80V Buck-Boost Lead-Acid and Lithium Battery Charging Controller Actively Finds True Maximum Power Point in Solar Power Applications

design features
80V Buck-Boost Lead-Acid and Lithium Battery Charging
Controller Actively Finds True Maximum Power Point in
Solar Power Applications
Tage Bjorklund
In solar power systems, the
bulk of the expense is in the
panel and batteries. Any
cost-effective solar power
solution maximizes the
capacity usage and lifetime
of these components.
For instance, a high
quality charger increases
battery run time, reducing
capacity requirements, and
extends battery lifetime,
minimizing maintenance
and replacement costs.
Likewise, using a DC/DC
controller that extracts the
maximum available energy
from the solar panel
reduces the size and cost
of the panels required.
The LT8490 is a charge controller for lead
acid and lithium batteries that can be
powered by a solar panel or a DC voltage
source. It includes true maximum power
point tracking (MPPT) for solar panels
and optimized built-in battery charging
algorithms for various battery types—no
firmware development required. 80V input
and output ratings enable the LT8490 to
be used with panels containing up to 96
cells in series. The power stage uses four
external N-channel MOSFETs and a single
inductor in a buck-boost configuration.
Unlike most charge controllers, the buckboost configuration allows the charger to
operate efficiently with panel voltages that
are below, above or equal to the battery
voltage. The minimum panel voltage is 6V.
Batteries live longer and run longer when
the charge algorithm is optimized for the
battery type. Likewise, a high performing MPPT charger, which tracks the solar
panel maximum power point during
partial shade conditions, allows the use
of a smaller and lower cost solar panel.
Creating a discrete-component charger
solution to perform all of these duties
would be costly and time consuming,
typically requiring a microcontroller, a
high performance switching regulator and
a lengthy firmware development cycle.
GATEVCC´
GATEVCC´
SOLAR PANEL
LOAD
TG1
BOOST1 SW1 BG1 CSP CSN
BG2 SW2 BOOST2 TG2
VBAT
CSPOUT
CSNOUT
EXTVCC
CSNIN
CSPIN
VIN
LT8490
GATEVCC´
AVDD
TEMPSENSE
+
–
RECHARGABLE
BATTERY
THERMISTOR
GATEVCC
INTVCC
STATUS
AVDD
FAULT
GND
AVDD
Figure 1. Simplified solar powered
battery charger schematic
July 2014 : LT Journal of Analog Innovation | 21
Dual local maximums are the downfall of the conventional
MPPT found in a number of controllers. In contrast, the
LT8490 finds the true MPP, yielding twice the charge
power—or even higher in other shade conditions.
250
FULL ILLUMINATION
PPANEL (W)
200
150
LOCAL
MPP
50
0
Since the power stage is external, it
can be optimized for the application.
Charge current limits (and input current limit when a DC voltage source is
used) can be configured as needed.
TRUE MPP
100
True Maximum Power Point Tracking
PARTIAL SHADE
0
10
20
voltage of 80V; a range corresponding
to 16 to 96 series-connected solar cells.
30
40
VPANEL (V)
Figure 2. The power curve of a 60-cell 250W solar
panel with entire panel illuminated and with a small
shadow partly covering one cell (Figure 3)
COMPLETE SINGLE-IC SOLAR
POWERED BATTERY CHARGER
SOLUTION
The LT8490 is an MPPT battery
charger controller with a long list
of features including:
•integrated MPPT algorithm (no firmware
development required) greatly reduces
time to market
•integrated buck-boost controller allows
VIN to be above, below or equal to VBAT
•supports lead-acid and lithium-ion
batteries
•6V–80V VIN and 1.3V–80V VBAT
The LT8490 can be powered by a solar
panel or any DC voltage source. For a
particular battery voltage, a wide range
of solar panel types can be used, as the
panel voltage can be lower or higher
than the battery voltage. The LT8490
accepts panel inputs from 6V to a maximum (cold temperature) open circuit
22 | July 2014 : LT Journal of Analog Innovation
When operating from a solar panel,
the LT8490 maintains the panel voltage
at the panel’s maximum power point.
Even during partial shade conditions,
when more than one local maximum
power point appears (an effect of bypass
diodes inside the solar panel), the LT8490
detects and tracks the true maximum.
Figure 2 shows the P-V curves for a common 60-cell 250W panel under two different lighting conditions. The maximum
power point (200W) occurs at 25V when
the panel is fully illuminated. In partial
shade (see Figure 3), the available power
at a 25V panel voltage drops to 50W, with
the new true maximum power point
(128W) appearing at 16V. Note that the
original 25V/200W power peak actually
moves to a local maximum ~32V/63W.
This dual local maximum effect is the
downfall of traditional MPPT functions
found in a number of controllers, for they
follow the initial 25V/200W peak as it
shifts to 32V/63W. In contrast, the LT8490
finds the true MPP at 16V/128W, yielding an additional 65W from the panel. It
does this by measuring the entire power
curve of the panel at regular intervals
and locating the true maximum power
Figure 3. The solar panel shaded in top right corner
peak at which to operate. In this case,
more than twice as much charge power
is extracted, with even greater gains
possible in other shade conditions.
