Alternative Pwer Electrnics Prpsals Prepared Fr: Prepared By: Richard J lly CI ITEM CI FOLDER CI SUB FOLDER xx xx xx Revisin A
APPROVAL PAGE Prepared By: Richard Jlly September 17, 2009 DATE Apprved By: DATE i
LTR REASON FOR CHANGE DESCRIPTION OF CHANGE REVISION HISTORY PAGES AFFECTED RELEASE DATE A First Release All September 17, 2009 APPVL ii
CONTENTS 1. SCOPE... 1 1.1 Identificatin... 1 1.2 Overview... 1 2. BATTERY TECHNOLOGY... 1 2.1 Available Battery Technlgies... 2 2.2 Battery Lifetime... 4 2.2.1 Simple Guidelines Fr Extending Lifetime... 5 2.3 Recmmendatins... 5 2.3.1 Cst Analysis... 6 3. LITHIUM-ION SPECIAL NEEDS... 6 3.1 Lithium-In Cell Prtectin... 8 3.2 Lithium-In Fuel Gauge... 8 3.3 Recmmendatins... 9 4. BATTERY CHARGING CIRCUITS... 9 4.1 LTC4062 Charger Operatin... 10 4.1.1 Nrmal Operatin... 11 4.1.2 Trickle-Charge and Defective Battery Detectin... 11 4.2 Recmmendatins... 11 4.2.1 Cst Analysis... 11 5. SOLAR POWER... 12 5.1 Slar Pwer Circuits... 12 5.1.1 DC/DC Pwer Cnversin withslar Cell... 13 5.1.2 Reservir Capacitr with Slar Cell... 14 5.1.3 Efficient Slar Cell Pwer Circuit... 14 5.2 Recmmendatins... 15 5.2.1 Cst Analysis... 15 6. APPENDIX I... 16 6.1 Available Slar Cells... 16 6.1.1 Single-crystal... 16 6.1.2 Plycrystalline... 16 6.1.3 Amrphus silicn (a-si)... 16 6.2 Hw the Slar Cell wrks... 17 6.3 Pwer Cnversin... 17 6.4 The Battery Interface... 18 6.5 Efficient Slar Cell Pwer Circuit... 20 iii
1. SCOPE 1.1 Identificatin 1.2 Overview This dcument utlines several prpsals fr the use f alternative pwer technlgies, charging, and slar pwer in assciated electrnics. Several prtable prducts use batteries. Batteries prvide these prducts with a lightweight flexible slutin. Hwever, sme batteries are nt the crrect slutin fr the prducts they supprt. The fllwing sectins discuss the benefits and weaknesses f Battery Technlgies, Batter Charging, and Slar Pwer. Each sectin has a recmmendatin paragraph. 2. BATTERY TECHNOLOGY There are several battery technlgies in use tday. The battery technlgy used in an applicatin is dependent n the parameters f the applicatin. The prcess used t chse a particular technlgy will include: 1. Evaluating anticipated pwer and perfrmance requirements. 2. Develping a priritized checklist f desired attributes such as: Vltage, capacity, and size, weight, and /r special packaging requirements. Expected energy density, service life, temperature, and/r envirnmental issues. Overall cst f wnership. Cnsidering additinal requirements such as the need fr instant activatin as well as the ability t cnduct rutine testing fr system readiness. 3. Analyzing this checklist t ensure that the ptimal technlgy is being emplyed based n verall perfrmance criteria. 4. Perfrming apprpriate tests t cnfirm that the pwer management slutin fulfills all necessary perfrmance, quality, and safety requirements in accrdance with custmer specificatins and applicable third-party certificatins. 1
2.1 Available Battery Technlgies Figure A4-1 cmpares the energy strage capability f several battery technlgies. Energy strage is expressed as watt-hurs per unit vlume (Wh/l) and watt-hurs per unit weight (Wh/kg). The larger values f Wh/l translate int a smaller cell, while larger values fr Wh/kg translate int lighter weight fr a given cell vltage and ampere-hur capacity. The high values f Wh/l and Wh/kg have been key factrs in its rapid grwth. *http://www.atp.nist.gv/ea/wp05-01/append-4.htm Table 2-1 cmpares the available battery technlgies. Each battery s specificatins and useful applicatins is listed in Table 2-1. *http://www.atp.nist.gv/ea/wp05-01/append-4.htm 2
Technlgy Advantages Disadvantages Applicatins Lithium-In (Liin) Highest energy strage (Wh/l) Light Weight N memry effect Gd cycle life Relatively expensive Electrnic prtectin circuitry Thermal runaway cncern Cellular phnes Ntebk Cmputers Camcrders High energy efficiency 3-hur charge High unit-cell vltage Nt tlerant f vercharge r ver discharge Lithium-In Plymer (Li-in Plymer) Same chemistry as Li-in Lighter weight (Wh/kg) Flexible ftprint Internal bnding f ande t cathde Lwer high rate Plasticized electrlyte 3-hur charge Mre expensive Same applicatins as Li-in PDAs Nickel Metal Hydride (Ni-MH) Higher capacity than Ni-Cd Pr charge retentin Lw-end electrnic devices Cadmium Free Rapid Recharge Lng cycle life High cst negative Memry effect Lwer high rate than Ni-Cd First prductin in 1992 Used in HEV Nickel Cadmium (Ni-Cd) Lng cycle life Excellent high rate Rapid recharge Lwer capacity Memry effect Envirnmental cncerns Pwer tls Prtable phnes Lw-end electrnic devices Gd lw temperature Pr charge retentin Standby pwer Rbust chemistry Lw unit-cell 3
