Building a DIY Wireless Charging Circuit for Smartphones Step by Step Guide

For reliable inductive energy delivery at 5W, use a Qi-compliant receiver coil (30mm diameter, 18 turns of 0.3mm Litz wire) paired with a Texas Instruments bq51013 IC. Match the transmitter with a 20V DC input and a half-bridge inverter driving an LC resonant tank (10µF + 33µH). This combination achieves 78% efficiency at 10mm spacing while staying under 60°C surface temperature.

To prevent overheating, integrate a NTC thermistor ( Vishay NTCLE100E3) directly beneath the receiver coil. Set the thermal cutoff threshold at 50°C via the bq51013’s internal ADC. For EMI compliance, add a 330nF Y1-rated capacitor between the primary and secondary grounds, ensuring radiated noise stays below 45 dBµA/m at 3m.

Optimize coil alignment with a ferrite layer (TDK IFL12) behind both coils–this reduces magnetic flux leakage by 42% compared to air-core designs. For input current sensing, use a 0.01Ω shunt resistor (Vishay WSL2010) on the high-side MOSFET source, feeding the bq51013’s ISENSE pin for precise FOD detection down to 200mW.

Building a Contactless Power Transmission Unit: Key Schematics

Begin with a resonant inductive coupling setup, using a 5V DC input derived from a standard USB source. A half-bridge inverter, built with two MOSFETs (e.g., IRFZ44N), converts this into a high-frequency AC signal at 100-200 kHz. The primary coil should consist of 15-20 turns of 0.8mm enameled copper wire wound around a 50mm diameter ferrite core for optimal flux concentration.

On the receiver side, a secondary coil mirrors the primary’s design but may require fewer turns (10-12) depending on the desired output voltage. A full-wave rectifier using Schottky diodes (e.g., 1N5817) ensures minimal voltage drop. Add a 10µF smoothing capacitor post-rectification to stabilize the DC output. A linear regulator (e.g., AMS1117) can refine this to 5V if precision is critical.

• Frequency tuning: Use a 10kΩ potentiometer in the oscillator circuit to adjust the resonant frequency. Matching primary and secondary coil inductances (measured with an LCR meter) prevents power loss.
• Coil alignment: Maintain a 5-10mm air gap for efficient energy transfer. Misalignment beyond 15mm degrades coupling efficiency by over 40%.
• Thermal management: Place a 10Ω NTC thermistor near the MOSFETs to limit current during overheating. Include a heatsink if ambient temperatures exceed 50°C.

For safety, integrate a 5V Zener diode (1N4733A) in parallel with the load to clamp overvoltage spikes. A 1A fuse in series with the primary input prevents catastrophic failure from short circuits. Test the configuration with a multimeter: measure primary AC RMS voltage (expect 2-3V) and secondary DC output (4.8-5.2V) before connecting to a device.

Optimize coil spacing empirically. Start with 7mm separation and adjust in 1mm increments while monitoring output voltage with an oscilloscope. Efficiency peaks at 85-90% under ideal conditions but drops sharply with metallic objects in the proximity. Use a non-conductive enclosure (e.g., ABS plastic) to minimize interference.

Alternative designs may incorporate a Class-E amplifier for higher efficiency (up to 95%) but require precise component selection. Replace the MOSFETs with Infineon BSC010NE2LS if switching losses exceed 0.5W. For extended range (up to 30mm), increase coil diameter to 70mm and add a 1µF resonant capacitor. Note that extended-range setups demand active feedback circuits to maintain regulation.

Document all component values and test results. A well-built unit should deliver consistent output for loads between 500mA and 2A. Below 300mA, regulation may falter due to insufficient current; above 2.2A, thermal shutdown will engage. Fine-tune the potentiometer until the primary current stabilizes at 400-600mA for typical load conditions.

Critical Parts for Constructing an Inductive Power Transfer System

Select a transmitter coil with a thickness of 0.5mm to 1mm copper wire, wound in 15-20 turns for optimal inductive coupling. Ferrite shielding beneath the coil reduces flux leakage, improving efficiency by up to 30%. Pair this with a receiver coil of matching diameter and turn count–deviation beyond 10% causes significant power drop.

Use a high-frequency switching regulator like the MAX77900 or BQ51013B as the primary control IC. These handle rectification, foreign object detection, and adaptive power adjustment. Their integrated buck converters ensure stable 5V/9V output with efficiency ratings above 85% under typical loads.

Capacitors play dual roles: tuning resonant frequency and smoothing transient spikes. For a 100kHz-200kHz operation, employ 47nF to 100nF high-voltage ceramic capacitors (X7R dielectric) in parallel with the coils. Temperature-rated variants prevent drift, maintaining consistent power delivery even after prolonged use.

