SamplexPower MSK-10A, Руководство пользователя

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12 | SAMLEX AMERICA INC. SAMLEX AMERICA INC. | 13
SECTION 4 | Operation
4.1 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.
4.2 PWM EXPLANATION
The output of the Solar Panel(s)/ Array 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 Solar 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 average 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 Solar 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.
4.3 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, Float and Equalization Charging Stages for different battery types. Battery
can, thus, be charged at the specified Voltage Regulation Set Points by PWM of the charging cur-
rent 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
difference 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 voltage is compensated
by raising the average current by raising the Duty Cycle. These fast updates on battery voltage
measurements and Duty Cycle corrections ensure charging of the battery at the specified Voltage
Regulation Set Point with minimum voltage deviation.