192 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 58, NO. 1, JANUARY 2011 Quasi-Z-Source-Based Isolated DC/DC Converters for Distributed Power Generation Dmitri Vinnikov, Member, IEEE, and Indrek Roasto Abstract—This paper presents new step-up dc/dc converter topologies intended for distributed power generation systems. The topologies contain a voltage-fed quasi-Z-source inverter with con- tinuous input current on the primary side, a single-phase isolation transformer, and a voltage doubler rectifier (VDR). To increase the power density of the converter, a three-phase auxiliary ac link (a three-phase inverter and a three-phase isolation transformer) and a three-phase VDR are proposed to be implemented. This pa- per describes the operation principles of the proposed topologies and analyzes the theoretical and experimental results. Index Terms—DC–DC power conversion, fuel cells (FCs), pulsewidth-modulated power converters, rectifiers. I. I NTRODUCTION D ISTRIBUTED power generation, when fully imple- mented, can provide reliable, high-quality, and low-cost electric power. As a modular electric power generation close to the end user, it offers savings in the cost of grid expansion and line losses. If connected to the power grid, the bidirectional transactions between the grid and the local generation result in grid capacity enhancement, virtually uninterrupted power supply, and optimum energy cost due to the availability of use/purchase/sales options [1]. Distributed power is a concept that covers a wide spectrum of schemes used for local electric power generation from renew- able and nonrenewable sources of energy in an environmentally responsible way. Basic schemes are mainly based on solar energy, wind energy, fuel cells (FCs), and microturbines. An FC is potentially the most efficient modern approach to distributed power generation. The efficiency of the conversion, i.e., the ratio of the electrical output to the heat content of the fuel, could be as high as 65%–70% [1]. In fact, its electrical ef- ficiency could be greater than 70% in theory. Current technolo- gies have only been capable of reaching efficiencies of around 45%. Combined cycles are intended to raise electrical efficiency up to 60% for plants based on high-temperature cells [2]. To interconnect a low-dc-voltage-producing FC (typically 40–80 Vdc) to residential loads (typically 230-Vac single phase or 3 × 400 Vac), a special voltage matching converter is required. A typical structure of a two-stage interface converter is shown in Fig. 1. Due to safety and dynamic performance Manuscript received July 8, 2009; revised October 12, 2009; accepted October 28, 2009. Date of publication February 8, 2010; date of current version December 10, 2010. The authors are with the Department of Electrical Drives and Power Elec- tronics, Tallinn University of Technology, 19086 Tallinn, Estonia (e-mail: dm.vin@ mail.ee; indrek.roasto@ttu.ee). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TIE.2009.2039460 requirements, the interface converter should be realized within the dc/dc/ac concept. This means that low voltage from the FC first passes through the front-end step-up dc/dc converter with the galvanic isolation; subsequently, the output dc voltage is inverted in the three-phase inverter and filtered to comply with the imposed standards and requirements (second dc/ac stage). The design of the front-end isolated dc/dc converter is most challenging because this stage is the main contributor of in- terface converter efficiency, weight, and overall dimensions. The low voltage provided by the FC is always associated with high currents in the primary part of the dc/dc converter (switching transistors and primary winding of the isolation transformer). These high currents lead to high conduction and switching losses in the semiconductors and therefore reduce the efficiency. Moreover, the large voltage boost factor requirement presents a unique challenge to the dc/dc converter design [2]. This specific requirement could be fulfilled in different ways: by use of an auxiliary boost converter before the isolated dc/dc converter [3]–[7] or by use of an isolation transformer with a large turns ratio [8]–[14] for effective voltage step-up. In the first case [Fig. 2(a)], the auxiliary boost converter steps up the varying FC voltage to a certain constant voltage level (80–100 Vdc) and supplies the input terminals of the isolated dc/dc converter. In that case, the primary inverter within the dc/dc converter operates with a near-constant duty cycle, thus ensuring better utilization of an isolation transformer. More- over, due to preboosted input voltage, the isolation transformer has the moderate turns ratio (1:7–1:8), which exerts a positive impact in terms of leakage inductance and efficiency. A very interesting solution is proposed in [5], where the conventional inductor in an auxiliary boost converter is replaced with a zero- ripple filter (ZRF). The ZRF comprises a coupled inductor- based filter for minimizing the high-frequency switching ripple and an active power filter for mitigating the low-frequency ripple. Despite evident advantages of the isolated dc/dc con- verter with an auxiliary boost converter, its main drawbacks are drawn from the multistage energy conversion structure, i.e., complicated control and protection algorithms and reduced reliability due to the increased number of switching devices. A direct step-up dc/dc converter without input voltage pre- regulation [Fig. 2(b)] is simpler in control and protection. Due to the reduced number of switching devices, the converter tends to have better efficiency and reliability. The varying voltage from the FC passes through the high-frequency inverter to the step-up isolation transformer. The magnitude of the primary winding voltage is controlled by the duty cycle variation of inverter switches in accordance with the FC output voltage and converter load conditions. The isolation transformer should 0278-0046/$26.00 © 2011 IEEE