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 T1, secondary windings T2 and T3, and a drive winding T4. A switching element Q1 (hereinafter, referred to as a first switching element) is connected in series with the primary winding T1. A control circuit CT controls the on-time of the first switching element Q1. Output from the secondary winding T2 is rectified by a rectification diode Ds1 and smoothed by a capacitor to be output as a first output Vo1. Moreover, output from the other secondary winding T3 is rectified by a rectification diode Ds2 and smoothed by a capacitor to be output as an output Vo2. In this example, the control circuit containing the drive winding T4 of the transformer T controls the on-time of the first switching element Q1, and moreover, causes the first switching element Q1 to oscillate autonomously. With this configuration employed, when the first switching element Q1 is on, an input voltage Vin is applied to the primary winding T1, making input current flow so that energy is stored in the transformer T. Furthermore, when the first switching element Q1 is off, the energy stored in the transformer T is released as output current via the secondary windings T2 and T3. 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 T4 is delayed to some degree by a resistor R4 and a capacitor C3 to be applied to the control terminal of the first switching element Q1, and moreover, is applied to a time constant circuit comprising a resistor R1 and a capacitor C1, so that a transistor Tr1 is turned on after a constant time-period, causing the first switching element Q1 to turn off. When the first switching element Q1 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 Ds1 and Ds2, 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 T4 of the first switching element Q1, voltage is applied to the control terminal of the first switching element Q1, so that the first switching element Q1 turns on again.
Moreover, an output voltage detection circuit DT is provided on the output side of the rectification circuit comprising the rectification diodes Ds1 and Ds2, and capacitors. The output voltage detection circuit DT detects only output voltage with respect to the first output Vo1. That is, the voltage of the first output Vo1 is detected by resistors R2 and R3. The voltage divided by the resistors R2 and R3 is input to a voltage comparison terminal Vr as a comparison voltage. A series circuit comprising a photodiode PD1, a shunt regulator ZD1, and a resistor 5 is connected between Vo1 and GND. The above detection voltage is input to the voltage comparison terminal (reference terminal) Vr of the shunt regulator ZD1. A phototransistor PTr1 arranged in opposition to the photodiode PD1 is connected between the base and collector of the transistor Tr1 in the above-described control circuit CT.
With the above arrangement, when the first output Vo1 is increased, the input voltage to the shunt regulator ZD1 is increased. Then, since the inflow current to the photodiode PD1 is increased, the transistor Tr1 is turned on earlier via the operation of the phototransistor PTr1 in the control circuit CT. As a result, the on-time of the first switching element Q1 becomes shortened, and thereby, the first output voltage Vo1 is reduced. In this way, the output voltage of the first output Vo1 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 Vo1 can be stabilized. Moreover, since the first output Vo1 is stabilized, the second output voltage Vo2 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 Vo1 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 Vo2 other than the output voltage Vo1.
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 Vo2 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.
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 be stabilized at the above-described desired control ratio only when the output voltage from the circuit connected to the Zener diode exceeds a predetermined voltage. Thus, the output voltages can be suppressed from increasing. Moreover, when the output voltage from the circuit connected to the Zener diode is less than the predetermined voltage, only the output voltage from the circuit not connected to the Zener diode is controlled for stabilization.
According to another aspect, the secondary winding comprises at least two secondary windings, the rectification output terminal of a predetermined secondary winding is connected to one end of the other secondary winding, and current through the other end of the other secondary winding is rectified, and is output.
In this embodiment, no influences are exerted due to of variations in voltage, caused by variations in current flowing in the rectification diode for the predetermined secondary winding. For this reason, correspondingly, the voltage accuracy of the other secondary winding is improved.
According to still another aspect, when the first switching element is on, input voltage is applied to the primary winding, causing current to flow so that energy is stored in the transformer, and when the first switching element is turned off, the energy stored in the transformer is released from the secondary windings.
In this embodiment, the switching power supply apparatus comprises a flyback type. Thus, it is not necessary to provide a choke coil or the like on the secondary side. Accordingly, a switching power supply apparatus having a small size, a high accuracy, and a high stability can be provided.
