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LIBRARY ALTERNATIVEFOCUS ON: ENERGY A compendium of technical articles from the editors of Electronic Design

Copyright © 2013 by Penton Media, Inc. All rights reserved. ELECTRONIC DESIGN LIBRARY FOCUS ON: ALTERNATIVE ENERGY WELCOME TO ELECTRONIC DESIGN’S E-BOOK ON ALTERNATIVE ENERGY. Here is what you will find in this edition: The final two articles in the book discuss In chapter 1, “Solar Energy Goes Beyond some of the circuitry needed to turn solar power Photovoltaics,” Don Tuite points out some of the generated by the panels into energy that is useful drawbacks of photovoltaics and then surveys sever- for the electrical grid. In his article, “Advanced Digi- al other ways to harness the sun’s power. He lumps tal Isolation Technologies Boost Solar Power Inverter the non-photovoltaic methods into the concentrated Reliability,” Paul Alfano of Silicon Labs explains how solar thermal (CST) category, which includes linear, traditional components may adversely affect the reli- power tower, and dish-engine systems. This is a ability of inverters over the life of the system, which very interesting read for anyone wanting a broader is about 25 years. One of these components is the understanding of solar power possibilities. optocoupler. As a substitute for optocouplers, Paul In chapter 2, Paul Schimel of International suggests modern CMOS isolators and shows how Rectifier, who is one of our experts on wind tur- they can be designed into a typical inverter circuit. bines, explains how these majestic machines In the concluding chapter, “No-Multiplier work and how electronics fit into this electrome- MPPT Monitors Power From Solar Array’s Switch chanical system for producing electricity. Converter,” author Giovanni Romeo explains how The next three chapters concern solar to simplify maximum power point tracking with energy produced by photovoltaics. In his article, a circuit that performs the function without the “What’s The Difference Between Thin-Film And need for a multiplier. This innovative circuit takes Crystalline-Silicon Solar Panels,” author Mat Dir- advantage of microcontrollers to maximize the jish writes about the history and research behind power from a solar array. each of these approaches. He then explains how We hope you enjoy this e-book and find it the two differ in their ability to produce electricity helpful in your own work in this exciting field. We from the sun. welcome any feedback you may have. electronicdesign.com SUBSCRIBE: electronicdesign.com/subscribe | 2 ELECTRONIC DESIGN LIBRARY FOCUS ON: ALTERNATIVE ENERGY CONTENTS

D2 U5 Voltage feedback (synch) MAX4376 + C14 C9 1 5 IOut Out RS– 10 μF 150 μF 2 D13 Gnd 3 4 + 5 V VCC RS C15 U1 C21 100 nF HV+ Full bridge Volta 0.47 μF 2 1 10 D8 VCC BST 2 9 IRF 3707Z DL DH 1 3 8 3 L2 PGND LX 40 μH D9 4 7 GND EN 5 V 5 6 + 5 V DLY PWM PWM 2 C28 C25 10 μF 180 μF High-side/ MAX8552 D12 Inductive 1 Digital IRL 7833 low-side ﬁlter 3 power gate IOut VOut controller driver

J1 R22 R18 5 V 1 2 + + 1k 1k 3 4 PGC–IOut + + 9 C12 5 6 PGD–V + + High-side/ k 100 nF Out 7 8 MCLR + + low-side 9 + + 10 Voltage C19 C17 SW gate 100 nF 100 nF driver PicFlash_CON U3 Current feedback 1 8 Current VDD VSS 2 7 AC OSC1 PGD PGD–VOut PAN+ A 3 6 current AN3 PGC PGC–IOut C10 8 4 5 MCLR CCP1 PWM 0.33 HV– sensor 00 nF MCLR PIC12F683

1 | Solar Energy Goes Beyond Photovoltaics 2 | Innovative Designs Keep Power Turbines Spinning 3 | What’s The Difference Between Thin-Film And Crystalline-Silicon Solar Panels 4 | Advanced Digital Isolation Technologies Boost Solar Power Inverter Reliability 5 | No-Multiplier MPPT Monitors Power From Solar Array’s Switch Converter

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GOESENERGY BEYOND PHOTOVOLTAICS

By Don Tuite, Analog/Power Editor

hen most people talk about solar power in the U.S., they tend to think Wnarrowly about photovoltaics. But it’s a big world

An experimental dish/engine system at Sandia National Laboratories in New Mexico represents the large end of the scale of dish/engine designs. The engine in most of these uses the familiar Stirling thermodynamic cycle. electronicdesign.com SUBSCRIBE: electronicdesign.com/subscribe | 5 1. With a peak output of 60 MW, the Olmedilla est photovoltaic plant in the U.S., Florida Photovoltaic Park in Olmedilla de Alarcón, Power and Light’s 25-MW Next Gen- Spain, is the world’s largest photovoltaic plant. eration Solar Energy Center in DeSoto It uses more than 160,000 conventional solar County, comprises more than 90,000 photovoltaic panels. SunPower solar panels on 180 acres. The world’s largest photovoltaic plant, out there. Even in the U.S., researchers the 60-MW Olmedilla Photovoltaic Park are working on interesting alternatives, in Olmedilla de Alarcón, Spain, uses particularly for big commercial bulk more than 160,000 conventional solar power facilities intended as alternatives photovoltaic panels (Fig. 1). to nuclear or fossil-fuel power plants. Olmedilla may represent a limit on One of the issues with photovoltaics the maximum capacity of photovoltaic is just how far they can scale. The larg- solar. Or it may not. These technolo- electronicdesign.com SUBSCRIBE: electronicdesign.com/subscribe | 6 ELECTRONIC DESIGN LIBRARY FOCUS ON: ALTERNATIVE ENERGY gies have a way of leapfrogging each approach has been used for decades other. Recently, First Solar in Tempe, in powering expensive military satellites, Ariz., announced that it would build a but the manufacturing cost put it out of 2000-MW solar power plant in China. reach for commercial use. First Solar makes thin-film solar cells in Beyond that, DuPont and the Univer- a continuous process, rather than on sity of Delaware have achieved 42% wafers. However, its cells use tellurium, efficiencies by using multiple band-gap the relative scarcity of which could limit collectors and spectral-splitting optics, product volumes. according to the university. Efficiency is another issue related to Until FirstSolar’s Chinese photovoltaic scaling. In terms of wafer-based cells, plant is built, photovoltaics seem lim- SunPower offers a conversion ratio of ited to a scale of tens of megawatts and 23.4%, while the average for the rest hundreds of acres. Yet as production of the market ranges from 12% to18%. economies of scale kick in, photovoltaic SunPower’s efficiency comes from met- panels appear ideal for “distributed re- alizing the collection grid on the back sources,” domestic rooftop systems like of the wafer, rather than the front, as the 2.5 kW on my own roof, or the dis- it is on other manufacturers’ wafers. tributed 2.5 MW that Intel announced it Back-collection increases the photon- would install in the first half of 2010 on capture area of the wafer. What makes factory roofs in Arizona, California, New this possible is SunPower’s process Mexico, and Oregon. technology, which allows the charge carriers created by the solar-cell junc- Linear Concentrated Solar tions when they are hit by photons to migrate all the way through the thick- Non-photovoltaic methods can be ness of the wafer. lumped into the concentrated solar ther- However, Sharp Solar has recently mal (CST) category. The common ap- announced cells with 35.8% conver- proaches include linear, power tower, sion efficiencies using a triple-junction and dish-engine systems. compound solar cell. (Multiple band- Linear systems generally comprise mul- gaps allow the junctions to respond to a tiple parallel rows of solar collectors. They broader range of photon energy levels— may be stationary, or they may be motor- that is, a broader light spectrum.) This driven to pivot around a north-south axis, electronicdesign.com SUBSCRIBE: electronicdesign.com/subscribe | 7 ELECTRONIC DESIGN LIBRARY FOCUS ON: ALTERNATIVE ENERGY following the sun from east to west through cloudy days using natural gas as a fuel, the day. Parabolic trough systems can providing continuous capacity. focus sunlight by 30 to 100 times, heating Linear systems are sometimes used a fluid in a pipe that runs down the focal with thermal energy storage (TES) sys- line of each trough. A heat exchanger then tems. This typically involves a salt mixture boils water to produce superheated steam of 60% sodium nitrate and 40% potas- to run turbine generators. It’s possible to sium nitrate, which melts at 221°C. The run the same turbines at night or during TES system keeps the mixture as a liquid

