One of the challenges of designing electronics circuits for space-constrained applications is the risk of interference from undesirable or parasitic magnetic fields. This is a particularly important issue to consider when the application involves the measurement of current using the induced magnetic field around the conductor, and when other conducting wires are near to the conductor being measured.
One simple solution for eliminating magnetic field interference is to increase the distance between the conductor and the other wires. Some applications, however, will not have sufficient space available to make this method possible.
In the case of measuring current in multiple wires which all occupy the same cladding, the wires must be spaced apart on the terminal interface, routed separately over their sensors, and then recombined back into the cladding. This again requires a considerable amount of space, which might not be available in every case.
This Design Note presents a different solution: shielding, to redirect the flux vectors of the nearby magnetic fields in a direction to which the sensor is not sensitive.
Simulation of the problem
By simulating the measurement circuit, it is possible to solve for the field vectors of the magnetic field induced by a nearby current-carrying trace. The QuickFields 6.0 Student Edition software tool aids the simulation, as it provides two-dimensional analysis of induced magnetic fields.
The magnitude of an induced magnetic field, B, generated by the current, I, in a straight and long wire at a distance, r, from the wire is expressed by Ampere’s equation:
In this equation, μ0 is the permeability of the medium. The direction of the field vector, B, is defined by the right-hand rule.
The case described in this Design Note uses the PCB as the insulator between the sensor and the current-carrying trace, as shown in Table 1. In this case, the magnetic sensor is mounted on the top of the PCB, with the current-carrying trace located on the other side of the PCB.
Table 1: Dimensions of the components used in the simulation
Numerical simulation of the problem
The next step is to specify the simulation environment. The physical set-up is defined as a cross-sectional view of the current-carrying trace on the bottom side of the PCB which is located exactly under a sensor chip package located on the top side of the PCB, as shown in Figure 1. In this simulation, current is considered to be going into the page with the magnitude of 20A DC in the current trace. Sensor A measures the current in the trace, and sensor B is another sensor placed at a distance, to simulate the interference in that location. In a real application, sensor B would be measuring the current in another current-carrying-trace, not shown in the picture, parallel to the current trace.
The results of this simulation are shown in Figure 2. We can see that although the magnitude of the magnetic field in the x direction at sensor B is very small, there is interference at this point induced from the current in the current trace.
By plotting the magnitude of the magnetic field in the x direction across a contour line (passing directly on top of the PCB), it can be seen that interference in this direction exists at the position of sensor B, as shown in Figure 3. With sensor B 10mm from sensor A, the graph shows that the magnetic field in the x direction is -11 around 5×10 T at the position of sensor B.
Numerical simulation of the solution
The solution to the interference problem involves the use of ferrite plates. Figure 4 shows the ferrite plate placed on top of sensor B.
The simulation results for this case, illustrated in Figure 5, show that the flux lines change direction as they pass through the ferrite plate thus disturbing the magnetic field.
As a result, the magnetic field in the x direction at the position of sensor B is markedly reduced, to near 5×10-13 T, shown in Figure 6.
Practical demonstration of the solution
The simulated solution described above may be demonstrated in an actual hardware implementation, a three-phase current-measurement circuit shown in Figure 7. In this set-up, current flows through the current trace under sensor A. It is possible to detect the induced magnetic field of the same current, at a lower magnitude, at a distance from the current-carrying trace.
The output of each sensor without shielding is shown in Figure 8, measured by an oscilloscope. The magnetic field induced by current trace A is detected by the two other sensors, B and C.
By shielding sensor B with a ferrite plate, the output of sensor B falls, as shown in Figure 9, around five-fold, thus proving the effectiveness of the shielding method as a means of negating the effect of interference on current-measurement circuits. This technique may be implemented when using compact, accurate current- measurement devices such as the CTSR218C-IS4 or CTSR218C-IQ2 from Crocus Technology.