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19
Investigation of the thermal drift of open-loop Hall
Effect current sensor and its improvement
Chen Xu, Ji-Gou Liu and Quan Zhang
ChenYang Technologies GmbH & Co. KG
Markt Schwabener Str. 8, 85464 Finsing, Germany
Email: jigou.liu@chenyang-ism.com
Chen Xu, Yongcai Yang
School of Optical-Electrical and Computer Engineering,
University of Shanghai for Science and Technology,
Jungong Rd. 516, 200093 Shanghai, China
Email: yangyc@usst.edu.cn
Abstract— Linear Hall IC can be used for the low price open-
loop Hall Effect current sensor thanks to its high sensitivity.
However, according to the experimental results, its thermal
performance is greatly related to the thermal drift coefficient of
linear Hall IC and the gain of the amplifying circuit. In this
paper, the thermal drift of the zero offset will be improved by
two methods. One is to use two Hall ICs, the thermal drift
coefficients of which are similar, to build up a differential
amplifying circuit to compensate the common thermal drift of the
two Hall ICs. Another one is to add a magnetic core for
concentrating the magnetic flux. In this way the gain of the
amplifying circuit is reduced by increasing the magnetic flux
density passed through the Hall IC. Experimental results show
that the thermal performance of the optimized current sensor is
reasonably improved by the proposed methods.
Keywords— linear Hall IC, open-loop Hall Effect current
sensor, thermal drift improvement, thermal drift coefficient,
amplifying circuit gain.
I. INTRODUCTION
Electrical current sensors are well known and find a wide
range of applications to the electronics industry [1]. There are
a lot of current sensors such as current transformers, shunt
resistors and Hall Effect current sensors, etc. [2-7]. Among
these current sensors, Hall Effect current sensors have more
advantages in good linearity, wide measuring range, high
isolation between input and output, relative high accuracy,
diverse sensor configurations and applications [5,6,7].
With the development of the semiconductor integrated
circuit industry, Hall IC, which integrates a Hall Effect
element and its signal processing circuit in one chip, is also
quickly developed. It has been widely used since it has a lot of
advantages, such as high sensitivity, long life, easy
installation, low power consumption, high frequency, shock-
resistant and low cost [8,9].
According to previous studies [7], the linearity of Free-
space Hall Effect current sensor by using a Hall Effect
element is not good and its sensitivity is not high enough for
industrial applications. In this paper, a higher sensitivity Hall
IC is used to improve the linearity and sensitivity of this
sensor. It is known that temperature is a major factor affecting
the accuracy of a Hall current sensor [10]. It is verified by
temperature experiments that the zero offset thermal
performance of the sensor depends on the thermal drift
coefficient of linear Hall IC and the gain of the amplifying
circuit.
In this paper, the thermal performance of low-cost current
sensors will be improved by two methods. The first one is to
use two Hall ICs to build up a differential amplifying circuit to
compensate the thermal drift of the two Hall ICs. The second
one is to integrate a magnetic core to increase the magnetic
flux density passing through the Hall IC, and consequently to
reduce the gain of the amplifying circuit. Hall Effect current
sensors developed according to these methods can be used in
solar energy system and other systems which need sensors to
have a wide operating temperature range.
II. OPEN LOOP HALL EFFECT CURRENT SENSOR BASED ON
LINEAR HALL IC
A. Free-space Hall Effect open-loop current sensor
A single linear Hall IC and a U-shaped copper wire are
used to build a Free-space Hall Effect current sensor. The
distance between IC and copper wire is 2mm and 10A primary
current flows through the U-shaped copper wire. The model of
the sensor is shown in Fig.1.
Fig.1 Current sensor model of using a single linear Hall IC and a U-shaped
copper wire
20
Fig.2 shows the relationship between the magnetic flux
density at the sensing point and the simulation region of the
solution domain. It indicates that as the solving region
gradually increases, the magnetic flux density becomes more
and more accurate. The magnetic flux density B is finally
equal to 0,98mT, which is approximate to the real value.
