Industrial heating applications look to solid-state devices to improve reliability and effectiveness, and save space

When it comes to delivering high-power high-frequency RF energy for industrial applications, valves or ‘tube’ devices such as travelling wave tubes (TWTs), klystrons and magnetrons have been the dominant technology for well over 30 years. Over that time there has been little innovation or improvement in the technology and, suffice to say, like most mature technologies, the price is now low. While previously solid-state devices could not compete with the power levels and efficiencies such tubes provide, the past couple of years have seen RF power transistors come to market offering significantly better capabilities. 

The solid-state process technologies of gallium nitride (GaN) and laterally diffused metal oxide semiconductor (LDMOS) have both started to penetrate industrial applications, with GaN being more suitable for very high-frequency applications above 4 GHz. LDMOS, a process technology that has been in use for some time, suits the spectrum below 4 GHz, and is ideally suited to industrial heating applications such as defrosting or heating of food. The magnetron has been the tube of choice in such cases for a very long time, but that’s not to say it has been the ideal candidate. A magnetron typically has an operating warranty of up to 5,000 hours, but this is compared to a transistor’s life expectancy in excess of 15 years. Clearly, a solid-state device is better matched to the service life traditionally associated with capital-intensive manufacturing operations, which are designed for 15–25 years’ operation.

Another aspect of the magnetron that is becoming a disadvantage is its size. Transistors offer a more scalable solution, take up less space and deliver a far more consistent performance in operation. Magnetrons not only suffer from a short service life, but their performance degrades over time, meaning that it will become necessary to adjust their operation during use. A factor closely associated with this is that the frequency of the energy makes a difference to the heating pattern, which is covered later in this article. Unfortunately, there isn’t a way to control either the output power or the operating frequency of the magnetron. The drive circuits are also high voltage, and require special attention, not to mention the additional transformers and high-frequency oscillator circuit required to provide an efficient drive to the inverter. The magnetron is certainly a high-power energy source, but unfortunately, it is rather ‘raw’ in operation, offering no control over power, frequency or phase, and at best these characteristics will not maintain operating stability. Also, there is no possibility to provide feedback on the energy absorbed or reflected by the food or item being heated. The instability of power delivery is partly caused by interactions with the resonant structure of the cavity. Figure 1 highlights how a magnetron-based industrial heating system is optimised and operates today. However, a solid-state-based RF heating approach provides a far more controllable solution. First, power, frequency and phase are all controllable; second, they offer a compact, reliable and far more stable approach to using RF energy for heating applications.

Figure 1 – Optimised industrial heating system using a magnetron (source: Ampleon)

In order to better understand what is going on in an oven cavity and how hotspots occur, it is necessary to delve into the standing wave patterns encountered. Figure 2 illustrates a simple 1D model of standing waves, with forward power (blue) and reflected power (red) and the sum of the two (green) when operating on 2.4, 2.45 and 2.5 GHz frequencies. Notice that the changing phases of the power sum will not only move the hotspot but will also have an impact on its power, requiring a more complex control environment. 

Figure 2 – Standing waves within a 30 cm cavity

In domestic microwaves a turntable is typically used to rotate the food being cooked through the RF field and the hotspots, in an attempt to achieve a more homogenous heating pattern. A ‘mode stirrer’ is used in other ovens that don’t use a turntable. Solid-state techniques can replace the need for a turntable by introducing electronic control of frequency, amplitude and phase. In the above industrial heating example, typically a conveyor moves the items to be heated through the energy field and hotspots – see Figure 3 – although this does not mean that a homogenous heating effect is achieved. 

Figure 3 – Industrial heating – using a single frequency

Introducing a second stable frequency source into the cavity will create a second group of hotspots – see Figure 4 (green). Changing the phase between the two frequencies introduces another dynamic to the RF energy pattern. Frequency and phase of course are closely linked, so switching quickly between frequencies, effectively phase-modulating the signal, will spread out the RF energy hotspots. 

Figure 4 – Using two frequencies in the cavity creates a second set of hotspots

The physics of heat flow shows that if you do it slowly enough you can heat homogenously with any kind of heat source. In the industrial example above, the time taken relates to the journey time of the item to be heated through the cavity on a conveyor belt. Spreading out the hotspots through multiple modulated RF energy ports means that more homogeneity can be achieved – see Figure 5 – potentially in less time. In this way more energy can be put into the cavity. The consequence of this is that the items being heated can reach the desired temperature in a shorter distance along the conveyor belt. The result might be that the size, specifically the length, of the industrial heating equipment could be reduced. 

Figure 5 – Spread of RF energy is more evenly distributed in the cavity

Clearly, the above approach is only possible by using solid-state devices. So how does an RF engineer start to prototype using such techniques? There area a number of building blocks that can help, and a review of the tools and evaluation platforms available from a potential supplier is recommended. High-power RF pallets, such as those from Ampleon – see Figure 6 – provide a quick means of incorporating the RF energy output stages into an initial design. Figure 6 shows the Ampleon BPC2425M9XS250 2.45 GHz pallet, which is capable of providing a 250 W continuous wave (CW) output. It has an integrated driver and final stage and can be driven from a frequency-agile small-signal source. 

Figure 6 – Ampleon BPC2425M9XS250 2.45 GHz pallet

In order to achieve the levels of RF energy required, multiple pallets can be combined using established microwave hardware such as planar or radial combiners, illustrated in Figure 7. For example – see left illustration – four 250 W pallets can be combined in a planar configuration within the PA stage to achieve a 1 kW output. A radial combiner – see middle illustration – provides another method of combining more pallets together, so with eight 250 W pallets 2 kW can be produced. A further approach, using multiple sources, provides a way of combining ‘in the air’ of the cavity – see right illustration – that allows trading the power levels for the desired frequency and phase required.

Figure 7 – Use of combiners to achieve desired power levels

In summary, there are significant advantages to using a solid-state approach. Not only does this relate to delivering the power levels that a magnetron can provide, but with a much longer life, achieving a more stable power source, and the stability and agility of frequency and phase modulation that delivers a high degree of homogeneity.