0885-8993 (c) 2018 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TPEL.2018.2810884, IEEE Transactions on Power Electronics IEEE TRANSACTIONS ON POWER ELECTRONICS 1 High Step-Up Quasi-Z Source DC-DC Converter Mohammad Mehdi Haji-Esmaeili, Ebrahim Babaei, Senior Member, IEEE, and Mehran Sabahi 1 Abstract—In this paper, a high step-up Quasi-Z Source (QZS) DC-DC converter is proposed. This converter uses a hybrid switched-capacitors switched-inductor method in order to achieve high voltage gains. The proposed converter have resolved the voltage gain limitation of the basic QZS DC-DC converter while keeping its main advantages such as continuous input current and low voltage stress on capacitors. Compared to the basic converter, the duty cycle is not limited, and the voltage stress on the diodes and switch isn’t increased. In addition to these features, the proposed converter has a flexible structure, and extra stages could be added to it in order to achieve even higher voltage gains without increasing the voltage stress on devices or limiting the duty cycle. The operation principle of the converter and related relationships and waveforms are presented in the paper. Also, a comprehensive comparison between the proposed and other QZS based DC-DC converters is provided which confirms the superiority of the proposed converter. Simulations are done in PSCAD/EMTDC in order to investigate the MPPT capability of the converter. In addition, the valid performance and practicality of the converter are studied through the results obtained from the laboratory built prototype. Index Terms—DC-DC converter, High step-up, Impedance network, Quasi-Z source I. INTRODUCTION Nowadays, power electronic converters play an important role as renewable energy interface devices [1]-[3]. Also, they are widely used in other applications such as distributed generation resources, power factor correction equipment, hybrid electrical vehicles, air-space industries, and HVDC [4]- [7]. Power electronic converters are generally classified as DC-DC, AC-DC, DC-AC and AC-AC. In some applications, multi-stage power conversion is required, and simultaneous use of several different types of converters is needed. This increases the number of elements which will result in lower efficiency, higher power loss, higher possibility of failure and lower reliability of the whole system. Impedance network based converters as an emerging technology in energy conversion are invented to overcome these disadvantages [8], [9]. They have capability of single-stage power conversion, and they could overcome the limitations of classical converters. Single-stage power conversion will result in important advantages such as fewer components, lower power loss, higher efficiency, higher reliability and lower cost compared to multi-stage conversion. Various impedance network based converters were proposed in the recent years. A comprehensive review of these structures is given in [10], [11]. Impedance networks proposed in recent years can generally be classified as: 1-Transformer/coupled inductor based (TCIB), and, 2-non-transformer/coupled inductor based Manuscript received on October 8, 2017; revised on November 29, 2017; accepted on February 12, 2018. Mohammad Mehdi Haji-Esmaeili, and Mehran Sabahi are with Faculty of Electrical and Computer Engineering, University of Tabriz, Tabriz, Iran. (e-mails: mm.hajiesmaeili@gmail.com, sabahi@tabrizu.ac.ir). Ebrahim Babaei is with Faculty of Electrical and Computer Engineering, University of Tabriz, Tabriz, Iran, and also with the Engineering Faculty, Near East University, 99138 Nicosia, North Cyprus, Mersin 10, Turkey. (e-mail: e-babaei@tabrizu.ac.ir). (n-TCIB). Comparing these two structures, TCIB structures have two main advantages as higher voltage gain and electrical isolation between input and output. On the other hand, n-TCIB converters have advantages such as lower volume, lower weight, lower cost, lower voltage and current stress on their elements, lower power loss, and higher efficiency. These advantages have made them more popular. However, their main weakness is their lower voltage gain which can limit their use in high gain applications. Generally, there are four different methods which have been proposed in order to increase the voltage gain of n-TCIB converters using: 1-diode-capacitor-inductor units [12], 2-switched-inductor units [13], 3-switched-capacitor units [14], and, 4-hybrid switched-capacitors switched-inductors units [15]. Among different types of n-TCIB network based converters, quasi-Z source (QZS) network has some key advantages such as continuous input current, low voltage stress on capacitors, and, common ground between input and output of the circuit. These advantages make this impedance network suitable for a variety of applications. However, similar to other n-TCIB converters, it suffers from low voltage gain which this issue can limit its use in high gain applications. Therefore, in recent years, different structures have been proposed in order to increase its voltage gain while keeping the main features of it unchanged. Basic QZS DC-DC converter structure is shown in Fig. 1. Fig. 1. Quasi-Z source DC-DC converter [16] High-frequency transformer has been used in [17] and [18], and coupled inductors have been used in [19] and [20], in order to increase the voltage gain of QZS impedance network. But the fact is that using transformers or coupled inductors results in some disadvantages which are listed below: 1- Peak of voltage/current stress on diodes and switches will be increased. 2- Even a small value of a DC voltage will cause the magnetic core to be saturated. This will disrupt the operation of the converter. 3- Saturation of magnetic core will result in a drastic decrease in the inductance value of transformer or coupled inductors, which this will cause a severe stress on the switch. 4- Higher weight, higher volume, higher power loss, and lower efficiency of the converter. Considering the above-mentioned drawbacks, using transformer or coupled inductors is not a proper method to improve the voltage gain of QZS network. Another method which has been presented in [21]-[24] is cascading several modules of QZS network to increase the total voltage gain. But, in this method, the number of components is notably increased. This results in high power loss, low efficiency, high possibility of failure, and low