22-10-2012, 12:17 PM
COUPLED -INDUCT OR AND VOLTAGE -DOUBLER CIRCUIT FOR HIGH STEP - UP DC-DC CONVERTER
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INTRODUCTION
BACKGROUND
A DC-DC converter converts directly from dc to dc and is simply known as a DC converter. A dc converter can be considered as dc equivalent to an ac transformer with a continuously variable turn’s ratio. Like a transformer, it can be used to step down or step up a dc voltage source. Better efficiency, reduced size, and lower costs have combined to make the Switching regulator a viable method for converting unfiltered DC input voltages into regulated DC outputs.
Dc converters are widely used for traction motor control in electric automobiles, trolley cars, marine hoists and mine haulers. They provide smooth acceleration control, high efficiency and fast dynamic response. Dc converters can be used in regenerative braking of dc motors to return energy back into the supply and this feature results in energy savings for transportation systems with frequent stops. Dc converters are used in dc voltage regulators; and also used in conjunction with an inductor to generate a dc current source, especially for the current source inverter.
A boost converter is generally used and it has several advantages such as simple structure, continuous input current, and clamped switch voltage stress to the output voltage. However, it is very difficult to satisfy both high voltage conversion ratio and high efficiency at once. This is primarily due to the parasitic resistances, which cause serious degradation in the step-up ratio and efficiency as the operating duty increases. Moreover, in high output voltage applications, a high voltage rating diode causes a severe reverse recovery problem and it requires a snubber. As a result, a general boost converter would not be acceptable for high step-up applications.
POWER ELECTRONIC CONVERTER
INTRODUCTION
Power electronics has applications that span the whole field of electrical power systems, with the power range of these applications extending from a few VA/Watts to several MVA / MW.The main task of power electronics is to control and convert electrical power from one form to another. "Electronic power converter" is the term that is used to refer to a power electronic circuit that converts voltage and current from one form to another.
The great strides taken in the industrial applications of power electronics during recent years have demonstrated that this versatile tool can be of great importance in increasing production, efficiency and control. Power electronic circuits are also called as power converters. A converter uses a matrix of power semiconductor switches to convert electrical power at high efficiency. The converter system comprised of switches, reactive components L, C and transformers. Switches include two terminal devices such as diodes and three terminal devices such as transistors or thyristors.
BOOST CONVERTER
The DC/DC converters are widely used in regulated switch mode DC power supplies. The input of these converters is an unregulated DC voltage, which is obtained by PV array and therefore it will be fluctuated due to changes in radiation and temperature. In these converters the average DC output voltage must be controlled to be equated to the desired value although the input voltage is changing. From the energy point of view, output voltage regulation in the DC/DC converter is achieved by constantly adjusting the amount of energy absorbed from the source and that injected into the load, which is in turn controlled by the relative durations of the absorption and injection intervals. These two basic processes of energy absorption and injection constitute a switching cycle. Intuitively speaking, if the energy storage capacity of the converter is too small or the switching period is relatively too long, then the converter would have transmitted all the stored energy to the load before the next cycle begins. This introduces an Idling period immediately following the injection interval during which the converter is not performing any specific task. The converter can therefore operate in two different modes depending upon its energy storage capacity and the relative length of the switching period. These two modes are known as the discontinuous conduction and continuous modes.
Basic Principle of Boost Converter
The boost is a popular non-isolated power stage topology, sometimes called a step-up power stage. Power supply designers choose the boost power stage because the required output is always higher than the input voltage. The input current for a boost power stage is continuous, or non-pulsating, because the output diode conducts only during a portion of the switching cycle. The output capacitor supplies the entire load current for the rest of the switching cycle.
Figure 1 shows a simplified schematic of the boost power stage. Inductor L and capacitor C make up the effective output filter. Resistor R represents the load seen by the power supply output.