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Solid-state technology for domestic microwave heating applications

Chaudhry, Kauser 2020. Solid-state technology for domestic microwave heating applications. PhD Thesis, Cardiff University.
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The use of solid-state power for microwave heating, first proposed in the late 1960’s and early 1970’s, is now an area of growing interest and research for a number of stakekholders; semiconductor device manufacturers, domestic and commercial microwave oven manufacturers, large-scale heating industry and consumers. The traditional way of generating power for these applications has been through the use of a high power magnetron source, the power is then coupled into the cavity via a waveguide. Although cost effective, the magnetron source is limited in that it has a relatively small bandwidth (20MHz) which means that only a small number of modes are excited. The different ingredients in a meal often have different dielctric properties and require multi-mode excitations over the entire (2.4-2.5GHz) band to distribute heat evenly and prevent uneven heating of the load. Its other limitations include no direct means of quantifying forward and reflected powers, the transit time becoming an appreciable portion of the signal cycle which decreases efficiency has been well documented. The recent advances and developments in semiconductor device technology (LDMOS, HVLDMOS and GaN-on-Si) has alleviated some of the earlier obstacles relating to lowpower and poor-efficiency. However, concerns remain about cost, device reliability and output power levels. For example the relatively recent, sufficiently high power levels from a single transistor (300W, CW), and the use of new power combining techniques are factors that are increasing the viability of solid-state power in microwave heating. This thesis focuses on a proposed application that involves the use of RF generated power from a solid-state amplifier, at the heart of the SSPA is the “power transistor”, which generates power to heat the loads (e.g. food) in resonant cavities. From an energy consumption perspective, the solid-state source is a key element that must be designed to satisfy stringent efficiency requirements. Device and circuit related efficiencies are required to maintain an efficient transfer of power into the cavity under variable loading conditions which poses an even greater challenge. The delivery of power into a cavity iii under variable loading conditions usually leads to impedance mismatch, highly reflective states and associated heating inefficiency. Addressing this set of unique challenges has led to continuous research to improve the efficiency and reliability of power transistors. For example, new power transistor technologies (GaN, SiC) offer increased performance compared to traditional silicon components. These transistors can operate at higher power levels, frequencies and temperatures with an improved energy efficiency with respect to that guaranteed by previous generation. From a solid-state heating perspective most of this research has focused on high power and high efficiency PA architectures and device reliability, there is little literature addressing the importance of a coupling structures and the potential performance enhancements they may offer in solid-state implementations. The coupling structure plays an important role in transferring available SSPA power into the cavity to heat the load. The novel work presented in this thesis includes capturing cavity impedance behaviour of different cavity geometries under variable loading conditions and introduces a coupling architecture through which these changes are identified and optimally matched to maintain system and heating efficiency. Following extensive research into means of transferring SSPA power into the cavity efficiently, maintaining device reliability and realizing the goal of homogeneous heating, the study has led to the development of a novel coupling structure. This structure ensures an optimised match under variable loading conditions by incorporating harmonic tuning elements for improved efficiency. The novel research introduced in this work shows how device reliability and efficiency can be improved along with improving heating uniformity.

Item Type: Thesis (PhD)
Status: Unpublished
Schools: Engineering
Uncontrolled Keywords: Solid State Technology for Microwave Heating; RF Power for Microwave Heating; Excitation of Resonant Cavity Modes; Microwave Oven Heating Efficiency; Coupling energy into a Cavity; PA design considerations for Microwave Heating.
Date of First Compliant Deposit: 17 September 2020
Last Modified: 17 Sep 2020 16:32

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