EEWeb Pulse - Volume 45 by EEWeb Magazines

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PULSE

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Issue 45 May 8, 2012

Anthony Catalano

TerraLUX Inc.

Electrical Engineering Community

EEWeb Electrical Engineering Community

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TA B L E O F C O N T E N T S TABLE OF CONTENTS

Anthony Catalano TERRALUX INC. Interview with Anthony Catalano - Founder and CTO

Advanced Thermal Control for Ensuring LED Lifetime BY ANTHONY CATALANO

Learn the root causes of LED light degradation and what factors assure maximum LED lifetime.

Featured Products Illogical Logic - Part 1: Boolean Algebra

15 17

BY PAUL CLARKE WITH EBM-PAPST

See how Boolean Algebra allows you to break complex logic to simply the elements that matter.

A System Perspective on Specifying Electronic Power Supplies: Efficiency

BY BOB STOWE WITH TRUE POWER RESEARCH

How energy conservation, package size and rise in temperature attribute to optimal power supply efficiency.

RTZ - Return to Zero Comic

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INTERVIEW

TerraLUX, Inc.

Anthony Catalano - Founder and CTO

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FEATURED INTERVIEW

Anthony Catalano

How did you get into electrical engineering and when did you start? I decided I wanted to be a chemist in the 6th grade, but got involved in electronics in high school by building a mass spectrometer and other electronic gadgets. I’ve always had one foot in electronics and another in chemistry. I got my undergraduate degree in chemistry from Rensselaer Polytechnic Institute and went on to receive my Ph.D. from Brown University, and did a post-doctorate in solidstate chemistry. While I was at Brown, I made my first LED, which really jumpstarted my career in electronics. Until then most of my solid state work had been done on passive materials and here was something that generated light! However, my career was quite separate from LEDs initially in that it dealt mostly with solar cells. I was on the staff at the University of Delaware’s Institute of Energy Conversion for several years working on Thin Film Solar Cells. I went from there to RCA Laboratories in Princeton and worked with Dave Carlson, the inventor of the amorphous silicon solar cell. While there, I developed the world’s first 10% conversion efficiency amorphous silicon solar cell, for which I received an award and a bunch of job offers in Japan. RCA decided it really wasn’t too interested in solar cells, and got out of the business. A group of us—including Dave Carlson—left and started a division of Solarex, Inc., which was already making crystalline silicon solar cells. We started the business with a filing cabinet a telephone and 30,000 square feet of warehouse space,

INTERVIEW

Solarex was later acquired by Amoco, which of course—long after I left—was purchased by British Petroleum (BP), which now has gone full circle and is divesting itself and closing down because of all of the worldwide competition in Photovoltaics. After leaving what was then Amoco, I came out to Colorado to be the Director of the National Renewable Energy Laboratory’s (NREL) photovoltaic division. We were working for The DOE (U.S. Department of Energy) to reduce the cost of solar energy to allow it to compete with other forms of electrical generation. In 2003 I realized that the white LED is really a game-changer. It was Craig Christensen from Harvard who coined the term “disruptive technology,” and if LEDs aren’t a disruptive technology, I don’t know what is. So I thought about exploring this technology, and said to myself, “You can stand on the sidelines or you can get wet.” And I decided to get wet and start TerraLUX to commercialize LED lighting technology. I really feel that LEDs will completely transform how lighting is produced during the 21st Century. Hence our company tagline: “Intelligent Lighting for the 21st Century.” I started the jump into LED lighting as both an inventor and angel investor in TerraLUX—spending my own money to get the company

started—I really wanted (and needed!) to have products that I could design and sell in fairly short order. The Company’s first entries into the LED lighting market were intended to generate revenue quickly. So we decided to build retrofits for flashlights that were then using incandescent bulbs. There were very few LED flashlights on the

Our general illumination products are little bit less than 50 percent, but it’s rapidly catching up. We expect it surpass the flashlight & upgrade business pretty quickly. market, and that’s how we got started. This all started in my garage in 2003. After it took over the kitchen, laundry room, garage and one of my daughter’s rooms we were asked to leave. Our flashlight upgrades were a considerable improvement to incandescent bulbs, and although they were expensive, there was a rather large market for them. The business ended up taking off, and really provided the revenue to do things like file patents and hire

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people to help us grow. The business was going well, but my eye has always been on general illumination. When I started the business, my vision—if I can justify that term—was for LED lighting to displace all of the then-current forms of lighting. It was efficient, potentially immortal in terms of its longevity, and it had so many features that were going to make it difficult for anything to stand it its way. I still feel that way. What are some technical challenges for LED lighting to be more universally adopted? Part of it is of course the present state of the economy. There isn’t a lot of new construction going on, so one of the technical challenges that we face regularly is compatibility with the existing infrastructure. The existing infrastructure in buildings—whether residential or commercial—involves legacy power supplies and legacy dimmers. One of the big challenges associated with that is making LED lighting work to the same level of performance that customers are used to. Flicker-free performance and dimming is an area where we have put a very large effort. Of course we have an eye to the future and want to be at the forefront of the technology. While the emphasis now is on energy efficiency, we are looking forward to how we’re going to deal with building information systems and things like that. We’re trying to see the whole universe of possibilities for LED lighting backward (in terms of compatibility) and forward (as regards building information

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and that was the beginning of my entrepreneurial experience. I ended up turning it into manufacturing and R&D space, and the business began to grow.

