Abstract
Wireless inductive-coupled power transfer is a very appealing technique for the battery recharge of autonomous devices like surveillance drones. The charger design mainly focuses on lightness and fast-charging to improve the drone mission times and reduce the no-flight gaps. The charger secondary circuit mounted on the drone generally consists of a full-bridge rectifier and a second-order filter. The filter cut-off frequency is usually chosen to make the rectifier output voltage constant and so that the battery is charged with continuous quantities. Previous works showed that an increase in power transfer is achieved, if compared to the traditional case, when the second-order filter resonant frequency is close to the double of the wireless charger excitation and the filter works in resonance. This work demonstrates that the condition of resonance is necessary but not sufficient to achieve the power increment. The bridge rectifier diodes must work in discontinuous-mode to improve the power transfer. The paper also investigates the dependence of the power transfer increase on the wireless excitation frequency. It is found the minimum frequency value below which the power transfer gain is not possible. This frequency transition point is calculated, and it is shown that the gain in power transfer is obtained for any battery when its equivalent circuit parameters are known. LTSpice simulations demonstrate that the transferred power can be incremented of around 30%, if compared to the case in which the rectifier works in continuous mode. This achievement is obtained by following the design recommendations proposed at the end of the paper, which trade off the gain in power transfer and the amplitude of the oscillating components of the wireless charger output.
Highlights
Wireless power transfer (WPT) systems based on inductive resonance are appealing solutions to recharge devices without connectors or cables [1,2,3]
The diagrams show a clear dependency on the WPT excitation frequency f SS, and two zones are identified
An unexpected power transfer increment was found in those papers by sizing the LC-filter capacitor to set the filter resonance frequency to the double of the WPT excitation
Summary
Wireless power transfer (WPT) systems based on inductive resonance are appealing solutions to recharge devices without connectors or cables [1,2,3]. Self-driving applications like surveillance drones use a WPT station to automatize the charging process [4,5]; no operator or sophisticated connectors and platforms are required. A WPT system is usually composed of two inductive-coupled circuits. The primary circuit is located in the landing platform on the ground, and it is composed of the power supply, the inverter, the compensation capacitor and the coil. The secondary circuit is installed on-board the drone. It is characterized by the coil, the compensation capacitor, the rectifier, the low-pass filter and the battery. The position of the compensation capacitor with respect to the coils determines the kind of WPT architecture: Series-Series (SS), Series-Parallel (SP), ParallelSeries (PS) and Parallel-Parallel (PP) [11].
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