Charge Control Functions
Charge algorithms can be configured
according to the requirements of each
application by adjusting the voltage on
two configuration pins. Lead-acid batteries
built with AGM, gel and wet cell technologies require slightly different charge
voltages for best lifetime, and Li-ion
and LiFePO4 cells have charge requirements that are different from lead-acid
batteries. Some of the built-in and configurable charge control functions are:
design features
½W
7mΩ
+
CIN3
2.2µF
×2
CIN2
2.2µF
×2
SOLAR
PANEL
VOC < 53V
–
10Ω
8.06k
110k GATEVCC ´
4Ω
3.24k
5.49k
2Ω
CSN
EXTVCC
CSPOUT
CSNOUT
3.01k
21k
100nF
LT8490
4.7nF
SYNC
8.45k
11.5k
4.7µF
+
FLOODED
LEAD
ACID
10k
AT 25°C
ß = 3380
NTC
100nF
SWEN
SWENO
CLKDET CLKOUT CHARGECFG2 STATUS FAULT CHARGECFG1
AVDD
1.3k
13k
AVDD
3.32k
68nF
10nF
DS
470pF
14.27V STAGE 2 (ABSORPTION) CHARGE VOLTAGE (VS2) AT 25°C
13.87V STAGE 3 (FLOAT) CHARGE VOLTAGE (VS3) AT 25°C
10A CHARGING CURRENT LIMIT
2.5A TRICKLE CURRENT LIMIT
7.2A INPUT CURRENT LIMIT
53V MAXIMUM PANEL VOLTAGE (VMAX)
NO TIMER LIMITS
TEMPERATURE COMPENSATION ENABLED
–20°C TO 50°C BATTERY TEMPERATURE RANGE
175kHz SWITCHING FREQUENCY
EXAMPLE SOLAR PANEL: SHARP NT-175UC1 175W
200k
90.9k
DF
549Ω
549Ω
M1, M2: INFINEON BSC028N06NS
M3, M4: INFINEON BSC042N03LSG
L1: 15µH COILCRAFT SER2915H-153KL
DB1, DB2: CENTRAL SEMI CMMR1U-02
CIN1: 33µF, 63V, SUNCON 63HVH33M
CIN2, CIN3, CIN4: 2.2µF, 100V, AVX 12101C225KAT2A
COUT1: 150µF, 35V NICHICON UPJ151MPD6TD
COUT2, COUT3: 10µF, 35V, MURATA GRM32ER7YA106KA12
COUT4: 1µF, 25V AVX 12063C105KAT2A
•charge voltage temperature
compensation (typically for lead-acid
batteries) using NTC sensor
•reduction of charge voltage to a lower
float voltage level when the battery is
fully charged
•over or under battery temperature stops
charge current to protect the battery
•charging time limits can be set when
operating from a DC voltage source
•dead battery detection stops the
charging, to avoid a hazard
CONCLUSION
•constant current charging that changes
to constant voltage charging as the
battery voltage reaches its final value
1µF
–
ECON
53.6k
•adjustable trickle charging of a deeply
discharged battery reduces risk of
damage
124k
23.2k
VDD
LDO33
SRVO_IIN
SRVO_FBIN
SRVO_FBOUT
SRVO_IOUT
IOR
IMON_OUT
VC
LOAD
0.082µF
26.1k
TEMPSENSE
AVDD
IOW
97.6k
8.2nF
274k
FBOR
FBOUT
FBOW
RT
SS
IIR
IMON_IN
32.4k
470nF
BOOST2 TG2
1.05k
249k
COUT4
1µF
220nF
GND BG2 SW2
COUT1
150µF
10Ω
DB2
MODE
93.1k
1µF
5mΩ
INTVCC
SHDN
VINR
FBIR
FBIN
FBIW
COUT2
10µF
×2
GATEVCC ´
3.3nF
TG1 BOOST1 SW1 BG1 CSP
CSNIN
CSPIN
VIN
GATEVCC
4.7µF
×2
35.7k
220nF
2Ω
CIN4
2.2µF
M3
10Ω
3.3nF
VBAT
COUT3
10µF
×2
10Ω
DB1
1W
5mΩ
M4
M2
GATEVCC ´
CIN1
33µF
×3
470nF
196k
L1
15µH
M1
Figure 4. Complete solar power system with
lead-acid battery charging/control
an inductor, allowing the charger to operate with VIN above, below or equal to the
battery voltage. All necessary functions are
included, with built-in battery charging
algorithms and MPPT control, requiring no
firmware development. n
The LT8490 is a full-featured true
MPPT charge controller that can operate
from a solar panel or a DC voltage source
with a voltage range from 6V to 80V,
charging lead-acid or lithium batteries
from 1.3V to 80V. The power stage is easily
configured by selecting four MOSFETs and
July 2014 : LT Journal of Analog Innovation | 23