Finding True Maximum Power Point
22 SOLAR POWER
www.linear.com
Finding True Maximum
Power Point
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. Tage Bjorklund,
Senior Applications Engineer, Power Products, Linear Technology Corp., USA
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. 80 V
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 buck-boost 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 6 V.
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.
Single-IC solar powered battery
charger solution
The LT8490 is an MPPT battery charger
controller with a long list of features
including:
䡲 integrated MPPT algorithm (no firmware
development required) reduces time to
market
Figure 1: Simplified solar powered battery charger schematic
Issue 8 2014 Power Electronics Europe
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SOLAR POWER 23
www.linear.com
Figure 2: The power
curve of a 60-cell 250
W solar panel with
entire panel
illuminated and with
a small shadow partly
covering one cell (see
Figure 3)
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. Thus
some of the built-in and configurable
charge control functions are charge voltage
temperature compensation (typically for
lead-acid batteries) using NTC sensor; over
or under battery temperature stops charge
current to protect the battery; dead battery
detection stops the charging, to avoid a
hazard; adjustable trickle charging of a
deeply discharged battery reduces risk of
damage; constant current charging that
changes to constant voltage charging as
the battery voltage reaches its final value;
reduction of charge voltage to a lower float
voltage level when the battery is fully
charged; and charging time limits can be
set when operating from a DC voltage
source.
䡲 integrated buck-boost controller allows
VIN to be above, below or equal to VBAT
䡲 supports lead-acid and lithium-ion
batteries
䡲 6–80 V VIN and 1.3–80 V 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 6 V to a maximum (cold
temperature) open circuit voltage of 80 V;
a range corresponding to 16 to 96 seriesconnected solar cells.
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 maximum power point tracking
When operating from a solar panel, the
LT8490 maintains the panel voltage at the
panel’s maximum power point (Figure 1).
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 250 W panel under two
different lighting conditions. The
maximum power point (200W) occurs at
downfall of traditional MPPT functions
found in a number of controllers, for they
follow the initial 25 V/200 W peak as it
shifts to 32 V/63 W. In contrast, the
LT8490 finds the true MPP at 16 V/128
W, yielding an additional 65 W from the
panel. It does this by measuring the entire
power curve of the panel at regular
intervals and locating the true maximum
power 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.
Figure 3: Solar panel shaded in top right corner
25 V when the panel is fully illuminated.
In partial shade (see Figure 3), the
available power at a 25 V panel voltage
drops to 50 W, with the new true
maximum power point (128 W)
appearing at 16 V. Note that the original
25 V/200 W power peak actually moves
to a local maximum ~32 V/63 W.
This dual local maximum effect is the
Conclusion
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 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.
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Issue 8 2014 Power Electronics Europe
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