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This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/3.0/).

Based on the magnetic resonance coupling principle, in this paper a wireless energy transfer system is designed and implemented for the power supply of micro-implantable medical sensors. The entire system is composed of the ^{3}.

Micro-implantable medical devices (IMDs) are becoming more and more popular in health and medical applications due to the ability to locally stimulate internal organs and communicate the internal vital signs to the outer world. The internal battery is not an ideal candidate for the power supply due to its limited life time, large volume, and possibility of leakage [

Recently, wireless power transfer schemes have often used in IMDs to not only to avoid transcutaneous wiring, but also to either recharge or replace the device battery. Wireless energy transfer can be divided into the near field and the far field transmission. The far field radiative transfer by microwaves or laser is limited by absorption and scattering in the atmosphere and requires a direct line of sight between the source and the device(s) [_{m} (_{m} = √_{1} × _{2}) of the transmitter and receiver coil radii (r_{1}, r_{2}) is commonly used as the performance metric for comparing different designs. The inductive coupling efficiency is lower than 40% and decreased with 1/d^{3} when d > r_{m} [

In 2007, Kurs reported [_{m}/d = 1.07). It is proved that the efficiency of resonant coupling is much higher than that of inductive coupling.

Based on the four-coil resonant circuit theory, a wireless energy transfer system (

This paper is organized as follows: the models of resonant coils and energy transfer efficiency are calculated and optimized in Section 2. Section 3 describes the design of peripheral circuit including the transmitter module and receiver module of the wireless power transfer system. The experimental setup and measurement results about the energy transfer efficiency and the whole system are presented and discussed in Section 4.

Energy transfer efficiency is the key issue for the four-coil resonant power link. It depends on the self-inductance, quality factor (Q factor) and the carrier frequency. The coils are wound with Litz wire in order to reduce the ac resistance and improve the Q factor. First, a theoretical model for designing transmitter and receiver coils based on multistrand Litz wire is applied.

In order to reduce and utilize the volume effectively, a multilayer solenoidal coil model is chosen. In addition, transmitter coils are composed of coil2 wrapped over coil1, and receiver coils are composed of coil3 wrapped over coil4. The inductance is derived by the summation of each turns' inductance and the mutual inductance between each turn.

For a solenoidal coil with _{a}_{t}_{i}_{a}_{ij}_{kl}_{ij}_{kl}_{l}_{0}

To achieve a high Q factor, consideration of different parameters such as the self-inductance of coil and operating frequency (

The self-resonance frequency _{self}_{a}_{self}

For a multilayer solenoidal coil with _{a}_{t}_{self}_{b}_{m}

The ac resistance of coils made up of multistrand Litz wires, including skin and proximity effect, can be approximated as [_{h}_{dc}_{s}, N_{s}, β_{a} is the area efficiency of coil with width b and thickness t. _{S}, N_{B}, N_{C}_{S}

At high frequency, skin and proximity effect increase the ac resistance. Since multistrand Litz wire can reduce the ac resistance, it is chosen to design the resonant coils. Litz wire (AWG44) with a number of 60 strands is used in transmitter coils, and with a number of seven strands it is used in receiver coils because of the size limit of the receiver coils. In the operating frequency (350 kHz∼850 kHz) of AWG44, the ac resistance is relatively low and a high Q factor can be achieved.

Substituting _{a}_{t}_{load}_{a}

The CMT [

The KVL equation can be captured in the following matrix from:
_{mn}, M_{mn}, and k_{mn} are the circuit equivalent impedance, mutual inductance, and coupling factor between coilm and coiln. R_{n}, L_{n}, and C_{n} are the equivalent resistance, inductance and capacitance of the nth resonance circuit.

The four coils are set to be resonant at the same frequency, _{n}C_{n}, n_{mn} = R_{n}. For small driver and load coil inductance and relatively large distance between coil1 and coil4, coil1 and coil3, and coil2 and coil4, the cross coupling factor k_{13}, k_{14}, k_{24} can be neglected. The current in coil1 (i_{1}) and coil4 (i_{4}) can be calculated from _{n} (Q_{n} = ωL_{n}/R_{n}). Therefore, the power transfer efficiency can be obtained as [_{2} and N_{3} are the turns of coil2 and coil3. _{2},a_{3},d_{2} and a_{3}, distance d) according to _{23} can be calculated from _{12}, k_{34} can aslo be derived as 0.5 from _{23} as a function of distance (d) between transmitter coils and receiver coils, and

The peripheral circuit mainly includes the transmitter module and receiver module of the wireless power transfer system. The transmitter module is used to generate a power signal with a Class-E amplifier and the receiver module is used to process the receiver signal with a rectifier and voltage regulator circuit. The receiving voltage signal is converted to a stable output voltage of 3.3 V and a current of 10 mA at the distance of 2 cm.

