Future Electronics — The Uses of Specialist RF Transistors in Industrial Heating Applications

By Yves François
Senior Specialist FAE (EMEA), Future Electronics

Read this article to find out about:

• The reasons to replace thermal with microwave heating techniques in industrial applications
• Why RF semiconductor equipment is preferable to the traditional magnetron
• New RF transistor products and tools from NXP for use in industrial heating and process applications

RF and microwave power transistors are essential components in high-power communications equipment such as cellular base transmitter stations and TV broadcasting equipment.

By contrast, solid-state RF components are rarely found in today’s factories or process plants. This is about to change, however, with the introduction of a new generation of high-voltage, high-power RF transistors operating at frequencies ideal for certain industrial processes and operations. Drying, curing, welding and bonding processes all derive huge benefits from the application of RF rather than thermal energy for targeted heating operations.

And as this article explains, the recent introduction of a broad set of evaluation and development tools for RF energy components smooths the path from older magnetron-based microwave equipment to semiconductor-based systems for designers who have limited experience in implementing solid-state RF circuits.
 

The Advantages of RF Energy Over Thermal Techniques

Some thermal processes used in industry today have barely changed for decades. For instance, drying of foodstuffs, fabrics and building materials is commonly performed by passing items through an oven, exposing them to hot air. Bonding of plastics or curing of adhesives often involves applying pressure to the location of the joint via a hot die.

If the material to be treated has a high enough dielectric loss factor, the application of RF/microwave energy can raise its temperature without exposing it to an external source of heat such as a die or hot air, as shown in Figure 1. The replacement of old thermal equipment with new RF energy techniques provides numerous benefits to the operators of industrial drying, curing and bonding processes:

• Heating time is typically between 2 and 20 times faster, as the heat is generated internally
• Reduced exposure time to high external sources of heat results in less deterioration of the treated material. In food processing for instance, heating through RF energy preserves more of the nutritional content of food than oven heating. In drying applications, the use of RF energy can reduce the risk of surface cracking.
• Since RF energy does not heat the ambient air, the heat transfer system is simpler. This can enable process operators to replace a batch process with a continuous process as well as to reduce floor space.
• The power of the RF energy applied to a material and the duration of the application of power may both be precisely controlled. Using information from moisture and temperature sensors, the system can stop applying RF power immediately when the target moisture level or temperature is reached.
   

Fig. 1: Dielectric loss factor of various common materials. The higher the factor, the easier it is to heat with RF energy

These benefits result in reduced energy consumption, operating costs, maintenance requirements and downtime. For example, to dry cotton from 55% moisture to 9% with RF heating requires 57% less energy than fresh air drying, and 23% less energy than pressure air drying.

Welding of plastics is another application which benefits hugely from conversion to RF energy. Conventional plastic welding requires a hot die to be applied to two plastic sheets until the temperature throughout the material is high enough for them to fuse together. They must then be held together under pressure until the die cools. This means that the highest temperature is applied to the faces of the material, and the die must be heated and cooled each time a weld is made.

By contrast, dielectric heating allows the weld to be made with cold dies the temperature of which rises only a little during the weld and falls rapidly after bonding. Because the die is cool, the hottest spot is at the interface between the two sheets, where it is needed. This reduces energy cost and greatly accelerates the process.
 

Growing Market for Replacement of Old Microwave Energy Generators

The huge cost-saving and operational benefits of using RF energy put this technology in pole position to replace thermal ovens and other thermal energy equipment. In fact, microwave energy is already used in industrial processes, for instance in drying or sterilizing food. Traditionally, the generator of microwave energy in industrial equipment has been the magnetron, a technology first brought into production in the 1940s. A magnetron is a cavity vacuum tube, a bulky and heavy device.

Weight and size reduction is one of the important benefits of using solid-state transistors to generate RF or microwave energy, but there are many more benefits as well.

A transistor offers much greater control over both power output and frequency. A low-end magnetron is either fully on or off, and even a high-end magnetron can only control power output at above 60% of full power. By contrast, an RF/microwave transistor offers full control of output power over a range from <1% to 100% of full power.

A solid-state RF or microwave generator also offers a certain amount of frequency agility. Figure 2 shows the effect of tuning the frequency of a transistor rated at a nominal 2.45GHz. In a material sample of mixed composition, different components of the material might have a different dielectric loss factor and generate heat at different frequencies. Dynamic frequency sweeping across a transistor’s frequency range redistributes hot and cold spots for even heating across the sample. Such dynamic, inprocess frequency sweeping is not possible with a magnetron.