According to still another aspect, the switching power supply apparatus further comprises an inductor connected in series with the primary winding, and a series circuit comprising a capacitor and a second switching element, connected in parallel to the series circuit comprising the inductor and the primary winding, wherein the control circuit turns the first and second switching elements on and off, alternately, so as to sandwich a time-period when both of the switching elements are off, and controls the on-time of the switching elements, whereby the outputs therefrom are controlled.
Such a switching power supply apparatus in which the primary of the transformer is configured as described above is disclosed in U.S. Pat. No. 6,061,252 and Japanese Unexamined Patent Publication No. 11-187664, both of which are assigned to the assignee of the present invention, and the disclosure of which are hereby incorporated by reference.
In this switching power supply apparatus, when the first switching element turns off, energy stored in the inductor connected in series with the primary winding is released as charging current into the capacitor. Then, directly after this, the second switching element turns on, and discharging is carried out based on the charge potential of the capacitor. With the discharging current, energy is stored in the primary winding of the transformer and the inductor. When the second switching element turns off after a predetermined time-period, the energy stored again in the inductor L flows via the primary winding and the input power source. In this operation, the inductor connected in series with the primary winding includes the leakage inductance of the transformer. Accordingly, generation of surge, caused by the leakage inductance at switching, can be prevented. Moreover, the discharging current, generated when the second switching element Q2 is on, becomes an resonant current. This is reflected by the secondary. That is, the secondary winding current output takes a part of the sinusoidal waveform starting from a zero voltage (mountainous waveform), so that surge in a leading edge can be practically neglected.
Since the current surge or the like is suppressed as described above, the output voltages can be prevented from increasing due to the surge current or the like. As a result, the voltage accuracy of an output produced at a small control ratio or a non-controlled output can be improved. Especially, when said load having a large control ratio is heavy, and the load having a small control ratio is light, conventionally surge current causes the output having a small control ratio to rise in voltage. However, this is considerably improved by this arrangement.
According to still another aspect, the switching power supply apparatus further comprises an inductor connected in series with the primary winding, and a series circuit comprising a capacitor and a second switching element, connected in parallel to the first switching element, wherein the control circuit turns the first and second switching elements on and off, alternately, so as to sandwich a time-period when both of the switching elements are off, and controls the on-time of the switching elements, whereby the outputs therefrom are controlled.
With this structure, the switching power supply apparatus carries out the same operation as expalained above.
According to still another aspect, the leakage inductance of the transformer is used.
In this embodiment, the inductor consists of a leakage inductance itself. This can reduce the number of parts, since it is not necessary to provide the inductor as a separate part.
According to still another aspect, the control circuit comprises a drive winding provided in the transformer to drive the first and second switching elements, respectively, and a control section provided with a time constant circuit for providing on-off signals to the control terminals of the first and second switching elements at a predetermined timing by use of a voltage substantially proportional to the voltage of the primary winding and generated in the drive winding, whereby the first and second switching elements oscillate autonomously.
In this embodiment, the first switching element and the second switching element are autonomously operated. Thus, oscillation IC""s or the like are not needed. The number of parts can be significantly reduced. Moreover, the first and second switching elements can be easily turned on and off alternately so as to sandwich a time-period when both of them are off, and the on-time of these switching elements can be simply controlled. The loss and breaking of elements, caused by the short-circuit current which flows when the two switching elements are simultaneously turned on, can be prevented.
According to still another aspect, the control circuit includes a rectification diode, and a capacitive impedance connected in parallel to the rectification diode.
Since the first switching element in the primary operates as a switch, surge-voltage and surge-current are produced in the output of the secondary. The capacitive impedance is connected in parallel to the rectification diode, which enables the voltage surge to be absorbed. Moreover, charges are supplied to the output via the capacitive impedance, so that the affects of the voltage-drop of the rectification diode can be reduced.
According to still another aspect, the rectification circuit includes an inductive impedance connected in series with the rectification diode.
In this embodiment, the inductive impedance is connected in series with the rectification diode, Thus, especially, current surge can be prevented.
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.