The Class-E power amplifier, known as the highest efficiency power amplifier, is used to reduce the power dissipation. In order to improve the efficiency of Class-E power amplifiers, three rules need to be considered when designing the Class-E: (1) Minimize the voltage across the device as the current flows through it; (2) Minimize the current flowing through the device when voltage exists; (3) Minimize the duration of any unavoidable condition in which appreciable current and voltage exist simultaneously [

The schematic of Class-E power amplifier introduced in this paper is shown in _{p}), load network (LC) and load (R_{L}) [_{p} [_{p}. Because the magnetic resonant coupling between the transmitter coils and receiver coils will reduce the resonant frequency of the system, a tuning capacitance (Cx) in series with LC is added to adjust the resonant frequency point of the transmitting circuit.

At the resonant working state, the voltage across C_{p} may reach 40 V. Because the maximum drain-to-source voltage and the current of power MOSFET (IRF530) are 100 V and 17 A, the on-resistance of which is less than 110 mΩ, it is chosen as the MOS switch. Due to the large gate capacitance, a driver circuit is needed in front of the Class-E power amplifier. As driving stage, the inverter has the advantages of low cost and simple circuit structure. The power MOSFET can be driven effectively by twelve CMOS inverters (74HC04) [

The receiver circuit module shapes the signal received from the receiver coils with the rectifier and voltage regulator circuit. It is used to provide a stable voltage source. This receiver circuit module is designed based on the CSMC 0.5 μm standard CMOS process with the Cadence simulation platform. It is composed of CMOS rectifier circuit and capacitor-less low dropout linear voltage regulator (LDO). In order to meet the demands of the area and power dissipation of the implantable chip, the rectifier chosen in this paper is compatible with the standard CMOS process and the LDO linear regulator has no off-chip capacitor. With the input power voltage changing from 3.5 V to 4.5 V, the LDO linear regulator can produce a stable 3.3 V power supply. When the output current of the LDO is 40 mA, it also has good stability and transient response.

The structure of the rectifier [_{P3}-M_{P6} are added to M_{P1} and M_{P2} to connect the N-well to V_{out}, coil1, or coil2 whichever is at a higher potential. Besides, the higher substrate potential reduces the threshold voltage of M_{P1} and M_{P2}. With the reduction of the threshold voltage, the power dissipation in the rectifier block decreases and the average rectified dc voltage available at the regulator input increases. Reducing the rectifier dropout voltage lowers the minimum receiver coil voltage, which in turn saves the required transmitted power or increases the maximum permissible coupling distance between the transmitter and receiver coils. The instantaneous voltage drop on the transistors of M_{P1} and M_{P2} can be found from:
_{D}_{ox}_{TH}

A capacitor-less LDO architecture [

Since the capacitor-less LDO does not have the off-chip capacitor, a sound compensation scheme for both the transient response and alternating current stability is proposed. It is crucial to regulate the compensation scheme shown as the differentiator circuit in _{f1}_{f2}_{ref}

Bandgap voltage reference circuit uses the negative temperature coefficient of emitter-base voltage in conjunction with the positive temperature coefficient of emitter-base voltage differential of two transistors operating at different current densities to make a zero temperature coefficient reference. The structure [_{T}_{1}_{3}_{EB1} and V_{EB2} are the emitter-base voltage of the transistor Q1 and Q2, respectively. If the size of M3 is equal to the size of M2 and the effect of channel-length modulation is neglected, we can obtain the output voltage of the bandgap reference:
_{EB1} is negative and the temperature coefficient of V_{T} is positive. The compensation of the temperature coefficientsV_{EB1} and V_{T} is ensured by choosing values of n and of the R_{3}/R_{1} ratio and the value of the output voltage is ensured by the _{4}/R_{3}

Since this circuit has a dead (zero current) operating point, a startup circuit shown in

The receiver circuit module rectifies the alternating signal from the receiver coils. It is convenient to integrate the receiver circuit module by using the CMOS rectifier circuit and capacitor-less LDO voltage regulator. Besides, because of the integration of the receiver circuit module, the volume of the receiver circuit is reduced and the stability of the system is increased.

