While battery range and charging times are getting the most attention when it comes to electric vehicle (EV) charging systems, safety and reliability are a critical part of the equation. Using the right current-sensing methodology can go far to address these concerns.
With the expanding adoption of EVs there is increasing pressure on the related power infrastructure to keep up. This pressure focuses mainly on accessibility related to battery range and the speed of charging. However, safety and reliability are critical in vehicular power systems, as human lives are involved. There are a variety of approaches being pursued when it comes to addressing the issue of charging EVs. Highway and city charging stations are currently being deployed, but the ability to charge at home is also a major piece of the EV charging market. Any catastrophic failure of any aspect of the power chain can result in a significant fire hazard and death, especially in a home-based system.
The power conversion system is key to EV performance and high-quality current sensing is a key component of that system. However, current sensing is also a significant contributor to safety and reliability. Optimizing automotive power conversion systems requires accurately measuring current, ensuring tight power-factor correction (PFC), and addressing thermal issues.
In addition to impacting the performance and efficiency of a power solution, current measurement can also help manage thermal performance, which if not addressed, can be both destructive and costly. When properly done, the right current-sensing methodology can significantly increase performance, safety, and cost-effectiveness. It is used for early fault detection and real-time performance information. This information can provide safety alerts, like the indication of an overcurrent condition or other loss of performance in order to predict and address things like potential thermal problems.
Beyond such obvious dangers as ground faults and short-circuits, operating at extreme power levels and at loading conditions beyond the system’s capability to support is also very dangerous. Charging systems rely on feedback loops for stability and optimum system performance — a current sensor is a vital part of that loop.
Methods of Current Sensing
A simple device like a resistive shunt can measure the voltage drop to determine the current flowing through the circuit. Although it offers good dynamic performance and linearity, it has limitations especially at high currents where the resistive power dissipation in the shunt itself becomes a thermal issue and is an unnecessary waste of power.
A Hall effect-based sensor is isolated, and it doesn’t affect the output in terms of voltage drop and power dissipation as much, but it lacks accuracy and bandwidth capabilities compared to other solutions.
A current transformer is isolated and can be made an accurate sensing solution, depending on the circuit topology, but because it is large and heavy it isn’t well-suited for automotive systems where space and weight are issues, and it also tends to be expensive.
The Model MCA1101 contactless isolated current sensor from ACEINNA is a single-chip solution using anisotropic magnetoresistive (AMR) technology. It provides galvanic isolation and essentially no power dissipation. Compared to the other methods of current sensing, it is compact as well as high-performing. The AMR technology offers a bandwidth of 1.5 megahertz and has low offset and noise. In addition, it enables fast readout while correcting offsets via active feedback loops, adjusting the gain parameters, and actively compensating the sensor offset. A further advantage is that since it is just a single chip, it offers significant footprint savings over board-assembled solutions. It offers the important combination of high accuracy and high speed.
AMR makes use of a common material, Permalloy, to act as a magnetometer. The alloy’s resistance depends on the angle between the magnetization and the direction of current flow. The film’s properties cause it to change resistance in the presence of a magnetic field. In the MCA1101, two of these resistors are connected in a Wheatstone bridge configuration to permit the measurement of the magnitude of the magnetic field produced by the current. The sensor voltage is fed into a differential amplifier followed by an instrumentation amplifier output stage that provides a voltage as a measure of the current.
A fully isolated U-shaped current path is provided by a low resistance copper conductor integrated into the package, making it suitable for both high-side and low-side bi-directional current sensing. It produces a voltage proportional to the magnetic field created by the positive or negative current in the current loop while rejecting external magnetic interference.
The high bandwidth of 1.5MHz (3dB) and low phase delay makes it ideal for current sense feedback loops in motor control, inverters, uninterruptible power supplies, battery management, power factor correction, high voltage distribution bus converters, and power supply applications, including those with fast switching wide-bandgap SiC- and GaN-based power stages.
EV Battery Charging
EV battery charging systems have critical needs because they are always on, use high switching speeds, and operate at high power. Current monitoring is required both for control and over- and under-current protection. At the speeds and power levels of EV charging systems, traditional fuses do not provide adequate protection except to prevent catastrophic failure in extreme situations. Intelligent fault management can also better address issues like user error and minor damage to cables and connectors.
High speed is vital for closed loop control of the fast switching currents found in DC-DC switching applications. The rule of thumb for any type of control loop is that the sensor should respond 10x faster than the switching speed of the transistors. For context, if transistors are switching at 120 kHz, you need about 1.2 MHz bandwidth — and the switching speed is likely to go higher in coming years.
Of course, fast response is absolutely critical for overcurrent trip-out sensing.
Another issue is that EV manufacturers are addressing advanced charging demands by increasing the working voltage, which can currently be as high as 800V. These higher-voltage systems provide the same amount of power with less current, resulting in lighter cables and a lower overall vehicle weight. If an EV has an 800-volt charging architecture and the charging station can handle high power levels, charging time drops significantly. For example, while 400-volt EVs can be charged at around 150 kW, an 800-volt charger can recharge a battery pack at a rate of 350 kW. The ACEINNA AMR-based integrated current sensor has a working rating of 1097 volts rms.
Although household systems may never reach the power levels of a service-station charger, they will be deployed at the highest possible power densities cost-effectively possible. These next-generation EV chargers need advanced current measurement to provide early fault detection and real-time performance information. Home systems must constantly watch for out-of-range current conditions or other loss of performance, to predict and address potential thermal and performance issues.
The dangerous fact is that many next-generation chargers are being driven at the edges of their performance envelope. This demands advanced current measurement to provide early fault detection and real-time performance information. EV battery-charging systems must have indication of an out-of-range current condition, or an over-current condition, or other loss of performance, to predict and address potentially dangerous issues.
Dangers to power electronics’ performance, and thereby system thermal issues, range from ground faults and short-circuits to operating at extreme power levels and at loading conditions beyond the system’s capability to support. Current sensors in advanced charging systems are deployed in each of the converter circuit feedback control-loop functions that regulate the performance, efficiency, and thermal linearity of the power systems in inverters.
In the case of high energy-density cells using lithium iron phosphate (LFP) or lithium-titanate (LTO), coulomb counting can determine battery State of Charge (SoC), State of Health (SoH), and State of Function (SoF). Only by closely monitoring those, can one confirm functionality and capacity.
Power quality is essential for efficient operation in advanced EV chargers, and the power factor is a big part of it.
Power Factor is the ratio of the true power of a load to the apparent power — a measure of the degree to which the voltage waveform and the current waveform are in phase with one another. So, it is a measure of how effectively you are using electricity and for optimum performance, it should as close to 1 as possible. For example, a load with a power factor of 0.90 means that only 90% of the power is being used effectively to do work. By measuring current and voltage at the same time, you can determine the phase difference and modulate the transistors to correct for it. Power factor correction (PFC) reduces the load on the electrical distribution system, increases energy efficiency and reduces electricity costs. It also decreases the likelihood of instability and failure of equipment.
The Value of AMR Current Sensing
While AMR current sensing can be useful in a range of industrial power and control applications, its high accuracy and high speed make it ideal for the rapidly expanding EV charging system market.