Wireless Power Transfer Explained: How It Works and How to Design It

_Wireless power transfer has a reputation for being complex. It's not. The physics is the same as a conventional transformer — alternating current through one coil induces current in another through mutual inductance. The only difference is that in wireless transfer, the two coils are separated by air instead of sharing a ferrite core._

_That air gap makes it both interesting and challenging. Efficiency drops without a core to guide the magnetic flux, and coil geometry, operating frequency, and resonant matching become the key design variables._

The physics: inductive coupling

AC through the transmitter coil creates an alternating magnetic field. A receiver coil placed in that field has a voltage induced in it — Faraday's law in action.

The coupling coefficient k describes how much of the primary's flux links with the secondary. k = 1 is perfect coupling (transformer with a core). k = 0 is no coupling. Wireless power systems operate at k = 0.1–0.8 depending on geometry and separation.

Coupling depends on: coil diameter and turns count, separation distance (falls roughly with the cube of distance), and alignment — coaxial alignment maximises coupling, angular or lateral offset reduces it.

Resonant vs non-resonant

Non-resonant: drive primary with high-frequency AC, receiver rectifies and regulates the output. Simple. Efficiency 50–70% at close range. Fine where simplicity matters more than efficiency.

Resonant: add capacitors to both coils to form LC resonant circuits tuned to the same frequency. At resonance, reactive power circulates in the coils rather than being dissipated — dramatically improving coupling efficiency. Well-designed resonant systems hit 85–95% at close range.

Resonant frequency: f = 1 / (2π × √(L × C))

Both coils must be tuned to the same frequency. Small detuning from component tolerances or coil movement causes efficiency to drop sharply — why commercial Qi chargers include alignment detection circuits.

Operating frequency

• 100–200 kHz (Qi standard): larger coils, simpler rectification, well-understood technology. Best for multi-millimetre separation. Pre-wound Qi coil modules are inexpensive and readily available — the practical starting point for most DIY designs.
• 1–10 MHz: smaller coils, more compact, but switching losses increase and rectification gets harder. Better for through-material transfer — powering sensors inside sealed enclosures or through non-metallic walls.

A simple DIY wireless power circuit

Transmitter: a Royer oscillator — two transistors cross-coupled — is a classic self-oscillating driver that automatically resonates with the tank circuit. Simple to build, frequency automatically adjusts. Alternatively, a 555 timer or dedicated driver IC (XKT-510, SG3525) with a MOSFET half-bridge.

Receiver: bridge rectifier (Schottky diodes for lower voltage drop) smoothed with a filter capacitor. Add an LDO or switching regulator for regulated output voltage.

For efficiency, use Litz wire for the coils — especially above 100kHz. Standard solid wire wastes most of its cross-section to the skin effect at RF. Litz wire (many individually insulated strands twisted together) presents much lower effective resistance at high frequencies.

Heat and efficiency: what to watch

Power not transferred appears as heat in the coil wire (I²R losses), switching transistors (switching losses), and rectifier diodes.

At 100kHz, even a 0.3V Schottky forward voltage dissipates significant power at several amps. Synchronous rectifiers — MOSFETs switching in sync with the AC — have milliohms of resistance instead of 0.3V drop. For anything above a few watts, synchronous rectification makes a meaningful efficiency difference.

Keep the coils aligned and at the minimum practical separation. The air gap is where most efficiency is lost — every millimetre of extra separation hurts.

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