Design and Realization of a 434 MHz, 200 Watts Solid-State Power Amplifier for a Microwave Hyperthermia System

Transcription

1 2011 International Conference on Circuits, System and Simulation IPCSIT vol.7 (2011) (2011) IACSIT Press, Singapore Design and Realization of a 434 MHz, 200 Watts Solid-State Power Amplifier for a Microwave Hyperthermia System Pakawat Kiatwarin, Suramate Chalermwisutkul The Sirindhorn International Thai-German Graduate School of Engineering (TGGS), King Monkut s University of Technology North Bangkok (KMUTNB), 1518 Pibulsongkram Rd., Bangsue, Bangkok, Thailand Abstract. This paper presents a class-b power amplifier (PA) for a microwave hyperthermia system. The power amplifier has been designed and fabricated for the operating frequency of 434 MHz with a Siliconbased LDMOS field effect transistor as the main power device. During the design process, source-pull and load-pull techniques were used for determining the optimal impedance in order to obtain maximum Power Added Efficiency (PAE) whereas the output power should be maintained higher than 53 dbm. The average power-handling capability (APHC) of the microstrip lines in the PA circuit and proper heat sink for the proposed power amplifier are taken into account and presented. As a result, our power amplifier achieves a peak PAE of 45.8% with a gain of 18.9 db and typical output power of 53 dbm (with 34 dbm input power). Keywords: RF power amplifier, class-b operation, microwave hyperthermia system. 1. Introduction Cancer is a leading cause of death worldwide. This disease accounted for 7.4 million deaths in World Health Organization (WHO) estimates that, without intervention, cancer will kill approximately 84 million people from 2005 to 2015 [1]. Therefore, treatments are necessary to prolong lives and to improve quality of life of the patients. Nowadays, Microwave (MW) hyperthermia is one of the most promising medical treatments for cancer patients. With this kind of treatment, cancerous tissue is exposed to a temperature of C in order to shrink and destroy this without harming noncancerous cells. It also does not have side effects in contrast to radiation and chemo therapy [2], [3]. The treatment is based on the fact that the healthy cells can withstand temperatures up to 45 C, whereas the cancer cells do not survive temperatures over 41 C [3]. A MW hyperthermia system can be described with the block diagram shown in Fig. 1. It is basically composed of a solid-state microwave source, low-loss coaxial transmission lines capable of transporting the necessary microwave power required, an applicator which radiates the electromagnetic energy to the treated tissue, thermometers for the monitoring of the temperature distribution in the tumour and a control unit which adjusts the output power of the MW in order to keep the temperature in the treated region constant at the desired level [4]. Power amplifier is an important component in the hyperthermia system described above which amplifies a low-level input signal from the signal source and delivers a high power signal to the output (see Fig. 1). This paper describes design and realization of a solid-state power amplifier for a hyperthermia system which can deliver an output power of 200 Watts. As an operating frequency, 434 MHz has been chosen since it resides in the so-called ISM (Industrial, Scientific and Medical) band. Transceivers and amplifiers for other applications using this frequency bands [5], [6], [7] have been reported in the literature. However, the output powers of such CMOS technology-based applications are relatively low. In comparison, the power amplifier proposed in this work aims for high output power (200 Watts) with high supply voltage (50V). 183

2 MICROWAVE SOURCE POWER AMPLIFIER INTERFACE BOARD CONTROL UNIT COAXIAL LINE APPICATOR o C THERMO METER TEMPERATURE PROBES TREATED TISSUE Fig. 1: Microwave hyperthermia system block diagram [4] Considering this specification, LDMOS is an appropriate power device for this application [8] and therefore, chosen for this work. Due to high output power, efficiency of the power amplifier is a critical issue. In case of low efficiency, the high loss would lead to dissipated heat which is unwanted. In this work, a class-b power amplifier is chosen since it can provide high efficiency whereas linearity is not so critical. Due to the high output power, thermal consideration must also be taken into account. In the next section, design of the proposed class-b power amplifier based on LDMOS transistor will be explained in more details. 2. Class-B High-Power Amplifier Design Power amplifiers are categorized in classes A, AB, B, and C depending on the conduction angle of the output current waveform. The different conduction angles from 0 to 360 can be set by changing the bias point [9]. This work concentrates on class-b due to its high power efficiency compared to class-a and -AB. Typical for class-b operation, the bias drain voltage is located midway between the knee voltage and the maximum drain voltage. The DC current point is located at zero drain current (pinch-off), so that the output drain current waveform is a half sinusoid. This represents the conduction angle of 180. The ideal maximum drain efficiency of a class-b amplifier reaches 78.5% [9]. The block diagram of the proposed class-b power amplifier is shown in Fig. 2. It consists of an Input Matching Network (IMN), an Output Matching Network (OMN), Input Bias Network, Output Bias Network and the main power device (transistor). The IMN and OMN are used for transforming input and output impedances of the transistor to the source and load impedances, respectively in order to achieve maximum power transfer and minimize the reflection [10]. The gate and drain bias networks provide DC voltages to operate the transistor while preventing the RF signals to go into the DC sources. By doing this, the transistor can act as a voltage controlled current source. For design of high power amplifiers, average power-handling capability of the circuit and thermal considerations must also be taken into account. In this paper, we selected MRF6V2300NR1 LDMOS field effect transistor from Freescale as the power device for the proposed power amplifier. This transistor can operate at frequencies up to 600 MHz with excellent thermal stability. According to the datasheet, this device can provide maximum output power of 300 Watts at 450 MHz. According to the class-b operation, the gate-source and drain-source bias voltages are set to 1.75V and 50V, respectively. As the substrate, Rogers RO4350B (ε r = 3.66, h = mm, tanδ = ) is used. The class-b power amplifier circuit was designed and simulated using Agilent s Advanced Design System (ADS). Load-pull and source-pull techniques have been applied to determine the optimum impedances presented to 184

