Switching power circuit

Details Switching power circuit Switching power circuit

2000-09-05

2001-07-24

Wong, Peter S. (Department: 2838)

Electric power conversion systems

Current conversion

Including d.c.-a.c.-d.c. converter

C363S133000

Reexamination Certificate

active

06266254

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a switching power circuit used for back-light of a liquid crystal display device or the like.
DESCRIPTION OF THE PRIOR ART
Conventionally, there has been used a switching power circuit shown in
FIG. 2. A
capacitor C
1
is a capacitor for smoothing a power supply voltage connected to an input terminal
1
. Two transistors Q
1
and Q
2
are push-pull connected switching transistors whose emitters are connected with each other and are grounded. Transformer T is composed of a primary winding N
1
, a secondary winding N
2
and a tertiary winding N
3
, wherein each end of the primary winding N
1
is connected to each collector of the transistors Q
1
and Q
2
respectively.
A bias resistor R is disposed between the input terminal
1
and a base of the transistor Q
1
to connect them with each other, and each base of the transistors Q
1
and Q
2
is connected to each end of the tertiary winding N
3
for feedback oscillation. A capacitor C
3
is a resonant capacitor connected parallel with the primary winding N
1
. A centertap of the primary winding N
1
is connected to the input terminal
1
through a choke coil L.
Each end of the secondary winding N
2
is connected to an output terminal
2
through diodes D
1
or D
2
respectively, and a centertap of the secondary winding N
2
is grounded. An end of a smoothing capacitor C
2
is connected to a middle point between the output terminal
2
and the diodes D
1
and D
2
and other end of the capacitor C
2
is grounded.
In the power circuit configured as described above, when the DC voltage Vin is applied to the input terminal
1
, a self-oscillation is generated by the tertiary winding N
3
. Thereby, the transistors Q
1
and Q
2
are switched by turns to generate AC voltage on the secondary winding N
2
. This AC voltage is full-wave rectified by the diodes D
1
and D
2
and is smoothed by the capacitor C
2
. As a result, DC voltage V
0
is provided for the output terminal
2
.
In the power circuit configured as described above, an operating point of the switching transistors Q
1
and Q
2
is determined by the resistor R. Thereby, there occurs a problem that, since an operating condition of the whole power circuit depends on a value of the resistor R, it is difficult to change the operating point over wide range.
Thus, the operation of the transistors Q
1
and Q
2
is generally within a range of class-AB to class-B operations. Accordingly, the object of the present invention is to provide a circuit capable of making the operation of the transistors Q
1
and Q
2
improved into class-B to class-C operations. Another object of the present invention is to provide a switching circuit which allows a thermal stability to be easily secured and also allows an operating efficiency to be improved.
SUMMARY OF THE INVENTION
The present invention provides a switching power circuit comprising: a pair of switching elements connected to an end of a DC power source; a transformer including a primary winding having a centertap, a secondary winding connected to a load and a tertiary winding for a feedback oscillation; and a resonant capacitor connected parallel with the primary winding;
wherein each end of the primary winding is connected to each of the pair of switching elements respectively; the centertap is connected to other end of the DC power source through a choke coil; each end of the tertiary winding is connected to each of the pair of switching elements respectively;
so that the switching power circuit is self-oscillated by a tertiary winding such that each of the pair of switching elements may be conducted alternately and thereby a current flowing from the centertap is switched so as to selectively flow alternately to one side of the primary winding or to other side of the primary winding to generate AC voltage on a secondary winding;
said switching power circuit characterized by that an operating point of the switching elements is changed from that in starting by using a DC voltage generated by a switching operation of the pair of switching elements as an inverse bias power source for the switching elements.


REFERENCES:
patent: 4755923 (1988-07-01), Maeba et al.
patent: 5930124 (1999-07-01), Otake
patent: 6009001 (1999-12-01), Otake
patent: 08126348-A (1996-05-01), None

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