Dear Jozef,

1. Please make sure, that the used 0E resistors have enough current capability. Please refer to this link.
OK. we have taken care of this.

2. Yes, this could happened if you place the serial resistors, but you can easily remove them and replace them with 0E resistors if this doesn't help. The current is very low in the communication lines. Please test it and if this doesn't help, we can go further.
Here the communication is working fine, as we are able to read cell voltages temperatures etc.., hence I feel that we should not disturb the trace by adding serial resistors on RDTX lines as of now.

However our major concern is with the cell balancing presently, so can you clarify that how these serial resistors can impact the cell balancing process?

3. Yes please. In our MC33771C EVB schematics the CSB, RESET and SPI_COM_EN are connected directly to GND without any pull-down.
We have replaced the 10K resistors by 0E resistors as you have suggested.  Still cell balancing issue not resolved.

4. I have checked your code, and I have only one note. If the BMS_MAX_CELLS is 20, please add = in the if condition. Instead of "if (i>13)", please use "if (i>=13)". Otherwise I don't see anything wrong with it.
Thank you for the suggestion.

Here another query I want add that, how can we know or check whether the internal MOSFETs of the AFEs are working fine or not? as these are responsible for cell balancing to happen from AFE side.

Please find the attached cell voltages measured across the cell balance resistors in different cell balancing scenarios. Please comment on this.

Thanks & regards,

Thaili Venkatesh.

[JozefKozon]

Dear Venkatesh,

this looks more like the software issue. Meaning that your MCU (S32K144) is not able to send a request to both AFEs for the voltages of the Cells. Please test to use the examples from the MC33771C&MC33772C Battery Cell Controllers - Software Driver from this.

Please also download the MC33771C/MC33772C (Battery Cell Controller) Evaluation GUI from this link and test it. 

Please set the communication in the GUI for TPL.

I have just tested my S32K144EVB+FRDMDUAL33664EVB+KIT33772CTPLEVB+BATT-6EMULATOR with the GUI. 

With Best Regards,

Jozef

[thailivenkatesh]

Hi Jozef kozon,

Thanks for the reply .We are using the below attached BCC software driver for our 2AFE board and added shared cell balancing code in the previous reply. Please support to resolve the issue.

Could you please answer the below question also

How can we know or check whether the internal MOSFETs of the AFEs are working fine or not? as these are responsible for cell balancing to happen from AFE side.

Thanks & Regards 

Venkatesh T

[JozefKozon]

Hi Venkatesh,

this can be easily checked in the GUI I mentioned in the previous answer. I have just tested it. For balancing/discharging the cells, the internal MOSFETs must be working. 

Please set the CBTimer bit in the GUI and set the CBDrvEn bit. After that please set the CBEn/CBSts bits for individual channels you want to check. For the channel you start balancing with the CBEn/CBSts bit, you must see a decrease in the voltage on the Cell about 20 to 30mV. This way you can be sure, if the internal MOSFETs work correctly. 

It is better to check the channels one by one.

With Best Regards,

Jozef

[JozefKozon]

Hi Venkatesh,

Decrease about 20 to 30mV for Channel 6.

Decrease about 20 to 30mV for Channel 3.

Decrease about 20 to 30mV for Channel 2.

Same way you can check all the Channels on your EVB kit.

With Best Regards,

Jozef