Wireless batteries to power low-power devices

　　There is a growing trend for portable consumer products to require wireless connectivity . Unfortunately, due to the short battery life, designs cannot achieve true mobility. Therefore, a power cord must still be connected to the device grid to obtain the necessary energy or to charge the battery. However, thanks to the low power requirements of today's electronic devices, it is possible to drive their wireless functions. This design concept describes a simple way to transmit wireless energy to low-power devices over a distance of more than 10CM. The design uses the principle of inductive harmonic coupling when operating at 13.56MHz . The system includes an RF power transmitter and an RF power receiver .

　　Figure 1 shows the transmission circuit that includes a 13.56-MHz oscillator. The oscillator, which includes a CMOS 4069 inverter, uses power from a 9-V battery to obtain a wide voltage swing. The oscillating signal then passes through a push-pull output stage consisting of two small-signal MOSFETS to obtain sufficient current at the output coil. Finally, the output signal is broadcast to the outside world by means of a series resonant LC circuit that has integrated a coil and a 60-pF variable capacitor tuned to 13.56 MHz.

　　Figure 2 shows a receiver circuit that includes an LC network tuned to a carrier frequency of 13.56 MHz. It consists of a coil and a 60-pF variable capacitor in parallel with the coil. A full-bridge rectifier consists of four diodes that correct the RF power.

The diode is composed of 1N4001. The frequency correction efficiency is about 50%. To achieve an output voltage of 3.3 V, a 9V point-to-point AC voltage is required through the coil pins. A shunt regulator integrates a 3.3V Zener diode, which can provide voltage clamping beyond 3.3V to prevent power level variations due to distance. Finally, a 1-nF capacitor after the full-bridge rectifier is used to weaken the power supply.

                                        

　　If your application requires longer range, as an improvement, you can increase the power supply to 15V to get a larger voltage swing at the transmitter coil. This is thanks to the CMOS process oscillator design used. In addition, designing a larger coil antenna on the transmitter and receiver can help increase the range operability.