Current Measurement and Power Supply Layout for MC33771b

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Current Measurement and Power Supply Layout for MC33771b

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Jenish_
Contributor II

So the dilemma I am facing is using the integrated coulomb counting feature of the MC33771b which has Isense+ and Isense- pins to implement the feature. 

In my case, the Battery I am using has two separate connectors i.e. Positive and Negative (48V) and second connector which is quasi cell-balancing connector with access to individual cells and a ground. For current sensing feature to be used according to the MC33771b data sheet, the pins should be connected across a shunt which is placed on the negative line of the battery and other cells are connected with the CT pins of the MC33771b. 

My question is, will the coulomb counting feature will still work as same if the shunt for Isense+ and Isense- is connected in the main power supply connector where actual current flows unlike the cell balancing connector which is mostly made for cell voltage measurements. If not the case which else design considerations needs to be taken into account to implement such thing on Hardware or Software level. 

Similarly if just SPI protocol is used should the ground of the Microcontroller and the MC33771b should be separated or it also works if they both share a common ground. 

Hoping to get any help possible.

Jenish__0-1686052224991.png

 


Thank you !

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1 Solution
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JozefKozon
NXP TechSupport
NXP TechSupport

Dear Jenish,

In my case, the Battery I am using has two separate connectors i.e. Positive and Negative (48V) and second connector which is quasi cell-balancing connector with access to individual cells and a ground. For current sensing feature to be used according to the MC33771b data sheet, the pins should be connected across a shunt which is placed on the negative line of the battery and other cells are connected with the CT pins of the MC33771b. 

My question is, will the coulomb counting feature will still work as same if the shunt for Isense+ and Isense- is connected in the main power supply connector where actual current flows unlike the cell balancing connector which is mostly made for cell voltage measurements. If not the case which else design considerations needs to be taken into account to implement such thing on Hardware or Software level. 

[A] Yes, the coulomb counting feature will still work. The ISENSE circuit measures the voltage on the ISENSE pins. The voltage is calculated by the Ohms law. V=Rshunt*I. You must choose the Rshunt value to make sure, that the voltage on the ISENSE pins will not exceed the limiting values stated in the Table 7. in the MC33771B datasheet. Any higher voltage will destroy the MC33771B. 

JozefKozon_0-1686115311430.png

Further, for correct function of the ISENSE circuit, you must choose the Rshunt, so the voltages will not exceed the VINC and VIND values stated in the Table 9. in the MC33771B datasheet.

JozefKozon_1-1686115446866.png

If Rshunt=100uOhm is used, the ISENSE circuit is capable to measure current up to +/-1500A. V=Rshunt*I=0.0001*1500=150mV.

The ISENSE circuit was intended for measurement of charge and discharge current of the Cells, but it will measure any current. You just must make sure, that the values mentioned above will not be exceeded. If ISENSE pins are not used, it is recommended to connect them to ground. Please see below a quote from an application engineer regarding ISENSE current measurement. 

"A charge current will be positive and a discharge current will be negative. If both are happening at the same time, the sum of the two currents will be measured by the AFE, which is the actual current that will be flowing inside the battery. This is the only current that needs to be taken into account by the SOC algorithm. The fact that it is the result of a charge and discharging current has no importance there."

Similarly if just SPI protocol is used should the ground of the Microcontroller and the MC33771b should be separated or it also works if they both share a common ground.

[A] Yes, it will work, however it is recommended to use different grounds for the MCU and the BCC (Battery Cell Controller). The BCC works with much higher voltages, than MCU, up to 61.6V operational voltage and up to 75V transient voltage. If any of this voltages accidentally gets to the common ground, it will destroy the MCU, which will be probably more expensive, than BCC. Therefore please use separate ground for the BCC and the MCU as recommended.

JozefKozon_2-1686115999763.png

With Best Regards,

Jozef

 

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Jenish_
Contributor II

Thank you Jozef for detailed explanation. This definitely will help improving the design.

Best Regards,
Jenish

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604 Views
JozefKozon
NXP TechSupport
NXP TechSupport

Dear Jenish,

In my case, the Battery I am using has two separate connectors i.e. Positive and Negative (48V) and second connector which is quasi cell-balancing connector with access to individual cells and a ground. For current sensing feature to be used according to the MC33771b data sheet, the pins should be connected across a shunt which is placed on the negative line of the battery and other cells are connected with the CT pins of the MC33771b. 

My question is, will the coulomb counting feature will still work as same if the shunt for Isense+ and Isense- is connected in the main power supply connector where actual current flows unlike the cell balancing connector which is mostly made for cell voltage measurements. If not the case which else design considerations needs to be taken into account to implement such thing on Hardware or Software level. 

[A] Yes, the coulomb counting feature will still work. The ISENSE circuit measures the voltage on the ISENSE pins. The voltage is calculated by the Ohms law. V=Rshunt*I. You must choose the Rshunt value to make sure, that the voltage on the ISENSE pins will not exceed the limiting values stated in the Table 7. in the MC33771B datasheet. Any higher voltage will destroy the MC33771B. 

JozefKozon_0-1686115311430.png

Further, for correct function of the ISENSE circuit, you must choose the Rshunt, so the voltages will not exceed the VINC and VIND values stated in the Table 9. in the MC33771B datasheet.

JozefKozon_1-1686115446866.png

If Rshunt=100uOhm is used, the ISENSE circuit is capable to measure current up to +/-1500A. V=Rshunt*I=0.0001*1500=150mV.

The ISENSE circuit was intended for measurement of charge and discharge current of the Cells, but it will measure any current. You just must make sure, that the values mentioned above will not be exceeded. If ISENSE pins are not used, it is recommended to connect them to ground. Please see below a quote from an application engineer regarding ISENSE current measurement. 

"A charge current will be positive and a discharge current will be negative. If both are happening at the same time, the sum of the two currents will be measured by the AFE, which is the actual current that will be flowing inside the battery. This is the only current that needs to be taken into account by the SOC algorithm. The fact that it is the result of a charge and discharging current has no importance there."

Similarly if just SPI protocol is used should the ground of the Microcontroller and the MC33771b should be separated or it also works if they both share a common ground.

[A] Yes, it will work, however it is recommended to use different grounds for the MCU and the BCC (Battery Cell Controller). The BCC works with much higher voltages, than MCU, up to 61.6V operational voltage and up to 75V transient voltage. If any of this voltages accidentally gets to the common ground, it will destroy the MCU, which will be probably more expensive, than BCC. Therefore please use separate ground for the BCC and the MCU as recommended.

JozefKozon_2-1686115999763.png

With Best Regards,

Jozef

 

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