This Design Note from Crocus Technology explains how high sensitivity magnetic sensors can be used to overcome the limitations of traditional coil sensors for measuring currents in power-electronic applications.
A common technique for measuring the current flowing in a conductor is based on Faraday’s Law of induction and involves placing a coil around the conductor; in this case the current flowing in a conductor produces an output equivalent to the rate of change of current. Integrating this output produces a voltage proportional to the current, which can then be monitored using an instrument such as an oscilloscope.
This approach has an advantage of being non-invasive and not requiring direct electrical connection, and hence can be put in place or removed quickly and easily. Since the coil is isolated from the current in the conductor being measured, the method is also safe for high currents ranging from a few amps to over a hundred amps. One drawback, however, is that the coil can only generate a response when an alternating current is present.
Engineers can now overcome this limitation and measure either direct or alternating currents by using a new type of magnetic sensor that incorporates an innovative technology called Magnetic Logic Unit (MLU) developed by Crocus. When MLU is used for magnetic-field sensing it typically involves a matrix of Magnetic Tunnel Junctions (MTJ).
The Magnetic Tunnel Junction
As the fundamental element of this new generation of magnetic sensors, the MTJ cell is constructed with two magnetic layers separated by a thin oxide which serves as a tunnel oxide. One of the two magnetic layers has a fixed magnetic orientation and it is called a reference layer. The other magnetic layer is called the sensing layer.
Figure 1 shows the basic MTJ stack. The sensing layer is able to change its orientation, relative to the reference layer, by up to 180° in the presence of a magnetic field, as illustrated in Figure 2.
This effectively modulates the resistance of the MTJ cell. The change of resistance can be directly correlated to the intensity and angle of the magnetic field. Crocus Technology has introduced multiple innovations around this basic principle in developing its magnetic-field sensor product line. A magnetic-field sensor is composed of multiple MTJ cells, as shown in Figure 3.
This structure delivers superior performance compared to conventional magnetic sensors for applications such as rotation and speed sensing, position sensing, linear and angular sensing, and current sensing. Compared to Hall devices, which have low sensitivity in general, the MLU sensor has several orders of magnitude higher sensitivity. Other advantages include low power, robust design, high stability and highly reliable performance over a wide range of operating temperatures up to 250°C. As such, this type of sensor is ideal for applications that are deployed in harsh environments, such as oil-drilling equipment or solar inverters.
Current Sensing with MLU-Based Sensors
The Crocus CTSX family of magnetic sensors based on the patented MLU technology is optimized for current-sensing applications. Analysis of the sensor’s performance in current measurement shows that the sensor has a linear response to current change, as shown in Figure 4. It is important to note that the dynamic range for current sensing based on CTSX sensors spans from a few milliamps to several hundred amps depending on the specific set-up.
Figures 5a and 5b show single and differential sensor-circuit configurations.
Table 1 shows the main parameters of CTSX high-sensitivity magnetic sensors.
|Output Resistance, RO||RO < 25kΩ|
|Input Resistance, RIN||30Ω|
|Voltage Supply (Typical)||5V|
|Input Bias Current||10mA|
|Linearity Range||(+/-) 1mT|