Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
IEC 62573-2011 Wireless Power Transfer General Requirements
Overview and Scope
IEC 62573-2011, titled “Wireless power transfer — General requirements and guidelines,” establishes the foundational framework for wireless power transfer (WPT) systems. This standard addresses the key technical aspects of WPT including system classification, performance metrics, safety requirements, and interoperability guidelines. As the first international standard specifically dedicated to wireless power technology, it paved the way for the widespread adoption of wireless charging in consumer electronics, industrial applications, and electric vehicles.
The standard covers both inductive coupling and resonant coupling methods, which represent the two dominant approaches to wireless power transmission over short to medium distances. It defines terminology, measurement methods for power transfer efficiency, electromagnetic compatibility (EMC) requirements, and guidelines for foreign object detection (FOD) and living object protection (LOP).
IEC 62573 classifies WPT systems by power level (micro-power < 5 W, low-power 5–50 W, mid-power 50–500 W, high-power > 500 W) and by coupling type (tightly coupled inductive, loosely coupled resonant). This classification framework enables manufacturers and regulators to apply appropriate safety and performance requirements based on the specific application category.
| Power Class | Power Range | Typical Applications | Coupling Type | Frequency Range |
|---|---|---|---|---|
| Micro-power | < 5 W | Wearables, hearing aids, medical implants | Inductive | 100–500 kHz |
| Low-power | 5–50 W | Smartphones, tablets, cameras | Inductive / Resonant | 100–300 kHz (Qi), 6.78 MHz (AirFuel) |
| Mid-power | 50–500 W | Laptops, drones, power tools, kitchen appliances | Resonant | 6.78 MHz, 85 kHz |
| High-power | > 500 W | Electric vehicles, industrial AGVs, robotic chargers | Resonant | 85 kHz (SAE J2954) |
Inductive Coupling Fundamentals
Inductive power transfer (IPT) relies on the principle of magnetic induction between a primary coil (transmitter) and a secondary coil (receiver). When alternating current flows through the primary coil, it generates a time-varying magnetic field that induces a voltage in the secondary coil. The power transfer capability depends on the mutual inductance between the coils, which is a function of coil geometry, alignment, separation distance, and the magnetic properties of the medium between them.
The standard defines the coupling coefficient k as the key figure of merit for inductive systems:
k = M / sqrt(L1 * L2), where M is the mutual inductance and L1, L2 are the self-inductances of the primary and secondary coils. For tightly coupled systems, k typically ranges from 0.3 to 0.9, while loosely coupled resonant systems operate with k as low as 0.01 to 0.3. The uncompensated power transfer capability is proportional to k², making coil design and alignment critical factors in system performance.
The choice of operating frequency involves a fundamental trade-off. Higher frequencies allow smaller coil dimensions for a given power level (advantageous for portable devices) but increase switching losses in the power electronics and eddy current losses in nearby conductive objects. For EV charging at 85 kHz, the coil diameters typically range from 200–600 mm, while smartphone chargers at 100–300 kHz use coils of 30–50 mm diameter.
Resonant Coupling and Compensation Networks
Resonant inductive power transfer (RIPT) introduces capacitive compensation networks on both the primary and secondary sides to improve power transfer efficiency and enable operation over larger air gaps. The standard describes four basic compensation topologies: series-series (SS), series-parallel (SP), parallel-series (PS), and parallel-parallel (PP). Each topology exhibits different characteristics in terms of output voltage regulation, load dependence, and tolerance to coil misalignment.
The SS topology is particularly popular for EV charging because the primary-side compensation capacitance is independent of the coupling coefficient and load condition, simplifying the system design. The SP topology offers better voltage regulation at the cost of more complex primary-side tuning. The standard provides detailed design equations for each topology, including calculation of resonant frequency, quality factor, and input impedance characteristics.
Foreign object detection (FOD) is a critical safety requirement for all WPT systems operating above 5 W. Metallic objects (coins, keys, tools) placed in the magnetic field can experience significant eddy current heating, reaching temperatures exceeding 100°C within seconds. IEC 62573 requires FOD systems to detect metallic objects of 10 mm diameter or larger within 100 ms and shut down power transfer before hazardous heating occurs.