3 the transistors at the drain and gate sides, respectively. From the simulations, the optimal input impedance of (0.407+j1.4) Ω and the optimal output impedance of (1.022+j1.475) Ω lead to a PAE of 58% and maximum output power of 54 dbm. Input and output matching network were designed based on the results of sourcepull and load-pull analysis in order to match the optimum input and output impedances to 50 Ohm. Fig. 2: Block diagram of the proposed class-b power amplifier. During the circuit design, the average power-handling capability (APHC) of a microstrip line was taken into account where the temperature rise of the strip conductor and the supporting substrate are considered. In general, APHC depends on several parameters such as transmission line losses, thermal conductivity of the substrate material, surface area of the strip conductor and temperature of the medium surrounding microstrip [11]. Losses in form of heat is well transferred to the surroundings if the thermal conductivity is high. So, substrate material with low loss tangent and high thermal conductivity contributes to a high APHC. Moreover, the wider the strip conductor, the higher APHC of the microstrip line can be achieved. The maximum average power for a given line can be calculated as P av = (T max - T amb )/ T (1) where T is the rise in temperature per watt, Tmax is the maximum operating temperature, and T amb is ambient temperature [11]. In this paper, the maximum operating temperature T max is assumed to 100 C, the ambient temperature T amb to 20 C. The rise in temperature per watt T can be calculated with material parameters e.g. tanδ, thermal conductivity K, relative dielectric constant ε r, etc. [11], [12] to C/W. According to equation (1), the maximum average power P av is equal to 1.73 kw. Thermal considerations of the power transistor are also important since the limited efficiency leads to heat generation due to the loss. The generated heat has to be removed in order to improve the performance and prevent the transistor from damaging. In order to improve the heat transport from the transistor, sufficient heat dissipation through a heat sink is needed. Thermal resistance of a proper heat sink can be determined with Θ SA = [(T j -T amb )/P d ] (Θ jc + Θ cs ) (2) where ΘSA is the thermal resistance of the heat sink, Tj is the transistor s junction temperature, Tamb is the ambient temperature, Pd is the dissipated power, Θjc is the transistor thermal resistance and Θcs is the thermal resistance case-to-heat sink [13]. For our proposed power amplifier, we set Tj = C, Tamb = 20 C, Pd = W, Θjc = 0.24 C/W, and Θcs = C/W, so that ΘSA = 0.28 C/W. 3. Experimental Results A photograph of the fabricated power amplifier is shown in Fig. 3. Fig. 4 shows the simulation and measurement results of the output power and gain versus the input power at 434 MHz. It is obvious that the simulation and measurement results are in good agreement. From the measurement results, the maximum output power of approx. 53 dbm can be reached with the maximum gain of 18.9 db and the input power of 34 dbm. Fig. 5 depicts the simulated and measured power added efficiency (PAE) and drain efficiency of the fabricated power amplifier versus input power. The 185

