Magnetic Current Sensing Drives Vehicle Electrification

Magnetic current sensors are replacing sense resistors and current transformers

Modern cars have become increasingly power hungry. Even the "least electrified" vehicles include advanced electronics for infotainment, safety, and engine control. Fully electric vehicles further add high-voltage power domains to charge and drive. Each of these systems shares the need for power monitoring and control, and this is where current sensors are used.

Safety systems and powertrain electrification OEMs are implementing electrified steering and braking systems to increase vehicle safety and enable automated driving. These systems encompass one or more high-current motors to actuate the steering rack or brake caliper. Safety systems require current sensors to be small and have low heat generation.

A basic electric vehicle powertrain encompasses a charging system and a drive system. A charging system may experience 100A at 400V or 800V, while the drive system may need to deliver several hundred amperes of current to the traction drives. Current sensors require isolation and high current capability, while also operating from DC to a high bandwidth for efficient power conversion and system protection.

Sense resistors and current transformers Current sensing needs have historically been met using sense resistors and current transformers. The sense resistor (and associated shunt amplifier) provides V=I×R but suffers due to large size, high power dissipation, slow bandwidth, and no inherent isolation. The current transformer provides an isolated, high- speed measurement, but has drawbacks of large component height, no DC sensing, a narrow frequency band, and needing many external components to operate.

Magnetic current sensors bring benefits Magnetic current sensors are non-contacting sensor integrated circuits. This means the sensor silicon is not touching the conductor carrying the current. Isolation does not require extra components, and ranges from 100V to several thousand volts. The sensor measures the magnetic field generated by the current and outputs a voltage proportional to that magnetic field. Some magnetic current sensors are placed close to a current- carrying wire (with or without a concentrator) while others integrate a conductor path inside the component for ease of implementation.

A magnetic current sensor can provide significant advantages over sense resistors and current transformers in automotive power systems. Compared to a sense resistor, the magnetic current sensor offers a smaller size, lower power dissipation, and higher bandwidth with integrated isolation. Advantages of magnetic current sensors over current transformers are smaller size, DC measurement, and minimal external components.

One example current sensor is the Allegro Microsystems ACS37010, which features 450kHz bandwidth, high accuracy, and integrated reinforced isolation in an enhanced SOIC-8 package. Other solutions such as the ACS71240 are optimized for smaller footprints, with low-or-no-isolation in SOIC-8 or 3×3 QFN packages.