INTERVIEW

What were some of the initial products TerraLUX was providing/selling? White LED technology in 2003 did not quite meet the needs of many illumination applications from a light output and light quality standpoint, so our very first products were in the portable space in the form of LED upgrade modules for flashlights. There are literally many millions of quality flashlights sitting around in toolboxes and kitchen drawers that quickly become obsolete from a performance stand-point with the introduction of LED technology. By offering LED upgrades for many popular flashlight models such as MAGlite® and Streamlight®, we gave technicians, mechanics, police officers, campers, and consumers the ability to “upgrade” their flashlight to LED performance for a

fraction of the cost of buying a new LED flashlight. The improvement in both light output and run-time is significant. The concept quickly caught-on, and

The technology we are developing is addressing the sector of the lighting market characterized by small form factor, high brightness and thermally challenging. A key element is various methods of providing protection to the LED and circuit components that ensures a long life. we now make upgrades for a wide range of flashlights. Eventually, we started designing and selling our own flashlights, and continue to expand our line with unique features such as penlights with high Color Rendering Index (CRI) LEDs for technicians to more clearly differentiate wire colors, and even flashlights that have been designed to accept an upgraded LED module in the future—because

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we will continue to see significant improvement in LED technology and performance for years to come. How much of your business focuses on flashlights versus the other lighting areas you’ve mentioned? Our general illumination products are slightly less than 50 percent, but are rapidly catching up. We expect it surpass the flashlight & upgrade business pretty quickly. Exactly what products do you sell to the OEMs? We have both line voltage and low voltage products. The low voltage products are MR16 halogen bulb replacements. We aren’t actually selling LED light bulbs though; what we’re selling is what we refer to as a light engine. They are form factor compliant, which means a little bit less than two inches in diameter so they can fit into customers’ fixtures that have been sized for an MR16. While they won’t fit into every fixture as is, it’s very easy for our OEM customers to adapt their products, and we help them do that. Most often this means little more than designing a small adapter that basically forms a thermal bridge between our light engine and the manufacturer’s fixture. Also, most of the lighting manufacturers are very skilled metal workers, and it’s usually easier for them to make it than it is for us. We actually use the fixture as a heat sink, which is essentially impossible for an ordinary LED light bulb because a bulb does not integrate mechanically and thermally with the fixtures. In many cases, that means that an ordinary LED bulb in a fixture will

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systems). Our customers are not the people who are typically buying LED light bulbs; we’re selling to the original equipment manufacturers (OEMs) and some of the major lighting fixture manufacturers. We’re trying to build features into our products—our LED light engines— features that will give our customers assurance that the products will fulfill the lifetime and energy that people expect of LEDs . One of the signature features of nearly all our products is thermal fold back, or LEDSense®. As the temperature of the LED gets higher, the allowable current is lower, so we adjust the current through the LED based on temperature measurements within the drive electronics. We use the LED Manufacturer’s LM-80 test data to do this. It’s all microprocessor based.

INTERVIEW

We have also developed a more versatile spotlight engine for applications like track lighting— one that can provide light output equivalent to a 50 watt halogen as well as several output levels below that. It also has features to alter the beam angle. So now customers only have to stock one product and change the output based on the specifications of the fixture or the lighting designer’s requirements. This product benefits everyone involved. It makes things easier for the fixture manufacturer, the lighting designer and the end user. This month we also started shipping our compact 120V linear engines as well. These work with current dimmers and infrastructure and are aimed at 120 to 180 degree output applications such as wall sconces, ceiling-mounted fixtures, and recessed step-lights like those found in theatres or outdoor landscape environments. Within a few months we will also offer a flexible voltage version of these 4 to 8-inch long modules, which can be powered with 100-277 Volts AC.

Do you develop custom lighting products for other companies if they request it? Yes, we’ve actually done that for many years. For example, a well-known healthcare company approached us in 2005—I guess because some of the engineers there bought our flashlight products—and asked us to design a product for them that eventually found its way into one of the world’s first LED medical scopes of its kind. That was an OEM product; it was proprietary to them and we continue to manufacture it for them to this day. Can you tell us more about TerraLUX, Inc. and the technology they are developing? The technology we are developing is addressing the sector of the lighting market characterized by small form factor, high brightness and is thermally challenging. A key element of our technology are the various methods of providing protection to the LED and circuit components that ensures a long life. Our goal is to provide a “Plug & Play” solution for both the OEM manufacturer and retrofit opportunities. Providing a completely engineered solution is our goal. Many of the OEMs are focusing on design and appearance and have few resources to devote to a product that requires expertise in electrical, optical, mechanical and thermal engineering. We do that so they don’t have to. We are now introducing a line voltage (100-277VAC) dimmable series of linear light engines that are completely integrated and have