2. Covering 7.9 square miles of California desert on nine separate sites, the Solar Energy Generating Systems (SEGS) solar-trough systems represent the largest non-photovoltaic solar installation the world today. Peak capacity is 364 MW. ELECTRONIC DESIGN LIBRARY FOCUS ON: ALTERNATIVE ENERGY and cycles it between “cold” storage at This is somewhat mature technology. 288°C and “hot” storage at 566°C. The plants were built between 1984 and In the solar plant, the hot salt is used 1990. SEGS VIII and IX, at Harper Lake, at night to take over the job of super- are the newest. They each have a peak heating steam for the turbine generator. capacity of 80 MW, which is more than A 100-MW turbine can be run for four double the capacity of most of the older hours by pumping molten salt between plants. (All except SEGS I have peak hot and cold tanks roughly 30 feet tall outputs of 30 MW. SEGS I has a maxi- and 80 feet in diameter. mum 14-MW output.) Linear Fresnel-reflector concentrating The concentrator mirrors are half-pipes. systems are an alternative to troughs. They The working fluid in the Rankin-cycle employ flat or slightly curved mirrors and steam turbine is water that is heated by Fresnel lenses to concentrate sunlight onto a synthetic oil, which is heated to more a pipe. They aren’t as optically efficient as than 400°C as it is pumped through the parabolic troughs, but they can be cheap- pipes at the foci of the troughs. The two- er because they can heat water for the tur- stage heating avoids over-pressurization bines directly. They also require less land in the heat-collection piping. because design engineers don’t have to worry about one trough shading another. Power Towers The largest non-photovoltaic installation in the world is a collection of nine Power tower systems use hundreds or trough systems in California. The Solar even thousands of large, sun-tracking Energy Generating Systems (SEGS) flat heliostat mirrors, covering acres to 364-MW collection of nine solar-trough concentrate sunlight onto a receiver at power plants, operated and partially the tops of tall towers. The sunlight heats owned by NextEra Energy Resources, pressurized water or molten salt to run comprises SEGS I-II (44 MW), located at steam generators. Daggett, SEGS III-VII (150 MW), located Tower systems are massive, and they at Kramer Junction, and SEGS VIII-IX cannot be separated into geographi- (160 MW), located at Harper Lake, all cally dispersed elements like the SEGS near Barstow, Calif., in the Mojave Des- trough farms. This can lead to conflict ert. Taken together, the nine plants cover with environmentalists who favor non- almost eight square miles (Fig. 2). fossil-fueled power on the one hand, electronicdesign.com SUBSCRIBE: electronicdesign.com/subscribe | 9 ELECTRONIC DESIGN LIBRARY FOCUS ON: ALTERNATIVE ENERGY but are concerned about wildlife on the Solar 10 (PS10) and 20-MW PS20, which other. That’s happening with a planned are located in Sanlucar la Mayor near 400-MW power tower that aims to use Seville, represent the first phase of a very some 6.25 square miles of desert near large construction project. The whole Las Vegas—desert that is habitat to the facility will be completed by 2013 and will desert tortoise. produce around 300 MW (Fig. 3). In February, The New York Times re- The facilities are co-located. PS10 ported that the developer of the first has 624 heliostats, each with a 120- new solar power plant in California in 20 m² surface-area parabolic mirror. The years had proposed revamping the proj- mirrors are focused on a 115-m tower, ect to defuse concern over its effect on heating water pipes that provide 200 m² the animal. BrightSource Energy now will of water-cooled energy-exchange sur- submit a new design plan that shrinks face area. The tower uses water rather the size of the 4000-acre Ivanpah Solar than molten salt, yet the steam retains Energy Generating Station by 12% and enough enthalpy for generation at half its energy generation from 440 to 392 capacity for an hour or more after dark. MW, the Times said. With 1255 two-axis sun-tracking helio- To get a sense of the scale of this stats and mirrors, PS20 has double the project, Ivanpah’s 4000 acres is much capacity of PS10. The PS20 tower is 165 bigger than FPL’s 180 acres of photo- m high and also uses steam, but it em- voltaics in DeSoto County. Another way ploys second-generation technology in to look at it is that 400 MW is roughly the receiver, in its control system, and in one-fifth the capacity of nearby Hoover energy storage. Dam. Lake Mead, behind Hoover Dam, covers about 250 square miles Dish/Engine Systems when it’s full—about 40 times more area than Ivanpah. Dish/engine systems are another Environmentalists in other countries photovoltaic alternative. Their optimum have raised less of an uproar about wild- size, however, seems to be more on life, but they’ve also been dealing with the scale of a distributed power gen- smaller-scale tower installations. The erator (1 to 40 kW) than a bulk-power largest solar power-tower installations in plant, though they can be scaled up in the world are in Spain. The 11-MW Planta arrays (opening figure). They are ex- electronicdesign.com SUBSCRIBE: electronicdesign.com/subscribe | 10 3. Shining sunlight from hundreds of heliostat mirrors at its two power towers in Sanlucar la Mayor, Spain, the 11-MW Planta Solar 10 (PS10) and 20-MW PS20, are the first of a cluster of solar towers that are expected to produce around 300 MW by 2013. ELECTRONIC DESIGN LIBRARY FOCUS ON: ALTERNATIVE ENERGY pected to find applications in emerg- fer medium and the working fluid for ing global markets. the engine. In a dish/engine system, a single con- Because of the high heat transfer ca- centrator tracks the sun, reflecting its pability of high-velocity, high-pressure solar energy onto a receiver where it is helium or hydrogen, direct-illumination absorbed, converted to heat, and ap- receivers can absorb approximately 75 plied to an engine/generator, which is W/cm2. When it is necessary to balance usually a Stirling-cycle engine. the temperatures between the cylinders Recall that in the Stirling cycle, the of a multiple-cylinder Stirling engine, working gas is alternately heated and liquid-metal, heat-pipe solar receivers cooled by constant-temperature and are used. constant-volume processes. Stirling en- In a heat-pipe receiver, liquid sodium gines typically incorporate an efficiency- metal is vaporized or evaporated on the enhancing regenerator that captures absorber surface of the receiver. Then, heat during constant-volume cooling it is condensed on the Stirling engine’s and replaces it when the gas is heated heater tubes. The working gas is hy- at constant volume. The best of the Stir- drogen or helium at temperatures over ling engines achieve thermal-to-electric 700°C and pressures up to 20 MPa. conversion efficiencies of about 40%. Several mechanical configurations Stirling engines are popular for dish/en- implement these constant-temperature gine systems because they run on “ex- and constant-volume processes. ternal combustion” (in this case some Some dish/engine designs in the fu- very distant nuclear fission) and be- ture may use gas turbines, in which cause of their high efficiency. efficiency is greater than 80%, instead A great deal of insightful information of reciprocating Stirling engines. In the about dish/engine design is available most successful of these designs to from SolarPACES (Solar Power and date, the receivers have used “volumet- Chemical Energy Systems), a program ric absorption,” in which the concen- of the International Energy Agency, at trated solar radiation passes through www.solarpaces.org/inicio.php. Ac- a fused silica “quartz” window and is cording to SolarPACES, the thermal absorbed by a porous matrix. Still, the receiver cooling fluid, usually hydrogen Stirling engine is today’s norm. Q or helium, may be both the heat trans- [RETURN TO THE TABLE OF CONTENTS] electronicdesign.com SUBSCRIBE: electronicdesign.com/subscribe | 12 FOCUS ON: LIBRARYALTERNATIVE ENERGY CHAPTER 2 Innovative Designs Keep Power Turbines Spinning

