Complete Schematic Guide for DIY Wireless Charging Circuit Design
Build your induction-based power delivery system using a Qi-compliant transmitter coil with inductance between 6.3 µH and 10 µH and a receiver coil pairing within ±10% tolerance. Ensure both coils operate at 100–200 kHz resonant frequency for optimal coupling efficiency of 70–85%. Place the transmitter and receiver 3–5 mm apart to avoid eddy current losses exceeding 15% in standard 5W configurations.
Integrate a half-bridge or full-bridge inverter with MOSFETs rated for ≥20V and ≥5A (e.g., Si4940EY or AO4606). Add a Schottky diode (SB240) at the receiver output to limit reverse current to . Use a resonant capacitor of 1 µF–4.7 µF (X7R dielectric) in series with the coil to maintain Q-factor ≥50 and prevent thermal drift beyond ±2°C under load.
For dynamic alignment compensation, implement a PWM controller (e.g., bq51013) with I²C communication to adjust drive current in 100 mA steps. Include overcurrent protection triggering at 1.2× nominal load and thermal shutdown at 85°C via a NTC thermistor or IC-integrated sensor. Test coupling coefficient k across misalignment ranges: ±10 mm laterally and ±5 mm vertically, targeting k ≥0.3 for consistent power delivery.
Optimize printed circuit layout with 1 oz copper thickness, separating high-current traces (>2A) by ≥2 mm from signal paths. Route ground returns as a star topology to minimize noise pickup below -60 dBm. Validate EMI compliance using FCC Part 15 or CISPR 11 standards, ensuring radiated emissions peak at 3 m distance during 5W transmission.
Key Circuit Design for Inductive Power Transfer
Select a half-bridge or full-bridge inverter topology based on power requirements–below 15W, a single MOSFET half-bridge (e.g., IRF640) with a 10μF output capacitor suffices; above 30W, a full-bridge (four MOSFETs) with current sensing (ACS712) ensures stable operation under varying load conditions. Position the transmitter coil (Litz wire, 10-30 turns, 10-30μH) no farther than 5mm from the receiver, with a ferrite shield (PC40 or PC95, 0.5-1mm thickness) to minimize EMI and edge losses. Use a Class D amplifier (TPA3116) for resonant coupling at 100-200kHz, paired with a half-bridge driver (IR2104) and bootstrap diodes (1N4148) to prevent shoot-through. Include a PFC stage (e.g., CRM boost converter with L6562) if efficiency drops below 75% under 50% load.
Critical Component Selection and Layout
- Resonant capacitors: Film (X7R) or NP0 ceramics, 5-10% tolerance; place within 10mm of coil terminals to reduce parasitic inductance. For 100kHz operation, aim for C = (1/(4π²f²L))–e.g., 10μH coil needs ~25nF.
- Current-limiting: Series resistor (0.1-0.5Ω) in the transmitter’s LC tank prevents thermal runaway; pair with a temperature sensor (NTC 10kΩ) mounted on the coil’s center.
- Rectification: Schottky diodes (SB560) for receiver output, with a low-dropout LDO (AP2112, ≤200mV dropout) if voltage regulation is critical (e.g., lithium-ion charging).
- PCB layout: Keep high-current traces (≥2oz copper) short (star grounding). Use vias (≥0.8mm diameter) for heat dissipation near MOSFETs.
- Testing: Verify resonance with an oscilloscope (Ch1: coil voltage, Ch2: MOSFET drain-source); mismatch >5kHz indicates incorrect L/C values. Measure efficiency with a precision meter (≤0.5% error) across load range (500mA–2A).
Key Components of an Inductive Power Transfer System
Start with a high-efficiency transmitter coil operating at 100–205 kHz for Qi-compliant designs; wind 18–24 turns of 0.3 mm Litz wire on a planar ferrite core (e.g., TDK PC40 or equivalent) to maximize coupling while minimizing eddy losses.
Pair the coil with a half-bridge resonant driver built around MOSFETs (e.g., Infineon BSC014N04LS) and a gate driver IC like the TI LM5111. Ensure dead-time of 50 ns between high and low-side switches to prevent shoot-through, and tune the series capacitor (100 nF, NP0 dielectric) for zero-voltage switching at the target frequency.
Power Management and Protection
Insert a synchronous rectifier on the receiver end using Schottky diodes (e.g., ON Semiconductor MBR10100CT) bypassed with low-RDS(on) FETs (Vishay SiR800DP) to drop forward voltage below 50 mV at 1 A. Add a 10 µF X5R input capacitor immediately after rectification to suppress ripple before the DC-DC converter.
Regulate output with a buck converter (TPS51218 or similar) set to 5 V/2 A; include thermal fold-back (125 °C threshold) via the IC’s junction temperature monitor and input overvoltage protection at 28 V using a Zener clamp (1N4752A) and series resistor. Keep layout traces under 2 mm wide for currents above 1 A to avoid copper etching losses.
