Resonant RF generator for high frequency induction heating

Induction heating allows the targeted heating of an applicable item for applications including surface hardening, melting, brazing, welding, cooking, sealing, heat treatment and plastic processing. Induction heating is used to heat liquid conductors such as molten metals, and also gaseous conductors such as a gas plasma. Induction heating can also be used to to heat pipes containing non-conductive liquids or crucibles containing any kind of material, both made of amorpous or sintered graphite. Finally induction heating is used extensively in the semiconductor industry for the heating of silicon and other materials.

Iron and its alloys respond best to induction heating, due to their ferromagnetic nature. Eddy currents can, however, be generated in any conductor, and magnetic hysteresis can occur in any magnetic material.

The basic setup is made of a control board, a power module, an impedance matching transformer and an L-C resonant tank. The generator provides high current at high frequency to resonant tank, and the workpiece, that is placed inside the coil, is heated by the alternating magnetic field that induces eddy currents inside it.

The frequency of the inductive current determines the depth that the induced eddy currents penetrate into the workpiece, and the effective penetration depth depends on several parameters among which the resistivity of the material (the higher the resistivity, the higher the depth) and the magnetic field frequency (the higher the frequency, the lower the depth). The equivalent resistance of the workpiece, and thus the efficiency, is a function of the workpiece dimension over the reference depth: it follows that, while thick materials can easily be heated to several hundred degrees with a few kilohertz, small workpieces usually require frequencies as high as hundred of kilohertz in order to be effectively heated, especially if the material is not ferromagnetic.

The controller provides PLL-based frequency modulated PWM to the power module in order to keep the best tuning, thus allowing zero-voltage-switching and zero-current-switching for best-in-class performances in terms of efficiency and electromagnetic compatibility.

The power module can switch up to 4 kW power at up to 500 kHz frequency, thanks to its ultra high speed MOSFETS and special low parasitics layout.