TIDUF88 October   2024

 

  1.   1
  2.   Description
  3.   Resources
  4.   Features
  5.   Applications
  6.   6
  7. 1System Description
  8. 2System Overview
    1. 2.1 Block Diagram
    2. 2.2 Design Considerations
      1. 2.2.1 Accuracy of Bus Voltage Measuring
      2. 2.2.2 Shunt Current Measuring
      3. 2.2.3 Insulation Impedance Monitor
    3. 2.3 Highlighted Products
      1. 2.3.1 BQ79731-Q1
      2. 2.3.2 TPSI2140-Q1
      3. 2.3.3 ISO7841
      4. 2.3.4 SN6507
      5. 2.3.5 TPS7B6950
  9. 3Hardware, Software, Testing Requirements, and Test Results
    1. 3.1 Hardware Requirements
    2. 3.2 Test Setup
    3. 3.3 Test Results
      1. 3.3.1 Bus Voltage Accuracy
      2. 3.3.2 Current Sensing Accuracy
      3. 3.3.3 Insulation Impedance Accuracy
  10. 4Design and Documentation Support
    1. 4.1 Design Files
      1. 4.1.1 Schematics
      2. 4.1.2 BOM
    2. 4.2 Tools and Software
    3. 4.3 Documentation Support
    4. 4.4 Support Resources
    5. 4.5 Trademarks
  11. 5About the Author

2.2.2 Shunt Current Measuring

The shunt current accuracy is a key parameter to calculate the SoC of the battery. The target shunt accuracy of this design is 0.1% when the current is no less than 10A, and maximum 10mA error when the current is less than 10A at room temperature. The shunt resistance is 150μΩ and current range is ±500A. A maximum of 10mA across the 150μΩ error when current is less than 10A requires the maximum error of CSADC to be less than 1.5μV when shunt voltage is less than 1500μV. The gain error of CSADC after calibration must be smaller than 0.1% or less.

The shunt resistance has temperature drift. The shunt resistance needs three test points to calculate the resistance-temperature curve and costs several minutes to get the stable shunt temperature. Some shunt manufactures can provide a resistance temperature curve for each shunt like the PCBS8518A050Q2AC00 device from C&B Electronics. Resistance temperature curve can be used to estimate Rshunt(T) (shunt resistance in temperature T).

The shunt calibration process in end-of-rack production line at room temperature:

  1. Current = 0A, read the CSADC data VCSADC0
  2. Current = 10A or larger
    • Read the CSADC data VCSADC0
    • Read the current ICSADC10, calculate the shunt voltage ICSADC10 multiplied by Rshunt(T)
  3. Calculate the offset. Offset = VCSADC0.
  4. Calculate the gain. Gain= (ICSADC10 × Rshunt(T) – Offset) / ( ICSADC10 × Rshunt(T)).
  5. Write offset and gain to EEPROM and flash