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By Paul L. Schimel, PE Contributing Technical Expert

Today’s wind power requires efﬁcient power trains and actuators to manage growing demands for alternative energy sources.

he giant turbines that generate wind power are among the most magnificent innova- tions that I’ve seen in the Midwest cornfields since the 16-row picker made its debut. Their size and scale are fascinating (see the opening figure). They can’t replace all of Tour existing power plants, but they are an attractive supplement that provides renewable energy. If we want to improve these machines, though, we first need to understand how they work. INNOVATIVE

DESIGNSKEEP POWER TURBINES SPINNING

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A Couple Of Unknowns cial” show up with an A weighted sound pressure level meter, stand 50 yards Having seized an opportunity or two away from the machine, point the me- to visit a large wind turbine site, lawfully ter straight up in the air so the distant and with proper permissions, it is clear machine is well into the side lobe of the to me that the impacts of this technology transducer’s polar sensitivity pattern, might not be completely understood. and proclaim that all is good. But is it? I’ve heard many claims of low-frequency What did he really measure? noise. Perhaps that repetitive low-fre- If the rotor has three blades spinning quency “whoosh whoosh” sound is the at 10 rpm or so, the whooshing is hap- culprit, perhaps not. pening 30 times a minute or at 0.5 Hz. I haven’t seen any substantive re- This fundamental is more than 160 dB search for or against this theory, but I down the A message weighting curve have seen the “appointed county ofﬁ- (Fig. 1). It is indiscernible! The hundredth har- 0 monic of this fundamen-

–20 tal noise is at 50 Hz, which is 32 dB down on –40 the A message weighting curve. How can that –60 instrument accurately –80 measure something Noise (dB) that far outside its pass –100 band? It says there’s no C weighting –120 B weighting audible noise pollution A weighting beyond some estab- –140 lished reference level.

–160 But what then of the low- 1 10 100 1000 10,000 100,000 frequency noise? Frequency (Hz) Another question that pops up is that of the 1. The poor guy at the site with the A weighted sound pressure long-term impact on wind meter doesn’t see the low-frequency noise at all! and weather patterns. electronicdesign.com SUBSCRIBE: electronicdesign.com/subscribe | 15 ELECTRONIC DESIGN LIBRARY FOCUS ON: ALTERNATIVE ENERGY

Again, this sort of study takes a lot of see that the electrical output power time and instrumentation—perhaps mil- at max wind is 2 MW. This occurs at a lions of sensors and a couple of decades rotor speed of 19 rpm. The gearbox in of measurement. Such studies won’t bear the machine multiplies this shaft speed any fruit in the fiscal quarter in which they up by a ratio of 120:1 to run the gen- are paid, purposed, and accounted for, erator in the rear of the machine. so they likely will be overlooked. The super-synchronous, doubly fed induction generator (SSDFIG) in the How Do They Work? rear of the machine produces this power level at 690-V, three-phase out- Unknowns aside, then, we have to put. On paper, that’s hardly amazing, ask what’s in that box and how it works. but when you approach the machine, When we drive past these majestic ma- it’s quite spectacular. The blade diam- chines or perhaps inquisitively walk up eter is 295 feet. This means the swept to them with proper permissions and area is roughly 68,500 square feet or personal protection equipment (PPE), 1.6 acres. we see their big blades spinning slowly The power train comprises a rotor, in the prevailing winds. The rotor cou- cascaded with a gearbox, cascaded ples to the nacelle. All of the necessary with an SSDFIG (Fig. 2). If the output of equipment to transform this mechanical the SSDFIG is 2 MW, the input is cer- energy flow into electrical power is in- tainly larger. Little information is avail- side the nacelle. able on the losses in the gearbox and I’ve seen two types of nacelles in the SSDFIG, but both of them have sub- Midwest: the more popular, elongated, stantial cooling mechanisms, so they and somewhat streamlined “shoebox” aren’t near the ideal. nacelle and the truncated, shorter, In the absence of any data, let’s ball- rounded cone “clipper” nacelle. I don’t park them at a throughput efficiency of have much experience with the clippers, 85% each. If we cascade these stages, since I haven’t found a way to have a we then need a mechanical input of 2.768 close look at them yet. MW at the rotor. With each of the three The easiest place to start exploring blades properly pitched to create full the shoebox nacelle is at its rotor. If we power in full wind, and a rotor speed of look at a Gamesa G90 machine, we 19 rpm, the rotor then sees a total torque electronicdesign.com SUBSCRIBE: electronicdesign.com/subscribe | 16 ELECTRONIC DESIGN LIBRARY FOCUS ON: ALTERNATIVE ENERGY of roughly 1 million foot pounds. And is spun faster than the locked synchro- that’s just steady state. What about wind nous speed, generating electricity at gusts? Indeed, they’re a prime mover line frequency. where the power train is located 325 feet Next, it includes “doubly fed” be- above the ground. cause the machine is fed from both the rotor and stator. The conventional The Gearbox shorting bars in the rotor are broken on one side and connected to slip rings. Looking at the power train compo- In simplest form, adding resistance to nents, one has to ponder the gearbox: these slip rings changes the negative 1 million foot pounds of torque input! slip of the machine, allowing a larger The gearboxes are typically planetary range of shaft speed to result in line arrangements, often set up in two or frequency output. three stages. The gearbox weighs be- The shiny load bank on top of some tween 40,000 and 60,000 pounds de- of the nacelles is the resistor bank pending on the machine. Lubricants used to add the slip. Beyond this, addi- found in the gearbox are usually on the tional control techniques are employed order of SAE 360, equipped with both wherein ac current is injected into the heaters (to keep the lubricant viscous rotor to control reactive power. This is on cold inactive days) and pumps, ra- typically done with an IGBT-based (in- diators, and fans to carry the heat away sulated gate bipolar transistor) inverter from the gearbox. and a torque vector control. This is not Gearboxes are the main point of fail- a trivial task. ure. Historically, Robert G. Letourneau If we consider the rotor of an induction echoed these findings in his prime motor, the frequency of current in the movers down in Peoria, Ill., many de- rotor increases as the motor is loaded. If cades prior. Very high torque is tough the machine stalls, the frequency of cur- on gears! rent in the rotor is that of the line. We can The SSDFIG converts the mechani- flip this across the synchronous speed cal power into electrical power. First, axis and say it’s similar for an induction “SSDFIG” includes the term “supersyn- generator. To generate anything, the ro- chronous” because the induction motor tor has to be spun faster than the rotat- is operated in a negative slip. The shaft ing fields in the stator. electronicdesign.com SUBSCRIBE: electronicdesign.com/subscribe | 17 ELECTRONIC DESIGN LIBRARY FOCUS ON: ALTERNATIVE ENERGY

