Robust Power Oscillator Design for Inductive-Power. Link Applications. Qingyun Ma, Mohammad Rafiqul Haider, and Yehia Massoud. Department of Electrical ...
Robust Power Oscillator Design for Inductive-Power Link Applications Qingyun Ma, Mohammad Rafiqul Haider, and Yehia Massoud Department of Electrical and Computer Engineering The University of Alabama at Birmingham Birmingham, AL 35294, USA E-mail: {maq, mrhaider, massoud}@uab.edu Abstract—Microelectronic devices are widely used in biomedical applications such as infusion pumps, artificial organs, dialysis machines, cochlear and dental implants, etc. For continuous operation of implantable medical devices, the implanted units need to be powered up from an external source. Use of implantable batteries poses potential battery fluid leakage and biohazard. Unlike the batteries, wireless power transmission shows better promises for implanted micro devices. Previously reported differential cross-coupled power oscillator based scheme showed more than 90% link efficiency designed in a 0.5μm standard CMOS process. However, the variation of mutual coupling between the link coils affects the resonance condition and lowers the power-added efficiency of the power oscillator. To make the power oscillator robust, injection-locking mechanism is incorporated with the differential power oscillator. The new injection-locked differential oscillator can lock the frequency with the variation of coupling coefficient by injecting weak differential current signals. Simulation results indicate that with the injection-locking, the oscillation frequency and the power-added efficiency are improved by 4.18% and 24.4%, respectively compared to the regular power oscillator structure for a coupling coefficient of 0.4.
I.
INTRODUCTION
An implantable micro device (IMB) is a kind of medical device designed with micro technology and features lowpower consumption, small size, and high sensitivity. IMBs such as neuromuscular stimulators, cochlear implants, and visual prostheses can help medical practitioners collecting accurate health information effectively [1]. In the recent years many research groups have focused on how to improve the performance of wireless power transmission, and lower the risk of the skin infection caused by tethered cables or biohazard from battery leakage [2]. Inductive-power-link system with the wireless power transmission provides the better solution. An inductive-link system consists of two loosely coupled resonating coils separated by a certain distance. The primary side resides outside of the human body and is driven by the external power source. The secondary side is placed just beneath the skin and captures the power in resonant mode. Previously reported works use solenoid coils to achieve better quality factors of the coils and higher link
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efficiency [3]. On the internal side of the inductive-link system, a cross-coupled MOSFET based rectifier rectifies and boosts up the recovered power signal. The recovered power is then used to power up the implanted sensor electronics. The power link coils can also be used for backward telemetry where usually the load-shift-keying (LSK) is used to back transmit the data to the external unit. The primary side of the inductive-power-link system is usually energized by a class-E power amplifier. Class-E power amplifier (PA) incorporates zero-voltage-switching and can provide theoretically 100% drain efficiency (DE). However, the drive requirement of class-E PA incurs power loss resulting in poor power-added efficiency (PAE). To overcome this problem, a single ended and a differential cross-coupled power oscillator structures have been reported with a PAE improvement by ~20% compared to the conventional class-E PA [4]. The differential power oscillator (POSC) as the core part of the inductive-link system, suffers from output frequency variation due to the variation of the coupling coefficient. Literature review reveals that coupling coefficient varies significantly with the variation of coil distance and the coil outer diameter [9, 10]. The coil geometry variation can be eliminated by fabricating planar spiral coils on a printed circuit board but the distance between the primary and the secondary coils is very hard to control especially for ambulatory monitoring. The variation of frequency as a result of coupling coefficient variation, drives the resonant link coils and the load networks of the POSC out of resonance and the PAE of the entire system is drastically reduced. To achieve the frequency stability without deteriorating the PAE of the system, an injection-locking mechanism has been incorporated in this work with the POSC structure. Injection-locking has been widely used to improve the efficiency and the power consumption of different RF circuits such as frequency dividers [5], frequency multipliers [6], transmitters [7], etc. In this work, two differential weak current signals are injected to stabilize the output frequency and withstand the coupling coefficient fluctuations under locking condition. The organization of the paper is as follows. Section II shows the brief description of the proposed injection-locked
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Fig. 1. Differential cross-coupled power oscillator
POSC structure, section III shows the performance of the proposed structure with the variation of coupling coefficient and finally section IV draws conclusion. II.
PROPOSED INJECTION-LOCKED DIFFERENTIAL POWER OSCILLATOR STRUCTURE
Fig. 1 shows the circuit schematic of the differential crosscoupled POSC used for the inductive-power-link system [4]. It consists of cross-coupled MOSFETs to provide positive feedback, resonant LC tanks to generate resonant frequency, load network to ensure zero-voltage-switching of the crosscoupled MOSFETs and finally a bottom LC tank to filter out 2nd harmonic component of the oscillating frequency signal. Due to the cross-coupled MOSFET structure and the resonating LC tank, the circuit can generate high frequency signal and obviate the need of a driver block. The free running frequency of the POSC can be depicted as,
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f osc = 2π
LRF 1C f
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where LRF and Cf are the inductance and capacitance of the resonant tank circuit, respectively. The load network ensures zero-voltage-switching and helps achieving higher power efficiency. Fig. 2 shows the setup of the POSC with an inductive-link
Fig. 3. Simulated drain voltage and current wave shapes of the differential cross-coupled power oscillator
coil system. The primary side of the link coils is energized by the POSC where the power is magnetically coupled to the secondary side in resonant condition. Use of series resonance on the primary side and parallel resonance on the secondary side ensures better power transmission. An equivalent resistance value of 1 KΩ has been used to simulate the deliverable power to the sensor electronics inside the human body. Fig. 3 shows the simulation results of the POSC with a supply voltage of 3 V. From the figure, it is clear that the differential cross-coupled POSC can maintain zero-voltageswitching and achieve PAE of more than 90% for a load resistance of 400 Ω. However, the link coil system of the POSC suffers from coupling coefficient fluctuations due to the misalignment, change of coil geometry, and body movement, and consequently, it disturbs the resonating resonant frequency of the resonant tanks and the load networks resulting in degraded efficiency. To overcome this problem, an injection-locking mechanism has been employed with the POSC structure. The functional block diagram of an injection-locked oscillator has been shown in Fig. 4. The core functionality of an injection-locked oscillator lies in a mixer block. Inside the mixer the injection signal beats with the free running oscillator signal and the resultant harmonic signals are filtered out by the band pass filter of the LC tank. Only that harmonic signal falls within the bandwidth of the band pass filter comes out of the band pass filter and the oscillator continues to oscillate at that frequency.
Fig. 2. Inductive-power-link using differential cross-coupled power oscillator
Fig. 4. Behavioral block diagram of an injection-locked oscillator
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the passive components are envisioned to implement using commercial off-the-shelf (COTS) components.
X-tal Oscillator
Vinj-
With a stable link coil system as well as load network, the differential POSC without injection-locking shows more than 90% link efficiency. In this case, all of the LC tank circuits are working in the perfect resonance condition. Changing coupling coefficient may create the main oscillation frequency to shift which disturbs the other three LC tank circuits to lose the resonant frequency. Deviation from the resonance condition increases the power loss and eventually results in a poor PAE.
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Fig. 5. Proposed injection-locked differential cross-coupled POSC
Fig. 5 shows the proposed injection-locked differential POSC where two parallel MOSFETs have been used to inject two differential current signals at the drain terminals of the cross-coupled MOSFETs of the main POSC. Under weak injection (Iinj
