Research On Key Issues Of Envelope Modulation Wireless Power Transfer Systems

Magnetic Coupling Wireless Power Transfer(MC-WPT)technology is a technology based on the alternating electromagnetic field.It has attracted researchers around the world due to its high value for market and research prospect.However,the promotion of this technology has been slow down by some shortcomings,such as complex controlling,low power factor,high cost,and low system efficiency.Nowadays,the research of envelope modulation MC-WPT technology is still not in-depth and sufficient.To expand its applications,some issues should be resolved,such as stability boundary,AC-AC envelope demodulation on the secondary side,steady-voltage operation and bidirectional power transfer without compromising the system complexity.Supported by the national high technology research and development program(863 program)"key technology research of public wireless power and information transfer system"(project number: 2015AA016201),the national natural science fund project "study on key technology of wireless power transfer network"(project number: 51277192),the national natural science fund project "the optimal efficiency control strategy research of high-power wireless power transfer system,"(project number: 51377183),this paper investigates the key problems of envelope modulation WPT.The main work in this paper is listed as follows:1.With the variation in the resonant network resistance,the envelope modulation WPT systems cannot work at the resonant frequency and the energy envelope valley values may depart from the zero-point,deteriorating the whole system performance.To solve the above problems,this paper introduces the time domain analyses of the state variables,derives the discrete iterative methods of the boundary conditions,obtains boundary conditions for envelope modulation WPT system,and fixes the theoretical boundary accuracy.The simulation and experimental results show that the DC envelope modulation WPT system works at the resonant frequency,soft-switching state within the boundary,and valley value of zero degree control is in the design scope.Once beyond the initial boundary,the system will work at partial resonant state.Otherwise,the envelope valley value of partial zero degree will exceed the scope of design goals.2.In the traditional WPT system,there exist issues when the load needs the AC power supply.In this paper,a novel energy mode based on the impulse theorem and the principle of conservation of energy is proposed to stabilize the output voltage.This method adjusts the inject energy on the primary side without increase in the circuit complexity.3.Aiming to low efficiency,complex control,and low power factor,AC-AC envelope modulation is adapted on the secondary side.The output of the energy transformation and envelope demodulation is analyzed and studied.This structure applies to not only AC load WPT systems,but also DC load WPT systems.4.Enormous switching devices of AC-AC inverters in the WPT system make the control strategy difficult.And,the output side of the AC-AC transformation and bidirectional transmission problem are seldom studied.This paper proposes a bi-directional envelope modulation WPT system based on the AC-AC converters to realize the bidirectional wireless power transmission.Simulation results verify the effectiveness of the proposed method.This method can downsize the switching devices,realize soft-switching and constant output current.The main innovation contributions are listed as follows:(1)The envelope modulation WPT systems cannot work at the resonant frequency with the change of the resonant network resistance,and this change may affect the power transmission performance.The discrete iterative method is proposed in this paper,the working boundary condition is obtained,and the accuracy of the theoretical boundary is also corrected.(2)When the load of the envelope modulation WPT system needs the AC power supply,it is hard to realize the system control and realize the bidirectional wireless transmission.This paper introduces an output voltage stabilizing method and proposes an AC-AC converters to the envelope modulation WPT system.The proposed method stabilizes the output voltage by adjusting the inject energy,and also realize the bidirectional wireless power transfer between the primary and secondary side.(3)This paper introduces an envelope modulation WPT system with an AC-AC converter in the secondary side,which takes out the converter rectifier bridge from the secondary side.This converter tracks the resonant energy envelope precisely,and applies to WPT system with inertia phase shift link.The system has higher efficiency and input power factor.