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

1.1 DC Charging Station for Electric Vehicles

To supply or drain the power from the vehicle battery to the grid, multiple conversion stages are necessary between the AC and the DC rails, as Figure 1-1 shows.

GUID-20220303-SS0I-T2RR-2VL0-RSJZ442Z6CFH-low.pngFigure 1-1 Current Sensing Points of an EV Charging System

AC/DC converters are responsible to convert AC into DC power by keeping under control the current Total Harmonic Distortion (THD) on the Point of Common Coupling (PCC) together with the DC voltage. At the same time, isolated DC/DC converters are mainly used for galvanic isolation between the grid and the car and to achieve Constant Current (CC) and Constant Voltage (CV) charging functionalities.

Figure 1-1 depicts typical current sensor locations of an EV charging system.

  • Power regulation and protections of the AC/DC stage are achieved by means of sensors placed in point A, B, C, and D:
    • Point A is the main connection point of the converter toward the PCC. By placing sensors at this location, the currents pushed into or pulled from the grid can be most accurately monitored and controlled, thereby achieving accurate control of active and reactive powers interchanged with the grid.
    • Point B has the capability to measure the switch current in the Switching Node (SN). By placing the current sensors in this location, protection of power switches and control loop speed can be improved. Furthermore, when an isolated power supply is required by the current sensing circuit, gate driver supply can be leveraged, thus reducing the total cost of the design. However, the measurement does not include the losses in the EMI filter, therefore this location is less suitable for reactive power compensation.
    • Point C is the measurement point of the DC bus current. Placement of the current-sensing circuit in this location allows cost reduction when the power supply is shared with the bottom switch-driver supply.
    • Point D is the measurement point of the DC bus current placed on the positive rail of the DC bus.
  • Power regulation and protection of the DC/DC stage are achieved by means of sensing placed in point G, F, and E.
    • Point G is required to control the windings currents.
    • Point F is the measurement point of the battery current located on the positive terminal.
    • Point E is the measurement point of the battery current located on the negative terminal. The benefit of sensing the current flowing to the negative terminal is that the gate-driver supply of the low-side FET can be leveraged for powering the current-sensing circuit.

In this application note, a study based on simulation results was conducted with the aim to define the minimal specifications required by the current sensors when used in DC charging applications for EVs. Optimal values of bandwidth, gain error, offset, and latency were derived for an 11-kW system presenting the system specifications listed in Table 1-1. Two different isolated DC/DC topologies are considered in this document: DAB (Dual Active Bridge) with phase-shift control and DAB with CLLLC resonant converter.

Section 2 discusses design considerations for AC/DC input current sensing Point A and B respectively with DC link current measurements in C and D. Section 3 details the requirements for the current sensing points in the DC/DC stage (G, F, E) in how proprieties as bandwidth, gain and offset errors impact the performance of the DC/DC stage.

Table 1-1 Target Specifications for EV Charger
ConditionDescription
Power ratings and power flow11-kW bidirectional operation to support V2G/V2H
AC ratings400 VAC (3-phase each 230 VAC)
16 ARMS (each phase)
Total harmonic distortion of AC current3.6% at PCC at full load
DC RatingsVDCBUS 800 V nominal ( from 650 V to 800 V)
IDCBUS 14 A ( from 14 A to 17 A)
VBAT 400 V (from 250 V to 450 V)
IOUT 27.5 A ( from 24 A to 44 A)
Switching frequency of AC/DC70 kHz (dead-time = 250 ns)
Switching frequency of DC/DC100 kHz for phase-shifted DAB
500 kHz nominal for resonant CLLLC DAB
Accuracy required by the power controlled in the DC sideVDCBUS ±1%
VBAT ±1%
IBAT ±1%
Implemented AC/DC bandwidths of the current and voltage loops3-kHz grid current loops (id, iq)
400-Hz DC bus voltage loop
Implemented bandwidths of the voltage measurements: grid and DC link 100 kHz