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.
Related Tools & Projects
Ready to put this into practice?
Try Wireless Power Transfer Calculator