RF Energy Radiates to New Apps

Powerful chips finally enable competition of RF energy in microwave ovens, spark plugs, growth lights and more as demonstrated by NXP at the IMS 2015.
By John Blyler, Editorial Director
Emerging applications for RF energy will change the way we cook our food, grow high-yield crops, conserve gas in our cars and more. Many of these applications were demonstrated at the NXP booth during IEE International Microwave Symposium (IMS) 2015. To learn more about this emerging technology, IoT Embedded Systems talked with Paul Wilson, VP Global RF & Wireless Solutions and Roger Williams, engineer at NXP Semiconductors. What follows are portions of those conversations during the show. – JB

Compelling quotes:
> “RF energy can perfectly cook a fish within a block of ice.”
> “Nobel metals or LD-MOS devices. Where would you rather spend your money?”
> “This time we’re focusing on the agricultural uses of plasma lighting instead of street lighting.”
> “All of these RF energy applications are possible with LDMOS transistor technology.”

 
I. RFIC Oven
Blyler: What advantage does RF technology have over the traditional microwave oven magnetron?
Wilson: The magnetron generates one power level at a time. In contrast, the solid-state cooker based on RF technology uses both power level control and frequency tuning to adjust the cooking conditions throughout the over. I’m told use can use RF energy to cook a fish to perfection while it is still embedded in a block of ice (see video and Figure 1).

Our customers in the food industry combine this technology with clever algorithms to change the heating state based on reflections. Thus, in a controlled way, they can sweep using various frequencies and phases to cook more homogenous. That is the potential.
Further down the road, you can envision a precooked meal with a NFC tag that tells the oven the spatial location of various parts of the meal, e.g., peas are over here, mash potatoes there and meat here. You can target the different food groups with the frequencies that they tend to absorb better. You can have homogenous cooking of each piece of food on the plate.
Blyler: When will actual consumer products be available?
Wilson: It’s maybe a year or two out. The exhibit that we have on display (see Figure 2) is only a mock-up. We took a commercial microwave cabinet and retro-fitted it with a few cool parts. It’s not the real thing – yet. One challenge in getting this application to market is the cost. We’re not talking about a $10k base station but rather a commercial microwave-like over. That’s why we want to work together with people in the value chain and this alliance to bring it to market. (see, “Does RF energy mean the end of the microwave oven?”)

 
II. RF Energy Alliance
Blyler: What is the RF Energy Alliance?
Spokesperson: The RF Energy Alliance is a non-profit technical association comprised of companies developing solid-state RF energy as a clean, highly efficient and controllable heat and power source.
The RF oven is the first prominent example of the application of RF energy. This technology has reached a tipping point where broad adoption can be realized in a variety of other applications including RF spark plugs for combustion engines and plasma lighting to grow more crops.
Key to the alliance will be identifying synergies between existing applications and developing roadmaps for all critical components of the RF energy delivery chain. This will involve creating component specifications per application and catering for equipment approval procedures.
 
III. Automotive – (RF Spark) Plugs
Blyler: Do we really need new spark plug technology? What does RF energy bring to the equation?
Williams: Today’s spark plugs ignite the fuel mixture. But RF spark plugs burn everything in the combustion chamber much more efficiently. The clear advantage in using plasma in the FR ignition is significantly improved fuel efficiency.

