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A Novel Contactless Current Sensor for HEV / EV and Renewable Energy Applications

The novel contactless current sensors consist of an integrated CMOS Hall effect sensor covered by a thin, ferromagnetic layer structured on its surface. The Integrated Magnetic Concentrator (IMC) layer acts as a magnetic flux concentrator, providing a high magnetic gain which increases the sensor’s signal-to-noise ratio. The sensor is particularly appropriate for DC and AC current measurement. Such measurements are characterized by the need for ohmic isolation, very low insertion loss, fast response, small package size and low assembly costs. Typically, current sensors are found in applications such as battery current monitoring, solar power inverters and power inverters that drive traction motors in hybrid electric and electric vehicles (HEV/EV). The presented IMC current sensors have no upper limit to the level of current measurable since their output level is only dependant on the conductor size and its distance from the sensor. This article explains how IMC current sensors are designed, how they perform, and how they are applied in these applications. Introducing Current Sensors Traditionally, electrical currents are either measured by resistive shunts, current transformers or magnetic sensors. Resistive shunts are popular however magnetic sensors offer many advantages at the system level. For instance, magnetic sensors operate contactless, and do not require galvanic isolation between the current conductor and the sensor. Resistive shunts transform electrical power into heat, creating in some applications thermal management costs and troubles, magnetic sensors avoid these issues. Additionally, CMOS Hall-effect based magnetic sensors can integrate advanced features to provide high-level output signals. Sophisticated magnetic sensors contain programmable memory and even microcontroller logic to allow for a fully custom calibrated output. It is even possible to implement standard interfaces simplifying communication with other circuits. Unlike coils in current transformers (CT), which measure the time-derivative of the magnetic flux, Hall sensors yield a signal proportional to the flux density itself. This makes practical measurement of both DC and AC current. The widespread use of electronics in automotive applications, renewable power conversion (solar and wind power), power supplies, motor control, and overload protection demands engineers use more effective current measurement techniques. These applications benefit from robust, reliable, isolated, high speed, and low-cost current measurement techniques provided by integrated IMC sensors. Current Sensor with Integrated Magnetic Concentrator The solution for these demanding applications lies in the open-loop Hall-effect current transducer, especially the IMC–Hall sensors like the MLX91205 and MLX91206. With rapid response times of less than 10 microseconds, galvanic isolation to avoid issues related to HF common mode voltages, thermal isolation, compact size, 5V operation, robustness, machine mountable standard packaging, and economical pricing, an IMC current sensor is an appropriate candidate for these applications. Some key features of this current sensor include sensitivity to magnetic fields parallel to the chip surface, linear output voltage proportional to the magnetic field, zero power loss in primary circuits, very high sensitivity, excellent nonlinearity, wideband (DC to 100kHz), low offset and offset drift, very low noise, and isolation from the current conductor. All this is housed in compact surface mount SOIC-8 package. In essence, it produces an analog linear, ratiometric output voltage that is proportional to the applied magnetic field parallel with the chip surface. Practically, the current sensor integrated circuit is fabricated using a standard CMOS process. The additional ferromagnetic layer, also known as the integrated magnetic concentrator (IMC), is added in a simple post processing step. The ferromagnetic layer amplifies the magnetic field and concentrates it on the Hall elements. Consequently, the IMC-Hall effect sensor features very high magnetic sensitivity, low offset, and low noise in a compact, cost efficient package. The IMC consists of a highly permeable, low-coercive-field, amorphous ferromagnetic layer directly bonded onto the Hall sensor chip’s surface. An IMC, which is approximately the size of the integrated circuit, “sucksin” the external magnetic flux lines and directly concentrates them onto the Hall elements, which are about one tenth the size of IMC. Figure 1 The two parts of the IMC collect and amplify the small magnetic flux parallel to the chip surface (e.g. generated around a current-carrying conductor) and locally rotate the in-plane component into a magnetic field perpendicular to the chip’s surface. Whereas a conventional Hall sensor is sensitive to a magnetic field perpendicular to its surface, the IMC-Hall sensor is sensitive to a magnetic field parallel to its surface. The IMC-Hall sensor is packaged in a standard plastic SOIC-8 package so that it can be mounted onto a printed circuit board (PCB) using standard pick-and-place machines and soldered using conventional soldering techniques. The measured current is sent either directly through a current track of the PCB located under the sensor (Figure 2, right), or the sensor is mounted at a given distance from a larger current conductor (Figure 2, left). In both the configurations, the magnetic flux around the current conductor enters the sensor on the outside of one octagonal IMC element, then passes through the Hall elements under the gap, and exits the other octagon on the opposite side. Since the material used for the IMCs has a very high permeability, the concentrator collects all the flux lines in its vicinity and focuses them on the Hall elements where the flux density is increased by a factor of about six. Applications and Results To demonstrate its usefulness in a variety of applications, a few examples are illustrated. Figure 3 shows a current sensor constructed for a hybrid electric vehicle (HEV) inverter application with a current range of 200 A. The sensor was placed into an inverter driving a 3-phase traction motor of an HEV (Figure 4). Figure 3 Figure 4 B