Introduction to RF Solid State Microwave Heating

Introduction to RF Solid State Microwave Heating Applications

Magnetron

Figure 1: Magnetron from Domestic Microwave Oven 

Although you probably don’t recognise it, there is a good chance you have one of these in your home. It is the engine and heart of your microwave oven. Its function is to generate RF energy in the microwave spectrum at around 2.4GHz with an output power of up to around 1kW for home use.

It was originally developed early in the last century and came to prominence in the Second World War as a microwave source for radar systems.

Since then, its design has been perfected, leading to improved efficiency and relatively low cost.

For cheap raw power, a magnetron remains a good solution.

RF Solid State Amplifiers

Solid state electronic devices are those which are constructed from a solid material where electrons and other charge carriers which are confined to the material are used to transfer charge and carry out a particular electronic function.

A well-known example of a RF solid-state device is a silicon LDMOS transistor. Such devices have been extensively developed over the last 10 to 20 years to generate high power microwave signals for use in the telecommunications industry, specifically for high reliability mobile phone base station transmitters.

An example of a solid state LDMOS transistor is shown below:

Figure 2: Solid State LDMOS Transistor for Solid State RF Amplifier 

Innovation with RF Solid State Heating

Power Control

A common misconception with microwave ovens is that food is heated from the “inside out”.  This is not the case, the actual dielectric microwave heating, which is effectively the heating due to the absorption of the microwave energy, takes place in the outer layers (approx.1.5cm) of the food and the centre is heated by conduction from these outer layers.

Figure 3: Diagram Showing RF Heating

The response time of magnetrons is determined by the time it takes for it to reach its operational temperature, much like a light bulb. Therefore, magnetrons achieve power control in a crude manner. In a low power setting, the power is turned on and off, with the power being turned on for example for 15 seconds and then off for a further 30 seconds. This method, coupled with the way microwaves heat food can lead to extreme temperature cycling in the outer layers which leads to degradation.

Solid state RF sources are routinely used in radar systems where the RF power has to be turned on and off very rapidly i.e in the order of a few 100 micro seconds. This high speed switching operation allows for pulse width modulation control of power which results in the ability to accurately modulate the heating power and provide tight control over heating rates.

Frequency Control

Microwave ovens typically operate in the 2.4 to 2.5GHz ISM (Industrial Scientific and Medical) license exempt band. Although magnetrons are tuned for operation at the centre of this band (2.45GHz), they are not inherently stable and cannot be easily tuned around in frequency.

Different foods and materials absorb RF energy by different amounts at different frequencies. A measure of the absorption of RF energy is called the return loss. This is a measure of how much power is reflected back as compared to how much was sent forwards towards the object to be heated. The return loss of foods is not constant with frequency. For example potatoes which are water rich may effectively absorb RF energy at all frequencies from 2.4 to 2.5GHz, whereas pop-corn may effectively absorb RF energy only at 2.3GHz. With RF solid state heating, it is very simple to change the frequency of the RF energy in a fast and flexible manner. With knowledge of the type of food, or by characterising the food, it is possible to tune the RF source to the best frequency for effective heating.

Lifetime and Reliability

The technology behind solid state RF heating was developed primarily for the telecommunications industry. Mobile phone base stations are required to typically function for 24 hrs every day in a consistent way for upwards of 15 years, solid state device have shown themselves to be extremely reliable in operation. Tests on microwave ovens show that output power from magnetrons falls steadily prior to complete failure. Although this is not a great concern in a domestic environment, it is potentially serious in a commercial heating application.

Phase Control

Precise control of the phase of the RF signal is possible with solid state RF sources. Phase control is often used in complex radar systems to provide free space power combining where multiple modules can be combined to provide a high power beam. These techniques, which are common place in the RF industry, can be used to prevent the formation of hotspots and if required, steer a focussed beam of RF energy to a particular location. Phase control of a magnetron is possible, but is not an inherent feature that can be implemented in a cost effective manner.

Feedback of Environment

Using solid state RF sources could also allow for some feedback of information about the object being heated. It may be possible to configure the system to measure forward and reverse power and from this information determine the optimum frequency within the 2.4 to 2.5GHz band to cook or heat the object. An example of such a system would be an RF source with a forward and reverse power detector built in (very low cost components). The food could initially be interrogated by a range of frequencies using a low power mode of operation. Based on the results, the system would then choose the optimum frequencies for cooking of the particular food.

Rugged

Another innovation from the wireless base station market that has benefits in solid state RF heating applications is the development of highly rugged RF transistors. In a serious fault condition, such as an antenna being damaged, it is possible for all of the RF power to reflect back to the RF transistor that is generating this power. Without the addition of protection devices, called circulators, such an event would historically often damage the transistor. With the desire to reduce component count and RF losses, solid state transistor manufacturers have developed transistors that are able to withstand these extreme conditions without the need for protection devices. Such features help to further improve the reliability and ruggedness of solid state RF heating solutions.

Summary of Innovation Potential

The fine control and precision of parameters such as power, frequency and phase in RF solid state heating has the potential to provide a vast range of innovative new methods for cooking and heating with microwaves.

Implementation of RF Solid State Heating

In the previous sections, we have highlighted a number of the features and benefits of solid state RF heating. In this section, we will take a look at the practical implementation and design considerations when actually using solid state RF amplifiers.

What does a Solid State Amplifier look like?

Figure 4: Solid State RF Amplifier (2.4 to 2.5GHz 250W Pallet) 

Figure 5: Schematic of Solid State RF Power Amplifier System 

The RF input power is provided from a low power synthesiser which can be frequency agile. This can be passed through a phase shifting network to control output phase and then passed through a switch to implement PWM (Pulse Width Modulated) power control. The output of the low power network is fed into the main pallet amplifier which amplifies to signal up to full RF output power.

Current State of the Art Power Levels at 2.4 to 2.5GHz

The table below provides a summary of some of the highest power transistors that are currently available for RF amplifier heating applications. It is expected that a level of power combining will be required to combine these individual amplifiers to achieve output powers in the region of 1kW.  Slipstream Engineering Design have expertise in the design and manufacture of high power RF combining networks.

Table 1: State of the Art Power Levels at 2.4 GHz to 2.5GHz 

Figure 6: RF Power Combined Amplifier – Multiple Modules Combined to Generate Increased RF Power 

Control, Interface and Testing

Integrating the LDMOS transistor into a modular power amplifier pallet is a good first step to simplifying the design of a solid state RF heating system.

The next step in develop a heating solution is to design the control electronics and power supply circuitry to power, control and monitor the amplifiers. Depending on the level of control required, the supporting circuitry may be relatively simple or potentially more complex with the use of phase shifting, PWM power control and frequency agility.

As well as the core RF elements, at Slipstream Engineering Design, we are often responsible for the complete design of the control electronics for complex RF systems.

We also have experience in the design of factory testing systems to support the amplifier through production.

Conclusion and Summary of RF Solid State Heating

The precise levels of signal control that are brought about by the introduction of solid state RF amplifiers into the microwave heating market opens up a myriad of opportunities for innovation and development which we believe will be a key area of growth in the RF industry over the coming years.

References

RF Solid State Cooking, Ampleon White Paper, Robin Wesson, System Architect, Ampleon RF Power Innovation