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.

Figure 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.