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Energy Conversion Technology (H/W)

Full-bridge dc/dc converters are extensively used in medium to high power level applications. For most of these applications, particularly those of the computer and telecommunication systems, the most desirable features of the converter are high efficiency, high power density, high reliability and low EMI.

Unfortunately, the standard full bridge topology operates in hard switching, and the hard switching converter is unable to achieve high efficiency and high power density, because of the following reasons. In order to achieve high power density, the switching frequency is normally increased. At increased frequency, the converter can employ smaller sized power magnetics and capacitors. However, as the switching frequency increases, the switching losses associated with the turning on and off of switches will become excessive. These losses remarkably reduce the converter's overall efficiency, and high power density can hardly be achievable due to the resultant high cooling requirements.

To solve these problems, this project is aiming at use of soft switching techniques. The experimental work in this project will center around two types of the soft switching techniques: zero voltage switching (ZVS) and zero current switching (ZCS). Either of the techniques can greatly reduce and even completely eliminate the switching losses in a converter, and then high efficiency will be obtained. However, preliminary experimentation has shown that ZVS is more advantageous for a MOSFET switch topology than is ZCS.

In the present project, high power level full bridge converters will use IGBT switches, due to IGBT's low conduction losses and high power capability. However, IGBT is not as fast as MOSFET, and its switching frequency is hardly above 100 kHz. Unlike IGBT, MOSFET is a resistive device when it is turned-on, and the conduction losses are higher as compared to IGBT. However, MOSFET is the fastest device and is able to be operated above 1 MHz.

The project's approach will be to operate the converter at a higher switching frequency to achieve high power density. Thus, for power levels below 3 kW, the full-bridge converters will be tried out with MOSFET switches. ZVS is proposed to be used to boost the overall efficiency and alleviate the thermal problems.