Fig. 2 The relationship between the magnetic flux density and the
solution domain
The gain of the amplifying circuit should be set to 45 so
that the sensor can give an output voltage of 2.5V±1V when
the primary current is 10A. Fig. 3 shows the linearity error of
the sensor, the measurement is repeated for 9 times to get a
reliable result. One can get that the linearity error is less than ±
0.40%, which is better than that of the current sensor based on
Hall Effect element in previous studies (± 0.60% ) [7].
Fig. 3 Linearity of current sensor of using a single Hall IC and a U-
shaped copper wire
B. Zero offset temperature experiment of the Free-space
Hall Effect open-loop current sensor
Experiments have been done in order to find the zero
offset thermal drift of the sensor. The results of current sensor
under using a Hall IC are shown in Tab І. The thermal drift
coefficient (TCVo) of the zero offset of the sensor is calculated
by considering 25°C as reference temperature. This parameter
is expressed in ppm/°C (ppm is an abbreviation for "part per
million") and is determined by:
16
1
25 10
25 25
oo
o
o
V T V C
ppm
TCV CV C T C
(1)
where Vo(25°C) is the zero offset output voltage at 25°C,
Vo(T1) is the zero offset output voltage at temperature T1.
The parameter in ppm/°C is a relative value and
independent on the output voltage. For instance, the thermal
drift coefficient of a current sensor with an output voltage of
+2.5VDC±2V under using a power supply of +5VDC is the
same as that of a current sensor with an output voltage of
+5VDC±4V under using a power supply of +10VDC.
Therefore it is better to use the unit ppm/°C than using the unit
µV/°C or mV/°C.
The results show that the thermal drift of zero offset Vo can
reach about 6533 ppm/°C as the temperature changes from -
40°C to 85°C.
Table I. THERMAL DRIFT COEFFICIENT OF A CURRENT SENSOR
BASED ON THE MODEL SHOWN IN FIG. 1
Similar experiments of Free-space current sensor under
using a Hall Effect element are also done to make a
comparison between the two types of sensors. A Hall Effect
element has two output pins, it can Fig. 4 shows the thermal
drift coefficients of the two output pins.
Fig.4 Thermal drift coefficients of two output pins of a Hall Effect
element
Tempera-
ture (°C)
Thermal drift coefficient of the sensor
Vzd(ppm/°C)
Vh(ppm/°C)
Vref(ppm/°C)
VO(ppm/°C)
-40
10,5229
226,056
81,5168
5829,88
-20
4,55994
261,133
103,946
6182,34
0
0,879417
275,236
108,831
6533,56
25
0
0
0
0
60
11,8652
209,348
-69,1458
5224,92
85
16,0412
222,088
-79,8486
5375,29
21
The thermal drift coefficients of the output pins are very
similar so that the thermal drifts can be compensated by a
differential amplifier circuit. However, due to its low
sensitivity of a Hall Effect element, the gain of the amplifying
circuit must be very high. It can reach 320 when using the
same U-shaped copper wire. Fig. 5 shows a comparison of the
sensors based on a Hall Effect element and a Hall IC under
using the same U-shaped copper wire. The thermal drift
coefficient of current sensor using a Hall IC is better than that
of current sensor by using a Hall Effect element.
Hence, an improvement of the thermal performance of
current sensors is necessary for the industrial applications.
Fig.5 Thermal drift coefficients of two types of current sensors
III. THERMAL PERFORMANCE IMPROVEMENT OF THE ZERO
OFFSET OF THE SENSOR
The circuit used in the current sensor is shown in Fig. 6.
Fig. 6 Circuit of the Hall Effect current sensor
The zero offset output voltage VO of the sensor can be
expressed as
O h ref h
V K V V V
(2)
Where K is the gain of the amplifying circuit, Vh is the
output voltage of linear Hall IC, and Vref is the constant
voltage reference.
The derivative of the sensor output with respect to the
temperature is given as:
1ref
Oh
dV
dV dV
KK
dT dT dT
(3)
From (2), it can be seen that the thermal drift coefficient
of the zero offset of the sensor is proportional to the gain of
the circuit and to the thermal drift coefficient of linear Hall IC
under using a constant reference voltage Vref. Therefore, the
thermal drift of zero offset should be reduced by using the
following methods.