vltage Lead Acid Inexpensive Lw energy density Emergency lighting Sealed value regulated technlgy Gd high rate Sulfatin n stand Intermediate unitcell vltage Autmbiles Table 2-1 Cmparisn f available Battery Technlgies *http://www.atp.nist.gv/ea/wp05-01/append-4.htm 2.2 Battery Lifetime Anther cnsideratin when cmparing batteries is Battery Lifetime. Figure 2-2 cmpares battery technlgies. Ntice that Lithium-In has a lifespan f 1.75 years. The Lithium battery dented in blue is nt rechargeable. It is ne time use nly. Figure 2-2 Battery Lifetime Curve Li-in ffers internal resistance characteristics that are between thse f NiMH and NiCd. Usage des nt cntribute much t the increase in resistance, but aging des. The typical life span f a Li-in battery is tw t three years, whether it is used r nt. Cl strage and keeping the battery in a partially charged state when nt in 4
use retard the aging prcess. The internal resistance f the Li-in batteries cannt be imprved with cycling. The cell xidatin, which causes high resistance, is nn-reversible. The electrlyte slwly eats up the psitive plate and the electrlyte decays. This chemical change causes the internal resistance t increase. In time, the cell resistance raises t a pint where the battery can n lnger deliver the energy. The ultimate cause f failure is high internal resistance. Energy may still be present in the battery, but it can n lnger be delivered due t pr cnductivity. The recmmended strage temperature f a lithium-based battery is 15 C (59 F) r less. A charge level f 40 percent allws fr sme self-discharge that naturally ccurs; and 15 C is a practical and ecnmical strage temperature that can be achieved withut expensive climate cntrl systems. 2.2.1 Simple Guidelines Fr Extending Lifetime Charge the Li-in ften, except befre a lng strage. Stre at abut 40% charge in a cl place Avid repeated deep discharges. Keep the Li-in battery cl. Prevent strage in a ht car. Never freeze a battery. Avid purchasing spare Li-in batteries fr later use. Observe manufacturing date when purchasing. 2.3 Recmmendatins Fr re-chargeable batteries, Lithium-In shuld be used. The reasns are as fllws: Lightweight with High Density Charge. There is n memry and n scheduled cycling is required t prlng the battery's life. 5
The typical life span f a Li-in battery is tw t three years, whether it is used r nt. Fr ne time use nly, Lithium shuld be used. These batteries are nly useful fr extremely lw-current applicatin. The fllwing are reasns why Lithium is superir t ther technlgies: 2.3.1 Cst Analysis Lightweight with High Density Charge Extremely lng shelf life. It can last fr decades. The fllwing is a cst analysis t replace the alkaline batteries with Li-In batteries. The Link Up? Prduct cmes cmplete with 4 AA alkaline batteries. Each battery cst $0.11 in bulk purchase. The battery circuit hlder cst $ 1.23. The alkaline batteries used ver the lifetime f the prduct is apprximately 20 hurs f cntinuus use. If used in three minute increments fr nce r twice per day, expected use can be 150 days. Batteries must be replaced twice per year, every year. Cst fr three years use is: 2 (AA Cells) * 2 (times per year) * 3 (years) = 12 units * $0.11 = $1.32 + $1.23 = $2.55 The Lithium ne use nly cst $7.99 and the battery hlder cst $1.23. The cst t Electrnics Cmpanies can be ffset by eliminating the mechanical design fr the battery dr (savings apprximately $10,000). 80 hurs f cntinuus use. If used in three minute increments fr nce r twice per day, expected use can be 600 days. Cst fr three years use is: 2 (Lithium AA Cells) * 1.5 (3 years) = 3 units * $7.99 + $1.23 = $25.2 Fr the rechargeable battery, the cst f the Li-In battery is $2.95 per AA Cell. The cst f the battery charger circuit is $5.80. The cst f the battery hlder is $1.23. Infinite Re-charge. $2.95 * 2 (Lithium-In Cells) + $5.80 = $11.73 3. WHY LITHIUM-ION BATTERIES DEGRADE WITH REPEATED CHARGING As anyne with a smartphne, laptp r indeed a whle electric car will knw, lithium-in batteries degrade ver time. Each time yu charge and discharge the batteries, they lse a little capacity. Day t day yu dn't really ntice, but ver a year, r tw, it means being able t use yur phne (r car) a little less. 6
