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Power Electronic Riddles No.14 - DC - Dc converter Principle & layout
Can anyone help me in bi directional DC-DC Converter operating principle with layout, please ? It will be used between 12V(Battery) & 24V (Supercaps)..for my thesis..

Author : Bigtheik - From: Myanmar
Thu, November 3rd, 2011 - 14:26
Approximately whole modern power electronic references consist a chapter about DC-DC converter which you can refer to them.
High-frequency electronic power processors are used in dc–dc power conversion. The functions of dc–dc converters are:
• to convert a dc input voltage VS into a dc output voltage VO;
• to regulate the dc output voltage against load and line variations;
• to reduce the ac voltage ripple on the dc output voltage below the required level;
• to provide isolation between the input source and the load (isolation is not always required);
• to protect the supplied system and the input source from electromagnetic interference (EMI);
• to satisfy various international and national safety standards.
The dc–dc converters can be divided into two main types: hard-switching pulse width modulated (PWM) converters, and resonant and soft-switching converters. This chapter deals with the former type of dc–dc converters. The PWM converters have been very popular for the last three decades. They are widely used at all power levels. Topologies and properties of
PWM converters are well understood and described in literature.
Advantages of PWM converters include low component count, high efficiency, constant frequency operation, relatively simple control and commercial availability of integrated circuit controllers, and ability to achieve high conversion ratios for both step-down and step-up application. A disadvantage of PWM dc–dc converters is that PWM rectangular voltage and current waveforms cause turn-on and turn-off losses in semiconductor devices which limit practical operating frequencies to a megahertz range. Rectangular waveforms also inherently generate EMI.
The step-down dc–dc converter, commonly known as a buck converter, is shown in Fig. (a). It consists of dc input voltage source VS, controlled switch S, diode D, filter inductor L, filter capacitor C, and load resistance R. Typical waveforms in the converter are shown in Fig. (b) under assumption that the inductor current is always positive. The state of the converter in which the inductor current is never zero for any period of time is called the continuous conduction mode
(CCM). It can be seen from the circuit that when the switch S is commanded to the on state, the diode D is reverse biased. When the switch S is off, the diode conducts to support an uninterrupted current in the inductor.
The relationship among the input voltage, output voltage, and the switch duty ratio D can be derived, for instance, from the inductor voltage vL waveform. According to Faraday’s law, the inductor volt–second product over a period of steady-state operation is zero. For the buck converter

It can be seen from Eq. above that the output voltage is always smaller than the input voltage.
The dc–dc converters can operate in two distinct modes with respect to the inductor current iL. Figure (b) depicts the CCM in which the inductor current is always greater than zero. When the average value of the input current is low (high R) and/or the switching frequency f is low, the converter may enter the discontinuous conduction mode (DCM).
In the DCM, the inductor current is zero during a portion of the switching period. The CCM is preferred for high efficiency and good utilization of semiconductor switches and passive components. The DCM may be used in applications with special control requirements, since the dynamic order of the converter is reduced (the energy stored in the inductor is zero at the beginning and at the end of each switching period). 
Author : Hamid - From: Iran
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