A. Hall Effect Current Sensor by using two Hall ICs
The linear Hall IC has only one output pin. Two Hall ICs
should be used to build a differential amplifying circuit (see
Fig. 7[11]). The zero offset output voltage VO of the sensor in
this case can be expressed by
12o h h ref
V K V V V
(4)
Where K is the gain of amplifying circuit, Vh1 and Vh
2
are
output voltages of linear Hall IC1 and IC2 respectively, and
Vref is the constant voltage reference.
Fig.7 Differential amplifying circuit [11]
The derivative of the sensor output with respect to the
temperature is given as:
12 ref
O h h dV
dV dV dV
K
dT dT dT dT
(5)
22
Under assuming that Vref is constant and K does not change
with temperature, the thermal drift coefficient of the sensor is
proportional to the difference between the thermal drift
coefficients of the two Hall ICs.
Fig.8 shows the thermal drift coefficients of the zero offset
of different ICs. Hall IC1 and IC2 have a different coefficient;
IC1 and IC3 have a similar coefficient. Theoretically, the
common thermal drift can be compensated when the output
signals of the two Hall ICs are processed with a differential
amplifier.
Fig.8 The thermal drift coefficient of different ICs
Furthermore by using two ICs the output signal of the
differential amplifier can be doubled in comparison to that by
using a single Hall IC. Therefore the gain of the amplifying
circuit can be reduced from 45 to 20. Fig.9 shows thermal drift
coefficients of the zero offset of different sensors. The first
sensor uses a single Hall IC and U-shaped copper wire. The
second sensor is built with the same U-shaped copper wire and
Hall IC1 and IC2, while the third sensor consists of Hall IC1
and IC3. The output signals of both Hall ICs are connected to
a differential amplifying circuit. Comparing these three
curves, it is obvious that the differential amplifier circuit can
improve the temperature characteristics of the sensor when the
thermal drift coefficient of two Hall ICs is approximately the
same. Otherwise the thermal drift coefficient will not be
reduced or even be worse.
Fig.9 Thermal drift coefficients of sensors
B. Hall Effect Current Sensor by using a Magnetic Core
In order to reduce the gain of the amplifying circuit of the
current sensor, a magnetic core should be used to concentrate
the magnetic flux generated by the primary current conductor
in the air gap of magnetic core, where the Hall IC is
positioned. From experimental results the gain of the
amplifying circuit is reduced from 45 to 4.8 with using a
magnetic core instead of the U-shaped copper wire. Fig.10
shows the linearity error of the sensor which uses a single Hall
IC with a magnetic core. The measurement here is also
repeated for 9 times to get a reliable result. The linearity is less
than ± 0.30%, which is better than that of using a Hall IC with
U-shaped copper wire (± 0.40%).
Fig.10 Linearity of the sensor using a single Hall IC with a core
23
Fig.11 Thermal drift coefficients of the sensors with using a U-shaped
copper wire and a magnetic core
Fig.11 shows the thermal drift coefficients of the sensors
that magnetic flux density is generated by using a U-shaped
copper wire and a magnetic core. The blue curve shows
thermal drift coefficient of the sensor of using a U-shaped
copper wire, while the red curve gives thermal drift coefficient
of the sensor of using a magnetic core. One can see that the
thermal drift of the sensor by using a magnetic core is much
lower than that of using a U-shaped copper wire. The reason is
that the magnetic flux density is increased by the magnetic
core, and consequently the gain of the amplifying circuit is
reduced reasonablely.
C. Hall Effect Current Sensor by using a Magnetic Core and
two Hall ICs
By combining the two sensors mentioned above, one can
build an optimized current sensor with using a magnetic core
and two Hall ICs. The gain of the amplifying circuit of this
sensor is reduced from 45 to 1.5 and the thermal drift
coefficient of the sensor can be greatly improved. Fig.12
shows the final results. The linearity error is less than ±0.20%.