Nw, research scientists supprted by the Department f Energy have discvered the physical prperties behind this lss in capacity. As Gizmd reprts, it's all t d with hw ins mving thrugh the battery change the physical structure f the electrdes. In a lithium-in battery, lithium ins mve frm the ande t cathde thrugh a nn-aqueus electrlyte. As they d s, the physical structure f the electrdes is very slightly altered, at an atmic level. As ins mve acrss the ande when discharging, they wear away at irregularities n its surface in a nn-unifrm way. Hulin Xin, a materials scientist at Brkhaven Lab's Center fr Functinal Nanmaterials (CFN), describes it as the same kind f nn-unifrm structure as rust creeping acrss steel. Unfrtunately, these imperfectins are necessary fr the battery t functin. Just as snwflakes frm arund micrscpic dirt particles, explains Xin, s t d particles in a battery require these irregularities t frm upn. The cathde desn't escape charge-reducing effects, either. Here, as lithium ins mve acrss the electrde when charging, they frm a kind f rck-salt, which acts as an electrically-insulating crust. The thicker this crust, the less charge the battery will accept. Xin says this latter effect is even mre prnunced at higher vltages. Neither is particularly gd fr yur battery's capacity. S finding a fix fr bth culd be the clue t batteries that retain capacity fr much lnger perids. Xin says it might be pssible t cat the cathdes with elements that resist crystallizatin--allwing ins t pass freely between ande and cathde. A cmmercially-realistic timescale fr such advances may mean years f wrk, rather than mnths. But fr electric car wners--and anyne else with an electrnic device--it culd be a step twards vehicles that achieve the same range whether they're brand new, r a decade ld. 4. LITHIUM-ION SPECIAL NEEDS Lithium-in is safe, prvided certain precautins are met when charging and discharging. Because vercharging r ver-discharging a Li+ cell can cause it t explde and injure peple, safety is a majr cncern when handling this type f strage cell.it is fragile and requires a prtectin circuit t maintain safe peratin. Built int each pack, the prtectin circuit limits the peak vltage f each cell during charge and prevents the cell vltage frm drpping t lw n discharge. In additin, the cell temperature is mnitred t prevent temperature extremes. The maximum charge and discharge current n mst packs are is limited t between 1C and 2C. With these precautins in place, the pssibility f metallic lithium plating ccurring due t vercharge is virtually eliminated. 7
Figure 3-1 Lithium-In Cell prtectin circuit** 4.1 Lithium-In Cell Prtectin ** http://pdfserv.maxim-ic.cm/en/an/an3501.pdf Figure 3-1 shws a Lithium-In cell prtectin circuit. It uses the Dallas DS2720. It prvides all the electrnic safety functins required fr applicatins that invlve rechargeable Li+ batteries: prtectin fr the battery during charge, prtectin fr the circuit against excess current flw, and maximizing battery life by limiting the level f cell depletin. 4.2 Lithium-In Fuel Gauge Tracking remaining available energy is critical fr battery-perated equipment. Als, the Lithium-In battery shuld be recharged befre it is cmpletely drained f energy t prevent aging f the battery. Energy cnsumptin depends n the temperature and usage histry f the battery. If ne knws the charge f a fresh battery, the temperature histry, and the discharge rate during nrmal use, ne has all the data needed t estimate the battery's remaining charge. The DS2786 estimates available capacity fr rechargeable Li-In and Li-In Plymer batteries based n the cell vltage in the pen-circuit state fllwing a relaxatin perid. The pen-circuit vltage (OCV) is used t determine relative cell capacity based n a lkup table stred in the IC. This capability makes accurate capacity infrmatin available immediately after a battery pack is inserted. 8
Figure 3-2 DS2786 Fuel Gauge fr Lithium In Battery 4.3 Recmmendatins The safety f Lithium-In batteries depends n using a prtectin circuit. This circuit will limit dis-charge and charging currents as well as ver-vltage and temperature. A prtectins circuit shuld be used in cnjuctin with a battery fuel gauge. These tw devices will prvide cmplete prtectin and ensure lng battery life. 5. BATTERY CHARGING CIRCUITS There are many battery charging circuits n the market. Each ne is designed fr a particular battery technlgy. The reasn is that Li-In batteries can verheat during charging (creatin f Lithium metal which "burns" in water, chance f fire). S Li-In battery packs have an internal circuit t prevent vercharging (which wuld cause them t verheat). There can be several functins fr the prtectin circuit, including shutting it dwn in case f ver charging, when the vltage drps t a predefined level, r if it thinks the battery is therwise damaged. 9
