Bench Talk

Magnetrons have powered microwave ovens since Percy Spencer accidently invented them at Raytheon in 1945. We take these appliances for granted even though they can’t do much more than heat coffee, explode popcorn kernels, heat up premade meals, and defrost foods. That’s about to change, if not immediately, then certainly in the next five years, as microwave ovens become true cooking appliances, as RF power transistors replace the magnetrons as their RF power source.

Magnetrons have never been an ideal solution for powering microwave ovens. They operate at a single frequency that can drift over time. Their RF output power can’t be varied so a microwave oven is either “on” or “off,” which makes it impossible to optimize cooking performance. A magnetron’s output power also declines with use, which isn’t a concern for consumers, but a huge issue for fast-food restaurants and other companies that use them almost continuously, requiring magnetron replacement at regular intervals.

That said, without magnetrons microwave ovens might never have become a commercial product at a price consumers could afford. They’re cheap and basically get the job done and anyway. Solid-state devices couldn’t produce enough power, were too fragile to withstand big changes in impedance, and cost too much, so they weren’t an option. But RF power transistors have come a long way, and the limitations that kept them from consideration have disappeared.

This presents interesting opportunities and the potential to create an entirely new type of kitchen appliance. It would combine the flexibility of solid-state power with digital signal processing and other techniques to transform the microwave oven into a full-fledged cooking appliance. White-goods manufacturers have taken notice.

Once these new appliances come to market, they’ll be able to cook several different kinds of food simultaneously, each one to exactly the right amount of “doneness” while retaining moisture and nutrients, which current microwave ovens don’t do very well. They’ll also eliminate the need to open a hot oven and poke foods with a thermometer to check their temperature: Thanks to the use of sensors, the oven will “know” when they’re done. Figure 1 shows a solid-state cooking appliance concept from NXP Semiconductors.

Figure 1: The Sage solid-state cooking appliance concept demonstrated by Freescale Semiconductor (now NXP Semiconductors) shows a complete cooked meal. (Source: NXP Semiconductors)

They’ll also be able to defrost foods far better than magnetron-powered ovens, which if you’ve use one to thaw a steak you’ve certainly experienced that some parts are thawed, other parts are cooked, and the rest is somewhere in between. The appliances will come with recipes that can be selected by a smartphone app and sent to the oven. Basically, all you do is press “start” and you’re done. No doubt the number of recipes will grow rapidly.

However, solid-state devices represent a new frontier and one that appliance manufacturers aren’t familiar with, which is precisely the situation that the RF Energy Alliance is helping to change. Since it was founded in 2014, the alliance has tripled its membership, and its members now include white goods goliaths like Whirlpool, Panasonic, and Miele, RF and microwave component manufacturers, and Worcester Polytechnic Institute.

In addition, manufacturers of RF power transistors such as NXP Semiconductors and most recently MACOM have designed devices dedicated to solid-state cooking. NXP has also worked with companies such as Goji Cooking Solutions, which developed the first commercially-available “software-defined” RF cooking appliance, and NXP has reference designs and other design tools to help OEMs build these new appliances.

Solid-state cooking may sound mouth-watering but don’t expect to find these appliances for sale during this year’s holiday season as they’re not fully cooked yet. They’re more than a design concept, though, and manufacturers simply need time to perfect them before releasing them into the commercial market.