Wireless Charging Sonic Electric Toothbrushes
WAS-1972EN V1.00
9 / 17
December 27, 2021
Coil Selection Description
Charging base coil
Inductance: 32.01µH
Coil turns: 10, multi-core enameled wire
Type: cylindrical, 8mm in height, 10mm in diameter, with magnetic column
Wire diameter: 0.6mm
Sonic electric toothbrush coil:
Inductance: 53.59µH
Coil turns: 18, single-core enameled wire
Type: cylindrical, 9mm in height, 15mm in diameter
Wire diameter: 0.3m
Charging Circuit Design Description
Toothbrush body input power protection design
Whether the lithium battery is fully charged or not, the charging base will both transmit the same
energy to the toothbrush body through the resonance circuit. Therefore, it is required to design a
front-end protection circuit. In order to prevent the toothbrush body MCU from being damaged
due to too high an energy level, it is necessary to design a full-bridge rectifier circuit and a Zener
diode at the front end.
This solution supports an operating current of about 45mA, an operating voltage of 5V and an
operating power of 0.225W. Therefore a 0.5W Zener regulator is selected. Refer to the application
note Wireless Charging Over Voltage Application Solutions for details (No. AN0532EN).
Parallel resonance frequency design
In this solution, the charging base coil inductance is 8.9µH, the default resonance frequency is
130kHz, the resonance capacitance is 168.41nF which is calculated by the LC resonance formula.
The LCFreq and PWMDuty parameters in the PWM program will be adjusted so that the PWM
output can switch at the zero point of the LC resonance waveform. The PWM frequency adjusted
by this solution is 114.3kHz and the toothbrush body inductance is 20µH. Here the resonance
capacitance of the toothbrush body is 96.97nF which is calculated based on the PWM frequency
of the charging base. For detailed adjustment methods and notes, refer to the application note
Wireless Charging Transmission Power Adjustment and Design (No. AN0558EN).
Software Description
Communication Mechanism Design
The communication method for this solution is based on load changes - changing load current. It will
include a resistor at the toothbrush body VIN and use a transistor to control it. When the transistor is
on, the load will increase and the charging base current will be larger. When the transistor is off, the
load will decrease and the charging base current will be smaller. Changes in current will allow the
charging base to receive these differences. The toothbrush body data signal will be decoded by circuits
such as a back-end amplifier and comparator.
