一种应用于射频热疗的新型功率电源设计

A new power supply design for radio-frequency hyperthermia

目的:目前大功率电容式射频热疗机在使用过程中经常需要根据患者的体质结构、环境温度变化等,通过调节电抗元件的参数去实现输出回路阻抗的匹配,从而获得最佳的加热效率。但这种调节既不灵便,又容易导致设备的可靠性下降。本文设计了一种新的基于单片机和直接数字频率合成(DDS)技术的射频功率电源。方法:射频信号来自DDS芯片,输出回路工作在谐振放大状态,单片机控制系统不断监测输出射频电压的幅度,当回路阻抗变化导致谐振电路失谐(或输出阻抗失配)时,射频输出电压幅度会大幅下降。单片机可控制DDS通过扫频的方式改变放大电路输入信号的频率,使电路在新的频率点上重新建立谐振,从而控制输出射频电压幅度的稳定。文中通过理论计算和电路仿真相结合的方式对设计进行了验证。结果:以初始谐振参数为参考值,当外电路电容参数变化在±5 p F内时,输出电压幅度变化约为20%,而单独调节DDS输出信号的频率变化在(?0-1.33)MHz^(?0+1.17)MHz内,输出电压幅度变化也可以达到18%。通过实时调节DDS输出信号的频率能够修正由于外电路参数变化引起的输出电压波动,使热疗过程中输出电压的幅度变化不超过设定值的2%。结论:当射频热疗机的负载和电路参数变化时,可以通过单片机自动调节DDS输出的信号频率来维持输出射频电压幅度的稳定。

Objective The current high-power capacitive radio-frequency （RF） hyperthermia apparatus needs to adjust the parameters of reactance element based on patient＇s physical structure and environmental temperature to realize output circuit impedance matching, achieving the best heating efficiency. The adjustment is inconvenient, easily decreasing the reliability of apparatus. A new RF power supply based on micro control unit （MCU） and direct digital frequency synthesis （DDS） technology is designed in this paper. Methods RF signals were generated by the DDS chip, and the output circuit was at resonance amplifying state. MCU system continuously monitored the amplitude of RF output voltage. When the changes of circuit impedance leaded to the detuning of resonant circuit or output impedance mismatch, the amplitude of RF output voltage would dramatically decreased. At this moment, MCU would control DDS to change the frequency of the input signal of amplification circuit by sweep frequency. And then, a new resonance would be established on another frequency to control the stability of output RF voltage amplitude. Finally, the theoretical calculation combined with circuit simulation was applied to verify the new design. Results With the reference value of initial resonance parameters, when the capacitance parameter of external circuit changed within 5 pF, the changes of output voltage was about 20%. However, if the frequency change of the DDS output signal was individually adjusted from f0-1.33 MHz to f0＋ 1.17 MHz, the variation range of output voltage could be up to 18%. The output voltage fluctuations caused by the change of external circuit parameters could be eliminated by adjusting the frequency of DDS output signals in real time, and the amplitude variation of output voltage during the hyperthermia would not exceed 2% of its initial value. Conclusion When the load or circuit parameter of RF hyperthermia apparatus changes, MCU will automatically adjust the frequency of DDS output signal to keep the amplitude stability of RF output voltage.