Wireless Charging Design Guide
Introduction
Wireless charging provides convenience and reliability for powering mobile devices. This guide covers Qi-compliant wireless charging design using Injoinic solutions.
Qi Standard Overview
Power Profiles
Qi defines two main power profiles:
Baseline Power Profile (BPP):
- Up to 5W output power
- Basic wireless charging
- Widest device compatibility
Extended Power Profile (EPP):
- Up to 15W output power
- Fast wireless charging
- Requires more sophisticated control
Operating Principle
Wireless charging uses magnetic induction:
Operating frequency: 110-205kHz
Transmitter Design
Coil Design
The transmitter coil is critical for performance:
A11 Coil (Standard):
- Diameter: 50mm
- Inductance: 6.3μH ±10%
- Construction: Litz wire recommended
- Ferrite backing for field shaping
Key Parameters:
- Inductance: Must meet Qi specification
- Q factor: Higher is better for efficiency
- Resistance: Lower reduces losses
Power Stage
Full-bridge inverter topology:
- Four MOSFETs in H-bridge configuration
- Drives alternating current through coil
- Switching frequency controlled by IC
- Requires proper gate driving
Resonant Tank
LC resonant circuit:
- Coil inductance (L)
- Series resonant capacitance (C)
- Resonant frequency: 100-205kHz
- High-Q capacitors for efficiency
Capacitance calculation:
``
C = 1 / (4π² × f² × L)
For 6.3μH at 150kHz:
C = 1 / (4π² × 150000² × 6.3×10⁻⁶) ≈ 179nF
``
Receiver Design
Receiver Coil
Receiver coil considerations:
- Size: Typically 30-40mm for smartphones
- Inductance: Matched to transmitter
- Shielding: Ferrite backing required
- Positioning: match transmitter coil
Rectification
Convert AC to DC:
- Synchronous rectification for efficiency
- Full-wave rectifier topology
- Output filtering capacitors
- Voltage regulation if needed
Communication
Receiver-to-transmitter communication:
- Load modulation on power signal
- Transmits power control packets
- Reports received power and errors
- Enables closed-loop control
Foreign Object Detection (FOD)
Why FOD is Critical
Foreign objects can:
- Heat up due to eddy currents (safety hazard)
- Reduce power transfer efficiency
- Damage the charging system
- Cause fire in extreme cases
FOD Methods
Power Loss Method:
- Compares transmitted vs. received power
- Significant loss indicates foreign object
- Required for EPP certification
Q Factor Method:
- Monitors resonant tank Q factor
- Metal objects reduce Q factor
- Supplementary detection method
FOD Calibration
Proper FOD setup requires:
- Baseline power loss measurement
- Testing with various foreign objects
- Threshold adjustment for sensitivity
- Validation across operating conditions
Thermal Design
Heat Sources
Identify heat sources:
- Coil resistance (I²R losses)
- MOSFET conduction and switching losses
- Capacitor ESR losses
- Receiver rectification losses
Thermal Management
Manage heat through:
- Adequate copper area for heat spreading
- Thermal vias to inner layers
- Component placement for airflow
- Temperature monitoring and derating
EMI Considerations
EMI Sources
Wireless charging generates EMI:
- Switching harmonics from power stage
- Magnetic field coupling to nearby circuits
- Communication signal modulation
EMI Mitigation
Reduce EMI through:
- Proper coil shielding with ferrite
- Input filtering on power lines
- Controlled switching edge rates
- Adequate spacing from sensitive circuits
Qi Certification
Certification Process
Steps to Qi certification:
Testing Requirements
Tests include:
- Power transfer capability
- Communication protocol compliance
- Foreign object detection
- Temperature limits
- Interoperability with reference devices
Design Guidelines
PCB Layout
Layout best practices:
- Place coil connections close to power stage
- Minimize high-current loop areas
- Use adequate copper for current carrying
- Separate analog and power grounds
- Provide shielding for sensitive circuits
Component Selection
Key component requirements:
- MOSFETs: Low RDS(on), adequate voltage rating
- Capacitors: High-Q, low ESR for resonant tank
- Coil: Proper inductance, high Q factor
- Ferrite: Appropriate permeability and saturation
Troubleshooting
Common Issues
No power transfer:
- Check coil alignment
- Verify resonance frequency
- Check communication packets
- Verify input power
Low efficiency:
- Check coil coupling (alignment/gap)
- Verify component quality (Q factor)
- Check for foreign objects
- Measure individual loss components
FOD false triggers:
- Recalibrate FOD thresholds
- Check for metallic enclosure effects
- Verify coil shielding
- Adjust sensitivity settings
Conclusion
Successful wireless charging design requires attention to coil design, power stage efficiency, FOD reliability, and EMI compliance. Injoinic's IP6808 and IP6806 provide integrated solutions for transmitter and receiver applications.
