Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2001-02-23
2002-04-09
Sterrett, Jeffrey (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S097000
Reexamination Certificate
active
06370040
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a switching power supply apparatus in which at least two output voltages are produced by means of one transformer.
2. Description of the Related Art
Generally, in a switching power supply apparatus having the configuration in which two output voltages are obtained by means of one transformer, only one output voltage is detected to control a first switching element connected to the primary of the transformer.
FIG. 1
is a circuit diagram of a conventional switching power supply apparatus configured as described above.
A transformer T is provided with a primary winding T
1
, secondary windings T
2
and T
3
, and a drive winding T
4
. A switching element Q
1
(hereinafter, referred to as a first switching element) is connected in series with the primary winding T
1
. A control circuit CT controls the on-time of the first switching element Q
1
. Output from the secondary winding T
2
is rectified by a rectification diode Ds
1
and smoothed by a capacitor to be output as a first output Vo
1
. Moreover, output from the other secondary winding T
3
is rectified by a rectification diode Ds
2
and smoothed by a capacitor to be output as an output Vo
2
. In this example, the control circuit containing the drive winding T
4
of the transformer T controls the on-time of the first switching element Q
1
, and moreover, causes the first switching element Q
1
to oscillate autonomously. With this configuration employed, when the first switching element Q
1
is on, an input voltage Vin is applied to the primary winding T
1
, making input current flow so that energy is stored in the transformer T. Furthermore, when the first switching element Q
1
is off, the energy stored in the transformer T is released as output current via the secondary windings T
2
and T
3
. Thus, the device is configured as an energy storage type switching power supply apparatus .
In the control circuit CT, output voltage from the drive winding T
4
is delayed to some degree by a resistor R
4
and a capacitor C
3
to be applied to the control terminal of the first switching element Q
1
, and moreover, is applied to a time constant circuit comprising a resistor R
1
and a capacitor C
1
, so that a transistor Tr
1
is turned on after a constant time-period, causing the first switching element Q
1
to turn off. When the first switching element Q
1
turns off, energy stored in the transformer T is released as electric current. When the release of the energy is completed, reverse voltages are applied to the rectification diodes Ds
1
and Ds
2
, respectively. The capacitive impedance, equivalent from the standpoint of these rectification diodes, and the winding inductor of the transformer T resonate, and with voltage generated in the drive winding T
4
of the first switching element Q
1
, voltage is applied to the control terminal of the first switching element Q
1
, so that the first switching element Q
1
turns on again.
Moreover, an output voltage detection circuit DT is provided on the output side of the rectification circuit comprising the rectification diodes Ds
1
and Ds
2
, and capacitors. The output voltage detection circuit DT detects only output voltage with respect to the first output Vo
1
. That is, the voltage of the first output Vo
1
is detected by resistors R
2
and R
3
. The voltage divided by the resistors R
2
and R
3
is input to a voltage comparison terminal Vr as a comparison voltage. A series circuit comprising a photodiode PD
1
, a shunt regulator ZD
1
, and a resistor
5
is connected between Vo
1
and GND. The above detection voltage is input to the voltage comparison terminal (reference terminal) Vr of the shunt regulator ZD
1
. A phototransistor PTr
1
arranged in opposition to the photodiode PD
1
is connected between the base and collector of the transistor Tr
1
in the above-described control circuit CT.
With the above arrangement, when the first output Vo
1
is increased, the input voltage to the shunt regulator ZD
1
is increased. Then, since the inflow current to the photodiode PD
1
is increased, the transistor Tr
1
is turned on earlier via the operation of the phototransistor PTr
1
in the control circuit CT. As a result, the on-time of the first switching element Q
1
becomes shortened, and thereby, the first output voltage Vo
1
is reduced. In this way, the output voltage of the first output Vo
1
is monitored by the output voltage detection circuit DT, and a control signal corresponding to the detection voltage is formed and fed back to the control circuit CT, whereby the first output voltage Vo
1
can be stabilized. Moreover, since the first output Vo
1
is stabilized, the second output voltage Vo
2
is stabilized to some degree.
However, the above-described conventional switching power supply apparatus shown in
FIG. 1
has the problem that, though the output voltage of the first output voltage Vo
1
to which the output voltage detection circuit is directly connected can be stabilized with high accuracy, a sufficiently high voltage accuracy can not be obtained for the output voltage Vo
2
other than the output voltage Vo
1
.
To solve this problem, in some cases, a voltage stabilization circuit such as a series regulator circuit or the like is inserted in the second output voltage Vo
2
circuit, or a dummy resistor is used. With such circuits, problems are caused such as an increase in number of parts, reduction of circuit efficiency, temperature rise of the power supply apparatus, and so forth.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a switching power supply apparatus in which, when at least two output voltages are provided, each of the output voltage accuracies can be stabilized at a predetermined control ratio by detecting the respective output voltages.
The switching power supply apparatus of the invention comprises a transformer having a primary winding and at least two secondary windings, a first switching element connected in series with the primary winding, a control circuit for controlling the output from the first switching element by control of the on-time thereof, a rectification circuit for rectifying at least two outputs from the secondary windings, and an output detection circuit for detecting the output voltages, and feeding back the output voltages as a control signal for the on-time to the control circuit, wherein the output voltage detection circuit comprises a control signal formation section in which the control signal for the on-time is formed, corresponding to voltage at a voltage comparison terminal, and plural voltage detectors connected between the at least two outputs of the secondary windings and the voltage comparison terminal, respectively.
In this switching power supply apparatus, output voltages from the respective outputs, generated by at least two secondary windings, are detected by the voltage detectors connected to the outputs, respectively, and are input together to the voltage comparison terminal. The extents of the influences of the variations in the respective outputs onto the voltage comparison terminal will be considered below. The extents of the influences, if resistors are used for the detection, are varied, depending on the resistances. Accordingly, the plural output voltages can be controlled at an optional ratio by designing the resistances of the voltage detection resistors corresponding to their specifications.
In this way, the plural voltage detectors are connected between at least two outputs of the secondary windings and the voltage comparison terminal, and the control signal for the on-time of the first switching element connected to the primary winding is formed, corresponding to voltage at the voltage comparison terminal. Therefore, the respective output voltages can be stabilized at a desired control ratio.
According to an aspect, for the plural voltage detectors, at least one Zener diode is used.
By appropriate selection of the Zener diode, the respective output voltages from the circuit can b
Hosotani Tatsuya
Minagawa Takahiro
Takemura Hiroshi
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