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 with compensating for a variation of a relationship between the primary side-current and the output current due to variation of duty ratio of the power switching elements, and variation of at least one of the DC output voltage and the DC input voltage of the voltage transforming apparatus.

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 explaining 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 showing a switching noise contained in the rectified primary-side current of the DC-DC converter;

FIG. 4 is a diagram showing a tooth 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 operation by the DC-DC converter is performed;

FIG. 8 is a diagram for explaining how a peak value of the primary-side current varies depending on a duty ratio of the switching transistors included in the DC-DC converter;

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;

FIG. 10 is a diagram showing a structure of a part of a controller included in a control apparatus according to a third embodiment of the invention, which is dedicated to generation of drive signals applied to switching elements included in the DC-DC converter;

FIG. 11 is a diagram showing a structure of a part of a controller included in a control apparatus according to a fourth embodiment of the invention, which is dedicated to generation of drive signals applied to switching elements included in the DC-DC converter;

FIG. 12 is a diagram showing a structure of a part of a controller included in a control apparatus according to a fifth embodiment of the invention, which is dedicated to generation of drive signals applied to switching elements included in the DC-DC converter;

FIG. 13 is a diagram showing a structure of a part of a controller included in a control apparatus according to a sixth embodiment of the invention, which is dedicated to generation of drive signals applied to switching elements included in the DC-DC converter; and

FIG. 14 is a diagram showing a structure of a part of a controller included in a control apparatus according to a seventh embodiment of the invention, which is dedicated to generation of drive signals applied to switching elements included in the DC-DC converter.

Claims

1. A control apparatus for controlling a voltage transforming apparatus including a transformer, power switching elements provided in a primary side thereof and operating to invert a DC input voltage to an AC voltage, synchronous-rectifying switching elements provided in a secondary side thereof 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; 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;wherein said judging circuit makes said judgment while compensating for a variation of a relationship between said primary side-current and said output current due to variation of duty ratio of said power switching elements, and variation of at least one of said DC output voltage and said DC input voltage.

2. The control apparatus according to claim 1, wherein said judging circuit includes a comparator comparing a detection value of said primary-side current with a threshold value corresponding to said specified current, and a compensating circuit compensating one of said detection value and said threshold value being inputted to said comparator on the basis of a compensation value by which said relationship between said primary-side current and said output current is compensated for.

3. The control apparatus according to claim 2, further comprising a periodic wave signal generating circuit generating a periodic wave signal repeating a cycle of monotone increase and monotone decrease, said compensating circuit compensating one of said detection value and said threshold value by use of said periodic wave signal.

4. The control apparatus according to claim 3, wherein said periodic wave signal is a triangular wave signal, said compensating circuit compensating one of said detection value and said threshold value by superposing said triangular wave signal onto said detection value.

5. The control apparatus according to claim 3, further comprising an error detecting circuit generating an error signal having a value depending on a value of said output voltage of said voltage transforming apparatus, and a PWM modulating circuit generating PWM-modulated drive signals to be applied to said power switching elements on the basis of comparison result between said error signal and a carrier signal produced from said periodic wave signal.

6. The control apparatus according to claim 2, wherein said compensating circuit compensates one of said detection value and said threshold value in accordance with said DC output voltage of said voltage transforming apparatus.

7. The control apparatus according to claim 1, wherein said inhibition circuit inhibits said synchronous-rectifying switching elements from performing said synchronous-rectifying control operation when said judging circuit judges that said output current immediately before said power switching elements are turned off is smaller than said threshold current.