Samlex America MSK-10A - Manual (Page 13)

12 | SAMLEX AMERICA INC. SAMLEX AMERICA INC. | 13
SeCtIOn 4 | Operation
PRinciPLe oF oPeRAtion oF SoLAR cHARGinG WitH SeRieS tYPe
PulSe wIDTH moDulATIoN (Pwm) CoNTRol
The design and operation of MSK-10A is based on Series Type PWM (Pulse Width
Modulation) control at PWM frequency of 25 Hz.
PWM exPLAnAtion
The output of the PV Panel(s) is connected to the battery in series with a Mosfet Switch
inside the controller. A Micro-controller controls the oN / off operation of the Mosfet
Switch to control the charging current and consequently, the State of Charge of the
battery.
A PV Panel is a current source that outputs constant current equal to its Short Circuit
Current (Isc) over a wide voltage range (provided Irradiance Level, Spectrum and Cell
Temperature remain constant). for example, at STC, a typical 12V nominal, 45W PV
Panel may provide constant Short Circuit Current (Isc) of around 3A over voltage range
from 0V to around 15V.
PWM consists of repetitive cycles of controlled duration of oN and off states of the
Series Connected Mosfet Switch inside the controller. The sum of oN and off times of
one cycle is called the Pulse Period. In PWM control, the duration of the Pulse Width
(oN time) 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 and will output the full instantaneous
Short Circuit Current “Isc” of the panel. 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 0A to the full Short Circuit Current Isc.
Thus, through PWM control, the Mosfet Switch inside the controller converts constant
Short Circuit Current (Isc) of the PV Panel(s) to controlled average charging current at
its output by varying the Duty Cycle. The average value of the charging current is equal
to the instantaneous input value of Short Circuit Current (Isc) of the panel multiplied by
the Duty Cycle.
PWM cHARGinG in MSk-10A
battery charging is a current based process. Current fed to the battery results in re-
charging of the cells and consequent rise in battery voltage. Controlling the current will
control battery voltage. for 100% return of capacity, and for prevention of excessive
gassing and sulfation, the battery charging voltage is required to be controlled at the
specified Voltage regulation Set Points for Absorption (boost), float and equalization
Charging Stages for different battery types. battery can, thus, be charged at the speci-
fied Voltage regulation Set Points by PWM of the charging current through control of
Duty Cycle as explained above. The controller checks the battery voltage and updates
the Duty Cycle regularly at a very fast rate. The Duty Cycle is proportional to the differ-
ence between the sensed battery voltage and the Voltage regulation Set Point. once
the specified Voltage regulation Set Point is reached, it is kept steady - rise in voltage
is compensated by reducing the average current by reducing the Duty Cycle and fall in