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IEC 63028: AirFuel Alliance Resonant Baseline for Wireless Power Transfer
System specification for 6.78 MHz resonant inductive charging with spatial freedom
1. Scope and Framework of IEC 63028
IEC 63028 specifies the AirFuel Alliance resonant baseline system specification for wireless power transfer (WPT). It defines the system architecture, communication protocol, power transfer control, and interoperability requirements for resonant inductive coupling WPT systems operating in the 6.78 MHz (ISM band) frequency range. Developed in collaboration with the AirFuel Alliance, this standard enables cross-vendor interoperability for wireless charging of consumer electronics, small appliances, and — through scaling — industrial and medical devices. The 6.78 MHz resonant approach offers significant advantages over tightly coupled inductive solutions: extended spatial freedom (multi-device charging on a single surface), tolerance to lateral and vertical misalignment, and the ability to charge through non-metallic obstacles.
The standard covers the full system stack: the power transmitter (charging surface or pad), the power receiver (in the device), and the bi-directional communication link that controls power delivery. Unlike the tightly coupled Qi standard (operating at 100–200 kHz), IEC 63028’s resonant approach uses loosely coupled magnetic resonance, where both the transmitter and receiver coils are tuned to the same resonant frequency. This enables simultaneous charging of multiple devices with different power requirements on the same transmitter surface — a feature known as “spatial freedom” or “free-positioning.”
The 6.78 MHz operating frequency is allocated as an ISM (Industrial, Scientific, and Medical) band globally, which means no special radio licensing is required for IEC 63028 compliant devices in most jurisdictions. This is a key practical advantage for global product deployment.
2. Technical Specifications and Communication Protocol
2.1 Power Transfer Categories and Coil Specifications
IEC 63028 defines five power categories, from Category 1 (up to 2 W, suitable for wearable devices and hearing aids) through Category 5 (up to 70 W, suitable for laptops, tablets, and small kitchen appliances). The baseline resonant coil geometry is specified as a planar spiral with a nominal outer diameter of 50 mm for the transmitter and 40 mm for the receiver, wound with Litz wire to minimize AC resistance at 6.78 MHz. The coil quality factor (Q) must be at least 100 at the operating frequency to achieve the resonant coupling efficiency. Table 1 summarizes the power categories.
| Category | Maximum output power | Typical applications | Receiver coil OD | Minimum system efficiency |
|---|---|---|---|---|
| 1 | 2 W | Hearing aids, smart rings, earbuds | 15 mm | 55 % |
| 2 | 10 W | Smartphones, wireless earbuds cases | 25 mm | 65 % |
| 3 | 30 W | Tablets, phablets, portable speakers | 35 mm | 72 % |
| 4 | 50 W | Ultrabooks, small power tools | 45 mm | 75 % |
| 5 | 70 W | Large laptops, monitors, kitchen scales | 55 mm | 78 % |
2.2 Bluetooth LE Communication and Foreign Object Detection
The control communication between transmitter and receiver uses Bluetooth Low Energy (BLE) advertising packets operating in the 2.4 GHz ISM band. The receiver broadcasts its power request, device identification, and status information; the transmitter acknowledges and adjusts its output accordingly. The BLE link operates independently of the power transfer channel, allowing the communication to function even before power transfer begins — essential for the initialization handshake. The standard specifies a maximum connection latency of 100 ms for power control commands, ensuring stable closed-loop regulation of the output voltage.
Foreign Object Detection (FOD) is a critical safety feature mandated by IEC 63028. The transmitter must detect the presence of metallic objects (coins, keys, paper clips) on the charging surface that could be inductively heated by the resonant field. FOD is implemented through a combination of methods: quality factor degradation measurement (a metallic object near the coil reduces its Q), power loss accounting (comparing transmitted and received power — a discrepancy exceeding 300 mW triggers shut-down), and, optionally, capacitive or infrared presence sensing. The standard requires FOD to trigger shut-down within 1 second of object placement.
The 6.78 MHz operating frequency can couple into the intermediate frequency (IF) stages of AM radio receivers and can interfere with some RFID systems operating in adjacent bands (6.765–6.795 MHz). IEC 63028 requires that transmitters implement active interference cancellation or adaptive frequency dithering to limit out-of-band emissions below CISPR 22 Class B limits.
3. Engineering Design Insights for Resonant WPT
3.1 Coil and Resonant Tank Design Optimization
The efficiency of an IEC 63028 resonant WPT system is dominated by the coil quality factor (Q) and the coupling coefficient (k) between the transmitter and receiver coils. At 6.78 MHz, the skin depth in copper is approximately 8 μm, making standard PCB trace coils impractical for categories above 2 due to excessive ohmic losses. Litz wire with strand diameters of 0.05 mm (50 μm) — finer than a human hair — is the standard conductor for transmitter coils. The number of strands required scales roughly with the power category: Category 3 requires approximately 100 strands of 0.05 mm, while Category 5 requires 300+ strands. The resonant capacitors (typically C0G/NP0 type for the transmitter and X7R for the receiver) must exhibit less than 0.5 % capacitance drift over the operating temperature range to prevent detuning.
