High Frequency Power Converters Based on Energy Injection Control for IPT Systems

Abstract

A contactless power transfer system has many advantages over conventional power transmission due to the elimination of direct electrical contacts. With the development of modern technologies, IPT (Inductive Power Transfer) has become a very attractive technology for achieving wireless/contactless power transfer over the past decade. IPT has been successfully employed in many applications for materials handling, lighting, transportation, bio-medical implants, etc. To transfer power across an air gap, an IPT system employs a primary power converter to generate a high frequency alternating current in a track/coil which is then magnetically coupled to one or more power pick-ups. The design and implementation of the primary side power converter and controller are of great importance in an IPT system because they affect the whole system performance under both steady state and transient conditions. Therefore, any development and control improvement within the primary power converter are desirable. This thesis focuses on the development of converters for IPT systems based on circuit resonance and energy injection control. A voltage fed DC-AC energy injection inverter is introduced. Basic characteristics and underlying principles of the inverter are studied. A variable frequency ZCS (Zero Current Switching) control strategy is developed for the switching operation. The new DC-AC inverter can achieve high frequency AC power generation using relatively low switching frequencies with predicable current ripple and transient behavior. Furthermore the soft switching operation ensures low switching losses and reduced EMI. A detailed analytical method has been developed and extended to a system level by taking system bifurcation and power transfer capability into consideration. The critical boundaries of both the loading and coupling conditions are determined for designing energy injection inverters without frequency bifurcation. It has been found there is an optimal coupling coefficient which results in the maximum power transfer capability of the inverter. Using such a system level inverter design process, a primary track current regulation method based on energy injection control has been proposed. Furthermore, as an example, a practical contactless slip ring system has been developed using the proposed control method. A new type of direct AC-AC converter is also proposed. A control strategy is developed based on energy injection control for the AC-AC matrix converter to achieve a high frequency current generation with soft switching operation. The converter operation is analytically analyzed in detail, including the current ripple and sag caused by the alternating input voltage. The theoretical analysis is proven by simulation studies and practical experiments. The energy injection concept is also applied to push pull resonant converters with a “dual” transformation in performance which leads to a boost operation. A mathematical model employing a stroboscopic mapping technique is developed for a push pull inverter to find the multiple ZVS operation points. It has been proven by both simulation and experimental studies that the power transfer capability of a traditional push pull inverter can be increased significantly by operating it at selected low ZVS frequencies without using any additional front-end boost circuits.