Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2000-04-12
2002-12-03
Sterrett, Jeffrey (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S091000, C363S093000
Reexamination Certificate
active
06490178
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a switching power circuit.
2. Description of Related Art
FIG. 5
is a block diagram showing a switching power circuit having a conventional MAGAMP magnetic amplifier system regulator. In
FIG. 5
, symbol T
1
denotes a switching power supply transformer, which is comprised of a primary winding N
11
, secondary windings N
21
, N
22
, and a primary auxiliary winding N
12
supplying power to a primary control circuit. A commercial power supply Vin (AC) is rectified by a diode bridge DB
1
, and is smoothed by a smoothing condenser C
1
to thereby acquire a direct current voltage Vin (DC). A field effect transistor (FET) Q
1
performs high-frequency switching of the direct current voltage Vin (DC) charged in the condenser C
1
, and the direct current voltage Vin is applied to the primary winding N
11
of the transformer T
1
. A pulse voltage synchronous with an output of the primary winding N
11
is generated at the secondary winding N
21
of the transformer T
1
. A rectifier circuit formed of diodes D
11
, D
12
, a choke coil L
11
and a condenser C
11
rectifies and smoothes the pulse voltage to thereby acquire a direct current voltage Vo
1
.
Resistances R
11
, R
12
, R
13
, a Zener diode Q
3
, a photo coupler Q
2
and the primary control circuit detect and feedback-control the direct current voltage Vo
1
by controlling the on/off time ratio for switching the field effect transistor Q
1
in such a manner that the direct current voltage Vo
1
can be a desired value. At the second winding N
22
, a saturable reactor L
22
, a transistor Q
22
, a Zener diode Q
21
, resistances R
21
, R
22
, R
23
, R
24
, R
25
and a diode D
23
control a direct current voltage Vo
2
to a desired voltage by a MAGAMP (Magnetic Amplifier) system. A forward-type rectifier circuit is comprised of a rectifier diode D
21
, a flywheel diode D
22
, a choke coil L
21
and a smoothing condenser C
21
.
The MAGAMP system is based upon a magnetic saturation operation of the saturable reactor. The saturable reactor is a device which has a sufficient initial inductance and is magnetically saturated to have an inductance L≈0 when an integrated value of a certain voltage * a time (which is generally called the “ET integrated value”) is applied to the saturable reactor. In
FIG. 5
, symbol L
22
denotes the saturable reactor.
FIG. 6
is a waveform chart showing waveforms at points A and B in the switching power circuit in FIG.
5
. As shown in
FIG. 6
, when a voltage V
1
is generated at the point A at a time T
0
, the impedance of the saturable reactor L
22
is high until a predetermined time Ti, so that no voltage is generated at the point B. When the ET integrated value (V
1
* (T
1
−T
0
) reaches a saturation ET integrated value of the saturable reactor L
22
at the time T
1
, the impedance of the saturable reactor L
22
is decreased to such a low value that the voltage at the point A passes through the point B. A reset current Ir is carried through the saturable reactor L
22
through the transistor Q
22
between times T
2
and T
3
, so that the saturable reactor L
22
is reset or released from its saturated state. Thereafter, the same process (the high impedance, the decrease in impedance, and the reset) is repeated from the time T
3
.
The saturation ET integrated value of the saturable reactor L
22
can be controlled by controlling the reset current Ir. Specifically, the resistances R
21
, R
22
detect the output voltage Vo
2
, and the reset current Ir corresponding to a difference of the detected output voltage Vo
2
from a reference voltage is carried through the saturable reactor L
22
, thus stabilizing the output voltage Vo
2
at a desired voltage. This is called the MAGAMP system regulator.
A synchronous rectifying system may be used instead of the MAGAMP system. A description will now be given of a synchronous rectifier circuit with reference to FIG.
7
.
FIG. 7
is a block diagram showing a switching power circuit provided with a synchronous rectifying system regulator. Field effect transistors (FET) Q
20
, Q
21
as semiconductor switching devices are connected in parallel with secondary rectifier diodes D
21
, D
22
, respectively. The field effect transistor Q
20
is turned on only while the diode D
21
is conducted. The field effect transistor Q
21
is turned on only while the diode D
22
is conducted. Consequently, a current is carried through the field effect transistors Q
20
, Q
21
with low ON resistance, and the current is rectified by a drop in voltage by a forward voltage Vf (≈0.5V) of the diodes D
21
, D
22
or less.
Since a certain limited time is required for turning on/off the field effect transistors Q
20
, Q
21
, there is a time-lag when drive signals for the field effect transistors Q
20
, Q
21
are acquired from a drive signal for the primary field effect transistor Q
1
in synchronism therewith. To address this problem, a PWM control circuit detects the output voltage Vo
1
regarded as a reference oscillation signal S
0
, and a signal S
1
with a time lag from the reference oscillation signal S
0
drives the field effect transistor Q
1
. A signal S
2
with a smaller time lag than the signal S
1
drives the synchronous rectifier field effect transistors Q
20
, Q
21
. This is called the synchronous rectifying system.
Thus, the synchronous rectifying system requires a complicated synchronous signal circuit. Further,. an additional circuit is needed for acquiring the drive signal for the primary switching device (field effect transistor Q
1
). Therefore, the synchronous rectifying system is not suitable for a method wherein a control IC is arranged at an upstream side (see FIG.
5
), which is now the mainstream.
In recent years, the operating voltage of digital ICs has been lowered, and the direct current voltage Vo
2
of the above-mentioned switching power circuit is set to 3.3V in many cases. In such cases, a power loss (=Vf * Io) due to the forward voltage Vf (≈0.5V) of the diodes D
21
, D
22
is relatively larger than in the case where the voltage Vo
2
is 5V. This deteriorates the power conversion efficiency.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to solve the above-mentioned technical problems.
It is another object of the present invention to provide a switching power circuit, which has improved power conversion efficiency with a simple structure and low costs.
To accomplish the above objects, according to a first aspect of the present invention, there is provided a switching power circuit comprising a transformer having a plurality of windings, one end of a primary winding of the transformer being connected to a voltage source, a first switching device, another end of the primary winding being connected to a return side of the voltage source through the first switching device, a magnetic amplifier connected to the secondary winding of the transformer, a forward-type rectifier circuit connected to the magnetic amplifier and including at least a flywheel diode, a second switching device connected in parallel with the flywheel diode, and a control circuit for turning on/off the second switching device according to an output of the secondary winding of the transformer or according to a signal acquired by inverting the output of the secondary winding.
To accomplish the above objects, according to a second aspect of the present invention, there is provided a switching power circuit comprising a transformer having a plurality of windings, one end of a primary winding of the transformer being connected to a voltage source, a first switching device, another end of the primary winding being connected to a return side of the voltage source through the first switching device, a semiconductor switching device connected to the secondary winding of the transformer, a forward-type rectifier circuit connected to the semiconductor switching device and including at least a flywheel diode, a second switching device connected in parallel with the flywheel diode, and
Canon Kabushiki Kaisha
Laxton Gary L.
Rossi & Associates
Sterrett Jeffrey
LandOfFree
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