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The International Conference on Renewable Energies “ICRE-2010” April 5 – 8, 2010 Damascus, Syria.
Design of High Efficiency DC-DC Converter for Photovoltaic Solar Home ApplicationsDiary R. Sulaiman Hilmi F. Amin Ismail K. Said Department of Electrical Engineering, College of Engineering, University of Salahaddin-Hawler, Iraq
Topics- Introduction to DC-DC converters- Analysis of the Buck-Boost Converters- Design of the Buck-Boost Converters - The converter control loop- Simulation Results- Conclusion- References
- The solar energy conversion system is very interesting alternative on supplement the electric system generation, due to the persistent cost reduction of the overall system and cleaner power generation.- Buck-boost converters make it possible to efficiently convert a DC voltage to either a lower or higher voltage.
- This paper analyzes and describes step by step the process of designing, and simulation of high efficiency low ripple voltage buck-boost DC-DC converter for the photovoltaic solar conversion system applicable to a (typical) single family home based on battery-based systems. The input voltage can typically change from (20V) initially, down to (5V), and provide a regulated voltage within the range of the battery (12V). The simulation results provide strong evidences about the high efficiency, minimum ripple voltage, high accuracy, and the usefulness of the system of the proposed converter when applied to either residential or solar home applications.
The Solar cell, module and array
The basic block diagram of the solar energy
-The solar energy conversion systems can be connecting to a large electrical transmission grid, or to the storage or auxiliary energy supply. If the photovoltaic route is chosen, extra electricity may be stored, usually in storage batteries, thereby extending the operating time of the system, the (typical 12V) storage batteries are ordinary used in the home solar conversion systems to satisfy its operation and maximize power tracking purpose.
-The converter and switching topology of the proposed buck boost DC-DC converter would result in higher efficiency, lower ripple voltage, and significant increase in the overall available power even in a sun lighting condition. Although a small amount of power is generated, given enough time, a battery will reach its full charge.
The basic schematic structure of the buck-boost converter, and the two operation states switch on and switch off are shown:
Current and voltage waveforms of the buck-boost converter are shown below:
Under the steady state operation of the converter in the CCM, the analytical expression of (Vo/Vg), (iL), and (Vo) can be obtained. Equating the integral to zero of the integral voltage over one time period yields [12,13],
For the buck-boost converter,
The inductor current at the beginning of the cycle is zero, its maximum value at (t=DT) is
And, during the off period (iL) falls to zero at the end of the off-state, and then, the load current (Io) equal to average diode current (ID,av), and the diode current equal to the inductor current during the off-state, therefore, (Io) is equal to,
Both (Vo/Vg), and (Io/Ig) are equal to
Therefore, the output voltage gain (Vo/Vg) in the continuous or CCM mode depends on the duty cycle (D) only, but in the DCM mode depends on the duty cycle (D), inductor value (L), input voltage (Vg), and the output current (Io) .
When MOSFET switch is on the voltage across the inductor is equal to (Vg) and when diode switch is on it is equal to (Vo). Then the duty ratio can be related to (Vo) by equation 3. The input and output currents are determined by the switching states of equation 4, [IoD=Ig(1-D)]. Theoretically, if (D=0), the output is zero, if (D=1) the output is infinity, and if (D=0.5), then the output is equal to the input voltage .
To find the switching frequency (f) for which (L>Lcr), the (Io) values, the time period, and (D) must be known, because the value of (Lcr) is dependant upon them. To determine (Io), the uses of analytical equations are possible . For a typical (12V) batteries, and (1kW) home devices shown in table 1, the output current of the converter will be (83.34A). Then the calculated results of duty ratio (D) and inductor current (IL=Io/1-D) of table 2 are obtained.
To make the regulator operate in continuous mode and the design will have a good load transient response with an acceptable output ripple voltage, and according to the simulation results, the (iL) considered to be (≤3%), so we can select its value equal to (3%)
The switching frequency of high efficiency buck-boost converters applicable in solar systems typically will be between (20kHz-100kHz) . Choosing the minimum and maximum input voltage values for only these two frequencies to determine the inductor value is shown in table 3.
The minimum or critical capacitor value (Ccr) for a desired output ripple and load current/voltage is
Then, the critical value of the capacitor (Ccr) operating on the frequency (46kHz) and minimum/maximum input voltages (5-20V) regarding the output ripple voltage (vo50mV), using (vo=40mV) is calculated as shown in table 4
The possible capacitance value for our design should be (C>Ccr), So, we can choose (C=1000F). The complete design parameters values of the proposed DC-DC converter are shown in table 5.
Digital PWM (DPWM) controllers can offer a number of advantages over analog controllers, including flexibility, lower sensitivity, high frequency switching, and programmability without external components
block schematic of DPWM, (b) The control signals
The simulations have demonstrated that, the design can achieve (88.20%) efficiency at (5V) input, scaled up to (96.55%) efficiency at (20V) input, producing acceptable ripple voltage (<11mV) for the inputs (5-20V), all at (46kHz) switching frequency under a (83.34A).
The advantages of this design are: the ability to choose the constant output voltage and current, the procedure is simple, the converter and controller has a simple structure, improved efficiency – up to (96.55%), reduced output ripple voltage-less than (11mV), the complete converter circuit is small and inexpensive, and finally, the designed converter circuit topology operates effectively for different input and output operating conditions.
It is, therefore, feasible for common solar DC-DC conversion applications.
This design procedure in principle opens the possibility to additional work in converter design and modeling, and could allow further improvements in efficiency, ripple, and usable power range. Other control schemes are also possible, and it could provide a way for controlling other converter topologies.
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