2234 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 25, NO. 9, SEPTEMBER 2010 Letters A Novel Integrated Single-Phase Inverter With Auxiliary Step-Up Circuit for Low-Voltage Alternative Energy Source Applications Ching-Tsai Pan, Member, IEEE, Ching-Ming Lai, Member, IEEE, and Ming-Chieh Cheng Abstract—This paper presents a novel integrated single-phase inverter with both high step-up ratio and buck-boost capabilities for low-voltage alternative energy source applications. An auxiliary step-up circuit is integrated with an isolated ´ Cuk-derived voltage- source inverter to achieve a much higher voltage-conversion ratio while avoiding using extreme duty ratios in both dc-side and ac-side switches. In addition, the proposed circuit possesses the automatic energy-transfer characteristic between parallel charging and se- ries discharging of two capacitors to achieve a much higher voltage level. Moreover, the voltage stress of dc-side active switches can be reduced to below one half as compared with that of a tradi- tional isolated ´ Cuk converter. Finally, steady-state characteristics, performance analyses, and representative experimental results are also made to show the merits of the proposed inverter. Index Terms—High step-up ratio, low-voltage alternative energy sources, single-phase inverter. I. INTRODUCTION I N RECENT years, the environmental pollution caused by fossil-fuel electricity generation has led to serious concerns about global warming and climate change. As a result, alterna- tive energy resources such as solar energy and fuel cells have gained considerable attention because of their environmental- friendly characteristics [1], [2]. These alternative energy sys- tems are often used to deliver electrical power to utility grids or in remote areas, as stand-alone power supplies [3]–[7]. As far as alternative energy sources are concerned, the photovoltaic modules or fuel-cell stacks usually supply a low-dc voltage that varies in a wide range depending on the load conditions. There- fore, in many applications, the dc power must be inverted to ac power and stepped up in order for it to be compliable with residential, industrial, or utility grid standards. The above tasks can be accomplished by using two basic power conditioner architectures: two-stage architecture and in- tegrated architecture. A two-stage power conditioner consists Manuscript received December 21, 2009; revised March 15, 2010; accepted April 17, 2010. Date of current version September 17, 2010. This work was supported by the National Science Council of Taiwan under Contract NSC-96- 2221-E-007-171-MY3 and Contract NSC-98-2218-E-007-011. Recommended for publication by Associate Editor B. Tamyurek. The authors are with the Center for Advanced Power Technologies, Department of Electrical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan (e-mail: cptan@ee.nthu.edu.tw; pecmlai@gmail.com; mjay.cheng@gmail.com). 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/TPEL.2010.2049123 of a dc–dc converter with a step-up voltage gain to low-voltage alternative energy sources; the dc–dc converter is then followed by an inverter. However, this two-stage architecture has some drawbacks such as complex control and high cost [8], [9]. In order to overcome this problem, the integrated architecture can be adopted as a solution [10]–[20]. Among several inte- grated topologies, for applications requiring both voltage buck and boost-power conversions, nonisolated-integrated inverters, which are derived from Sepic, ´ Cuk, or Zeta dc–dc converters, have been proposed [15]–[20]. In order to realize galvanic isola- tion and obtain a large voltage step-up ratio, a family of isolated buck-boost converters has also been presented [15]–[19]. Un- fortunately, the aforementioned circuits can be only operated in a limited number of practical applications because of the large reduction in the conversion efficiency as the duty ratio of both dc-side and ac-side switches approaches unity. Furthermore, the resulting voltage stress of the dc-side switch is rather high and renders the low-voltage and high-performance devices unsuit- able [20]. Another serious problem with the aforementioned circuits is that the input-current ripple is large due to the use of a single switch for the dc-side operation. This renders those converters unsuitable for low-voltage and high-current input ap- plications [11]–[20]. In order to solve the aforesaid drawbacks, an integrated single-phase inverter with a high step-up ratio and voltage buck- boost capabilities is presented for low-voltage alternative energy source applications. An auxiliary step-up circuit is integrated into an isolated ´ Cuk-derived voltage-source inverter (VSI). As a result, a high-voltage gain can be achieved while avoiding using extreme duty ratios in both the dc- and inverter-side switches. Note that a similar conceptual solution, e.g., diode-assisted net- work, had previously been used for nonisolated dc–ac energy conversion application [21], where a novel circuit topology can be derived and integrated into an isolated inverter for meeting such requirements. In addition, the proposed circuit possesses the automatic energy-transfer characteristic between parallel charging and series discharging of two capacitors to achieve a much higher voltage level. Therefore, one can adopt devices with a lower voltage rating in order to further reduce both switch- ing and conduction losses of the dc-side switches with the aim to improve the overall conversion efficiency of the proposed inverter. In addition, due to the two-phase architecture on the dc-side of the proposed inverter, the input current can be shared; the size and heat dissipation of the energy storage inductors can also be reduced easily. The steady-state characteristics are 0885-8993/$26.00 © 2010 IEEE