High-five to high-side current sensing

Richard Dickinson
Bill Bentley
Allegro Microsystems Inc.
Worcester, Mass.

This ACS754 Halleffect current sensor from Allegro Microsystems measures isolated high-side currents up to 200 A using less space than a quarter.

An exploded view of an Allegro Microsystems ACS75x current sensor depicts the main current path, the Hall-effect sensor with integrated electronics, and a magnetic concentrator which tightly couples the magnetic field of the measured current to the Hall-effect device.

The Hall-effect device makes up only a small portion of the electronics in the ACS75x family of current sensors. This block diagram illustrates the major elements comprising the sensor circuitry. The sensor also incorporates over 3-kVrms isolation between the measured current pins and the output without transformers or optoisolators.

For over a century the standard method of measuring amperage has used a current-shunt ammeter in series between the load and power source. The shunt, basically a resistor with very low resistance, bypasses the majority of load current around the measuring instrument. Only a proportional fraction of overall load current actually passes through the meter. The technique was called high-side current sensing.

However, high-side shunts require isolation in modern electronic control systems because of their direct connection to the high-side voltage rail. Current transformers perform this service in ac power systems. But the size and cost of isolated high-side dc sensing precluded its use in many applications.

To get around this limitation designers moved the shunt from the high-side to the low or ground side of the load. With one terminal of the shunt grounded, control systems easily monitored current demands. But low-side sensing raises the load above ground potential, thus creating possible noise and EMI problems. Also, low-side sensing will not detect up-stream short circuits. A lack of physical access in some applications prevents the insertion of shunt devices between a load and ground. All of these factors make low-side sensing unattractive from a measurement viewpoint.

High-side current sensing returns with a new generation of smaller and more cost-effective Hall-effect-based current sensors. Hall-effect devices measure current via the intensity of the magnetic field generated by current flow. Higher currents produce stronger magnetic fields. They now provide the means to measure high-side current in applications where it was neither physically nor economically feasible.

Hall-effect devices provide voltage isolation without optoisolators or current transformers. There is no electrical connection between the current path and sensor. Unlike current transformers, Hall-based sensors measure dc as well as ac currents. Many operate from a single 5-Vdc supply so their output does not need level shifting before connecting to an a/d converter, microprocessor, or microcontroller.

Insertion resistance of Hall-effect sensors typically ranges two to three orders of magnitude less than that of shunts or sense resistors. The lower resistance dramatically reduces power losses created by the sensing elements. For example, 20 A passing through a typical 5-mΩ shunt resistor dissipates 2 W and generates a 100-mV drop. In contrast, 20 A through a typical Hall-effect-based current sensor generates only a 2-mV drop dissipating 40 mW. Motor windings remain more evenly balanced when only two of the three phases are measured. Typical power consumption of the current sensor itself is only 35 mW.

Another benefit over shunt resistors is that Hall-effect sensors operate from power circuits independent of the measured current. This is not the case when coupling shunt resistors with op-amps to measure current. Parasitic resistance in the ground lines may introduce large errors in amplifier output.

A typical example of Hall-effect-based current sensors is Allegro Microsystems Inc. ACS7xx family of sensors. The ACS7xx line sports ratiometric gain and offset that scale with variations in VCC. Variations in VCC are eliminated as an a/d conversion error source as long as the converter connects to the same 5-Vdc supply. Operating temperatures up to 150°C affirm the ruggedness of these devices.