Integrated Magnetic Transformer for ZVS Phase Shift Full Bridge Converter -By Xinlan Li-GYEONGSANG NATIONAL UNVERSITY Department of Electrical Engineering

Integrated Magnetic Transformer for ZVS Phase Shift Full Bridge Converter A Dissertation submitted to the Faculty of the Graduate School of the Gyeongsang National University By Xinlan Li In partial fulfillment of the requirements for the degree of Master of Engineering July, 2008

 Abstract
 Integrated Magnetic Transformer for ZVS Phase Shift Full Bridge Converter Xinlan Li Department of Electrical Engineering Graduate School Gyeongsang National University Supervised by Professor Hwi-Beom Shin
 Integrated magnetic (IM) has been widely used in power electronics system. The design of IM transformer is considered in a zero voltage switching (ZVS) phase shifted full bridge converter. The purpose of this design is to improve the performances, like power density, packaging, size and cost of IM transformer that can be applied to high frequency converter.
 In this dissertation a new IM transformer is proposed. The transformer is located on the center leg; the output inductor is located on the outer legs with air gaps. The design equations and equivalent magnetic circuit are presented. The analysis of each timing interval is included for ZVS. For reducing the core size, EE core is redesigned. The applications of these circuit are verified on a 1.2kW prototype converter through experimental results. The performance of the proposed IM transformer is compared with conventional IM transformer.
 The efficiency of proposed IM is similar to the one of conventional IM but the size of proposed IM is reduced up to 20%. Therefore, with the proposed IM transformer, the power density can be largely increased. I. Introduction The IM transformers begin to be used in the low output voltage and high current power electronics system [1]. With the IM technique, two or more magnetic components are integrated in one magnetic structure. The EE or EI core is commonly utilized. The magnetic parts in the converter may be reduced, so that the size and power density can be improved. In some case, overall efficiency may be increased and an EMI problem may be reduced. For a PSFB converter, several IM transformers are proposed [2]. The transformer winding is typically located on the two outer legs in series connection and the inductor is wound on the center leg. The AC flux of transformer and a half of the DC flux of inductor flows in outer legs but the inductor flux only flows in the center leg. The flux density in outer legs is much higher than one in center leg because the center leg has nearly two time cross-sectional area of the outer leg. This IM transformer has a high magnetizing inductance. Since the transformer coupling is low, EMI may be high. A new IM transformer is proposed for ZVS PSFB converter in this paper. The transformer winding is located on the center leg for close magnetic coupling and the inductor is wound on two outer legs in series connection. The proposed IM transformer is analyzed electrically and magnetically. An E-core is redesigned and implemented.
 The proposed IM transformer is experimentally compared with the conventional one through a 1.2kW prototype ZVS PSFB converter and the performance is discussed.

II. ZVS PSFB converter and IM transformer
 ZVS PSFB converter is shown in Fig. 1. The center-tapped transformer
and the output inductor are usually integrated in one EE or EI core. Fig. 2(a) shows the proposed IM transformer and the circuit connection in the output stage. The transformer and output inductor are located on the center leg and two outer legs, respectively. The conventional IM transformer given in [2] is shown in Fig. 2(b).

2.1 Operation Principle 
When the PSFB converter operates in ZVS, there are 8 operating modes during each cycle as shown in Fig. 3. The ZVS transition intervals are magnified in Fig. 3 and these intervals can be neglected when analyzing the magnetic circuit. Hence, 4 modes only are considered for analysis. The duty ratio D is limited below 0.5 because of the alternating current. The magnetic states within each core leg are modeled by using the capacitive modeling method [3] ~ [5]. For simplicity, all the devices are assumed to be ideal. The permeance of core is assumed to be infinite. The operation is explained as follows.