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Title: | Power distribution and efficiency analysis of quasi-resonant converters using regulated unified model |

Authors: | Choi, Che-tat |

Degree: | M.Phil. |

Issue Date: | 2005 |

Abstract: | This thesis analyzes the power distribution of the unified model in quasi-resonant converters. The major contribution is that the internal equivalent resistance of each component is taken into consideration for analysis so that the current and voltage waveforms can be found accurately. The switching waveforms of this near-practical model will be derived. However, an equation, which represents the resonant stage, cannot be written in explicit form. In order to achieve zero voltage or zero current switching condition, the equations must be solved numerically. By using the averaging techniques, the regulated large signal model can be predicted under different supply voltage and load current. In other words, the variation of switching frequency, which is a controller parameter, can be determined while output voltage is kept constant. Another contribution is to determine the switching frequency in order to regulate the output voltage under varying the supply voltage or load current. The results are very useful for predicting the performance of the quasi-resonant converters by using large signal models. The power distribution in the ideal frequency-modulated zero-voltage switching quasi-resonant switch (FM ZVS QRSW) is obtained. The internal resistance of each component of the unified quasi-resonant switch model (QRSW) is taken into consideration. The power dissipation is analyzed so that the efficiency of the QRSW can be estimated. Moreover, the conduction loss of each component can be found and the maximum theoretical efficiency can be predicted by using the large signal model. In order to achieve high efficiency, finding the critical components in power dissipation is shown. The large signal models of Buck, Boost, Buck-Boost, Cuk, Sepic and Zeta QRC are analyzed in this thesis. The output load is assumed to be resistive only. The loading current can be varied according to the analysis. The regulated model means that the output load voltage will keep constant under regulation even though the operating conditions or loading current are changed at any time. The system is assumed to be in steady state. Only the active power is considered in the power distribution analysis because the active power dissipation is the major component and there is no power transformer in the models. Only the equivalent resistances of the components will be considered. The parasitic inductance in the capacitors and parasitic capacitance in the inductors will be neglected. The transient value, transient analysis and stability are not in the scope of analysis. The state-space averaging techniques are used to derive the system equations in each switching stage. There are two state variables: VCr(t) and ICr(t) in the system equations. Because the large signal model is derived, only the steady state value or averaging value is used. A mean value is used to represent a varying value over a time period or a switching cycle. The transient value, transient analysis and stability are not considered when calculating the state variables. The MATLAB simulation tools [6] are used to implement the derived system equations by using numerical methods to find the numerical switching frequency in order to regulate the output voltage even thought the operating conditions say output current and supply voltage are changed. Having determined the system parameters, the switching frequency, current in each voltage node and voltage across each current branch can be found. The results are represented by numerical values. In order to present the numerical results easily, 2-D and 3-D diagrams are used to describe the trend and shape of the findings. The power distribution and power loss in each circuit component can be predicted under different operating conditions because the current passing through the voltage nodes and the voltage arcoss the current branches are calculated by using the derived system equations. They are presented in 3-D diagrams, so view angle of the diagrams can be rotated in the simulation tools. Moreover, by comparing the input and output power, the efficiency of the regulated large signal model can be represented in a 3-D diagram. The independent variables are supply voltage and load current respectively. |

Subjects: | Hong Kong Polytechnic University -- Dissertations Power electronics Electric power supplies to apparatus Electric current converters |

Pages: | xi, 156 leaves : ill. (some col.) ; 30 cm |

Appears in Collections: | Thesis |

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