An adaptive feedback for implantable wireless power transmission system

Abstract

The safety of powersupply for biomedical implants is critical in an implantable system.In this paper, an adaptive feedback with pulse width modulation (PWM) and load shift keying (LSK) is introduced and used in the design of a wireless power tra nsmission system. Experimental results show that the system can receive stable power wh en the distance bet\Wen the transmitter and receiver changes. Compared to the system without adaptive feedback, the efficiency is improved by 5.7times with the dis tance unde r 20mm.Keywords----a daptive feedback; biomedical implants; efficiency I. I NTRODUCTION Wireless powering of biomedical implants obviates the need for batteries which must be periodically replaced and constitutes a health risk, but faces a new problem of power supply safety since it may transfer excessive power and hurt human body. Kinds of solutions have been studied so far. Liu W.T. et al. achieved the feedback control of power by using analog-to -digital converter (ADC) to sample the received power and send it to the transmitter [I]. The systems in [2] and [3] kept the received voltage stable by using the variable capacitor and electro nic-control inductor. Yang T. and Wang B.implemented the feedback by changing the frequencies and duty cycle of the driving signal [4,5]. However, they are complex and not easy to implement for biomedical implants.In this paper, an adaptive feedback with pulse width modulation (PWM) and load shift keying (LSK) is introduced into the wireless power supply system. It makes the recovered power more stable and the structure of the parts inside the body is more simple, and thus it is more suitable for biomedical implants. TT. T HE T HEORYOF T HE F EEDBACKThe blo ck diagram of the wireless power transmission system for biomedical implants is shown in Fig. 1. The signal source is amplified by the power amplifier, then drives the transmitting coil and transmits power to the receiver. The system consists of two feedback loops, including the PWM feedback lo op and the LSK feedback loop. A. The Pulse Width Modulation Feedback LoopThe functional blo ck diagram of the PWM feedback loop is shown in Fig. 2. It is made up of the receiving coil, the full-wave rectifier, the PWM Mo dulation circuit and the LSK modulation circuit.L is the inductance of the receiving coil and C is the shunt capacitor. When the MOS transistor M1 is turned off, the 978-1-4673-2523-3113/$31.00 mOl3 IEEE recelvmg coil is set to be tuned at the same resonance frequ ency with the transmitting coil and the current of the load will achieve the maximum value iM When MI is turned on, the inserted capacitor Cn will make the circuit deviate from the resonance and the load current will decrease to a lower value iL• Fig. 1. The wireless power transmission system ,---Itcccl"llI� I L Cull I I I I M nl:I�� �lon ) ,----------" Fig. 2. The PWM feedback loopThe received rectifier output voltage Urecis sampled and put into the PWM unit, producing a signal Upwm whose duty cycle D is proportional to Urec• Then the average load current iav...