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

2.1.1 AC Current Control Loops

Controlled power conversion between AC and DC is achieved by synchronizing the control unit with the grid frequency, achieving controlled amplitude and phase of the currents drained by the grid. To get the MCU synchronized with the grid, grid voltages (V_L1_A, V_L2_A, V_L3_A) are sampled by the MCU and fed to a phase-locked loop (PLL)11. By using the outcomes of the PLL (cos(φ), sin(φ)), plus Clarke and Park transforms, the three-phase system can be controlled by using the rotating frame reference technique (dq frame control), which allows the control to be simplified and improved.

Figure 2-2 is a schematic representation of the current control loop implemented in an AC/DC by using the rotating frame where measured Id and Iq can be derived by applying the Clarke and Park transformation to either I_L1_A, I_L2_A, I_L3_A or I_L1_B, I_L2_B, I_L3_B.

Figure 2-2 dq Current Control (Feedforward, PI Proportion Integral Control)

Figure 2-2 illustrates that the currents can be controlled by means of two PI controllers:

  • By changing the reference current called direct current (I_d*), the three-phase currents can be controlled in phase with respect to the corresponding grid voltages. This feature allows a direct control of the active power drained or sourced by the AC/DC, as represented in Equation 8, where P_Rec is the three-phase active power. Changing the sign of the reference current makes it possible to drain and source power correspondingly.
  • By changing the reference current called quadrature current (I_q*), the three-phase currents can be controlled 90° phase shifted with respect to the corresponding grid voltages. By changing this value, a direct control of the reactive power can be achieved, as shown in Equation 2, where Q is the total three-phase reactive power. By changing the sign of the reference current, it is possible to change the capacitive or inductive power drained by the equipment.
Equation 1. P_Rec=32VdId*
Equation 2. Q=32VdIq*

In addition to the PI controllers, feedforwards (FFW) are typically implemented in the current loops to decrease the response time and remove dependencies of the control loop bandwidths when variables in the system change (for example, if V_DC is not compensated, when V_DC decreases, control loop bandwidth can increase causing possible instabilities).