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Will the Microwave Oven’s Magnetron Soon be Obsolete?

Ask almost any electrical engineer (except an antique collector) if they have any vacuum tubes in their house and the answer is likely to be somewhere between “you have to be kidding -- of course” or “not since I got rid of my CRT TV.” Well, think again: if you have a microwave oven in your house, you do have a vacuum tube – the magnetron.

What’s a magnetron? It’s the first famous vacuum tube ever, right up there with the first amplifying vacuum tube, the triode invented by Lee De Forest in 1906. The physics and the electromagnetics of the magnetron are complicated, it uses a strong external magnetic field which influences the flow of electrons in a resonant circular cavity to develop tens and hundreds of watts at frequencies well into the gigahertz (GHz) range (Figure 1). See References for various perspectives on the magnetron.

Developed in England during WWII, it was a closely guarded secret as it was the key to radar, which was compact enough for aircraft while operating at the high enough frequency of 500 megahertz (MHz) (that was a super-high frequency in those days) to provide meaningful resolution.

Figure 1: The magnetron is a specialized vacuum tube which uses the interaction of electric and magnetic fields in a conductive cavity to generate microwaves at relatively high power levels. (Image source: Hyperphysics/Georgia State University)

The need for the magnetron has declined with the development of solid-state power amplifiers (SSPAs) that can reach into the GHz+ parts of the spectrum, and it has largely been relegated to the electronics history museum, except for outputs at multi-kilowatt (kW) levels (and even there, solid-state devices are dominating).

Still, there’s one place where the magnetron lives on and thrives, and that is as the core of the consumer microwave oven, as well as many commercial ovens used for baking and drying. How is this possible?

In brief, it’s very cost-effective at doing a good enough job as a high volume, low-cost, mass market RF source of several hundred watts at 2.45 GHz (Figure 2). It’s both ironic and a demonstration of the power of high volume manufacturing that the top-secret microwave energy source of WWII is now a mass market component at the heart of an oven selling for under $100 for a basic unit, and $500 - $1000 for a larger, more powerful one.

Figure 2: This Panasonic magnetron plus associated waveguide is a single integrated assembly which generates and disperses 2.45 GHz energy in consumer microwave ovens. (Image source: Encompass Supply Chain Solutions, Inc.)

But even the magnetron’s days in this role may be coming to an end – maybe.

Manufacturers of solid-state power amplifiers (SSPAs) see it as a strong potential growth market, and not just because these devices can replace the magnetron’s function there. The reality is that the magnetron-based microwave oven has some serious weaknesses, which become apparent as you learn more about it.

For example, it’s hard to modulate the output amplitude. When you set the oven for an intermediate power level, the magneton is pulse-width modulated (PWM) to provide that level as its average power, but the PWM duty cycle is fairly long – on the order of tens of seconds – and so is ineffective for shorter heating times. There are also issues with directing the RF output such that it fully and evenly fills the oven cavity. That’s why you’re supposed to stop during the heating cycle and stir the food, which most people don’t bother to do. Even with the integral turntable of many microwave ovens, there will be hot and cold spots.

Go with a solid-state PA?

If you think this is just a qualitative assessment of the mediocre performance of the magnetron-based unit, take a look at the detailed evaluations in the “RF Solid State Cooking White Paper” from Ampleon, a leading proponent of the use of SSPAs for microwave ovens. While as a vendor they may be biased, the technical details and test images in the report are impressive and unambiguous.

Ampleon offers SSPAs which are a good fit for standard ovens, such as its BLC2425M10LS500PZ (Figure 3). This 500 watt, LDMOS power transistor measures approximately 16 × 32 × 2 millimeters (mm) and is designed for continuous wave (CW) operation from 2.4 to 2.5 GHz, straddling the critical 2.45 GHz frequency used in consumer ovens.