Rotor hub Nacelle Oil cooler Coupling

Heat exchanger Gear box

Spinner Rotor shaft Control panel

Mainframe Ventilation

Soundprooﬁng

Bearing bracket

Generator Pitch drive Yaw drive

2. The inside of the nacelle is a crowded place. Setting foot inside with the rotor spinning is strictly verboten.1

If it is stalled, generating nothing, the quires rotor currents on the order of 1000 drive that processes the power, wave- A. Whether steering into a resistive load forms, and phasing into the rotor has bank to simply add slip or driving appro- to handle full transformer action. Those priate excitation and phasing to control voltages can be substantial. Further, power factor, the control needs to safely maximum output of these machines re- accommodate these currents (Fig. 3). electronicdesign.com SUBSCRIBE: electronicdesign.com/subscribe | 18 ELECTRONIC DESIGN LIBRARY FOCUS ON: ALTERNATIVE ENERGY

5000 nacelle, and the exhaust is on the top. 4000 Additional Actuators 3000 2000 In addition to the primary power train, 1000 a couple of other actuators merits dis- 0 cussion. The rotor blade pitch control is one of the most interesting. Its func- –1000 tion is pretty straightforward. Each of Torque (foot-pounds) Torque –2000 the three blades needs to be rotated to –3000 proper pitch for the wind input and pow-

–4000 er output conditions. Various position encoders maintain the same pitch for all –5000 0 2000 4000 6000 8000 three blades. Speed (rpm) The machine of choice to do this is an induction motor with a substantial gear- 3. Most anyone who has stalled a table saw or head and a variable frequency drive. a bench grinder has developed a good feel for The gearhead mates with ring gears on the torque/speed characteristics of an induction the rotor blades. That’s easy enough to motor. The turbine’s induction generator is understand. The interesting part arises similar, except it generates power. when we consider that the rotor spins, which isn’t very eventful. Gearhead mo- Finally, the induction generator in the tors and drives can handle that without SSDFIG is a three-phase induction ma- much trouble. chine overexcited to operate as a gener- The required operation compounds ator. The SSDFIG typically is air-cooled these needs. In the event of a power and requires a fair amount of ventila- outage, the rotor blades must pitch tion. I’ve seen multiple blowers on the full into the wind. So, a battery bank is order of 20 hp per unit. A typical insula- needed to run the drives in the event tion system for an SSDFIG is a class F of power failure. The battery pack has or B system with a maximum ambient to rotate in the rotor as well. While most temperature of 40°C. The intake for the sealed battery chemistries appear to be blowers is on the bottom or rear of the able to do this, few are rated for the job. electronicdesign.com SUBSCRIBE: electronicdesign.com/subscribe | 19 ELECTRONIC DESIGN LIBRARY FOCUS ON: ALTERNATIVE ENERGY

The pitch information, power source Wind Turbine Classifications (as available), and command come in through slip rings on the rotor. While the Under IEC61400-1, large wind tur- drives in the rotor are quite secure in a bines are classified into three main 1-in. thick steel faraday cage, the slip categories. Type III machines are used rings and related wire harness are fairly in low-wind areas, defined as an aver- susceptible to lightning events. Light- age wind speed at the height of the ro- ning often hits the highest blade. The tor hub of 16.8 mph. These machines current is earthed through conductors in stand a little taller and have blade diam- the blade that tie to the rotor. eters slightly larger than the remaining We certainly wouldn’t want that 10- classes of machines. Type II machines kA to 100-kA current impulse flowing are used in medium-wind areas with an through the bearings and the races. average wind speed of 19.0 mph at the So, sharp and pointy spark gaps height of the hub. Type I machines are that are near the rotor arc over and used in high-wind areas with an aver- conduct this current to earth ground. age wind speed of 22.4 mph or greater. While fairly deterministic, all of those The classifications bear no correlation to lines are certainly near that lightning maximum output power. Q event. The good news is that they usually tie into the controls toward REFERENCES the rear of the nacelle, not in the 1. Plantier, Keith; Mitchell Smith, Karen; shortest earthing path, but they still Electromechanical Principles of Wind Tur- see some energy and failures from bines for Wind Energy Technicians, TSTC these events. Publishing, December 2009, ISBN 978-1- The yaw position motors are like the 934302-54-5 rotor position motors. They are slightly 2. Andy Erbach, Department Head, Alterna- larger and often operated as multiple tive Energy Program, Elgin Community Col- units in parallel, but they are induction lege, [email protected] machines with gearheads nonetheless. Nearly all large-scale wind turbines that I’m aware of control yaw so the blades are on the windward side and the wind then flows over the nacelle. [RETURN TO THE TABLE OF CONTENTS] electronicdesign.com SUBSCRIBE: electronicdesign.com/subscribe | 20 FOCUS ON: LIBRARYALTERNATIVE ENERGY CHAPTER 3 What’s The Difference Between Thin-Film And Crystalline-Silicon Solar Panels

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WHAT’S THE DIFFERENCE BETWEEN THIN-FILM AND CRYSTALLINE- SILICON SOLAR PANELS?

