10 | SAMLEX AMERICA INC.
SAMLEX AMERICA INC. | 11
Principle of operation of
PWM (Pulse Width Modulation)
Battery charging
A Lead Acid battery is normally charged to
full capacity using the following 3 stages
of charging (See Fig. 2.1):
•
First Stage: Constant Current bulk
Charge Stage
•
Second Stage: Constant Voltage
Absorption Stage
•
third Stage: Constant Voltage
float Stage
PWM (Pulse Width Modulation) battery
charging is the most efficient and effec-
tive method for recharging a battery in
a solar system. The Solar panel is con-
nected to the battery through a series or
parallel (called Shunt) connected MOSFET
Switch. During the Constant Current Bulk
Charge Stage,the MOSFET Switch is “on”
continuously till the next Absorption
Stage is reached. As the MOSFET switch is
in“on”condition continuously, the Solar
Panel is connected to the battery continu-
ously and it delivers a constant current
nearly equal to the Short Circuit Current
Isc. (actually equal to the current on the
V-I curve of the panel corresponding to
the battery voltage).
The subsequent Absorption and Float
Stages are constant voltage stages. The
MOSFET switch can NOT be kept in “oN”
condition continuously as it will result
in continuation of the Constant Current
Stage due to the V-I characteristic of
the Solar Panel. In order to convert the
constant current characteristic of the Solar
Panel to a constant voltage character-
istic for battery charging purposes, the
constant current from the Solar Panel is
not fed to the battery continuously but
in pulses. The MOSFET Switch turns“on”
and “off” at a fixed frequency where the
width of the “On Time” is controlled (in
SCC-1208L, the frequency is 30 Hz). This
is called PWM or Pulse Width Modula-
tion. The ratio of On Time of the MOSFET
Switch to the sum of On and Off Times is
called the Duty Cycle and is specified in
percentage. For example, if the MOSFET
Switch remains in “on” condition continu-
ously, the Duty Cycle will be 100% and if
it remains in “on” condition for half the
time out of the sum of “on” and “off”
times, the Duty Cycle will be 50%. When
the MOSFET switch is in the “on” condi-
tion, a short pulse of constant current will
be fed to the battery corresponding to
the voltage of the battery at that time.
During this pulse of constant current, the
battery voltage will rise to a higher level
if the duration of the “On Time” pulse
of the MOSFET Switch is longer and will
rise to a lower level if the duration of the
“On Time” pulse of the MOSFET Switch is
shorter.The controller checks the voltage
of the battery during the “Off Time” of
the MOSFET Switch and adjusts the next
Duty Cycle (width of the “On Time”) to
ensure that the battery voltage is kept
constant. This technique allows the cur-
rent to be effectively “tapered” as the
battery gets charged and the result is
equivalent to constant voltage charging.
for a battery entering the Absorption
Stage at around 80% charged state, the
“On Time” pulses would be very long and
almost continuous. As the charge state
approaches the end of Absorption and
beginning of the Float Stage, it may just
“tick” and send very short pulses to
the battery.
SeCtIOn 3-1 |
Supplementary Information
