Introduction
Interest in wireless devices has increased demand for smaller gadgets and electronics, along with higher security communication and more efficient power transfer. Far electric fields are susceptible to stray signal interference and malicious remote read. Furthermore, the required antennae length limits miniaturization. Magnetic induction offers the promise of higher security due its field strength decaying with cubic distance. This is advantageous for applications such as wireless pay or medical device authentication. High-density coil inductors closely spaced and aligned can also provide efficient wireless power transfer.
Circuit Fabrication
Specialized high density coil manufacturing techniques are needed to make full use of magnetic induction advantages. With interlayer and intercoil thicknesses as little as 5 µm, advanced additive circuit fabrication can produce single and multilayer coils that offer smaller package sizes and higher copper densities than wire winding or traditional flex circuits. Traditional flex circuits are limited in copper trace density due to subtractive manufacturing techniques. Additive circuits are made with photolithographically-defined electroplated traces, sandwiched between thin layers of photosensitive dielectric materials. Physical vapor deposited seed layers provide the ability to make interlayer vias and multilayer structures.
These roll-to-roll additive manufacturing techniques offer MEMS-like capability, but have the advantages of lower cost, faster time to market, and higher customization than wafer-based processes. These techniques can fabricate small footprint, high-copper density coils with integrated resonant tuning capacitors, along with temperature and strain/force sensors.
Wireless Power
Two coiled inductors closely spaced and aligned provide efficient wireless power transfer. This is beneficial for applications such as wireless battery charging, powering ICs, or LEDs. Table one shows modeled results of similar footprint coils, manufactured to the leading-edge capabilities of traditional flex circuit and additive circuit manufacturing capabilities. The results show how high aspect ratio additive circuits can provide three to four times more power than traditional flex circuits. This is a direct result of trace aspect ratio greater than one and thin dielectric spacing.
Product Authentication Overview
Device companies commonly install memory chips in a disposable or semi-reusable attachment within a product assembly. Attachment examples include single-use medical diagnostic sensors, disposable printer ink cartridges and tooling inserts. The memory chip provides information about the attachment to a hosting device that controls or operates the attachment. This stored information may include the attachment’s number of connections or uses, calibration coefficients and the manufacturer or patient identification and date. This attachment data can then be used to enforce product requirements regarding reliability, accuracy, safety and brand. For example, the host device can alarm the user, shutdown, or prevent attachment operation when the memory data is outside the product’s validated or tested limits.
The traditional one-Wire communication protocol is a host architecture for reading and writing information to a memory chip that’s commonly terminated with pins or sockets. These electrical contact terminations wear out with repeated insertions, are prone to environmental contamination and require precise positioning within a mating connector assembly. A wireless radio frequency identification (RFID) communication protocol solves these kinds of wired connection problems, but may introduce different design issues. If multiple memory chips are within the host’s detection range, the host may not be able to correctly identify the connected attachment. Additionally, increasing the separation distance between the host transmitting antenna and attachment receiving antenna may cause the antenna dimensions and transmitting power to become prohibitively large.
This article describes a mechanically and electrically robust wireless connection that prevents misidentification of a connected device attachment, while minimizing both transmitting and receiving antenna size. The miniature antenna size and close separation distance allow inconspicuous placement within a device’s attachment directly at the interface. Both antennas can be molded or encapsulated within plastic or other non-magnetic materials to create a non-contact hermetically sealed electrical connector interface. The limited broadcasting and listening range of the coupled inductor antennas minimizes RF power emission while maximizing communication security.
An optimized assembly configuration for the miniature inductor coil assemblies. Both transmit and receive coils are 3 x 4 x 0.11 mm. The Max66240 wafer package EEPROM is 1.9 x 1.9 x 0.2 mm and the 0201 package capacitor is 0.6 x 0.3 x 0.3 mm. (Image Credit: TDK)
Antenna Design and Operation
Figure One depicts the optimized components used for wireless secure authentications. The antennas are custom planar coil circuits that are 110 microns thick, constructed on a 50-micron stainless steel (SST) base. The transmit coil’s SST base, spanning the entire flexible circuit from coil spiral to the electrical interface pads (not shown), can be grounded to limit electrical field emissions. Copper traces, at least 40-micron tall, wide, and having a 50-micron center-to-center spacing, are spiraled for at least 20 turns within a rectangular area of 3 x 4 mm. Polyimide layers, about 10 micron thick, separate copper from the SST base and surrounding environment. The inductance of both antennas measured at 13.56 MHz is about 1 µH.
