Wireless Power Cutting the cord in today’s mobile world Contents Introduction…………………………………………………………………………………….... A Brief History of Wireless Energy Transfer…………………………………………………. Wireless Energy Transfer Techniques……………………………………………………….. Transformers and Induction…………………………………………………………….... Radio Coupling……………………………………………………………………………. Electro-dynamic Induction……………………………………………………………….. Regulatory Issues………………………………………………………………………………. Communication……………………………………………………………………………. Industrial, Scientific, and Medical (ISM) Bands, and ISM Equipment……………….. Vehicle Standards Development………………………………………………………… Modular Approval…………………………………………………………………………. Testing Methods………………………………………………………………………….......... Radiated Power…………………………………………………………………………… Radiated Spurious Emissions and Harmonics………………………………………… AC Line-Conducted Emissions………………………………………………………….. Frequency Stability……………………………………………………………………….. Human RF Exposure……………………………………………………………………… FCC Inquiry………………………………………………………………………………… Methods of Demonstrating RF Exposure Compliance………………………………. Measurement…………………………………………………………………………….. SAR Testing……………………………………………………………………………… Calculation……………………………………………………………………………….. Mathematical Modeling…………………………………………………………………. Conclusion…………………………………………………………………………………….. About Intertek………………………………………………………………………………….. Introduction With the proliferation of battery-operated portable electronic devices in today’s society, consumer demand for wireless power solutions is growing at a rapid pace. Consumers are looking for products which offer convenience and ease of use. Eliminating the power cord can reduce the hassle of cable tangles, wire routing or wear issues in systems with moving subsystems, and can offer improved marketability and flexibility in form factor by removing the need for an input cable or charging terminal on the device exterior. Wireless charging can also offer improved user safety by eliminating the possibility of electric shock from devices which are used near water. In applications where a power cable is undesirable or impractical, wireless power transfer techniques can provide innovative solutions for extending the battery lifetime of devices which would otherwise be inaccessible. A Brief History of Wireless Energy Transfer The concept of wireless energy transfer is not a new one and is as old as the field of electromagnetism itself. Wireless energy transfer was actually demonstrated before Marconi proved that radio communication was possible in 1895. As early as 1820, Andre-Marie Ampere demonstrated that a current in a wire produces a magnetic field, and in 1831 Michael Faraday showed that a time-varying magnetic flux induces a current on a wire. The transformer, developed in 1836 by Nicholas Callan initially used these concepts to step up the voltage from batteries, but was an early form of wireless energy transfer. Perhaps the most famous historical example of wireless energy transfer is Nikola Tesla’s demonstration in 1893 at the Chicago World’s Fair, where light bulbs were lit wirelessly with high frequency ambient electric fields. Medical Implants have been using wireless charging systems since the 1960s. Another common example of wireless charging systems is the electric toothbrush, which has been charged wirelessly since the 1990s. More recently, wireless charging mats and other wireless charging accessories for cell phones and related portable devices have appeared, and this technology will soon be a standard feature in many cell phone, laptop, and tablet computer models. Wireless Energy Transfer Techniques Transformers and Induction Perhaps the most familiar example of a wireless energy transfer system is a transformer. In a transformer, a coil of wire with alternating current generates a time-varying magnetic flux, which couples into an adjacent coil of wire and generates a corresponding current on the secondary coil through magnetic induction. The coils are often wound around a ferromagnetic core or may only be separated by an air gap. Use of a core concentrates the bulk of the magnetic field in the core and provides an efficient “bridge” between the two coils, improving transformer efficiency. However, ferrite cores add significant weight to devices, and a solid core between the two coils makes it difficult to remove one when desired. In a wireless inductive charging system, the primary coil resides in the charging device, and the secondary coil is located in the portable device. Therefore it is necessary to use either an air gap transformer, or a split-core transformer with an air gap between the two cores to improve the efficiency. When the secondary coil is brought in close proximity to the primary coil, the transformer is formed and energy transfer occurs. This method of wireless energy transfer can be fairly efficient as the majority of the magnetic flux resides in the core of the transformer, and therefore losses due to leakage fields are low except at the air gap between the two cores. Air gap transformers are not as efficient as magnetic core transformers due to interaction of the magnetic field with nearby objects, dissipating additional power and thus reducing efficiency. This interaction is a source of additional heat in the nearby boards as the induced currents in the circuitry dissipate resistively, and can impact thermal design considerations. Due to the need for a small air gap even in split core transformers, the efficiency is lower than more traditional transformer designs, and as the air gap increases the effectiveness of the coupling is decreased significantly. For these reasons this method is only useful for energy transfer over short distances. Air gap transformers which do not use magnetic cores are generally more effective over larger distances, but performance will suffer as the coils are separated, and losses due to interaction with nearby objects are larger since the field is not concentrated in a core. Therefore the potential for heating nearby boards increases correspondingly. Use of coils with small diameters concentrates the power but requires more precise coupling between them. In contrast, larger coils will transfer some energy even if they are not optimally aligned, but suffer from the aforementioned increased interaction with the host device and other nearby objects. Radio Coupling Radio communication utilizes energy transfer to convey information, by producing a time- varying electromagnetic field at a specific frequency which is modulated in some fashion, and which radiates outward from the source and induces a corresponding current in nearby receiving antennas. In theory, this induced current can be used to trickle charge a battery or power a product directly if the device power requirements are not large. Depending upon the antenna type used, the radiation pattern can either be directional or isotropic. Radio coupling methods suffer from much more inefficiency than transformers as the electromagnetic field strength drops rapidly as distance increases from the source. Unless extremely directional antennas are used, much of the energy is radiated into free space rather than at the intended recipient. Directional antennas can improve this, but achieving optimum coupling at large distances with two directional antennas can be difficult. The radio coupling method has the benefit of allowing multiple devices to be powered from the same source simultaneously - if the device power requirements are low enough – and also allows devices to be charged at longer distances than transformers, including applications such as charging “hot spots” or room-wide charging fields. Isotropic receiving antennas can be used to allow coupling at multiple angles which is an important consideration for portable devices. Electro-dynamic Induction Electro-dynamic Induction is a variation on transformer design which uses tuned resonant coils to transfer energy from the primary to the secondary coil at high efficiency over short distances (usually less than a wavelength of the frequency being used). A capacitor is used to form an LRC oscillator that rings at the frequency used. When the primary coil is driven, a large amount of energy is stored in the capacitor and coil which dissipates slowly. Since both coils resonate at the same frequency the coupling efficiency between the primary and secondary coils is high, and while losses through distance do occur, a large amount of energy can still be transferred. Electro-dynamic induction is especially suitable for high power applications, where large charging currents are needed. In these situations the high power stored in the coil can be significantly dissipated by dielectric and resistive losses in the coil windings and cores, so air-core designs may be beneficial. Regulatory Issues Wireless charging systems have a unique set of regulatory considerations. As devices which intentionally generate and radiate energy, they are similar to transmitters, but since they do not always communicate, they often fall under different rules. Due to the fact that wireless charging systems need to be of a much higher power than most transmitters, issues such as Human RF exposure become even more important. Systems which radiate high power also run a higher risk of interfering with adjacent devices. Testing which quantifies the radiated output power and harmonic emissions, as well as any other unwanted emissions, is generally required for most inductive charging systems. Communication One important decision when implementing a wireless energy transfer system is