Solid State RF Generators for Induction Heating Applications

Radio frequency power for induction heating has traditionally been generated by vacuum tube oscillators. These generators use high voltage tubes and generally operate in the frequency range of 100 - 500 kHz, 1 KW - 400 power output at approximately 50 - 60% efficiency. Although thyristor SCR's and bipolar transistors are marketed at this time, they are unsuitable due to either slow switching or low power performance. Until just recently, solid state devices with both high-frequency and high-power capabilities were not available. A new power semiconductor - the power MOSFET - a highspeed field effect transistor, can now meet these criteria. The power MOSFET switching time is in the order of 50 - 100 nanoseconds and can generate many kilowatts of power at frequencies to 500 kHz. This paper describes a solid state RF generator using MOSFET transistors for the power semiconductors. The RF generator is a lpad tracking, resonant inverter capable of full power output over a frequency range of 100 kHz to 500 kHz. Output power levels are in the kilowatt range with induction coil KVA's to 200 KVA (400 V and 500 A) and more. This solid state RF generator has similar characteristics to the more familiar low frequency power converters presently used in the industry; namely, all solid state, high efficiency (approaching 90%), small and compact, and has no vacuum tubes or moving parts.