Design notes for switches

In Figure 1, the relationship of the magnetic flux density B to the distance d from the magnet surface of the square magnet (5mm x 5mm x 3mm) is actually calculated and plotted. The assumed material here is ferrite. Its residual magnetic flux density Br is 300 mT. It is shown that the magnetic flux density decreases rapidly near the magnet surface with distance, and, as the distance from the magnet surface increases, the magnetic flux density gradually decreases.
The ideal design involves placing the Hall effect IC so that it turns on and off in the region where the magnetic flux density rapidly changes, and it is not recommended to locate the operating point in an area where the change in the magnetic flux density with distance is small.
There are two reasons for this.

Figure 1: Relation between the magnetic flux density and the distance from the magnet surface

In the case of a normal Hall effect IC, it has a certain width Bop and Brp. When there are Hall effect ICs with 5 mT (1) and 2 mT (2) of Bop and Bop changes 1mT, the dispersion of the position where the Hall effect IC activates is small for the Hall effect IC (1) as shown in Figure 1.
This also applies for the change in Bop and Brp due to thermal characteristics.

The magnetic flux density may vary depending on the lot of the magnet. For instance in the Hall effect IC (2)in the example above, and when the magnetic flux density produced by a magnet is 20% less, the activation point will move as much as 1.0 mm, while, in the case of the Hall effect IC (1), it is 0.6 mm.

Furthermore, it is necessary to take care when positioning the sensor. Bop and Brp described so far are the values on the semiconductor film surface, which is at the position of the sensing surface of magnetism of the Hall effect IC, and reference is not to values on the surface of the Hall effect IC package. Please refer to the table below for the approximate distance from the surface of the package of our Hall effect IC to the magnetic sensing surface.

When the magnet is brought close to the side of the Hall effect IC without the marking, ensure that the distance is different from the above.

Note 1: About the dispersion of sensitivity of the Hall effect IC
The sensitivity of the Hall effect IC (dynamic sensitivity) has dispersion. It is a good design practice to absorb or neglect this dispersion. For example, EW-750B, Unipolar Hall effect switch is considered. Table 2 shows the electrical characteristics of EW-750B. It is indicated in Figure 2.

Figure 2: Dispersion of sensitivity of EW-750B

This means that some EW-750Bs require a magnetic flux density of 10 mT maximum to change the output from H to L, and require reduction of the magnetic flux density to 2.5 mT at minimum. To avoid problems for all Hall effect ICs on the market, it is necessary to select a magnet in which 10 mT is secured when the magnet is close and 2.5 mT is secured when the magnet leaves the Hall effect IC.

Note 2: About the thermal characteristics of the Hall effect IC
As with other semiconductors, the Hall effect IC has thermal characteristics. The thermal characteristics of the EW-750B, Unipolar switch type, are shown as an example in Figure 3. The number of elements measured is three. Generally, a strong magnet is used in consideration of a Hall effect IC whose sensitivity is poor. While this is not a problem in the case of the Bipolar Latch type, it should be noted that the magnetic flux density may not decrease to the value Brp when the movement of the magnet is small, and the Hall effect IC will not be turned off in the case of the Unipolar switch type. In addition, the dispersion of sensitivity mentioned above should be taken into account.

Figure 3: Temperature dependence of Bop,Brp of the EW-750B (for reference only)