## 6 Results and Discussion
### Reliability Assessment  
According to prior studies, semiconductor devices used in power electronic converters have always been considered a major factor in reducing reliability and lifetime [57]. Thus, the reliability of the most fragile components of the system has been evaluated in the first stage while undertaking lifetime assessments. To analyze the semiconductor devices in terms of reliability, the above-mentioned relationships are used, and a thermal cycling test must be performed on the converters under study. The extracted results of this simulation on the boost converter are shown in Figure 16.  
As is clear from Figure 16, a constant power has been applied to the power module in the first part of the cycle (0.5 s), and in the next 0.5 s, the applied power to the semiconductor devices is interrupted and the module is allowed to cool down by the heatsink (or cooling system). During this test, depending on the cooling capacity of the heatsink, the junction temperature may reach the ambient temperature over the cooling phase or may not. For instance, in the boost converter case, the junction temperature did not reach the ambient temperature (25 \({}^{\circ}\)C). Figure 16: The temperature variations of the IGBT used in the boost converter during the power cycling test. Figure 15: The junction temperature of the power semiconductors within the Cuk converter. What is important from this test for reliability analysis is the maximum and minimum temperatures during the thermal cycling, as the lifetime of the module is also measured by these values (these maximum and minimum values are specified in Figure 16). To determine the parameters \(\Delta T_{j}\) and \(T_{j\mu}\), these maximum and minimum temperatures are adequate; then, by performing the thermal test for all the converters under the mentioned conditions, the data listed in Table 1 can be found. Therefore, the Norris-Landzberg parameter (the number of cycles) can be calculated by substituting values in Equation (24). It is clear from Table 1 that the conventional boost converter is quite failure-prone due to its improper performance in terms of reliability. In the meanwhile, the most competitive topologies are interleaved, floating interleaved, and multi-switch. Although there seems to be no significant temperature difference between these converters, the number of cycles can vary enormously even with a slight difference in temperature values, so the floating interleaved converter has almost three times better performance than the interleaved converter. In order to have a better comparison between the studied converters, it is necessary to examine all indicators related to thermal performance, reliability, number of components, and cost at the same time. The first important factor in choosing the best converter for fuel cell applications is the number of cycles, because a higher number of cycles means a greater useful lifespan. The second most important factor among the chosen topologies is the highest junction temperature among the semiconductor devices (\(T_{jmax(Q)}\) and \(T_{jmax(D)}\)); these values need to be as low as possible for more efficient operation and an optimal cooling system. The diodes and IGBTs corresponding to the highest temperature are specified for each converter as follows:  
* Conventional Boost: D2 and Q1
* Floating Interleaved Boost: D3 (or D4) and Q1 (or Q2)
* Multi-switch Boost: D3 and Q2
* Cuk: D2 and Q1  
The next decisive factor in a fuel cell system is the cost of the converter, which is even preferred in some cases to the performance. To determine the cost indicator, four different levels are specified: very low (1), low (2), average (3), and high (4). The next factor is the number of circuit components; a smaller number of components would be better. Reducing the number of circuit components--particularly power switching devices and the required drive circuit--is important because it reduces the weight, size, cost, and installation area [58].