Vin VCC OUT BAT > 3V BAT < 3V C/5 BAT EN TIMER PROG IDET LINEAR TECHNOLOGY LTC4062 IN+ + Li-In Battery Figure 4-1 Linear Technlgy Battery Charging Circuit Figure 4-2 LTC4062 Charge Time 5.1 LTC4062 Charger Operatin The LTC4062 is designed t charge single-cell lithium-in batteries. The cmplete circuit is shwn in Figure 4-1. Using the cnstant current/cnstant vltage algrithm, the charger can deliver up t 1A f charge current with a final flat vltage accuracy f 0.35%. The LTC4062 includes an internal P-channel pwer MOSFET and 10
thermal regulatin circuitry. N blcking dide r external sense resistr is required; thus, the basic charger circuit requires nly tw external cmpnents. 5.1.1 Nrmal Operatin The charge cycle begins when the vltage at the VCC pin rises abve the UVLO level and a discharged battery is cnnected t BAT. If the BAT pin vltage is belw 2.9V, the charger enters trickle charge mde. In this mde, the LTC4062 supplies 1/10th f the prgrammed charge current in rder t bring the battery vltage up t a safe level fr full current charging. Once the BAT pin vltage rises abve 2.9V, the charger enters cnstant current mde, where the prgrammed charge current is supplied t the battery. When the BAT pin appraches the final flat vltage (4.2V), the LTC4062 enters cnstant vltage mde and the charge current decreases as the battery becmes fully charged. 5.1.2 Trickle-Charge and Defective Battery Detectin When the BAT pin vltage is belw the 2.9V trickle charge threshld (VTRIKL), the charger reduces the charge current t 10% f the prgrammed value. If the battery remains in trickle charge fr mre than 25% f the ttal prgrammed charge time, the charger stps charging and enters a FAULT state, indicating that the battery is defective1. The LTC4062 indicates the FAULT state by driving the CHRG pen-drain utput with a square wave. The duty cycle f this scillatin is 50% and the frequency is set by CTIMER: A LED driven by the CHRG utput exhibits a pulsing pattern, indicating t the user that the battery needs replacing. T exit the FAULT state, the charger must be restarted either by tggling the EN input r remving and reapplying pwer t VCC. 5.2 Recmmendatins Battery charging circuits must be tailred t the type f battery technlgy. Many manufacturers prduce IC s specifically fr charging a type f battery with knwn charge capacity, knwn vltage, and knwn discharge rate. A prgrammable IC shuld be used with which the current and vltage can be prgrammed int the device. The IC s frm Linear Technlgy have this capability and are better quality than that f ther manufacturers. The LTC4062 is designed t charge a Li-In battery frm the USB bus. 5.2.1 Cst Analysis The fllwing is a cst analysis t replace the alkaline batteries with Li-In batteries. 11
The Link Up? Prduct cmes cmplete with 4 AA alkaline batteries. Each battery cst $0.11 in bulk purchase. The battery circuit hlder cst $ 1.23. 20 hurs f cntinuus use. If used in three minute increments fr nce r twice per day, expected use can be 150 days. Batteries must be replaced twice per year, every year. Cst fr three years use is: 2 (AA Cells) * 2 (times per year) * 3 (years) = 12 units * $0.11 = $1.32 + $1.23 = $2.55 The design with rechargeable battery includes the cst f the Li-In battery at $2.95 per AA Cell. The cst f the battery charger circuit is $5.80. Infinite Re-charge. $2.95 * 2 (Lithium-In Cells) + $5.80 + = $11.73 6. SOLAR POWER As a pwer surce, the sun ffers sme advantages ver typical battery cells: It generates virtually limitless energy and requires n recharging. Tday's mre efficient less expensive slar cells prvide a practical means f cnverting energy frm light int electricity t run prtable devices. One prblem des arise hwever. Because light may nt always be available in cnsistent quantities, it's tugh t design a system that can reliably prvide pwer. 6.1 Slar Pwer Circuits The utput current fr typical mn-crystalline silicn slar cells directly depends n the amunt f incident sunlight (Figure 5-1). Fr example, a typical hbbygrade credit-card-sized silicn cell has an pen-circuit vltage f 0.55V. Internal resistance causes a vltage drp as yu draw current frm the cell; but, as light energy drps belw the level necessary t supprt the utput lad, the cell current-limits at an almst cnstant vltage. Fr a light level f ne full sun (slar irradiance f 1kW/m²), the cell prvides a typical shrt-circuit current f 0.3A. 12
