Current Sensing Techniques: a Review Silvio Ziegler, Robert C

354 IEEE SENSORS JOURNAL, VOL. 9, NO. 4, APRIL 2009 Current Sensing Techniques: A Review Silvio Ziegler, Robert C. Woodward, Herbert Ho-Ching Iu, Senior Member, IEEE, and Lawrence J. Borle, Member, IEEE Abstract—This paper provides a thorough review of state-of- The aim of this paper is to provide the reader with a fun- the-art current sensing techniques. It catalogues the current damental understanding of the techniques available for current sensors according to the underlying physical principle in order to monitoring and some understanding of the performance and point out their strengths and weaknesses. New current sensing principles based on either established limitations of each technique. Therefore, we have classified the techniques such as Hall-effect sensors and fluxgate principles current sensing techniques based upon the underlying funda- or emerging technologies such as magneto resistance effect and mental physical principle. These principles are the following. fiber-optical techniques provide attractive alternatives for current 1) Ohm’s law of resistance. sensing, although generally at a higher price than traditional 2) Faraday’s law of induction. techniques like shunt resistors. 3) Magnetic field sensors. Index Terms—Current measurement, current transformers, 4) Faraday effect. fluxgates, Hall-effect sensors, magnetoresistance, shunt resistor. In addition, we will discuss known sensing configuration, such as open-loop, closed-loop, with particular reference to magnetic field sensors, and the use of combinations of sensors in order to I. INTRODUCTION meet more demanding performance requirements. NFORMATION regarding current flow is required in a wide I range of electrical and electronics applications. Each appli- II. CURRENT SENSORS BASED UPON cation has different performance requirements in terms of cost, OHM’S LAW OF RESISTANCE isolation, precision, bandwidth, measurement range, or size and Ohm’s law of resistance is basically a simplification of the many different current measurement methods have been de- Lorentz law that states veloped to satisfy these demands. Today, current information increasingly needs to be available in digital form for digital (1) control or monitoring purposes. This requires the output signal is the current density, the electric field, the charge ve- of a particular current sensing technique to be acquired by an locity, the magnetic flux density acting onto the charge, and analog-to-digital converter. the material conductivity. In most cases, the velocity of the As an example, shunt resistors have been used extensively moving charges is sufficiently small that the second term can be in power electronics due to their low cost, small size, and neglected relative simplicity, while providing reasonable accuracy. To achieve higher integration, higher efficiency and enable more (2) sophisticated control techniques, the analog control loops are increasingly replaced by digital control loops. Accordingly, This equation is known as Ohm’s law of resistance and states the current information needs to be digitalized. The small that the voltage drop across a resistor is proportional to the voltage drop across a shunt resistor needs amplification, which flowing current. This simple relationship can be exploited to alters the bandwidth, and increases the device size and cost. sense currents. These current sensors often provide the advan- Power converters, moreover, have reached a power density tage of lower costs compared with other sensing techniques, and level, where the power losses inside shunt resistor start to be- have the reputation of being reliable due to their simple working come troublesome. Therefore, power electronics engineers are principle. seeking alternatives for shunt resistors with similar accuracy but lower power losses, and with an output voltage that can be A. Shunt Resistor directly sampled by an analog-to-digital converter. A common approach due to its simplicity is the use of a shunt resistor for current sensing. The voltage drop across the shunt resistor is used as a proportional measure of the current flow. Manuscript received September 20, 2008; accepted October 27, 2008. Current It can be used to sense both alternating currents (AC) and di- version published February 27, 2009. This work was supported in part by the rect currents (DC). The shunt resistor is introduced into the cur- company Power-One, Inc. The associate editor coordinating the review of this paper and approving it for publication was Dr. Kailash Thakur. rent conducting path, and can, therefore, generate a substantial S. Ziegler, H. H. C. Iu, and L. J. Borle are with the School of Electrical, amount of power loss. The power loss can be calculated via Electronic and Computer Engineering, University of Western Australia, Ohm’s law and increases with the square of the current. Crawley, WA 6009, Australia (e-mail: [email protected]; herbert@ ee.uwa.edu.au; [email protected]). This power loss may restrict the use of shunt resistors in high R. C. Woodward is with the School of Physics, University of Western Aus- current applications. tralia, Crawley, WA 6009, Australia (e-mail: [email protected]). 