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

3.3.1 Bus Voltage Accuracy

The test setup follows Figure 3-1. This design measures the VBAT channel in the board connected to the 1500V DC source. The source is set as 500V to 1500V and the working voltage of ESS is assumed to be 1100V to 1500V. VF1_7 is the BQ79731 reading data. VBAT_7 is the VBAT voltage calculated by BQ79731. VBAT_7 equals Ratio multiplied by VF1_7. The ratio is the voltage gain which equals 361.7214429 in this design. The calibration process uses test points of 1100V and 1500V to remove the offset error caused by BQ79731 VF1 ADC and gain error caused by resistor error.

Table 3-10 shows the bus voltage accuracy data measured by BQ79731 VF1. The ladder and sensing resistors of BAT voltage have 1% accuracy. The accuracy before calibration is 0.76% maximum. After calibration, the accuracy is less than 0.1% when the bus voltage ≥ 500V, and a maximum 0.3V absolute error when the bus voltage is 500V. This easily meets the requirement in GBT34131-2023 as 0.5% when the bus voltage is ≥ 500V and 5V when the bus voltage is < 500V.

Table 3-10 Bus Voltage Accuracy Data
VF1_7 (V) METER READINGS
VBAT_M (V)		 BEFORE CALIBRATION AFTER CALIBRATION
VBAT_7 (V) ACCURACY VBAT_7_c (V) ACCURACY
1.3894 499.898 502.5758 0.53% 500.2247 0.07% (Abs 0.3V)
2.225 799.881 804.8302 0.61% 799.7497 –0.02%
2.7834 999.868 1006.815 0.69% 999.911 0.00%
3.0624 1099.92 1107.736 0.71% 1099.92 0.00%
3.6197 1299.92 1309.323 0.72% 1299.687 –0.02%
4.1783 1499.92 1511.381 0.76% 1499.92 0.00%

Figure 3-2 shows the bus voltage accuracy curve measured with BQ79731 VF1.

TIDA-010272 Bus Voltage Accuracy Curve Figure 3-2 Bus Voltage Accuracy Curve