Holdup time improvement methods for DC-DC switching mode power supplies

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Holdup time improvement methods for DC-DC switching mode power supplies


Author: Ng, Weng-fai
Title: Holdup time improvement methods for DC-DC switching mode power supplies
Degree: M.Sc.
Year: 2007
Subject: Hong Kong Polytechnic University -- Dissertations.
DC-to-DC converters.
Switching power supplies.
Power electronics.
Department: Dept. of Electrical Engineering
Pages: 122 leaves : ill. ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2199000
URI: http://theses.lib.polyu.edu.hk/handle/200/2934
Abstract: For 2 typical forward topologies - 2-FET forward and catching winding forward, the duty cycle is always limited to 50% or has other serious drawbacks (i.e. the voltage stresses on the drain of power mosfet and secondary rectifiers are higher; using higher voltage rating parts will increase the cost and reduce the efficiency). The 50% maximum duty cycle is a constraint for power supply to use the stored energy in the bulk capacitor for hold up time. If one wants to have longer holdup time and maintain the same efficiency, it needs to increase the cost for semiconductor parts or bigger bulk capacitor. Bigger bulk capacitor consumes space and this is a barrier for power supply to achieve higher power density. Three circuits are introduced to achieve longer holdup time without increasing much in cost and space or using smaller bulk capacitor to save money or space. The concept is to break the constraint of duty cycle with mode switching. The power supply has 2 modes, normal mode and holdup mode. When the input power is normal, it is normal mode. The operation is the same as conventional forward topologies. During the input power failure, the bulk voltage is dropping, when it drops to the defined voltage level, the power supply switches to holdup mode. At holdup mode, the maximum duty cycle is allowed to more than 50%. Because the bulk voltage is lower so the voltage stresses on power devices are still below their ratings. Because the power supply can operate at higher than 50% duty cycle therefore it can achieve longer holdup time. For 2-Fet topology, some US patents declared the similar concept, but it has different snubber reset circuit; besides, that patent did not specify the method to implement the concept. In this research, besides the power circuits, 2 control circuits are built and verified. The circuits are shown in the section of each new circuit. In order to increase the duty cycle while keeping the frequency unchanged in holdup mode, the control circuit uses 2pcs PNP transistors as switches to control the duty cycle and frequency separately. For the 2 new circuits in 2-FET forward and catching winding forward topologies, the mode switch mosfets are at the locations of high voltage. The control circuit needs to have a gate drive transformer to turn on and off the remote mosfet switch. The control circuit and switch mosfet need to act fast but consumes little power. Two control circuits are proposed. In three new circuits, the improvement on holdup time for the new flyback output choke design is the lowest. The reason is studied in this report. It provides a room for further improvement, i.e. using faster control circuit to reduce the turn on time loss.

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