## 1 Introduction  
Population growth and the excessive use of electrical appliances have an enormous influence on electricity consumption mainly coming from fossil fuels. Alternative energy sources have gained considerable attention in many countries in recent decades, owing primarily to greenhouse gas emissions, global warming, and ozone layer depletion. Although there is a great effort to replace all the traditional energy sources with wind and solar energies, fuel cells are becoming even more important and have gained a favorable position as a leading source of power. Moreover, fuel cells are one of the effective technologies to integrate green hydrogen into the power system. Given the fact that the fuel cell is compact, lightweight, and non-polluting, it is very suitable for applications such as electric vehicles and emergency power systems [1]. Compared to wind and solar power generation, the advantage of using a fuel cell is that it can be used anywhere as there are no environmental or geographical constraints to use it [2]. These features have enabled fuel cells to be used from watts to megawatts for applications such as the space industry [3], portable applications [4], remote area power supplies [5], and communication applications [6]. Among the different types of fuel cells, proton exchange membrane fuel cells (PEMFCs), with characteristics such as low operating temperature, high power density, low cost, and fast response, are the most attractive option for electric vehicles and residential applications [7]. A PEM fuel cell uses a solid polymer as an electrolyte which only needs hydrogen and oxygen from the air. After generating electrochemical energy, the only resulting by-product is water [8]. However, a power conditioning system for fuel cells still encounters several problems due to the small and non-regulated voltage produced at the output of the fuel cell. Therefore, the use of power electronic converters in order to step up/regulate the output voltage level to the desired voltage and reduce the current ripple is necessary [9]. Using these converters as an advantage, the number of cells used within the fuel cell can be reduced [10]. The behavior of the fuel cell is nonlinear, which is why its output voltage should be initially increased by a DC-DC boost converter to feed the load, or a grid-tie inverter can be connected to the boost converter through a DC-link. Although transformer-based DC-DC converters are commonly used to step up the voltage level, these converters will increase the weight and cost of the entire fuel cell system, which is not desirable [11]. Another important issue to consider is that a high-level current ripple can accelerate the aging of the electrodes since the efficiency of transportation is reduced in an aged electrode [12]. The input current for a buck-boost power converter is discontinuous or pulsating; thus, it is recommended to use step-up converters with an input inductor for a fuel cell to minimize the current ripple [13]. Nevertheless, it is important to choose a power electronic converter that has a longer lifetime and greater reliability while meeting the cost and performance requirements. The primary purpose of this paper is to evaluate five different non-isolated (transformerless) DC-DC step-up converters--including the conventional boost, interleaved boost, floating-interleaved boost, multi-switch boost, and Cuk converter--in terms of both thermal and reliability aspects. In the first stage, a detailed thermal model will be used to estimate the accurate values of junction temperatures. Then, a power cycling test will be done by applying a periodic power with a frequency of 1 Hz to the power semiconductors, after which, the gathered temperature variations will be used to calculate the number of cycles to failure as perhaps the most significant factor concerning their reliability and lifetime.