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1、Class-E Oscillators as Wireless Power Transmitters for Biomedical ImplantsAnthony N. LaskovskiMehmet R. YuceSchool of Electrical Engineering andSchool of Electrical Engineering andComputer ScienceComputer ScienceThe University of NewcastleThe University of NewcastleCallaghan, NSW, Australia 2308Call
2、aghan, NSW, Australia 2308Email: alaskoieee.orgEmail: Mehmet.Yucenewcastle.edu.auAbstractThis paper presents the use of Class-E oscillators as inductive power transmitters for implanted telemetry devices that transmit information sensed by biosensors. Several Class-E oscillators are compared with an
3、 equivalent Class-E amplifier, showing higher efficiency while maintaining frequency clarity, stability and accuracy. Energy has been successfully transferred to a receiving inductor 1.5cm away, which has been rectified to produce a 1VDC signal.I. INTRODUCTIONWireless power transmitters form an impo
4、rtant role in sup-plying energy to implanted electronic devices and biosensors. Inductive coupling was introduced to biomedical implants to recharge implanted batteries for devices such as pacemakers in order to avoid periodic surgery to replace flat batteries 1 2.As biomedical implant technology pr
5、ogresses with devel-opments such as cochlear and retinal prosthesis, attention is increasingly being focused on the supply of constant wireless power within tight space and power constraints. As space restrictions tighten, so does the allowable size of components in the implanted environment. This i
6、mplies that higher fre-quencies of operation are required such that the size of circuit components is reduced. However, the transfer of energy to implanted devices becomes less efficient as the frequency of transmission increases 3. This makes the design of highly efficient higher frequency power tr
7、ansmitters a point of inter-est.The concepts behind inductive power transfer may be un-derstood by considering a weakly coupled transformer, with a primary coil from which energy is transmitted and a secondary coil at which energy is received. The core of this transformer is a combination air and se
8、veral layers of human tissue between the two coils. Once the energy is received at the secondary coil, it is rectified and regulated as a DC supply for the implanted electronics and biosensors. The primary coil external to the body is driven by a power transmitter.Given that space restrictions cause
9、 a natural progression towards smaller coils and therefore higher transmission fre-quencies, power transmitters have been developed at higher frequencies. One particular field of interest has been switching power amplifiers. They operate efficiently at higher frequen-Fig. 1.The Class-E amplifier 5Fi
10、g. 2.The Class-E oscillator 6cies, which is why the Class-E power amplifier has been widely adopted as the means by which energy is inductively transferred to an implanted device 4.Shown in Fig. 1, the Class-E amplifier comprises an in-ductor L2. This inductor represents the primary coil for the tra
11、nsmission of wireless energy to an implanted device. The amplifiers efficiency at high frequencies is attributed to its ability to hold zero charge across the terminals of the transistor while it is switching. Not only is the voltage designed to be zero, but so is the rate of change of voltage (dv/d
12、t). It allows for a power amplifier which efficiently transmits energy across L2 according to the switching frequency supplied by the input voltage vi. The capacitor C1 also absorbs parasitic978-1-4244-8132-3/10/$26.00 2010 IEEEcapacitance that exists between transistor terminals, which becomes incr
13、easingly significant at higher frequencies where design parameters are in the order of parasitic impedances 5, 7, 8.The Class-E amplifier works efficiently at high frequencies, however the fact that it is a switching power amplifier means that it requires a high frequency square-wave input in order
14、to operate effectively. This assumption does not consider the energy that is required to produce the square-wave input using a dedicated oscillator, be it a crystal or LC oscillator.The idea of feeding an oscillated output signal back to the input of the amplifier implies that the circuit would be s
15、elf-oscillating, similar to the commonly known Colpitts oscillator, while operating with zero switching conditions. This is the concept behind the Class-E oscillator as shown in Fig, 2 6.Additional circuit elements have been added to form the Class-E oscillator, namely the feedback elements C3, C4 a
16、nd L3. It was designed by Ebert et al. to constructively shift the phase of the feedback point of the oscillator 6. The diode D1 is placed at the input of the transistor in order to clip the input signal such that it appears as a square wave, satisfying the requirement of the Class-E circuit to have
