Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2000-06-21
2002-10-01
Patel, Rajnikant B. (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C307S125000
Reexamination Certificate
active
06459594
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a switching power supply apparatus provided with a converter, such as a DC-DC converter or an AC-DC converter.
2. Description of the Related Art
FIG. 9
shows an example of a circuit construction of a converter which is a basic circuit element of a conventional switching power supply apparatus. The converter shown in
FIG. 9
is an isolated and has a transformer
1
therein. A primary winding N
1
of the transformer
1
is connected to a primary circuit
2
while a secondary winding N
2
thereof is connected to a secondary circuit (output circuit)
3
.
The primary circuit
2
includes a primary switching element Q (a MOS-FET in the example shown in
FIG. 9
) and a resistor
4
and has an input thereof connected to an input power supply
5
. The secondary circuit
3
includes diodes
6
and
7
, a choke inductor
8
, and a smoothing capacitor
10
and has output terminals
3
a
and
3
b
thereof connected to a load (not shown). The direct-current input voltage Vin of the input power supply
5
is applied across the primary winding N
1
of the transformer
1
in accordance with on-off operations of the primary switching element Q, and an output voltage is produced from the secondary winding N
2
in accordance with the input voltage Vin. This output voltage from the secondary winding N
2
is rectified and smoothed by the secondary circuit
3
, and a direct-current voltage Vout is applied across the load.
As shown in
FIG. 9
, the gate of the primary switching element Q is connected to a control circuit
11
. A tertiary winding N
3
is provided in the transformer
1
, and this tertiary winding N
3
is connected to a detection circuit
12
. This detection circuit
12
includes diodes
13
and
14
, a choke inductor
15
and a smoothing capacitor
16
. These circuit elements form the same circuit as the secondary circuit
3
. Since a voltage substantially corresponding to a voltage generated across the secondary winding N
2
is generated across the tertiary winding N
3
, the voltage across the tertiary winding N
3
is rectified and smoothed by the same type circuit as the secondary circuit
3
, whereby a voltage corresponding to the output voltage Vout of the secondary circuit
3
is generated across the smoothing capacitor
16
. In addition to the above construction, the detection circuit
12
includes a diode
17
and voltage-dividing resistors
18
and
19
. The voltage across the smoothing capacitor
16
is voltage-divided by the voltage-dividing resistors
18
and
19
, and the divided voltage is applied as a detection voltage of the output voltage Vout to the control circuit
11
.
The control circuit
11
controls the on-off operations of the primary switching element Q so that the output voltage Vout, which is output based on the detection voltage from the secondary circuit
3
, has a stable set voltage value. In
FIG. 9
, N
1
′ denotes a primary winding and reference numeral
20
denotes a diode.
As described above, the switching power supply apparatus shown in
FIG. 9
performs stabilization control on the output voltage Vout of the secondary circuit
3
by causing the detection circuit
12
to indirectly detect the output voltage Vout of the secondary circuit
3
, and by causing the control circuit
11
to perform switching control of the primary switching element Q based on the detection voltage.
However, in the circuit shown in
FIG. 9
, since the switching power supply apparatus is constructed such that the output voltage Vout of the secondary circuit
3
is indirectly detected, there has been a problem that the output voltage Vout deviates from a set output voltage value Vsp because of a change of the input voltage Vin.
This problem may arise for the following reasons. For example, there is a case in which the number of turns of the secondary winding N
2
and that of the tertiary winding N
3
are different. In this case, a voltage induced across the secondary winding N
2
and a voltage induced across the tertiary winding N
3
are different. Therefore, it is necessary to use devices individually having various different characteristics, such as an endurance voltage, a forward voltage, and a reverse current, as the diodes
6
and
7
of the secondary circuit
3
and the diodes
13
and
14
of the detection circuit
12
. Even when the number of turns of the secondary winding N
2
and that of the tertiary winding N
3
are the same, in the same manner as described above. there are some cases in which devices having various different characteristics are used as the diodes
6
,
7
,
13
, and
14
.
When the input voltage Vin is changed, for example, the reverse current of each of the diodes
6
,
7
,
13
, and
14
is changed. When the characteristics of each of the diodes
6
,
7
,
13
, and
14
are different, differences in changes of the reverse currents occur among the diodes
6
,
7
,
13
, and
14
.
For example, when the input voltage Vin has a voltage value “a”, circuit constants of the control circuit
11
and the like are supposed to be set so that the output voltage Vout having the set output voltage value Vsp is stably output in accordance with switching control operations of the control circuit
11
based on the detection voltage of the detection circuit
12
. Even though the control circuit
11
is formed as such, when the input power supply
5
supplying the input voltage Vin having a voltage value “b”, which is different from the voltage value “a”, is connected to the primary circuit
2
, concerning a case in which the input voltage Vin has the voltage value “a” and a case in which the input voltage Vin has the voltage value “b”, there is a deviation between the detection voltages of the detection circuit
12
to the output voltage Vouts in accordance with a difference in the change of the reverse current of each of the diodes in response to the change of the input voltage Vin.
Since the control circuit
11
controls the on-off operations of the primary switching element Q based on the detection voltage, the output voltage Vout is obtained having a voltage value which deviates from the set output voltage value Vsp. Therefore, the output voltage Vout having the set output voltage value Vsp cannot be stably output.
As the input voltage Vin increases, the deviation of the detection voltage of the detection circuit
12
to the output voltage Vout increases. Therefore, as indicated by the dashed lines in FIG.
2
and
FIG. 4
, the greater the input voltage Vin becomes, the greater the deviation of the output voltage Vout to the set output voltage value Vsp becomes.
Recently, the operating voltages in ICs and LSIs tend to be lower, and lowvoltage driven and high-current outputs are demanded. Because of this demand, there are some cases in which, instead of the diodes
6
,
7
,
13
, and
14
, a synchronous rectifier, such as a power MOS-FET, is used. In this case, when the input voltage Vin is changed, the drive voltage of the synchronous rectifier is changed, and the resistance of the on-resistor of the synchronous rectifier is changed, whereby the voltage drop of the synchronous rectifier is changed. Likewise, because of this, there arises a problem in that the output voltage Vout cannot be stably output due to the change of the input voltage Vin.
SUMMARY OF THE INVENTION
The present invention is made to solve the foregoing problems, and an object of the present invention is to provide a switching power supply in which an output voltage having a set voltage value can be stably output regardless of changes in the input voltage.
To achieve the foregoing objects, this invention provides a method for solving the above problems using the construction described below. That is, a first aspect of the invention provides a method for solving the above problems using a construction in which in a switching power supply apparatus is provided for outputting a voltage from the secondary winding of a transformer based on an input voltage in accordance with on-off operations of a primary
Moromizato Eito
Tsuji Hitoshi
Murata Manufacturing Co. Ltd.
Ostrolenk Faber Gerb & Soffen, LLP
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