Samlex America SCC-30AB - Manuel (Page 22)

22
4
PRINCIPLE OF OPERATION & FEATuREs
A solar panel is a current source that outputs almost constant current equal to its
Short Circuit Current (I
SC
) over a wide voltage range (provided incident light, cell
temperature and air mass remain constant). For example, please refer to Fig 2.2 at
page 6 which shows Current versus Voltage Curve of a 12V nominal, 42.5W solar panel
at Standard Test Conditions (Incident sunlight of 1000 W/m², cell temperature of 25°C
and Air Mass of 1.5). It will be seen that Short Circuit Current (I
SC
) of 2.7A is almost
constant over voltage range from 0V to around 15V.
In the example shown in Fig. 4.0 above, the switch is fed with constant Short Circuit
Current (I
SC
) of 2.7A of the 42.5W panel. 2 PWM output current pulses of the Switch at
frequency of 300 Hz and Duty Cycle of 25% are shown.
PWM consists of repetitive cycles of controlled duration of ON and OFF states of the
Switch. The Pulse Period of one cycle of 300 Hz PWM is the total combined duration
of ON and OFF states of the Switch which is 3.33 ms. Number of cycles of switching per
second is called the PWM Frequency. Mathematically, Frequency = (1 ÷ Pulse Period
“T”) and is 300 Hz in this case (1÷ 33.3 ms = 300 Hz). The duration of ON state is also
called the “Pulse Width”. In PWM control, the duration of the Pulse Width is varied
(modulated) and is defined by “Duty Cycle” which is the ratio of the “ON Time” to
the “Pulse Period ”. Duty Cycle is normally specified in %. Thus, 0% Duty Cycle will
mean that the switch is constantly OFF (will output 0A) and 100% Duty Cycle will mean
that the switch is constantly ON (will output the full Short Circuit Current (I
SC
) i.e. 2.7A
in the above example). For Duty Cycles > 0% and < 100%, the switch will alternate
between ON and OFF states in a controlled manner in every cycle and will output
variable current within a range of slightly higher than 0A to slightly less than the full
Short Circuit Current (I
SC
) i.e. 2.7A in the above example.
Fig. 4.0: Series Type PWM Control - PWM Frequency = 300 Hz
2.7A
A
0
PULSE PERIOD = 3.33 ms
OFF = 2.5 ms ON = 0.83 ms
AVERAGE = 0.675A
Pulse Width = ON Time
FREQUENCY = % DUTY CYCLE =
1
PULSE
PERIOD
1
3.33 ms
= 300 Hz
PULSE WIDTH
PULSE PERIOD
0.83 ms
3.33 ms = 25%
= =
SOLAR
PANEL
BATTERY
SERIES
MOSFET
SWITCH