Figure 3: The compact BLC2425M10LS500PZ SSPA can deliver up to 500 watts continuously in the microwave oven’s 2.4 to 2.5 GHz range. (Image source: Ampleon)

Why 2.45 GHz? See Eric Bogatin’s “Why do Microwave Ovens Operate at 2.45 GHz?” blog to understand why. And, before you say it, you’re wrong, it’s not because that’s the resonant frequency of water molecules, which is a common misconception. Note that many commercial ovens operate at lower frequencies (and thus longer wavelengths) such as 900 MHz, to more efficiently fill their larger internal working volumes.

A clearer picture of the output power against frequency for the BLC2425M10LS500PZ SSPA is shown in Figure 4.

Figure 4: Power gain and drain efficiency as function of output power; typical values for the BLC2425M10LS500P power LDMOS transistor (Image source: Ampleon)

Ampleon is not the only RF SSPA vendor that sees the potential of this market. MACOM Technology Solutions, for example, outlines the opportunity in its note “How GaN is Transforming RF Energy and Cooking Applications”. Its claim that, “It has been demonstrated how a steak can be cooked on the same plate as ice cream without it melting, showing the precision of the directed RF energy,” certainly got my attention, and the ability to precisely control the energy level and distribution is an attractive benefit. The note includes a useful table showing the attributes of magnetron-based ovens versus those of PA-based ones (Table 1).

Table 1: Key attributes of solid-state power amplifier vs. magnetron approach at 2.45 GHz. (Image source: MACOM Technology Solutions)

The technical benefits are fairly clear with respect to overall AC line to end efficiency, RF efficiency, output level control, and even DC voltage needed (28 volts versus 4 kV). There’s also a long-term reliability issue, as magnetrons—being vacuum tubes—do degrade over time and then burn out; some high usage commercial ovens actually replace their magnetrons every few weeks, as standard maintenance practice. Of course, there’s more to an SSPA-based system than the SSPA itself, and that impacts cost and other design factors (Figure 5).

Figure 5: The SSPA-based RF energy cooking system requires a substantial amount of support circuitry in addition to the PA. (Image source: MACOM Technology Solutions)

Conclusion

So, will the SSPA-based microwave soon replace the magnetron-based one in your home? The higher cost of the SSPA (for now) is certainly an issue, especially as people seem content, if not thrilled, with the $100 to $500 ovens. Even if the unit does burn out after a few years, well, it’s now considered a disposable item in most homes, and is working well enough for those who just want to heat up leftovers or popcorn.

It’s likely that the biggest initial SSPA adoption will come in commercial settings where their much higher efficiency, lower operating costs, and longer life outweigh the higher up-front cost. Perhaps the long-term road map will be like that of cars, where the technical advances first appeared in the higher end models and then gradually migrated down to mid and lower range models; after all, features such fuel injection rather than carburetors were once confined to luxury cars only, and are now standard on all cars.

Perhaps the top-end microwave ovens will have a label on the front panel declaring “Solid State Power Amplifier Inside” similar to those first solid-state radios labeled “all transistor” or those later generation CD players which told the buyer (and their friends) that they had a “1-bit DAC inside” – not that the consumer knew what that was!

Magnetron References

About this author

Image of Bill Schweber

Bill Schweber is an electronics engineer who has written three textbooks on electronic communications systems, as well as hundreds of technical articles, opinion columns, and product features. In past roles, he worked as a technical web-site manager for multiple topic-specific sites for EE Times, as well as both the Executive Editor and Analog Editor at EDN.

At Analog Devices, Inc. (a leading vendor of analog and mixed-signal ICs), Bill was in marketing communications (public relations); as a result, he has been on both sides of the technical PR function, presenting company products, stories, and messages to the media and also as the recipient of these.

Prior to the MarCom role at Analog, Bill was associate editor of their respected technical journal, and also worked in their product marketing and applications engineering groups. Before those roles, Bill was at Instron Corp., doing hands-on analog- and power-circuit design and systems integration for materials-testing machine controls.

He has an MSEE (Univ. of Mass) and BSEE (Columbia Univ.), is a Registered Professional Engineer, and holds an Advanced Class amateur radio license. Bill has also planned, written, and presented on-line courses on a variety of engineering topics, including MOSFET basics, ADC selection, and driving LEDs.

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