Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2001-09-06
2002-11-05
Patel, Rajnikant B. (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C323S222000
Reexamination Certificate
active
06477063
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a multi-output switching power source apparatus for supplying stabilized DC voltages to a plurality of loads in industrial and consumer electronic appliances. As this kind of multi-output switching power source apparatus, the multi-output switching power source apparatus disclosed in Japanese Patent Publication No. 2803186 discloses is available.
FIG. 9
is a circuit diagram of the multi-output switching power source apparatus disclosed in Japanese Patent Publication No. 2803186. The reference numerals in the Japanese Patent Publication No. 2803186 are changed so as to be matched with those in the explanations of the present invention.
FIG. 10
shows operation waveforms at various portions in the circuit shown in FIG.
9
.
The operation of the conventional multi-output switching power source apparatus will be described below by using FIG.
9
and FIG.
10
.
First, when a main switching circuit
202
is ON, an input voltage Ei is applied to the primary winding
231
of a transformer
203
. At this time, a current Id flows through the primary winding
231
. When the main switching circuit
202
turns OFF at time t
1
, a current Is
1
flows from an output winding
233
via a diode
241
, and a current Is
2
flows from an output winding
234
via a diode
261
. A switching device
260
connected in parallel with the diode
261
is turned ON by a drive pulse signal Vg
6
from a control circuit
213
during a period between time t
1
and time t
2
. The current Is
2
becomes zero at time t
2
but flows continuously in the opposite direction via the switching device
260
. Hence, an output voltage Eo
2
is applied to the output winding
234
. On the other hand, when the current Is
1
becomes zero, the diode
241
turns OFF, and the current Is
1
stops flowing.
The control circuit
213
controls the ON period of the switching device
260
. When the switching device
260
turns OFF at time t
3
, the voltage across each winding of the transformer
203
is reversed. At this time, a diode
221
conducts on the primary side. A drive circuit
209
detects that the voltage across a drive winding
232
has been reversed, and outputs a drive pulse signal Vg
2
for turning on the switching device
220
of the main switching circuit
202
. The current Id flowing through the main switching circuit
202
flows so as to regenerate electric power for a DC power source
1
.
This current Id becomes zero at time t
4
as shown at a part (b) of FIG.
10
. After time t
4
, the current Id flows from the DC power source
1
to the primary winding
231
and the switching device
220
. At time t
5
, the switching device
220
turns OFF, and the above-mentioned operation after time t
1
is repeated.
When the number of turns of the primary winding
231
is N
31
, when the number of turns of the output winding
233
is N
33
, when the number of turns of the output winding
234
is N
34
, and when the ON period and the OFF period of the main switching circuit
202
are Ton and Toff respectively in the above-mentioned operation, output voltages Eo
1
and Eo
2
are represented by the following expressions (1) and (2) respectively:
Eo
1
≈(N
33
/N
31
)·(Ton/Toff)·Ei (1), and
Eo
2
≈(N
34
/N
31
)·(Ton/Toff)·Ei (2).
The relationships shown in the expressions (1) and (2) are similar to the relationships between the input and output voltages of an ordinary flyback converter. In the case of the conventional multi-output switching power source apparatus shown in
FIG. 9
, the drive circuit
209
sets the ON period Ton, and the control circuit
213
adjusts the OFF period Toff so as to stabilize the output voltage Eo
1
.
Generally in a multi-output switching power source apparatus, the output voltages of unstabilized outputs, other than outputs controlled directly so as to be stabilized, fluctuate depending on input and output conditions. This kind of voltage fluctuation is caused by induced voltages due to leak inductances of a transformer and voltage drops due to line impedances. In the case of the output voltage (Eo
2
) not controlled so as to be stabilized for example, when its output current (Io
2
) increases, a voltage drop due to a line impedance increases, and the output voltage (Eo
2
) lowers. On the other hand, in the case of the output voltage (Eo
1
) controlled so as to be stabilized, when its output current (Io
1
) increases, the ON period (Ton) of the main switching circuit
2
becomes long, and the output voltage (Eo
2
) rises. The output voltage (Eo
2
) fluctuates up and down as described above to stabilize the output voltage (Eo
1
) in spite of the voltage drop due to the line impedance. In particular, when the output current (Io
2
) is very small, an induced voltage due to a leak inductance, which is superimposed on the winding voltage of the transformer
3
, raises the output voltage (Eo
2
) further.
In the conventional multi-output switching power source apparatus shown in
FIG. 9
, however, the output current Io
1
serving as a stabilized output is large; even if the output current Io
2
serving as an unstabilized output is zero, the above-mentioned rise of the output voltage Eo
2
does not occur. This is because the current Is
2
flows during the whole range of the OFF period Toff regardless of input and output conditions, whereby a condition wherein the output current Io
2
is equivalently large is maintained. Furthermore, the induced voltage due to the leak inductance, which is superimposed on the output winding
34
, is controlled, and the voltage drop due to the line impedance is not affected by the output current Io
2
.
However, in the conventional multi-output switching power source apparatus shown in
FIG. 9
, when the output current Io
1
is small, the voltage drop due to the line impedance is also small, and the ON period Ton becomes short. In this case, the output voltage Eo
2
rises, thereby causing a problem.
To cope with the diversification of loads in industrial and consumer electronic appliances, multi-output switching power source apparatuses capable of stably supplying a plurality of output voltages in all conditions are demanded. The present invention is intended to provide a multi-output switching power source apparatus capable of controlling the fluctuations of output voltages in any load conditions excluding overloads.
BRIEF SUMMARY OF THE INVENTION
A multi-output switching power source apparatus in accordance with the present invention comprises:
a main switching circuit for converting the voltage of a DC power source into an AC voltage and for inputting the voltage to the primary winding of a transformer,
the transformer having a plurality of output windings including the primary winding,
a plurality of secondary switching circuits to which AC voltages induced across the plurality of output windings are input and which turn ON/OFF complementarily with the main switching circuit,
a plurality of smoothing circuits connected to the plurality of secondary switching circuits respectively, and
a control drive circuit for turning ON each of the secondary switching circuits after the main switching circuit is turned OFF, for turning ON the main switching circuit after each of the secondary switching circuits is turned OFF, for detecting one of a plurality of DC output voltages and for adjusting the ON period of the main switching circuit and the ON period of each of the secondary switching circuits to stabilize the DC output voltage.
In the multi-output switching power source apparatus in accordance with the present invention configured as described above, each of the secondary switching circuits turns ON during the OFF period of the main switching circuit, whereby the outputs are short-circuited via the transformer. For this reason, factors for fluctuating each output voltage can be limited to a voltage drop due to the line impedance of each output. Hence, the present invention can control the fluctuations of the output voltages in any load conditions including electric power supply from a load but
Ishii Takuya
Nagase Hisanori
Nakajima Yasufumi
Okura Hideki
Matsushita Electric - Industrial Co., Ltd.
Patel Rajnikant B.
Pearne & Gordon LLP
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