State-of-the-art magnetic field-based current sensors generally include a magnetic field sensor, an iron core, and an amplifier. The iron core surrounds an electrical conductor, preferably comprising copper for carrying the current to be measured, the current causing a magnetic field corresponding to the intensity of the current to be generated in the iron core. The magnetic field sensor is arranged in a gap in the iron core. Thus, the field lines of the magnetic field induced in the iron core by the current to be measured pass through the magnetic field sensor. The magnetic field sensor generates an electrical output signal generally proportional to be current to be measured, and this signal is amplified by an amplifier, such as an operational amplifier.
The iron core in conventional current sensors primarily provides two functions. First, the conducting of the magnetic field induced by the current to be measured through the magnetic field sensor, and second, the screening of the magnetic field sensor from extraneous electric and magnetic fields which are known to cause measurement error. However, the use of iron cores results in a sensor cost that is too expensive for many applications.
A known improved current sensor design disclosed in EP 1192473/WO 00/79291 dispenses with the iron core and thus its expense. According to the sensor design disclosed, the primary conductor which carries the current to be measured comprises an electrically conducting ferritic material generally mixed with an electrically conducting material (e.g. copper). The primary conductor at least partially surrounds the magnetic field sensor which assumes the functions of the iron core. The magnetic field sensor is centered near a notch in the primary conductor which serves to focus the magnetic flux lines from the primary current in the primary conductor that reaches the magnetic field sensor. The current sensor design disclosed in EP 1192473/WO 00/79291 generally has the disadvantage that the electromagnetic field associated with the current to be measured generates a magnetic field in the primary conductor, which can prevent precise measurement of the magnetic field associated with the current to be measured in the notch by adding another magnetic field component. The remanance of the primary conductor also causes non-linearity and magnetic saturation in the region of the notch which degrades the effect of the focussing notch. If only magnetic permeable material is used for the primary conductor (without copper or other electrical conductor), then a large cross-sectional area of this expensive material is generally needed because of its relatively high electrical resistivity.
Thus, although sensors disclosed in EP 1192473/WO 00/79291 having a primary conductor including magnetic permeable material mixed with electrically conducting material can provide good performance and a reduced cost compared to conventional iron core-based current sensors, in certain important applications more precise measurement of electric current is required. Precise measurement of electric current requires improved linearity (between the current to be measured flowing as primary current and the magnetic flux reaching the magnetic sensor) and reduced resistive impedance. Lower cost would also be helpful.
Some present current sensor applications include measurement of electric current include motor current control in frequency inverters, phase current control in servo-drives, overload protection for motors and power semiconductors, control of energy systems (e.g. wind energy), current control and monitoring in welding equipment, current regulation in switching mode power supplies, current control in uninterruptible power supplies, battery current diagnosis in motor vehicles, ground fault detection and laboratory test equipment. Some of these applications generally require precise current measurements.
Small sensor size is required in certain applications, such as those where current is measured directly on a printed circuit board. Moreover, low cost is also required in most automotive applications and other high volume applications. What is needed is a new current sensor design that provides improved sensor performance while also providing a low cost.