Small Signal Modeling and Closed Loop Control Design of ZVS Current-fed Half-Bridge L-L Type Dc/Dc Converter with Active-Clamp

Abstract

Abstract-This I. INTRODUCTIONA high-frequency (HF) transformer isolated DC-DC converter is part of a fuel cell power conditioning system for standalone or utility interface [1][2] used to boost low fuel cell stack voltage (that can vary with fuel pressure [3]) higher than the peak of the grid or rated load voltage. This stepped up DC voltage is converted into line-frequency grid/load compatible AC power supply using an inverter stage [1][2]. Soft-switching is necessary to operate the converter at higher switching frequency to achieve small size, light weight and low cost converter. Although an active-clamped DC-DC current-fed converter and its analysis and design have been recently published [2] [4][5], this converter can not maintain ZVS for wide variation in supply voltage and load variations. Therefore, a modified L-L type active-clamped current-fed ZVS DC-DC converter, shown in Fig. 1 has been proposed for this application [6]. This converter always operates with Manuscript received September 20, 2009. This work was supported by a grant from the Natural Sciences and Engineering Research Council (NSERC) of Canada.Akshay K Rathore is with the Department of Electrical and Computer Engineering, University of Illinois, Chicago, IL 60607 USA. (phone: 312-996-9548; e-mail: arathore@ uic.edu).Ashoka K. S. Bhat is with Department of Electrical and Computer Engineering, University of Victoria, Victoria, BC V8W 3P6 Canada. (e-mail: bhat@engr.uvic.ca).Ramesh Oruganti is with the Department of Electrical and Computer Engineering, National University of Singapore, Singapore (e-mail: eleramsh@nus.edu.sg).the duty cycle of main switches greater than 50% and in continuous current mode. The two main switches S 1 and S 2 are controlled by gating signals phase-shifted by 180 o .Auxiliary switches S a1 and S a2 are gated by signals complementary to main switch gating signals. The two inductors are identical but uncoupled to each other.This front-end DC-DC converter should be controlled to produce a constant voltage at the input of the inverter for all fuel pressure values. This requires small signal modeling and closed loop control design of this converter and they are not yet reported in literature and therefore form the objectives of this paper. The peak current control requires slope compensation for stability and introduces error at high ripple currents at light load and high input voltage. Therefore, average current control of the two inductor currents is used in this paper. In literature, the current control of a single inductor and coupled inductor converter topologies are presented [7][8][9][10]. The current control of two independent (uncoupled) inductor topology was presented in [11] for active-clamped ZVS two-inductor current-fed isolated DC-DC converter. Detailed small signal analysis was presented while taking into account the effect of auxiliary clamp circuit [11] for ZVS. The series inductor current state variable was discontinuous (assuming high magnetizing inductance) and therefore, was omitted in the analysis. ...