Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2002-11-26
2004-08-24
Riley, Shawn (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
Reexamination Certificate
active
06781852
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a switching power supply, and more specifically, to a switching power supply that can prevent an output voltage Vout from undershooting and fluctuating when the operation of the switching power supply is stopped.
DESCRIPTION OF THE PRIOR ART
Switching power supplies are widely used as power supplies for electrical and electronic equipment such as computers.
FIG. 7
is a circuit diagram showing a conventional switching power supply.
As shown in
FIG. 7
, the conventional switching power supply is composed of a transformer T
1
, a switching circuit located on the primary side of the transformer T
1
, and a rectifier of the self-drive type and a smoothing circuit located on the secondary side of the transformer T
1
. The switching power supply lowers a DC (direct current) input voltage Vin supplied to the switching circuit located on the primary side to generate a DC output voltage Vout and supplies it to a load. In
FIG. 7
, the load is represented by a resistance component RLoad, capacitance component CLoad, and reactance component LLoad.
A control circuit
10
controls main switches Q
1
and Q
2
included in the switching circuit of the primary side based on the output voltage Vout. Specifically, the control circuit
10
lowers the duty factor of the main switches Q
1
and Q
2
when the output voltage Vout increases relative to the desired voltage so as to decrease the electric power supplied to the load and raises the duty factor of the main switches Q
1
and Q
2
when the output voltage Vout decreases relative to the desired voltage so as to increase the electric power supplied to the load. Thus, the output voltage Vout supplied to the load can be always stabilized at the desired voltage. Because the control circuit
10
belongs to the primary side, the control circuit
10
cannot receive the output voltage Vout directly. The control circuit
10
is therefore supplied via an isolation circuit
20
with a voltage Vout′ associated with the output voltage Vout.
Operating voltage Vcc for the control circuit
10
is generated by an operating voltage generation circuit consisting of a transistor Tr
1
, resistor R
1
, and zener diode Z
1
. A capacitor C
3
is connected between power terminals of the control circuit
10
for stabilizing the operating voltage Vcc. The operating voltage generation circuit is activated when an operation switch S
1
is in the ON state and inactivated when the operation switch S
1
is in the OFF state. The operation switch S
1
can be controlled from the outside. When the operation of the switching power supply shown in
FIG. 7
is to be started, the operation switch S
1
is turned ON; when the operation of the switching power supply is to be terminated, the operation switch S
1
is turned OFF.
Rectifying switches Q
3
and Q
4
included in the rectifier of the secondary side are self-driven by the secondary voltage of the transformer T
1
. Further, resistors R
2
and R
3
are inserted between the gate electrodes and the source electrodes of the rectifying switches Q
3
and Q
4
, respectively, so as to prevent the gate electrodes of the rectifying switches Q
3
and Q
4
from being in a floating state.
Next, the operation of the conventional switching power supply shown in
FIG. 7
will be explained.
FIG. 8
is a timing chart showing the operation of the conventional switching power supply shown in FIG.
7
.
As shown in
FIG. 8
, when the operation switch S
1
is in the ON state, the gate-source voltages V
GS
(Q
1
) and V
GS
(Q
2
) of the main switches Q
1
and Q
2
are alternately activated to a high level at a predetermined switching frequency under the control of the control circuit
10
. As a result, the polarity of the primary voltage V
LP
of the transformer T
1
is alternately inversed, so that primary side capacitors C
1
and C
2
are alternately charged and discharged.
Synchronously with the operation of the primary side, the polarity of the secondary voltage appearing at secondary coils Ls
1
and Ls
2
of the transformer T
1
is alternately inversed, so that the rectifying switches Q
3
and Q
4
are alternately brought into ON state in turn at the predetermined switching frequency. More specifically, while the main switch Q
1
is in the ON state owing to the gate-source voltage V
GS
(Q
1
) being at a high level, the gate-source voltage V
GS
(Q
3
) of the rectifying switch Q
3
is raised to a voltage greater than the threshold voltage thereof by the voltage (secondary voltage) appearing at secondary coil Ls
2
, whereby the rectifying switch Q
3
turns ON. On the contrary, while the main switch Q
2
is in the ON state owing to the gate-source voltage V
GS
(Q
2
) being at a high level, the gate-source voltage V
GS
(Q
4
) of the rectifying switch Q
4
is raised to a voltage greater than the threshold voltage thereof by the voltage (secondary voltage) appearing at secondary coil Ls
1
, whereby the rectifying switch Q
4
turns ON.
As a result, the secondary voltage of alternately inversed polarity is rectified. The rectified voltage is smoothed by the smoothing circuit, which consists of an output reactor Lout and output capacitor Cout so that the stabilized output voltage Vout is generated.
On the other hand, when the operation switch S
1
is turned OFF at a certain time, the operation of the control circuit
10
is stopped because the transistor Tr
1
turns OFF, so that both the main switches Q
1
and Q
2
are put in the OFF state. That is, the switching operation is stopped.
However, because the operation of the switching circuit of the primary side is stopped when the operation switch S
1
is turned OFF, one or the other of the rectifying switches Q
3
and Q
4
is kept in the ON state and a reverse current begins to flow from the output capacitor Cout and the capacitance component CLoad of the load to the output reactor Lout.
FIG. 8
shows the case where the rectifying switch Q
3
is kept in the ON state at first in response to the operation switch S
1
being turned OFF. In this case, because the switching circuit of the primary side is stopped, the discharge path for the electric charge of the gate electrode of the rectifying switch Q
3
is substantially only the resistor R
2
. Therefore, the gate-source voltage V
GS
(Q
3
) of the rectifying switch Q
3
falls gradually owing to the current flow through the resistor R
2
. During this period, the reverse current flowing to the output reactor Lout continues.
On the other hand, when the rectifying switch Q
3
turns OFF because the gate-source voltage V
GS
(Q
3
) of the rectifying switch Q
3
falls below the threshold voltage thereof owing to the decrease of the output voltage Vout and the secondary voltage by discharge of the output capacitor Cout and the capacitance component CLoad of the load and discharge of the electric charge from the gate electrode of the rectifying switch Q
3
via resistor R
2
, a flyback voltage rises at the transformer T
1
. The flyback voltage boosts an internal voltage Vp in the switching circuit via the transformer T
1
and boosts the gate-source voltage V
GS
(Q
4
) of the rectifying switch Q
4
. Therefore, the rectifying switch Q
4
stays ON.
As shown in
FIG. 8
, because the direction of the current flowing to the output reactor Lout via the rectifying switch Q
4
becomes forward temporarily, the output capacitor Cout and the capacitance component CLoad of the load are charged during this period, so that the output voltage Vout is increased.
Then, when the direction of the current flowing to the output reactor Lout becomes reverse, the gate-source voltage V
GS
(Q
4
) of the rectifying switch Q
4
falls gradually owing to the decrease of the output voltage Vout and the secondary voltage by discharge of the output capacitor Cout and the capacitance component CLoad of the load and discharge of the electric charge from the gate electrode of rectifying switch Q
4
via resistor R
3
. Then, when the rectifying switch Q
4
turns OFF because the gate-source voltage V
GS
(Q
4
) of the rectifying switch Q
4
falls
Hatakeyama Haruhiko
Hatta Masaharu
Watanabe Masato
Riley Shawn
Seed IP Law Group PLLC
TDK Corporation
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