Output Characteristics of Voltage-Tuned Type High-Frequency Power Supply with MOS-FETs

Abstract

DC-RF MOS-FET power inverters, each providing an output power of 3 to 10kW, operating at frequencies of up to 1, 600kHz have actually been used in the RF power supplies in the broadcasting applications with an efficiency of as high as 80_??_90%. However, as the operating frequency increases beyond 10MHz, the power loss caused by the drain-source capacitance of each MOS-FET remarkably increases unless a tank circuit is built containing the drain-source capacitance. Thus, the DC-RF MOS-FET power inverters of single-ended voltage-tuned type are discussed hereafter.
The expressions for the drain efficiency, output power and power gain are given in terms of the ON-resistance, number of MOS-FETs connected in parallel, frequency, phase angle, the wiring inductance, and the load impedance. The calculated values well agree with the measured ones.
Based on the above theory, the tradeoff between the output power and drain efficiency is experimentally and theoretically confirmed to find the optimum design conditions in constructing a DC-RF MOS-FET power inverter.
The phase angle and load impedance are required to be optimized because they affect the power loss. Higher drain efficiency can be obtained for higher load impedance, and however the load impedance is usually specified as low value to increase the output power.
This paper also shows that the drain efficiency is about 70% in a frequency range of 14_??_28 MHz at best if the drain voltage is sinusoidal.