Both antennas (inductor coils) are closely coupled such that the magnetic field created from alternating current (AC) in the transmit coil induces an alternating voltage in the nearby receive coil. The induced voltage provides power to the memory chip that is directly mounted to the receive antenna. Within the memory chip is an internal switching circuit that modulates the chip’s impedance load at a sub-carrier frequency. This repeated on and off load switching creates variable current flow through the receiving antenna resulting in a radiated voltage signal traveling back through the transmit antenna to the host RF electronics. Both the carrier and sub-carrier frequencies are amplitude modulated in order to produce digitized communication between the host and memory chip.
Memory Chip and Host RF Electronics
The receiving antenna is mounted directly to a memory chip that has embedded encryption, a 256-bit secure hash algorithm that further enhances secure authentication and communication integrity. The chip’s generated sub-carrier frequency is 423.75 KHz.
The RF-generating electronics excites the remote transmit antenna with ±5 AC volt dual in phase RF drivers, each rated for 100 mA peak current. The carrier frequency, 13.56 ± 0.007 MHz, is a FCC-regulated industrial, scientific and medical (ISM) band for mobile low power communication. A two-meter length of 50-ohm impedance co-axial cable was spiced to the transmit coil’s electrical connection pads located about 40 mm from the spiral center.
Resonant tuning capacitors, 150 pF and 240 pF, were mounted in parallel and across the respective transmit and receive coil circuits to maximize peak voltage and power the memory chip.
The inductor coil voltage signals decay with separation distance. The Max66240 requires peak about 2.9 volts to operate. Write function ceased at 0.9 mm coil separation with both coils dimensionally centered. Beyond 0.9 mm the read function ceased. (Image Credit: TDK)
Communication Performance Versus Antenna Positions
An experimental study was performed to determine how separation distance and lateral alignment between the inductor coils influences communication performance. The positional tolerance results are useful for designing the connector components that would house and mate both coils.
To vary antenna separation distance, a controllable Z-axis stage micrometer was affixed to the top surface of a 3 x 4 mm area memory chip that was soldered, then glued to a receiving coil having a side mounted resonant tuning capacitor. For antenna alignment changes, a controlled XY-axis micrometer stage was attached to the transmit coil bottom SST surface. Operating software was used to test both read and write functions with the memory chip at each coordinate position. The separation distance studied ranged 0.05 to 0.90 mm with both coils nearly concentric, having no obvious misalignment. At 0.20, 0.50, 0.65 and 0.80 mm separation distances, the XY positions that delineate successful and failed communication function were mapped.
The coordinates are mapped for successful read and write function of the EEPROM coil assembly at various coil separation distances. At the optimal signal center, left shifted from dimension center, the transmit coil can be ± 0.5 mm misaligned from the receive coil. The receive coil’s SST bus bar, located across the spiral path, may have changed the magnetic field center. (Image Credit: TDK)
When both antenna coils are concentrically aligned at the spiral’s dimensional center, communication fails at approximately a 0.9 mm separation distance, Figure Two. At a practical 0.65 mm separation distance, the XY positional tolerance is ± 0.5 mm from a signal center that is left shifted along the long turn 4 mm length dimension, Figure Three. The shift between dimension and signal center may be from the SST layer bus bar that it connects, by way of plated vias, the inside copper spiral to the outside connection pad.
Table 1. Simulated peak current performance results for two mutually coupled square spiral coils separated at 1 mm. For comparison, all square spirals have inside and outside widths of 10.0 and 11.0 mm respectively. The high aspect ratio additive process produces higher density copper traces which in turn produce more resonance current at 13.56 MHz to efficiently drive higher power devices, such as LEDs.
|
Copper Coil Attribute |
High Aspect Additive |
Narrow Additive |
1/3 oz Traditional Flex |
1 oz Traditional Flex |
|
Thickness (micron) |
60 |
10 |
12 |
36 |
|
Width (micron) |
43 |
15 |
30 |
90 |
|
Spacing (micron) |
7 |
15 |
30 |
90 |
|
Turns, per 0.5 mm width |
10 |
16 |
8 |
3 |
|
Inductance (µH) |
2.9 |
7.8 |
1.9 |
0.27 |
|
Resistance (Ω) |
2.8 |
76.3 |
15.9 |
0.66 |
|
Reactance to Resistance Ratio |
87.9 |
8.7 |
10.1 |
34.7 |
|
Tuning Capacitor (pF) |
47 |
15 |
72 |
550 |
|
Peak Diode Current (mA) |
16 |
3 |
4 |
5 |
Filed Under: Capacitors, M2M (machine to machine)