Implement foreign object detection via amplitude shift monitoring: sample the transmitter coil current at 1 ms intervals and trip if the RMS value deviates >15 % from baseline; use a 12-bit ADC (ADS7042) for precision. Add a 555 timer circuit to force a 2 s shutdown if abnormal readings persist.
Mount both coils with 2 mm air gap clearance; align centers within ±3 mm for ≥80 % efficiency. Use 35 µm copper foil on FR4 substrate for the receiver coil to balance cost and losses; shield the bottom side with a 0.5 mm ferrite sheet to reduce EMI and improve coupling factor (k) above 0.6.
Step-by-Step Assembly of a Qi-Compatible Transmitter Coil
Select a ferrite sheet with a thickness of 0.2–0.3 mm for the coil base to minimize magnetic flux leakage. Cut the sheet to match the target device’s footprint, ensuring a 10–15% margin around the edges to prevent interference from adjacent components. Adhere the sheet to a non-conductive substrate, such as FR-4 or polyimide, using a high-temperature epoxy rated for at least 150°C to withstand thermal cycling during operation.
Wind 15–25 turns of Litz wire (AWG 38–42) in a spiral pattern, spacing each loop by 0.5 mm to reduce parasitic capacitance. Use a mandrel with a diameter of 30–40 mm for consistency; variations beyond ±2 mm affect resonance tuning. Secure the wire ends with solder tabs rated for 2A+ to handle peak currents without oxidation. Verify resistance between terminals–target values should fall within 0.3–0.8 Ω; deviations indicate poor winding or damaged insulation.
Align the coil’s center with the receiver’s expected position, offsetting it by no more than 3 mm to maintain coupling efficiency above 70%. Shield the underside with an additional ferrite layer if PCB traces run beneath, using a 0.1 mm gap to avoid eddy currents. Connect the coil to a Qi-compatible driver IC (e.g., Texas Instruments BQ500212A) via twisted pair cables, keeping traces shorter than 5 cm to minimize inductance losses.
Component Specifications
| Component | Specification | Notes |
|---|---|---|
| Litz Wire | AWG 38–42, 105-strand | Strand count reduces skin effect at 100–200 kHz |
| Ferrite Sheet | TDK IFL04, 0.2 mm | Permeability μ’ ≥ 200 for 100–300 kHz |
| Driver IC | BQ500212A (or STWBC-MC) | Supports Q-factor > 100 for stable power transfer |
| Capacitor (Resonant) | MLCC, C0G, 10 nF ±5% | Voltage rating ≥ 50V to handle back-EMF spikes |
Test the assembly with a Qi-compatible receiver positioned at varying offsets (±15 mm horizontally). Measure power transfer efficiency using an oscilloscope with a differential probe across the coil terminals. Optimal performance yields a sinusoidal waveform at 110–140 kHz; harmonics above 1% total harmonic distortion (THD) suggest misalignment or poor shielding. Adjust loop spacing or ferrite thickness iteratively until THD drops below 0.5% and output stabilizes at 5W ±2%.
Selecting the Right Rectifier and Voltage Regulator for Stable Output
Opt for a Schottky diode-based rectifier (e.g., 1N5822) for low-forward-voltage drops (0.2–0.4V) to minimize power dissipation in high-frequency inductive coupling systems. Silicon PN diodes (1N4007) introduce higher losses (0.7–1.1V drop) and should only be considered for low-current applications under 100mA or where cost outweighs efficiency.
For voltage regulation, LDOs with ultra-low quiescent current (e.g., TLV710xx, 5μA IQ) are ideal for battery-powered inductive transfer circuits, while switching regulators (e.g., LM2596) suit high-power designs requiring >1A output. Avoid linear regulators like 7805 series in high-load scenarios–they waste excessive energy as heat (>1W dissipation at 500mA).
Thermal and Load Considerations
Match rectifier current ratings to peak coil currents–underrated diodes (e.g., 1A-rated for 1.5A peaks) risk thermal runaway. For 5V/2A outputs, use dual independent rectifiers (e.g., B380 dual Schottky) to split current and reduce junction temperatures by 25–35%. Regulator selection must account for dropout voltage: LDOs like AP2204K (200mV dropout) work for input voltages as low as 5.2V, while LD1117V33 (1V dropout) requires ≥4.3V minimum.
Transient response in regulators is critical–opt for devices with fast load regulation (e.g., RT9080, 100nF ceramic capacitor at the regulator input and output to suppress high-frequency noise; polymer tantalum capacitors (e.g., 22μF/6.3V) improve low-ESR stability over electrolytic types.
EMI/RFI Mitigation
Post-rectification, add a pi-filter (e.g., 2x 10μF + 10Ω series resistor) to attenuate switching harmonics from the controller IC. Regulators with built-in soft-start (e.g., MCP1700) reduce inrush current by 40%, preventing input voltage sag. For medical-grade applications, select regulators with RMS output noise (e.g., LT3045) and isolate feedback traces from high-current paths to avoid coupling interference.