Efficient Wireless Power Transfer Topology for Phone Charging Application

Abstract

Wireless power transfer scheme used on phone chargers are expected to have high efficiency, and reliability. However, existing AC-DC converter schemes were shown to be susceptible to leakage inductance and have lower regulation or excessive power losses. Output voltage regulation is a concern for wireless power converter topologies due to the need for additional shunt devices at DC output. In this paper, literature survey on various wireless power transfer scheme is performed with specific focus on phone charging applications. A modified topology using impedance matching transformer is presented with circuit analysis and LT spice simulation results. It is shown that the proposed topology can deliver better output voltage regulation at various coupling coefficients indicating a superior circuit performance for various separation distances between transmitting and receiving coil.

Introduction

I. INTRODUCTION

Wireless power transfer systems are increasingly adopted for various power transfer applications like electrical vehicle charging and smart phone chargers. In this paper, literature survey was performed on various wireless power transfer topologies. Based on the literature study, traditional design approach was seen to be susceptible to varying leakage inductance and magnetic coupling between primary transmitter coil and secondary receiver coil. In this paper, a simplified model with mathematical equation is discussed along with excel based analytical results. Circuit leakage inductance was seen to impact output voltage regulation.  Since the power transfer system was susceptible to leakage inductance and lower magnetic coupling, a modified power topology with impedance matching transformer is proposed. In this paper, analysis of impedance matching transformer with wireless power transfer system is presented along with mathematical model and excel analysis. LT spice simulation results show that the proposed model improves output voltage regulation and retain zero voltage switching of power devices. Based on theoretical analysis and simulation results, the proposed model is well suited for wireless power transfer systems like phone charges, which required high reliability at low cost.

II. LITERATURE SURVEY

The magnetic induction based wireless power transfer scheme is quite popular in phone charges since the technology is best suited to transmit power between short distances. Although the implementation of power circuit can get extremely difficult, the magnetic resonance is able to transfer 60 W power is transferred at a 2-m distance [1]. As explained in [3], the high-frequency primary current will induce magnetic field, on the receiving coil and by resonating with the secondary compensation network, the transferred power and efficiency are significantly improved. A phone charger device was tested for a power transfer capability of 0.5W at 2.5meter distance [13]. Ultrahigh frequency of operation using split-ring loop design is shown to significantly increase the Q-factor and maximizing power transfer efficiency [5]. Studies also show that inductive magnetic coupling devices operating at non resonant frequencies are still able to achieve higher efficiency over long transmission distances [6]. To achieve higher power transfer efficiency through various load conditions, a compact planar & low-cost wireless power transfer link using matching circuit was analysed in [7]. Use of coupled capacitor based wireless power transfer topology was successfully verified [8]. Several theoretical models are also presented in literature including the leakage length models for E-core and U-core transformers with concentric windings [9]. Sensitivity studies were performed to accurately model leakage inductance using Finite element analysis [10]. Hardware testing with USB2.0 charging system, shows the effect of charging distance with the power generated from multiple transmitters and output DC power management circuit was required to ensure stable output [14]. A modified wireless power transfer topology was proposed using impedance matching LCC resonant circuit and 3.3 kW power was transmitted over 20cm distance with 92.7% system efficiency [20].  In this paper, a similar impedance matching technique is applied to low voltage wireless charging system and a power stage design is presented with mathematical analysis and simulation results.

III. THEORETICAL MODEL and Simulation

Figure 1. shows a theoretical model for AC-DC converter for a wireless power transfer system, between a transmitting side and receiver coil. Below analysis will refer to transmitting section as primary coil or winding, and receiver section as secondary coil or winding. Magnetizing inductance (Lm) is directly proportional to core (relative) permeability value, ranging from a value of several thousands (Ferrite core) to 1.0 (coreless or air).

IV. ACKNOWLEDGMENT

The author acknowledges that the research is conducted independently and there is no conflict of interest regarding the publication of this paper.

Conclusion

As wireless charging applications, such as Electric Vehicle (EV) chargers and smartphone charges, become increasingly prevalent, the demand for efficient power transfer topologies rises. In this paper, we conduct a literature review to explore various techniques used for low voltage wireless power transformer scheme, including the effect of leakage inductance on circuit operation and voltage regulation. To achieve superior output voltage regulation, a theoretical model for an impedance matching magnetic circuit is developed and analyzed. To validate the effectiveness of the impedance matching method, a simulation model is developed, and empirical evidence provided. Device voltage and current waveforms were shown to retain soft switching operation, proving the effectiveness of the proposed impedance matching approach.

References

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Copyright

Copyright © 2023 Santosh Arcot. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.