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the necessary UL, FCC and other approvals. Although the product is effectively a luminaire by itself, it’s intended for OEMs to include in sconces, ceiling lights and other fixtures they manufacture. It can also be used in retrofits of existing luminaires in the field. Because it is truly Plug & Play, it only requires 3 wire nuts and a screwdriver to install. I say this from personal experience; I’ve installed them in outdoor sconces at home. It’s almost a DIY product. We have a low voltage (12 VAC) product coming onto the market that produces the light output equivalent of a 50 W halogen. This product has variable beam angle and adjustable brightness levels that can be set. In essence, a lighting manufacturer can stock one SKU and with little effort either the lighting designer or manufacturer can change beam angle and light output either on the factory floor or in the field. It’s also dimmable using our microprocessor-based Dynamic Transformer recognition (DTR) and contains a high level thermal fold-back feature that is tied to the LED’s LM-80 data. It’s a pretty sophisticated product. What direction do you see your business heading in the next few years? Lighting is, from a small company perspective, a semi-infinite market. Maintaining focus during a period of extraordinary growth is the greatest challenge. That said, we have very ambitious goals and expectations of continuing to expand our portable product (flashlight) business while also realizing explosive growth with our LED engines for the general illumination market. Interestingly

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reach a destructive temperature. At any given time, LEDs must be maintained within a safe operating temperature range. This is nearly impossible for a sealed fixture with a LED replacement bulb, but fairly straight-forward using one of our modular light engines. One area where we’ve become pretty successful, is landscape lighting. Landscape lighting fixtures are sealed, and can pretty easily accommodate our product.

INTERVIEW We as an industry must figure out how to ensure an experience which is cost-effective and satisfying.

What challenges do you foresee in our industry? The legacy infrastructure for lighting is quite diverse and complex. The various dimming schemes and power sources (both line and low voltage) represent a huge challenge. LEDs react nearly instantaneously, which is often a curse. The slow response time of incandescent sources prevents many problems from becoming apparent. When LEDs are used the often messy state of the electrical infrastructure becomes evident. The LED light source is usually faulted.

Is there anything that you have not accomplished yet, that you have your sights on accomplishing in the near future? I think we as a team want to see TerraLUX recognized as a leader in LED lighting. As my high school physics teacher implied, we need to turn our “potential” into “kinetic.” Personally, having started the Company back in 2003 it has been very satisfying to see the original vision turned into products, patents and perhaps most importantly-jobs. ■

FEATURED INTERVIEW

enough, we’ve found the technology development for these two different markets to be very complimentary. For example, we initially developed our LEDSense® Thermal Management technology for flashlights, which allowed us to realize significant light output from a flashlight with minimal heat-sinking during the typically short “ontime” that a flashlight experiences, while also ensuring that the system does not overheat or degrade the LEDs if left on for a longer period of time—either purposely or by accident. Now we use LEDSense® technology in our illumination engines to assure long-term lumen maintenance in potentially adverse conditions such as a landscape lighting fixture being accidentally covered in dirt and not able to dissipate heat. With LEDSense®, the system will simply pull back on power until the fixture us uncovered. We expect to continue to leverage core technology like this in more and more lighting applications, leading to significant growth for many years to come.

Do you have any tricks up your sleeve? We have a lot of technology we have not deployed in products. There are methods of LED control that get to the basic physics of how LEDs operate that would be exciting to implement, but it’s probably too soon. Other features such as making the light source part of the information superhighway are exciting. Since LEDs are very controllable, and require drive electronics anyway, taking the next step of integrating lighting into a building information system is not a significant stretch. This can offer significant new features from tuning light to meet the demands of the users to a higher level of control for the purpose of conserving energy. By integrating technology such as wireless control into our LED engines and modules, we can even bring these new features to older buildings.

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Avago Technologies LED Lighting Solutions

One LED. Infinite colors. World’s first waterproof package. Avago Technologies Tri-color High Brightness PLCC6 SMT LEDs gives you a reliable, long life product for ease of design in full color interior and exterior signs

Tri-color High Brightness SMT LEDs from Avago Technologies Avago’s PLCC-6 SMT LEDs are high brightness, high reliability, high performance, IPX6 compliant and are water and dust proof. They are designed with a separate heat path for each LED die, enabling it to be driven at higher current. They deliver super wide viewing angle at 120° together with the built in reflector pushing up the intensity of the light output.

Applications • Indoor and outdoor full color display • LED advertisement panels • Decorative lighting

Features • Water-resistance (IPX6*) per IEC 60529:2001 • Very small PLCC6 package dimensions – 3.4 x 2.8 x 1.8mm • In-line RGB dies configuration • Available in White Surface, Black-Surface and Full Black-Body • Wide operating temperature range: -40° to +110° For more information or to request a sample please go to:

www.avagoresponsecenter.com/led

Advanced Thermal Control for Ensuring LED Lifetime Anthony Catalano

Founder and CTO TerraLUX Inc.

Background White light emitting LEDs have proven to be a disruptive technology challenging all older forms of light generation. The potential for: 1) very long life (>35,000 hrs), 2) extremely high efficacy (theoretically ~250 Lumens/ Watt) and 3) low temperature operation has taken the lighting market by storm. These great expectations of the technology have lead LED manufacturers and the industry as a whole to devise testing standards to ensure lighting products embody the performance that is expected by consumers, whether they be the end user or the OEM manufacturer. Unlike ordinary filament-based incandescent lamps, LEDs do not “burn out” but instead may gradually experience a decrease in light output depending on operating conditions. Why Do LEDs Degrade? LEDs are complex solid state devices. Figure 1, illustrates a cross-section of a typical device, showing the various structures that comprise a packaged LED.

