used for electrical current sensing are determined by the nature of the
application. If current needs to be measured directly from the power mains or
from a high-voltage dc power supply, as shown in Figure 1, then the current
sensor solution requires galvanic (voltage) isolation. In these applications
the system node carrying the sensed current is a high-voltage bus, while the
output of the current sensor is a low-voltage signal interfaced with a low-voltage
signal processing circuit.
primary options are available to a designer who requires a high-side current
A current transformer, burden resistor and voltage amplifier.
An optical isolator and external discrete biasing circuits.
A Hall-effect IC (integrated circuit) based current sensor.
most common solution for systems that employ low-side current sensing is simply
a shunt resistor and a voltage amplifier. Low-side sensing, however, does not
offer protection for short circuits from high-side system nodes to
lower-potential nodes. This can be important in systems such as those requiring
short circuit protection for semiconductor devices.
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Hall sensors have suffered from insufficient accuracy and slow output response
times, but recent advances in Hall sensor technology have made Hall sensors a
preferred current sensing solution in both high-side and low-side applications.
Advanced Hall Current Sensor IC Solutions
explain these advanced Hall current sensor ICs, I'll use Allegro MicroSystems'
integrated current sensor packages designed for motor and power supply
applications as an example. In each of these sensor packages, the IC itself is
the most important component and contains a precision Hall-effect element
coupled to a low-offset, high-accuracy amplifier. Both the Hall element and the
amplifier are chopper-stabilized for enhanced accuracy and offset drift
performance (Allegro developed proprietary chopper stabilization techniques
that also enable >120 kHz bandwidth operation and analog output response
times of less than 4 µs).
2 shows a patented packaging technique for Hall-effect current sensor IC that
provides a small form factor, especially useful where space is limited. The current to
be sensed flows through the conductive copper path between the four pins at the
lower left corner of the package and the four pins at the lower right corner of
the package. The Hall-effect IC is assembled into the package using standard
flip chip assembly techniques. Two major advantages result from this
The Hall-effect sensing element is in close
proximity to the current-carrying conductor, thereby maximizing magnetic
coupling and device accuracy.
The Hall-effect IC is not in contact with the
integrated conductor, thereby maintaining voltage isolation and enabling
working voltages as high as 500 VRMS. Solder bumps attached to the low-voltage
input/output pads on the Hall IC make contact with the lead fingers shown in
the upper portion of Figure 2.
block diagram in Figure 2 shows that advanced features can be integrated into
the device to reduce overall bill of material costs; for example, having an
overcurrent fault output that responds in < 2 µs with a user-programmable
trip point. The end result is a small form-factor, galvanically isolated
current sensor solution that can sense up to 40A of continuous current. The
internal conductor has a 1 mOresistance for low power loss - approximately 2
to 10x lower than the resistance value typically used in shunt resistor-based
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applications that require sensing currents above 200A, as in hybrid electric
vehicle (HEV) inverters, designers can use linear Hall sensors in the gap of a
simple steel "C" core concentrator as shown in Figure 3.
Hall-Effect Solutions in Inverter Applications
most popular uses for inverter applications are motor current monitoring in
consumer white goods, industrial motor controllers, and HEVs, as well as power
supplies and UPS systems. Figure 1 shows an example of how current sensors can
be used in inverter-based control of a 3-phase brushless dc motor. Sensors A, B
and C measure the phase currents of the motor and the outputs are used by a
microprocessor to alter the PWM control of the switching elements, thereby
using intelligence to alter the motor speed and torque as required. Using these
control schemes, highly efficiency, high-performance motor controllers can be
Recognizing that applied current and voltage isolation
ultimately determine the optimum sensor configuration for a given application,
is an important factor that should not be overlooked in systems design.