Nonintrusive Installation of the TMR Busbar DC Current Sensor

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Introduction
Due to its noncontact isolation measurement, high precision, and low cost [1,2], current sensors are widely used in power electronic systems, such as photovoltaics, wind power, and electric vehicles, and the use of current sensors has increased year by year. A magnetic field is generated by the measured current. The magnetic field can be measured using different sensor technologies including Hall effect sensor, magnetic fluxgate, and magnetoresistance (MR) sensor. The MR sensors have grabbed the attention of researchers in recent years due to their high sensitivity. The latest research in this field mainly includes anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR), and TMR. Hall, AMR, GMR, and TMR have been used in current sensors [3][4][5][6]. Hall has the inherent defects of low sensitivity, high power consumption, and poor linearity. Although the sensitivity of AMR and GMR is much higher than that of hall, their linear range is narrow. Compared with hall, AMR, and GMR, TMR has better temperature stability, higher sensitivity, lower power consumption, and better linearity. The conventional busbar current sensor with differential structure adopts a pair of hall sensors on the conductor surface, which partially suppresses the external magnetic field [7][8][9]. A busbar with an amphitheater hole is shown [10], and an AMR busbar current sensor is described [6]. Circular sensor array is also a commonly used measurement method. Its accuracy is less dependent on the position of the conductor, and it has better resistance to external magnetic field interference [11,12]. The disadvantage of circular arrays is that they are too large. In previous research literature, each magnetic sensing device is separately amplified and measured, and then the measured current value is obtained by comprehensive calculation and fitting [13]. The existing method is complicated, and the cost is high. Since the TMR itself is a magnetoresistance device, the magnetoresistances can be connected in parallel. In this paper, four TMRs are directly connected in parallel.
The TMR sensor is a thin film element in which a barrier layer consisting of a thin insulator is sandwiched between two ferromagnetic layers (free layer/pin layer). Due to the wave-like nature of electrons, electrons in the ferromagnetic layer can tunnel through the insulator layer, and the possibility of tunneling through is related to the magnetization of pin and the free layer. The magnetization direction of the pin layer is fixed, and the magnetization direction of free layer changes with the direction of the external magnetic field.
When the direction of magnetization of the pin layer is parallel to that of the free layer, the resistance is minimal, since the electron tunneling is the largest and vice versa. Obviously, the TMR element has a sensitive orientation due to pin layer magnetization fixation. In this paper, the sensitive direction of all TMR chips is parallel to the surface and set to the x direction ( Figure 1). The structure of the TMR sensor is a magnetic multilayer film material, as shown in Figure 2(a). There is a thin layer of insulation that can be tunneled by electrons between the magnetic layers. This structure is called a magnetic tunnel junction (MTJ). When the free layer is magnetically aligned with the pinned layer, electrons are more likely to pass through the barrier layer [14,15]. Therefore, the resistance (R) of the sensor depends on the external magnetic flux field (B), as shown in Figure 2(b). In    Journal of Sensors Figure 2(c), the TMR chip consists of Wheatstone bridge and has a good temperature characteristics and anticommonmode signal interference. The bridge circuit contains four resistances as shown. V+ represents the forward half-bridge terminal output of TMR, and V-represents the negative halfbridge terminal output of TMR. The voltage between V+ and V-is proportional to the adjacent magnetic flux density, also to the current under measurement.
The new type of current sensor proposed in this paper adopts the TMR array, with flexible differential structure and no magnetic core, and is suitable for current measure-ment in different ranges. The use of nonmagnetic core solution can effectively reduce the volume and weight. Differential structure current sensors are used to partially suppress the external magnetic field interference and improve the test accuracy. We present a new small volume current sensor with the characteristics of the nonintrusive installation. The supply voltage is 5 V, the rated current measurement range is ±300 A, and the peak current measurement range is ±600 A, corresponding to 2:5 ± 2 V output at ±600 A dc current. It uses a differential technique based on tunnel junction magnetoresistance (TMR)   Journal of Sensors technology placed on the surface of the busbar. The current sensor module is consisted of TMR chips, programmable gain amplifier (PGA). Because of busbar's skin effect, which is relatively complex, this paper focuses on dc current measurement, not AC current measurement.

Sensor Design
2.1. Busbar Current Sensor. The TMR and PGA chips are soldered on the printed circuit board (PCB). The four TMR chips are connected in differential mode and then connected to the PGA input port. PGA is used to adjust the gain and  Figure 6: The equivalent circuit of TMR in the magnetic flux density.   Journal of Sensors temperature characteristics of the current sensor. For the current sensor, TMR and PGA used in this paper manufactured by Sinomags Technology Co., Ltd. in China. We design and test the performance of TMR and PGA and give the test data of the current sensor in this paper. Figure 1 shows the photo of the current sensor and the schematic diagram of the differential structure of the module. The V + terminals of TMR1, TMR2, TMR3, and TMR4 are connected with the IN+ terminal of PGA. The V − terminals of TMR1, TMR2, TMR3, and TMR4 are connected with the IN-terminal of PGA. This connection forms a differential structure. The use of a differential structure has the advantage of being resistant to external magnetic fields. I is the measured busbar current, and the cross-sectional area (a × b) of the busbar is 20 mm × 3 mm, whose length is 80 mm, and the vertical distance (y-axis) between the TMR and the center of the busbar is 4.8 mm, which is determined by the thickness of the parts. The size of the busbar is 20 mm × 3 mm × 80 mm, and the TMR sensitivity direction is parallel to the x-axis in Figure 1. . I ex is the interference busbar current. The distance (D) between the centers of two busbars with the same size is 60 mm, which is three times the busbar width.
The sensitivity of TMR is typically 0.1 mV/V/Gs, and the saturation magnetic field is 200 Gs. In the 200 Gs magnetic field range, the TMR chip has good linearity, beyond 200 Gs, and the TMR chip output tends to be saturated. When the measured current is 600 A, the working magnetic field of the sensor is within 200 Gs ( Figure 3); so, the current sensor has good linearity. Four TMR chips are connected in a differential mode and mounted on the sides of the PCB. The current sensor adopts an open-closed structure for easy installation. The sensor is installed at busbar via 2 set screws.

