This paper deals with a unique solution related to failure detection in power supply units of large scale computers using LSI's, particularly power supply unit circuits. A large amount of current and low voltage are special features of large scale computers. When the power supply unit is operated in parallel, a failure can be detected easily if blocking diodes are connected in series through the output terminal of each power supply unit. With low voltage and large amounts of current, however, considerable power loss occurs because of the diode forward voltage drop. Although it is preferable to avoid the use of blocking diodes in the output of a power supply unit from the stand point of efficiency, failure detection is difficult when a failure occurs in units connected in parallel. Power supply units presented in this paper ease maintenance by employment of a failure detection system.