When in 1945 Raytheon’s engineer Percy Spencer
accidently discovered the potential of RF energy for heating food (or in Spencer’s case, for melting a chocolate bar in his pocket), the device generating that energy was a magnetron.
Sixty years later, this venerable “vacuum electron device” is still powering everyone’s microwave ovens, making it the only consumer product still using a vacuum tube rather than a semiconductor for any purpose. However, this last bastion of vacuum tubes in consumer electronics may soon be relegated to history now that RF power transistors have achieved the required RF output power, efficiency, and ruggedness required to replace them.
just introduced its first two devices designed exclusively for RF heating applications in early October. The MHT1002N that delivers 350 W CW at 915 MHz and the 250-W MHT1003N are the first silicon LDMOS FETs to target this market, although other manufacturers will likely soon follow. The first consumer microwave ovens are likely to appear sometime next year.
You might be wondering why other than creating a potentially enormous new market for RF semiconductor manufacturers does this accomplishment merit acclaim. After all, magnetrons cost $20 (making microwave ovens dirt cheap) and current magnetron-powered ovens work just fine for as long most people care to keep them, so who cares?
The answer is that solid-state devices provide several benefits that magnetrons can’t deliver. Magnetrons cook by turning on and off rather than providing a variable level of power, which is not true for transistors. The result is potentially more accurate cooking and defrosting. The RF power section is smaller and lighter than a magnetron-based design, which provides room for other functions.
Freescale’s LDMOS FETs operate from 28 to 50 V rather than the 4,000 V that magnetrons require, which reduces the size of the power supply and makes servicing it much safer. Last but not least, the operating life of an LDMOS-powered amplifier in this application is cited as 20 years versus the 500 to 1000 hours of a magnetron, and its power output does not degrade over time as does that of a magnetron.
A magnetron’s decline in power with time isn’t all that big a deal for most consumers who use microwave ovens to reheat and defrost, pop corn, and prepare pre-cooked meals. However, it’s a huge expense for fast-food restaurants whose commercial ovens are in use almost continuously. Replacement is a regular maintenance task, and commercial magnetrons are not $20 items.
And there’s this other little tidbit: One manufacturer has designed an LDMOS-powered oven that can cook four different kinds of food simultaneously through a combination of proprietary techniques. From what I’ve heard they all came out perfect.
Barry Manz is president of Manz Communications, Inc., a technical media relations agency he founded in 1987. He has since worked with more than 100 companies in the RF and microwave, defense, test and measurement, semiconductor, embedded systems, lightwave, and other markets. Barry writes articles for print and online trade publications, as well as white papers, application notes, symposium papers, technical references guides, and Web content. He is also a contributing editor for the Journal of Electronic Defense, editor of Military Microwave Digest, co-founder of MilCOTS Digest magazine, and was editor in chief of Microwaves & RF magazine.
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