A Novel Control Method for Magnetic Wireless Charging System Xiaoyin Bai, Zhi-Hui Kong, Liter Siek School of Electrical and Electronic Engineering Nanyang Technological University Singapore
[email protected] Abstract—Wireless charging makes charging of portable devices more flexibly. However, this technology is limited by its charging distance. The resonant wireless charging technology @ MHz is an effective technology to extend the charging distance. However, as the charging distance increases, the frequency split effects the power transfer efficiency and voltage gain. This paper proposes a frequency/phase-shift tuning method to keep the system both in high efficiency and fixed voltage gain. Keywords—wireless charging; power transfer efficiency; voltage gain.
I. INTRODUCTION Wireless charging is considered as a suitable way for adoption contactless power transfer applications for its safety and flexibility. However, the low charging distance is a bottle neck of this new technology. In 2006, researchers in MIT developed [1] a 4-coils resonant wireless charging technology with charging frequency at 10MHz. In the experiment, they light a bulb over 2 meters. This technology increase the charging distance significantly. However, according to [2], this resonant wireless charging suffers from a frequency split problem. In [2], a key distance for maximum power transfer is derived. If the charging distance is below this distance, the voltage gain of the resonant wireless charging system could drop dramatically. Obviously, it is more attractive that if customers can put their devices freely while charging, which means the charging distance could be larger or smaller than the key distance. In that case, the wireless charging system suffers from insufficient voltage gain at low charging distance. In this paper, a novel tuning method is proposed. The followed sections are arranged as below: in section II, an analysis of both power transfer efficiency and voltage gain will be proposed for resonant wireless charging, in this section, the frequency split phenomenon will be explained in details; in section III, a novel tuning method will be presented. This method takes both charging efficiency and voltage gain into consideration; in section IV, the simulation results will be shown; in section V, a conclusion will be given about this method.
II. ANALYSIS OF RESONANT WIRELESS CHARGING SYSTEM A simplified wireless charging system is illustrated in Fig.1. The AC source Vin is a replacement of a full or half bridge DC-AC converter with high frequency. Primary coil and the secondary coil are replaced by L1 and L2, which are also the self-inductance of the two coils. K is the coupling coefficient between them. C1 and C2 are implanted as series compensation capacitors to make the system work under magnetic resonant wireless charging mode. R1, R2 are equivalent series resistor of both the windings of the coils and compensation capacitors. RL is the load. There are two key specifications of resonant wireless charging system: efficiency and voltage gain, which are shown in (1) and (2):
Fig. 1. Simplified wireless charging system
η=
PR
(1)
L
PV
in
A=
Vout Vin
(2)
In (1), η is the charging efficiency, PR is real power transferred to load RL and PVin is real power generated by the input AC source Vin; In (2) and Vout is the voltage cross the load RL , and Vin is the root mean square voltage of input AC voltage. K is coupling coefficient of the two coils. L
For detailed analysis, the (1) and (2) are specified as (3) and (4).
RL
η=
( R2 + RL ) 2 + (ωL2 RL
1 2 ) ωC 2
ω2 M 2
(3)
(jωL1 +
III. DESCRIPTION OF THE CONTROL METHOD
+ R2 + RL
jωMR L
A=
1 1 + R 1 )(jωL2 + + R 2 + R L ) + ω2 M 2 jωC1 j ωC 2
real application, this is not accepted. A novel control method is developed in the next sections to solve this problem.
(4)
As discussed in the last section, frequency split happens when the couple is strong. The efficiency is always at its peak when the operating frequency equals the resonant frequency. However, when the coupling coefficient is high, it can be seen from Fig. 3 that the efficiency drop is small near the resonant frequency. So the control method is described in Fig. 4.
Fig. 2 and Fig. 3 show the voltage again and efficiency verses the operating frequency from (3) and (4).
Fig. 2. Voltage gain analysis
Fig. 4. Control Flow Chat
At beginning, the system works at resonant frequency fres, and if the voltage cross the load is larger than 5V, which means the A is larger than 0.5, (in this design, Vin=10V and Vout=5V, so A=0.5 is taken to judge whether the voltage is enough). If A≥0.5, the system works under loose couple mode, then the system chooses fres as operating frequency and move to phaseshift tuning process [3] to tune the Vout to 5V (A = 0.5). If A
