Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2002-05-23
2003-11-18
Sterrett, Jeffrey (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S065000, C323S266000
Reexamination Certificate
active
06650552
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a switching power supply unit, and more particularly to a synchronous rectifying switching power supply unit, a switching power supply unit having a half-bridge circuit, and a switching power supply unit using a plurality of converters connected in series.
(Related Art 1)
Conventionally, DC/DC converters are known as a switching power supply unit. A typical DC/DC converter converts an alternating current input to direct current once by using a switching circuit, then transforms (boosting or lowering) the voltage by using a transformer, and further converts the direct current to alternating current by using an output circuit, whereby alternating current output having voltage different from the input voltage can be obtained.
In some cases, switching elements such as transistors are employed in an output rectifier for use in the DC/DC converter, so that the switching elements may be synchronously controlled with the switching circuit on the input side. The DC/DC converter having such an output rectifier is generally called as a synchronous rectifying switching power supply unit.
FIG. 15
is a circuit diagram of a conventional synchronous rectifying switching power supply unit.
As shown in
FIG. 15
, the conventional switching power supply unit includes a transformer
1
, a half-bridge circuit
2
provided on the primary side of the transformer
1
, a rectifier circuit
3
provided on the secondary side of the transformer
1
, a rectifier-transistor driving circuit
4
provided on the secondary side of the transformer
1
, a smoothing circuit
5
provided at the following stage of the rectifier circuit
3
, and a control circuit
9
for controlling on/off of a first main switch
7
and a second main switch
8
provided in the half-bridge circuit
2
based on the result of monitoring output voltage Vo via an insulating circuit
6
.
The half-bridge circuit
2
includes a first input capacitor
11
and a second input capacitor
12
connected in series between both ends of an input power supply
10
in addition to the first and second main switches
7
and
8
. The primary winding
20
of the transformer
1
is connected between a node where the first and second main switches
7
and
8
are joined and a node where the first and second input capacitors
11
and
12
are joined. The rectifier circuit
3
has a first rectifier transistor
13
and a second rectifier transistor
14
. The drain of the first rectifier transistor
13
is connected to the first secondary winding
21
of the transformer
1
, whereas the drain of the second rectifier transistor
14
is connected to the second secondary winding
22
of the transformer
1
. As shown in
FIG. 15
, as the source of the first rectifier transistor
13
and the source of the second rectifier transistor
14
are short-circuited, a voltage waveform appearing between the common source node of both transistors and a node where the first and second secondary windings
21
and
22
of the transformer
1
are joined forms an output from the rectifier circuit
3
. The rectifier-transistor driving circuit
4
has a first diode
15
connected between the gate and source of the second rectifier transistor
14
and a second diode
16
connected between the gate and source of the first rectifier transistor
13
. The third secondary winding
23
of the transformer
1
is connected between the cathode of the first diode
15
and the cathode of the second diode
16
. Further, the smoothing circuit
5
has a smoothing inductor
17
and a smoothing capacitor
18
.
With the arrangement above, the first and second main switches
7
and
8
are turned on alternately under the control of the control circuit
9
at intervals of predetermined dead time, whereby the output voltage Vo determined by the input voltage Vin and the turn ratio of the transformer
1
is applied to a load
19
.
FIG. 16
is a timing chart showing the operation of the conventional synchronous rectifying switching power supply unit. In
FIG. 16
, Vgs
7
and Vgs
8
mean the gate-source voltages of the first and second main switches
7
and
8
respectively; Vds
13
and Vds
14
mean the source-drain voltages of the first and second rectifier transistors
13
and
14
respectively; and Vgs
13
and Vgs
14
means the gate-source voltages of the first and second rectifier transistors
13
and
14
respectively.
As shown in
FIG. 16
, in the conventional synchronous rectifying switching power supply unit, the first and second main switches
7
and
8
are driven alternately under the control of the control circuit
9
at intervals of predetermined dead time, and in response to the operation, the secondary voltage is generated across the source and drain of the second rectifier transistor
14
during the interval the first main switch
7
is “on”, whereas the secondary voltage is generated across the source and drain of the first rectifier transistor
13
during the interval the second main switch
8
is “on”.
In this case, in the rectifier-transistor driving circuit
4
, the first diode
15
is turned on during the interval the first main switch
7
is “on” and the second diode
16
is turned on during the interval the second main switch
8
is “on”. Consequently, during the interval the first main switch
7
is “on”, the gate-source channel of the first rectifier transistor
13
is driven and turned on, and during the interval the second main switch
8
is “on”, the gate-source channel of the second rectifier transistor
14
is driven and turned on. Further, as the gate of the first rectifier transistor
13
and the gate of the second rectifier transistor
14
are short-circuited via the third secondary winding
23
of the transformer
1
during the interval the first and second main switches
7
and
8
both are “off”, the gate-source voltages of the first rectifier transistor
13
and the second rectifier transistor
14
each become intermediate voltages.
As the first rectifier transistor
13
is turned on during the whole interval the second main switch
8
is “off” and as the second rectifier transistor
14
is turned on during the whole interval the first main switch
7
is “off”, no current is practically allowed to flow into the body diode of the first rectifier transistor
13
and the body diode of the second rectifier transistor
14
, so that rectification can be carried out with a small loss.
(Related Art 2)
There have been proposed so-called two-stage converters for electronic systems like computers and as one example of a switching power supply unit for efficiently and stably supplying voltage, a preceding-stage buck converter and a following-stage half-bridge converter are combined in such a two-stage converter.
The buck converter is used for stepping down input voltage to a certain voltage level, whereas the half-bridge converter employs a half-bridge circuit for converting the input voltage to AC voltage, insulating, rectifying and smoothing the AC voltage to generate DC voltage.
A rectifying-smoothing circuit comprises a self-drive type synchronous rectifying circuit formed with a synchronous rectifying switch element connected to the secondary winding side of a transformer, capacitors and an inductor.
As described in a document under the title of “Buck+Halfbridge (d=50%) Topology Applied to very Low Voltage Power Converters” by P. Alou, J. Oliver, J. A. Cobos, O. Garcia and J. Uceda in the IEEE Applied Power Electronics Conference (APEC), 2001, a two-stage converter arrangement is made through the steps of fixing to 50% the duty ratio of a main switch element provided in a following-stage half-bridge converter and controlling the duty ratio of a switching element provided in a preceding-stage buck converter so as to make the duty ratio of the switching element variable in accordance with output voltage.
(Related Art 3)
There has been proposed a technique recently for exciting the primary winding of a transformer by using a half-bridge circuit, wherein a buck converter circuit and the half-bridge circuit
Shimizu Katsuhiko
Takagi Masakazu
Yamamoto Jun-ichi
Zaitsu Toshiyuki
Sterrett Jeffrey
TDK Corporation
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