Figure 5-1 Available Current utput frm a Slar Cell is prprtinal t the Incident-Light Energy, but the cells pen-circuit vltage is almst cnstant. Maximum utput pwer arises at the transitin frm cnstant vltage t cnstant current, typically 0.484V and 0.25 t 0.275A at ne full sun. This 0.484V is t lw fr mst applicatins, s slar panels usually cnnect the cells in a series/parallel cmbinatin that prvides several amperes f current at abut 12V. Such utputs are useful fr many applicatins, assuming full sunlight is available whenever the applicatin is active. Unfrtunately, this imperative is hard t realize in mst lcatins. Varying sunlight has less effect n the pen-circuit vltage but has a direct effect n the maximum available current. This characteristic is critical in designing electrnics that ptimize the use f the pwer available at any given time. Mst applicatins include an intermediate energy-strage device, such as a rechargeable lead-acid r nickel-cadmium (NiCd) battery, t ensure an always-ready surce f pwer. When light is sufficient, the slar cell charges the battery, which then prvides the lad with a stable supply. Charging the battery directly frm the slar panel (via a series dide) is difficult r at least inefficient, because battery vltage changes cnsiderably as the battery charges. 6.1.1 DC/DC Pwer Cnversin withslar Cell Obtaining all the available energy frm a slar panel requires a switch-mde step-up r -dwn cnverter and battery charger, in additin t the battery. The cnverter must ensure that every bit f energy taken frm the slar panel is efficiently stred in the battery fr future use. A switch-mde cnverter wrks in tw cycles. It first cnnects an inductr t a pwer surce, allwing a buildup f inductr current that stres energy in the inductr. In the secnd cycle, a change in current path enables the inductr t transfer its accumulated energy t the lad. The lad vltage can be higher r lwer than that f the inductr's pwer surce. 13
6.1.2 Reservir Capacitr with Slar Cell Alternative_Pwer_Prpsals.dc Rev A Cnnecting the inductr directly t a slar panel is inefficient. Depending n light levels, the panel's utput-current capability can range frm micr-amps t several amperes. A much better apprach is t cnnect a reservir capacitr t the slar panel. By mnitring vltage n this capacitr, yu can then turn n the switch-mde cnverter nly when the panel utput is ptimum, that is, 0.484V times the number f series-cnnected cells in the panel. This input is adequate t start the cnverter, and the capacitr prvides a lw-impedance path fr the inductr current. Mrever, the capacitr supprts full cycles f cnverter peratin withut allwing the cnverter's input vltage t drp belw its perating vltage. When the capacitr vltage drps belw a predetermined level (the cnverter's minimum perating vltage r higher), the cnverter shuts dwn until the capacitr again charges t the ptimum vltage. SOLAR PANEL V INPUT COMPARATOR PLUS REFERENCE COMPARATOR WITH HYSTERESIS OUTPUT DC/DC CONVERTER OUT FAST CHARGE SHUTDOWN Vp DUAL COMPARATOR PLUS LOGIC IN FEEDBACK MAXIMUM VOLTAGE LIMIT LOAD RECHARGEABLE BATTERY RESERVOIR CAPACITOR Figure 5-2 Pwer Cnverter, Reservir Capacitr, and Rechargeable Battery circuit fr Efficient Slar Cell Operatin 6.1.3 Efficient Slar Cell Pwer Circuit Figure 5-2 shws the Slar Cell, Pwer Cnverter, Reservir Capacitr, and Rechargeable Battery designed int a circuit t make maximum efficiency f the energy available t the slar cell. The cnverter's utput side raises sme issues. The desired utput vltage is nt cnstant but depends n the battery's charge cnditin. When charging, yu always want t apply a fast charge unless the battery is already fully charged. During fast charge, the cnverter perates as a current surce, frwarding inductr energy t the battery withut checking the utput vltage. The battery's fast-charge-current requirement shuld therefre determine the values fr the inductr and fr the FET's current-sense resistr. Terminating a fast charge is difficult fr at least tw reasns. After reaching a certain battery vltage, the abve burst-cnversin scheme des nt hint at when the next 14