Fig.12 The linearity of the sensor using two Hall ICs with a magnetic core
Fig.13 Thermal drift coefficients of four different sensors
The thermal drift coefficients of the zero offset of different
sensors are shown in Fig.13. The first sensor consists of a
single Hall IC and U-shaped copper wire. The second sensor
uses a single Hall IC and a magnetic core. The third sensor is
built with two Hall ICs and U-shaped copper wire. The fourth
sensor is the optimized sensor with two Hall ICs and a
magnetic core. It is obvious that the thermal drift coefficient of
the optimized current sensor is much better than that of other
three sensors. The maximum of the thermal drift coefficient of
the optimized sensor is 289ppm/°C. This is the effect of using
two Hall ICs and a magnetic core for reducing the thermal
drift and improving the linearity.
If using another two Hall ICs, which have a similar
thermal drift coefficient, do the same experiment, one gets the
result shown in Fig.14. The thermal drift coefficient of the
sensor is reduced in this case.
Fig.14 Thermal drift coefficients of an optimized current sensor under
using two Hall ICs, which have a similar thermal drift coefficient
24
IV. CONCLUSIONS
In this paper, the zero offset thermal performance of open-
loop Hall Effect current sensors is improved by the proposed
methods, which have been proved by experiments. The
following conclusions can be drawn:
The sensitivity of Hall IC is higher than that of Hall
Effect element. It is suitable for current sensors.
The thermal performance of an open-loop Hall Effect
current sensor depends on the thermal drift coefficient
of the linear Hall IC and the gain of the amplifying
circuit
Using two linear Hall ICs, the thermal drift coefficients
of which are approximately the same, to build a
differential amplifying circuit in the current sensor, it
can not only compensate thermal drifts of the two Hall
ICs but also increase the sensitivity of current sensor
and reduce the gain of the amplifying circuit.
Current sensor by using a magnetic core has a lower
thermal drift than that of sensor by using a U-shaped
copper wire thanks to the concentration of magnetic
flux at the sensing point of the Hall IC.
The linearity of the sensor can also be improved by
using two Hall ICs and a magnetic core.
V. REFERENCES
[1] Larwrence D. Radosevich et al. United States Patent[P]. United States
:US 09/133,782 ,Aug.12,1998
[2] Y. Wang et al., “Split core closed loop Hall effect current sensors and
applications”. Int. Exhibition and Conference for Power Electronics,
Intelligent Motion, Power Quality, Nuremberg, Germany, 8-10 May,
2012
[3] Honeywell Inc., Hall Effect Sensing and Application, Micro Switch
SensingandControl,Chapter5,P.33–41,
http://www.honeywell.com/sensing
[4] P.E. Bill Drafts, Magnetoresistive Current Sensor Improves Motor Drive
Performance, Pacific Scientific-OECO, 4607 SE International Way,
Milwaukie, USA, http://www.oeco.com
[5] E. Ramsden, “Hall Effect Sensors – Theory and Application”. Elsevier
Inc., Amsterdam, London, New York etc., 2006.
[6] Ji-Gou Liu et al., “ Error Compensation of Closed Loop Hall Effect
Current Sensors”. in Aachen, Germany, IEEE International Workshop
on Applied Measurements for Power Systems (AMPS), September 26-
28, 2012
[7] Jianhai, Qiu et al. “Simulation and Optimization of Conductor Structural
Parameters of Free-space Hall Effect Current Sensor” [R]. in Aachen,
Germany:The 5th IEEE International Workshop on Applied
Measurements for Power Systems, 2014.
[8] Shaojian Hu et al.,“Design of GaAs integrated hall switch “[J]. Journal
Of Functional Materials And Devices, 2003, 9(1): 1-1.
[9] Tao He et al., “Signal condition circuit of Hall sensor”[J]. Journal of
Traducer Technology,2001,20(12):16.
[10] Min, Gao, ect. Temperature compensation of Hall current sensor based
on two-dimensional regression analysis[J]. Journal of electronic
measurement and instrument, 2009, 23(2): 1-1
[11] Habiro et al. United States Patent[P]. United States : 5,065,088,Nov. 12,
1991.
[12] Quan Zhang et al., “A new complementary symmetrical structure of
using dual magnetic cores for open loop Hall Effect current sensors”,
PCIM Europe 2015, May 19-21, 2015, Nuremberg Germany
