Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2001-12-19
2002-12-24
Vu, Bao (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S025000, C363S056100, C363S134000
Reexamination Certificate
active
06498735
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to switching power supply units, and more specifically, to a switching power supply unit that improves efficiency and performance in a light load operation.
2. Description of the Related Art
Switching power supply units related to the present invention are disclosed in U.S. Pat. Nos. 6,061,252 and 6,201,713.
FIG. 1
shows a diagram of a switching power supply unit disclosed in U.S. Pat. No. 6,061,252.
In the switching power supply unit, a transformer T having a primary winding T
1
and a secondary winding T
2
, a first switching circuit S
1
, and an input power supply E are connected in series, and a series circuit of a second switching circuit S
2
and a capacitor C is connected in parallel to the primary winding T
1
of the transformer T. Furthermore, a rectifying and smoothing circuit including a rectifier element Ds is connected to the secondary winding T
2
of the transformer T, and a capacitor Cs is connected in parallel to the rectifier element Ds. Furthermore, the transformer T includes a first drive winding T
3
and a second drive winding T
4
. The first drive winding T
3
is connected to a first control circuit
11
and the second drive winding T
4
is connected to a second control circuit
12
. The control circuits
11
and
12
which constitute a switching control circuit control the ON/OFF of the first and the second switching elements Q
1
and Q
2
, respectively.
The first switching circuit S
1
is defined by a parallel connection circuit including a first switching element Q
1
, a first diode D
1
, and a first capacitor C
1
. The second switching circuit S
2
is defined by a parallel connection circuit including a second switching element Q
2
, a second diode D
2
, and a second capacitor C
2
. L indicates a leakage inductor in the primary winding T
1
or an inductor that is connected separately.
In the configuration described above, the first and the second control circuits
11
and
12
constituting the switching control circuit control the first switching circuit S
1
and the second switching circuit S
2
to alternately turn the switching circuits S
1
and S
2
on and off with an intermediate period in which both of the switching circuits S
1
and S
2
are turned off, such that energy is stored in the primary winding T
1
of the transformer T during an ON period of the first switching circuit S
1
and the energy is discharged from the secondary winding T
2
of the transformer T during an OFF period of the first switching circuit S
1
. The operation cycle is repeated such that energy is received from the secondary winding T
2
and a power is supplied to the load. The first and the second control circuits
11
and
12
include transistors connected to the control terminals of the switching elements Q
1
and Q
2
, and time constant circuits connected to the control terminals of the transistors, so that the circuits control when the switching elements Q
1
and Q
2
are turned on and off.
FIG. 2
is an operation waveform chart of the switching power supply unit. Referring to
FIG. 2
, Q
1
and Q
2
indicate when the switching elements Q
1
and Q
2
are turned on and off, respectively. Vds
1
and Id
1
indicate a voltage across and a current through the switching element Q
1
, respectively. Vds
2
and Id
2
indicate a voltage across and a current through the switching element Q
2
, respectively. Vs and Is indicate a voltage across the rectifier element Ds and a current through the secondary winding T
2
, respectively.
In the configuration described above, when the first switching element Q
1
is turned off, a voltage is generated on the drive winding T
4
for the second switching element Q
2
, whereby the second switching element Q
2
is turned on. Then, the transistor in the control circuit
12
is turned on after a certain period of time determined by the time constant circuit has elapsed, whereby the second switching element Q
2
is turned off. At this time, a voltage is generated in the drive winding T
3
for the first switching element Q
1
when the rectifier element Ds on the secondary side is turned off if the rectifier element Ds has been turned on, or when the second switching element Q
2
is turned off if the rectifier element Ds has been turned off. That is, a voltage is generated when both of the second switching element Q
2
and the rectifier element Ds are turned off, whereby the first switching element Q
1
is turned on. By the operation described above, the first switching element Q
1
and the second switching element Q
2
are controlled to alternately turn on and off with an intermediate period in which both of the switching elements Q
1
and Q
2
are turned off, such that an energy which is stored on the primary winding of the transformer T during an ON period of the first switching element Q
1
is output from the secondary winding T
2
as electric energy during an OFF period of the first switching element Q
1
.
In the switching power supply unit described above, a zero-voltage switching operation is performed in which the first and the second switching elements Q
1
and Q
2
are turned on after voltages applied across each of the switching elements drops to zero. This reduces switching loss and prevents switching surge, to thereby improve efficiency and performance.
However, in the switching power supply unit described above, when the load is light, a portion of the energy stored in the primary winding T
1
of the transformer T during an ON period of the first switching element Q
1
is regenerated on the input side. In
FIG. 3
, regenerative current is shown as an area indicated by A. The regenerative current becomes a circulating current that does not contribute to output. If the ON period of the switching element Q
2
is fixed, the circulating current increases as the output power decreases (as the load becomes lighter). A large circulating current greatly increases conduction loss in the first and the second switching elements Q
1
and Q
2
and the transformer T, resulting in greatly reduced efficiency under light load operation.
Accordingly, in a switching power supply unit according to U.S. Pat. No. 6,201,713, circulating current is reduced by shortening the ON period of the second switching element Q
2
in a light load operation, thereby improving efficiency. However, a reduction in circulating current causes an increase in the switching frequency. This increase in switching frequency produces an increase in switching loss including loss in drive circuits for switching elements.
That is, if circulating current is large, switching loss is small because the switching frequency is low. However, conduction loss associated with the circulating current increases. On the other hand, if circulating current is reduced, conduction loss is reduced. However, switching loss increases due to the higher switching frequency.
SUMMARY OF THE INVENTION
To overcome the above-described problems, preferred embodiments of the present invention provide a switching power supply unit that reduces circulating current and thereby reduces conduction loss under light load operation or no load operation, and that also reduces switching loss and switching surge, thus improving the efficiency and reducing the size and weight of a switching power supply unit.
A preferred embodiment of the present invention provides a switching power supply unit including a first switching circuit including a parallel connection circuit that includes a first switching element, a first diode, and a first capacitor; a second switching circuit including a parallel connection circuit that includes a second switching element, a second diode, and a second capacitor; a capacitor connected in series to the second switching circuit, the series circuit of the capacitor and the second switching circuit are connected to one end of the first switching circuit; a transformer including a primary winding and a secondary winding; a rectifying and smoothing circuit including a rectifier element, connected to the secondary wind
Hosotani Tatsuya
Takemura Hiroshi
Keating&Bennett, LLP
Murata Manufacturing Co. Ltd.
Vu Bao
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