The 4-leg inverter can adjust the load current or output voltage even under unbalanced load conditions, but it is known that the additional switch arm to the 3-leg inverter can increase the overall cost and the failure rate. This paper aims to analyze the failure rate and mean time between failures (MTBF) of 3-leg inverters and 4-leg inverters using part count failure analysis (PCA) and fault-tree analysis (FTA), and to compare the price of the inverters. The FTA can analyze the failure rate, including the type, number and connection status of the circuit components, and moreover the redundancy effect of the 4-leg inverter. For more accurate failure-rate prediction, the failure rate and MTBF of the 4-leg inverter according to the lifecycle of the controller are analyzed. Finally, by comparing the price of 3-leg inverters and 4-leg inverters using the cost model of major parts, the degree of reliability improvement against price increase is quantitatively analyzed.

Clamped-single submodule (CSSM) has DC short circuit current protection function to improve the safety and stability of high voltage, direct current (HVDC) system. In order to carry out the protection, it needs an additional number of insulated gate bipolar transistors (IGBTs) and diodes compared to the conventional half-bridge submodule (HBSM). In general, the failure rate tends to increase in proportion to the number of circuit components. Also, complex operation of the submodule may increase the failure rate, so accurate reliability analysis considering these points is required to apply CSSM in a practical HVDC system. We estimate the failure rate and the mean time between failures (MTBF) of CSSM using a fault tree. Fault-tree analysis (FTA) is possible to analyze the failure rate more accurately than the prior part count failure analysis (PCA) that considers only the number of parts, the type of parts, and the connection status of each circuit component. To provide guidelines for submodule selection under various conditions, we compare the economic cost of a CSSM with HBSM, FBSM, and clamped-double submodule (CDSM), and analyze the failure rate according to the voltage margin of the parts.

The single-ended and double-ended forward converters are compared using the failure-rate and the mean time between failures (MTBF), which are representative indices of reliability. In addition to the part count analysis (PCA), which considers only the type of parts, the number of parts, and the connection status of parts, the reliability is analyzed by applying the fault-tree concept to reflect the operational risks of the converter. In the proposed fault-tree based analysis, the lowest failure event of the fault-tree, which indicates the cause and effect of the converter fault, uses the failure library of MIL-HDBK-217F. The cause and meaning of the failure-rate difference obtained by the PCA and the proposed FTA are explained, and finally we draw a conclusion that the single-ended forward converter is more advantageous in the viewpoint of reliability than the double-ended forward converter.

The forward converter is divided into two circuit topologies depending on the method of resetting the energy stored in the magnetizing inductance of the transformer: a single-ended structure using a reset circuit and a double-ended structure called a two-switch forward converter and, accordingly, a difference occurs in the operational mode. Generally, forward converters are designed with low power of less than 500 W and are sold at low prices, so it is common to replace them in case of failure. However, in terms of environmental conservation through resource saving, it is desirable to design with as high reliability as possible. In this paper, reliability is analyzed for the two representative circuit topologies of forward converters. The conventional PCA (part count failure analysis) method determines the failure rate only by the number of circuit components constituting the converter and the correlation between the components, but the FTA (fault-tree analysis) method can additionally consider the operation risk of the converter according to the circuit configuration and operation. Therefore, it is possible to analyze the failure rate according to the operating characteristics of the two converters from the difference in the failure rate obtained by the two methods. In addition, the two converters were manufactured as a 300 W prototype, respectively, and the volume, weight, and power loss were compared, and the economic feasibility was analyzed based on the cost model.