With the development of science and technology, more and more kinds of electronic products come out, and the demand for charging is becoming more and more important. The compound annual growth rate of wireless charging exceeds 40％. For the demand for wireless charging equipment, the nitride developed by our team Gallium power transistors can achieve high-efficiency wireless charging through power levels ranging from 25 watts to 2500 watts and meet fast charging. It is one of the best applications for exhibiting the characteristics of gallium nitride transistors.
Wireless charging transfers energy through inductive coupling. Like a transformer without an iron core, wireless charging devices currently on the market use electromagnetic induction technology, which needs to be closely fitted and accurately positioned, and only allows one-to-one charging, causing users The inconvenience of resonant wireless charging is based on the principle of frequency resonance for power transmission, allowing multiple receivers and a looser distance limit. The AC power supply is converted to 300VDC direct current and input to a class E amplifier. By controlling the class E The amplifier is in the best state of resonance, which converts electrical energy into a magnetic field and transmits it to the receiving end, and the receiving end uses a high-frequency rectifier circuit to convert alternating current to direct current for electronic products or to charge batteries.
Among them, the control of Class E amplifier is more difficult, because its resonance state is easily affected by the changes of various component parameters, such as the output capacitance Coss change when the transistor is switched, and the impedance change of the load. The team proposed to adjust the frequency and duty cycle to control the circuit state. However, it is currently manually adjusted, and there is still something can be improved, such as using a microcontroller or a chip.
The frequency and duty cycle of the class E amplifier are controlled by two voltages. After the voltage-controlled oscillator generates a signal, the output signal is modulated by the comparator circuit. In this way, the current state of the class E amplifier can be compared to the frequency and The duty cycle is adjusted to achieve optimal control of the transmission power.
Nowadays, most wireless charging systems are based on the inductance (Qi) standard, and operate through inductive coupling with a frequency in the range of 100 to 300 kHz, but they can only charge a single device, and the device must be placed in a specific direction and very close to the charging Device. Alternative Class E topology realizes more free wireless power transmission design through resonant coupling. Utilizing the Class E topology, high efficiency can be achieved in the operating frequency range of 1 to 10 MHz.The switching frequency is 6.78MHz or 13.56MHz, GaN will be the best choice. In the 6.78MHz-based resonant wireless charging, by using resonant inductive coupling, its high-Q factor resonator can make good use of the significantly weaker magnetic field in the surrounding area to transmit power over a longer distance.
The principle of electromagnetic resonance technology is to exchange electromagnetic energy through magnetic field resonance. The charging distance can reach more than 10 cm. It supports "one-to-many" charging. However, compared with electromagnetic induction technology, its charging efficiency is usually less than 70％. The GaN transistor developed by NYCU has a wireless charging efficiency of over 95％.

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