Complete Guide to Building a Wireless Phone Charger Circuit Schematic
The most reliable approach to designing an inductively coupled energy transmitter begins with selecting a resonant frequency between 100–200 kHz for optimal efficiency. Use a half-bridge or full-bridge inverter topology with MOSFETs like IRF540N or IRLML6401, paired with a dedicated driver IC (e.g., UCC27211 or IR2110) to minimize switching losses. A primary coil wound with 0.5 mm enameled copper wire–10–15 turns on a 50–60 mm diameter ferrite core (e.g., EFD25)–ensures sufficient magnetic flux density. Secondary coils should match the primary’s inductance (±10%) with identical wire gauge and turn count, but include a Schottky diode (1N5822) and a low-dropout linear regulator (AMS1117-5.0) to stabilize output at 5V/1A.
For impedance matching, insert a series capacitor (100 nF–1 µF, 25V X7R) in both transmitter and receiver circuits to achieve resonance at the chosen frequency. Use a rectifier module (MB10S) on the receiver side to convert AC to DC, followed by a 10 µF tantalum capacitor for ripple suppression. Avoid metal shielding near coils–maintain a 5 mm air gap between the transmitter and any conductive surfaces to prevent eddy current losses. Test load stability with a 10 Ω power resistor before integrating the device.
Transmitter control relies on a microcontroller (STM32F030 or ATTiny85) generating PWM signals at the resonant frequency. Implement foreign object detection (FOD) via a current sensing resistor (0.1 Ω, 1W) and ADC input on the MCU–halt transmission if measured input current exceeds 120% of nominal value. For safety compliance, include thermal shutdown (NTC thermistor) and overvoltage protection (Zener diode BZX84C5V6) on both sides. Prototyping should use 2 oz copper PCB traces (min. 3 mm width) for the primary coil to handle currents up to 1.5A.
Designing an Inductive Power Transfer Circuit for Mobile Devices
Begin with a transmitter coil rated for 5V–12V input; wind 15–20 turns of 0.5mm enameled copper wire on a 40mm ferrite core to maximize flux density while minimizing resistive losses. Pair it with a N-channel MOSFET (e.g., IRFZ44N) driven by a 555 timer in astable mode, set to 100–200 kHz–this frequency balances efficiency and regulatory compliance (Qi standard threshold).
On the receiver side, use a matching coil with identical turns and diameter; spacing between coils should not exceed 5mm for optimal coupling. Integrate a Schottky diode (1N5822) and a 22µF low-ESR capacitor to smooth rectified output before feeding a linear regulator (AMS1117-5.0) to ensure stable 5V/1A output. Overvoltage protection requires a Zener diode (5.1V) across the output.
Thermal management dictates a 2 oz copper pour on both sides of the PCB beneath the transmitter coil, extending to the MOSFET’s drain pad. Add 10–15 thermal vias per square centimeter to the ground plane. For EMI suppression, place a 100nF capacitor between the coil’s tap and ground, and route traces at 45° angles to reduce parasitic inductance.
A resonant capacitor (10nF–100nF, NP0 dielectric) in parallel with the transmitter coil forms an LC tank circuit; adjust its value empirically to match the coil’s inductance (typically 10–30µH). Use a gate driver IC (e.g., TC4427) if switching losses exceed 0.5W, ensuring the MOSFET’s gate voltage peaks at 10V with
Receiver efficiency peaks when the load resistance is 5–15Ω; test with a variable resistor and oscilloscope to confirm on the output. For foreign object detection, monitor the transmitter’s input current–if it deviates >20% from the baseline (~200mA), trigger a shutdown via a comparator (LM393) connected to the MOSFET’s gate.
Regulatory testing demands FCC Part 15/CE EN300 330 compliance; shield the transmitter with a 0.1mm mu-metal sheet if radiated emissions exceed -40dBm at 1 meter. Use EMI filters (e.g., Murata BLM18PG121SN1) on the input and output leads. For alignment tolerance, embed Hall-effect sensors (A1302) on both PCBs to disable power transfer when misaligned >10mm.
Power density scales with coil separation; at 2mm gap, expect 80% efficiency, dropping to 60% at 5mm. Copper thickness should be >1oz for currents >1A to prevent I²R losses >0.3W. For multi-device support, stack three receiver coils with 30° angular offset and autonomous load detection via I²C (ATTiny85).
Final calibration involves sweeping the drive frequency ±10% around resonance while logging input current and output voltage; the peak efficiency point must align with Qi’s 110–205kHz band. Document all component derating factors (e.g., 70% for capacitors, 50% for inductors) and ensure ESD protection (TVS diode, 15V) on USB output to meet IEC 61000-4-2 Level 4.
Core Parts for Building an Inductive Power Transfer System
Start with a transmitter coil wound from enameled copper wire (0.3–0.5 mm diameter). Opt for a 20–30 turn circular loop with an outer diameter around 60 mm–this matches most receiver pads in handheld devices. Ensure minimal resistance by using Litz wire if targeting high-efficiency (>80%) builds.
A power stage requires a high-frequency inverter, typically a half-bridge or full-bridge MOSFET array (e.g., IRF730, IPW60R041). Drive it with a dedicated gate driver IC like DRV2700, capable of switching at 100–200 kHz. Add dead-time control to prevent shoot-through and thermal runaway in the FETs.
The receiver module hinges on a similar coil but sized 40–50 mm in diameter. Pair it with a rectifier–Schottky diodes (SB560) reduce voltage drop compared to standard silicon. A low-dropout linear regulator (AP2112K) stabilizes output to 5V/1–2A, while a 10 µF ceramic capacitor smooths transient spikes.