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For simplicity we will categorize the components into 3 areas. First, a semiconductor device containing a p/n junction that, in the case of conventional white LEDs, actually generates blue light at a wavelength of approximately 450 nm. Second is a phosphor layer that absorbs the blue light and coverts it to a broad band of colors that the eye perceives as white in much the same manner as a fluorescent tube. Lastly, there are a series of clear layers that encapsulate the semiconductor and a lens that collimates the exiting light. Each of these three regions may participate in the degradation of the device, albeit through different mechanisms. The Semiconductor Junction. LED manufacturers hold dear their process and composition of the semiconductors that comprise the diode’s p/n junction. However, at present, all devices are comprised of materials classified as III-V materials: Ga, In, or Al combined with N, P, As from the respective columns 3 and 5 of the Periodic Chart. Virtually all commercial devices are heterojunctions, meaning they are combinations of dissimilar chemical

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PROJECT Lens

Figure 1: Simplified Cross-section of an LED illustrating the important components associated with degradation. EN Conduction Band

EC Photon

- electron - hole Defect EV

Valence Band

Figure 2: Band Diagram Illustrating radiative emission and nonradiative recombination via defects.

compounds. Moreover, they are single crystal structures relying on epitaxial growth via chemical vapor deposition for their formation. These layers are grown on substrates such as sapphire or silicon carbide. Often they are complex layered structures,-so called “quantum wells” that carefully manipulate electronic processes to maximize the conversion of electrical charges into light. One consequence of these combinations of dissimilar materials are defects that arise due to mismatches in the atomic lattice dimensions and thermal expansion coefficients among the layers. The consequence of these imperfections are atomic defects in the lattice structure, both in the bulk of the material as well as at the interfaces between the different materials. To create light electrons injected from the majority carrier, n-type doped layer recombine with holes injected from the p-type contact within the junction to form blue light. However, not all electrons and holes recombine to generate light, otherwise we would have vastly higher performance! Non-radiative recombination of carriers may happen via several mechanisms, but the most important from the standpoint of reliability occurs at these defects within the semiconductor. Because these defects lie at a lower

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Phosphors. Phosphors convert the 450 nm blue light from the LED to the various colors of the visible spectrum to create white light. They do so by absorbing the blue light and losing a portion of the photon’s energy in a controlled fashion, down-converting the blue to red, green and blue over a broad band of wavelengths. These phosphors are often complex rare-earth silicates or oxides, and may be doped to ensure specific wavelengths of emission. While these materials are polycrystalline and already contain numerous atomic defects, recombination as described in the previous section is active here too. In addition chemical processes such as reaction with water vapor or other chemical compounds can lead to degradation. Because these effects are highly dependent on the chemical composition of the phosphors, and the phosphors used are part of proprietary designs, there may be considerable variation among LEDs. Often even within a manufacturer’s product line different phosphors are used, or they are applied in a different fashion that results in a particular behavior. Lens & Encapsulant. The clear lens that acts to collimate light emanated from the semiconductor die-phosphor structure and the protective encapsulant material must remain highly transmitting throughout the life of the LED. Because LEDs operate at elevated temperature and humidity, degradation may occur here as well. In addition, the blue light exiting the LED phosphor also may play a role in the darkening process. Once more, the specific chemical composition and structure of the lens will determine its behavior under normal and adverse circumstances and is highly process and composition dependent. Conclusion. The complex electrical and chemical processes that occur during the operation of an LED and give rise to decrease in light output are difficult to quantify via a simple analytical expression. While a

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Encapsulant Phosphor Semiconductor Die

energy level than the conduction and valence band of the semiconductor, they act as a means by which electrons and holes recombine non-radiatively, giving off heat instead of light. This energy can be quite large, on the order of the energy of the chemical bonds and thereby creates more defects through the displacement or rupture of chemical bonds. This initiates a “snowballing effect” that accelerates with time. Figure 2 illustrates a simplified band diagram of the semiconductor showing the various recombination processes.

PROJECT • Interpolation Between Temperatures Are Based on “Activation Energy” In mathematical terms the decrease in light output can be stated as,

Standards: LM-80 and TM-21

• Ambient and LED Case Temperature and Orientation • Drive Voltage, Current and Waveform • Instrumentation The standard calls for measurement of light output at an ambient air temperature of 25oC, but LED case temperatures of 55oC, 85oC and another temperature selected by the manufacturer. The drive current is specified by the manufacturer as this varies with the die area of the LED. Measurements take place over a minimum of 6000 hrs of operation (10,000 hrs is preferred) and at intervals of at most 1000 hrs. It is common practice among first-tier manufacturers to employ several different drive currents. As we shall see later this is very important to our method of ensuring LED lifetime. Although LM-80 provides a uniform method for measuring light output over time under standardized conditions, in practice LEDs may be used at temperatures that differ substantially from the LM-80 values. Moreover, because this is not an accelerated testing method, very long times are needed to reach a conclusion on reliability. Another more recent IES Standard, TM-21 helps solve this dilemma. The standard is effectively an “ad hoc” model of LED degradation that allows the interpolation of timetemperature data between temperatures and formalizes the extrapolation of data into the future to predict output over extended times. The major points of the standard are: • Exponential Decrease in Light Output (LOP) is assumed • LOP May be Extrapolated Maximum of 6x in Time

