IDEAS FOR DESIGN
aren t always easy to apply. These
Circle 521
sources are characterized by both strin-
gent peak-power limitations and  use it
or lose it availability. Successful appli-
Maximum-Power-Point-
cation of sustainable energy sources
therefore depends on strict attention to
Tracking Solar Battery Charger efficiency in both power conversion and
energy storage.
W. STEPHEN WOODWARD For small systems, workable en-
Venable Hall, CB3290, University of North Carolina, ergy-management schemes usually in-
Chapel Hill, NC 27599-3290; e-mail: [e-mail]. clude a rechargeable battery and bat-
tery charger. A shortcoming of this
ustainable electrical sources like environmentally friendly alternatives solution is that ordinary battery
solar photovoltaic arrays are be- to fossil fuels. But, while they re nice for chargers, even efficient ones, do an im-
Scoming increasingly important as the environment, sustainable sources perfect job of squeezing the last milli-
+
+12 V @ 2 A max
From
+
50 F
1N4148 IRF9Z34
2 To load
"12 V"

50 V
5 to 20 W
VIN 1
16
solar
CAP P-GATE 100 H
array
0.068
0.047
F
F
4
3
R8
P-DRIVE RT Couple
VCC
+ 49.9k
thermally
10 F
5 13
T
12 V
1% +
VCC N-GATE
16 V
1N5819
~10 Ahr
IRFZ34
R7
+ lead-acid
VP

LTC1149
150 F 16 V
226k
+10 V
R1 1k
1% OS-CON
15
SHUTDOWN 2 VFB 10
100
C1
R5 R6 26.1k RSENSE
R9
pF
1 F
2M 360k 1%
0.05
Load GND 2M
7 9 4
SENSE
ITH
3 5
1000
C3
S3
3300
pF 7
6 8 R2
0.1 F
pF
CT SENSE
C2
Null
9
0.01 F
1M
GNDS
2
+10 V R3
1k
CT 11 12 14
2.4M 
1
A1 1M
150 pF
VC 3
+
R4
+10 V 13
470 2M
1M
2
20M
Panel 16
pF 8
6 10 14
15 11
GND S2 S1
7 
RT = PNT122-ND (50k @ 25°C)
A2 8
1
5
6
+
12
LMC6062 Duty-factor
4
dither
(50 Hz)
4053B
470
2M
pF
1. This Maximum-Power-Point-Tracking charger, used in small solar power systems, overcomes the shortcomings of ordinary battery chargers.
114
ELECTRONIC DESIGN / SEPTEMBER 14, 1998
IDEAS FOR DESIGN
2. The I/V and P/V curves are given for a typical photovoltaic array 3. The Maximum-Power-Point-Tracking (MPPT) technique also can be
when exposed to  standard sunlight intensity of 1 kW/m2. Standard applied to other sustainable energy sources like small water turbines,
design approaches dictate an increased number of cells to provide such as the  Pelton-wheel impulse turbine (above), due to its similar
usable charging currents for  normal ranges of solar insolation. power output versus loading characteristics.
watt from sustainable sources over re- limit set by RSENSE) at a voltage work. Once reached, A2 saturates with
alistic combinations of ambient and around 12 V. If U1 drives Q1 to a 100% zero output and normal LTC1149 con-
battery conditions. duty factor, inefficient operation at the stant-voltage regulation takes over. RT
The circuit shown addresses this direct-connect point  A will result. provides temperature compensation
problem in small solar power systems However, the optimization circuit appropriate for typical lead-acid bat-
(Fig. 1). It works by continuously opti- doesn t let that happen. Instead, 50-Hz tery chemistry. R2 allows for A1 offset
mizing the interface between the solar multivibrator S1/S2 causes A2 to con- nulling, which is particularly important
array and battery. The principle in tinuously dither Q1 s duty factor by at low panel output levels. The circuit
play, sometimes called Maximum about Ä…10%. The result is a dither of makes no provision for preventing re-
Power Point Tracking, is illustrated in approximately Ä…1 V in VIN. There s verse current from being drawn from
the I/V and P/V curves for a typical also a corresponding 50-Hz modulation the battery under no-light conditions,
photovoltaic array (Fig. 2) exposed to of the average power extracted from but since the drain even in total dark-
 standard sunlight intensity (insola- the solar panel as reflected in the re- ness is less than 3 mA (comparable to
tion) of 1 kW/m2. turn current through RSENSE. typical battery self-discharge rates),
To accommodate a useful range of in- The 50-Hz ac waveform across adding a blocking diode would actually
solation and battery voltage variation, RSENSE is filtered by R1C1 and syn- reduce overall efficiency.
