Patent ID: 7109438

Claim:
A power supply apparatus for an arc-utilizing apparatus, comprising: an input rectifier for rectifying an AC voltage applied to an input terminal thereof, said AC voltage being one of higher and lower two AC voltages; first and second inverters connected in an output of said input rectifier; high-frequency transformers connected to outputs of respective ones of said first and second inverters; output rectifiers connected to respective ones of outputs of said high-frequency transformers; a switching circuit operating to connect said first and second inverters in series with each other in the output of said input rectifier when said higher AC voltage is applied to said input terminal of said input rectifier, and operating to connect said first and second inverters in parallel with each other in the output of said input rectifier when said lower AC voltage is applied to said input terminal of said input rectifier, whereby a predetermined DC voltage is applied to inputs of said first and second inverters; and voltage balancing control means for suppressing imbalance between the input voltages applied to said first and second inverters when said first and second inverters are connected in series with each other in the output of said input rectifier, wherein said voltage balancing control means comprises: a first capacitor connected in the input of said first inverter and in parallel with said first inverter; a second capacitor connected in the input of said second inverter and in parallel with said second inverter; a first semiconductor switching device having a first conduction path connected in parallel with said first capacitor, said first semiconductor switching device having a first control electrode for controlling the conduction of said first conduction path; a second semiconductor switching device having a second conduction path connected in parallel with said second capacitor, said second semiconductor switching device having a second control electrode for controlling the conduction of said second conduction path; a first drive circuit connected to said first control electrode for changing the conduction of said first conduction path; a second drive circuit connected to said second control electrode for changing the conduction of said second conduction path; and a control circuit connected between a node at which a voltage having a magnitude half of an output voltage of said input rectifier is developed, and a junction of said first and second capacitors connected in series when said switching circuit connects said first and second inverters in series, said control circuit detecting imbalance between the input voltages to said first and second inverters connected in series, and controlling said first and second drive circuits in accordance with the detected imbalance.