SBAA541 December   2022 AMC1202 , AMC1302 , AMC1306M05 , AMC22C11 , AMC22C12 , AMC23C10 , AMC23C11 , AMC23C12 , AMC23C14 , AMC23C15 , AMC3302 , AMC3306M05

 

  1.   Abstract
  2.   Trademarks
  3. 1Introduction
    1. 1.1 DC Charging Station for Electric Vehicles
    2. 1.2 Current-Sensing Technology Selection and Equivalent Model
      1. 1.2.1 Sensing of the Current With Shunt-Based Solution
      2. 1.2.2 Equivalent Model of the Sensing Technology
  4. 2Current Sensing in AC/DC Converters
    1. 2.1 Basic Hardware and Control Description of AC/DC
      1. 2.1.1 AC Current Control Loops
      2. 2.1.2 DC Voltage Control Loop
    2. 2.2 Point A and B – AC/DC AC Phase-Current Sensing
      1. 2.2.1 Impact of Bandwidth
        1. 2.2.1.1 Steady State Analysis: Fundamental and Zero Crossing Currents
        2. 2.2.1.2 Transient Analysis: Step Power and Voltage Sag Response
      2. 2.2.2 Impact of Latency
        1. 2.2.2.1 Fault Analysis: Grid Short-Circuit
      3. 2.2.3 Impact of Gain Error
        1. 2.2.3.1 Power Disturbance in AC/DC Caused by Gain Error
        2. 2.2.3.2 AC/DC Response to Power Disturbance Caused by Gain Error
      4. 2.2.4 Impact of Offset
    3. 2.3 Point C and D – AC/DC DC Link Current Sensing
      1. 2.3.1 Impact of Bandwidth on Feedforward Performance
      2. 2.3.2 Impact of Latency on Power Switch Protection
      3. 2.3.3 Impact of Gain Error on Power Measurement
        1. 2.3.3.1 Transient Analysis: Feedforward in Point D
      4. 2.3.4 Impact of Offset
    4. 2.4 Summary of Positives and Negatives at Point A, B, C1/2 and D1/2 and Product Suggestions
  5. 3Current Sensing in DC/DC Converters
    1. 3.1 Basic Operation Principle of Isolated DC/DC Converter With Phase-Shift Control
    2. 3.2 Point E, F - DC/DC Current Sensing
      1. 3.2.1 Impact of Bandwidth
      2. 3.2.2 Impact of Gain Error
      3. 3.2.3 Impact of Offset Error
    3. 3.3 Point G - DC/DC Tank Current Sensing
    4. 3.4 Summary of Sensing Points E, F, and G and Product Suggestions
  6. 4Conclusion
  7. 5References

3.2.3 Impact of Offset Error

This chapter investigates offset error on the DC/DC converter. The same control-loop settings, current-sensor bandwidth of 100 kHz, and 0% gain error of the current sensor were assumed in the simulation for the settling time simulation shown in Figure 3-5. The offset error has been varied from 0%, 1%, to 2%.

Figure 3-5 Steady State Output Current Errors vs Current Sensor Offset Errors

Again, settling time is unaffected by offset error. The settled output current is significantly affected. For 1% offset error the current output is 1.5% or 0.3 A lower (for 2% offset the output shows 3% or 0.6 A error, respectively).

Like the Gain Error, the Offset Error is specified to the full-scale error. In our example, the full-scale current was 32 A. This means at a 1% error, the absolute error is 0.3 A (for 2%, absolute 0.6 A). The simulation indicates these results are precise.

Unlike the gain error that scales relative to the output, the offset error adds in absolute to the output current that is set in a converter. Offset error is either calibrated out or compensated by feedforward techniques (adding the known error to the output).

In summary, both gain and offset error do not impact the settling time of the control loop as long as the current sensor has a high enough bandwidth not to limit the control-loop bandwidth. Both gain and offset error impacts the accuracy of the DC-charger output. For the target specifications of the EV-Charger defined in Table 1-1, this means the current sensor needs to have a bandwidth between 10 kHz and 100 kHz and total error (for both gain and offset) smaller than 1%. Use offset calibration to achieve the target.