Driving Light
Posted date : Dec 13, 2010

SOI is poised to take center stage in the impending lighting revolution, with companies like NXP leading the charge. Here’s why.

Incandescent bulbs are being phased out or banned worldwide: European bans started taking effect in 2009; the US, Canada, Japan and Russia will start in 2012. India, Brazil, China and many more have all taken action.

Compact Fluorescent Lamps – CFLs – are poised to take the relay in the short term, followed later by LEDs (as they become more cost-competitive).

But consumers are in an uproar about the quality of most CFL lamps – and with good reasons: even though 4 billion a year are sold, they’re usually too slow to get up to full brightness, they don’t last as promised, they’re expensive, the color’s cold and unfriendly…and so the list goes on.

Lighting designers need chip solutions that solve these challenges without compromising time-to-market and cost. With SOI-based processes, lighting components leader NXP’s got the answer.

But to understand these challenges, it helps to understand a few basics about CFL lighting technologies past, present and future.

Incandescent vs. CFL basics

The meaning of “incandescent” is to produce light by heat. The modern incandescent light bulb, with its swirl of tungsten filament, has been doing that more or less unchanged in our homes for the last 100 years.

Electricity enters the bulb on a metal wire, comes up against the resistance of the tungsten (also a metal) filament, causing said filament to glow: ie. produce light. But it does this very inefficiently: only about 10% of the energy going in to an incandescent bulb comes out as light; most is dissipated as heat. The rule of thumb is that it takes an incandescent bulb one watt to produce around 15 lumens of light, so a 60-watt bulb produces about 900 lumens.

The fluorescent light – and its more recent incarnation, the CFL – works on a very different principle. Essentially, you have to get electrical current flowing through a tube containing mercury vapor and coated on the inside with a phosphor. The electrons around the mercury atoms get excited by the current, jumping into  higher orbitals then dropping back, thereby releasing ultra-violet (UV) photons (which are invisible). These UV photons in turn excite the phosphor, which emits visible light photons.

With just one watt, a CFL can produce 50-70 lumens. So, to produce 1000 lumens takes the CFL as little as 20 watts.

It sounds straightforward, but when it comes to making light with electricity, gas has one very different characteristic from metal. Metal – the heart of incandescent lights – is by its very nature resistant. And the resistance in metal is very constant and predictable – affected only by the kind of metal, how thick it is, and temperature.

With gas, however, the more current you run through it, the more it loses its resistivity. If you were to just plug a fluorescent tube into the mains, the amount of current flowing through the gas would quickly climb and climb til the bulb – literally – blew out. So you have to control the current very carefully:  this is where ballasts come in.

All fluorescent lamps must have ballasts at the ends of the tube, which get the current flowing through the gas and then control it. Old fluorescent lamps where not terribly good at this. They had a complex preheat system to ionize the gas in the tube and get the current flowing. But those old ballasts were based on electromagnetics (think: wire coils): the unsteady current produced those awful flickering, humming fluorescent lights of our childhood.

To make fluorescent bulbs that would fit into the same size light sockets as incandescents required a much smaller solution. Enter the electronic ballast: a little circuit board with independent passive and active components, including rectifiers, capacitors, switching transistors and inverters. They have to kick off with a very high-frequency jolt of electricity – about 40,000 Hz to ionize the mercury vapor – then settle right back to a normal operational levels (such as 50 Hz in Europe, 60 Hz in the US).

Drivers on SOI

However, one of the challenges for lighting product and electrical design engineers is that the circuitry in each ballast has to be custom-tailored to the shape and size of each CFL tube design, with all its twists and turns.

NXP recognized that integrating as many of the discrete components of a light ballast as possible into a single driver IC could drastically simplify lighting ballast design while improving overall system performance (lifetime, form factor, switching cycle) and quality (reliability, robustness, early failure, and so forth).