1) High-Performance Coaxial Shunt: Shunt resistors have Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. been used extensively to measure transient current pulses with Digital Object Identifier 10.1109/JSEN.2009.2013914 fast rise-times and high amplitudes. In such applications, the 1530-437X/$25.00 © 2009 IEEE ZIEGLER et al.: CURRENT SENSING TECHNIQUES: A REVIEW 355 Fig. 1. Equivalent circuit diagram for a shunt resistor. Fig. 3. Bandwidth and voltage drop of shunt resistors based on a series of ex- emplary SMD resistors at a power dissipation of 1 W. stantial, which results in bulky shunt resistors that may not be suitable for device integration. Unfortunately, the higher integration comes at the cost of substantial higher parasitic inductance compared with optimized heavy-duty shunt resistors. Due Fig. 2. Impedance measurement of a typical SMD shunt resistor (WSL2512, 3 m – image courtesy of Vishay Dale, Inc.). to the small physical dimensions of SMD resistors, the skin effect becomes secondary, and a first-order model incorporates only the ohmic resistance and the parasitic inductance . high-frequency behavior of the shunt resistor is of critical im- The accurateness of this model is verified in the impedance mea- portance. Fig. 1 shows the equivalent circuit diagram of a shunt surement in Fig. 2 of a typical SMD shunt resistor. It has to be resistor with a nominal resistance , including a parasitic induc- noted that the frequency response will be deteriorated if the area tance and the series resistance due to skin effect. The par- enclosed by the sense wires is increased. Accordingly, it is im- asitic inductance is often a source of confusion since it is fre- portant to understand how the manufacturer of the shunt resistor quently assumed to be related to the self-inductance of the shunt measured the parasitic inductance in order to predict the perfor- resistor. In reality, the parasitic inductance is determined by the mance for the intended application. The resistor then shows a mutual inductance between loop built by the sense wires and 20 dB/decade rise in the impedance value after the corner fre- the loop built by the main current [1]. Hence, the connection of quency is reached as predicted by the circuit model. The corner the sense wires is crucial to achieve good performance. Signif- frequency can be calculated according to icant research has been conducted to reduce in order to in- crease the measurement bandwidth. Geometrical embodiments, (3) e.g., coaxial resistive tube, have been found, which significantly reduce the parasitic inductance by reducing the flux that cou- This formula is only valid if the skin effect is negligible small. ples into the sense wires [2]–[4]. For coaxial shunt resistors, In this case, it is feasible to improve the frequency response by the skin effect is notable since the parasitic inductance is very employing a first-order low-pass filter [8]. The corner frequency small due to the superior coaxial construction. For heavy duty is defined, where the reactance of the inductance is equal to shunts that measure pulse currents with magnitudes of 100 kA, the ohmic resistance. For this first-order system, the bandwidth the skin effect can become the limiting factor that determines is equal to the corner frequency. In Fig. 3, the bandwidth to- the measurement bandwidth [5]. Ironically, it has been found gether with the voltage drop across the shunt resistor has been that by ensuring that a certain amount of flux couples into the plotted against the current for a series of SMD resistors. The measurement wire the skin effect can be compensated by the resistance values were chosen to maintain a constant power dis- induced voltage [4], [5]. Another technique uses a flat strap ge- sipation of one watt. The experimentally derived parasitic in- ometry [6]. These methods allow the measurement of current ductance data was obtained from the supplier Vishay Dale, Inc. pulses with rise times of a few nanoseconds and magnitudes of (WSL 2512 Series), or for the 0.2 resistor (BVS-Z-R002) several kA. from Isabellenhuette GmbH. Fig. 3 demonstrates that measuring 2) Low-Cost Surface-Mounted-Device (SMD): For highly high currents with a shunt resistor leads to low bandwidth and integrated electronic devices, coaxial shunt resistors are not suit- low output voltages. Naturally, the lower the voltage drop the able since they are bulky and expensive, and their usefulness more gain is required to provide satisfactory output voltage for is generally limited to the measurement of fast current pulses. the analog-to-digital converter. As operational amplifiers have a In the majority of cases, thick film structures are used that can constant gain-bandwidth-product this means that at high gains be integrated into surface-mounted-devices (SMDs) [7].

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