olar cells have been around a Lighter than c-Si panels, SoloPower’s long time, although most of that flexible thin-film solar panels install easily on “time” was and still is devoted to commercial rooftops. research and development. When light hits a solar cell, it generates voltaic effect. The amount of electric- Selectricity, producing the photo- ity depends on a number of factors: electronicdesign.com SUBSCRIBE: electronicdesign.com/subscribe | 22 ELECTRONIC DESIGN LIBRARY FOCUS ON: ALTERNATIVE ENERGY cell material (silicon, thin-ﬁlm, other), Light Antireﬂective e– cell size (larger means more individual coating cells translating into either more volt- Transparent Front e– age or current), and the intensity and adhesive quality of the light source. The most ef- contact Cover glass fective and desirable light source is the Load sun, which is the most available and costs nothing. t"#SJFG4PMBS#JP t$SZTUBMMJOF4JMJDPO4PMBS1BOFMT t5IJO'JMN4PMBS1BOFMT – Electron t4JMJDPO7FSTVT5IJO'JMN + t"QQT Hole t4FNJ$PODMVTJPO n–type semiconductor t3FGFSFODFT p–type semiconductor Back contact e–

A Brief Solar Bio 1. One example of a crystalline silicon cell consists of seven material layers, two of If sources bode accurate, French which—the outer electrical contact—hold the physicist Alexandre-Edmond Becquerel entire package together. inadvertently observed the photovoltaic effect around 1839 while manipulating and gold—put a bit of tarnish on his ac- an electrode inside a conductive fluid complishment. exposed to light. American inventor As the story progresses, 1888 found Charles Fritts is credited with fabricating Russian physicist Aleksandr Stoletov the first photovoltaic (solar) cell around assembling a photoelectric cell based 1883. His approach involved coating on a photoelectric effect discovered selenium with a thin layer of gold, cre- by Heinrich Hertz in 1887. In 1905, Al- ating a cell that was less than 1% ef- bert Einstein explained the photoelec- ficient at best, but it worked. Of course tric effect. American engineer Russell the high cost of the materials—selenium Shoemaker Ohl patented the junction electronicdesign.com SUBSCRIBE: electronicdesign.com/subscribe | 23 ELECTRONIC DESIGN LIBRARY FOCUS ON: ALTERNATIVE ENERGY

Transparent 2. Consisting of six conducting coating Light layers in this case, a thin-film solar cell is not much different Antireflection coating in construction than its c-Si counterpart Junction formed between two and operates on the semiconductor same photovoltaic materials of opposite principle. were integrated Substrate Top n–type into emerging sat- “window” layer ellite designs such Ohmic contact formed by the first as Bell Labs’ Tel- semiconductor material star. Bottom p–type “absorber” layer Solar-cell prog- fromed by second ress, however, semiconductor material was quite slow for semiconductor solar cell in 1946 while about 20 years until Exxon’s Eliot Ber- performing research that would lead to man made some price and efficiency the invention of the transistor. breakthroughs. Around 1969, Berman Bell Laboratories gets credit for devel- first noted that solar cells were fabricat- oping the first efficient photovoltaic cell ed using a semiconductor manufactur- JO6TJOHBEJGGVTFETJMJDPO1/ ing process. Rather than cut and polish junction gave the device an efficiency silicon devices from scratch and apply boost, but not enough to get it into a antireflective coatings, he noted that the cost-effective price range. Four years industry’s scrap silicon wafers already MBUFS UIF7BOHVBSE*TBUFMMJUFMBVODIFE had anti-reflective front surfaces. They into space with its outer body covered only needed to be cut to size with the with solar cells. The goal was to extend printed circuits applied to the anti-re- mission time, which was usually limited flective surface. by the amount of battery run time avail- Berman’s approach eliminated two BCMF1SPWJOHUPCFFGGFDUJWF TPMBSDFMMT costly processes while enabling a vast electronicdesign.com SUBSCRIBE: electronicdesign.com/subscribe | 24 3. Crystalline silicon solar panels primarily are used in power generating and harvesting applications, particularly where higher efficiency is necessary.

electronicdesign.com SUBSCRIBE: electronicdesign.com/subscribe | 25 electronicdesign.com Subscribe: electronicdesign.com/subscribe ELECTRONIC DESIGN LIBRARY FOCUS ON: ALTERNATIVE ENERGY recycling of scrap silicon. He realized Two types of c-Si are in common that the silicon need not be perfect for use: monocrystalline and multicrys- solar apps and that the minor imperfec- talline silicon. Cut from a high-purity tions of discarded stock had no signifi- single crystal, monocrystalline silicon cant effect on its performance as a solar consists of 150-mm diameter wafers cell. Long story shorter, 1973 saw Ber- measuring 200 μm thick. Despite gain- man and crew cranking out silicon solar ing more favor, multicrystalline silicon panels at a cost of around $10/W and seems more involved to make, i.e., selling them for more than $20/W. sawing silicon blocks into bars and then wafers. In either case, one c-Si Crystalline-Silicon Solar Panels DFMMHFOFSBUFTBQQSPYJNBUFMZ7 and multiple cells are connected in se- Crystalline silicon (c-Si) solar cells are ries to boost output voltage. currently the most common solar cells in use mainly because c-Si is stable, it Thin-Film Solar Panels delivers efficiencies in the range of 15% to 25%, it relies on established process Even in the form of scrap, crystalline technologies with an enormous data- silicon wafers are not exactly inexpen- base, and, in general, it has proven to sive based on the efficiency levels they be reliable. Ironically, c-Si is a poor ab- achieve. Enter thin-film solar cells, which sorber of light and, what might be a sin are potentially cheaper than traditional in this micro-miniature age, it needs to panels but less efficient, in the realm of be fairly thick and rigid. 20% to 30% of light-to-voltage conversion. A basic c-Si cell consists of essentially Typical thin-film solar cells are one seven layers (Fig. 1). A transparent adhe- of four types depending on the mate- sive holds a protective glass cover over rial used: amorphous silicon (a-Si) and the antirefelective coating that ensures thin-film silicon (TF-Si); cadmium tel- all of the light filters through to the silicon luride (CdTe); copper indium gallium crystalline layers. Similar to semiconductor deselenide (CIS or CIGS); and dye- UFDIOPMPHZ BO/MBZFSTBOEXJDIFTBHBJOTU sensitized solar cell (DSC) plus other B1MBZFSBOEUIFFOUJSFQBDLBHFJTIFME organic materials. together with two electrical contacts: posi- /PUUPPEJGGFSFOUUIBOD4JDPNQP- tive topside and negative below. nents, thin-film solar cells consist of electronicdesign.com SUBSCRIBE: electronicdesign.com/subscribe | 26 ELECTRONIC DESIGN LIBRARY Focus on: Alternative Energy

4. Extending the Nuna series of solar-powered cars, the Nuna 6 spreads 1690 monocrystalline silicon solar cells over its body. The cells work with a 21-kg Li-ion battery and deliver an efficiency of 22%.