4 simulated and measured maximum PAEs are 56% and 45.8%, respectively. Accordingly, the simulated and measured maximum drain efficiencies are 56.7% and 46.4%, respectively. In addition, this power amplifier has been tested with other bias points: first, with Vgs = 2.75V and Vds = 50V (class-ab bias condition) and then, with Vgs = 0V and Vds = 50V (class-c bias condition). The results are presented in Table 1. At the same output power of 200 Watts, class-c bias condition provides the highest PAE and drain efficiency of 47.09% and 48.66%, respectively whereas class-ab bias condition provides the highest gain of 20.5 db. Class-B is a compromise between class-ab and class-c which provides PAE of 45.81%, drain efficiency of 46.4% and gain of 18.9 db. Fig. 3: Fabricated 434 MHz power amplifier using a lateral MOSFET Fig. 4: Simulation and measurement results of output power and gain versus varied input power at 434 MHz with gate bias voltage of 1.75V and drain bias voltage of 50V. Fig. 5: Simulation and measurement results of power added efficiency (PAE) and drain efficiency versus varied input power at 434 MHz with gate bias voltage of 1.75V and drain bias voltage of 50V. TABLE I MEASUREMENT RESULTS OF OUTPUT POWER, GAIN, PAE AND DRAIN EFFICIENCY WITH VARYING BIAS POINTS CLASS-AB: VGS = 2.75V AND VDS = 50V, CLASS-B: VGS = 2.1V AND VDS = 50V, AND CLASS-C: VGS = 0V AND VDS = 50V P out (dbm) Gain (db) PAE (%) Drain Eff. (%) Class-AB Class-B Class-C

5 4. Conclusion A 434 MHz class-b power amplifier for microwave hyperthermia system has been proposed. The proposed power amplifier was designed using Si-LDMOSFET from Freescale as the power device and source-pull/load-pull techniques. Thermal issues including APHC of the microstrip structures on the circuit and proper heat sink for heat transfer from the transistor were also considered. Our experimental results show that our proposed power amplifier can achieve the output power of 200 Watts, PAE of 45.8%, drain efficiency of 46.4% and gain of 18.9 db which satisfy the specification required for a microwave hyperthermia system. 5. Acknowledgment The authors wish to thank MSc. Jesus Cumana, MSc. Jan Vrba and the members of the Chair of Electromagnetic Theory (ITHE) at RWTH Aachen University in Germany for providing great support. The authors also would like to thank the members of the Communication Engineering team at the Sirindhorn International Thai-German Graduate School of Engineering (TGGS), King Monkut s University of Technology North Bangkok (KMUTNB), Thailand for encouragement and suggestions. 6. References [1] World Health Organization. World Cancer Day [online]. last checked 5 June 2010 [2] Guillermo Jacqueline L. Long, The Gale Encyclopedia of Cancer: Guide to Cancer and Its Treatments, 2 nd ed., THOMSON Gale, a part of the Thomson Corporation, 2005, pp. 583 [3] Safarik, J.; Vrba, J., "Slotted Applicator for Microwave Local Hyperthermia," Microwave Techniques, COMITE th Conference on, pp.1-4, April 2008 [4] P. Togni, J. Vrba, and L. Vannucci, System to study the effects of microwave hyperthermia on in-vivo melanoma model, in Microwave Conference,2008. EuMC th European, volume , [5] F. Zhao, X. Gao, and et al., "A CMOS 434/868 MHz FSK/OOK transmitter with integrated fractional-n PLL," Wireless and Microwave Technology Conference, WAMICON '09. IEEE 10th Annual, pp.1-4, April 2009 [6] Melly, T.; Porret, A.-S.; Enz, C.C.; Vittoz, E.A., An ultralow-power UHF transceiver integrated in a standard digital CMOS process: transmitter, Solid-State Circuit, IEEE Journal, vol.36, no.3, pp , Mar 2001 [7] S. Yoo, H. J. Ahn, and et al., "The design of 433 MHz class AB CMOS power amplifier," Mixed-Signal Design, SSMSD Southwest Symposium, pp.36-40, 2000 [8] MRF6V2300N Datasheet, RF Power Field Effect Transistors, N-Channel Enhancement-Mode Lateral MOSFETS Rev.2, Freescale Semiconductor Inc. May 2007 [9] A. Raghavan, N. Srirattana, J. Laskar, Modeling and Design Techniques for RF Power Amplifiers, John Wiley & Sons, 2007, pp [10] G. Gonzalaz, Microwave Transistor Amplifier Analysis and Design, 2 nd ed., Prentice Hill, 1996, pp [11] I.J. Bahl and K.C. Gupta, Average power-handling capability of microstrip lines, Optics and Acoustics, in IEEEJournal on Vol. 3, No. 1, pp. 1-4, January [12] David M. Pozar, Microwave Engineering, 2 nd ed., John Wiley and Sons, New York [13] Helge Granberg Norman Dye, Radio frequency transistor principles and practical, 2 nd ed., Newnes, pp