Importance

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Abstract

When a second switching element Q.sub.2 is changed over from ON state to OFF state for each switching cycle while a first switching element Q.sub.1 remains ON in a unit weld period T.sub.a for example, a primary current I.sub.1 does not come to a stop at once under the influence of inductance of a welding transformer 16 but it flows as a transient current i through a primary circuit until it is off. The transient current i makes a closed circuit through which it flows from a primary coil of the welding transformer 16 via a third diode D.sub.3 and then a first switching element Q.sub.1 again into the primary coil of the welding transformer 16. More specifically, due to the first switching element Q.sub.1 being kept ON, the transient current i which has passed through the third diode D.sub.3 flows through the first switching element Q.sub.1, without passing through a capacitor 12, and comes back into the primary coil of the welding transformer. Little or substantially no current flows through the capacitor 12.

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First Claim