Qi TX design FAQs

Common questions engineers ask when designing Qi wireless charger transmitters, from bridge choice and resonant control to FOD tuning, thermal sensing and coordination with adapters or USB Type-C PD sources.

• Q1 How should a designer choose between half-bridge and full-bridge for a Qi TX around 5 W, 15 W or 30 W? ▸

  For 5 W class designs a half-bridge normally gives adequate voltage swing, lower cost and simpler layout. At 15 W many designs still use half-bridge but margin for cable loss and misalignment becomes tighter. Around 30 W and above a full-bridge offers higher coil voltage, better efficiency at higher current and more FOD headroom.
• Q2 What gate-driver features are most critical for soft-switching and low EMI in a wireless charger transmitter? ▸

  A suitable driver provides programmable gate strength, clean Miller clamp and tight dead-time control so switches see zero-voltage or near zero-voltage transitions. Robust CMTI and short propagation delay help maintain timing at high dv/dt. Integrated bootstrap management and under-voltage lockout prevent half-bridge shoot-through and reduce EMI from mis-timed transitions.
• Q3 How does ASK or FSK demodulation from the RX side typically connect into a Qi protocol controller on the TX side? ▸

  The RX modulates load on the coil, and the TX senses this as small amplitude or frequency changes on the resonant current or voltage. A demodulator front end filters and rectifies the signal into a digital envelope that feeds the Qi controller or MCU, which then decodes packet timing and bit framing according to the profile.
• Q4 Which TX-side parameters most strongly influence foreign object detection accuracy and false positive rate? ▸

  FOD relies on matching transmitted power to received power estimates, so accurate TX current and voltage sensing is essential. Coil quality factor, calibration of baseline losses for each charger and table entries for different RX types also have strong impact. Poor thermal coupling, noisy sense signals or loose component tolerances increase false triggers or missed events.
• Q5 Where should temperature sensors be placed around the TX coil and switching devices to manage hotspot risk effectively? ▸

  One sensor should sit close to the hottest part of the TX coil or shielding stack, not just on the PCB edge. Additional sensors near the main MOSFET or GaN devices inside the power stage track silicon temperature. For car docks or furniture, a sensor near the user touch surface helps enforce conservative derating limits for skin contact safety.
• Q6 How should the Qi transmitter coordinate with an upstream adapter or USB Type-C PD source during power ramp-up? ▸

  The TX should first request an appropriate PD or adapter voltage, then soft-start the resonant stage while monitoring input current and bus voltage sag. Power must ramp in steps tied to protocol negotiation rather than a single jump. Current limit, brownout detection and fast shut-down protect both the adapter and the transmitter during unstable connections.
• Q7 How can the TX power path be designed so the phone sees stable charging even with cable losses and supply noise? ▸

  A low-impedance input path, adequate bulk capacitance and well-compensated control loop help keep the DC bus stable under dynamic coil loading. Sense points should be placed at the resonant bridge rather than at a distant connector. Coordinating current limit and voltage droop behavior with the adapter or PD source prevents repeated resets and charge interruptions.
• Q8 When is it justified to move from silicon MOSFETs to GaN devices in a Qi TX power stage? ▸

  GaN becomes attractive when switching frequency and power level push MOSFET losses or thermal margins too high, such as compact 30 W or higher designs with tight airflow. If efficiency targets or thermal limits cannot be met with reasonable silicon devices, GaN allows faster edges and smaller magnetics in exchange for stricter gate drive and layout discipline.
• Q9 How can intermittent charging paused messages from phones be debugged on a Qi transmitter prototype? ▸

  Logging input voltage, coil current and temperature versus time while reproducing the issue reveals whether problems come from FOD trips, thermal derating or unstable handshake. Checking coupling alignment, foreign objects and adapter capability is important. Scope captures around the moment of pause should confirm that ASK packets and power steps follow the profile without unexpected gaps.
• Q10 What protection measures reduce the risk of metal foreign objects overheating on a desk, car dock or furniture surface? ▸

  Effective protection combines tuned FOD thresholds, time-based energy limits and conservative temperature cutbacks. The design should treat unknown loads as suspicious when received power does not match transmitted estimates and reduce power quickly. Placing sensors near likely metal locations and enforcing lower temperature limits for household or automotive interiors further reduces risk of user complaints and damage.
• Q11 How should current sense resistors and voltage sensing ranges be chosen for reliable power control and FOD calculation? ▸

  Sense resistor values must give enough signal swing for the ADC without excessive dissipation at full power. Voltage sense dividers should map the highest expected bus and coil voltages into the linear ADC range with headroom. Bandwidth and filtering must preserve the envelope needed for power estimates while attenuating high frequency switching noise that corrupts FOD comparison.
• Q12 How can interfaces between the TX module, system MCU and USB Type-C or CAN be planned for future feature upgrades? ▸

  A clean separation between real-time power control and higher level management allows later changes in protocol or connectivity. Exposing status, fault codes and power setpoints over a simple digital interface keeps integration flexible. Reserving pins or buses for future sensors or communication standards helps adapt the same hardware to new Qi revisions or product families.