Resonant tuning demands precision: combine the transmitter coil with a series capacitor (1–10 nF, C0G dielectric). Calculate capacitance using C = 1/(L × (2πf)²), where L is coil inductance (~10–20 µH) and f the switching frequency. Mismatched values degrade coupling efficiency by 20–30%.
Implement foreign object detection (FOD) via an LC resonant sensor–a small auxiliary coil (5 turns) beneath the primary. Monitor phase shift between excitation and response; deviations >5% indicate metallic debris. Use a microcontroller (STM32F030) to cut power within 50 ms to comply with Qi safety protocols.
| Component | Recommended Part | Key Parameter | Failure Impact |
|---|---|---|---|
| Full-bridge FET | IRF730 | RDS(on) = 0.4 Ω | Thermal shutdown at >1W loss |
| Gate driver | DRV2700 | Dead-time = 30 ns | Cross-conduction destroys FETs |
| Schottky diode | SB560 | VF = 0.55 V | Excessive heat at >1.5A load |
Thermal management is non-negotiable: apply 1–2 W/K thermal adhesive between FETs and a 30×30 mm aluminum heat spreader. Limit case temperatures to
Feedback loops require a current-sense resistor (0.01 Ω, 1% tolerance) in series with the primary coil. Amplify the differential signal with an op-amp (LM358), then feed it to the MCU’s ADC. Adjust PWM duty cycle dynamically–target a 100–150 mA ripple at nominal load to maintain stable resonance.
Shielding minimizes EMI: place a ferrite sheet (2–3 mm thick) behind both coils, ensuring
Step-by-Step Transmitter Coil Assembly for Power Transfer Units
Select a ferrite core with a diameter of 45–55 mm and thickness of 2–3 mm to maximize magnetic flux density while minimizing eddy current losses. Use a core with relative permeability (μr) above 800 for 100–200 kHz operation. Stack two discs if single-core thickness exceeds thermal dissipation limits under sustained 5W output.
Wind the coil in a spiral pattern with enameled copper wire (AWG 28–30) spaced 0.5 mm between turns to prevent parasitic capacitance. Begin the inner turn at 10 mm from the core center and maintain consistent tension (20–30 g) to avoid wire deformation. Complete 20–25 turns depending on target inductance (40–60 μH); verify with an LCR meter at 150 kHz after each 5-turn increment.
- Apply Kapton tape under each layer if stacking exceeds 3 mm height to insulate against core edge abrasion.
- Secure the outer turn with cyanoacrylate adhesive–avoid solder near coil to prevent shorted turns.
- Test inductance vs. frequency sweep (100–300 kHz) to detect resonance drift caused by inadequate spacing.
Solder the coil leads to a 220 pF series tuning capacitor (COG/NP0 dielectric) mounted perpendicular to the PCB to reduce loop area. Terminate wires with tinned copper braid strips (3 mm wide) to handle RMS currents of 1.2–1.5 A without overheating. Shield the assembly with 0.3 mm mu-metal foil covering the sides, leaving a 2 mm gap at the center to prevent flux shunting.
Validate performance by loading the coil with a dummy receiver (30 mm diameter, 10 turns, 8 Ω resistive load). Measure efficiency (≥80%) at 5 mm separation and confirm frequency stability (±2 kHz) across 3.5–8 V input. Rewind the coil if Q-factor drops below 80 or if harmonic distortion exceeds 3% at full load–typically caused by uneven turn distribution or core saturation.
Integrating Receiver Coil with Mobile Device Power Management
Connect the inductive receiver’s output directly to a synchronous rectifier with a Schottky diode bypass for redundancy. Target a forward voltage drop under 300mV to minimize thermal losses–use Vishay’s VBQ30100 for sub-200mV efficiency. Route the smoothed DC through a low-ESR capacitor (Nichicon UHW or Panasonic FM series) rated ≥25V to suppress ripple below 50mVpp at 6.2 Arms.
Bridge the rectified output to a buck converter operating at 1.2 MHz–Texas Instruments’ TPS51218 delivers 95% peak efficiency at 2A loads. Configure the feedback network with a 2.21kΩ (1% tolerance) resistor in series with a 34.8kΩ resistor to regulate the 5V rail; ensure PCB traces exceed 1.5 oz copper thickness to prevent voltage sag under pulsed currents. Isolate the enable (EN) pin via a GPIO-controlled MOSFET–microcontrollers like STM32G0xx toggle this in
Embed a Coulomb counter IC (e.g., Maxim MAX17260) between the buck regulator and the battery PMIC to monitor charge cycles. Feed its I²C output into the device’s fuel-gauge firmware–Android’s `BatteryStats` API parses this data to adjust charge termination thresholds dynamically. For iOS, map it to `/var/wireless/charger` sysfs nodes using a kext with `com.apple.driver.AppleUSBCharger` entitlements to override default USB-C PD profiles when inductive fields are detected.
Implement a ferrite bead (Murata BLM15P-series) in series with the data lanes (D+ and D-) if the receiver shares a connector with USB ports. Position it within 10 mm of the connector shank to attenuate RF noise above 150 MHz–USB-IF compliance tests (Chapter 6-8) require ≤15 dB SNR degradation under concurrent charging and data transfer. Bypass capacitors (100 pF NPO dielectric) on the IC side filter residual switching noise without compromising signal integrity.
Validate the integration by stress-testing the receiver with a Gle140A-class load box while measuring thermal rise via FLIR E4–target 5A/ms).