The need to transition from gasoline and diesel (fossil fuels) powered vehicles to electric vehicles (EVs) to reduce carbon dioxide levels has revealed many controversial issues. Common issues include implementation timing, required infrastructure (including fast charging multi-vehicle systems), legislated rather than consumer-based adoption, ongoing system enhancements, and sources of new critical raw materials.
One of the more straight-forward, yet still open technical issues for design engineers to resolve is the current sensing technology. Safely and effectively switching the current levels involved in electric vehicle propulsion (motor control), voltage conversion, battery management, and charging (onboard charger (OBC), home, and infrastructure-based) systems all require current sensors.
Batteries are the sole energy source for electric vehicles and have a limited lifetime. To extend their life, a battery-monitoring system estimates the state of health (SOH), state of charge (SOC) and state of function (SOF) using current and temperature sensors, often called IVT (current, voltage and temperature) sensors. According to Research Reports World, the electric vehicle battery current sensor market alone has a market size valued at USD 1310.0 million in 2021, increasing at a compound annual growth rate (CAGR) of 12.17% during the forecast period, and reaching USD 2610.0 million by 2027.
The good news is system designers have choices that do not require governments to dictate how they sense current or involve exotic materials. Among the choices are a resistive (current) shunt, a variety of Hall-effect sensor designs with supplier choices, current transformers, new technology options such as magnetoresistance and even diamond quantum sensors. System design issues for current sensors include high side vs low side current sensing, interface circuitry, bandwidth, response time, shielding, flux concentrator design, signal to noise ratio (SNR), crosstalk, programmability, and more.
Current Shunt
A standard current shunt is the simplest solution to sense current. It is a high precision, low value, high-power resistor. Current routed through the shunt results in a voltage drop proportional to the current (E=I*R). However, the voltage across the shunt needs to be amplified, isolated, and measured to determine the current level. Since the shunt resistor is in series with the load, the voltage drop results in a power loss, so increasingly smaller resistors values are used. This means that the analog front end (AFE) for amplifying and compensating the voltage drop must be very accurate to measure these very small values. The AFE can also provide isolation for improved safety.
Hall Effect Sensing
Hall-effect current sensors measure the magnitude of the magnetic field around a current-carrying conductor. Unlike shunts that are directly connected to the high current, Hall-effect devices are isolated from the load. With their inherent galvanic insulation, Hall-effect current sensors can measure both DC and AC currents, have low power loss, and are thermally decoupled from the power electronics.
In a Hall-effect current sensor, the Hall element is mounted in the gap of a ferrite magnetic core (flux ring or collector) placed around the current conductor. The AFEs in commercially available devices include amplifiers to boost and convert the Hall voltage to a usable level and other circuitry. Differential current sensor measurements using two Hall cells provide high accuracy even in a noisy environment where crosstalk can occur from adjacent current lines or magnetic stray fields.
Part 2 will discuss other technologies for current sensing in EVs.
References
Understanding Current Sensing in HEV/EV Batteries
Image source: Isabellenhutte_EV Shunt Resistors_tech article_final (isabellenhuetteusa.com)
Featured Image source: Hall Effect Current Sensing in Hybrid Electric Vehicle (HEV) Applications | Allegro MicroSystems
Current sensors | XENSIV™ – high-precision coreless current sensors – Infineon Technologies
Filed Under: Sensor Tips
