Electromagnetism & electronics

Qi wireless charger

Electromagnetism · Inductive power transfer

How does a phone charge just by lying on a pad, with no wire plugged in?

The charging pad and the phone each hide a flat coil of wire. Electricity rushing back and forth in the pad’s coil makes an invisible magnetic field that flips direction over a hundred thousand times a second. That flipping field passes through the phone’s coil and makes electricity flow in it, which charges the battery.

A Qi wireless charger is an air-gap transformer: an alternating current in a flat transmitter coil produces a changing magnetic field, and the part of that flux that threads the phone’s receiver coil induces a voltage there. Resonant capacitors, ferrite sheets and close alignment keep the coupling and efficiency high enough to charge a battery at 5–15 W.

Key facts

Operating frequency

≈ 110–205 kHz (Qi Baseline Power Profile)

Power levels

5 W (BPP), up to 15 W (EPP, Qi2 MPP)

Coil-to-coil distance

≈ 3–5 mm through pad top and phone back

Coupling coefficient k

≈ 0.3–0.7 when aligned; falls fast with offset

End-to-end efficiency

≈ 70–80 % aligned (wired charging ≈ 85–95 %)

Feedback link

Receiver → pad by load modulation, ≈ 2 kbit/s

Parts you can explore (11)

• Transmitter coil
• Ferrite shield
• Inverter board
• Charging pad housing
• Power input
• Magnetic field
• Receiver coil
• Receiver ferrite
• Rectifier and charger IC
• Battery
• Phone body

In the 3D model

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