Electricity: power supply or regulation systems – Output level responsive – Using a three or more terminal semiconductive device as the...
Reexamination Certificate
2000-10-23
2001-10-23
Berhane, Adolf Deneke (Department: 2838)
Electricity: power supply or regulation systems
Output level responsive
Using a three or more terminal semiconductive device as the...
Reexamination Certificate
active
06307360
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a technique in a switching regulator to detect and restrain inrush current at turn-on or in the initial operation, or excessive current caused at a short-circuit failure in the capacity for smoothing the output voltage.
BACKGROUND ART
Generally, in a switching regulator which converts a DC power-supply voltage into another DC voltage, the restraint of inrush current or excessive current has been one of the significant challenges. Here, the inrush current refers to the current that flows abruptly to charge the capacity for smoothing the output voltage when a switch connected to the power source is turned on in the initial operation such as at power-on. The excessive current, on the other hand, refers to the current that flows excessively from the power source towards the load when the switch connected to the power source is in the ON state. Without restraining the inrush current and the excessive current, the current flowing through the inductor within the smoothing circuit would exceed the maximum rating value, thereby causing the deterioration of the performance or even a breakage. It must be noted that in the present specification the inrush current and the excessive current are collectively called inrush current except for the case where it might be misleading.
In order to restrain the inrush current, the following circuit structure has been suggested.
FIG. 11
shows an example of the circuit structure of a conventional switching regulator. In this example, an inrush current detection circuit
50
includes a resistance element
51
for inrush current detection and an inrush current detector
52
which detects the presence or absence of the inrush current by means of the voltage across the resistance element
51
. The inrush current detector
52
places the switching element
53
with a small on resistance in the ON state in a steady operation, and turns the element
53
off to flow the current to a resistance element
54
for inrush current restraint upon the inrush current detection. The resistance element
54
for inrush current restraint is composed of a resistor with a large resistance value or a temperature fuse resistor which interrupts a current flow in response to overheating (Refer to Japanese Laid-Open Patent Application No. 9-121546).
FIG. 12
shows another example of the circuit structure of the conventional switching regulator, which controls the switching pulse width variably with a sawtooth wave in the initial operation. Under the direction of a switch control circuit
57
, a selector
56
selects and outputs the sawtooth wave in the initial operation and a reference potential in the steady operation. Consequently, as shown in
FIG. 13
, early in the initial operation, the on signal of the p-type MOS transistor switch SW
1
has a short pulse width due to the comparison between the sawtooth wave and the output voltage Vout, and later the pulse width becomes gradually longer with increasing output voltage Vout. As a result, the inrush current in the initial operation is successfully restrained.
FIG. 14
shows a further another example of the circuit structure of the switching regulator which restrains the inrush current in the initial operation by means of a sawtooth wave in the same manner as in FIG.
12
. In the structure shown in
FIG. 14
, the on-off control of switching elements
61
,
62
are done by using a loop
1
in the initial operation and a loop
2
in the steady operation. A sawtooth wave generating circuit
63
generates a sawtooth wave STW so as to perform the initializing operation while gradually changing the switching on-off duty, thereby restraining the inrush current in the initial operation (Refer to Japanese Laid-Open Application No. 5-56636).
Problems to be Solved
However, the conventional switching regulator with the above-described structures has the following problems.
The structure with the inrush current restraint circuit shown in
FIG. 11
has a drawback of a low power conversion efficiency. To be more specific, the resistance element
51
for inrush current detection causes a resistance loss because it is interposed with a current path from the power source, and therefore, the power conversion efficiency of the switching regulator deteriorates.
As another drawback, the structures where the initial operation is performed with a sawtooth wave as shown in
FIGS. 12 and 14
, it is possible to restrain the inrush current in the initial operation; however, it is impossible to detect and restrain the excessive current in the steady operation. There is further another drawback of increasing the time required for the initialization. Moreover, there is a necessity to generate a signal that distinguishes between the initial operation and the steady operation due to the nature of the circuit structure.
DISCLOSURE OF THE INVENTION
The present invention has an object of providing a switching regulator which restrains the inrush current and the excessive current while maintaining a high power conversion efficiency and which requires a shorter time for initialization than conventional switching regulators.
To be more specific, a switching regulator of a synchronous rectifying mode of the present invention comprises: a first switch and a second switch arranged in series between a first power source which supplies a first potential and a second power source which supplies a second potential lower than the first potential, the first switch being positioned closer to the first power source than the second switch; a switch control unit for controlling an on-off operation of the first and second switches; and a smoothing circuit for smoothing a potential of an output node between the first switch and the second switch, said switch control unit turning off the first switch when the potential of said output node is below a first reference potential which becomes a reference to detect an occurrence of inrush current or excessive current while the first switch is in an ON state.
According to the present invention, when the potential of the output node goes below the first reference potential while the first switch is in the ON state, the first switch is turned off by regarding it as the occurrence of the inrush current or the excessive current. To be more specific, when the occurrence of the inrush current and the excessive current has been detected by making use of a voltage drop due to the on resistance of the first switch, the first switch is turned off so as to interrupt the inrush or excessive current. This approach can realize the detection of the inrush or excessive current without setting up a resistance element for detection or without causing a decrease in the power conversion efficiency of the switching regulator. Moreover, the initial operation does not need a different control from the steady operation, which makes it possible to restrain the inrush current in the initial operation and the excessive current in the steady operation in the same manner.
The switch control unit of the switching regulator of the present invention preferably comprises: a first reference potential generating circuit for generating the first reference potential; a first potential comparator for comparing the first reference potential generated by the first reference potential generating circuit with the potential of said output node; and an available period setting circuit for setting an available period during which the comparison results of the first potential comparator is valid. The switch control unit turns off the first switch when the first potential comparator detects that the potential of said output node is below the first reference potential during said available period set by said available period setting circuit.
It is also preferable that the available period setting circuit sets the start of said available period at a time when a predetermined time period has elapsed since the first switch is turned on.
It is further preferable that after turning off the first switch, said switch control unit places the
Kajiwara Jun
Kinoshita Masayoshi
Ootani Katsuhiro
Sakiyama Shiro
Satomi Katsuji
Berhane Adolf Deneke
Matsushita Electric - Industrial Co., Ltd.
Nixon & Peabody LLP
Robinson Eric J.
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