The present invention relates generally to induction heating, and more particularly, to a coil with an integrated resonant capacitor for induction heating, as well as to methods of fabrication thereof and to induction heating systems employing the same.
Induction heating is a known method of heating an electrically conductive load using an alternating magnetic field to induce currents in the load. Induction heating can be beneficial in applications where direct contact with a load is undesired or unattainable, and is efficient since the majority of heating energy appears directly within the load. Further, it is possible to precisely control the depth that the heating energy penetrates into the load.
In a majority of induction heating applications, an alternating current (AC) magnetic field is generated in the load by means of a coil, known as the working coil, heating coil, or induction coil which is placed in close proximity to the load, and may even surround the load. In electrical terms, the working coil forms the primary of a transformer and the load becomes the secondary. However, the transformer thus formed often has a very high leakage inductance, and thus, a resonant circuit is usually formed by the addition of a series or parallel capacitor. This resonant capacitor has three main functions. First, the capacitor is used to cancel high leakage inductance and thus make it easier to drive the required power into the transformer, and therefore the load. Second, the combination of the load, working coil and capacitor forms a tuned network that allows simple control of the load power by adjustment of the input frequency. Third, the tuned network formed acts to reduce the harmonic content of the magnetic field, minimizing the electromagnetic interference (EMI) generated.
The resonant capacitor for an induction heater must often store a great deal of reactive power, meaning that the capacitor must handle high voltage, high current, or both. This typically requires an expensive and physically large capacitor. Furthermore, capacitor losses must be taken into account to prevent overheating and capacitor failure.
There are several approaches used for resonant capacitors. For industrial applications, custom capacitors are often placed in metal cans filled with dielectric fluid. A benefit of this approach is simplification of the thermal management. In other applications, custom capacitors can be constructed using many smaller capacitors in order to handle the defined requirements. In all these cases, however, the capacitor(s) is(are) a separate physical entity, which necessarily increases the overall size of the induction heater. Thus, recognized herein is a need in the art for an enhanced coil for induction heating which is designed with an embedded capacitance that reduces the overall cost and size of the resultant induction heating apparatus.