The generation of RF energy by solid-state components also produces equipment that is easier to install and operate in a factory. It provides a rapid response to changes in power requirement, and starts up instantly, with no delay for warming up or cooling.

A transistor system uses reliable, compact, and efficient switch-mode power supplies operating from a low-voltage supply.

Fig. 2: NXP’s Lab box RFEL software enables the user to sweep across phase and frequency to find the points with the best match to the load

For a power output of 800W, a magnetron requires a 4kV supply, whereas a microwave transistor runs from a safer 50V supply.

In addition, the solid-state technology requires no complicated electromechanical controls and sequencing, and is insensitive to vibration.

This also contributes to the transistor’s high reliability: the lifetime of a high-end 915MHz magnetron is between 2,000 and 6,000 hours, or up to eight months of continuous operation. When a magnetron needs to be replaced or serviced, a factory operator can expect downtime of between several hours and several days.

By contrast, a transistor’s rated lifetime is 100 years, and it suffers no performance degradation over time. Even if a fault occurs, a hot-swap architecture allows a failed transistor to be quickly replaced with minimal downtime.

Finally, the transistor-based system will be smaller and lighter than the equivalent magnetron-based design: at 915MHz, a transistor-based RF generator can be half the size and weight of a magnetron system.
 

New Components and New Tools Ease Implementation of RF Energy System Designs

The scope for improved design outcomes from the replacement of a magnetron with solid-state components is clear. But how easy is it for industrial equipment designers to find the components and the tools that they need to meet their specific application requirements?

Fig. 3: The NXP RFEP24-250 power amplifier reference design features three stages of amplification, and is supplied as a board with a footprint of just 50mm x 76mm

Component availability has been much enhanced by the introduction by NXP Semiconductors of a range of RF power transistors intended for use in heating and process applications. Importantly, the NXP devices cover the frequencies of most interest to industrial applications:

• MRFX1K80H: 1MHz to 400MHz, 1,800W rating, 65V LDMOS device, 76% efficiency
• MRF13750H: 915MHz, 750W, 50V LDMOS device, 67% efficiency
• MRF24300N: 2.45GHz, 300W, 32V LDMOS device, 60% efficiency

All three devices offer higher power density and wider safety margins than previous generations of high-voltage RF transistors, and are thus able to deliver a greater power output in the application. Manufactured in a standard silicon Laterally-Diffused Metal-Oxide Semiconductor (LDMOS) fabrication process, these transistors benefit from a substantial cost advantage over competitors’ equivalent RF transistors fabricated in an alternative semiconductor material, Gallium Nitride (GaN).

And now NXP supports these devices with a new portfolio of development tools for its RF energy transistors, as shown in Figure 4. The range of development tools addresses the requirements of design teams with different levels of RF circuit design expertise. At the most basic level, NXP supplies the transistor with a reference circuit schematic. The RF Energy Pallet (RFEP), shown in Figure 3, adds RF transistor drivers and a 50Ω matching circuit.

Fig. 4: NXP’s RFEL24-500 RF energy lab box is a 2 x 250W, fully integrated RF development system designed for use by RF or non-RF engineers

On top of this, the RF Energy Module (RFEM) includes a microcontroller with embedded RF source, power, current and temperature sensors, circulator and coupler, and a command interface.

For a complete demonstration RF energy system, designers should use the RF Energy Lab Box (RFEL): this includes a power supply, heat-sink and fans, and provides a USB connection to a PC, as shown in Figure 4. NXP supplies RFEL control software with auto-tuning features to run on a PC. This software provides a rich demonstration of the capability of solidstate RF energy systems to use data to optimize system performance. The RFEL collects data about parameters such as reflected energy, and can dynamically adjust the frequency, power and phase generated by the RF transistor to maximize the transferred energy, or to distribute it to specific locations.
 

Hundreds of New Applications for Solid-State RF Energy

The number of existing drying, curing and bonding applications to which RF energy could be applied is vast. But the ready availability of small, high-power RF transistors opens up many new applications for which a bulky, heavy magnetron is unsuitable. An example is handheld surgical equipment for skin ablation, but many others are sure to emerge as the advantages of solid-state RF energy equipment drive the replacement of magnetron technology in factories and process plants worldwide.