Engineering Design Insights
Designing a high-performance WPT system requires careful optimization across multiple domains: electromagnetic, thermal, power electronic, and control systems. The coil design is the most critical element. For a given application, engineers must optimize the coil geometry (number of turns, winding pattern, layer structure), ferrite shielding configuration, and aluminum backplate design to achieve the target coupling coefficient while minimizing stray fields and meeting EMC limits.
The power electronic converter topology significantly influences system cost and performance. Full-bridge resonant converters with zero-voltage switching (ZVS) are the dominant topology for mid-to-high power WPT systems, achieving peak efficiencies of 95–98%. The control strategy typically employs primary-side frequency control for output regulation, with secondary-side communication providing feedback via in-band (load modulation) or out-of-band (Bluetooth, NFC) channels.
Thermal management is often the limiting factor in WPT system design. The coils and ferrite shields experience both ohmic losses (proportional to I²R) and core losses (proportional to frequency and flux density). At 85 kHz and 11 kW (typical EV charging), total losses in the coil assembly can reach 300–500 W, requiring active cooling solutions such as liquid-cooled coils or forced-air ventilation. The standard’s thermal design guidelines recommend keeping ferrite temperatures below 100°C to prevent demagnetization and coil temperatures below the insulation class rating.
The interoperability framework defined in IEC 62573 has been instrumental in the commercial success of wireless charging. By specifying minimum performance requirements rather thanmandating specific coil geometries, the standard enabled the development of competing ecosystems (Qi, AirFuel, SAE J2954) while ensuring that devices from different manufacturers can achieve basic charging functionality. This “minimum interoperability” approach has been widely adopted as a model for other wireless power standards.
EMC and Safety Requirements
Electromagnetic compatibility is a major consideration for WPT systems due to the strong magnetic fields they generate. IEC 62573 references IEC 61000 series standards for emission limits and immunity requirements. The standard specifies that WPT systems must comply with human exposure limits defined in ICNIRP guidelines, which set maximum magnetic flux density limits of 27 µT at 85 kHz for general public exposure and 100 µT for occupational exposure.
Safety requirements cover electrical safety (insulation coordination, touch current limits), thermal safety (maximum surface temperatures), mechanical safety (enclosure strength, moving parts protection), and functional safety (fail-safe operation, fault detection). The standard requires a minimum of two independent protection mechanisms for each identified hazard, ensuring that a single fault cannot lead to a dangerous condition.
Frequently Asked Questions
How does wireless charging affect battery life?
Wireless charging itself does not inherently reduce battery life compared to wired charging. The battery degradation effects are primarily determined by the charging profile (constant current / constant voltage), charge termination voltage, and thermal conditions — all of which are controlled by the battery management system (BMS), not the power transfer method. However, wireless charging typically generates more heat in the device due to coil and power electronic losses, which can accelerate battery aging if not properly managed. Well-designed WPT systems maintain receiver temperatures within 5–10°C of wired charging temperatures through efficient coil design and thermal management.
What is the maximum distance for wireless power transfer?
For practical applications, the maximum efficient transfer distance depends on the coil size relative to the air gap. Generally, efficient power transfer is achievable at distances up to approximately one coil diameter for inductive coupling and up to 2–3 coil diameters for resonant coupling. Beyond these distances, efficiency drops below useful levels. For EV charging (coil diameter ~400 mm), the practical gap is 100–250 mm. For smartphone charging (coil diameter ~40 mm), the gap is typically 3–8 mm. Long-range WPT (kilometers) using microwave or laser beams exists for specialized aerospace applications but is not covered by IEC 62573 due to safety and efficiency concerns at scale.
Do multiple devices on one charger reduce efficiency?
Modern multi-device chargers use either time-division multiplexing (charging one device at a time in sequence) or multi-coil arrays (multiple independent transmitter coils). In time-division systems, total efficiency is similar to single-device charging since only one coil is active at any moment. Multi-coil arrays can experience crosstalk between adjacent coils and reduced efficiency when multiple devices charge simultaneously, typically dropping from 75–80% (single device) to 65–72% (three devices). The standard provides guidelines for coil array design to minimize crosstalk through proper spacing, orthogonal winding orientations, and active cancellation techniques.