Magnetic-Field Analysis.
For the busbar whose crosssection is rectangular, a and b are the length and width dimensions of the cross-section of the busbar. Current I is passed through the busbar. B x represents the magnetic flux density parallel to the TMR sensitivity at coordinates (f x, f y) in Figure 1. According to Maxwell equation, the magnetostatic equation can be written as equation (1).
COMSOL software was used for finite element simulation. Figure 3 shows 600 A current densities at the busbar and the magnetic flux density near busbar. Take the magnetic flux density of B 1 and B 2 in the positive direction. Referring to equation (1), when current I is 600 A, the absolute magnetic flux density is show in Figure 3.

Sensor Circuit Design.
The equivalent circuit diagram of the current sensor is shown in Figure 4. The TMR chip con-sists of four resistors connected as a Wheatstone bridge. Each TMR chip is connected to PGA through the V + terminal and V − terminal. Ideally, the sensitivity of TMR1, TMR2, TMR3, and TMR4 is the same and parallel to the x-axis. The V + terminals of TMR1, TMR2, TMR3, and TMR4 The equivalent circuit of each TMR and PGA connection is shown in Figure 5.
Ideally, the resistance values of the four resistors of TMR chips in the same wafer are equal. The same goes for the sensitivity.
Each V + and V − output for each TMR chip is shown in equations (4) to (12). The equivalent circuit of TMR in the magnetic flux density is shown in Figure 6.

Journal of Sensors
The equivalent circuit of the busbar current sensor is shown in Figure 7.
B 1 represents the magnetic flux density generated by the measured current I at TMR1 according to equation (1). K I is the coefficient between B 1 and current I.
K A is the magnification of PGA. Equations (4)- (6) and (17)-(20) can be calculated to obtain equation (21). The V OUT of the busbar current sensor is shown in equation (21).     Journal of Sensors The equivalent circuit of the busbar current sensor in the external magnetic flux density is shown in Figure 8.
Therefore, it can be concluded from equation (24) that under the constant external magnetic field interference, the output of the current sensor does not change, which meets the requirement of the resisting external constant magnetic field interference.
In Figure 1, when the measured busbar is passed with 600 A current without an external interference busbar, it can be obtained from Figure 3 that the equivalent differential magnetic field strength of the current sensor is 19.52 Gs. When the measured busbar and the interference busbar with the same size are passed with 600 A current in the same direction, the equivalent differential magnetic field strength of the current sensor is 19.80 Gs. The interference error of the current sensor is 1.4%. When D takes other values, the same method can be used for analysis and calculation.

Results
The maximum magnetic flux density generated by the dc 600 A current near TMR is less than 200 Gs, in the good working linear range of TMR. Figure 9 shows the measurement system which includes current source, power supply, multimeter, PC, and data acquisition card which includes microcontroller unit (MCU), analog-to-digital converter (ADC), RS232, and RS485. The MCU is S9S08DZ60F2MLH made by NXP semiconductors, and the ADC is AD7194BCPZ made by Analog Devices, Inc. The dc current source is XF30ZA-1 K, which is manufactured by Tsinghua Science and Technology Development Co., Ltd., China, with a typical accuracy of ±0.02%, which can meet the requirements of current sensor testing in Table 1. Figure 10 shows the I-V characteristic, and the linearity error of the current sensor is less than ±1% at 25°C.
Using the default configuration of the multimeter DMM4050, a sampling period of 0.5 s, sampling for 50 s, to test the noise of the current sensor. Figure 11 shows the noise RMS (root mean square) of the current sensor that is about ±1 mV. The good noise characteristic of the sensor is the guarantee of the good accuracy of the sensor. Figure 12(a) shows the offset of the current sensor in the full temperature zone. The change of offset in the full temperature zone relative to 25°C is not more than ±10 mV. The sensitivity change in the full temperature zone relative to 25°C is less than ±1% in Figure 12(b). Figure 12(c) shows the relative accuracy of current sensor relative to 25°C in the full temperature zone. The relative accuracy of the current sensor in the full temperature range is within ±1%. So, the current sensor has good temperature characteristics.

Conclusions
This paper designs a new type of small volume dc current sensor, which has the characteristics of small volume, flexible installation, and high precision. The current sensor adopts a new differential combination scheme, which has good linearity and anti-interference ability. The current sensor adopts an open-closed structure for easy nonintrusive installation. Compared with the traditional magnetic current sensor and ring array sensor, the new sensor has a smaller volume. The size of the current sensor can be adjusted flexibly according to the size of the busbar. The relative accuracy of the current sensor in the full temperature range is within ±1%. It is very suitable for the dc busbar power system and has a broad application prospect.

Data Availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of Interest
The authors declare that they have no conflicts of interest.