Electronics Guide

Acoustic Power Transfer

Transmit energy using sound waves. This section addresses ultrasonic power transfer, acoustic transducer design, impedance matching layers, acoustic focusing techniques, through-metal power transfer, through-water power transfer, implantable device powering, acoustic rectification, parametric acoustic arrays, beamforming for acoustic power, safety limits for acoustic power, acoustic power for sensors, multi-frequency acoustic transfer, and acoustic energy storage.

Inductive Power Transfer

Transfer power via magnetic coupling between coils. Topics include loosely coupled systems, tightly coupled systems, resonant inductive coupling, Qi wireless charging standard, A4WP/Rezence standard, foreign object detection, coil design optimization, ferrite core materials, shielding techniques, dynamic wireless charging, wireless power for vehicles, implantable device charging, underwater wireless power, and three-dimensional wireless power.

Magnetic Resonance Coupling

Extend wireless power range using resonant magnetic coupling between tuned transmitter and receiver coils. Topics include resonance theory and quality factor optimization, loosely coupled resonators, impedance matching networks, frequency tracking, multi-coil relay systems, AirFuel resonant standard, and applications in electric vehicle charging, robotics, and consumer electronics requiring greater spatial freedom.

Microwave Power Transmission

Beam power over long distances using focused microwave energy. Coverage includes rectenna (rectifying antenna) design, high-efficiency microwave sources, beam forming and steering, atmospheric propagation, safety considerations, power beaming demonstrations, and ambitious applications such as space-based solar power satellites and drone recharging in flight.

Laser Power Transmission

Deliver power via concentrated laser beams to photovoltaic receivers. This section addresses laser source selection, beam collimation and pointing, specialized photovoltaic cells for laser wavelengths, thermal management, safety interlocks, and applications including powering unmanned aerial vehicles, remote sensors, and underwater systems.

Capacitive Power Transfer

Transfer energy through electric field coupling between capacitive plates. Topics include coupling capacitance optimization, high-frequency inverter design, compensation networks, comparison with inductive approaches, and applications in rotating machinery, underwater systems, and situations where magnetic interference is problematic.

Dynamic Wireless Charging

Charge electric vehicles while in motion on specially equipped roadways. This section covers in-road coil infrastructure, vehicle-mounted receivers, high-power electronics, segmented transmitter activation, vehicle detection and communication, alignment tolerance, efficiency at speed, grid integration, and pilot project experiences worldwide.

Near-Field versus Far-Field Transfer

Wireless power transfer mechanisms divide into near-field and far-field categories based on the relationship between transmission distance and wavelength. Near-field techniques, including inductive and capacitive coupling, operate within the reactive near-field region where electromagnetic fields do not propagate as waves but rather oscillate in place. Energy transfer occurs through direct field coupling, with efficiency highly dependent on coupling coefficient and thus on distance and alignment.

Far-field power transfer uses propagating electromagnetic waves to carry energy over distances much greater than a wavelength. Microwave and laser systems operate in this regime, potentially spanning kilometers but facing fundamental challenges including beam divergence, atmospheric absorption, and the need for precise pointing. Far-field systems achieve distance at the cost of reduced efficiency and increased safety complexity compared to near-field approaches.

Coupling Coefficient and Quality Factor

Two parameters fundamentally govern near-field wireless power transfer performance: coupling coefficient (k) and quality factor (Q). The coupling coefficient describes what fraction of magnetic flux generated by the transmitter coil passes through the receiver coil, ranging from 0 (no coupling) to 1 (perfect coupling). Closely spaced, well-aligned coils with similar sizes achieve high coupling, while separation, misalignment, or size mismatch reduce k.

Quality factor quantifies the ratio of energy stored to energy dissipated per cycle in a resonant circuit. High-Q resonators maintain strong oscillations with minimal loss, enabling efficient energy transfer even with weak coupling. The product kQ determines system performance: traditional inductive transfer relies on high k, while magnetic resonance coupling compensates for lower k with very high Q values, extending effective range.

Resonance and Frequency Selection

Resonant wireless power transfer operates transmitter and receiver circuits at their natural resonant frequency, maximizing energy storage and transfer while minimizing losses. The operating frequency profoundly impacts system design: lower frequencies reduce skin effect losses and simplify electronics but require larger coils and may limit power density, while higher frequencies enable compact coils but increase switching losses and electromagnetic interference concerns.

Regulatory bodies allocate specific frequency bands for wireless power applications. The Industrial, Scientific, and Medical (ISM) bands, including 6.78 MHz and 13.56 MHz, see common use for wireless power systems. Consumer electronics standards typically operate in the 100-300 kHz range for inductive charging, while high-power applications may use lower frequencies to manage switching losses in power electronics.