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L (t) = B [e - at] where L(t) is the Lumen output at time t, B is the normalized light output at 0 or 1 hours, and α is the decay rate, which is a function of temperature. The value of α varies with temperature according to the Arrehenius expression,

a (T) = Ce -E /kT a

where Ea is the activation energy, k is Boltzmann’s constant and T is the temperature in degrees Kelvin, and C is a constant. The TM-21 standard only allows for the interpolation of temperature data, so for two temperatures T1 and T2 we can calculate the activation factor, Ea/k as,

ln [ aa ] E =(k) [ T1 - T1 ] 1

Once the activation factor is known then it is straightforward to calculate the decay rate from the Arrehenius expression for the intermediate temperature and the resulting time dependence. Figure 3 shows a graph of both real LOP data and the extrapolated and interpolated values based on these calculations (reference: Mark Richman LEDs Magazine). 1.1

Normalised Light Output

The Illumination Engineering Society (IES) has developed standards for testing LEDs so performance and reliability can be characterized in a consistent fashion to assess their practical life. One standard, LM-80, provides a description of the method to be used in determining the “Lumen Maintenance” of LEDs, that is the light output as a function of time. The essential features specified by the testing protocol are:

1 0.9 0.8 105C1 Amp Data

0.7

TM-211 Amp 105C 85C1 Amp Data

0.6 0.5

TM-211 Amp 85C 0

5000

10000

15000

20000

25000

Hours

Figure 3: LED degradation data (triangles and diamonds) used to extrapolate and interpolate temperature vs time Lumen maintenance information.

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mathematical description might be possible for one or two of these processes, the complex overlapping and interdependence of the processes makes it impossible at this time.

PROJECT The drive current of the LED plays an important role in determining Lumen maintenance. As discussed at the outset of this article, recombination in the semiconductor leads to the multiplication of defects and a reduction of the radiative efficiency. So it stands to reason that drive current must play an important role in Lumen depreciation. Unfortunately, no current standards recommend testing in this regime, and it is left to the LED manufacturer to choose the values of current used in testing. Fortunately, the top-tier manufacturers have chosen wisely and substantial data is available. Figure 4 provides a dramatic illustration of the importance of drive current, particularly at elevated temperatures. In this chart TM-21 has been used to extrapolate the Lumen maintenance data for an LED at a semiconductor junction temperature of approximately 127oC. It can clearly be seen that there is a very large increase in the rate of degradation as the current rises from 0.35 A to 1 Amp. It can also be seen from this chart that the L70 value (where light output decreases to 70% of its initial value) decreases from over 91,000 hours at 0.35 Amp to 22,900 hrs at 1 Amp, a huge reduction in effective useable life. (Strictly speaking TM-21 only permits a 6-fold extrapolation beyond the time-dependent data; I have clearly gone beyond this to make a point)

Normalised Light Output

0.9 L70=91,400 hrs

Tj≈127ºC 0.8

L70=52,200 hrs

0.7

1 Amp Data 0.7 Amp Data 0.35 Amp Data

0.6

1 Amp Calc 0.7 Amp Calc

L70=22,290 hrs

0.35 Amp Calc 0.5 1000

10000

Hours

Figure 4: Chart showing the influence of drive current on LED degradation.

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The Importance of LEDSense® Technology Given the dramatic reduction in Lumen maintenance that results from operation at high current levels, it is important that a means be provided in the drive electronics to safeguard the operation of LED lighting products. Thermal fold-back provides that safety net. Thermal-fold back is important because: • It allows the maximum brightness and maximum lifetime regardless of conditions • It provides a “worry-free” solution to OEMs • It virtually eliminates the consequences of a bad installation in the field TerraLUX’s implimentation of thermal fold-back, called LEDSense®, employs a microprocessor-controlled constant current driver. Figure 5 illustrates a block diagram of the essential features of the LEDSense circuit. Various buck or boost topologies are used depending on the input voltage range and the length of the LED string which determines the required output voltage. The temperature of the LED is measured via a thermistor which is located in the thermal path of the device so that the operating temperature of the LED can be determined. The microprocessor, via an A/D converter measures this value and compares it to the known operating characteristics of the LED via a built-in algorithm. The processor then sets the current of the driver through its D/A converter. With a properly designed and operating luminaire-lightengine combination, the current (and temperature) remains at the predetermined safe level. If temperature exceeds a preset threshold, an algorithm in the processor gradually reduces the current set point via the firmware’s algorithm to assure the longevity of the LED. The microprocessor is also used to analyze the waveform of the power source in order to tailor the driver operation to the particular dimmer-transformer combination in use, proving additional benefit to the OEM manufacturer. The resulting performance characteristic is shown in Figure 6. This figure compares the drive current to the temperature of the LED for two examples of an LED lightengine, the first (blue squares) without LEDSense® technology and the other (red circles) with LEDSense® operating. In this example the temperature that is being externally controlled is that of the back of the lightengine heat sink, but we have plotted the LED’s slug

