An Improved Multi-layer PCB Winding and Circuit Design for Universal Contactless Charging Platform Hao Ma, Lingni Ma College of Electrical Engineering, Zhejiang University E-mail:
[email protected] Abstract- Contactless battery charging platform is an emerging technology that brings safety, reliability and convenience to low-profile charging applications. In this paper, an improved three-layer circular coil array PCB winding is proposed, which can generate the near-uniform magnetic field over a specific operation area. Moreover, an optimal circuit design procedure is developed to maintain the approximately constant output voltage, regardless of the magnetic coupling variation. The coil structure is analyzed with finite-element simulation and verified with EMC scanner test results. Measurements and experiments are further carried out to confirm the almost uniform magnetic coupling and validate the circuit design procedure. The design proposed in this paper is well adaptable to a universal battery charging platform.
I.
INTRODUCTION
Recently, the planar contactless battery charging platform with its distinct advantage over the conventional charging approach has attracted great attention. Utilizing the principle of inductively coupled power transfer, the charging platform removes direct electrical connections and provides great convenience, good reliability and low maintenance to battery charging. In addition, the PCB transformer is adaptable to low-profile applications and several loads can be charged simultaneously regardless of their positions. To implement such a platform, a high-quality factor PCB coil should be designed so as to offer a near-uniform magnetic field over a considerable operation area. And to further ensure approximately constant output, the resonant compensation tank also deserves careful considerations. In previous researches, some PCB windings have been proposed and discussed. In [1-4], platforms which consist of only one spiral winding are introduced. The single spiral winding is well capable of contactless power transfer, especially with single load. However, as [5] points out the magnetic field of single spiral winding demonstrates a convex manner that its magnitude drops off from the center to the peripheral. Therefore, the secondary coil couples different amount of magnetic flux and induces different output at different positions. Thus, if the coil is not properly placed at the center, charging maybe ineffective in particular with multi-load application. To solve this problem, a hybrid winding structure is proposed in [5], in which a coil and a spiral winding are combined so that the concave distribution of the former and the convex distribution of the latter can compensate each other to offer a near-uniform magnetic field. Multi-layer coil array structures are proposed in [6-9] as a
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universal winding solution. A three-layer hexagonal coil array structure is introduced in [6] and another tow double-layer square coil array structures are also proposed in [7]. These overlapped designs can be easily applied to platforms with any size. Moreover its near-uniform magnetic field over the entire charging area ensures several loads to be charged simultaneously. However, as [9] points out fully overlapped windings also cause ‘central-field sag’ problem. To enhance charging performance, the design of the high-frequency resonant compensation tank is also widely investigated. Some basic compensation design considerations and comparison between different topologies are well developed in [10-13]. These studies mainly concentrate on core transformer with single load and there is lack of systematical design procedure proposed. Therefore a multi-load design procedure based on PCB transformer is proposed in [5]. While this method ensures efficient power transfer to each load, the reactive power can be much higher than the active power as the number of loads increases. Thus an improved design method is proposed in [3], which enhances the power factor. But still while operating frequency is considered to evaluate circuit performances, the impact of magnetic coupling variation is not discussed. In this paper, an improved PCB winding for universal charging platform is proposed, which consists of circular coil arrays arranged in three layers. This design does not only generate an almost uniform magnetic field over a considerable operation area, but also mitigates the ‘central-field sag’ problem. To enhance the charging performance, an optimal circuit design procedure is also developed with which approximately constant output can be maintained. The coil structure is analyzed with finite-element tool in section II and the design procedure is developed based on circuit modeling in section III. Experimental verifications and conclusions are presented in section IV and V respectively. II.
SIMULATION STUDY OF THE PROPOSED COIL DESIGN
A sketch of the proposed planar PCB winding is shown in Fig. 1, which consists of circular coil arrays arranged in three parallel PCB layers. The circular winding is adopted, because circles demonstrate three advantages over other geometry. First, for a given length of perimeter, circles have the largest area. Hence, less wire is required to form the PCB winding and its parasitic resistance can be reduced. Secondly, within the
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same layer circles can be arranged tangent to each. While current flows in the same direction in each spiral, it flows in the opposite direction in the adjacent winding which causes current cancellation phenomenon to weaken the magnetic field. Because only circles can contact each other at single points, less cancellation influence is imposed. To further analyze and verify this design, a finite-element simulation is carried out with Ansoft Maxwell 3D v10.0. In all simulations, every coil is excited with a 5A sinusoidal current of 500 kHz. A plane, parallel to the platform and 8mm high above the surface, is used to demonstrate the distribution of the magnetic field as well as the direction of the magnetic flux. A. Simulation of a Single Spiral Winding To generate a near-uniform magnetic field, the magnetic field distribution of a single spiral winding is first studied. The spiral winding shown in Fig.2 (a) is simplified into several coaxial circles as presented in Fig.2 (b). Due to the limitation of simulation computing speed, the actual number of turns is reduced to three. The detailed dimension of the simulation model is illustrated in Fig.2 (b) and the same dimension is used for all equivalent spiral models in this paper. The simulation results are shown in Fig.3. As expected, the magnitude of the magnetic field drops off from the center to the periphery, forming a convex distribution. Also it can be observed that the perpendicular magnetic flux which is capable to induce the voltage in secondary coils is only distributed in the central region. This convex distribution leads to strong charging position dependence so that charging is not effective, if the device is not placed properly in the central.
