Wireless charging system can handle compact Li-ion secondary batteries

A contactless wireless charging system for low power applications that is capable of charging compact lithium-ion (Li-ion) secondary batteries has been developed by Renesas Electronics. The company says the solution will find applications in healthcare and wearable products that need to be water and dust resistant.

The design consists of a power receiver IC(RAA457100) and a power transmitter IC (RAA458100), each of which includes the functions for wireless charging on a single chip.

The company says that wireless charging is of particular interest in low power applications such as hearing aids that require small form factors as well as resistance to water and dust.

However many wireless charging technologies are considered unsuitable for charging systems employing compact Li-ion secondary batteries because the difficulties in achieving smaller form factors with the antenna size mandated by the current standards and heat dissipation due to the large charging currents that are required to charge low power applications.

The RAA457100 Power Receiver IC used in the system has a synchronous rectifier circuit that rectifies the AC power used to excite the antenna coil of the power receiver into direct current and a charging control circuit for charging a Li-ion secondary battery.

A 12bit A/D converter is used to monitor the battery voltage and current during charging and this data is supplied to the power transmitter to maintain the optimal charging power level. Protection features for the Li-ion secondary battery and a DC/DC regulator are also incorporated on a single chip.

Also employed in the design is an on-chip DC/DC converter with a claimed efficiency of 85% when the system is operating at a low load level of around 1mA. This is important because low power applications generally use compact batteries with small capacities.

The RAA458100 Power Transmitter IC integrates the circuits needed for wireless power transmission and support for single 5V power supply voltage from mobile batteries.

Generally speaking contactless power transmission involves applying a 125 kHz alternating current to the antenna coil of the power transmitter to excite the antenna coil of the power receiver and generate AC power.

Renesas’ newly developed power transmitter IC drives a bridge circuit and controls the alternating current to obtain the transmission power value required by the power receiver. The power transmitter IC integrates a bridge circuit over current protection function and a two-line external overheating protection function.

The design advantage her for system manufacturers is they can modify parameter values by programming the read registers with external EEPROM data via an I2C interface to meet their application needs. In addition, they can further customise the solution to their requirements by connecting an external MCU.