about six layers (Fig. 2). In this case, And, obviously, the operating principle a transparent coating covers the anti- (photovoltaic) is the same as reflective layer. These are followed by c-Si cells. UIF1BOE/UZQFNBUFSJBMT GPMMPXFE One would think, and rightfully so, that by the contact plate and substrate. with a name like thin-film cell, the com- electronicdesign.com SUBSCRIBE: electronicdesign.com/subscribe | 27 ELECTRONIC DESIGN LIBRARY FOCUS ON: ALTERNATIVE ENERGY ponent in question would be thinner and terms of initial cost. Also, they have a low lighter than other cell technologies. Oth- absorption coefficient and are rigid and erwise identical in function and struc- fairly fragile. ture, the singular difference between On the other side of the fence, thin- thin-film and c-Si solar cells is the thin film soar cells are less expensive than and flexible pairing of layers and the older c-Si wafer cells. Available in thin photovoltaic material: either cadmium wafer sheets, they are more flexible telluride (CdTe) or copper indium galli- and easier to handle. They’re also less um deselenide (CIGS) instead of silicon. susceptible to damage than their silicon rivals. Silicon Versus Thin Film The main disadvantage of thin-film solar components is their lower effi- So, c-Si technology has been around ciency, which in some applications can a while and proven its worth and mettle, offset the price advantage. They also while thin film is still pretty much in its have a more complex structure. Flex- infancy but has the potential to be sig- ible versions require unique installation nificantly less expensive and at least skills. And, not yet at least, they aren’t comparable in efficiency and reliability. viable for aerospace applications. With that said, which does one choose? Advantages of c-Si cells include a Apps high efficiency rate of about 12% to 24.2%, high stability, ease of fabrication, Both c-Si and thin-film solar panels and high reliability. Longevity is another suit a wide range of similar power appli- plus: c-Si modules deployed in the cations. Based on their pros and cons, 1970s are still in operation, and single you’ll see more c-Si in apps requiring crystal panels can withstand the harsh higher efficiency, and thin-film panels conditions associated with space travel. will take on more cost-effective and flex- Other benefits include high resistance ible situations. to heat and lower installation costs. And, Crystalline silicon solar panels are silicon is more environmentally friendly a common fixture in power harvest- come disposal/recycling time. ing systems as well as general utility On the downside, c-Si cells are the designs (Fig. 3). They also figure into most expensive solar components in unique scenarios such as the solar- electronicdesign.com SUBSCRIBE: electronicdesign.com/subscribe | 28 ELECTRONIC DESIGN LIBRARY FOCUS ON: ALTERNATIVE ENERGY

QPXFSFE/VOBSBDJOHDBSCVJMUCZUIF and in use. As of now, however, there %VUDI/VPO4PMBSUFBN(Fig. 4). are no recycling plans for these com- 6OWFJMFEJO+VMZ /VOBJTUIF ponents. Q MBUFTUJOUIF/VOBTFSJFT*UTNPOP- crystalline silicon solar cells cover an REFERENCES area of 6 m2. The cells work in conjunc- i1PMZDSZTUBMMJOF4JMJDPOw IUUQFOXJLJQFEJB tion with a 21-kg lithium-ion (Li-ion) bat- PSHXJLJ1PMZDSZTUBMMJOF@TJMJDPO tery. Solar-cell efficiency in this app is 2. “Thin film solar cell” (http://en.wikipedia. specified at 22%. Overall, the car weighs PSHXJLJ5IJO@mMN@TPMBS@DFMM 145 kg, which is significantly lighter than 3. “How Thin-film Solar Cells Work” (http:// previous vehicles in the series. science.howstuffworks.com/environmental/ Thin-film solar panels are also viable for green-science/thin-film-solar-cell.htm), by outdoor, energy-garnering applications. William Harris 4PMP1PXFSPG4BO+PTF $BMJG PGGFST 4. “Crystalline-Silicon Solar Cells for the a line of flexible panels for commercial 21st Century” (www.nrel.gov/docs/fy99o- rooftops using thin-film solar cells made sti/26513.pdf), Y.S. Tsuo, T.H. Wang, and from a combination of copper, indium, T.F. Ciszek gallium, and selenium integrated on a 5. “Crystalline Silicon Solar Cells” (www. flexible foil. These panels are lighter XPSMETDJCPPLTDPNFUFYUCPPLQQ@ than glass-encased c-Si panels and chap4.pdf), Martin A. Green install quickly. 6. “Advances in crystalline silicon solar cell technology for industrial mass production” Semi Conclusion (www.nature.com/am/journal/v2/n3/full/ am201082a.html), Tatsuo Saga There seems to be a feeling in the i5IJO'JMN4PMBSWT$SZTUBMMJOF4PMBS1BO- market that thin film will not only catch els - home applications” (www.youtube. up with c-Si components, but also will DPNXBUDI W;/H1@6B #FUTZ'FSSJT surpass them on all levels, which truth- Wyman, et al. fully are just cost and efficiency. One 8. “GE places solar bets on thin-film cells” way to cut cost in thin-film solar cells IUUQOFXTDOFUDPN@ is to use an environmentally unfriendly 20000695-54.html), Martin LaMonica material like cadmium. The makers claim it’s safe as long as it’s encased [RETURN TO THE TABLE OF CONTENTS] electronicdesign.com SUBSCRIBE: electronicdesign.com/subscribe | 29 FOCUS ON: LIBRARYALTERNATIVE ENERGY CHAPTER 4 Advanced Digital Isolation Technologies Boost Solar Power Inverter Reliability

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By Don Alfano Silicon Labs

oday’s photovoltaic (PV) ADVANCED power systems offer a sustainable alternative to fossil-fueled power DIGITAL ISOLATION plants, providing lower long-term operating TECHNOLOGIES Tcosts, modular scalabil- ity, higher efﬁciency, and a smaller carbon footprint. These power systems use BOOST PV panels to convert sunlight into dc voltage and solar power inverters to transform the dc voltage into ac volt- SOLAR age suitable for a commercial grid. In addition to performing POWER this essential dc-ac conversion, the solar inverter provides grid-disconnect ca- pabilities to prevent the PV INVERTER system from powering a dis- connected utility. An inverter remaining online during grid RELIABILITY disconnect or delivering power through an unreliable both the system and utility workers. Upon recon- connection can cause the nect, the inverter cannot deliver power until it de- PV system to back-feed lo- tects rated utility voltage and frequency over a ﬁve- cal utility transformers, creat- minute period. ing thousands of volts at the The inverter compensates for environmental utility pole and endangering conditions that affect power output. PV-panel out- electronicdesign.com SUBSCRIBE: electronicdesign.com/subscribe | 31 ELECTRONIC DESIGN LIBRARY FOCUS ON: ALTERNATIVE ENERGY

Voltage feedback (synch) Galvanic isolation

HV+ Full bridge Voltage

High-side/ Inductive Digital low-side ﬁlter power gate controller driver 60-Hz transformer Grid High-side/ low-side gate Voltage Voltage driver Current feedback Current AC current A C HV– sensor

