**Abstract:**
With the release of the iPhone X, which supports wireless charging, many users have found it challenging to find a compatible wireless charger. Although the iPhone X is compatible with the Qi wireless charging standard, there are still limited options available on the market. In this article, we will explain the basics of wireless charging technology, the Qi communication standard, and introduce five recommended wireless chargers for the iPhone X.
**Introduction to Wireless Charging Technology**
Wireless charging technology (also known as wireless power transfer) is derived from the concept of transferring energy without physical connections. It can be categorized into low-power and high-power wireless charging. Low-power wireless charging typically uses electromagnetic induction, such as the Qi standard used in smartphones. High-power wireless charging, often used in electric vehicles, usually relies on magnetic resonance. The power supply device transfers energy to the receiving device, which then charges the battery and powers itself simultaneously.
Because the energy is transmitted through a magnetic field between the charger and the device, there is no need for direct electrical contact. This makes wireless charging safer and more convenient, especially in environments where wires could pose a risk.
**Basic Principles of Wireless Charging**
**Electromagnetic Induction**
This method involves two coils: one in the charger and one in the device. When alternating current flows through the primary coil, it induces a current in the secondary coil, transferring energy wirelessly. This is the most common method used in smartphones, including the iPhone X. The technology is not new—Chinese automaker BYD had already developed non-contact inductive charging as early as 2005.
**Magnetic Field Resonance**
This technique involves tuning both the transmitter and receiver to the same frequency so they can efficiently exchange energy. Researchers at MIT, led by Professor Marin Soljacic, have explored this method, even lighting a 60-watt bulb from two meters away using a large coil. While promising, the size of the components currently limits its practical use in consumer devices.
**Radio Wave Energy Transfer**
This method uses microwave transmitters and receivers to capture and convert radio wave energy into usable power. It’s a more mature technology and similar to how old radios worked. A wall-mounted transmitter sends energy, while a small receiver in the device converts it into stable DC power. This system offers flexibility but is less efficient than other methods.
**Introduction to the Qi Wireless Charging Standard**
The Qi (pronounced "chee") standard is a global specification for wireless charging, developed by the Wireless Power Consortium (WPC). It ensures compatibility between different manufacturers’ products. All Qi-certified chargers must meet strict technical requirements, ensuring safe and efficient power transfer.
**Qi Wireless Charging Communication Standard**
Under the WPC standard, wireless charging typically operates at power levels between 0 and 5 watts. The system uses inductive coupling between two planar coils to transfer power from the transmitter to the receiver. The distance between the coils is usually around 5 mm.
Communication between the charger and the device occurs via backscatter modulation, where the receiver adjusts its load to send information back to the transmitter. This process includes several steps:
1. **Analog Ping**: The transmitter detects the presence of a device.
2. **Digital Ping**: The transmitter sends a signal, and the device responds with a signal strength packet.
3. **Identification & Configuration**: The device provides setup and configuration data to the transmitter.
4. **Power Transmission**: The receiver sends control packets to adjust the power level, and the transmitter continuously monitors and adjusts accordingly.
5. **Termination**: If the device stops communicating or sends a “terminate charging†message, the transmitter enters a low-power state.
This protocol ensures that wireless charging is both efficient and safe, making it an ideal choice for modern smartphones like the iPhone X.
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