C10-5
A 6.78 MHz Wireless Power Transfer System for Simultaneous Charging of Multiple Receivers
with Maximum Efficiency using Adaptive Magnetic Field Distributor IC
Hao Qiu1,2, Makoto Takamiya1

The University of Tokyo, Tokyo, Japan; 2Now with Nanjing University, Nanjing, China. Email: qiuhaonju@gmail.com

1

Abstract
We developed a 6.78 MHz wireless power transfer (WPT)
system for simultaneous charging of multiple receiver (RX) coils.
On the basis of the transmitter (TX)-RX and RX-RX coupling
distinguished by the adaptive magnetic field distributor (AMFD)
IC, the distribution of magnetic fields from the TX coils wasoptimized at each RX coil for the maximum efficiency. A 2-TX 2RX WPT system was implemented with the AMFD ICs fabricatedin 1.8 V, 180 nm CMOS process. Compared with the conventional
method, the system efficiency is increased from 8.9 % to 61 %with the load power of 173 mW.
Introduction
With an explosive increase of users, as shown in Fig. 1, a WPTsystem that supports simultaneous wireless charging of multiple
RXs with maximum efficiency is highly desirable. The
conventional method like turning on a single TX coil, as shown inFig. 2(a), cannot effectively transfer power wirelessly to the RXcoils when the coupling between TX coil 2 and the RX coils isweak. Using multiple TX coils without control, as shown in Fig.
2(b), is not a good method either since the magnetic fields
generated by the TX coils can cancel out at the RX coils. The worst
case is no power can be transferred to the RX coils.
On the other hand, as shown in Fig. 2(c), by controlling thecurrent in each TX coil on the basis of the TX-RX and RX-RX
coupling, the distribution of magnetic fields from the TX coils canbe optimized at different RX coils to achieve the maximum
efficiency. Several previous works on a multiple TX WPT systemhave been reported [1-3]. However, all these works only support
wireless charging of a single RX coil. When the number of the RX
coils is greater than 1, taking [3] as an example, since they arecoupled with the TX coils at any point in time, the integrated ksensor cannot distinguish between them. Moreover, the RX-RX
coupling cannot be known either. As a result, the optimizedcurrent in each TX coil cannot be correctly computed for theoptimized distribution of magnetic fields at different RX coils.
In this work, by distinguishing between the coupling
coefficients (ks) between each pair of coils (TX-RX and RX-RX)
using the AMFD IC, we developed a WPT system forsimultaneous charging of 2 RX coils with the maximum efficiency.
Adaptive Magnetic Field Distributor ICFig. 3 shows a block diagram of the WPT system, consisting of2 TX coils driven by 2 AMFD ICs (TX1 and TX2), 2 RX coils,
and a top controller. According to the theoretical analysis in [4],the TX-RX and RX-RX coupling can be calculated by tentatively
turning on TX coils and measuring the current and voltage of each
TX coil. To achieve this without using current and voltage probes
which are bulky and costly, a current and voltage sensor is
integrated in each AMFD IC, which also consists of two halfbridges (HBs) and related digital control. AMFD ICs TX1 andTX2 firstly measure the currents and voltages in TX coils 1 (I1, V1)
and 2 (I2, V2) and output the results to the top controller for thecalculation of the TX-RX and RX-RX coupling. After that, theAMFD ICs drive HBs [3] to adjust the current in each TX coil to
its optimal value for the maximum system efficiency.
For the coil current sensing, we obtain it by differentiating thevoltage across the capacitor in the coil, considering that thesensing resistor may introduce power loss and the transformer
cannot be integrated. However, the current sensing accuracy canbe degraded due to the process variation of the RC time constantin the operational amplifier (op-amp) differentiator. To solve thisproblem, a calibration technique is proposed, as shown in Fig. 4(a).
During the calibration, the RX coils are absent. Take TX coil 1
as an example. Since it is resonated at the resonance frequency (f0),the amplitude of voltage across the inductor L1 (VL) equals thatacross the capacitor C1 (VC). These high-level voltages areattenuated and result in VL1 and VC1. VL2 is obtained by integrating
VL1 and VC2 is obtained by differentiating VC1 over time. As VL2and VC2 are AC voltages, their peak values are stored as VL3 andVC3 by the sample-and-hold (S&H) circuits at proper instants. Thecorresponding equations are listed in Fig. 4(b). The calibration isexecuted and will end up with VL3 equal to VC3, as shown in Fig.
4(c). According to the comparison results, the 5-bit R is controlled
by the logic circuits to calibrate the RC time constants of both opamp integrator and differentiator. After calibration, R remains
unchanged and the RC time constant equals 1/(2πf0), and the coilcurrent (I1) can be accurately obtained with the peak value asAMP_I, since L1, C1, and the attenuation ratio (β) have beenknown. The calibration procedure guarantees a correct current
measurement for the calculation of the TX-RX and RX-RX
coupling and the optimized current in the top controller.
The coil voltage (V1) passes the attenuator, and its peak value isobtained as AMP_V by the S&H circuit. The phase between V1
and I1 is obtained as PHASE by two zero-crossing detectors
followed by an XOR gate and a low pass filter (LPF).
Experimental Results
Fig. 5 shows a die photograph of AMFD IC fabricated in 1.8 V,
180 nm CMOS. The die size is 1.90 mm x 1.83 mm. All TX andRX coils are 10-turn flexible PCB coils with the outer diameter of50 mm. The load resistances (RLs) are 10 Ω. f0 is 6.78 MHz. Fig.
6 shows the measured waveforms during the calibration of the coilcurrent sensor. β is 0.01. EN is the calibration enabled signal andis generated off-chip. STATE indicates the state of calibration andgoes to LOW when the calibration ends. The system enters normal
operation after calibration.
Figs. 7(a), 8(a), and 9(a) show the measurement setups, inwhich the RX coils are perpendicular to the TX coils. 3 cases arelisted, in which RX coil 1 is perpendicular to (case 1) or parallelwith (cases 2 and 3) RX coil 2. On the basis of the measurement
results of the AMFD ICs, the top controller calculates the ks asshown in Figs. 7(b), 8(b), and 9(b) and the optimized current foreach TX coil. The optimized currents in TX coils 1 and 2 aremeasured and shown in Figs. 7(c), 8(c), and 9(c). The measured
load power is shown in Figs. 7(d), 8(d), and 9(d), and the measured
system efficiency (ηSYS) is shown in Figs. 7(e), 8(e), and 9(e).
Compared with all three conventional methods (TX coil 1 on,TX coil 2 on, and TX coils 1 and 2 on without control), ourproposed method shows great performance improvement. In case1, ηSYS is increased from 8.9 % to 61 % with the load power of
173 mW. Similarly, ηSYS is increased from 3.9 % to 19 % with theload power of 68 mW in case 2 and increased from 2.6 % to 47 %with the load power of 183 mW in case 3. Relatively highperformance can be obtained by selectively turning on TX coil 1
or 2. However, this requires that the status of RLs be transmitted tothe TX side and will make the system complex. On the other hand,
no TX-RX communication link is required in this work. Table I
shows a comparison table summarizing related works on multiple
TX WPT systems. This work achieves simultaneous charging ofmultiple RX coils with maximum efficiency.