A detachable stent with variable dimensions was designed to accommodate different sizes of the coils, which can be removed to further reduce the volume of the coil. The structure of the coils is described in

The implanted coils and the IC chip are integrated on a PCB board as the _{3}N_{4} is a good sealing material, a layer of 5000 Å Si_{3}N_{4} is firstly deposited on the surface of the coils and PCB board by Plasma Enhanced Chemical Vapor Deposition (PECVD). Then the whole ^{3}. After packaging, the whole

As shown in

_{m}/d = 0.9), which is much higher than 43% of the two-coil system. The highest η is 86% at the distance of 1 cm. What's more, η is little affected by the distance when the distance is less than 2 cm. Even if the distance is 3 cm, the efficiency is as high as 24%.

In order to improve the energy transfer efficiency at the same distance, two magnetic enhanced resonators (5th and 6th) are added as shown in

In order to characterize the relationship between energy transfer efficiency and location parameters, η

The performance of the whole system is measured on the experimental platform as depicted in

A wireless energy transfer system with resonant four coils is presented in this paper. The whole system is composed of a Class-E amplifier, transmitter coils, receiver coils, and signal shaping chip which includes the rectifier module and LDO voltage regulator module. The electrical and geometrical parameters of the coils are theoretically optimized. The energy transfer efficiency is modeled and optimized based on the resonant circuit theory and it is measured based on the designed experimental setup. Experimental results show that the energy transfer efficiency of the resonant four coils is much higher than that of two coils. At the carrier frequency of 742 kHz, the measured coupling efficiency is 85% at the distance of 1.5 cm. The highest efficiency of 86% is obtained at the distance of 1 cm. Even if at the distance of 3 cm, the efficiency is as high as 24%. In addition, the power transfer efficiency can be improved by adding magnetic enhanced resonators. The system measurement results show that the receiving voltage signal is converted to stable output voltage of 3.3 V and a current of 10 mA at the distance of 2 cm. In addition, the output current is changed with the distance.

This work was supported by Beijing National Science Foundation (4122058), the National Natural Science Foundation of China (60706031), the Fundamental Research Funds for the Central Universities (2011JBM202 and 2011JBZ002), and the “Talents Project” of Beijing Jiaotong University.

(

Optimized Q factor _{1} _{a}, N_{t} for coil1; (_{2} _{a}, N_{t} for coil2; (_{3} _{a}, N_{t} for coil3; (_{4} _{a}, N_{t} for coil4.

The equivalent circuit of power transfer system.

(_{23} _{12} = k_{34} = 0.5, L_{2} = 25 μH, L_{3} = 28 μH); (

Schematic of the Class-E power amplifier and driving circuit.

The diagram of the standard CMOS rectifier.

The structure of the capacitor-less LDO.

The schematic diagram of the bandgap voltage reference.

Description of the coils: (

The experimental setup for the four-coil power transfer system.

(

The diagram of adding magnetic enhanced resonators.

(

(

Optimized coil geometry parameters by theory model.

_{t} |
_{a} |
||||||
---|---|---|---|---|---|---|---|

Driver Coil | 1 | 36 | 34 | 10 | 2 | 25 | 1.8 |

Primary Coil | 2 | 38 | 36 | 10 | 2 | 26.5 | 75 |

Secondary Coil | 3 | 16.5 | 14 | 6 | 5 | 26.4 | 29 |

Load Coil | 4 | 19 | 16.5 | 6 | 5 | 24.8 | 2 |

Specification of the four coils (Measured).

1 | 0.05 mm | 2 | 10 | 34 | 36 | 7 | 22 | 2 |

2 | 0.05 mm | 2 | 10 | 36 | 38 | 7 | 24 | 173 |

3 | 0.05 mm | 5 | 6 | 16 | 18 | 2 | 26 | 42 |

4 | 0.05 mm | 5 | 6 | 14 | 16 | 2 | 25 | 2.17 |