The coupling coefficient between the transmitter and receiver coils in a resonant WPT system is typically 0.1–0.4 — much lower than the 0.7–0.95 typical of tightly coupled inductive systems. Despite this, the system can achieve end-to-end efficiencies above 75 % for Category 4 and 5, because the resonant tank stores reactive energy efficiently, and only the real power consumed by the load is drawn from the transmitter. This is the fundamental advantage of resonant coupling: the “reactive power” circulates in the resonant tank, not through the power source.
3.2 Power Amplifier Topology for the Transmitter
The transmitter’s power amplifier is the most critical and design-intensive component. IEC 63028-compliant transmitters almost universally use Class-D or Class-E switching amplifiers operating at 6.78 MHz. Class-E amplifiers offer the highest theoretical efficiency (100 % in an ideal switch) by shaping the voltage and current waveforms so that the switching device turns on at zero voltage (zero-voltage switching, ZVS). In practice, GaN (gallium nitride) FETs are the preferred switching devices due to their low output capacitance (Coss) and fast switching edges at 6.78 MHz. A typical 70 W (Category 5) transmitter uses a half-bridge Class-D amplifier with two 650 V GaN FETs, a spiral-wound Litz wire coil, and a digitally tuned capacitor bank for impedance matching. The DC supply voltage is typically 48 V for higher categories.
Thermal management of the power amplifier is a significant challenge: even at 90 % efficiency, a 70 W system must dissipate 7–8 W of heat within the transmitter enclosure. Without proper thermal design, the resonant capacitors’ temperature rise can cause detuning and a positive-feedback efficiency collapse. IEC 63028 requires that the transmitter maintain rated power output for at least 30 minutes at 35 °C ambient temperature without exceeding 85 °C on any component surface accessible to the user.
Multi-coil transmitter arrays (using 3–16 overlapping resonant coils) enable “free-positioning” charging surfaces where a device can be placed anywhere within a defined area and still achieve Category 3+ power levels. IEC 63028 provides the coil switching and drive sequencing guidelines necessary for implementing such arrays without violating radiated emission limits.
4. Frequently Asked Questions
Q1: How does IEC 63028 differ from the Qi standard for wireless charging?
Qi uses tightly coupled inductive charging at 100–200 kHz, requiring precise coil alignment (within 5 mm). IEC 63028 uses loosely coupled resonant charging at 6.78 MHz, offering 10–20 mm of spatial freedom in all directions. Qi is more efficient for single-device charging of phones; IEC 63028 excels at multi-device charging and higher-power applications.
Qi uses tightly coupled inductive charging at 100–200 kHz, requiring precise coil alignment (within 5 mm). IEC 63028 uses loosely coupled resonant charging at 6.78 MHz, offering 10–20 mm of spatial freedom in all directions. Qi is more efficient for single-device charging of phones; IEC 63028 excels at multi-device charging and higher-power applications.
Q2: Can an IEC 63028 receiver charge on a Qi transmitter, or vice versa?
Not directly — the operating frequencies differ by two orders of magnitude, and the communication protocols are incompatible (BLE vs. in-band load modulation). However, dual-mode receivers supporting both standards are common, and the industry is working toward a unified standard under the “Ki” initiative for wireless kitchen appliances.
Not directly — the operating frequencies differ by two orders of magnitude, and the communication protocols are incompatible (BLE vs. in-band load modulation). However, dual-mode receivers supporting both standards are common, and the industry is working toward a unified standard under the “Ki” initiative for wireless kitchen appliances.
Q3: What is the maximum practical charging distance for IEC 63028?
The standard assumes a coupling distance of 5–20 mm between the transmitter and receiver surfaces. Beyond 20 mm, the coupling coefficient drops below 0.05, and efficiency degrades rapidly. For “through-counter” charging (30–40 mm distance through granite or wood), extended coil geometries and higher Q factors are needed, but these are not covered by the baseline specification.
The standard assumes a coupling distance of 5–20 mm between the transmitter and receiver surfaces. Beyond 20 mm, the coupling coefficient drops below 0.05, and efficiency degrades rapidly. For “through-counter” charging (30–40 mm distance through granite or wood), extended coil geometries and higher Q factors are needed, but these are not covered by the baseline specification.
Q4: Does the standard address electromagnetic safety for human exposure?
Yes — IEC 63028 requires compliance with ICNIRP 2020 guidelines for both general public and occupational exposure at 6.78 MHz. The transmitter must demonstrate that the incident magnetic field (H-field) at a distance of 20 cm from the surface does not exceed 21 A/m (rms) for general public exposure, with additional testing for metallic implant safety.
Yes — IEC 63028 requires compliance with ICNIRP 2020 guidelines for both general public and occupational exposure at 6.78 MHz. The transmitter must demonstrate that the incident magnetic field (H-field) at a distance of 20 cm from the surface does not exceed 21 A/m (rms) for general public exposure, with additional testing for metallic implant safety.
