Patent ID: 6956427

Claim:
An electrically insulated type of switching element drive circuit, for controlling on/off operation of a switching element in accordance with an externally supplied control command signal, said switching element drive circuit comprising: a control electrode drive circuit for supplying a drive signal to a control electrode of said switching element, to control said on/off operation based on said control command signal; a power supply circuit for supplying electric power to operate said control electrode drive circuit; a transformer for supplying an AC voltage to said power supply circuit from a secondary side of said transformer, said power supply circuit performing rectification and smoothing of said AC voltage from said secondary side of said transformer to thereby obtain a DC voltage, and supplying said DC voltage to operate said control electrode drive circuit; and an AC voltage output circuit for supplying said AC voltage to a primary side of said transformer; a power transmission system that extends from said AC voltage output circuit to said power supply circuit comprises a resonant circuit, wherein: said control electrode drive circuit comprises means for detecting a condition of said AC voltage relating to a frequency of said AC voltage and for altering a magnitude of a drive voltage applied to said control electrode, based upon results of said detection of said condition of said AC voltage, said AC voltage output circuit is responsive to an on status of said control command signal, indicating that said switching element is to be set in said on state, for establishing a frequency of said AC voltage that is substantially identical to a resonance frequency of said resonant circuit, and said AC voltage output circuit is responsive to an off status of said control command signal, indicating that said switching element is to be set in said off state, for establishing a frequency of said AC voltage that differs from said resonance frequency by a predetermined amount.