A wireless electricity transmitter is a high-frequency inverter circuit that drives a primary coil to generate an alternating magnetic field, transferring power across an air gap to a secondary receiver coil without physical contacts. In a real circuit, it replaces a direct galvanic connection with a loosely coupled transformer, forcing you to manage coupling coefficients (k), mutual inductance (M), and strict resonant tuning rather than just voltage and current. Makers commonly confuse near-field inductive or resonant magnetic coupling (which powers your phone) with far-field radiative RF transfer (like Wi-Fi or microwaves), incorrectly assuming you can beam watts of power across a room like a radio signal. In reality, wireless power transfer (WPT) is strictly a near-field magnetic phenomenon bounded by the physical dimensions of your coils.
The Physics of the Air Gap: Coupling and Resonance
When you separate two inductors by an air gap, the magnetic flux from the primary coil ($L_1$) that successfully links the secondary coil ($L_2$) drops dramatically. This relationship is defined by the coupling coefficient, k = M / √(L_1 × L_2). In a tightly coupled wired transformer, k is near 0.98. In a wireless transmitter pad sitting 10mm away from a receiver, k might drop to 0.3 or lower.
Because the leakage inductance (the flux that doesn't cross the gap) is so high, a raw inductive link is terribly inefficient. To fix this, we add capacitors to both the transmitter and receiver circuits to create LC resonant tanks. By tuning both sides to the exact same resonant frequency, the reactive impedances cancel out, allowing real power to cross the gap while the leakage inductance merely stores and releases energy locally.
Worked Numeric Example: Tuning the Transmitter Coil
Suppose you are building a custom Qi-compatible wireless electricity transmitter operating at the standard 110 kHz. You wind a primary coil on a ferrite shield and measure its inductance with an LCR meter at 20 µH. What tuning capacitor do you need?
Using the resonant frequency formula: f = 1 / (2π√(LC)), we rearrange to solve for C:
C = 1 / (4 × π² × f² × L)
- f = 110,000 Hz
- L = 0.000020 H
- C = 1 / (39.478 × 12,100,000,000 × 0.000020)
- C = 1 / 9,553,684 ≈ 104.6 nF
You would select a standard 100 nF or parallel a 100 nF + 4.7 nF NP0/C0G ceramic capacitor array to handle the high circulating currents without dielectric heating.
Where You Meet Wireless Transmitters in Practice
You will encounter wireless electricity transmitter topologies in three distinct tiers of modern electrical design:
- Consumer Electronics (The Qi Standard): Governed by the Wireless Power Consortium (WPC), this covers 5W to 15W transfers across 3mm to 10mm gaps. It relies on strict communication protocols where the receiver sends backscatter modulation to the transmitter to request more or less power.
- Electric Vehicles (SAE J2954): Operating in the 85 kHz band, these transmitters push 3.6 kW to 11 kW across a 150mm to 250mm ground clearance gap. They use massive Litz-wire coils and aluminum shielding to manage the intense magnetic fields and foreign object detection (FOD) requirements.
- Industrial AGVs and Robotics: Automated Guided Vehicles in warehouses use high-power resonant transmitters (often 500W to 2kW) embedded in the floor. Unlike Qi, these often operate open-loop or with simple optical alignment, prioritizing ruggedness over multi-device interoperability.
Decision Tree: Selecting Your Transmitter Topology and IC
Choosing the right transmitter controller depends entirely on your power target and mechanical air gap. Use this decision matrix to lock in your architecture.
