Hybrid Microfluxgate and Current Transformer Sensor

In this work, we develop the hybrid current sensor comprising a microfluxgate and a current transformer (CT) for wide-bandwidth current measurement. The microfluxgate consists of two microcoil chips 3.5 mm $\times1.75$ mm in dimension made by using the standard Complementary Metal-Oxide Semiconductor MicroElectroMechanical Systems (CMOS-MEMS) $0.35 ~\mu \text{m}$ process. The aluminum wire-bonding technique is used to construct the top excitation coil surrounding the amorphous soft magnetic core patterned by photolithography and wet etching techniques. Under the optimal excitation condition, the response of the microfluxgate current sensor becomes linear with a maximized sensitivity. The in-house made CT is combined with the microfluxgate driving system to form the hybrid current sensor by using a summing amplifier. The hybrid current sensor has a spectral current noise of 0.4 mA/ $\surd $ Hz at 1 Hz. The maximum sensing range is ±10 A, and the non-linearity is less than 2% within the ±4.6 A range, corresponding to a linear dynamic range of 60 dB in a 10 Hz bandwidth. The hybrid microfluxgate-and-CT sensor has a flat bandwidth from dc to 100 kHz. It has the potential to become a practical miniature device featuring a high-dynamic range, wide bandwidth, and low-temperature drift.

I. Introduction

The applications of magnetic field sensing in industry and consumer electronics are largely electronic compass [1]–[4] and non-contact current measurement [5]–[11]. The major advantage of non-contact current measurement is the non-invasive and isolated nature, which would not induce a load to the circuit under test. Among various kinds of magnetic sensors, the fluxgate and microfluxgate can detect the static and alternating magnetic fields with a very low-temperature drift [3]. It has been reported that the low-noise fluxgate and microfluxgate sensors can be implemented by using the widely available amorphous magnetic ribbon as the core material [13]–[16], which allows further reduction in the cost for the emerging widespread applications, including current monitoring in dc microgrids [11] and biomagnetic sensing [15]. The fluxgate in a rod-core structure is known to be immune to the cross-field effect [3], which is important for accurate current measurement in the unshielded environment. When driven by an excitation frequency above 1 MHz, the microfluxgate sensor can achieve a bandwidth of more than 100 kHz [12]. The bandwidth limit is usually determined by the cutoff frequency of the low-pass filter for suppressing the excitation interference. The excitation frequency to minimize the noise level of the fluxgate sensor is generally below 100 kHz, which makes the typical flat bandwidth of a fluxgate system less than 1 kHz when a precise dc response is required. To extend the flat bandwidth, the hybrid combination of a dc sensor with a coil-based ac sensor is a viable solution [5], [19]–[21]. The current transformer (CT) [17], [18] and Rogowski coil [10] are suitable for accurate measurement of alternating current. Unlike the dc sensor comprising a soft-magnetic core, the hysteresis and remanence of the core material [7] is not an issue for the coil-based sensor intended for ac measurement. Furthermore, the simplicity in the construction of a coil-based sensor is attractive for building a hybrid wide-bandwidth current sensing system with integrated driving circuits.