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Sustainable electrical sources likesolar photovoltaic arrays are be-coming increasingly important as

environmentally friendly alternativesto fossil fuels. But, while theyre nice forthe environment, sustainable sources

arent always easy to apply. Thesesources are characterized by both strin-gent peak-power limitations and use itor lose it availability. Successful appli-

cation of sustainable energy sourcestherefore depends on strict attention toefficiency in both power conversion andenergy storage.

For small systems, workable en-ergy-management schemes usually in-clude a rechargeable battery and bat-tery charger. A shortcoming of thissolution is that ordinary batterychargers, even efficient ones, do an im-perfect job of squeezing the last milli-

Maximum-Power-Point-Tracking Solar Battery Charger

W. STEPHEN WOODWARDVenable Hall, CB3290, University of North Carolina,

Chapel Hill, NC 27599-3290; e-mail: woodward@net.chem.unc.edu.

Circle 521

+

+

+

SENSE

SENSE

VFB

N-GATE

P-DRIVE

P-GATEVIN

LTC1149

0.068F

16

13

1015

11 12 14CT150 pF

CAP

VCC

VCC

ITH

CT

GNDS

From

"12 V"5 to 20 W

solararray

10 F16 V

+10 V

+10 V

+10 V

3300

pF

1k

PanelGND

26.1k1%

Couplethermally

150 F 16 VOS-CON

RSENSE0.05

+12 V @ 2 A max

To load

12 V~10 Ahrlead-acid

R52M

50 F50 V

100H

1N5819

R849.9k1%

R7226k1%

R6360k

SHUTDOWN 2

1N4148

0.047F

VP

100pF

1000pF

IRF9Z34

IRFZ34

4053B

Duty-factordither

(50 Hz)

470pF

2M

2M

2M

R420M

470pF

RT = PNT122-ND (50k @ 25C)

Null C20.01 F

1M

1M

1M

R2

R9Load GND

R1 1k

C11 F

R32.4M

VCA1

C3

0.1 F

A2

LMC6062

+

+

+

T

4

53

1

7

8 6

54

2

3

15 14

13

11

12

102

8 1 6

16

7

9

RT3

+

5

21

4

9

8

7

6

S2 S1

S3

1. This Maximum-Power-Point-Tracking charger, used in small solar power systems, overcomes the shortcomings of ordinary battery chargers.

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watt from sustainable sources over re-alistic combinations of ambient andbattery conditions.

The circuit shown addresses thisproblem in small solar power systems(Fig. 1). It works by continuously opti-mizing the interface between the solararray and battery. The principle inplay, sometimes called Maximu mPower Point Tracking, is illustrated inthe I/V and P/V curves for a typicalphotovoltaic array (Fig. 2) exposed tostandard sunlight intensity (insola-tion) of 1 kW/m2.

To accommodate a useful range of in-solation and battery voltage variation,designers of solar panels make the num-ber of cells large enough so that a usefullevel of charging current is providedeven when the light level is low and thebattery voltage is high. Consequently,when lighting conditions happen to bemore favorable, these panels can pro-duce up to 50% more voltage and 30%more power than the battery wants.Simple direct connection of panel to bat-tery will therefore cause inefficient op-eration at point A, with the excesspower lost as heat in the solar panel.

Figure 1 does better than that bycombining a high-efficiency (95%)SMPS circuit (LTC1149) with an ana-log power-conversion optimizationloop. To understand how it works, as-sume battery B1 is in a state of dis-charge. In this condition, E1 will ac-cept all of the current the SMPS cansupply (subject to the 2.5-A current

limit set by RSENSE) at a voltagearound 12 V. If U1 drives Q1 to a 100%duty factor, inefficient operation at thedirect-connect point A will result.

However, the optimization circuitdoesnt let that happen. Instead, 50-Hzmultivibrator S1/S2 causes A2 to con-tinuously dither Q1s duty factor byabout 10%. The result is a dither ofapproximately 1 V in VIN. Theresalso a corresponding 50-Hz modulationof the average power extracted fromthe solar panel as reflected in the re-turn current through RSENSE.