The rectifier stage demands low forward-voltage diodes (Schottky SS14 or similar) to minimize losses. Bridge configurations work, but single diodes offer simpler layout for compact designs. Fast recovery types (UF4007) handle the AC waveform without excessive heat buildup.

• Transmitter and receiver alignment tolerances must not exceed 5mm vertically or 20mm horizontally for Qi-compliant systems. Larger gaps require increased coil diameter or signal amplification.
• MOSFETs (IPP60R099) drive the transmitter coil with 1MHz switching capability, essential for fast response to load changes.
• Current sensing resistors (0.01Ω 1%) monitor power flow, triggering shutdown if thresholds exceed 1.2A for safety.

Coil winding orientation directly impacts magnetic field symmetry. Transmitter coils should follow a clockwise pattern, receiver coils counterclockwise, ensuring flux lines align optimally. Uneven turns or overlapping windings create dead zones, reducing output by 15-20%.

Foreign object detection (FOD) relies on impedance monitoring. Implement NTC thermistors near the receiver to detect metallic debris–small clips or coins alter the impedance curve measurably. Advanced ICs like the STWBC-MC include proprietary algorithms for real-time FOD without additional sensors.

Thermal management dictates PCB trace widths and enclosure design. Use 2oz copper for power traces and distribute vias around heat-generating components. Polyimide tape or thermal pads between PCB and ferrite substrate improve dissipation, preventing efficiency drops from overheating.

Step-by-Step Assembly for Energy Transfer Coil Pairing

Select copper wire with a gauge between 22-28 AWG for both induction elements–thicker wire reduces resistance but increases bulk, while thinner wire suits compact designs. Wind the transmitter loop tightly around a non-conductive cylindrical core (e.g., plastic or ferrite rod) with at least 15-20 turns for 5V output; the receiver should mirror this configuration but allow 5-10% more turns to compensate for coupling inefficiencies. Secure each winding with polyimide tape to prevent unraveling and ensure consistent spacing between turns, as inconsistent gaps disrupt magnetic field uniformity.

Align the coil centers precisely during mounting–misalignment beyond 30% of the coil diameter reduces efficiency by over 40%. Use adhesive-backed ferrite sheets (0.5mm thickness) on the underside of both coils to direct flux and minimize radiation losses. For rigid setups, embed the transmitter beneath a non-metallic surface (acrylic, MDF) at least 3mm thick; metal enclosures create eddy currents that degrade performance unless separated by a 10mm air gap or insulating layer.

Connect the transmitter coil to a half-bridge inverter using MOSFETs (e.g., IRFZ44N) with a gate driver (TC4427) for clean switching. Route traces with at least 3oz copper thickness on a PCB to handle currents exceeding 1A without overheating. The receiver coil should feed a full-wave rectifier (1N5822 Schottky diodes) followed by a 10μF-47μF smoothing capacitor–values below 10μF cause voltage ripple exceeding 5%, while higher caps slow transient response.

Component	Transmitter Spec	Receiver Spec
Coil turns	15-20 (5V)	16-22
Wire gauge (AWG)	24-26	24-26
Resonance cap (nF)	100-220	100-220
Ferrite thickness (mm)	>0.3	>0.5

Tune both coils to 100-200kHz using polypropylene film capacitors (100-220nF) in parallel with each winding–ceramic capacitors introduce parasitic effects at these frequencies. Verify resonance with an oscilloscope by probing the coil voltage; a clean sine wave indicates proper matching, while distortion suggests detuning or poor coupling. For adjustable designs, use a trimmer capacitor (10-100pF) in series with the main tuning cap to fine-tune the frequency within ±5kHz.

Test under load by placing the receiver 5-15mm above the transmitter; distances beyond 20mm reduce output by 60%. Use a multimeter to confirm DC output matches the target (e.g., 5V ±0.2V) and measure input current–efficiency above 75% is achievable with proper alignment and minimal thermal drift. Add a thermal cutoff (100°C bimetallic switch) if continuous operation exceeds 80% load to prevent coil insulation failure. For dynamic positioning, implement an FSK feedback loop between pairs to auto-adjust frequency based on coupling strength.

Final Validation Checks

Inspect for short circuits between coil turns after assembly–even microscopic contact causes local heating exceeding 80°C within minutes. Verify insulation resistance between windings and core exceeds 100MΩ at 500V DC; lower values indicate contamination or damaged wire coating. Confirm electromagnetic emissions comply with FCC Part 15 limits (