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The Important Role of Drive Current on Lumen Maintenance

PROJECT

Microprocessor ROM

Conclusion

Firmware

Current Temperature Data

Waveform Detection Based Dimming Algorithm

Digital/Analog Converter

Analog/Digital Converter

Driver IC Figure 5: Schematic diagram showing the major functional areas of the microprocessor based LEDSense thermal fold back

Drive Current, (Amps)

Comprehensive standards have been developed by the industry to test LEDs and describe their degradation over time. So far however, these standards only allow prediction as a function of operating temperature. We have shown that the drive current in combination with operating temperature has an extremely important influence on LED lifetime as defined by L70. Thermal fold-back provides a useful method to ensure control within the safe operating envelope of LEDs ; TerraLUX’ LEDSense® technology has the added benefit of a quantitative control algorithm that assures both maximum LED lifetime and light output. References

1.1 1 L70=30,500

L70~13,900

0.9 0.8

L70>326,000 hrs Die Temp=123ºC

0.6 Thermal Foldback 0.5

No Thermal Foldback

IESNA LM-80-08: “Method for Measuring Lumen Maintenance of LED Light Source”, American National Standards Institute IES TM-21-11: “Projecting Long Term Lumen Maintenance of LED Light Sources”, American National Standards Institute

0.7

0.4

the eyes’s insensitivity at high brightness. Nonetheless, even in the region where there is only a slight overtemperature condition, the LEDSense algorithm will keep the LED safe.

105

115

125

Eric Richman, “The Elusive ‘Life’ of LEDs: How TM-21 Contributes to the Solution”, LEDs Magazine, November/ December 2011 ■

LED Slug Temperature (ºC)

Figure 6: Comparison of the projected lifetime of LEDs with and without LEDSense based thermal fold back..

Also shown is the L70 time calculated for the LED according to the TM-21 standard (although once again I have extended this beyond the recommended time for the low current condition). The case where the LED temperature is uncontrolled, but is operated at the maximum current shows a dramatically shortened value of only 13,900 hours. In contrast, the LEDSense feature has reduced current to the extent that it has effectively “neutralized” the negative lifetime effect of the existing high temperature condition. Although in this instance the user will notice significant dimming, less dramatic overtemperature conditions may be hard to detect because of

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temperature; the dashed blue line is meant to illustrate the temperature offset that is the result of the thermal resistance of the assembly and the drive power level.

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2012

WI NNER

Illogical Logic

Part 1 - Boolean Algebra Paul Clarke

Electronics Design Engineer

When it comes to logic, we know it’s all supposed to make sense. For newcomers, it can also be very confusing to wrap your head around these concepts. This is why I have decided to do a short series on understanding the illogical world of logic! For the most part, understanding basic logic gates is easy enough. They explain what they are clearly; an AND gate just says the output is: logic ‘1,’ then input ‘x’ AND ‘y’ are logic ‘1.’ If you’re not 100% on this and other logic gates, then a quick read on Wikipedia will help set you straight. What becomes confusing is that we use lots of logic gates together, like in an FPGA. So, how do you work out what you need and why do some people seem to use so few gates for such complex tasks? Boolean algebra is a way of explaining logic in a written form without having to draw out all the logic gates. In place of an AND gate, you simply write A.B (note the full stop), and for the OR function you use the plus symbol (for example: A+B). This means you can turn complex logic into one written line of Boolean (Figure 1). What’s shown in Figure 1 quickly becomes…

However, Boolean allows you to do things you cannot see or work out from a logic circuit. Boolean allows you to apply simple rules that will enable you to break down the logic to simply the elements that matter.

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4. The “because it works” rule These can be explained, but last time I tried it took ages! So, like the lecturers and teachers that introduced me to Boolean, I’ll say “just write these down because they work.”

Figure 1

There are three sets of rules that I use to help break down Boolean into more simple logic: 1. Break (or make) the line – change the sign (DeMorgan’s Theorem)

allows you to remove gates completely from the circuit and replace them with either a fixed logic level or carry the logic signal forward.

(And also to say this post would become very long!)

If you have a simple logic gate like the NAND gate, then you write:

What you will find is that if you were to invert (NOT) the logic levels of A and B to get the same logic result you would OR the inverted A and B lines. By breaking the line above the symbol and changing the sign, it keeps the logic true:

The following is an example of breaking down the logic into its simplest form:

2. Disappearing gates When a gate has fixed inputs say at logic ‘1’ or ‘0’, then the output also becomes fixed. That’s because you are restricting the combination of outputs available. So the following

3. Adding fixed logic levels This allows for making a logic gate have more than n inputs. By adding a fixed logic ‘1’ or ‘0’ input, then

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Consider the second and third terms—the A.B is common to both. Now imagine that the third term also has a third input. This would— for the AND gate to work—have to be at logic ‘1’ as explained in my third set of rules. This allows you to combine the Boolean together into:

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TECHNICAL ARTICLE

that function of the gate remains the same. However, it allows for simplification of two gates with common inputs (explained in example at the end).

