The future of microwave cooking is solid-state

Microwave cookers are about to change as magnetrons are replaced by more flexible solid-state energy sources – or so says RF power transistor maker Ampleon, which was part of NXP until last year.

Ampleon cooker PA

It’s all about cooking speed and cooking control.

The venerable magnetron does a fine job delivering microwave power, many would say, and manufacturers have making them down to a fine art.

But Rob Hoeben of Ampleon argues solid-state power sources will make technically superior ovens, particularly if there is more than one in each cooker.

“The magnetron has limitations,” said Hoeben: “It only has on-off control and the combination of the magnetron and cooking cavity makes hot-spots and cold-spots in the food.”

Brick-shaped 3D standing waves set up in the cavity are what causes these temperature differences, and are the reason a turntable sweeps the food through the RF field. Even with the turntable, residual temperature differences are one of the reasons food instructions say ‘leave to stand for one minute after cooking’.

With solid-state sources, frequency can be shifted (2.4-2.5GHz, for example) to move nodes and anti-nodes around the cavity. and power can also be modulated.

Once control over frequency and amplitude is available, some form of feedback mechanism becomes viable. Hoeben proposes measuring reflected power as fraction of radiated power and feeding this back through suitable algorithms.

Have two or more RF sources, each with its own port into the cavity, and relative phase between the two sources can be altered to shift nodes and anti-nodes just about anywhere – all controlled on-the-fly by cooking algorithms.

Hoeben is proposing two 500W sources or four 250W sources.

“We can get faster cooking, and more delicate cooking,” he said, crowning it with: “We have cooked a whole egg.”

For the uninitiated, cooking a whole egg might not seem very clever, but don’t try this at home because whole eggs explode rather than cook in conventional microwave ovens.

Better microwaves is not just an internal project at Ampleon. Its customers are well into product development and, last summer, RF power transistor maker Freescale at its technology forum announced a solid-state ‘RF cooking’ concept called Sage.

NXP Freescale Sage solid-state cooker

NXP/Freescale Sage solid-state cooker

Ironically, the whole reason Ampleon is an independent company is that in the last six months NXP acquired Freescale along with RF cooking and Sage, and in order to make the acquisition NXP had to divest its own RF power business – now Ampleon.

From customer feedback, Ampleon is expecting to see the first professional cooking appliances with solid-state microwave sources in the second half of this year, and high-end consumer products in 2017.

“There is a race to be first in the market,” said Hoeben. “In the mid-range, $400-700 combination convection/microwave cookers are being developed now – we think – our customers don’t tell us everything.”

And at the low end? “$50 microwave ovens are going to have magnetrons.”

Even within premium microwave cookers, the packages used for traditional microwave power transistors are too expensive, which is why plastic alternatives to existing low-loss high-reliability air-cavity ceramic packages are being developed.

The move from +125°C to +85°C ambient operation helps, but 250W at 2.4GHz means hot leads and high transient electric fields.

Ampleon has developed low-loss air-cavity plastic packages, and is working on solid plastic package that will cost less at the expense of higher losses. NXP is also offering plastic package RF power transistors.

A choice of hollow or solid packages is one area oven companies can make trade-offs, another is what goes inside the package.

If phase, frequency and amplitude control is to be applied to multiple power ports, there is going to have to be a multi-output signal generator feeding several power amplifier via gain/driver stages.

According to Hoeben, how manufacturers will partition this signal chain is not yet clear. “Integration is the million dollar question,” he said. “Bluetooth 10-15 years ago started with modules, and now the sweet-spot is a small SoC with external RF power amplifier.”

Silicon LDMOS is the technology Ampleon is offering to cooker makers (as is NXP), so each package will at least have several finger-like transistor die connected in parallel, plus input and output matching component. Time will tell if the driver stages, phase shifters of oscillators end up in there as well or somewhere else.

To get customers going, for the moment Ampleon is offering a VCO and control registers in a single-channel signal-generator chip producing 10-15dBm, and a choice of pre-amplifiers to take this up to the 10mW to few-Watts range needed to drive 250W and 500W output stages. A four output signal generator chip is in the pipeline, plus a power amplifier reference design (see photo above). Modules may follow for customers with less RF design expertise.

RF grows plants

Microwave cooking is not the only market RF power transistor makers are looking to enter.

Perhaps surprisingly, horticulture is a possibility, where artificial lighting is used to extend the growing day in commercial greenhouse.

A bunch of technologies are vying for the market, with 1kW high-pressure sodium (HPS) lamps like Philips SON-T a popular choice.

A subtlety when discussing grow-lamps is that lumens/watt is a poor way to compare efficacy because the spectral sensitivity curve of the human eye is built-into lumens, while which wavelengths best turn energy into tomatoes, for example, is more crops important to growers.

The plant equivalent to lm/W is PAR – photosynthetically active radiation – whose curve has peaks at red and blue, and a dip in green where chlorophyll reflects rather than absorbs – although plants still need some green, insists Hoeben.

That said, greenhouse workers still need to see what they are doing, so an entirely tomato-centric spectrum may not be the complete answer.

‘Ceramic discharge metal-halide’ (CDS) is the established competitor to HPS, with LEDs a more recent entry.

Ampleon is pushing plasma lighting, which is similar to CDS lighting except in CDS lighting a mixture of materials is whipped into a photon-emitting plasma by electrodes in intimate contact with the mixture, while in plasma lights the mixture is sealed away in a capsule and RF energy is pumped in through electrodes on the outside of the capsule. The immediate advantage of the second arrangement is longer life – the electrodes are not gradually eaten away by the plasma.

The jury is still out on the ‘best’ grow lamp.

A mixture of LEDs can be chosen to match any required growing spectrum, but Hoeben argues LEDs do not deliver the raw power of the other three and a better suited to germinating seedlings.

In November last year, Ampleon set up a greenhouse experiment with Hogeschool University in the Netherlands to compare all four technologies, with results expected in April.

Ampleon grow lampsFrom the left in the photo, separated by white sheets, are HPS, ceramic and plasma lighting. LEDs illuminate the bottom-right hand side. The plants are: radish, tomatoes, red lettuce and roses, plus brassica seedlings.

A third application for RF power transistors is to improve fuel efficiency in internal combustion engines. The idea that a plasma can help lean fuel mixtures burn, for example.

“It is in an early state,” said Hoeben. “You use spark plug, then RF plasma for complete combustion.”

The antenna might one day be part of the spark plug, he added.

Ampleon?

Ampleon is a fabless RF power semiconductor company, formerly part of NXP.

Its main technology is LDMOS, which is made in NXP’s 0.15µm 200mm fab – the firm is seeking a second LDMOS source in Asia. It is also looking at GaN technology with two fab partners.


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