17、 a square-wave input.Given that low power consumption is advantageous in biomedical systems, it is useful to consider a self oscillating Class-E oscillator as a wireless power transmitter rather than a Class-E power amplifier. Similar to the power amplifier, the oscillator would transmit energy thro
18、ugh L2.II. DESIGN AND IMPLEMENTATION OF CLASS-E TRANSMITTERSA. Comparison of Class-E TopologiesIn order to determine whether using a Class-E oscillator is advantageous over a Class-E amplifier, similarly designed circuits have been constructed and tested with the main circuit elements chosen to be a
19、s similar as possible. The transistor used for all of the five circuits is BC547B from Fairchild Semiconductors. The elements of the Class-E circuits have been determined according to 8, and all of the circuits are supplied with a 3V supply. The inductor L2 is in fact a spiral inductor, forming the
20、primary coil for the transfer of inductively transferred power.The first power transmitter circuit uses the Class-E amplifier, as shown in Fig.3. The 27-MHz clock input signal to the amplifier is produced by the crystal oscillator ACHL-27MHZ-EK. Small 1 measurement-resistors Rmeas have been posi-tio
21、ned to measure the total current and current through the transmission coil such that the input and output power can be measured. The 27MHz frequency of transmission is in the allowable limits of the Industrial, Scientific and Medical (ISM) band.The power used by the circuit is 77mW (18.9dBm), which
22、is determined by multiplying the supply voltage with the r.m.s. value of the total current. The output power of 11.2dBm is calculated by multiplying the r.m.s. voltage and current acrossFig. 3. A 27MHz Class-E amplifier constructed in hardware, driven by a crystal oscillator.Fig. 4. Schematic diagra
23、m of a Class-E oscillator with an LC feedback network and element between the transistors base and emitter terminals, being either a 100k resistor or a Schottky diode.and through the inductor L2. This corresponds to an efficiency of 17%.The next step of the comparison involves the construction of th
24、e Class-E oscillator circuit, shown in Fig.4. This differs from the Class-E oscillator shown in Fig.2 in that the load is represented by the inductor L2 rather than a resistor. The feedback point is also taken between L2 and C2, and consists of an LC pair in order to reverse the phase difference tha
25、t is incurred at the point across L2. The element between the base and emitter of Q1 is a 100k biasing resistor. The power used by this circuit is 35mW (15.5dBm) and 11.4dBm is transmitted, corresponding to an efficiency of 39%. While this is much higher than the efficiency of the Class-E Amplifier
26、of Fig.3, the 34MHz frequency of the output is higher than the desired 27MHz value. This is due to the fact that the accuracy of the circuits frequency is controlled solely by the accuracy of its individual inductor and capacitor values.A variant of this circuit has been implemented by inserting a 2
27、7MHz crystal (Citizen America CS1027.000MABJ-UT) as978-1-4244-8132-3/10/$26.00 2010 IEEEFig. 5. Schematic diagram of a Class-E oscillator with a 27MHz Crystal feedback network and element between the transistors base and emitter terminals, being either a 100k resistor or a Zener diode.the feedback n
28、etwork in order to create a stable frequency. This crystal is not an oscillator itself, but an accurate impedance that ensures the input frequency to the transistor is 27MHz. The schematic of this circuit is shown in Fig.5, with the base-emitter element being 100k resistor. The power used by the cir
29、cuit has been measured to be 112mW (20.5dBm) and the output power is also 20.5dBm, which corresponds to a measured efficiency of 100%. This is attributed to negligibly small losses in the biasing circuit of the amplifier and zero-switching conditions at the collector of the transistor.B. Class-E Osc
30、illators biased with DiodesClass-E oscillators presented in literature have employed Schottky diodes across the base and emitter of the circuits transistors 6. This is usually done to clip the input signal such that it approximates a square-wave input, which is preferred as the transistor continues
31、to operate as a switch.The LC feedback Class-E oscillator of Fig.4 has been imple-mented using a Schottky diode from Fairchild Semiconductors (MBR0520L), which is the same diode used in 6. The power used by this circuit is 42mW (16.2dBm) and an output power of 9.9dBm is produced, corresponding to an
32、 efficiency of 23%, which is considerably lower than the 39% efficiency achieved with a 100k resistor in place of the diode. Zener diodes are a more suitable option than the Schottky diode used, however the voltage level of the output is not high enough to switch the transistor.A 3V Zener diode (NXP
