Technical Field
This Patent Document relates generally to fluxgate magnetic field sensing, either single ended or differential, such as can be used for contactless current sensing.
Related Art
Fluxgate sensors measure (sense) a magnetic field generated by an external field source (field of interest) at the location of the sensor. This external field of interest can be referred to as an external, target field BEXT.
Differential fluxgate sensor topologies, with dual fluxgate elements, can be configured to sense differential components of a target BEXT field. In such applications, any stray field BSTRAY (for example, the Earth's magnetic field) will appear in common mode, and will be rejected by the differential fluxgate sensor.
A fluxgate sensor includes a fluxgate magnetics element and fluxgate readout circuitry. The fluxgate magnetic element includes a magnetically susceptible core with an excitation coil, and a sense coil coupled to the fluxgate readout circuitry. The excitation coil is driven by an electrical excitation current IEXC at an excitation frequency fEXC, driving the core through alternating cycles of positive and negative magnetic saturation (B-H loop), and inducing corresponding VSENSE voltage pulses in the sense coil.
The fluxgate sensor is disposed at a location/source of the target external field, with the fluxgate magnetics element arranged for magnetic coupling to the target BEXT field, with the magnetics geometry oriented to measure the target BEXT field in a selected direction, which can be referred to as the axis of sensitivity (for example, the x-axis). Fluxgate core saturation will then align with BEXT, resulting in a net flux imbalance in the fluxgate core, which is reflected in a corresponding change in VSENSE output. Phase synchronous detection is used to convert this VSENSE output signal to sensor data representative of the BEXT field of interest.
Fluxgate readout circuitry can be configured for open or closed loop operation. For closed loop operation, the fluxgate magnetics element includes a compensation coil. The fluxgate readout circuitry implements a fluxgate control loop including a feedback compensation path that injects an internal compensation current ICOMP into the fluxgate compensation coil, inducing in the fluxgate core a feedback-controlled compensation field BCOMP to null the target field BEXT, so that the internal compensation current ICOMP required for field nulling is a measure of BEXT, as represented by the sensor data output of the fluxgate control loop.
While single ended sensing can be used if BSTRAY fields can be ignored or shielded, a differential fluxgate sensor topology with dual fluxgate sense elements enables differential measurement of the target BEXT field, with the BSTRAY field appearing in common-mode. The differential fluxgate sensor includes a common-mode loop that nulls (suppresses) the common-mode BSTRAY field in the fluxgate cores, and a differential-mode loop that measures a differential field BEXT by generating feedback-controlled differential BCOMP fields to null the respective differential components of the BEXT field appearing in respective fluxgate cores.
One application for fluxgate sensing is contactless current sensing, eliminating the need for a shunt resistor or other in-circuit element. For example, the fluxgate sensor can be placed on a PCB over or adjacent to a PCB trace to effect magnetic coupling between the fluxgate core and the BEXT field induced by current flow through the PCB trace.