Output Voltage Control by Frequency Regulation in Class-DE Power Amplifier

A Class DE or Class D Zero-Voltage Switching amplifier is a high-efficiency, radio-frequency power amplifier which also finds application in various power conversion industrial circuits such as dc/dc converters as well as dielectric and induction heaters. The output voltage (current or power) in these circuits can be controlled by varying the Class-DE-amplifier operating frequency and maintaining high efficiency over a wide range of output power. In the paper the computer-aided first-harmonic method is used to find relations defining parameters of the Class-DE amplifier vs. its operating frequency for the constant load resistance. It has been shown that by decreasing the operating frequency of the Class-DE-amplifier in the optimum mode its output voltage is increased. Unfortunately, then the efficiency is reduced because the transistors leave the ZVS region and their power losses rise. In contrast, by increasing the amplifier operating frequency its output voltage is decreased maintaining the ZVS operation and high efficiency (sub-optimum mode). The operating frequency can be increased up to the value corresponding to the second optimum operation mode of the Class-DE amplifier. The theoretical results have been verified by computer simulation with SPICE program as well as by experimental tests carried out for 1MHz 265W Class-DE amplifier (measured efficiency exceeded 96%).