Wireless Power Transfer / NFC Antennas Sheets (Shield)

Solution Guide Total Solutions for NFC Circuits

Total Solutions for NFC CircuitsNFC is an abbreviation of Near Field Communication; it is a type of short-range wireless communications.
NFC is a function that can perform data communications and authentication when two NFC-compatible devices are brought close together, and adoption in smart phones has been growing rapidly in recent years.
Increased use in peripheral devices including wearable terminals, such as smart watches, has also been observed.
NFC is used in many situations including cashless payment and authentication of connections with peripheral devices, and it is expected to be used for even more diverse applications as we move towards a touchless society.
This article introduces the main components used in NFC circuits: NFC antenna, magnetic sheet, LC filter inductor, single-end circuit balun, and electric double-layer capacitor (EDLC/supercapacitor).

Main Components Used in NFC Circuits

Figure 1(a) Diffrencial mode

Figure 1(a)

A circuit used in an NFC circuit is shown in Figure 1(a).

NFC communication operates by using an electromagnetic induction method, and consequently, loop antennas are generally used.
An antenna matching circuit and an LC filter made up of an inductor and a capacitor are inserted between the antenna and the NFC controller IC.

As shown in Figure 1(b), however, in some instances, a single-end antenna is connected. With this format, the single-end signal is converted to differential mode, and as a result, it is necessary to use a balun to perform the mode conversion.

Figure 1(b) Single-end mode

Figure 1(b)

NFC Antenna and Magnetic Sheet

NFC communication adopts an electromagnetic induction method, and when the antenna receives a carrier wave from a reader/writer, an IC chip performs signal processing (Figure 2a).
When this antenna is installed in an electronic device such as a smart phone, there may be metal present close to the antenna.
In this case, an induced current flows in the metal and a magnetic field is produced the opposite the magnetic field generated by the reader/writer. This cancels the magnetic field from the carrier wave, shortening the communication distance and making communication impossible (Figure 2b).
As shown in figure 2c, if magnetic material is present between the coil and the metal, the high permeability has the effect of confining the magnetic flux generated from the reader/writer. As a result, the generation of the induced current in the metal can be curtailed, and good communication conditions can be maintained.
TDK provides magnetic sheets for installation between the antenna and metal and antenna units that integrate the magnetic sheet and antenna.

Table 1. NFC Antenna and Magnetic Sheet
Series NFC Antenna
(Thin Type)
NFC Antenna
(WPC Rx Integrated Type)
Magnetic Sheet
P# MCS series Under development WR series IFL/IBF series
Product Exterior View
Features Antenna design technology and magnetic sheet forming technology are used to create products that are thinner and have higher characteristics than earlier products. Components ideal for installation in mobile devices are created by integration with an electromagnetic induction type wireless power transfer coil. High permeability and low magnetic loss at 13.56 MHz
Figure 2. Communication Status When Metal is Near the NFC Antenna
Figure 2. Communication Status When Metal is Near the NFC Antenna

Inductors for NFC LC Filters

Inductors for LC filters are required to produce narrow tolerances in order to reduce loss due to impedance mismatch with the antenna.
As shown in Table 2, TDK’s MLF/MLJ series are both product lineups with J tolerances (± 5%).

To prevent a decrease in antenna output, it is also important to control inductor loss at 13.56 MHz, the communications frequency.
To do this, it is necessary that the AC resistance (Rac) be kept low and that a low Rac be maintained even when current is applied.
TDK’s MLJ-W series achieves low Rac, while the new MLJ-H series achieves low Rac even when a large current is applied.
As can be seen in Figure 3, the Rac of the MLJ1005H remains low even in the high current range.
On the other hand, the Rac of the MLJ1005W is low in the low current range, and the optimal product can be selected depending on the actual current values used.

Table 2. Inductors for NFC LC Filters
Series MLF1608D
MLF1005V
MLJ1608W
MLJ1005W
MLJ1005HUnder development
type STD High Current
Low Loss
Super High Current
Super Low Loss
size [mm] 1608 / 1005 1608 / 1005 1005
L tolerance [%] +/- 5 +/- 5 +/- 5
L value [uH] 0.10 – 0.82 (1608 size)
0.10 – 0.56 (1005 size)
0.10 – 0.56 (1608 size)
0.075 – 0.56 (1005 size)
0.056 - 0.20
Current [mA] 70 – 200 (1608 size)
120 – 180 (1005 size)
400 – 800 (1608 size)
250 – 550 (1005 size)
480 - 950
Figure 3. AC Resistance vs. sine wave current @13.56MHz
Figure 3. AC Resistance vs. sine wave current @13.56MHz Figure 3. AC Resistance vs. sine wave current @13.56MHz

NFC Single-End Circuit Baluns

A balun is an element used to switch between a balanced circuit and unbalanced circuit. It is also possible to convert the impedance by changing the turn ratio of two coils.
Baluns used in NFC circuits often have a turn ratio of 1:1, and the keep insertion loss in the 13.56 MHz band low.
TDK’s proprietary technology makes possible slimmer and more compact products.

Table 3. NFC Single-End Circuit Balun
Series ATB1610HD-20011-T06
Under development
Product Exterior View
size [mm] 1610
height [mm] 0.65 max
UB/B Imp [ohm] 20 : 20
Rated Current [mA] 400mA
Features Compact, low-profit type ideal for NFC use

Electric Double-Layer Capacitors (EDLCs/Supercapacitors) for Smart Cards

Thin electric double-layer capacitors (EDLCs/super capacitors) are ideal for contactless IC cards that use NFC.
Not only can they quickly store the energy required to rewrite the display on electronic paper, they also have the advantage of being a very safe power storage device compared to lithium-ion batteries.

Figure 4. Example of the Structure of a Battery-Free Next-Generation Smartcard
Figure 4. Example of the Structure of a Battery-Free Next-Generation Smartcard