The 50-Hz ac waveform acrossRSENSE is filtered by R1C1 and syn-chronously demodulated by S3. This dcerror signal, whose polarity indicatesthe slope of the solar panel I/V curvewherever VIN happens to be sitting, isintegrated by A1 to close a feedbackloop around A2. For example, if theSMPS happens to be operating at a VINbelow the maximum power point (V

IN VMPPT re-verses the dither phase relationshipand A1 ramps toward higher duty fac-tors and lower VIN. Either way we getconvergence toward VMPPT and maxi-mum charging current for B1.

This mode of operation continues asB1 charges and its voltage rises to the14.1-V terminal-voltage setpoint de-termined by the R6-R7-R8-RT net-

work. Once reached, A2 saturates withzero output and normal LTC1149 con-stant-voltage regulation takes over. RTprovides temperature compensationappropriate for typical lead-acid bat-tery chemistry. R2 allows for A1 offsetnulling, which is particularly importantat low panel output levels. The circuitmakes no provision for preventing re-verse current from being drawn fromthe battery under no-light conditions,but since the draineven in total dark-nessis less than 3 mA (comparable totypical battery self-discharge rates),adding a blocking diode would actuallyreduce overall efficiency.

The MPPT technique has muchwider application than just photo-voltaics alone. Thats because concep-tually similar functionality of poweroutput versus loading can be seen inthe I/V curves of other sustainable en-ergy sources. Such sources are smallwater turbines (e.g. the Pelton-wheel impulse turbine of Figure 3)and fixed-pitch-rotor wind-power tur-bines, when either is combined withconstant field alternators.

The voltage, current, and powerproduced by any of these sources ishighly variable in response to ambientconditions (insolation, hydrostatichead, or windspeed) and dramaticallydependent on the electrical impedanceof the imposed load (V vs. I). Underany combination of ambient conditions,each of these sources is characterizedby exactly one ideal load impedance,

2. The I/V and P/V curves are given for a typical photovoltaic arraywhen exposed to standard sunlight intensity of 1 kW/m2. Standard

design approaches dictate an increased number of cells to provideusable charging currents for normal ranges of solar insolation.

3. The Maximum-Power-Point-Tracking (MPPT) technique also can beapplied to other sustainable energy sources like small water turbines,

such as the Pelton-wheel impulse turbine (above), due to its similarpower output versus loading characteristics.

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which will result in operation at VMPPTand maximum power transfer. Also ofbenefit is the simplifying absence ofconfusing local maxima in the powerversus voltage curves.

Of course, the actual physics behindthe I/V curves for the various sources

are very different. In the case of photo-voltaics, the primary energy-producingprocess is recombination of photoelec-tric charge carriers and how the rate ofsuch recombination varies with outputvoltage, temperature, and insolation.For wind-power generators, the domi-nant parameter is the interaction of

Tip Speed Ratio (defined as turbineperipheral velocity divided by windspeed) with the aerodynamic design ofthe turbine. For small hydroelectricgenerators, its the fluid dynamics ofthe turbine or runner as they relateto the pressure and volume of the avail-

able water source. But the MPPTcharger really doesnt care about thesedetails. It just blindly climbs the I/Vcurve to the VMPPT summit.

Figure 1s circuit can therefore beeasily adapted to any of these systems.The only modification necessary is abigger C2 (0.1 F to 1 F) to slow the

dither rate to 5-Hz to 0.5-Hz frequen-cies compatible with the inertial timeconstant of mechanical power sources.In addition, wind-power applicationswill benefit from an overspeed preven-ter. This VIN-limiting circuit is basically

just a big Zener diode connected across

the input terminals that dumps excesspower in conditions of high wind speedsand low battery demand. Consequently,it prevents overrevving of the turbineand alternator. For higher power appli-cations (25 W and up) or other outputvoltage ranges, consult Linear Technol-ogy LTC1149 application literature.