TECHNICAL ARTICLE

Again ANDing a ‘1’ also vanishes so in this case, we end up with just:

Don’t believe me – try it! Our logic now looks like this:

Once again we can combine inputs in this case with the logic A input to get:

Which then becomes:

The logic ‘1’ disappears in the AND gates leaving us with just:

We can now see that the logic A input has no effect on the circuit and can just be removed. It’s using these simple rules that allows us to reduce complex requirements down to simple logic. I’m not saying that all logic can be reduced this much, being that this was an example, but it’s important to remove this dead logic. When I started electronics it meant reducing the numbers of chips on a PCB. Now in the modern day of high speed electronics it means you can reduce switching times and the number of gates used in an FPGA. In fact, if you are an FPGA programmer and wonder why it takes so long to generate the logic, you can now see why, because it’s doing all of this for you.

the other rules to it. Next time I’ll be looking at Karnaugh maps… About the Author Paul Clarke is a digital electronics engineer with strong software skills in assembly and C for embedded systems. At ebm-papst, he develops embedded electronics for thermal management control solutions for the air movement industry. He is responsible for the entire development cycle, from working with customers on requirement specifications to circuit and PCB design, developing the software, release of drawings, and production support. ■

I’ve not shown an example using my first set of rules, known as DeMorgan’s Theorem, however, you can use it to great effect on its own in many cases. It can also be used as a method to generate an expression that allows you to apply

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You will now see that the C element of this Boolean is irrelevant as ORing anything with a ‘1’ vanishes. So:

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PWM DC/DC Controllers with VID Inputs for Portable GPU Core-Voltage Regulator ISL95874, ISL95875, ISL95876

Features

The ISL95874, ISL95875, ISL95876 ICs are Single-Phase Synchronous-Buck PWM regulators featuring Intersil’s proprietary R4 Technology™. The wide 3.3V to 25V input voltage range is ideal for systems that run on battery or AC-adapter power sources. The ISL95875 and ISL95876 are low-cost solutions for applications requiring dynamically selected slew-rate controlled output voltages. The soft-start and dynamic setpoint slew-rates are capacitor programmed. Voltage identification logic-inputs select four (ISL95875, ISL95876) resistor-programmed setpoint reference voltages that directly set the output voltage of the converter between 0.5V and 1.5V, and up to 5V with a feedback voltage divider.

• Input Voltage Range: 3.3V to 25V

Compared with R3 modulator, the R4 modulator has equivalent light-load efficiency, faster transient performance, accurately regulated frequency control and all internal compensation. These updates, together with integrated MOSFET drivers and Schottky bootstrap diode, allow for a high-performance regulator that is highly compact and needs few external components. The differential remote sensing for output voltage and selectable switching frequency are another two new functions. For maximum efficiency, the converter automatically enters diode-emulation mode (DEM) during light-load conditions, such as system standby.

• Output Voltage Range: 0.5V to 5V • Precision Regulation - Proprietary R4™ Frequency Control Loop - ±0.5% System Accuracy Over -10°C to +100°C • Optimal Transient Response - Intersil’s R4™ Modulator Technology • Output Remote Sense • Extremely Flexible Output Voltage Programmability - 2-Bit VID Selects Four Independent Setpoint Voltages for ISL95875 and ISL95876 - Simple Resistor Programming of Setpoint Voltages • Selectable 300kHz, 500kHz, 600kHz or 1MHz PWM Frequency in Continuous Conduction • Automatic Diode Emulation Mode for Highest Efficiency • Power-Good Monitor for Soft-Start and Fault Detection

Applications • Mobile PC Graphical Processing Unit VCC Rail • Mobile PC I/O Controller Hub (ICH) VCC Rail • Mobile PC Memory Controller Hub (GMCH) VCC Rail

RVCC CVCC

PHASE

SREF

PGOOD

CIN QHS

12 11

10 QLS

CBOOT

ROFS

VIN

3.3V TO 25V

RPGOOD

13 VCC

14 PVCC

15 LGATE

UGATE

OCSET

4 CSOFT

ROFS1

GPIO

BOOT

RTN

GND

RFB1

FSEL

RTN1

PGND

CPVCC

ROCSET

+5V

VOUT 0.5V TO 3.3V CO

CSEN

RTN1

RFB

FIGURE 1. ISL95874 APPLICATION SCHEMATIC WITH ONE OUTPUT VOLTAGE SETPOINT AND DCR CURRENT SENSE

March 2, 2012 FN7933.1

Intersil (and design) is a registered trademark of Intersil Americas Inc. Copyright Intersil Americas Inc. 2011, 2012 All Rights Reserved. All other trademarks mentioned are the property of their respective owners.

T C H N I C Perspective A L A R T I C L E on AESystem

Efficiency A

B Bob Stowe

Power Supply Design Consultant

n the last installment of this series entitled “A System Perspective on Specifying Electronic Power Supplies,” we discussed the effects of source characteristics upon power supply specification. In this installment, we will learn about the importance of efficiency for your system and how to specify it.