33、-BZX384-C3V0) has been imple-mented as the element between the base and emitter of Q1 in a Class-E oscillator based on Fig.5. The circuit works in that the feedback voltage is high enough to drive the transistor and Zener diode. The circuits power usage has been recorded as 63mW (18dBm), with an out
34、put power of 18dBm. This mathematically corresponds to 100% efficiency, and is attributed to negligibly small losses in the biasing circuit ofFig. 6. Voltage at the collector of Q1 in Fig. 5 with a 9V supply and a 3V Zener diode between the base and emitter.the amplifier and zero-switching condition
35、s at the collector of the transistor.The results of these experiments are summarised in Table I. The first significant point is that using a feedback network in the Class-E circuit improves the efficiency of the transmitter, in that a dedicating oscillating unit is no longer required.Another signifi
36、cant point is that including a crystal in the feedback network allows the frequency of the transmitter to be controlled, rather than relying on the accuracy of individual inductors and capacitors, which may cause unstable or inac-curate transmission frequencies.The use of zener diodes on the base of
37、 the circuits transmit-ter makes little difference on the performance of the amplifier, however it may be an important to regulate the output of the transmitter in the case that the feedback voltage becomes too high.C. Class-E Oscillator with Crystal FeedbackA higher-power Class-E oscillator based o
38、n Fig.5 has been tested, operating at 9V and using the Zener diode (NXP-BZX384-C3V0). The voltage at the collector of the transistor is shown in Fig.6, which ranges from 3V to over 15V. Ideally the lowest point should be 0V, as this would remove the wasted energy discharged on the capacitor C1.The i
39、nput voltage at the transistor is shown in Fig.7, where the intended function of the Zener diode can be seen as the input signal is clipped at 3V. The output voltage across VL2 is shown in Fig.8, measuring at 18V. As indicated by the waveform, it is a reasonably clean 27MHz sinusoidal wave.The input
40、 power used by this circuit measures at 794mW (29dBm), with an output power of 28dBm. This corresponds to a measured efficiency of 80%. The likely cause of the circuits deviation from 100% efficiency is the voltage at the collector not reaching zero during the transistors switching points, causing a
41、 discharge of energy from C1.D. Wireless Power TransferThe higher-power Class-E oscillator of Section II-C has been tested as a wireless power transmitter, with L2 rep-978-1-4244-8132-3/10/$26.00 2010 IEEETABLE ICOMPARISONS BETWEEN CLASS-E CIRCUITSClass-E CircuitB/E ElementfPower UseVL2ppOutput Powe
42、rAmplifier of Fig.327MHz18.9 dBm1.6 V11.2 dBmOscillator with LC feedback, Fig.4100k34MHz15.5 dBm0.9 V11.4 dBmOscillator with Crystal Feedback, Fig.5100k27MHz20.5 dBm7.0 V20.5 dBmOscillator with LC feedback, Fig.4Schotky Diode34MHz16.2 dBm1.2 V9.9 dBmOscillator with Crystal Feedback, Fig.5Zener Diode
43、27MHz18.0 dBm6.7 V18.0 dBmFig. 9.Stacked spiral receiving inductor 9.Fig. 7. Voltage at the input of Q1 in Fig. 5 with a 9V supply and a 3V Zener diode between the base and emitter.Fig. 10.Voltage received on a spiral coil from the transmitter shown in Fig.5 with a 9V supply and a 3V Zener diode bet
44、ween the base and emitter.Fig. 8. Voltage across output spiral L2 in Fig. 5 with a 9V supply and a 3V Zener diode between the base and emitter.resenting the transmitter spiral. The receiving coil is the stacked spiral inductor of Fig.9, which was designed for use in biological implants.At a distance
45、 of 1.5cm a voltage of 1.5Vpp is received on the coil. This signal has been rectified to produce the 1V signal shown in Fig.11, which is a level at which current IC technologies and biosensors operate at.III. CONCLUSIONThis paper proposes the use of Class-E oscillators as in-ductive power transmitte
46、rs for implanted telemetry devices that transmit information read by biosensors. Several Class-E oscillators were compared with the commonly used Class-E amplifier. The Class-E oscillators were determined to be moreFig. 11. Rectified voltage received on a spiral coil from the transmitter shown in Fi
47、g. 5 with a 9V supply and a 3V Zener diode between the base and emitter.978-1-4244-8132-3/10/$26.00 2010 IEEEefficient when considering the power used by the input signal to the amplifier.Different feedback options are compared amongst Class-E oscillators, and it is proposed that a crystal feedback network ensures a more efficient circuit with a more stable and better controlled frequency. The use of Zener diodes at the input of the oscillator is determined to be advan