Wireless electricity supply is the transfer of electrical energy from a power source to a load across an air gap using time-varying electromagnetic fields, without physical conductors. While pop culture and sci-fi often confuse this concept with far-field radio frequency (RF) energy harvesting or microwave power beaming—where energy radiates outward in all directions like a lightbulb—practical wireless power relies almost entirely on near-field magnetic or electric coupling. In the near-field, the electromagnetic field does not radiate into space; instead, it oscillates locally to induce a current in a specifically tuned receiving coil.
The Physics of Near-Field Magnetic Coupling
At the bench level, wireless electricity supply is essentially an air-core transformer. It operates on Faraday’s Law of Induction: a changing magnetic field in the transmitter coil induces an electromotive force (EMF) in the receiver coil. The efficiency of this transfer is dictated by the mutual inductance ($M$) between the two coils and their individual self-inductances ($L_1$ and $L_2$).
Think of two water pipes separated by a flexible rubber membrane; pulsing water in the primary pipe pushes the membrane, which pushes water in the secondary pipe, transferring energy without the fluids ever mixing. The stiffer the membrane and the closer the pipes, the better the transfer.
When the air gap increases or the coils misalign, the coupling coefficient drops, leakage inductance rises, and the system must work harder to push the same amount of power across the gap. This is why precise coil alignment and magnetic shielding are non-negotiable in high-power designs.
Worked Example: Calculating Loss in a 15W Qi Charger
To understand what this means for real-world thermal management, let us look at a modern 15W smartphone charger operating under the Wireless Power Consortium (WPC) Qi2 standard.
- Target Output Power: 15W DC delivered to the phone battery.
- System Efficiency: ~70% (a realistic figure for 15W at a 4mm air gap with slight lateral misalignment).
- Required Input Power: $15W / 0.70 = 21.4W$ drawn from the wall adapter.
- Power Lost as Heat: $21.4W - 15W = 6.4W$.
That 6.4W of lost energy does not disappear; it dissipates as heat, primarily in the copper windings (I²R losses) and the ferrite shielding (core losses). In a small, enclosed plastic charging puck with no active cooling, a continuous 6.4W heat load will easily push internal coil temperatures past 60°C. This is why Qi chargers embed NTC thermistors directly against the coil to throttle the power delivery when thermal limits are reached.
Where You Meet Wireless Electricity Supply in Practice
You will encounter near-field wireless power across several distinct engineering domains, each with different frequency and distance requirements. The SAE International J2954 standard, for instance, governs the heavy-duty side of this spectrum for electric vehicles.
| Application | Governing Standard | Typical Power Level | Operating Frequency | Air Gap Distance |
|---|---|---|---|---|
| Smartphones & Wearables | Qi / Qi2 (WPC) | 5W - 15W | 110 - 205 kHz | 2 - 5 mm |
| Electric Vehicles (EVs) | SAE J2954 | 3.6 kW - 11 kW | 85 kHz | 100 - 250 mm |
| Medical Implants | Proprietary / ISO | 10 mW - 100 mW | 1 MHz - 10 MHz | 5 - 20 mm |
| Industrial AGVs & Robotics | Custom / WPC | 100W - 1 kW | 20 - 100 kHz | 10 - 30 mm |
What Changes in a Real Circuit or Installation
When you replace a physical copper wire with a wireless electricity supply link, the underlying circuit topology changes drastically. You cannot simply feed DC or 60Hz mains AC into a transmitter coil and expect power to cross an air gap. The circuit must be redesigned around high-frequency AC switching and resonance.
High-Frequency Inversion and Litz Wire
To generate a rapidly changing magnetic field, the DC input is chopped into high-frequency AC (typically 100kHz to 300kHz) using an H-bridge inverter. At these frequencies, standard silicon MOSFETs suffer from massive switching losses. Modern designs use Gallium Nitride (GaN) FETs, such as the EPC2045, which can switch at 200kHz with minimal gate charge loss.
Furthermore, the coils themselves cannot be wound with standard solid magnet wire. At 200kHz, the skin effect forces alternating current to travel only on the extreme outer edge of a solid conductor, effectively choking the wire and spiking its AC resistance. To defeat this, transmitter and receiver coils are wound with Litz wire—cables made of hundreds of individually insulated micro-strands (e.g., 1050-strand 46 AWG)—which forces the current to distribute evenly across the entire cross-section.
Resonance and Ferrite Shielding
To maximize power transfer across a loose coupling coefficient, capacitors are added in series or parallel with the coils to create an LC tank circuit. By tuning this circuit to resonate exactly at the H-bridge switching frequency, the reactive impedance cancels out, allowing real power to flow efficiently.
Frequently Asked Questions About Wireless Electricity Supply
Is wireless electricity supply safe for human exposure?
Yes, when designed to standard specifications. Near-field magnetic coupling operates at non-ionizing frequencies (typically 85kHz to 200kHz). Unlike ionizing radiation (X-rays), these magnetic fields do not have enough energy to break chemical bonds or damage DNA. However, the primary safety concern is thermal: the induced eddy currents can heat up metal objects (like jewelry or coins) left on the charging pad. This is why compliant systems include Foreign Object Detection (FOD) circuits that monitor the Q-factor of the coil and shut down power if an unexpected metallic mass is detected.
Can I use wireless electricity supply for long-distance power transmission?
No. Near-field inductive coupling follows an inverse-cube law for magnetic field strength, meaning the power transfer capability drops off aggressively once you move beyond a distance roughly equal to the diameter of the transmitter coil. While far-field technologies like microwave or laser power beaming can transmit energy over kilometers, they suffer from severe atmospheric attenuation, require precise line-of-sight targeting, and pose significant eye and tissue safety hazards. For practical DIY and commercial applications, wireless electricity supply is strictly a short-gap solution.
Why does my wireless charger get hot even when the phone is fully charged?
Even when the phone's battery management system (BMS) stops drawing current to charge the battery, the phone's internal receiver coil remains magnetically coupled to the transmitter. The transmitter continues to ping the receiver with low-power communication pulses to maintain the handshake, and the receiver's rectifier diodes and voltage regulators still draw a small quiescent current to keep the phone's logic awake. Additionally, any slight misalignment causes the transmitter to increase its drive voltage to maintain the magnetic field, which increases I²R heating in the transmitter coil even if no real power is being transferred to the battery.
What is the difference between inductive and resonant wireless power?
Basic inductive coupling (like early electric toothbrush chargers) relies on tight physical alignment and a very small air gap, operating slightly off-resonance. Resonant wireless power (used in modern Qi2 and EV chargers) adds tuning capacitors to both the transmitter and receiver to create matched LC resonant circuits. Resonance allows the system to transfer power efficiently across much larger air gaps and with significant lateral misalignment, as the energy oscillates back and forth between the magnetic field and the electric field of the capacitors, effectively "pulling" the power across the gap rather than just pushing it.
