Inductor for short-range wireless communicationInductor for short-range wireless communication

In recent years, the number of products equipped with NFC features has been increasing in the field of miniature mobile devices such as smart phones and tablets. “NFC” is an abbreviation of Near Field Communication, which is a feature that uses a magnetic field at a frequency of 13.56 MHz to enable dedicated readers/writers and other equipped devices to wirelessly communicate when they are close to each other (Figure 1).

Murata Manufacturing provides the LQW18C series (winding type) and the LQM18J series (multilayer type) of NFC inductors. In particular, the new LQM18J series of products achieve the optimal electrical properties for NFC equipped devices.

Short-range wireless communication broadly refers to close-range wireless communication protocols such as Bluetooth and Zigbee, but here we are referring to NFC, which is an abbreviation of “Near Field Communication.” In recent years, the number of smart phone, tablet, and other miniature mobile device products that support NFC has been increasing, and the demand is expected to continue to grow in the future.
Figure 2 shows a schematic drawing of a typical circuit between an NFC antenna and a control IC. The LC filter is included as a low-pass filter between the antenna and the control IC. This LC filter is inserted to efficiently transmit just the 13.56 MHz signal that is the NFC operating frequency by cutting the higher harmonics.

When designing an NFC circuit, an inductor is used to perform impedance matching as shown in the circuit of Figure 2. However, because the LC filter described above affects the impedance matching, the inductor needs to have an inductance with a small deviation (within +/- 5%).

A large-amplitude current of 13.56 MHz flows through an NFC inductor. For this reason, different points must be considered when selecting an NFC inductor as compared to a typical matching inductor. This section explains the characteristics of an NFC circuit and the key points for selecting an inductor.

Previously, it was common to use an IC with a low output during transmission as the NFC control IC. However, as NFC antennas continue to grow smaller in order to be mounted in mobile devices, the NFC communication performance decreases with the smaller current amplitude. Therefore, the number of ICs with a high output during transmission is increasing in order to achieve high NFC communication performance. When selecting an NFC inductor, it is important that the inductance does not change even when there is a high current amplitude.
Figure 3 shows the dependency of the inductance on the current amplitude for the LQW18CNR16J00 (winding type), LQM18JNR16J00 (multilayer type: new), and the LQB18NNR22J10 (multilayer type: old). During NFC communications, an alternating current with a current amplitude of approximately 100 to 700 mA (peak to-peak) flows through the inductor. The inductance of the LQB18N roughly doubles at a current amplitude of 400 mA (peak to-peak), but the LQW18C and the LQM18J achieve an inductance that does not change even when the current amplitude exceeds 1 mA (peak to-peak).

Not only does the LQM18J maintain a constant inductance while conducting a current, it also exhibits favorable NFC communication performance. As shown in Figure 4, the NFC Forum (industry organization which formulates NFC standards and common specifications) measured the communication performance at the measuring point indicated by the red dot during their performance tests. The results of the LQW18C and LQM18J communication performance measurements are shown in Figure 5. The 4.1 V line is the passing standard for this test, and higher voltages indicate favorable NFC communication performance. The LQM18J and the LQW18C are able to achieve equivalent communication performance.

The mounting density of electronic components is increasing as smart phones and other mobile devices add more functions. The LQW18C uses an open magnetic structure, which causes magnetic flux leakage around the components. In the case of high-density mounting, there was a possibility that the magnetic flux would cause interference between inductors and change their properties. Therefore, it was necessary to arrange the inductors in a T-shape to prevent interference due to magnetic flux (Figure 6).
The newly developed LQM18J uses a magnetic shield structure, so there is no magnetic flux leakage around the components. This enables parallel mounting even under high-density mounting conditions to achieve a decrease in the mounting space (Figure 6).
Figure 7 shows the different coupling coefficients when the distance between inductors is set to 200 um, and when the LQW18C is mounted in parallel, mounted in a T-shape, and the LQM18J is mounted in parallel. When the LQW18C is mounted in parallel, the inductors are strongly coupled. However, when the LQW18C is mounted in a T-shape and the LQM18J is mounted in parallel, they are able to create a state in which inductor coupling does not occur.

Inductors are used for matching in NFC circuits, but they must have properties such as an inductance specification with a small deviation, stable inductance when conducting a current with a large amplitude, and measures to deal with external magnetic coupling to enable high-density mounting. The LQM18J series of multilayer inductors are miniature inductors that satisfy these conditions and are recommended for use with NFCs.