Contact LiTong for design support and Qi certification guidance.
💡 FAE Insights
⚠️ Common Pitfalls
- ✗ Inadequate FOD testing leading to certification failures or safety issues
- ✗ Using copper wire instead of Litz wire, causing excessive AC losses
- ✗ Insufficient EMI filtering causing regulatory compliance issues
- ✗ Poor coil shielding affecting nearby electronics
- ✗ Inadequate thermal design causing power derating
📋 Customer Cases
Accessory Manufacturer
Consumer Electronics
Challenge
The customer was developing their first wireless charging product and struggled with FOD calibration. Their prototype either failed to detect foreign objects or falsely triggered during normal charging, depending on threshold settings.
Solution
We provided detailed FOD calibration procedures specific to their mechanical design. Our FAE team guided them through baseline measurement, object testing, and threshold optimization. We also recommended minor mechanical modifications to reduce metallic interference.
Results
- Successfully detected all test foreign objects without false triggers
- Passed Qi certification on first submission
- Achieved 78% end-to-end efficiency
- Product received positive reviews for reliability
- Customer launched additional wireless charging products
Frequently Asked Questions
1. What is the difference between wireless charging and wired charging?
Wireless charging uses magnetic induction to transfer power through the air, while wired charging uses direct electrical connection. Wireless charging offers convenience (no connector wear, easy placement) but typically has lower efficiency (70-80% vs 90-95% for wired) and slower charging speeds. Wired charging provides higher power, better efficiency, and lower cost. Wireless charging is ideal for convenience-focused applications like charging pads, while wired charging remains preferred for high-power and cost-sensitive applications.
2. How efficient is wireless charging?
Wireless charging efficiency typically ranges from 70-80% end-to-end, compared to 90-95% for wired charging. Efficiency depends on coil alignment, distance between coils, coil quality (Q factor), and power level. Better alignment and closer coupling improve efficiency. Higher power levels (EPP) are generally more efficient than low power (BPP) due to reduced relative fixed losses. The efficiency loss manifests as heat in both transmitter and receiver.
3. What is the maximum power for wireless charging?
The Qi standard currently supports up to 15W for consumer devices (EPP). Some proprietary implementations offer higher power (up to 40-65W), but these are not Qi-compliant and only work with specific devices. For universal compatibility, design for 15W EPP maximum. Higher power wireless charging requires larger coils, better thermal management, and more sophisticated control. The industry is working on extending Qi standards to higher power levels.
4. Does wireless charging work through phone cases?
Wireless charging works through most phone cases up to 3-4mm thickness. Cases thicker than 4mm may significantly reduce efficiency or prevent charging entirely. Metal cases block wireless charging completely. Cases with metal plates (for magnetic mounts) can interfere with charging. Leather, plastic, and silicone cases generally work fine. Some cases have designated alignment marks for optimal charging position.
5. What causes wireless charging to stop unexpectedly?
Unexpected charging interruption can be caused by: Foreign object detection triggering (metallic objects nearby), overheating protection activation, poor coil alignment causing communication loss, incompatible device or protocol, power adapter insufficient for requested power, or fault in transmitter or receiver. Check for these issues systematically. The IP6808 provides status indicators that can help diagnose the cause.
6. How do I position devices for optimal wireless charging?
For optimal wireless charging: Center the device on the charging pad, align the device coil with the transmitter coil (usually centered on device back), remove thick cases if charging is slow, avoid metal objects between device and pad, and ensure the device is properly seated without movement. Some chargers have alignment guides or multiple coils to improve positioning tolerance. LED indicators typically show when proper alignment is achieved.