Sensing elements for current measurements – Part 3
Introduction
The fundamentals to translating the analog world into the digital domain reduces to a handful of basic parameters. Voltage, current and frequency are electrical parameters that describe most of the analog world. Current measurements are used to monitor many different parameters, with one of them being power to a load.
There are many choices of sensing elements to measure current to a load. The choices of current sensing elements can be sorted by applications as well as the magnitude of the current measured. This write up is part three of a three part series that discusses different types of current sensing elements. The focus of this paper is measuring current by way of a Hall Effect Sensor. A Hall Effect Sensor measures the strength of a magnetic field from a nearby conductor in determining the magnitude of the current passing through a conductor.
Hall Effect sensors that measure current are commonly found in lossless and very high current applications. A Hall Effect sensor remotely measures current passing through a conductor by measuring the magnitude of the magnetic field sourced from a trace. Systems in which employ Hall Effect sensors are considered lossless since the sensor remotely measures current. Applications above 200A may use Hall Effect sensors because the power dissipation from a sense resistor is large for high current applications. Figure 1 illustrates the basic concept of a Hall Effect current measuring application.
Element: Hall Effect Sensor
Figure 1: Hall Effect Sensor Example.
The circuit in Figure 1 measures current through the trace by measuring the magnetic field, B, emitted from the current flowing through the trace. The B field is directly proportional to the magnitude and direction of the current flowing through the trace. The B field is perpendicular to the current flow. The direction of the B field with respect to current flow is illustrated in Figure 2.
Figure 2: An illustration of current flow with respect to the magnetic field direction.
The mathematical relation between the magnitude of the current and the magnetic field is represented in Equation 1 for a wire. A strip line trace has a slightly different equation. For simplicity, this paper uses Equation 1 to discuss the relationship between current and magnetic fields.
Equation 1: The mathematical relationship between current and magnetic field for a wire.
µ0 is the permeability of the magnetic field. The permeability value, μo, of free space equals 4π*10-7 H/m. The value r is the distance in meters between the conductor and the linear Hall Effect sensor. The variable I is the current flowing in amps through the conductor. B is the magnetic field in Gauss.
Figure 3: A side profile of the circuit in Figure 1.
From Equation 1, the strength of the magnetic field diminishes as the spacing between conductor and sensor increases. A linear Hall Effect sensor converts the magnetic field measured into either a current or a voltage. The gain of the sensor is reported as either mV/G or mA/G. Some manufactures report the gain in Teslas. A Tesla equals ten 10,000 Gauss.
Suppose a 200A current flows through a trace that is 0.03m from the center of the trace to the center of the of Hall Effect chip. What is the expected magnetic field at the center of the sensor? If the sensor has a gain of 5mV/G, what is the output voltage of the sensor?
Using the simplified relationship in Equation 1, the magnetic field is 13.33G. The output from the sensor is calculated to be equal to 66.67mV.
Linear Hall Effect sensors are active circuits that draw between 3mA to 10mA of current. The noise levels of the sensor averages around 25mV or 5G. In applications that either have low currents or large trace to sensor spacing, the linear Hall Effect is not a good choice. This is due to the noise level of the sensor as well as the current draw from the sensor.
The environment in which the current bearing trace and the sensor are subjected to is important for measuring weak magnetic fields. A linear Hall Effect sensors measure the total available magnetic field at the set location. Current bearing traces routed near the sensor will change the magnetic field at the sensor and ultimately change the accuracy of the measurement. The sensor will also measure changes in the environmental magnetic fields. A change in the environmental magnetic field could be caused by a switching motor or any device that radiates energy.
A magnetic shield that encapsulates the current bearing trace and the Hall Effect sensor is a means to controlling the magnetic field in the sensor’s environment. Figure 4 illustrates a metal case enclosing the trace and sensor. The enclosure is known as a Faraday cage.
Figure 4: Shielding the conductor and the sensor will improve low magnetic field measurements.
The shield in Figure 4 should be grounded since ground is usually the most stable and lowest impedance to which most circuits are referenced.
Recently, Hall Effect sensors that integrate the current conduction path, provide environmental shielding and temperature compensation circuitry in a single package have been released to market. The integration of the current conduction path simplifies the gain calculation between the current flowing through the conductor and the output voltage. The single chip solution simplifies the layout and the design of a Hall Effect sensor measuring application because a user does not have to worry about the conductor to sensor spacing and the environment the sensor is in. Figure 5 is a simplified circuit of the integrated solution.
Figure 5: A simplified circuit diagram of a Hall Effect sensor that integrated the current conduction path.
The integrated conduction path (IP+, IP-) has a resistance ranging from 0.1mΩ to 2mΩ. The current sense in Figure 5 is not a lossless system because of the loss associated with integrated conduction path.
Summary:
Hall Effect technology has improved recently allowing for easier design ins, better accuracy and better noise immunity. While there have been advancements, the strength of the technology resides with high current applications. A Hall Effect sensor dissipates less power than a shunt resistor.
Conclusion:
In the three part series of the evaluating current sensing elements, it was learned that no one sensor is the choice solution for all applications. Shunt resistors are the most widely used current sensing element due to the simplicity of the design, precision of the measurement and cost of the solution. Direct Current Resistance (DCR) sensing is useful in switching regulator applications with low regulation voltages because current is measured remotely. Finally, Hall Effect sensors are suitable for high current applications where the power dissipation of a shunt resistor is greater than a Hall Effect solution.
For every positive about a sensing element, there is a drawback. Shunt resistors dissipate power resulting in power efficiency reductions. The voltage drop associated with current flowing through the shunt resistor consumes valuable voltage headroom in low voltage applications. A DCR sensing circuit specialty is to sense current remotely in switching power applications. A DCR circuitry is dependent on matching of a capacitor and an inductor. Both components have loose tolerances and high temperature coefficients. A Hall Effect sensor is susceptible to environment noise and design challenges. Advancements have been made in the technology but measurement accuracy is still a limitation.
Part 1 and Part 2 of this three-part series.
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