1. The single-stage, single-phase inverter incorporates a PWM full-bridge converter. put voltage and current are susceptible operation at the maximum power point to variations in temperature and light when the product of voltage and current intensity per cell unit area (called “ir- is at its highest value. The inverter also radiance”). The cell output voltage is provides manual and automatic input/ inversely proportional to cell tempera- output disconnect for service opera- ture, and cell current is directly propor- tions, EMI/RFI conducted and radiated tional to irradiance. Wide variations in suppression, and ground fault interrup- these parameters cause the optimum tion. Encased in a ruggedized package, inverter voltage/current operating point the inverter is designed to remain in full- to move significantly. power operation for more than 25 years. The inverter addresses this issue by The single-phase PV inverter uses using closed-loop control to maintain a digital power controller and isolat- electronicdesign.com SUBSCRIBE: electronicdesign.com/subscribe | 32 ELECTRONIC DESIGN LIBRARY FOCUS ON: ALTERNATIVE ENERGY ed high-side/low-side gate drivers to The PV inverter’s exposure to extreme pulse-width modulate a high-wattage, heat and cold for 25 years should cause full-bridge power stage (Fig. 1). Us- the designer to take a major pause ing feedback from the local ac mains when considering inverter components. and the power stage, the controller For example, electrolytic capacitors that modulates the MOSFETs to synthesize a filter out ripple, as well as optocouplers discrete-time, grid-synchronized 60-Hz that provide galvanic isolation, have no high-voltage ac switching pattern. This chance of “going the distance” in life- discrete switching pattern is then filtered time longevity. Electrolytic capacitors to remove high-frequency noise compo- dry out and fail, and the optocoupler’s nents and delivered to the grid through LED brightness gradually fades in the an isolation transformer. face of excessive heat and input current. A PV inverter’s hardware design is full Workarounds for these delicate compo- of trade-offs that can be problematic if nents include replacing electrolytic ca- the designer makes the wrong choice. pacitors with costly film capacitors and For example, PV systems are expected substituting modern CMOS isolators for to operate reliably and at full rated out- the optocouplers. put for at least 25 years, and yet they CMOS technology offers high reliabil- must be competitively priced, forcing ity, cost effectiveness, high-speed op- the designer to make tough cost/reliabil- eration, small feature size, low operating ity tradeoffs. power, and operating stability over volt- The high efficiency requirements of age and temperature extremes. Unlike inverters often require the use of more the gallium-arsenide (GaAs) technology expensive gate drivers, power switches, used in optocouplers, CMOS devices and magnetic components. Twenty-five- have no intrinsic wear-out mechanisms. year system life in an outdoor environ- The CMOS isolation cell is capacitive, ment increases the cost of inverter sys- fully differential, and optimized for tight tem packaging due to the higher costs timing performance, low-power opera- of advanced insulation and potting com- tion, and high immunity to data errors pounds. These considerations and the caused by external fields and fast com- cost-competitive nature of the inverter mon-mode transients. market combine to create difficult deci- The advantages of CMOS, combined sions for the designer. with proprietary silicon product design, electronicdesign.com SUBSCRIBE: electronicdesign.com/subscribe | 33 ELECTRONIC DESIGN LIBRARY FOCUS ON: ALTERNATIVE ENERGY

5-kV Digital feedback data CMOS Pulse-width Attenuator V digital modulator IN isolator Grid feedback circuit Gate 2.5-kV 2.5-kV 2.5-kV control CMOS CMOS CMOS A signals isolated isolated isolated Grid driver driver driver A B C B 6 3 Ø Digital 2.5-kV 2.5-kV 2.5-kV transformer power CMOS CMOS CMOS C controller isolated isolated isolated driver driver driver

Power stage

Cycle-by-cycle reset 1-kV isolated ac current sensor Overcurrent protection

2. The transformer-based three-phase inverter uses CMOS isolation components as part of the gate-driver circuitry. enable robust isolation devices that of- topologies vary, they share the need fer higher functional integration, superior for the same component solutions. The reliability (greater than 60-year isolation block diagram in Figure 2 shows several barrier lifetime), –40°C to 125°C continu- CMOS isolation devices used in a trans- ous operation at maximum VDD, and former-based, three-phase inverter. substantial gains in performance, power In this classic closed-loop architec- efﬁciency, board-space savings, and ture, the digital controller modulates the overall ease-of-use. power-switch duty cycle to force the PV PV-inverter architectures do not end system’s output-voltage amplitude and with the single-phase, transformer-based phase to match that of the grid. Individ- inverter shown in Figure 1. Other com- ual 2.5-kV isolated gate drivers are typi- mon types include high-frequency, bi- cally preferred in inverter designs be- polar, three-phase, transformerless, and cause they simplify printed-circuit board battery-powered inverters. While these (PCB) layout. electronicdesign.com SUBSCRIBE: electronicdesign.com/subscribe | 34 ELECTRONIC DESIGN LIBRARY FOCUS ON: ALTERNATIVE ENERGY

Current feedback to the controller is capacity, lower cost, and higher reli- provided by a single, CMOS, isolated- ability. As this happens, PV inverters ac current sensor that offers a wider will expand in functionality, and de- temperature range, higher accuracy, signers will demand more integrated, and higher reliability than current-sense application-speciﬁc component-level transformers. The sensor is reset on a devices that will further leverage and cycle-by-cycle basis using the inverter drive innovation in CMOS isolation. As gate-control signals generated by a these events unfold, PV power sys- digital controller, eliminating the need tems will become more widespread for external reset circuitry. and ultimately a viable member of the Like other emerging technologies, utility mainstream that will signiﬁcantly PV systems will continue to evolve reduce our dependence on fossil fu- to meet market demands for higher els. Q

[RETURN TO THE TABLE OF CONTENTS] electronicdesign.com SUBSCRIBE: electronicdesign.com/subscribe | 35 FOCUS ON: LIBRARYALTERNATIVE ENERGY CHAPTER 5 No-Multiplier MPPT Monitors Power From Solar Array’s Switch Converter

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By Alessandro Iarocci and Giovanni Romeo INGV, Rome, Italy

esigners working on solar power arrays can simplify maximum power point NO-MULTI- tracking (MPPT) with a circuit that Dperforms the function with- PLIER MPPT out the need for a multiplier. The advantage of this approach is very evident in analog MPPTs. MONITORS Everybody would avoid analog multipliers if at all possible. Doing so decreas- POWER es component count and eliminates the error intro- duced by the multiplier in the computing of the MPPT. And FROM in a digital circuit you would save one analog-to-digital SOLAR ARRAY’S converter (ADC) channel and some code. SWITCH CONVERTER The principle is simple: The goal is to maximize the power you get from the solar Since the power supplied by the solar array is the array. For the MPPT to work, same (or at least proportional to) as the power you it must look at the curve of get from your switching converter (usually a buck power versus buck duty cy- converter), you can perform MPPT by looking at cle. But another curve will do the output power. Non-exotic loads (for example, if its maximum coincides with a resistor or a battery to be charged) usually offer the power curve. voltage and current curves satisfying this require-