1 978-4-86348-780-2 ©2021 JSAP

2021 Symposium on VLSI Circuits Digest of Technical Papers

Acknowledgement

This work was partially supported by JST
ERATO Grant Number JPMJER1501, Japan.

References[1] B. H. Water et al., IEEE Trans. Power Elec.,
pp. 6298-6309, 2015. [2] D.-H. Kim et al.,IEEE Trans. Power Elec., pp. 11391-11400,
2020. [3] H. Qiu et al., VLSI Circ., 2020, pp. 12. [4] G. Yang et al., IEEE Trans. Signal Fig. 1. Wireless charging
of multiple RXs.
Process., pp. 2860-2874, 2017.

Fig. 2. (a)-(b) Conventional and (c) proposed WPT method.

Fig. 3. Block diagram of the proposed WPT system.

Fig. 4. (a) Block diagram ofcurrent and voltage sensor withtheproposed
calibration
technique. (b) Corresponding
equations. (c) Operation principle.

Fig. 5. Die micrograph.

Fig. 6. Measured waveforms during calibration.
Table I. Comparison with Prior Art.

(a) Setup. (b) Calculated k. Measured (c) optimized coil current, (d) load power,and (e) system efficiency. Conventional (1) TX coil 1 on, (2) TX coil 2 on, (3)Both TX coils on w/o control. Proposed: Both TX coils on w/ control.

2 978-4-86348-780-2 ©2021 JSAP

2021 Symposium on VLSI Circuits Digest of Technical Papers