Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2000-05-31
2001-03-06
Han, Jessica (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S056090
Reexamination Certificate
active
06198637
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to switching power supply circuits, and more particularly, the invention relates to switching power supply circuits in which heat generation and power loss in the switching element can be reduced by performing intermittent switching operations under light load.
2. Description of the Related Art
FIG. 3
shows a conventional switching power supply circuit disclosed in Japanese Unexamined Patent Application Publication No. 4-12666, which can perform intermittent switching operations under light load.
In
FIG. 3
, a switching power supply circuit
1
has a DC power supply E, a switching unit
2
, a switching control unit
3
, a transformer T which includes a primary winding N
1
, a secondary winding N
2
, and a feedback winding Nb, a rectifying/smoothing circuit
4
, a first photo coupler (PC)
5
, a current detecting unit
6
, an intermittent/continuous switching signal generating unit
7
, and a second photo coupler
8
. In this case, the power supply E is connected to the primary winding N
1
of the transformer T via the switching unit
2
. The switching control unit
3
is connected to the feedback winding Nb of the transformer T, and is also connected to the switching unit
2
. The secondary winding N
2
of the transformer T is connected to the rectifying/smoothing circuit
4
, whose outputs are connected to the switching control unit
3
via the first photo coupler
5
, and are also connected to output terminals Po via the current detecting unit
6
. The current detecting unit
6
is connected to the intermittent/continuous switching signal generating unit
7
, whose output is connected to the switching control unit
3
via the second photo coupler
8
.
In the switching power supply unit
1
having such a structure, an output current is detected by the current detecting unit
6
. First, when the current value detected by the current detecting unit
6
is large, that is, under rated load or heavy load, the intermittent/continuous switching signal generating unit
7
outputs a signal for directing continuous operations. This signal is input to the switching control unit
3
via the second photo coupler
8
, and, according to the control of the switching control unit
3
, the switching unit
2
performs continuous switching operations.
Meanwhile, when the current value detected by the current detecting unit
6
is small, that is, under light load, the intermittent/continuous switching signal generating unit
7
outputs a signal for directing intermittent operations. This signal is input to the switching control unit
3
via the second photo coupler
8
, and, according to the control of the switching control unit
3
, the switching unit
2
performs intermittent switching operations.
Usually, there are problems in that since the switching frequency significantly rises under light load, the switching element generates heat and switching loss increases. However, these problems can be solved by performing intermittent switching operations as described above.
In the switching power supply circuit
1
, however, the signals for directing continuous operations and intermittent operations are output by using the current detecting unit
6
and the intermittent/continuous switching signal generating unit
7
, and the output signals are fed back to the switching control unit
3
to perform the switching of intermittent/continuous operations. In this case, since it is necessary to dispose the current detecting unit
6
and intermittent/continuous switching signal generating unit
7
, there are problems in that the structure of the switching power supply unit becomes complicated, and miniaturization of the circuit is thereby hindered and cost increases.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a switching power supply circuit having a simple structure, in which intermittent switching operations can be performed under light load.
The switching power supply circuit comprises a transformer which has a primary winding, a secondary winding, and a feedback winding, a switching element connected in series to a first end of the primary winding, a control circuit disposed between a control terminal of the switching element and the feedback winding, a starting circuit connected between a second end of the primary winding and the control terminal of the switching element, and a rectifying/smoothing circuit connected to the secondary winding.
The control circuit includes a first delay circuit for determining the time that elapses before the switching element is turned on by a voltage generated at the feedback winding, a control element driven to prevent the switching element from turning on or forcibly turn off the switching element, and a second delay circuit for changing a delay time such that, under rated load or under heavy load, the switching element is turned off by driving the control element after the switching element is turned on by the voltage generated at the feedback winding, and, under light load, the turn-on of the switching element is blocked by driving the control element before the switching element is turned on by the voltage generated at the feedback winding.
With this arrangement, the switching power supply circuit of the present invention has a simple structure, in which intermittent switching operations under light load can be performed without the need for intermittent/continuous switching signals. As a result, heat generation and power loss of the switching element under the light load can be reduced.
In addition, the switching power supply circuit in accordance with the present invention is formed by connecting the output of the rectifying/smoothing circuit to an output stabilizing power supply circuit.
With this arrangement, in the switching power supply circuit of the present invention, even when intermittent switching operations are performed under light load, a high quality output in which output voltage ripple is small can be obtained.
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.
REFERENCES:
patent: 5012399 (1991-04-01), Takemura et al.
patent: 5978234 (1999-11-01), Nagahira
patent: 6038143 (2000-03-01), Miyazaki et al.
patent: 412666 (1992-04-01), None
Hosotani Tatsuya
Takemura Hiroshi
Han Jessica
Murata Manufacturing Co. Ltd.
Ostrolenk Faber Gerb & Soffen, LLP
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