Six ways to improve microwave cooking using solid-state devices

Ampleon’s Robin Wesson describes in detail how solid-state RF components will replace the magnetron for a more efficient, effective and reliable microwave oven.

Microwave ovens are about to be updated, as the venerable magnetron RF source is replaced with solid-state components. This change, coupled with the introduction of signal-processing techniques borrowed from the communications industry, will enable the creation of more efficient, effective, flexible and reliable ovens that will change the way we cook for ever.

Why solid state?

The shift to solid-state sources will overcome many of the drawbacks of magnetrons. Today’s magnetrons generate high-power RF signals with good efficiency at relatively low cost, but their performance changes with temperature, the output frequency varies, and it is impossible to control their amplitude or phase accurately. You can see this in Figure 1, which shows the variability of results between three experiments in which a magnetron-based oven heats the same load for 30s.

Magnetron-based microwave cooking is also challenging because the way that RF energy is absorbed depends on its frequency and the dielectric properties of the food. In a consumer microwave oven operating at around 2450MHz, this means that food is only directly heated to a depth of a couple of centimetres. The second issue is that the magnetron-sourced microwave energy interacts with the cavity to set up standing waves that create hot and cold spots in the food being heated.

Six innovation opportunities enabled by solid-state microwave sources

Much of the solid-state RF technology that is becoming available for use in domestic and commercial microwave ovens was originally developed to serve the needs of the communications industry. Savvy oven designers will also borrow techniques from that industry to design the next generation of microwave ovens. Here are six such techniques that provide innovation opportunities.

1 - Power control

Conventional microwave ovens adjust the rate at which energy is delivered to the food in their cavities by simply turning the magnetron on and off in a regular pattern. The switching rate is limited by the time it takes for the magnetron to reach its operational temperature, which can mean poor cooking due to thermal cycling at the edges of some foods, as shown in Figure 2.

Solid-state systems can use pulse-width modulation with periods measured in microseconds, so the rate at which energy is delivered can be varied much more linearly than with a magnetron, as shown in Figure 3.

The solid-state approach can also be subject to closed-loop control, to even out any variations in the source’s output power.

2 - Frequency accuracy and stability

Solid-state RF amplifiers can use modulator technology and digital signal-processing techniques borrowed from the communications industry to closely control their output phase and amplitude. Low-cost, frequency-agile synthesiser technology, developed to stop signals from multiple Bluetooth devices operating in the same band from clashing, can also be used to help spread a solid-state RF source’s output power across its operating frequency spectrum.

This is something that you can’t do with a magnetron, and it is useful in microwave cooking because different foods absorb RF energy and turn it into heat in different ways, depending on the frequency and phase of the radiation applied. Figures 4 and 5 show what happens if you put different loads, in this case microwave popcorn in Figure 4 and potatoes in Figure 5, into a microwave oven and subject them to different frequencies. The red and white lines show how a spectrum of RF energy from two different sources is absorbed by the food in the cavity, while the green line shows the ‘compound’ energy absorption.

The conclusion is that RF solid-state amplifiers should be able to deliver peak power and efficiency at any frequency in their band in order to be able to heat a wide variety of foods efficiently.

3 - Multisource phase locking

The Bluetooth synthesisers mentioned above can also coordinate multiple RF amplifiers to emit their energy at the same frequency. This makes it possible to combine power from multiple sources within the oven’s cavity, and to reduce the direct-heating effect in which food absorbs RF energy straight from the antenna, rather than after it has been reflected from all sides of the cavity.

4 - Phase control

Shifting the phase of the signal from one or more of the RF source antennas in the oven, relative to the others, will change the way that signals interact in the cavity and so alter the energy distribution within it. This approach could be used to reduce hot and cold spots in an oven, by sweeping the relative phase of the RF sources so that the position of any standing waves created in its cavity is constantly moving.

5 - Cavity sensing and algorithms

When you change the phase of any RF source in a microwave oven, you change its return loss (in other words, how much of the energy it transmitted is absorbed in the cavity), the return loss experienced by the other RF sources, and the compound return loss.

This feedback means that it is possible to build closed-loop control systems that sense the amount of power reflected back from the cavity and then adjust the signals to each RF source to compensate.

This could lead to the development of new heating strategies, such as sweeping the sources’ phases, to alter the field distribution pattern, and sweeping their frequency, to ensure that RF energy is provided at the right frequencies to couple most efficiently into various types of food. It should also be possible to use feedback loops to find the highest return-loss modes to ensure the maximum power is retained within the cavity. These feedback systems will also enable designers to write control algorithms that can compensate for the way in which foods change their moisture content and energy absorption characteristics as they cook.

6 - Lifetime and reliability

Consumer magnetrons have a limited lifetime of around 500 hours, whereas the solid-state devices being repurposed from the communications industry have been designed to operate continuously for up to 15 years. This gives designers an opportunity to build much greater reliability into their ovens, which will change consumer perceptions of the expected lifetime and reliability of a microwave oven, from that of a consumable to an investment, like a washing machine or dishwasher. This in turn could prompt the development of new business models to replace the revenue lost from regular replacements.

For example, consumer oven makers could offer a subscription-based service to keep the oven up to date with new recipes and their related cooking algorithms. Alternatively they could source and sell crowd-scale nutritional information, by gathering data about what people are eating and when, based on what their ovens sense that they are heating.

Suppliers making ovens for commercial use will welcome the enhancement to their brands from being able to offer much more reliable ovens to an industry for which an oven failure can represent a major setback to a business.

Conclusions

Today’s microwave ovens have a lot of shortcomings, such as the way that their ability to heat food depends on their age, the temperature of the magnetron, the type of food being heated, and the pattern of standing waves created by the interaction between the source, the food and the cavity.

Moving to solid-state RF heating, which offers enhanced power control, greater frequency accuracy and stability, opportunities for phase control and locking, cavity sensing and adaptive algorithms, and increased reliability, will enable designers to overcome many of the magnetron’s shortcomings and build better microwave ovens that will change how we cook.

Ampleon