Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2001-10-09
2003-07-08
Sherry, Michael (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S097000, C363S131000, C363S021080
Reexamination Certificate
active
06590790
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a switching power supply device which achieves noise reduction during switching.
2. Description of the Related Art
In a switching power supply device having a switching device connected to the primary of a transformer, such that the switching device is used for a self-excited or separately-excited oscillation to provide an output from the secondary of the transformer, a pulsed voltage across the switching device is provided when the switching device is turned on and off, and noise is radiated externally when the pulse increases and decreases. Radiation noise having a predetermined level or higher of radiation energy might adversely affect external devices, and must be reduced by some technique. A conventional method of effectively reducing the radiation noise is to connect capacitors across both ends of the switching device so as to mitigate a rapid change of the voltage so that high frequency noise components may be eliminated.
FIG. 1
illustrates a conventional switching power supply device of this type, to which capacitors are connected.
In the switching power supply device shown in
FIG. 1
, a transformer T having the primary winding N
p
and the secondary winding N
s
is connected in series to a first switch circuit S
1
and an input power source E. One end of a series circuit comprising a second switch circuit S
2
and a capacitor C is connected to one end of the first switch circuit S
1
. A rectifier smoothing circuit is connected to the secondary winding N
s
of the transformer T. The switching power supply device is a self-excited oscillation device. The details of the switching power supply device shown in
FIG. 1
are disclosed in Japanese Unexamined Patent Application Publication No. 11-187664. In the switching power supply device disclosed, the first switch circuit S
1
includes a first switching device Q
1
and a capacitor C
1
which are connected in parallel to each other, and the second switch circuit S
2
includes a second switching device Q
2
and a capacitor C
2
which are connected in parallel to each other. The capacitors C
1
and C
2
allow a rapid change of the voltages generated across the first and second switching devices Q
1
and Q
2
, respectively, to be mitigated, so that high frequency noise components may be eliminated.
However, in the switching power supply device shown in
FIG. 1
, the capacitors C
1
and C
2
must have a high voltage rating. Furthermore, the capacitance of the capacitors C
1
and C
2
must be a predetermined level or higher in order to constantly reduce the radiation noise. Therefore, a problem occurs in that the capacitors C
1
and C
2
are so large that a compact and low cost switching power supply device may not be achieved.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a switching power supply device which uses a less capacitive impedance to provide higher radiation noise reduction performance.
To this end, the switching power supply device includes a transformer having primary and secondary windings, a first switch circuit, an input power source, a second switch circuit, and a rectifier smoothing circuit connected to the second winding of the transformer. The transformer, the first switch circuit, and the input power source are connected in series. The second switch circuit and a capacitor are connected in series to form a series circuit, and one end of the series circuit is connected to one end of the first switch circuit. The first switch circuit includes a parallel connection circuit comprising a first switching device and a first diode, and the second switch circuit includes a parallel connection circuit comprising a second switching device and a second diode. The switching power supply device further includes a switching control circuit for controlling each of the first and second switching devices so as to be alternately turned on and off with an off-time interposed between on-times. The transformer further has a third winding wound in the same direction as that of the primary winding, and a capacitive impedance element is connected between the termination of the third winding and the input power source.
In
FIG. 1
, if the primary winding N
p
of the transformer T has an inductance L
p
the resonant frequency f
r
while the voltage across the first switching device Q
1
varies is found by equation (1) as follows:
f
r
=
1
2
π
L
p
C
1
(
1
)
where the parasitic capacitance of the first switching device Q
1
is negligible.
From equation (1), as the capacitance of the capacitor C
1
increases, the resonant frequency f
r
decreases, and high frequency noise components are reduced.
On the other hand, as shown in
FIG. 2
, in a power supply device according to the present invention, a transformer T includes a third winding (in
FIG. 2
, a second driving winding N
b2
having turns a turn Nb
2
corresponds to the third winding) which is wound in the same direction as that of the primary winding N
p
(having turns N
p
). A capacitive impedance element Ca is connected between the termination of the third winding and an input power source E, so that the capacitors C
1
and C
2
shown in
FIG. 1
may be removed, or, otherwise, may have lower capacitances. If the capacitive impedance is indicated by C
a
, and the inductance of the third winding is indicated by L
a
, the resonant frequency f
r
while the voltage across the first switching device Q
1
varies is found by equation (2) as follows.
f
r
=
1
2
π
(
L
p
+
L
a
)
C
a
(
2
)
It is understood from comparison between equations (1) and (2) that the capacitor C
a
can be more compact than the capacitor C
1
if the resonant frequencies f
r
are the same. Therefore, according to the present invention, a capacitive impedance element is connected between the termination of a third winding and an input power source so that the radiation noise caused by a rapid change of the voltage across a switching device can be reduced. This allows the capacitive impedance to be lower, thereby achieving a compact and low-cost switching power supply device.
Preferably, the capacitive impedance element includes a series circuit comprising a capacitor and an inductor.
This prevents a current from rapidly flowing into the capacitor, thereby achieving noise reduction.
The inductor may be a ferrite bead. A compact and low-cost ferrite bead would prevent a current which rapidly flows into the capacitor, in particular, a high frequency current, thereby achieving noise reduction.
Preferably, a voltage generated at the third winding is used to turn on the second switching device. If a driving winding for allowing the second switching device to be turned on is used as the third winding, the transformer may be more compact.
At least one of the first and second switching devices may be a field effect transistor. Therefore, the parasitic diode of the field effect transistor can be used instead of the first and/or second diode. In this case, the first and/or second diode may be removed, thereby making the switching power supply device more compact and light-weight.
Preferably, the transformer includes a driving winding which allows a voltage to turn on the first switching device to be generated to provide self-excited oscillation. Therefore, there is no need for ICs such as oscillation circuits and control circuits, thereby achieving a compact, light-weight, and low-cost switching power supply device.
The transformer may include either a leakage inductor connected between the primary and secondary windings, or an inductor connected in series to the transformer. The resulting inductor and the capacitor form a resonant circuit.
The resonance by the inductor and the capacitor allows the energy accumulated in the inductor to be output without being dissipated, thereby providing high efficiency. Furthermore, the second switching device can perform a zero current turn-off operation, thereby reducing switching loss.
Preferably, the rectifier smoothing ci
Hosotani Tatsuya
Takahashi Kentaro
Yamada Tomohiro
Keating & Bennett LLP
Laxton Gary L.
Murata Manufacturing Co. Ltd.
Sherry Michael
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