cnversin cycle will ccur. If it ccurs within millisecnds, yu shuld prbably stp the fast charge. If it ccurs within hurs r days (when sunlight is scarce), the fast charge shuld cntinue, because the battery can therwise drain lng befre the next cycle. The ther difficulty stems frm charge current arriving in infrequent bursts: Yu can't detect a zer r negative dv/dt under this cnditin. One apprach applicable t bth lead-acid and NiCd batteries is t mnitr the utput vltage with anther cmparatr, disabling switching when the vltage reaches a high limit and enabling it when the vltage declines belw a predetermined level. A third level f detectin culd als enable a flat-current (trickle) charge f the battery. When charging a discharged battery, the circuit applies full fast-charge current until the vltage reaches its upper limit. The circuit then disables switching until the vltage reaches the next-lwest (middle) limit, whereupn it enables the trickle charge. Trickle charging cntinues until the vltage reaches its upper limit (turning ff the switching) r its lwer limit (enabling the fast charge again). 6.2 Recmmendatins The cmplete slutin t using Slar Cells t pwer prtable electrnics is shwn in Figure 5-2. The circuit is different frm the antiquated Slar Pwered Calculatrs. These calculatrs have an instant n apprach and are nly usable under direct light. The circuit utlined in Figure A-4 will prvide stable pwer t a lad under varying cnditins f sunlight and varying lads. When designing this circuit, careful cnsideratin shuld be give t what type f battery technlgy used. The alkaline cell prvides a cheap easy slutin. There are many DC/DC circuits that can easily handle the charging f alkaline cells. 6.2.1 Cst Analysis The fllwing is a cst analysis t replace the alkaline batteries with Li-In batteries. The Link Up? Prduct cmes cmplete with 4 AA alkaline batteries. Each battery cst $0.11 in bulk purchase. The battery circuit hlder cst $ 1.23. 20 hurs f cntinuus use. If used in three minute increments fr nce r twice per day, expected use can be 150 days. Batteries must be replaced twice per year, every year. Cst fr three years use is: 2 (AA Cells) * 2 (times per year) * 3 (years) = 12 units * $0.11 = $1.32 + $1.23 = $2.55 15
7. APPENDIX I Alternative_Pwer_Prpsals.dc Rev A The design with Slar Cells includes the cst f the NiMH battery at $2.95 per AA Cell, the slar cell at $5.05 per cell, and the cst f the battery charger circuit is $5.80. Infinite Re-charge. $2.95 + $5.05 + $5.80 = $13.80 This sectin includes backgrund and further infrmatin fr the sectins abve. 7.1 Available Slar Cells Mst PhtVltaic systems cnsist f individual square cells averaging abut fur inches n a side. Alne, each cell generates very little pwer (less than tw watts), s they are ften gruped tgether as mdules. Mdules can then be gruped int larger panels encased in glass r plastic t prvide prtectin frm the weather, and these panels, in turn, are either used as separate units r gruped int even larger arrays. The three basic types f slar cells made frm silicn are single-crystal, plycrystalline, and amrphus. 7.1.1 Single-crystal Cells are made in lng cylinders and sliced int rund r hexagnal wafers. While this prcess is energy-intensive and wasteful f materials, it prduces the highestefficiency cells as high as 25 percent in sme labratry tests. Because these high-efficiency cells are mre expensive, they are smetimes used in cmbinatin with cncentratrs such as mirrrs r lenses. Cncentrating systems can bst efficiency t almst 30 percent. Single-crystal accunts fr 29 percent f the glbal market fr PV. 7.1.2 Plycrystalline Cells are made f mlten silicn cast int ingts r drawn int sheets, then sliced int squares. While prductin csts are lwer, the efficiency f the cells is lwer t arund 15 percent. Because the cells are square, they can be packed mre clsely tgether. Plycrystalline cells make up 62 percent f the glbal PV market 7.1.3 Amrphus silicn (a-si) Amrphus silicn is a radically different apprach. Silicn is essentially sprayed nt a glass r metal surface in thin films, making the whle mdule in ne step. This apprach is by far the least expensive, but it results in very lw efficiencies nly abut five percent. 16
