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The energy efficiency of an inductive wireless power transfer (WPT) system will deviate from the possible peak value if the magnetic coupling of the transmitter and receiver coils or the load resistance changes. To achieve maximum-efficiency tracking regardless of coupling or load changes, on–off keying (OOK) modulation is used. However it has the issue of large surge current in the circuits during OOK modulation. The current overshoots of a series–series (SS) compensated WPT system at the startup stage might be much higher than the rated current. The current overshoots not only increase the stress on the power switch devices but also cause high voltage spikes across the windings and the resonant capacitors which is a challenge for insulation design. A soft-start scheme is proposed to address the current overshoot problem. The principle of the soft-start scheme can be essentially applied to WPT systems with different compensation topologies. The experimental results of a 3.3-kW prototype verify that the proposed soft-start scheme effectively suppress the current overshoots. [ABSTRACT FROM AUTHOR]

This paper presents a discrete sliding mode control (DSMC) scheme for a series–series compensated wireless power transfer (WPT) system to achieve fast maximum energy efficiency (MEE) tracking and output voltage regulation. The power transmitter of the adopted WPT system comprises a dc/ac converter, which incorporates the hill-climbing-search-based phase angle control in achieving minimum input current injection from its dc source, thereby attaining minimum input power operation. The power receiver comprises a buck–boost converter that emulates an optimal load value, following the MEE point determined by the DSMC scheme. With this WPT system, no direct communication means is required between the transmitter and the receiver. Therefore, the implementation cost of this system is potentially lower and annoying communication delays, which deteriorate control performance, are absent. Both the simulation and experiment results show that this WPT system displays better dynamic regulation of the output voltage during MEE tracking when it is controlled by DSMC, as compared to that controlled by the conventional discrete proportional-integral (PI) control. Such an improvement prevents the load from sustaining undesirable overshoot/undershoot during transient states. [ABSTRACT FROM PUBLISHER]

Optimum energy efficiency in a wireless power transfer (WPT) system usually occurs only at a specific load value while the load such as a battery is normally not constant. A major challenge in WPT is, therefore, to achieve high energy efficiency over a wide load range. Previously, impedance transformation methods such as dc–dc converters and use of a relay coil have been used to transform the practical load resistance to the optimum (or near-optimum) value. In this paper, a new approach based on reconfigurable magnetic resonant structures is proposed to achieve high energy efficiency and low volt-amp ratings. This basic principle is to create more than one efficiency–load curve. The WPT system is controlled to be operated within the top regions of the energy efficiency curves of the reconfigurable circuits so that high energy efficiency can be achieved over a very wide load range. The principle of this new approach is explained with an analysis and verified with practical measurements. [ABSTRACT FROM PUBLISHER]

Maximum energy efficiency in wireless power transfer (WPT) systems can be achieved through the use of magnetic resonance technique at a certain load resistance value. However, practical load resistance is not constant. Previously, a switched mode dc–dc converter was used in the receiver circuit to emulate an equivalent load resistance for maximum energy efficiency. In this paper, a new approach based on the On–Off Keying is proposed to achieve the high energy efficiency operation over a wide range of load power without using an impedance-matching dc–dc power converter. This simple and effective method has reduced average switching frequency and switching losses. It can be applied to any series-series resonant WPT system designed to operate at a constant output voltage. Practical measurements have confirmed the validity of the proposal. [ABSTRACT FROM PUBLISHER]

A charging time control method for wireless power transfer systems with a secondary-side hysteresis output power control is presented. It is a primary-side control method that adopts the combined use of three concepts, namely 1) an intermediate capacitor in the receiver circuit as a power flow indicator, 2) hysteresis switching actions of a shunt decoupling power switch in the receiver circuit to regulate the dc voltage of such intermediate capacitor, and 3) the turn-on and turn-off times of the decoupling switch detected on the primary side for closed-loop control. This method has the advantage of eliminating the need for 1) precise information of the mutual inductance between the transmitter and the receiver coils and 2) wireless communication system for feedback purpose. Practical results obtained from a hardware prototype are included. They confirm the proposed operating principles and indicate that the method can automatically lead to optimal energy efficiency operation. [ABSTRACT FROM PUBLISHER]