Transmitter Design

Wireless power transmitters convert DC or line-frequency AC into high-frequency AC to drive the transmitting coil or antenna. Power inverters using switching transistors (typically MOSFETs or GaN devices for higher frequencies) generate the AC drive signal. Resonant inverter topologies including series resonant, parallel resonant, and LLC configurations achieve soft switching, reducing losses and electromagnetic interference.

The transmitting coil or antenna design optimizes magnetic field generation while minimizing losses. Litz wire construction reduces AC resistance from skin and proximity effects at typical operating frequencies. Ferrite shielding concentrates magnetic flux toward the receiver and reduces stray fields. Foreign object detection circuitry identifies metallic objects that could dangerously heat in the alternating magnetic field, triggering protective shutdown.

Receiver Design

The receiver captures transmitted energy and converts it to the DC voltage required by the load. Receiving coils must match transmitter characteristics for optimal coupling, with similar considerations for wire construction and shielding. Rectification circuits convert the high-frequency AC to DC, with synchronous rectification using actively controlled switches improving efficiency over passive diode rectifiers.

Voltage regulation downstream of the rectifier provides stable output despite variations in coupling, transmitter power, and load requirements. Communication between receiver and transmitter, often using modulation of the power carrier itself, enables closed-loop power control, authentication, and safety signaling. Battery charging applications integrate specialized charging profiles for lithium-ion or other battery chemistries.

Compensation Networks

Compensation networks tune transmitter and receiver circuits to resonance and transform impedances for optimal power transfer. Series compensation places a capacitor in series with the coil, while parallel compensation uses a shunt capacitor. More complex topologies including LCC and LCL configurations provide additional design freedom for impedance matching and voltage or current source behavior.

Proper compensation network design maximizes efficiency across the expected range of coupling coefficients and load conditions. Bifurcation, where multiple stable operating points exist, must be avoided through appropriate topology selection and control. Adaptive tuning can compensate for component variations and changing coupling conditions, maintaining optimal operation as the receiver moves or environmental conditions change.

Control and Communication

Sophisticated control systems manage wireless power transfer for efficiency, safety, and interoperability. Transmitters regulate output power based on receiver feedback, reducing power when the battery nears full charge or increasing it during high-demand periods. Control loops maintain stable operation despite coupling variations from receiver movement or changing loads.

In-band communication modulates the power carrier to exchange data between transmitter and receiver without requiring separate radio transceivers. Standardized protocols enable interoperability between devices from different manufacturers. Authentication mechanisms prevent unauthorized devices from drawing power, while encrypted communication protects against tampering with power control messages.

Qi Standard

The Qi (pronounced "chee") standard, developed by the Wireless Power Consortium, dominates the consumer electronics wireless charging market. Qi specifies inductive power transfer at frequencies between 80 and 300 kHz, with baseline power profiles from 5 W to extended power profiles reaching 15 W or higher for compatible devices. The standard defines coil dimensions, communication protocols, and foreign object detection requirements.

Qi's widespread adoption ensures that smartphones, earbuds, smartwatches, and other devices from different manufacturers can charge on any Qi-certified pad. The standard continues evolving with Qi2, incorporating magnetic alignment (derived from Apple's MagSafe technology) for improved positioning and higher power transfer efficiency. Certification programs ensure interoperability and safety compliance.

AirFuel Standards

AirFuel Alliance promotes both resonant and RF-based wireless power standards. The AirFuel Resonant standard operates at 6.78 MHz, enabling greater spatial freedom than tightly coupled inductive systems. Multiple devices can charge simultaneously with more tolerance for positioning, though the higher frequency presents different design challenges.

AirFuel RF enables power delivery over greater distances using radio frequency energy harvesting. While power levels are lower than near-field approaches, RF power can reach devices scattered throughout a room without precise positioning. This approach suits low-power IoT sensors and other applications where convenience outweighs power transfer efficiency.

Electric Vehicle Charging Standards

SAE International's J2954 standard specifies wireless power transfer for light-duty electric vehicles at power levels from 3.7 kW (WPT1) through 11 kW (WPT3) and beyond. The standard defines ground and vehicle assembly requirements, interoperability testing, and safety provisions. Operating at 85 kHz, J2954 systems must accommodate substantial ground clearance and alignment tolerance inherent in automotive applications.

Heavy-duty vehicle applications require even higher power levels for commercial fleet charging. Standards development continues for megawatt-class wireless charging suitable for electric buses and trucks. Dynamic wireless charging for vehicles in motion remains an active area of standardization, with various pilot projects informing specification development.

Sources of Power Loss

Wireless power systems incur losses at multiple stages. Inverter switching and conduction losses consume power in the transmitter electronics, with efficiency depending on semiconductor technology and switching frequency. Coil losses from AC resistance (skin and proximity effects) and core losses in ferrite materials reduce magnetic coupling efficiency. Rectifier and regulation circuits add further losses on the receiver side.

