Patent ID: 8564975

Claim:
An AC-DC converter comprising: a rectification circuit ( 2 ) for commutating an AC voltage (E A ) from an AC power source ( 1 ), a boosting chopper circuit ( 3 ) that comprises at least a reactor ( 4 ) connected to an output terminal of the rectification circuit ( 2 ), a step-up switching element ( 5 ) connected in series to the reactor ( 4 ) and a capacitor ( 7 ) connected in series to the reactor ( 4 ) and also in parallel to the step-up switching element ( 5 ) for increasing an output voltage from the rectification circuit ( 2 ) through the reactor ( 4 ) by on-off operation of the step-up switching element ( 5 ), a DC-DC converter ( 10 ) that comprises a primary winding ( 8 a ) of a transformer ( 8 ) connected to the boosting chopper circuit ( 3 ) and at least one main switching element ( 9 ) for producing a DC voltage (E 0 ) to a load ( 14 ) through a rectification smoother ( 11 ) connected to a secondary winding ( 8 b ) of the transformer ( 8 ) by on-off operation of the main switching element ( 9 ), a voltage detecting circuit ( 21 ) for detecting a voltage (V N ) appearing on any winding of the transformer ( 8 ) by on-off operation of the main switching element ( 9 ) in the DC-DC converter ( 10 ) to produce an output signal (V CP1 ) when the voltage (V N ) on any winding has one polarity, a waveform shaper ( 23 ) for generating chopping signals (V RC ) from the output signal (V CP1 ) of the voltage detecting circuit ( 21 ), and a PWM circuit ( 27 ) for comparing an output voltage (V RC ) from the waveform shaper ( 23 ) and output voltage (V CH ) from the boosting chopper circuit ( 3 ) to supply drive signals (V G1 ) to the step-up switching element ( 5 ) in the boosting chopper circuit ( 3 ) when the output voltage (V RC ) from the waveform shaper ( 23 ) exceeds the output voltage (V CH ) from the boosting chopper circuit ( 3 ), wherein during a shorter on-period of the main switching element ( 9 ) under a light mode of the load ( 14 ), the DC-DC converter ( 10 ) inhibits the waveform shaper ( 23 ) from producing a triangular wave signal (V RC ) of voltage reaching the output voltage (V CH ) of the boosting chopper circuit ( 3 ) to keep the step-up switching element ( 5 ) in the boosting chopper circuit ( 3 ) in the off condition.