Contactless Electrical Energy Transmission System
A Contactless Electrical Energy Transmission System for Portable-Telephone Battery Chargers
A high-performance contactless electrical energy transmission (CEET) technique which employs the inductive energy transmission principle is described. The proposed technique enables the implementation of high-efficiency high-power-density fully regulated CEET systems suitable for applications with a wide input and load range. A high efficiency of the system is achieved by recovering the energy stored in the leakage inductances of the transformer by incorporating them in the operation of the circuit, and by employing high-frequency-inverter and controlled-rectifier topologies that allow a controlled bidirectional power flow through the transformer. In addition, a feedforward variable-switching-frequency control of the inverter is used to maintain approximately constant power transfer through the transformer with the input voltage changes, whereas the output-side rectifier employs a local pulsewidth-modulation control to achieve a tight regulation of the output in the presence of load variations. Specifically, the described CEET system is suitable for use in universal-input battery chargers. Index Terms—Contactless charger, inductive charger, local pulsewidth modulation (PWM) control, resonant inverter. IN MANY applications, contactless electrical energy transmission (CEET) has distinct advantages over the conventional energy transmission which uses wires and connectors. For example, CEET has been the preferred power-delivery approach in hazardous applications such as mining and underwater environments due to the elimination of the sparking and the risk of electrical shocks . Also, a number of CEET systems have been developed for electric vehicle battery-recharging applications because of their potential enhanced safety, reliability, and convenience. In addition, CEET has been considered in some medical applications since it makes it possible to transfer electric energy, which is required for running implanted electrical circulatory assist devices, through the intact skin of a patient . Finally, CEET has been used in cordless electric toothbrushes and portable telephones to increase their reliability by eliminating the contacts between their battery charger and the battery. Generally, CEET is implemented by using magnetic induction, i.e., by employing specially constructed transformers. In such transformers, the energy from the primary to the secondary is transferred inductively through the air. Because of safety requirements and/or mechanical constraints, the transformers for CEET applications usually have a relatively large separation be tween the primary and secondary winding. Therefore, the characteristics of these transformers are very different from those of the conventional transformers that have good coupling between the windings.