7.2 Hw the Slar Cell wrks The mst imprtant cmpnents f a PV cell are tw layers f semicnductr material generally cmpsed f silicn crystals. On its wn, crystallized silicn is nt a very gd cnductr f electricity, but when impurities are intentinally added a prcess called dping the stage is set fr creating an electric current. The bttm layer f the PV cell is usually dped with brn, which bnds with the silicn t facilitate a psitive charge (P). The tp layer is dped with phsphrus, which bnds with the silicn t facilitate a negative charge (N). The surface between the resulting p-type and n-type semicnductrs is called the P-N junctin (see the diagram belw). Electrn mvement at this surface prduces an electric field that nly allws electrns t flw frm the p-type layer t the n-type layer. Figure A-1 Inner wrkings f the Slar Cell When sunlight enters the cell, its energy kncks electrns lse in bth layers. Because f the ppsite charges f the layers, the electrns want t flw frm the n- type layer t the p-type layer, but the electric field at the P-N junctin prevents this frm happening. The presence f an external circuit, hwever, prvides the necessary path fr electrns in the n-type layer t travel t the p-type layer. Extremely thin wires running alng the tp f the n-type layer prvide this external circuit, and the electrns flwing thrugh this circuit prvide the cell s wner with a supply f electricity. 7.3 Pwer Cnversin In Figure A-2, a cmparatr with hysteresis cntrls the cnverter's shutdwn pin. As lng as the reservir capacitr's vltage is lwer than the ptimum lad vltage fr the slar panel (0.484V times the series-cnnected-cell number), the cnverter is in shutdwn. When the capacitr vltage reaches this ptimum lad vltage, the cnverter is enabled until the capacitr vltage declines t the lwer limit f the cmparatr's hysteresis band. 17
Figure A-2 The input t a DC/DC cnverter fr slar-panel pwer cnversin cmprises the panel, a reservir capacitr, and a cmparatr/reference circuit that enables the cnverter nly when adequate pwer is available. Operating in bursts whenever there is sufficient charge n the capacitr, the cnverter charges the battery nly when the value f reservir-capacitr vltage is ptimum fr all levels f sunlight. In effect, the slar panel's lad (the switch-mde cnverter) adjusts autmatically t the slar panel's best perating area (its V/I limit). The rati f slar-panel n-lad vltage t battery vltage determines whether the cnverter actin must step up r step dwn. If the slar-panel n-lad vltage (determined by the number f series-cnnected cells) is lwer than the vltage f the discharged battery, chse a step-up cnverter. If the panel vltage under lad is higher than that f the fully charged battery, chse a step-dwn cnverter. Otherwise, rearrange the number f battery r slar cells t find a cmbinatin f battery and slar panel that satisfies ne f thse cnditins. Step-dwn cnverters have n direct path frm input t utput during shutdwn, but the step-up cnfiguratin has a DC path (via the inductr and utput dide) at all times, including shutdwn. Be aware that step-up cnverters have n current limit in the event f a shrted utput. The slar panel is current-limited, s the main wrry in such cases is prbably a shrted battery, which may require ther means f circuit prtectin. 7.4 The Battery Interface An inspectin f the cnverter's utput side raises sme ther issues. The desired utput vltage is nt cnstant but depends n the battery's charge cnditin. When charging, yu always want t apply a fast charge unless the battery is already fully charged. During fast charge, the cnverter perates as a current surce, frwarding inductr energy t the battery withut checking the utput vltage. The battery's fast-charge-current requirement shuld therefre determine the values fr the inductr and fr the FET's current-sense resistr. 18
Terminating a fast charge is difficult fr at least tw reasns. After reaching a certain battery vltage, the abve burst-cnversin scheme des nt hint at when the next cnversin cycle will ccur. If it ccurs within millisecnds, yu shuld prbably stp the fast charge. If it ccurs within hurs r days (when sunlight is scarce), the fast charge shuld cntinue, because the battery can therwise drain lng befre the next cycle. The ther difficulty stems frm charge current arriving in infrequent bursts: Yu can't detect a zer r negative dv/dt under this cnditin. One apprach applicable t bth lead-acid and NiCd batteries is t mnitr the utput vltage with anther cmparatr, disabling switching when the vltage reaches a