Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2001-05-29
2002-09-17
Sterrett, Jeffrey (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S025000, C363S097000, C363S098000
Reexamination Certificate
active
06452816
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a switching power supply comprising a DC-DC converter for supplying a stabilized DC voltage in an electronic equipment for industrial and consumer use, and relates, in particular, to a soft-switching power supply for soft switching such as zero-volt switching (ZVS).
A switching power supply converts an input DC voltage into an output of constant DC voltage, and is used in an electronic equipment such as a television set, a VTR and a personal computer. In a switching power supply, semiconductor devices, such as MOS-FET, IGBT and thyristor, are used as switches, whereby the ratio between the input and output voltages can be set through the duty ratio of turning ON and OFF. Accordingly, a switching power supply can stably output a predetermined DC voltage by controlling the turning ON and OFF. Since the electric power loss (switching loss) due to turning ON and OFF is generally small, a switching power supply is often used for the purpose of energy saving.
Reactance elements, such as transformer, inductor and capacitor, included in a switching power supply can be downsized and weight-reduced by turning ON and OFF at a higher frequency (switching frequency). On the other hand, the ratio between the input and output voltages of a switching power supply depends substantially only on the duty ratio of turning ON and OFF. Accordingly, a switching power supply can be rather easily downsized and weight-reduced with keeping its output voltage.
In recent years, there are rapidly increasing demands for energy saving, downsizing and weight reduction of various electronic equipments. Also regarding to switching power supplies, there are strong requirements for higher efficiency, smaller size, lighter weight and more stable output.
In order to meet such requirements, a higher switching frequency is necessary. However, a higher switching frequency causes a larger switching loss. Further, a part of the electric power dissipated as a switching loss causes surge current and voltage, which result in an adverse influence of noise on electronic equipments in the periphery.
Accordingly, the increase of switching frequency requires a switching technology for suppressing the switching loss. Known as such a technology is the soft switching. In soft switching, a switch in a transitional state turning from ON to OFF or vice versa is provided with a resonance voltage or current, whereby the switch turns from ON to OFF or vice versa when the voltage or current is at zero. In particular, zero-volt switching (ZVS) is the switching carried out when the voltage applied across the switch is at zero, whereas zero-current switching (ZCS) is the switching carried out when the current applied across the switch is at zero.
In accordance with soft switching, no electric power is applied across the switch at the instance of turning between ON and OFF. Accordingly, no switching loss in the electric power occurs in principle. In particular, in accordance with ZVS, no charge remains in the parasitic capacitance of the switch at the instance of turning ON. Therefore, no surge current occurs.
In a so-called isolation type switching power supply using a transformer for stopping a direct current between the power supply side and the output side, a prior art, such as disclosed in Japanese Laid-Open Patent Publication No. Hei 11-89232, is known, as a circuit for performing zero-volt switching using the energy stored in the transformer.
FIG. 15
shows a circuit constituting of a known switching power supply. The known example is of a full-bridge type converter, in which full-wave rectification is carried out in the secondary of a transformer
3
.
FIG. 16
shows the pulse waveform of the current or voltage at each part indicated by an arrow in
FIG. 15
of the known circuit.
As shown in
FIG. 15
, a switching control circuit
7
outputs switching signals G
1
, G
2
, G
3
and G
4
to four switching devices
11
S,
12
S,
13
S and
14
S, respectively. As shown in
FIG. 16
, the switching signals G
1
, G
2
, G
3
and G
4
are rectangular waves having predetermined widths. The switching devices
11
S,
12
S,
13
S and
14
S are ON when the switching signals G
1
, G
2
, G
3
and G
4
are at a high potential (H), respectively, whereas the switching devices
11
S,
12
S,
13
S and
14
S are OFF when the switching signals G
1
, G
2
, G
3
and G
4
are at a low potential (L), respectively.
As shown in
FIG. 16
, the switching signal G
1
changes from H to L at time T
1
, whereby the first switching device
11
S turns OFF. Then, a resonance occurs among the leakage inductance of a primary winding
3
a
, a first capacitor
11
C in a first switching section
11
, and a second capacitor
12
C in a second switching section
12
. That is, a current I
3
flowing through the primary winding
3
a
causes the first capacitor
11
C to charge and the second capacitor
12
C to discharge. Thus, the voltage V
11
across the first switching device
11
S increases from zero, while the voltage V
12
across the second switching device
12
S decreases from a maximum value Vin.
The voltage V
11
across the first switching device
11
S reaches the maximum value Vin, and, at the same time, the voltage V
12
across the second switching device
12
S reaches zero. Then, a second diode
12
D connected to the second switching device
12
S in parallel turns ON. At time T
2
immediately after that, the switching control circuit
7
changes the switching signal G
2
from L to H, thereby turning ON the second switching device
12
S. In such a manner, ZVS is carried out for the turning ON of the second switching device
12
S.
Similarly, a resonance occurs among the leakage inductance of the primary winding
3
a
, the first capacitor
11
C and the second capacitor
12
C during the interval from the time T
7
when the second switching device
12
S turns OFF to the time T
8
when the first switching device
11
S turns ON. After the voltage V
11
across the first switching device
11
S reaches zero, the first switching device
11
S turns ON. In such a manner, ZVS is carried out for the turning ON of the first switching device
11
S. Further, regarding to the turning ON of the third switching device
13
S at time T
4
and the turning ON of the fourth switching device
14
S at time T
6
, ZVS is carried out similarly with a resonance among the leakage inductance of the primary winding
3
a
, a third capacitor
13
C and a fourth capacitor
14
C.
In addition to the above-mentioned full-bridge type converter of the prior art, switching power supplies with ZVS include a half-bridge type, a push-pull type and modifications thereof combined with an auxiliary winding, as disclosed in Japanese Laid-Open Patent Publication No. Hei 9-163740. In each of these, ZVS is carried out with, a resonance among the leakage inductance of the primary winding and the parasitic capacitors of the switches.
In a switching power supply disclosed in the Japanese Laid-Open Patent Publication No. Hei 9-163740, a bi-directional switching device is provided in parallel with a primary winding or an auxiliary winding. The bi-directional switching device comprises two switching devices interconnected in series. Each switching device is connected with a diode in parallel. The ends of the switching devices on the anode or the cathode side of the respective diodes are interconnected. The bi-directional switching device serves as a switching snubber (also called an active clamp). That is, the bi-directional switching device absorbs surge currents and voltages occurring when a switch for conducting electricity between the transformer and the input power supply is turned ON and OFF. Thus, the surge current and voltage are prevented from exerting the adverse influence of noise on other circuits in the periphery.
Recently, there is a growing number of the apparatus to be energized even in periods out of driving and the apparatus held on standby for a long time with being energized. The former apparatus include a personal computer and a facsimile machine, while
Kuranuki Masaaki
Yoshida Koji
Akin Gump Strauss Hauer & Feld L.L.P.
Sterrett Jeffrey
LandOfFree
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