Improving Energy Efficiency with Hall-Effect ICs

Techniquesused for electrical current sensing are determined by the nature of theapplication. If current needs to be measured directly from the power mains orfrom a high-voltage dc power supply, as shown in Figure 1, then the currentsensor solution requires galvanic (voltage) isolation. In these applicationsthe system node carrying the sensed current is a high-voltage bus, while theoutput of the current sensor is a low-voltage signal interfaced with a low-voltagesignal processing circuit.

Threeprimary options are available to a designer who requires a high-side currentsensor:

Themost common solution for systems that employ low-side current sensing is simplya shunt resistor and a voltage amplifier. Low-side sensing, however, does notoffer protection for short circuits from high-side system nodes tolower-potential nodes. This can be important in systems such as those requiringshort circuit protection for semiconductor devices.

Historically,Hall sensors have suffered from insufficient accuracy and slow output responsetimes, but recent advances in Hall sensor technology have made Hall sensors apreferred current sensing solution in both high-side and low-side applications.

Advanced Hall Current Sensor IC Solutions

Toexplain these advanced Hall current sensor ICs, I'll use Allegro MicroSystems' family ofintegrated current sensor packages designed for motor and power supplyapplications as an example. In each of these sensor packages, the IC itself isthe most important component and contains a precision Hall-effect elementcoupled to a low-offset, high-accuracy amplifier. Both the Hall element and theamplifier are chopper-stabilized for enhanced accuracy and offset driftperformance (Allegro developed proprietary chopper stabilization techniquesthat also enable >120 kHz bandwidth operation and analog output responsetimes of less than 4 mus).

Figure2 shows a patented packaging technique for Hall-effect current sensor IC thatprovides a small form factor, especially useful where space is limited. The current tobe sensed flows through the conductive copper path between the four pins at thelower left corner of the package and the four pins at the lower right corner ofthe package. The Hall-effect IC is assembled into the package using standardflip chip assembly techniques. Two major advantages result from thisconfiguration:

Theblock diagram in Figure 2 shows that advanced features can be integrated intothe device to reduce overall bill of material costs; for example, having anovercurrent fault output that responds in < 2 mus with a user-programmabletrip point. The end result is a small form-factor, galvanically isolatedcurrent sensor solution that can sense up to 40A of continuous current. Theinternal conductor has a 1 mOresistance for low power loss - approximately 2to 10x lower than the resistance value typically used in shunt resistor-basedsolutions.

Forapplications that require sensing currents above 200A, as in hybrid electricvehicle (HEV) inverters, designers can use linear Hall sensors in the gap of asimple steel "C" core concentrator as shown in Figure 3.

Hall-Effect Solutions in Inverter Applications

Themost popular uses for inverter applications are motor current monitoring inconsumer white goods, industrial motor controllers, and HEVs, as well as powersupplies and UPS systems. Figure 1 shows an example of how current sensors canbe used in inverter-based control of a 3-phase brushless dc motor. Sensors A, Band C measure the phase currents of the motor and the outputs are used by amicroprocessor to alter the PWM control of the switching elements, therebyusing intelligence to alter the motor speed and torque as required. Using thesecontrol schemes, highly efficiency, high-performance motor controllers can bedeveloped.

Recognizing that applied current and voltage isolationlevelsultimately determine the optimum sensor configuration for a given application,is an important factor that should not be overlooked in systems design.

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