Measuring Current with IMC Hall Effect Technology

Hall effect sensors have been used to measure electric current for many years. Because of their relatively low sensitivity, conventional Hall sensors need magnetic circuits that serve to increase their cost, size, and weight. The Hall effect current sensor presented in this article features an integrated magnetic concentrator (IMC) that boosts its sensitivity enough to measure current directly, without a ferrite core.

Configuration and Operation
The IMC Hall effect sensor is fabricated by means of standard CMOS technology, with an additional ferromagnetic layer characterized by high permeability and low coercive field (very soft) bonded to the chip surface (see Figure 1).

This layer functions as a magnetic flux concentrator; it provides additional magnetic gain of ~10, increasing the output signal without doing the same to the inherent sensor noise. Another distinction is that the IMC Hall sensor responds to a magnetic field parallel to the chip surface, whereas a conventional Hall sensor detects a field perpendicular to its surface (see Figure 2).

IMC sensors are also more sensitive than conventional Hall technology, making them similar to magnetoresistive sensors but without the associated drawbacks such as nonlinearity, hysteresis, and insensitivity to field direction.

Figure 3. The results of placing the IMC Hall sensor in a homogeneous magnetic field parallel with the magnetic concentrator are given in (A). An expanded view of the air gap between the two concentrators illustrates the vertical component of the magnetic fields near the Hall elements (B).

In addition, the IMC sensor?s 100 kHz bandwidth and typical response time of 3?6 µs cover a wide range of applications, such as the requirements of PWM controllers and overload protection.

The ferromagnetic layer is structured so that a narrow gap is created in the approximate middle of the chip, where the perpendicular component of the field is strongest (see Figure 3). The concentrator locally converts the magnetic field parallel to the chip surface into a field perpendicular to it. The two sets of Hall elements placed near the gap have reverse outputs because the field direction into one set is opposite that into the other set. Their outputs are summed together to enhance the SNR and also to make the device insensitive to fields perpendicular to the chip surface. The result is a sensitivity of 300 V/T (30 mV/G). Furthermore, because the sensor is sensitive to fields parallel to its surface, it is ideal for measuring current in conductors above or below itself.

How Hall Effect Sensors Measure Current

Figure 4. Current flowing into the page creates a magnetic field that is clockwise around the conductor, proportional to the current, and inversely proportional to the distance from the conductor.

Hall effect sensors measure current by converting the magnetic field generated by current flowing through a conductor into a voltage proportional to that field. The sensor output is linear to the magnetic field, and because the field is linear to the current in the conductor, the output voltage will provide a linear voltage that is directly proportional to the current. The magnetic field at distance r from an ideally thin, straight, and infinitely long conductor carrying a current I (see Figure 4), is given by:

(1)

In a vacuum (or air), the magnetic induction (or flux density) B can be calculated from H by multiplication with the permeability:

(2)

If,

then

(3)

where:

B	=	flux density (teslas)
I	=	current in conductor (amps)
r	=	distance (meters)

For example, flux density at a distance r = 2 mm from a current conductor carrying

20 A is:

(4)

With the IMC Hall sensor?s sensitivity of 300 V/T,

(5)

The IMC Sensor at Work
Because the magnetic field strength drops off very sharply with distance, the closer the sensor can be placed to the conductor, the higher the output voltage. IMC sensors? sensitivity to fields parallel to the package surface allows the conductor to be positioned very close to the sensing element. This feature, and a sensitivity ~10 × that of conventional Hall devices, provide an output ~20 × greater than that of standard Hall sensors.

For instance, the IMC sensor can be mounted on a PCB to measure the current in traces directly under the IC (see Figure 5).

Bidirectional output allows the sensor to measure both AC and DC currents. The only coupling between the IC and the conductor is a magnetic field; the sensor is thus completely isolated from the conductor and no optical isolators are required.

Because the upper limit of the current an IMC sensor can measure is determined by the maximum field, larger currents can be handled by simply moving the conductor farther away from the sensor. With a trace directly under the IC, the optimum current for the sensor is 10?20 A. To handle higher levels of current, multiple layers of copper traces can be incorporated in the PCB trace to distribute the current density and thereby increase the total current capability. The differential output voltage for this configuration can be approximated by:

(6)

Figure 6. Current flow under and around the sensor increases the magnetic flux density in the chip to ~120 mV for a 1/32 in. thick PCB.

The sensitivity can be increased by configuring a PCB layout. The IC is placed in a loop created by using traces on both sides of a PCB and a jumper wire mounted over the IC. The current flows from the left wire, through a trace on the bottom of the PCB and directly under the sensor, then up and over the sensor through the jumper wire, back under the sensor through the top layer trace, and out through the right wire (see Figure 6). Each conductor creates a magnetic flux that is sensed by the IC (see Figure 7). The three fields add together and raise the output signal by ~3. The actual gain depends on the thickness of the PCB. The output sensitivity for a 1/32

Figure 7. Magnetic flux generated by the jumper wire, upper trace, and lower trace add together at the sensitive area of the chip.

in. thick PCB will be ~120 mV/A. A ¹/₁₆ in. thick PCB will have a slightly lower level because the bottom conductor will be farther away from the sensor and its contribution to the total flux density at the chip will be less.

High currents can be measured by placing a formed copper bus bar over the IC. The standoff distance and size will depend on the full-scale output desired. An approximation of differential output voltage can be obtained with:

then

(7)

where:

d	=	distance (mm) between conductor center and sensor surface
I	=	current (amps) in conductor

For example, if I = ±80 A and d = 1 mm, then:

then

(8)

Fiture 8. Adding a magnetic concentrator significantly increases the sensitivity of the output by shortening the magnetic path in air and increasing the flux at the sensor.

The output sensitivity of this configuration can be increased by placing a small piece of magnetic material on top of the bus bar directly over the IC (see Figure 8). The material shortens the magnetic path around the conductor and increases the flux density in the sensor, thereby increasing the output voltage for a given current.

Another way to measure high currents on PCBs is to use a large, thick-gauge copper trace capable of carrying the higher currents on the opposite side of the PCB. An advantage of this configuration is that the PCB material provides additional electrical isolation between the sensor and the current-carrying conductor. The sensor should be located near the center of the trace.

Typical Applications
Among the potential applications for surface-mount current sensors are motor control, overload sensing, fault detection, current present detection, phase monitoring, PWM controllers, DC/DC converters, diagnostics, and instrumentation.

Summary
IMC Hall effect technology provides a unique solution to current sensing by combining all the advantages of Hall sensors, such as high linearity and low hysteresis, with the high sensitivity found in magnetoresistive devices. Packaged in a small 8-pin SMD, the technology lends itself in particular to applications where space and weight are a concern. Although this article has focused on current sensing, which is the primary use for these devices, they are good choices for other areas as well, including magnetic field measurement and long-range proximity sensing.