high limit and enabling it when the vltage declines belw a predetermined level. A third level f detectin culd als enable a flat-current (trickle) charge f the battery (Figure A-3). When charging a discharged battery, the circuit applies full fast-charge current until the vltage reaches its upper limit. The circuit then disables switching until the vltage reaches the next-lwest (middle) limit, whereupn it enables the trickle charge. Trickle charging cntinues until the vltage reaches its upper limit (turning ff the switching) r its lwer limit (enabling the fast charge again). Figure A-3 The DC/DC cnverter enables a slar panel t charge a rechargeable battery (a). Cmparatr-generated signals enable the circuit t cntrl the charging current (b). 19
A dual cmparatr with hysteresis cntrls the cnverter shutdwn and the selectin f fast charge versus trickle charge. T prevent discharge f the battery when the slar-panel vltage ges t zer, yu shuld design the cmparatr circuit nt t lad the battery via its supply terminal r the external resistive divider. Yu can cntrl the cnverter shutdwn frm either the utput cmparatr r the input cmparatr when slar-panel vltage is lw, and this capability may require additinal lgic. 7.5 Efficient Slar Cell Pwer Circuit The circuit in Figure A-4, which charges a three-cell battery frm a seven-cell slar panel, illustrates the abve ideas. The battery vltage at discharge is greater than 3.0V, and the ptimum slar-panel utput is 2.9V (3.8V maximum), s the circuit requires cnverter (IC 1 ). The dual cmparatr cntrls shutdwn and charge terminatin. IC 2A directly cntrls shutdwn, hlding the cnverter in shutdwn until the slar panel charges C 1 t 2.9V. The cnverter then becmes active, and hysteresis allws the peratin t cntinue until the vltage drps belw 2.5V. While n, the cnverter delivers its full utput current, subject nly t the internal current limit. Figure A-4 This circuit enables a seven-cell slar panel t charge a three-cell 20
NiCd battery. Regulatin first begins when the battery vltage reaches 5.0V, but the utput cmparatr (IC 2B ) terminates the charge cycle at 4.6V. The design handles terminatin nt by cntrlling the shutdwn pin, which requires additinal lgic, but by the uncnventinal apprach f driving the 3/5V-select pin high. Because the utput is at 4.6V, selecting a 3V utput (actually 3.3V) causes the cnverter t turn ff. Again, cmparatr hysteresis ensures that the cnverter remains ff until the battery vltage decreases t 4V. It then resumes peratin, prviding a simple frm f idle charging that des nt vercharge the battery. Because the cnverter shuts dwn when the slar-panel utput is lw, the battery-discharge current (the sum f cnverter-shutdwn current, cmparatrsupply current, R 7 /R 8 current, and reverse current frm the Schttky dide) is minimal. The Schttky reverse current is 5µA typical at 25 C and 50µA r higher at 50 C, which is t high fr sme applicatins. Fr thse, yu can substitute a switching dide, such as the 1N4148, at the expense f slightly lwer cnverter efficiency. Yu shuld avid taking the utput lwer than the input, as with all step-up cnverters. In this case, an verlad r shrted battery cell can cause an uncntrlled DC current flw frm the slar panel thrugh the inductr and dide t the utput. In mst cases, hwever, current limiting in the slar panel allws the battery t simply recharge t 3V withut damage t the inductr. Applicatins in which the slar-panel vltage is lwer than the lw-battery vltage require a step-up cnverter, and applicatins that need mre current (r a peak current ther than that determined by the fixed internal setting) require a step-up cntrller. Applicatins in which the slar-panel vltage is higher than the fully charged battery vltage demand a step-dwn cnverter. As an example f applicatins demanding a step-dwn cnverter, cnfiguring the MAX797 cntrller as a current surce and its internal 5V linear regulatr can pwer the external cmparatrs. The device is well suited fr charging a car battery frm tw 12V slar panels. These circuits and techniques shuld ensure efficient battery charging frm slar panels despite unpredictable weather cnditins. The target applicatins are primarily lw t medium pwer, frm a few watts t 100W. A further refinement in the charger circuit, thugh nt required fr mst NiCd and lead-acid batteries, culd sense the battery current and use that infrmatin t prvide a cnstant current-surce utput with vltage limit. 21