Control apparatus for voltage transforming apparatus

Abstract

The control apparatus for controlling a voltage transforming apparatus having a transformer, power switching elements disposed in a primary side, and synchronous-rectifying switching elements disposed in a secondary side includes a judging circuit making a judgment as to whether or not an output current of the voltage transforming apparatus is smaller than a specified current on the basis of a primary-side current of the transformer and an inhibition circuit inhibiting the synchronous-rectifying switching elements from performing their synchronous-rectifying control operation when the judging circuit judges that the output current is smaller than the specified current. The judging circuit makes the judgment on the basis of the primary-side current flowing through the primary coil of the transformer immediately before the power switching elements are turned off.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a diagram showing a structure of a DC-DC converter as a voltage transforming apparatus, and a structure of a control apparatus according to a first embodiment of the invention used for controlling the DC-DC converter;

FIG. 2 is a diagram showing temporal changes of various signals in the DC-DC converter;

FIG. 3A is a diagram showing how a rectified primary-side current of the DC-DC converter varies depending on a duty ratio of switching elements included in the DC-DC converter;

FIG. 3B is a diagram explaining a switching noise contained in the rectified primary-side current of the DC-DC converter;

FIG. 4 is a diagram showing a sawtooth wave signal superimposed onto the rectified primary-side current;

FIG. 5 is a diagram showing a structure of a part of a controller included in the control apparatus according to the first embodiment of the invention, which is dedicated to generation of drive signals applied to the switching elements included in the DC-DC converter;

FIG. 6 is a diagram showing a structure of a sawtooth wave signal generating circuit included in the controller of the first embodiment of the invention;

FIG. 7 is a diagram for explaining in which timing switching between permission and inhibition of a synchronous-rectifying control by the DC-DC converter is performed;

FIG. 8 is a diagram explaining a timing to sample a detection value of a primary side current of the DC-DC converter in the first embodiment;

FIG. 9 is a diagram showing a structure of a part of a controller included in a control apparatus according to a second embodiment of the invention, which is dedicated to generation of drive signals applied to switching elements included in the DC-DC converter; and

FIG. 10 is a diagram explaining a timing to sample a detection value of a primary side current of the DC-DC converter in the second embodiment.

Claims

1. A control apparatus for controlling a voltage transforming apparatus including a transformer, power switching elements provided in a primary side of said transformer and performing on-operation and off-operation to invert a DC input voltage to an AC voltage, a synchronous-rectifying switching elements provided in a secondary side of said transformer and performing synchronous-rectifying control operation in synchronization with said power switching elements in order to rectify said AC voltage transformed by said transformer into a DC output voltage, and a smoothing circuit smoothing said DC output voltage, said control apparatus comprising: a judging circuit making a judgment as to whether or not an output current of said voltage transforming apparatus is smaller than a specified current on the basis of a primary-side current flowing through a primary coil of said transformer immediately before said off-operation; andan inhibition circuit inhibiting said synchronous-rectifying switching elements from performing said synchronous-rectifying control operation when said judging circuit judges that said output current is smaller than said specified current.

2. The control apparatus according to claim 1, further comprising an error signal generating circuit generating an error signal indicative of a difference between a detection value of said DC output voltage and a reference voltage, a PWM modulating circuit generating a PWM signal having a duty ratio depending on said error signal, and a driver circuit generating, through power conversion of said PWM signal, drive signals to be applied to said power switching elements, said judging circuit making said judgment at a timing in synchronization with said PWM signal appearing upstream of said driver circuit.

3. The control apparatus according to claim 1, further comprising an error signal generating circuit generating an error signal indicative of a difference between a detection value of said DC output voltage and a reference voltage, a PWM modulating circuit generating a PWM signal having a duty ratio depending on said error signal, and a driver circuit generating, through power conversion of said PWM signal, drive signals to be applied to said power switching elements, said judging circuit making said judgment at a timing in synchronization with said drive signals being outputted from said driver circuit.

4. The control apparatus according to claim 2, wherein said judging circuit makes said judgment, while compensating for at least one of effect of variation of said duty ratio of said power switching elements, effect of variation of said DC output voltage, and effect of variation of said DC input voltage on a relationship between said primary-side current and said output current of said voltage transforming apparatus.

5. The control apparatus according to claim 3, wherein said judging circuit makes said judgment, while compensating for at least one of effect of variation of said duty ratio of said power switching elements, effect of variation of said DC output voltage, and effect of variation of said DC input voltage on a relationship between said primary-side current and said output current of said voltage transforming apparatus.