SOURCE

Power supply efficiency, then, is a measure of how much input power is transferred to the load as useful and desirable power, rather than dissipated in the form of heat in the power supply. Efficiency is expressed either as a percentage figure, or as a decimal figure. When expressed as a percentage, 100% is perfect efficiency and typical efficiencies for power supplies range

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POWER SUPPLY

Power Out

LOAD

Waste Power

What is Power Supply Efficiency? Figure 1 shows the typical power flow from a source, through the power supply, and on to the load. Power supplies are not ideal, so not all of the power supply input power is transferred to the load as useful power. A portion of the input power is instead dissipated to the power supply environment as heat — as represented by the red highlighting around the power supply.

Power In

To Power Supply Environment

Figure 1

anywhere from 35% to 96% depending on the type and application of the supply. When expressed as a decimal, 1 is perfect efficiency and typical efficiencies range anywhere from 0.35 to 0.96. The following equation summarizes power flow:

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Specifying Electronic Power Supplies:

TECHNICAL ARTICLE The importance of efficiency has three main aspects: 1) energy conservation, 2) package size, and 3) temperature rise. Energy Conservation Energy conservation is an important consideration in: 1. Systems where the source is in the form of stored energy such as batteries or ultracapacitors, 2. Energy harvesting applications. 3. Green applications. With stored energy systems, power supply efficiency has a major impact upon battery life or battery and ultracapacitor discharge time. A perfect example is how long a notebook computer will be able to run while on battery. An efficient power supply will maximize battery discharge time. Energy harvesting is a relatively recent application for power electronics whereby energy is “captured” from the environment and converted to useful energy. Since, with present technology, the rate of capturing energy from the environment is relatively low, power supply efficiency plays a major role in converting as much of that energy as possible into useful power. Green applications are a long term vision for transforming our culture into responsible users of energy. Efficiently transforming energy is a main effort for green design. An example is the effort to switch from inefficient incandescent light bulbs to efficient flourescent which require efficient power supplies. Package Size and Temperature Rise To the engineer not well versed in power electronics, a less obvious effect of efficiency is power supply package size and/or power supply temperature rise. In today’s design world, the power supply is often thought of as a necessary complement to the main function of the product in design. But since the power supply is not the main function, it often takes a back seat in the design process. One of the natural results is that the power supply must be unobtrusive with small size. However, efficiency is one of the main determiners of power supply size. The reason for this is that the

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less efficient a power supply is, the more waste heat generated. For a given package surface area, the package temperature increases as the waste heat increases. Package temperature directly affects reliability and life. (An old thumb rule for reliability and life vs. temperature is that each 10 degree C increase in temperature reduces reliability and life by a factor of two.) In other words, for a given package temperature which results in a given reliability and life, greater waste heat requires greater package surface area or a more sophisticated and more expensive waste heat removal method. As an example, for a given input power, an 80% efficient power supply requires twice the surface area to maintain the package at a given temperature compared to a 90% efficient power supply. Required surface area is proportional to the complement of efficiency (1-Efficiency) when expressed as a decimal. How to Calculate Efficiency Calculating efficiency in all cases can be done by dividing the power output by the power input:

Efficiency = h =

Pout Pin

Power is simply RMS voltage times RMS current multiplied by the power factor if the input is AC, or just voltage multiplied by current if the input or output is DC. Power factor is the distortion factor multiplied by the displacement factor. The distortion factor accounts for the effect of a non-linearly changing power supply input impedance over one line cycle. The displacement factor accounts for an effective phase difference between line voltage and line current due to reactance at the input of the power supply. Linear Regulator Efficiency In the special case of a DC input linear regulator, the efficiency calculation is simply the output voltage divided by the input voltage. Switching Power Supply vs. Linear Power Supply Efficiency In most applications, switching power supplies are more efficient than linear power supplies, and therefore offer smaller size due to the lesser waste heat. Switching

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Importance of Efficiency

TECHNICAL ARTICLE

One case where the linear power supply can offer comparable or even better efficiency than a switching power supply is the case of a low drop-out regulator with a relatively high output voltage. In this case, the voltage dropped across the regulating pass element is a very small part of the input voltage, and the output voltage is near the input voltage. Efficiencies can easily be over 90%. How to Specify Efficiency Efficiency is best specified for applications requiring energy conservation as discussed earlier. Efficiency varies depending on input voltage and load current. Optimal efficiency should be specified at the operating point(s) where the product is most likely to operate. For other operating points, a tolerable minimum should be specified. For other applications not involving energy-limited

BeStar

sources such as batteries, ultra-capacitors, energy harvesting, or green applications, it is best not to specify efficiency. Instead, other requirements such as package size limits, and/or temperature rise limits should be specified. Efficiency in these cases should be left to the power supply designer as a design variable that is determined based on other requirements such as package size limits and temperature rise limits that are more typically specified by the application demands. About the Author Bob Stowe is currently working at True Power Research as a Power Supply Design Consultant. He has over 21 years of experience in the various disciplines as related to electronic energy conversion, possesses a master’s degree in power electronics, and is a member of IEEE in good standing. He also has obtained his certification in power electronics from the University of Colorado (COPEC). Additionally, he graduated from the United States Naval Academy in 1984 with a Bachelor’s degree in Electrical Engineering and served for five subsequent years as a United States Naval Officer. As a former military officer, he is familiar with military project requirements. ■

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power supply efficiencies typically range from 70% to 96%. Linear power supplies efficiencies can be 35% or even lower, requiring larger sizes to handle the greater waste heat.

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