If Jeopardy! host Alex Trebek asked, “What is RF power used for other than sending radio waves through space,” would even Ken Jennings, who won 74 games in a row, have the correct answer—or any answer at all? Maybe not because the power of RF energy to create heat is buried within industrial systems most people don’t even know exist.
However, if you’ve cursed those nasty, thick, cut-inflicting plastic enclosures protecting products like thumb drives, you’ve experienced the power of RF energy—in dielectric heating—to melt and merge two plastics together. That crunchy food that Fido loves so munch? It didn’t start out crunchy: It had to be dried, and chances are RF energy was the heat source. There are a lot more applications for dielectric heating, dozens in fact, as you can see in the following list:
Applications for dielectric heating span many industries
|Increasing seed fertility||RF transfer printing on textiles||Pasteurizing hams and soft drinks|
|Drying and backing carpet||Drying synthetic fibers||Drying water-based printing inks|
|Drying agricultural products||Drying leather||Pasteurizing milk|
|Drying textiles||Drying sand cores||Preheating rubber|
|Accelerating concrete curing||Bonding textiles||Deactivating flour|
|Heat setting nylon rope||Laminating flexible polycarbonate boards||Curing and preheating PVC sheet|
|Drying sizing on glass fiber||Drying timber and veneers||Drying pasta|
|Printing carpet tiles||Welding vehicle PVC interior trim||Fabricating life jackets and life rafts|
|Curing resin-bonded abrasive wheels|| |
|Drying tobacco||Welding camera cases||Preheating thermoset plastic|
|Drying pharmaceutical powders and pills||Drying adhesives on cartons||Post-baking of biscuits and cereals|
|Heating wool bales||Inserting metal pieces into eyeglass frames||Welding and embossing PVC|
|Drying gelatin||Profiling of moisture on paper||Drying soup powder|
|Dye fixation||Drying potato chips||Cracking concrete|
|Drying water-based adhesives||Drying matchbox striking edges||Mending asphalt roads|
RF energy probably seems a lot more complicated than other methods like heating an electrode or burning fuel, but it’s typically less costly than the massive electrical power required to heat electrodes, is a lot more environmentally “friendly” than fossil fuels, and has unique benefits that the alternatives can’t deliver.
The RF power amplifiers used in RF heating have been powered for decades by vacuum tubes such as magnetrons. Even though solid-state devices (RF power transistors) might seem a more modern solution, they haven’t been tough enough to withstand high impedance mismatches and couldn’t produce enough RF power per transistor to make them viable.
That’s no longer true. Laterally Diffused Metal Oxide Semiconductor (LDMOS) RF transistors today can deliver more than 1.5 kW of RF power and do so into an impedance mismatch close to a direct short without performance degradation or failure. An amplifier using a half dozen of these devices can produce enough power to satisfy many industrial heating applications, and much higher power is achievable as well.
The MRF1K50GNR5 LDMOS RF power transistor, for example, is one of NXP’s latest devices for industrial and other high-power applications. It delivers RF output power of 1.5kW CW from 1.8 to 500MHz, and can withstand an impedance mismatch (VSWR) greater than 65:1 even when driven by twice its rated input power.
Unlike magnetrons that are either “on” or “off,” the output of transistors can be infinitely varied by controlling input power, gain, and drain voltages to deliver the correct amount of heat almost instantly. They also operate at 50VDC rather thousands of volts so they’re a lot safer, their operating life is millions rather than hundreds of hours, and their output doesn’t decline with time.
So The $64,000 Question, then, is: Why haven’t manufacturers of RF heating systems jumped at the opportunity to differentiate their products as “powered by solid-state devices?” First, RF technology is a means to an end in these machines, so manufacturers typically don’t have RF engineers who can design solid-state amplifiers. This has been frustrating for proponents of solid-state heating as they attempt to convince an industry steeped in vacuum tube technology to make the leap to solid-state. Second, it’s only recently that RF power transistors have been up to the task.
While it’s likely that in applications where hundreds of kilowatts of RF power are required, magnetrons will continue to be the power generation source of choice, systems requiring less power are fair game for solid-state devices. A few industrial heating system manufacturers have already taken the plunge and more are likely to follow, as the RF Energy Alliance spreads the word about how RF power transistors can be used in more industrial, medical, and cooking applications.