ANALYSIS OF PLANAR BALUNS FOR 1 kW SOLID-STATE AMPLIFIERS AND POWER COMBINING FOR 1 . 8 kW *

Solid-state transmitter for booster and storage ring in synchrotron would be composed of hundreds of amplifier modules. The amplifier module is biased at class AB and constructed in push-pull operation. Recent trend of amplifier module design features higher power up to 800 Watts and equipped planar balun (balance-unbalance converter) for push-pull operation. In NSRRC, the exclusive round planar design has encountered high temperature situation at kW range. Therefore, further study on this thermal condition is carried out in this study. Four types of planar balun design and two laminate materials are used for heat analysis. The typical coaxial balun is also applied on actual amplifier design. The results bring the better design with proper laminate choice and leads to acceptable thermal distribution with 1kW output power at 500MHz. Besides, for a more compact module with higher output power, the combination of two chips on the same circuit reaching 1.8kW is also presented. INTRODUCTION Since the usage of solid-state technology for high power transmitter application has become more and more popular in accelerator facilities application [1-2], the study of the basic property of solid-state amplifier module in maximum available power is still required. In UHF band, the common used transistors are Freescale MRF6VP41KH and NXP BLF578 serial chips [3-4]. The above chips can operate reliably with properly water cooling below them. Although these chips claim them to have maximum output power of more than 1200W in pulse operation, for CW operation, due to thermal dissipation limitation with the increasing temperature, the practically available maximum power is still limited within 1000Watt [3]. In 2013, NSRRC has developed an exclusive kind planar balun with low loss that can greatly reduce the generated heat on it [5]. However, the thermal study still lacks systematic experiments to diagnosis what is the key issue affecting the thermal distribution and RF performance. In this article, four versions of planar balun and one coaxial balun on two kinds of microwave laminate are manufactured with BLF578 chip and tuned to have 1kW saturation power. The thermal distribution at output parts of the amplifier circuits are thus discussed in a more detail manner. PLANAR BALUNS WITH BALANCE TUNING MECHANISM Several versions of printed planar balun have been designed and applied on the push-pull amplifier circuit design in the past two years in NSRRC. Although the printed balun can successfully deliver 1kW output power continuously without problem, certain power degradation and higher driving power is also observed during tests. Unlike coaxial balun soldered on circuit which has better thermal transmission path through substrate and heat sink, the printed balun is actually floated without touching base heat sink for efficient thermal transmission path. Besides, the proposed planar baluns is modified from Marchand balun [6] and forming circulator shape, some tuning is still necessary for better amplitude and phase balance. In order to have flexibility of tuning, the tuning mechanism is kept in each version as shown in Fig. 1.