Bench Talk

At the core of a ubiquitous household kitchen appliance lies an old piece of technology, the magnetron. Although the first magnetron dates back to the 1920s, credit for the first kitchen microwave goes to American engineer Percy Spencer (1894-1970) in the 1940s. Spencer would later go on to patent the microwave oven in the 1950s.

Figure 1: One of the drawings of the high-energy magnetron developed in the 1940s by Percy Spencer, who went on to perfect the microwave oven while working at Raytheon. (Source: explainthatstuff.com.)

With the exception of changes to the exterior design, to match the flavor of the decade’s décor and the addition of digital readouts, the microwave oven has pretty much stayed the same since its inception. That is, until now, with the advent of new high-power laterally diffused metal oxide semiconductor (LDMOS) RF transistor technology, that outdated kitchen appliance we’ve all come to depend on for heating up our lunches might just be getting a new lease on life.

Figure 2: Present day kitchen microwave oven. (Source: Mouser Electronics)

As you can see from Figure 2, at the heart of every microwave oven lies a magnetron. The magnetron is a type of vacuum tube, in some ways very similar to the cathode ray tube (CRT) from bygone television sets. In a CRT, when a current is applied to the electron gun (cathode) an electron beam is generated. After some beam focusing, using accelerating voltages, the beam hits the phosphorous screen and magically an image appears (very simplified.) When you turn on a microwave oven, the magnetron gets a direct current (DC) applied to its cathode (-), which in turn radiates streams of electrons which then pass through a magnetic field (permanent set of top and bottom magnets) on their way to a series of cavities in the anode (+). This causes the magnetron to act like and oscillator (frequency generator) and oscillate at a resonant frequency (cavity dependent.)

Figure 3: Typical elements of a magnetron.  "magnetron: typical elements". Art. Encyclopædia Britannica Online. Web. 02 Apr. 2015. (Source: https://www.britannica.com/EBchecked/media/137/Typical-elements-of-a-magnetron)

This frequency leaves the magnetron via an output antenna, in the form of high-frequency electromagnetic waves (microwaves). The microwaves are routed to the cooking chamber of the microwave via a waveguide. Once there, the microwaves bounce all over the cooking chamber and do their magic of warming (or as I call it nuking) your food in a process known as dielectric heating. This dielectric heating occurs as a by-product of molecular dipole rotation in food products, especially those with a high water or fat content. As the food molecules are exposed to the electromagnetic waves from the magnetron, the food molecules begin to rotate trying to magnetically align themselves with the rotating microwave field around them, similar to how two magnets behave when they come in close proximity to each other. This movement in turn causes the food molecules to get excited, rotate, bump into one another, creating friction which leads to heating of the food.

Of course, this a very simplified version of how a magnetron works to heat up our food, but moving off from the actual theory of operation, magnetron technology has remained pretty much unchanged since the first consumer microwave was introduced back in the 1960s by Amana.

Recently, however, a new breed of potential magnetron replacement suitors have been developed in the form of high power metal oxide semiconductor field-effect transistors (MOSFETs). Companies like Freescale and NXP are now making breakthrough developments with new LDMOS RF semiconductors that are able to provide the power needed to cook food while achieving acceptable efficiencies. This, along with multiple other advantages over the aging magnetron technology leads me to believe that in the near future, maybe we will be replacing the relic microwave oven with new and more powerful RF cookers! 

According to engineers at Freescale, other benefits associated with shifting from the magnetron technology to smaller Solid-State RF semiconductors are outlined in the table below.

 Table 1: Advantages of Solid State RF transistors over Magnetron technology

Besides the previous aforementioned advantages of microwaves using these solid state RF solutions, there are other applications that could benefit from these new RF semiconductors.

• RF plastic welding
• Water purification
• Food rethermalizing and tempering
• Drying and moisture profiling
• Deodorizing and sanitizing
• Pasteurization and sterilization
• Medical applications requiring sterile environments and highly accurate temperature control

Video 1: RF Energy Revolutions: Clean, Efficient, Controllable Power.  NXP.com

So, when can we expect these new types of microwaves to appear in the consumer marketplace? Well, that I’m not sure, but what I am sure of is that like many of you I too rely on microwave ovens, and any change to it that will make it more versatile will be a change that I would welcome.