Series Connected Class-E Push-Pull Converters using GaN HEMT for High-Efficiency RF Generators in Float Zone Silicon Production

Ultra-pure Float Zone silicon production uses induction heating to melt poly-crystal silicon ingot into mono-crystal structure. Radio-frequency (RF) generators used for induction heating operate at around 2.4 MHz. The generators currently used by the industry are based on vacuum-tube triode and thus suffer from low efficiency. Wide bandgap devices such as gallium-nitride (GaN) can achieve the high switching frequency however the power levels required by such RF generators cannot be achieved by a single high frequency converter. Series connection of Class-E push-pull converters in a multi-cell architecture is a way to achieve higher power, however challenge arises when multiple Class-E push-pull converters operating at 2.4 MHz are connected in series leads to instability in the system. In this paper the authors have identified this instability mechanism as the parallel resonance between GaN HEMT output capacitance and external capacitor and presents a solution to mitigate the issue. Finally, the paper demonstrates three Class-E push-pull converters operating in series. Output of 4.1 kW at 98% efficiency is achieved.