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D2 U5 PAN+ MAX4376F + C14 C9 1 5 IOut Out RS– 10 μF 150 μF 2 D13 Gnd 3 4 + 5 V VCC RS C15 U1 C21 100 nF 0.47 μF 2 1 10 D8 5 V VCC BST 2 9 IRF 3707Z C22 C23 DL DH 1 4.7 μF 0.47 μF 3 8 L2 RSHU 3 D9 F2 PGND LX 40 μH 0.05 4 7 Thermal fuse GND EN 5 V Out 5 6 + R20 1.8 A 5 V DLY PWM PWM 2 C28 C25 10 μF 180 μF 200k MAX8552 D12 VOut 1 IRL 7833 R21 3 50k IOut VOut

5 V J1 R22 R18 5 V 1 2 + + 1k 1k PGC–I 3 + + 4 C12 Out R15 R19 5 + + 6 D14 100 nF PGD–VOut 1k 1k 7 + + 8 LED MCLR 5 V 9 + + 10 C19 C17 SW3 C5 100 nF 100 nF 100 nF R16 PicFlash_CON 300 U3 U4 1 8 LM7805 VDD VSS 2 7 13 TTL_OUT OSC1 PGD PGD–VOut PAN+ VIn VOut 5 V 3 6 T AN3 PGC PGC–IOut C10 Gnd C11 C8 4 5 R17 MCLR MCLR CCP1 PWM 0.33 μF 2 0.1 μF 1k 100 nF PIC12F683

1. The multiplierless MPPT system incorporates a buck converter and a control stage. RSHU and the current sense amplifier, U5, sense the system’s output power. ment. This means the MPPT can work the solar panel to the battery, a buck simply by looking at a variable like out- converter is placed between them, con- put voltage or current (see “Achieve trolled by the MPPT. The input of the MPPT Control Without Power Calcula- control stage is only the current ﬂowing tion” at www.electronicdesign.com). to the battery. The output is the buck If the application involves charging a converter’s pulse-width modulation battery (a very common task), the best (PWM) signal. variable to maximize is the current, be- The MPPT uses a “perturb-and-ob- cause the voltage, since it is clamped serve” algorithm. It slightly modiﬁes the by the battery, offers a very small range. PWM value, monitoring the effect on To transfer the maximum power from the output current. If it observes a cur- electronicdesign.com SUBSCRIBE: electronicdesign.com/subscribe | 38 ELECTRONIC DESIGN LIBRARY FOCUS ON: ALTERNATIVE ENERGY

1.0

0.8

0.6

IMPPT = 0.78 A I (Amps) 0.4

IOut with perturb and observe maximizing IOut 0.2 IOut with perturb and observe maximizing POut

0.0

0.00 0.10 0.20 0.80 1.00 PWM

2. Using a perturb-and-observe algorithm, the system showed a good correlation between the power test program and the current. Total system efficiency was 91%. rent increase, the MPPT understands RSHU and the current sense ampliﬁer, it’s doing the right thing and continues U5, sense the output current. The micro- modifying the PWM in the same direc- controller receives this signal, makes the tion. Otherwise, it decides to go in the analog-to-digital conversion, and com- opposite direction. putes the proper PWM signal. The signal Figure 1 shows the MPPT system, is then sent to the MOSFET driver, U1, comprising the buck converter and the which drives the buck converter’s pair of control stage. Connector J1 is not need- power MOSFETs (D8 and D12). ed for the circuit to work, but makes it The microcontroller also provides in- easier to program microcontroller U3. puts for the temperature and the output (When the microcontroller programmer voltage measurements, so the circuit is not connected, jumpers are needed can safely charge the battery. U3 also between pins 1, 3, 5, 7, and 2, 4, 6, 8 of provides UART communications (TX) connector J1.) through pin 2 (TTL_OUT). electronicdesign.com SUBSCRIBE: electronicdesign.com/subscribe | 39 ELECTRONIC DESIGN LIBRARY FOCUS ON: ALTERNATIVE ENERGY

The power curve is derived by power- system efficiency is 91%. ing the prototype with a 10-W solar ar- A critical aspect to consider for a solar ray (with a 21-V unloaded output), load- powered system is that it works under ing it with a 6-V lead-acid battery, and twilight conditions. In fact, microproces- running a simple test program (ingvlink) sors often suffer from unstable or slow to vary the duty cycle from 0 to 1. The rising edges at power up. For this rea- curve has a maximum at 6.2 W, which son, during testing, we verified that the corresponds to IMPPT = 0.77 A and system worked correctly in the presence VMPPT = 8.05 V. of smooth edges on power-up. Figure 2 shows the results obtained by The critical conditions were simulated feeding the perturb-and-observe algo- at the input by smoothly varying the rithm with the power test program and voltage and the current of a power sup- with the current. The two current curves ply. The system started working prop- converge at the same value, demon- erly under these conditions (see “‘Un- strating the feasibility of the multiplier- stable’ Power Supply Simulates Solar less approach. The measured buck ef- Panel Behavior” at www.electronicde- ficiency is about 94%, while the entire sign.com). Q

[RETURN TO THE TABLE OF CONTENTS] electronicdesign.com SUBSCRIBE: electronicdesign.com/subscribe | 40 ELECTRONIC DESIGN LIBRARY FOCUS ON: ALTERNATIVE ENERGY AUTHORS

DON ALFANO serves as director of isolation products at Sili- con Laboratories and was cofounder and vice president of marketing for Cygnal Integrated Products, which was ac- quired by Silicon Laboratories in 2003. He has worked for TelCom Semiconductor, NVE, Dallas Semiconductor, and Mostek. He holds a master’s degree in electrical engineering from Villanova University.

MAT DIRJISH, former Electronic Design Editor, Components

ALESSANDRO IAROCCI is an electronics engineer with the Istituto Nazionale di Geoﬁsica e Vulcanologia, Rome, Italy. He received a laurea in electrical engineering from the University of Rome, “La Sapienza.”

GIOVANNI ROMEO, technical director with the Istituto Nazio- nale di Geoﬁsica e Vulcanologia in Rome, received a laurea in physics from Rome University, “La Sapienza.”

PAUL SCHIMEL is a senior ﬁeld applications engineer at Inter- national Rectiﬁer. He has more than 15 years experience in power electronics design, EMC, and transformer design.

DON TUITE covers analog and power issues for Electronic Design’s magazine and Web site. He has a BSEE and an MS in technical communication, and he has worked for compa- nies in aerospace, broadcasting, test equipment, semiconductors, publishing, and media relations, focusing on developing insights that link technology, business, and communications. Don is also a ham radio operator (NR7X), private pilot, and motorcycle rider, and he’s not half bad on the ﬁve-string banjo. electronicdesign.com SUBSCRIBE: electronicdesign.com/subscribe | 41 (QJLQHHULQJ ,6 (9(5<:+(5( DQGZH·UHDOORYHULW