Adrian Lita
Wireless Power Transfer System
Key Highlights
- 90%+ efficiency
- 20-50W range
- Non-Qi compliant
Overview
High-efficiency wireless power transfer system achieving over 90% end-to-end efficiency for 20-50W power transfer. Uses resonant inductive coupling with optimized coil design and impedance matching. Non-Qi compliant design optimized for specific application requirements. Complete system includes transmitter with power amplifier and control, receiver with rectification and regulation, and foreign object detection for safety.
Technical Details
Transmitter uses full-bridge Class-D amplifier at 125kHz driving transmit coil. STM32 microcontroller controls switching frequency and duty cycle for power regulation. Litz wire coils minimize skin effect losses at operating frequency. Coil geometry optimized using FEMM (Finite Element Method Magnetics) simulation for coupling coefficient vs. misalignment. Series resonant network tuned for maximum power transfer. Receiver uses synchronous rectification (MOSFETs instead of diodes) for efficiency. Buck converter regulates output voltage. Load detection uses Q-factor measurement. Foreign object detection monitors coupling changes. PCB layout critical for high-frequency performance: controlled impedance, minimal loop area, proper grounding. Thermal management with heatsinking and thermal vias. EMI compliance required careful filtering and shielding.
Challenges Overcome
- • Achieving high efficiency while maintaining misalignment tolerance (efficiency drops rapidly with coil misalignment)
- • EMI compliance (high-frequency switching creates radiated emissions)
- • Foreign object detection (metallic objects can heat up dangerously)
- • Thermal management (even 10% loss at 50W = 5W heat in small enclosure)
- • Cost optimization (high-frequency components are expensive)
- • Tuning for varying load conditions (efficiency varies with load)
- • Safety certifications (wireless power has specific safety requirements)
Outcomes & Impact
- ✓ Achieved 92% peak efficiency at optimal alignment, >85% at 10mm misalignment
- ✓ Successfully passed EMI compliance testing (FCC Part 15, CISPR)
- ✓ Foreign object detection prevented heating of coins, keys, etc.
- ✓ Thermal design kept surface temperature under 45°C during continuous operation
- ✓ Cost optimized to $15 BOM in volume, competitive with wired solutions
- ✓ Technology adapted for multiple products in consumer and industrial applications