Coupling efficiency, determined by the coupling coefficient and coil quality factors, represents the fundamental wireless link efficiency. Misalignment, increased separation, and metallic objects in the field all reduce coupling and thus overall efficiency. End-to-end efficiency from wall plug to battery or load typically ranges from 70-90% for well-designed, well-aligned systems, decreasing significantly with distance or misalignment.

Optimization Strategies

Maximizing wireless power transfer efficiency requires optimization across the entire system. High-quality-factor coils using appropriate wire construction and shielding materials minimize magnetic losses. Wide-bandgap semiconductors such as GaN and SiC enable higher switching frequencies and lower losses in power electronics. Resonant converter topologies achieve soft switching, dramatically reducing switching losses compared to hard-switched alternatives.

System-level control optimizes operating point for instantaneous conditions. Maximum efficiency point tracking adjusts frequency, phase, or duty cycle to maintain optimal operation as coupling varies. Power matching ensures that transmitted power matches receiver needs, avoiding wasted energy. Thermal management becomes critical at higher power levels, with heat sinking and active cooling maintaining semiconductor and coil temperatures within safe operating ranges.

Electromagnetic Field Exposure

Wireless power systems generate electromagnetic fields that must remain within regulatory limits for human exposure. Near-field systems create strong magnetic fields in the immediate vicinity of coils, requiring careful design to limit exposure of nearby body parts. International Commission on Non-Ionizing Radiation Protection (ICNIRP) guidelines specify frequency-dependent limits that wireless power systems must respect.

Coil shielding and field shaping concentrate magnetic flux in the intended coupling region while minimizing stray fields. Active field cancellation techniques can further reduce exposure. Far-field systems using microwave or laser beams present different hazards requiring exclusion zones, interlock systems, and beam power reduction when obstructions or personnel are detected.

Foreign Object Detection

Metallic objects in the alternating magnetic field of a wireless power transmitter can experience induced currents, causing significant heating. Keys, coins, or other metal items left on a charging pad could reach dangerous temperatures. Foreign object detection (FOD) systems identify such objects and reduce or halt power transmission to prevent fire or burn hazards.

FOD techniques include monitoring for anomalous power draw, measuring quality factor changes, and using auxiliary sensor arrays to detect object presence. Thermal sensors provide backup protection. Standards mandate FOD capability, with certification testing verifying detection sensitivity across object sizes and positions. Living tissue detection addresses the specific case of fingers or other body parts entering the charging zone.

Electromagnetic Compatibility

Wireless power systems must coexist with other electronic equipment without causing harmful interference. The high-power, high-frequency switching inherent in wireless power can generate significant electromagnetic emissions across a broad spectrum. Shielding, filtering, and spread-spectrum techniques reduce emissions to comply with regulatory limits.

Medical devices present particular concerns, with pacemakers and other implantable electronics potentially susceptible to wireless power fields. Warning labels and distance guidelines address this risk for consumer devices, while medical-grade wireless power systems undergo extensive testing and incorporate additional safety features. Industrial applications must similarly ensure compatibility with surrounding equipment and control systems.

Consumer Electronics

Wireless charging has become standard in smartphones, smartwatches, earbuds, and other personal electronics. The convenience of simply placing a device on a charging pad, without fumbling with cables or worrying about connector wear, drives widespread adoption. Integration into furniture, vehicles, and public spaces extends charging availability throughout daily life.

Electric Vehicles

Wireless charging eliminates the need to physically connect electric vehicles to charging infrastructure. Stationary wireless charging in parking spaces and garages provides convenient home and destination charging. Dynamic wireless charging from electrified roadways could enable indefinite range for vehicles, though infrastructure costs remain challenging. Autonomous vehicles particularly benefit from wireless charging, enabling fully automated fleet operations without human intervention for charging.

Medical Implants

Implantable medical devices including pacemakers, cochlear implants, neurostimulators, and ventricular assist devices can receive power wirelessly through the skin, eliminating the infection risk of percutaneous power connections. Transcutaneous energy transfer systems must operate within strict safety limits while providing reliable power for life-critical functions. Advances in wireless power enable more capable implants that would be impractical with battery power alone.

Industrial Automation

Automated guided vehicles, robots, and rotating machinery benefit from wireless power that eliminates cable wear and connector maintenance. Sealed equipment operating in harsh environments can receive power without exposed electrical contacts susceptible to contamination or corrosion. Wireless power enables new automation architectures with greater flexibility and reduced maintenance requirements.

Internet of Things

Distributed sensors and IoT devices can receive power wirelessly, eliminating battery replacement logistics that limit deployment scale. RF energy harvesting powers devices throughout a room from a central transmitter. Combined with energy harvesting from ambient sources, wireless power delivery can maintain perpetual operation of sensor networks in buildings, infrastructure, and industrial facilities.