(a) Spiral PCB winding
(b) Simplified simulation model
Fig. 2. Geometry of a single spiral
Fig. 3. Stimulated magnetic field distribution of a single spiral winding
(a) An overlapped simulation model
B. Simulation of a Coil Array Unit To compensate the weak magnetic field of the peripheral area of a single spiral winding, an overlapped topology is proposed as shown in Fig.4 (a). The inner spiral coil is overlapped with the other six outer coils, thus the central winding is divided into two parts as Fig.4 (b) illustrates. As a result, the weak magnetic field of the overlapped area is e n h a n c e d w h il e th e s tr o n g ma g n e t ic f i e ld o f th e none-overlapped area is reduced. With a proper proportion, the magnetic field of the entire area can be near-uniform. This idea is confirmed with simulation as presented in Fig.5, from which it can be observed a desirable uniform magnetic field is formed over the central winding area. This overlapped structure forms a three-layer coil array unit that lays the foundation for charging platforms with any operation area.
(b) The equivalent model of the central winding Fig. 4. Simulation model of a coil array unit
Fig. 5. Stimulated magnetic field distribution of a coil array unit
Fig. 1. Proposed three-layer circular spiral coil array structure
C. Simulation of the Proposed Coil Array Platform To form a considerable operation area, the coil array unit is expanded as shown in Fig.6 with its simulation result given in Fig.7. As expected, the platform region, which is indicated by
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the black rectangular, presents a near-uniform magnetic field distribution.
capacitance ratio. Therefore the voltage gain can be calculated as (11).
III. CIRCUIT ANALYSIS AND DESIGN MV =
Besides PCB coil, the resonant compensation tank should also been carefully designed so as to enhance the charging performance. Among all the possible compensation topologies, the series compensation at both sides of the transformer is an easy and effective way to nullify the leakage inductance and ensure desirable power transfer [3,5]. Therefore a circuit as Fig. 8 is adopted.
⎧⎪⎛ ω 2 + k − 1 ⎞ 2 Vout 2 = n ⎨⎜ n 2 ⎟ + [ (1 − k )Qnω n ] Vi ⎠ ⎪⎩⎝ kω n
⎡ 1 γ ω 2 + k − 1⎤ × ⎢ m (1 − 2 ) + (1 − 2 ) × n 2 ⎥ ωn ωn kω n ⎦ ⎣
2
⎫⎪ ⎬ ⎪⎭
−0.5
(11)
A. Circuit Modeling and Analysis The conventional T model is well adapted to loosely coupled transformers with multiple secondary windings [3]. In this paper, assume all loads are the same and purely resistive, thus define: LS 1 = LS 2 = " = LSm = mLSS
(1)
C S 1 = C S 2 = " = C Sm = C S / m
(2)
R1 = R2 = " = Rm = mR
(3)
Fig. 6. Simulation model of the proposed three-layer coil array structure
where m is the number of loads, LSi (i=1, 2 … m) is the secondary leakage, CSi (i=1, 2 … m) is the secondary compensation capacitance and Ri (i=1, 2 … m) is the load resistance. Lss, Cs and R are equivalent parameters of secondary side. A simplified equivalent circuit can be plotted as Fig. 9, in which 2
Vi =
π
Vin .
(4)
LM = kLP , LSP = (1 − k ) LP
LSS ' =
LSS R , C S ' =n 2 C S , R ' = 2 n2 n
(5)
Fig. 7. Simulated magnetic field distribution of the proposed design
(6)
where k is the coupling coefficient, LM is the magnetizing inductance, LP is the primary inductance, LSP is the primary leakage inductance, n is the secondary to primary turn ratio and LSS’, CS’, R’ are LSS, CS, R referred to the primary side respectively. Define: ω0 = ωn = Qn =
γ=
1 LSPCP
=
1 (1 − k ) LPCP
(7) Fig. 8.
ω ω0
Circuit topology for multi-load application
(8)
ω0 LP ' 1
R
=
n 2ω0 LP R1
CP C = 2P ' CS 1 n CS 1
(9) (10)
Fig. 9.
where ω0 is the resonant frequency, ωn is the normalized frequency, Qn is the quality factor and γ is the compensation
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Equivalent Circuit for multiple identical purely resistive loads
To ensure constant output, the variable magnetic coupling should also be taken into consideration. Because in most cases while the operating frequency is fixed, the magnetic field of even an optimized platform cannot be ideally uniform. As loads added to the platform, they also slightly distort the field distribution. Therefore the analysis of how variable magnetic coupling influence the charging performance is essential to circuit design. Assume k varies from k0 to k’, the resonant frequency defined in (7) is then adjusted to ω0 ' =
1 − k0
(1 − k0 ) (1 − k ' ) LPCP
= ω0
1 − k0 . 1− k'
(12)
As a result, the normalized frequency is also changed to ωn' =
ω ω 1− k' 1− k' = ⋅ = ωn . ' ω0 ω0 1 − k0 1 − k0
(13)
Substitute (13) into (11), the relationship between the voltage gain MV and the coupling coefficient k can be obtained. Define FMV = M V (max) − M V (min)
(14)
as the fluctuation of MV over a range of k. Suppose k0=0.25 and k=0.1~0.3, for different value of compensation capacitance ratio γ, quality factor Qn and number of loads m, the fluctuation of MV (assume n=1) is presented in Fig.10.
It is shown in Fig.10 that for ω≥ω0, voltage gain fluctuation decreases as Qn and γ increase, while for ω
