Resonant charging – wireless power transfer

Resonant wireless charging provides a range of benefits including enhanced user-friendliness – you can place a device anywhere in the vicinity of the transmitter (usually with up to 30 mm of vertical freedom) – and the ability to charge multiple devices of varying sizes and power at the same time. It is operating at 6.78 MHz and the standard is driven by the AirFuel Alliance. Resonant wireless charging systems communicate using Bluetooth Low Energy, while proprietary resonant solutions utilize in-band communication. Two main topologies can be used to develop resonant solutions: Class-D (full-bridge or half-bridge) topology and class-E (single-ended or differential) topology.

Resonant wireless charging operates at very high frequencies. This creates significant challenges for standard silicon power technologies in transmitter and receiver devices, especially in higher power designs. Class-D zero voltage switching (ZVS) and differential class-E are the main topologies used for resonant wireless charging systems. Both approaches reduce switching losses by transitioning between the on- and off-switching positions of power devices at zero volt across the respective power switch.

Class-D topologies can use a half or full bridge with a bus voltage under 100 V, whereby the switching nodes switch between zero volts and the bus voltage. The circuit is tuned for zero voltage switching over the required range of output impedance. In the Class-D ZVS topology, lower breakdown voltage devices can be used to increase overall system efficiency.

Class-E topologies have single-switch and dual-switch differential options. In each case, the drain is connected to the resonant circuit. When correctly tuned for zero-voltage switching with no body diode conduction, the drain voltage peaks at 3.56 times the DC bus voltage. Under non-ideal conditions, the drain voltage may rise much higher. A simpler driver architecture (low-side only) and a single switch per Class-E branch can help reduce system costs.

CoolGaN™ is ideally suited to both topologies as it can maximize overall system performance (in class-D implementations) and also reduce overall system solution costs (in class-E implementations). Due to its significantly reduced parasitic capacitances, CoolGaN™ is the ideal choice when switching at frequencies in the MHz range (e.g. 6.78 MHz); it provides low, almost linear, C_OSS without a large increase at low V_DS, enabling ZVS over a wide load impedance range. In addition, CoolGaN™ offers very low Q_G and Q_OSS compared with equivalent silicon MOSFETs, which, together with the Driver IC, helps switch power faster.