Some Design Considerations for Coupled Inductors for Integrated Buck-Boost Converters J. Zakis, D. Vinnikov, L. Bisenieks Department of Electrical Drives and Power Electronics of Tallinn University of Technology janis.zakis@ieee.org, dmitri.vinnikov@ieee.org Abstract- This paper presents some design considerations for magnetically coupled inductors in buck-boost converters for distributed power generation. As compared to the conventional approach, the proposed integrated magnetic structure with coupled inductors enables reduction in core loss, core size, winding losses and in the number of turns. This paper discusses rough and simple design of the coupled inductor to achieve desired objectives. The methodology for the selection of inductance, core geometry, number of turns, air gap and isolation is described. In addition, general operation principles and conditions for the continuous conduction mode (CCM) and the discontinuous conduction mode (DCM) are presented. In order to verify the theoretical assumptions a prototype has been built. The experimental results are presented and discussed. I. INTRODUCTION Today, green power generation systems powered by photovoltaic (PV), wind generators and fuel cells (FC) are becoming more promising in different applications due to their increased efficiency. However, it is not only the efficiency of an energy source, but also the selection of a suitable power electronic interface that are essential. It is known that the output voltage of distributed energy sources can vary in a wide range depending on the load and externals. It means that fast, efficient and reliable buck-boost interface converters should be implemented in order to keep constant output voltage. The voltage fed quasi-Z-source inverter (qZSI) has been quoted as one of the most challenging technologies for distributed power generation systems [1-4]. The qZSI could be simply represented by the quasi-Z-source (qZS) network coupled with the pulse width modulated (PWM) inverter (Fig. 1). The qZS-network could be realised with two separate inductors (Fig. 1a) or with coupled inductors (Fig. 1b). Fig. 1. General power circuit of the qZSI: with two separate inductors (a) and with coupled inductors (b). If the voltage from the source is at the rated level, then the qZSI works in the non-shoot-through mode as a traditional VSI, but if the rated voltage of the voltage source suddenly drops, then the qZSI operates in the shoot-through mode and performs as a boost converter. The voltage boost can be achieved by introducing an additional switching state of the qZSI. It is the so-called shoot-through state [1, 2] that actually means simultaneous conduction of both semiconductor switches of one inverter leg. This operation state is forbidden in traditional VSI because it will short circuit the voltage source and DC-link capacitors. Since the PWM inverter is coupled with the qZS-network (unique connection of two capacitors C 1 and C 2 , two inductors L 1 and L 2 and diode D) the shoot-through state is allowed and is used to boost the DC-link voltage. The voltage buck-boost operation in a single stage and continuous input current is the most significant advantage of the proposed topology. This paper discusses possibilities for the optimization of the qZS-network in order to increase power density of the proposed converter. In addition, theoretical analysis of the continuous conduction mode (CCM) and discontinuous conduction mode (DCM) conditions is presented. II. GENERAL OPERATING PRINCIPLE OF QZSI Generally, in the CCM the qZSI has two types of operational states on the DC side: the non-shoot-through (i.e. the six active states and two conventional zero states of the traditional three-phase inverter) and the shoot-through state (i.e. both switches in at least one leg conduct simultaneously). To simplify the analysis the PWM inverter was replaced by a switch S (Fig. 2). When the switch S is closed, the shoot- through state occurs and the converter performs the voltage boost action. When the switch S is open, the non-shoot- through state emerges and previously stored inductive energy in turn provides the boost of voltage seen on the load terminals. Fig. 2. Simplified power circuit of the qZSI used in the analysis and experiments. The operating period T of the qZSI in the CCM consists of a shoot-through and a non-shoot-through states with durations t A and t S , respectively [5]: 1 = + = + S A S A D D T t T t , (1) where D A and D S are the duty cycles of a non-shoot-through and shoot-through states, correspondingly. Fig. 3 shows the Proceedings of the 2011 International Conference on Power Engineering, Energy and Electrical Drives Torremolinos (Málaga), Spain. May 2011 978-1-4244-9843-7/11/$26.00 ©2011 IEEE