Using Superimposed Fields for Current Sensing

DN Staff

July 17, 2006

3 Min Read
Using Superimposed Fields for Current Sensing

Two of the most widely used approaches for current sensing have shortcomings that limit their use in highly integrated power electronics applications. Hall-effect current sensors use a flux concentrating core that restricts size reductions. In addition, these sensors have a bandwidth of about 200 kHz at 90 percent accuracy (-1 dB). Integrated shunt current sensors have added loses and galvanic isolation schemes add size and cost and limit the bandwidth. As a result, alternate approaches using Giant Magnetoresistance (GMR) and Anisotropic Magnetoresistance (AMR) have been researched as point field detectors for future motor control applications.

In a GMR sensor, a sheet of nonmagnetic material separates thin-film magnetic layers to achieve a 10 to 20 percent change in resistance in response to a magnetic field. Initial results from an open-loop current sensor using a GMR field detector in a Wheatstone bridge configuration detected a magnetic field with 99 percent accuracy up to 300 kHz. More recently, efforts dealt with determining the ideal sensor location inside a power electronics module and developing techniques to optimize dynamic performance. The researchers found encouraging results from a systematic evaluation of magnetic fields around conductors. The analysis includes the dynamic behavior of source currents and induced eddy currents.

Experimental results for the GMR field signal compared to a high bandwidth current shunt showed excellent agreement for a location between the center and edge of a

Time domain current signals recorded from a high bandwidth shunt resistor compare very favorably to GMR signals, especially in location between the center and the edge (location B). To achieve superior performance from the field detector, the dynamic behavior of the magnetic field (the Flat Bandwidth) must be analyzed.

rectangular conductor. To determine the best location for the detector in a system, an optimization function was defined that combines the information from a spatially dependent flat bandwidth (within some percentage of the dc value) and the field intensity. This function provides information regarding the tradeoff between high bandwidth and high field intensity. The 5 percent flat bandwidth measurements provided a value approaching 300 kHz, similar to the previous open-loop results.

Before the point field detector can be used as a current sensor, additional development effort is required to analyze the decoupling of internal and external cross-coupled fields. The results of this research could be reported within the next year.

Contact: Erik Olson, [email protected]

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