Study On Soft Switching Operating Points Of Contactless Power Transfer Systems And Their Application

Contactless Power Transfer (CPT) is a novel techonology developed to deliver power contactlessly from power supply to one or more movable loads using modern power electronics, control theory and magnetic coupling techniques. The main advantages of this technology are safe, flexible, easy to maintain, highly reliable and environmentally friendly.In CPT systems, because of the loosely coupling condition between the primary and secondary sides, resonant technology is usually employed to increase power transfer capability. Therefore, soft-switching resonant technology is usually used in the main circuit of a CPT system. In CPT system design and analysis, the soft-switching operating points and their steady-state characteristic are the most concerned points. However, it has been shown from literature reveiew that there is not such one method so far which can accurately and quickly determine all the possible soft-switching operating points and steady-state response waveforms. In order to get all the possible operating points, complicated numerical analysis or repetitive simulation is unavoidable. But both methods are very time-consuming and low efficiency.For the soft-switching operating points analysis of a CPT system, this paper is devoted to proposing a method suitable for analyzing the periods and steady-state response waveforms of all the soft-switching operating points of a common resonant converter. And the method is applied in practical CPT systems.The main works of this paper are as follows.1. Proposed an accurate analysis method for soft-switching operating pointsA general stroboscopic mapping model of an autonomous nonlinear oscillating system which can be piecewise linearized is derived based on stroboscopic mapping method and fixed points theory. And then, both the voltage-fed and current-fed load resonant circuits which widely used in CPT systems are studied respectively. Their periodic fixed point functions and soft-switching boundary condition equations are derived, based on which, the equations for calculating the periods and the steady-state waveforms of the soft-switching operating points have been obtained. The steps and characteristics of the proposed soft-switching operating point analysis method have been summarized briefly. Afterwards, the relationship between the periodic fixed point function and the system parameters has been studied in a LCR series tuned resonant circuit.2. Analyzed the characteristics of multiple soft-switching operating points of a CPT systemThe proposed method has been applied to analyze an LCL-type CPT system. The steady-state characteristics of the soft-switching operating points have been investigated in detail. Bifurcation diagrams are employed to study the influence of the system key parameters on the soft-switching operating points. The theoretical results and the proposed method have been verified by both simulation results and experimental results. More interesting phenomena and potential practical applications have been found by studying the waveforms of multiple soft-switching operating points. For example, the subharmonic oscillation characteristic at the low frequency operating points can be used to achieve high frequency oscillation by relatively low frequency switching. And the power transfer capability difference among the operating points can be used to design a novel power flow controller.3. Analyzed the resonant operating points (a special soft-switching operating point) and their stability of a CPT systemTo study the existence of multiple resonant operating points and their autonomous oscillation stability of a CPT system, a push-pull CPT system has been taken as an example. The definition and stability criterion of resonant operating points in a CPT system have been described. A Poincarémapping model has been built up and its Jacobian matrix has been derived with the implicit function derivative rule and chain rule. Then the stability of the fixed points can be determined according to the eigenvalues of the the Jacobian matrix. It has been used to study an example push-pull CPT system with three resonant operating points. The results have been verified by PLECS simulation results. The steady-state response waveforms of the three resonant operating points under fixed frequency operating mode have been simulated and analyzed. A soft start-up method for fixed frequeny operating mode is proposed to solve the over-voltage problem of the conventional method. The floating frequency operating modes of the three points have also been studied and the results verified their local stability obtained from the Jacobian matrix. Particularly, an operating point jumping phenomenon has been studied when the system is under pick-up short-circuit decoupling control.4. Proposed a novel power flow control methodAs the difference of power transfer capability among the soft-switching operating points is concerned, a novel power flow control method has been proposed. The essential idea is to control the system switching among the selected soft-switching operating points according to the feedback error information. This method has been applied to a series tuned CPT system. The main concerned aspects, such as operation principle, design procedure and control characteristics etc., have been studied in detail. Both simulation results and experimental results have verified the proposed control method.The main contributions of this paper are listed as follows.1. To accurately and quickly explore all possible soft-switching operating points of a CPT system, based on stroboscopic mapping method and fixed points theory, an accurate method for calculating soft-switching operating points has been proposed. This method simplifies the steady-state analysis significantly and can be applied to general load resonant circuits.2. To analyze the multiple resonant operating points of a CPT system brought by its high order nonlinear characteristic, a stability criterion of resonant operating points has been proposed based on the Jacobian matrix eigenvalues of the Poincarémapping model. It is found that the existence of multiple resonant operating points and their stability difference may cause operating points jumping phenomenon in a CPT system.3. Based on the power transfer capability difference among the multiple soft-switching operating points, a Multiple Soft-Switching Points (MSSP) power flow control method has been proposed. The transferred power can be adjusted continuously by switching the soft-switching operating points of the system without any additional switching circuit.