500 W, Class E 27.12 MHz Amplifier Using A Single Plastic MOSFET

By Richard Frey, P.E.
Microsemi Corporation

Abstract
In this paper, we report on the design and evolution of a 500W, 27 MHz Class E amplifier. It doubles the operating frequency of previous high efficiency amplifiers using MOSFET transistors in the TO-247 package. Device criteria, circuit design, and amplifier performance characteristics are presented and compared to a HEPA computer model.

Introduction
As the semiconductor industry moves to larger size wafers to raise its productivity, they need more control of the processing. This translates into the need to produce a "harder" and more uniform plasma in vapor deposition and etching operations that rely on RF plasma. Typical operation is at 13.56 MHz but 27.12 MHz produces better results and is gaining increased popularity.

One of the challenges facing designers is finding devices suitable for service at the higher frequency. Around 1990, plasma equipment designers discovered that some of the inexpensive plastic high voltage MOSFETs they used in their switchmode power supplies were capable of operating in high efficiency Class E service at 13.56 MHz. Since the switchmode devices operate at high voltage and cost much less than the alternative purpose-built RF parts, acceptance was almost immediate.

Purpose-built RF devices operate on supply voltages below 50 Vdc. Suitable switchmode devices with drain breakdown voltage of 1 kV can run on supply voltages up to 300 Vdc. They also demonstrated system level benefits operating at higher voltage. Power Combining and matching are easier since the drain impedance is higher. Lower RF current is less stress on the series elements and enables the power supply design to be more efficient, smaller and lighter.

This RF application challenges most standard switchmode devices due to their packaging and die layout. The drain of the MOSFET is connected to metal back heat spreader, used for heat sinking the device. This requires either an insulator between the package heat spreader and the heat sink or grounding the drain and changing the circuit topology to accommodate it. Either method adds assembly cost of the PA and increases the potential for failures due to incorrect installation not to mention the poor thermal transfer characteristics of the insulator.

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