Abstract: A un-interruptible power supply (UPS) apparatus for a diffusion furnace used in semiconductor device fabrication. The diffusion furnace consists of a power electronic subsystem coupled to a three-phase utility power source via two series-connected normally-open main circuit breaker switches, an automatic control unit, and a direct-current power supply. The automatic control unit controls the operation of the diffusion furnace. The UPS apparatus includes a UPS unit having a power input connected to a node between the two series-connected normally-open main circuit breaker switches. A first normally-open circuit breaker has a switch connected between the output of the UPS unit and the power input of the automatic control unit. A second normally-open circuit breaker has a switch connected between a first phase power line of the three-phase utility power source and the input of the direct-current power supply. A third normally-closed circuit breaker has a switch connected to the output of the UPS unit.