A driver IC bridges the analog and digital worlds, taking a logic signal output from the control system’s microcontroller (which can be integrated onchip or can be on a separate chip), and provides the appropriate current and voltage to turn power devices on and off. As such, it has to be extremely robust – especially in a high-temperature environment – yet very cost-competitive.

Leveraging its EZ-HV SOI technology (see sidebar), NXP is integrating an increasing number of components into high-voltage power ICs intended to drive and control electronically ballasted CFLs with few external components needed.

For example, the NXP UBA2024 family and its newly release successor, the UBA2211, is a 550 V lamp controller and half-bridge IC.

The high level of integration reduces the number of external components to only 17 (compared to the 27 typically required by a discrete driver solution). The IC supports NXP’s patented preheat and a series of protections, which the company says enables CFL lamps to reach lifetime performance of up to 15,000 hours.

Rapid ramp ahead

As of  2009, lighting components market leader NXP had sold 300 million fluorescent lighting driver ICs. Now with regulations taking effect worldwide over the next few years, the market is set to explode.

“With 20 to 25 percent of the electricity produced in the world consumed in lighting applications, and over 5 billion energy saving bulbs to be sold in 2012 according to Datapoint Research, NXP’s next generation of CFL lighting drivers break open the next ‘killer application’ for power management ICs. Backed by [SOI-based] GreenChip technology, and NXP’s ability to manufacture to scale, the economics are staggering,” says John Croteau, senior vice president & general manager, Business Lines High Performance RF, Power & Lighting, NXP Semiconductors.

That certainly makes for a very bright, SOI-enabled future.

The first stage of the 2005/32/EC Directive took effect in 2009. By 2013, the eco-design requirements take full effect.


Thinking Thin: NXP’s EZ-HV-SOI

First announced a decade ago, NXP’s EZ-HV™ is process technology for the production of commercial high-voltage (HV) SOI-enabled ICs.  It is at the heart of NXP’s GreenChip technology, and is ideal for implementing optimal solutions for a wide range of lighting applications. In high-power systems, it allows sophisticated control logic and high voltage drive circuits to be integrated into a single IC (replacing the separate high and low voltage chips), halving the cost of the overall unit.

It represents a radical departure from “thick SOI” solutions, in which a 10 to 20 micron layer of silicon overlaying an insulating material limits the electric field strength to prevent a regenerative, killer effect called ‘avalanche breakdown’.

Instead, the EZ-HV process uses SOI wafers with a relatively thin layer of top silicon (only 0.5 microns thick), which is much cheaper to produce. Even with 650 volts applied across this layer there is still insufficient distance between the upper and lower layers of the silicon for charge carriers to be accelerated to avalanche breakdown energy levels.

Other advantages include:

  • lower cost
  • higher integration density
  • latch-up immunity, even with multiple power components on a single chip
  • significantly reduced parasitic capacitance for far faster switching
  • RDS(on) values halved enables handling of higher currents
  • drift region fully depleted doubles effectiveness of RESURF, simplifying high-voltage design trade-offs

Cross Section of EZ-HV LDMOS Structure: Producing this critical high-voltage component requires the use of only 13 mask steps, making the production of basic EZ-HV ICs cost-effective even for price sensitive applications.


Newest Drivers Reach New Levels of Integration

Annual sales of CFLs with integrated ballast

NXP’s UBA2211 for the 230V and 110V markets is the latest driver IC family in the company’s CFL IC portfolio. Built on NXP’s EZ-HV SOI technology, it features the highest level of integration available on the market today, including a current controlled preheat function, enabling more compact designs, highly efficient power conversion, and extended CFL lifetimes in the range of 12,000 to 15,000 hours.

“As incandescent bulbs are phased out around the world and with the recent EU ban on 75W incandescent bulbs in effect, the time is now for CFLs to prove they can offer better quality of light at lower costs. NXP driver ICs such as the UBA2211 offer breakthrough quality, performance and features that enable CFL manufacturers to match consumer expectations for quality lighting in the home,” says Jacques LE BERRE, director of marketing and business development, Lighting Solutions, NXP Semiconductors.

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