Blyler: Why is an RF plug so much better?
Wilson: There are two main reasons: One is that you’re able to continue to pump energy into combustion chamber for a longer period of time. High-voltage spark plugs create a millisecond impulse event where a single spark spreads the flame from the center outward. It is a self-developing plasma. But a microwave plasma, because of the continuous pumping, allows you to continue to pump energy into it and maintain a constant, stable combination cycle.
The other thing is that microwave is pumping into a larger volume. Instead of a high-voltage spark that starts in the middle and grows out, you can start pumping into a 6 to 10 mm sphere because the wavelength is long. Naturally, it depends upon your antenna design. But the end result is that you can deliver energy into a quite a large volume simultaneously, i.e., quite a distance away from the center of the spark plug.
Blyler: How much more efficient is the RF implementation?
Wilson: The current RF technology has a 10% fuel efficiency improvement over the Japanese vehicle JC08 drive cycle. This is a drive cycle where the car is idling for a given amount of time, then accelerating slowly under various load conditions. [Editor’s Note: Driving cycles are produced by different countries and organizations to assess the performance of vehicles in various ways, as for example fuel consumption and polluting emissions.] The other improvement you get is from the recirculation of exhaust gases. Every car has exhaust gas recirculation. That’s your EGR value, which has been used as an emission control feature since the early 70s. Traditional high-voltage combustion systems have a lot of fuel mixture that isn’t burned. These unburned petro-carbons would go out the exhaust pipe. Instead, they are recirculated thus improving gas efficiency while reducing emission. Also, it means that there are fewer noxious emissions to remove in the catalytic convert. This can potentially make the job of the catalytic converter much easier and reduce its cost. That’s good as the RF electronics cost more money compared to high-voltage emissions. Reducing the costs of the palladium feeds in the catalytic converters helps to offset the cost of buying the LPMOS transistor used in RF plugs.
It’s a good tradeoff: Nobel metals or LD-MOS devices. Where would you rather spend your money?
 
IV: Lighting – Plasma for Agriculture
Blyler: You’ve had plasma lighting display at past shows. How is this one different?
Wilson: This time we’re focusing on the agricultural uses of plasma lighting instead of street and warehouse illumination. In either application, RF energy is used to excite plasmas that then give off light (see Figure 4). The color of the light can be turned by the composition of the plasma gas.

The nice thing about plasma lighting is that you don’t need sunlight to grow planes. Lettuce can be produced in a controlled environment in a warehouse and then delivered to super markets and restaurants with minimal transportation costs. Current shipment of produce across the country is expensive – including petroleum and cooling costs for the produce – and may still results in spoilage. That’s not the case if the produce can be grown locally, near the super markets and restaurants.
Blyler: If you live in Portland like I do, you also have a limited number of sunny days.
Wilson: Yes – a greenhouse environment can be used to extend the growing phase. Plus the yield impact on the produce is significant when using RF technology. Lettuce, for example, can be grown to twice its usual weight. On the other hand, the RF technology is about $300 or so more expensive than traditional methods.
 
IV. LDMOS RF transistor technology
Blyler: Is there a common element behind all of these RF energy applications?
Wilson: All of the applications we’ve just seen (cooking, automotive, agriculture) are being enabled by the development of high-power Laterally diffused metal oxide semiconductor (LDMOS) RF transistor technology (see Figure 5 for an example). [Editor’s Note: Silicon-based LDMOS FETs are widely used in RF power amplifiers for base-station.] This is the same technology used in wireless base station and defense applications. At IMS 2013, we introduced a complete RF power transistor portfolio for the 2.45GHz ISM (Industrial, Scientific, and Medical) frequency band. The portfolio enables RF energy to be used as a clean, highly efficient and very controllable energy source.

This year at ISM 2015, we’ve incorporated addition innovation into the basic chips. Pretty much every base station uses the Doherty circuit technique to increase its efficiency. [Editor’s Note: The Doherty amplifier design uses a combiner to improve efficiency compared to balanced amplifiers.] The challenge is that Doherty circuits have a lot of matching configurations that typically go on the outside of the chip. Now, we’ve put all of the external devices into the chip. We’ve integrated the Doherty circuitry to get rid of all of this matching by using bond wires and other concepts inside (see Figure 6).

Blyler: During the show, there was talk about the benefits of GaN over LDMOS and vice-versa. What are your thoughts on these different implementations?
Wilson: The applications and technology that I’ve shown you so far is based on LDMOS technology. There has been a lot of talk at this show about gallium nitride as an up and coming technology for RF power especially where larger bandwidths and higher efficiency are needed. GaN started in military communications for jammer and other applications that needed large bandwidths of frequency. Now it is starting to be used in some base station application because it offers high efficiency over more bands with one amplifier. Also, it has better performance at high frequencies.
We (NXP) have both technologies – GaN and LDMOS – so our customers can chose which is best for their applications. Still, we continue to innovate on LDMOS, achieving 48% efficiency and getting better. GaN is more expensive than LDMOS but better in some applications. So there are tradeoffs.