04-03-2013, 12:04 PM
Designing And Simulating Of Microcontroller Based on PWM Solar Charge Controller
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Abstract:
In this paper, we present a design and simulation of an efficient solar charge controller. This solar charge controller works with a PWM controlled DC-DC converter for battery charging. The system is implemented using an inexpensive PIC microcontroller and simulated by using Proteus ISIS ® Professional package and the simulation results for different PV cell and battery voltage levels.
Introduction
The primary function of a charge controller in a stand-alone PV system is to maintain the battery at highest possible state of charge while protecting it from overcharge by the array and from overdischarge by the loads[1]. Although some PV systems can be effectively designed without the use of charge control, any system that has unpredictable loads, user intervention, optimized or undersized battery storage (to minimize
initial cost) typically requires a battery charge controller. The algorithm or control strategy of a battery charge controller determines the effectiveness of battery charging and PV array utilization, and ultimately
the ability of the system to meet the load demands. Additional features such as temperature compensation, alarms, meters, remote voltage sense loads and special algorithms can enhance the ability of a charge controller to maintain the health and extend the lifetime of a battery, as well as providing an indication of operational status to the system caretaker.
PIC Microcontroller
An overview
The PIC (Programmable Interface Controller) line of microcontrollers was originally developed by the semiconductor division of General Instruments Inc. The first PICs were a major improvement over existing microcontroller because they were programmable, high output current, input/output controllers built around a RISC (Reduced Instruction Set Code) architecture. The first PICs ran efficiently at one instruction per internal clock cycle, and the clock cycle was derived from the oscillator divided by 4. Early PICs could run with a high oscillator frequency of 20 MHz. This made them relatively fast for an 8-bit microcontroller, but their main feature was 20 mA of source and sink current capability on each I/O (Input/Output) pin. Typical micros of the time were advertising high I/O currents of only 1 milliampere (mA) source and 1.6 mA sink [5].
PIC16F887 Microcontroller
The PIC16F887 is one of the latest products from Microchip. It features all the components which modern microcontrollers normally have. Because of its low price, wide range of application, high quality and easy availability, it is an ideal solution in applications such as: the control of different processes in industry, machine control devices, measurement of different values etc. [6]
External oscillator modes
The external oscillator modes (see figure 1) support the usage of internal oscillator for configuring clock source. The frequency of this source is determined by quartz crystal or ceramic resonators connected to the oscillator pins. Depending on features of the component in use, select one of the following modes[5]:
LP mode (Low Power) is used for low-frequency quartz crystal only. This mode is designed to drive only 32.768 kHz crystals usually embedded in quartz watches. It is easy to recognize them by small size and specific cylindrical shape. The current consumption is the least of the three modes.
XT mode is used for intermediate-frequency quartz crystals up to 8 MHz. The current consumption is the medium of the three modes.
HS mode (High Speed) is used for high-frequency quartz crystals over 8 MHz. The current consumption is the highest of the three modes.
Analog Modules
Apart from a large number of digital I/O lines, the PIC16F887
contains 14 analog inputs. They enable the microcontroller to
recognize, not only whether a pin is driven to logic zero or one (0 or
+5V), but to precisely measure its voltage and convert it into a
numerical value, i.e. digital format. The whole procedure takes place
in the A/D converter module which has the following features:
• The converter generates a 10‐ bit binary result using the method of
successive approximation and stores the conversion results into the
ADC registers (ADRESL and ADRESH);
• There are 14 separate analog inputs;
• The A/D converter allows the conversion of an analog input signal
to a 10‐ bit binary representation of that signal; and
• By selecting voltage references Vref‐ and Vref+, the minimal
resolution or quality of conversion may be adjusted to various needs
Conclusions
The overall cost of a stand-alone PV system can be reduced with proper battery-charging control techniques, which achieve high battery state of charge and lifetime, under continuously varying atmospheric conditions, which give rise to intermittent PV energy production. In this paper, a novel battery charging regulation system has been presented, consisting of a DC/DC converter controlled by a low-cost microcontroller unit. Advantages of the proposed method are:
(a) the PWM technique employed in the control algorithm assures maximization of the energy transferred to the battery bank, and thus a better exploitation of the PV source is achieved.
(b) the battery lifetime is increased because the battery is operating at a higher state of charge.