designers of solar panels make the num- chronously demodulated by S3. This dc The MPPT technique has much
ber of cells large enough so that a useful error signal, whose polarity indicates wider application than just photo-
level of charging current is provided the slope of the solar panel I/V curve voltaics alone. That s because concep-
even when the light level is low and the wherever VIN happens to be sitting, is tually similar functionality of power
battery voltage is high. Consequently, integrated by A1 to close a feedback output versus loading can be seen in
when lighting conditions happen to be loop around A2. For example, if the the I/V curves of other sustainable en-
more favorable, these panels can pro- SMPS happens to be operating at a VIN ergy sources. Such sources are small
duce up to 50% more voltage and 30% below the maximum power point (VIN < water turbines (e.g. the  Pelton-
more power than the battery wants. VMPPT), then there will be a positive wheel impulse turbine of Figure 3)
Simple direct connection of panel to bat- correlation between VIN and ISENSE, and fixed-pitch-rotor wind-power tur-
tery will therefore cause inefficient op- and A1 will ramp toward lower average bines, when either is combined with
eration at point  A, with the excess duty factors and higher VIN. By con- constant field alternators.
power lost as heat in the solar panel. trast, operation at VIN > VMPPT re- The voltage, current, and power
Figure 1 does better than that by verses the dither phase relationship produced by any of these sources is
combining a high-efficiency (H" 95%) and A1 ramps toward higher duty fac- highly variable in response to ambient
SMPS circuit (LTC1149) with an ana- tors and lower VIN. Either way we get conditions (insolation, hydrostatic
log power-conversion optimization convergence toward VMPPT and maxi- head, or windspeed) and dramatically
loop. To understand how it works, as- mum charging current for B1. dependent on the electrical impedance
sume battery B1 is in a state of dis- This mode of operation continues as of the imposed load (V vs. I). Under
charge. In this condition, E1 will ac- B1 charges and its voltage rises to the any combination of ambient conditions,
cept all of the current the SMPS can H" 14.1-V terminal-voltage setpoint de- each of these sources is characterized
supply (subject to the H" 2.5-A current termined by the R6-R7-R8-RT net- by exactly one ideal load impedance,
116
ELECTRONIC DESIGN / SEPTEMBER 14, 1998
IDEAS FOR DESIGN
which will result in operation at VMPPT  Tip Speed Ratio (defined as turbine dither rate to 5-Hz to 0.5-Hz frequen-
and maximum power transfer. Also of peripheral velocity divided by wind cies compatible with the inertial time
benefit is the simplifying absence of speed) with the aerodynamic design of constant of mechanical power sources.
confusing local maxima in the power the turbine. For small hydroelectric In addition, wind-power applications
versus voltage curves. generators, it s the fluid dynamics of will benefit from an overspeed preven-
Of course, the actual physics behind the turbine or  runner as they relate ter. This VIN-limiting circuit is basically
the I/V curves for the various sources to the pressure and volume of the avail- just a big Zener diode connected across
are very different. In the case of photo- able water source. But the MPPT the input terminals that dumps excess
voltaics, the primary energy-producing charger really doesn t care about these power in conditions of high wind speeds
process is recombination of photoelec- details. It just blindly climbs the I/V and low battery demand. Consequently,
tric charge carriers and how the rate of curve to the VMPPT summit. it prevents overrevving of the turbine
such recombination varies with output Figure 1 s circuit can therefore be and alternator. For higher power appli-
voltage, temperature, and insolation. easily adapted to any of these systems. cations (25 W and up) or other output
For wind-power generators, the domi- The only modification necessary is a voltage ranges, consult Linear Technol-
nant parameter is the interaction of bigger C2 (0.1 µ F to 1 µ F) to slow the ogy LTC1149 application literature.
118
ELECTRONIC DESIGN / SEPTEMBER 14, 1998