| Application Scenario | Power Level | Air Gap | Topology | Concrete IC Pick |
|---|---|---|---|---|
| Wearables, IoT sensors, medical patches | < 5W | < 5mm | Tight Inductive (WPC Qi) | STMicro STWBC86 (Highly integrated, minimal external passives) |
| Smartphones, tablets, power banks | 5W - 15W | 5mm - 20mm | Resonant (WPC Qi EPP) | Texas Instruments bq501212A (Robust FOD, excellent thermal management) |
| Drones, AGVs, power tools | 50W - 300W | 15mm - 40mm | High-Power Resonant | Infineon TLE7259 driver paired with discrete SiC MOSFETs (e.g., IMW120R045M1) |
| EV Charging Pads | 3kW - 11kW | 150mm - 250mm | High-Power Resonant (SAE J2954) | Custom DSP (e.g., TI TMS320F28388D) controlling full-bridge IGBT/SiC modules |
Common Design Failures and How to Avoid Them
Building a wireless electricity transmitter on the bench is easy; making it reliable in the field is where engineers stumble. Watch out for these three specific failure modes.
1. Litz Wire Thermal Runaway
At the standard 110 kHz Qi frequency, the skin depth in copper is roughly 0.2 mm. If you wind your transmitter coil with solid 18 AWG wire (1.02 mm diameter), 80% of the copper cross-section carries zero current. The wire acts like a high-value resistor, and the coil will melt under a 15W load. The Fix: You must use 175/46 Litz wire (175 strands of 46 AWG enameled wire braided together). This provides massive surface area, keeping the coil temperature below 40°C at full load.
2. Foreign Object Detection (FOD) False Trips
Transmitters detect metal objects (like coins or keys) by monitoring the Q-factor (quality factor) of the resonant tank. A coin placed on the pad absorbs magnetic flux, dropping the Q-factor from ~80 down to ~30. If your microcontroller's ADC sampling rate is too slow, or your baseline calibration drifts with ambient temperature, the transmitter will either fail to detect the coin (causing a fire hazard) or false-trip on the receiver's own ferrite shield. The Fix: Implement a moving-average digital filter on your Q-factor ADC readings and calibrate the empty-pad baseline at every power-on cycle.
3. Loss of Zero Voltage Switching (ZVS)
High-power transmitters use half-bridge or full-bridge inverters. To prevent the MOSFETs from destroying themselves via switching losses, the circuit must achieve ZVS—meaning the voltage across the MOSFET drops to zero before it turns on. If the receiver is suddenly removed, or the air gap shifts laterally, the reflected impedance changes, shifting the phase angle and causing 'hard switching'. The Fix: Use a phase-shifted full-bridge topology with hardware dead-time generation, and monitor the VDS (drain-to-source voltage) via a high-speed comparator to fault-out the gate drivers if ZVS is lost.
Frequently Asked Questions
Can I transmit power through a metal enclosure?
No. Solid metal enclosures will absorb the alternating magnetic field, generating massive eddy currents. This will heat the metal (acting like an induction stove) and drop your transfer efficiency to near zero. If you must pass power through a barrier, use a plastic, glass, or wood window, or machine a precise slot in the metal and fill it with a non-conductive epoxy.
Does lateral misalignment destroy efficiency?
Yes. In a standard tightly coupled system, shifting the receiver coil laterally by just 50% of its diameter can drop the coupling coefficient (k) by half, which squares the efficiency loss. This is why modern 15W Qi chargers use arrays of overlapping coils (like Apple's MagSafe or multi-coil pads) and mechanically switch to the coil with the highest mutual inductance.
Why do my resonant capacitors keep cracking?
Standard X7R or Y5V ceramic capacitors exhibit severe microphonic effects and dielectric heating under high RF currents. The mechanical vibration from the piezoelectric effect literally cracks the ceramic. You must use C0G/NP0 dielectric ceramics, or for currents above 5A RMS, switch to high-voltage film capacitors (like polypropylene) rated for high dV/dt.
When prototyping a new wireless power link, do not waste time trying to build a custom discrete inverter from scratch unless you are targeting the >100W industrial tier. For the vast majority of makers and product designers building a 15W charging pad, the default and most reliable pick is the Texas Instruments bq501212A. It handles the complex ping, identify, and power transfer state machines required by the WPC specification, integrates robust foreign object detection, and saves you months of firmware debugging, letting you focus on optimizing your Litz-wire coil geometry and thermal management.
