JP3286234B2 - Self-excited switching power supply - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a self-excited oscillation type power supply circuit which indirectly controls a voltage generated in another winding by controlling a voltage generated in a specific winding of a transformer. The present invention relates to a technique for stabilizing an output voltage with respect to a change in an input voltage.

[0002]

2. Description of the Related Art When a power supply circuit requires a plurality of output voltages or an AC voltage, a voltage generated in a specific winding of a transformer provided in the circuit is generally converted to a DC voltage. After the conversion, the DC voltage is monitored to control the output voltage. As an example of such a power supply circuit, a switching power supply having a circuit as shown in FIG. The circuit configuration of the switching power supply shown in FIG. 5 and its schematic operation are as follows. In FIG. 5, reference numerals 1 and 2 denote a high potential side input terminal and a low potential side DC output terminal, respectively. The input terminal and the output terminal on the reference potential side are assumed to be connected to the ground, and are not shown.

A capacitor C1 and a series circuit of a switching transistor Q1 and a primary winding N1 of a transformer T are connected between an input terminal 1 and the ground. Secondary winding N of transformer T
2 are both AC output terminals 3a,
3b, 3c. Switching transistor Q1
Is connected to one end of a feedback winding N3 of a transformer T via a series circuit of a resistor R2 and a capacitor C2, and the other end of the feedback winding N3 is connected to one end of a main current path on the input terminal 1 side of the switching transistor Q1 ( Emitter). The resistor R3 and the main current path of the transistor Q2 as a control element are connected between the base of the switching transistor Q1 and the ground. The base of the transistor Q2 is connected to the input terminal 1 via the resistor R1. A series circuit of a diode D1 and a smoothing capacitor C3 is connected between the connection point of the switching transistor Q1 and the primary winding N1 and the ground.
A connection point between the diode D1 and the capacitor C2 is connected to the DC output terminal 2, and a constant voltage diode DZ1 is connected between the DC output terminal 2 and the base of the transistor Q2.

In the circuit having the above configuration, the transformer T
In each of the windings, a voltage is generated by a current flowing through the primary winding N1. Here, the voltage generated in the feedback winding N3 of the transformer T is input to the base of the switching transistor Q1 as a feedback signal.
1 is driven. Then, the switching transistor Q
5, the circuit shown in FIG. 5 performs a self-excited oscillation operation by a signal feedback loop formed by the primary winding N1 and the feedback winding N3. The voltage generated in the secondary winding N2 of the transformer T by the self-excited oscillation operation is supplied to the AC voltage output terminals 3a, 3
It is supplied to the outside as AC voltage V _O3 via b and 3c. On the other hand, the voltage generated at both ends of the primary winding N1 is rectified and smoothed by the diode D1 and the smoothing capacitor C3, and then supplied to the outside as the negative DC voltage V _O2 via the DC output terminal 2. .

The DC voltage V _O2 appearing at the DC output terminal 2 is applied to the base of the transistor Q2 via the constant voltage diode DZ1. The transistor Q2 changes the bias of the switching transistor Q1 according to the reference voltage provided by the constant voltage diode DZ1 and the value of the DC voltage V _O2 . The on-duty of the self-excited oscillation operation of the switching transistor Q1 changes due to the change in the bias, and the DC output voltage V _O2 is controlled to be constant. Specifically, for example, when the absolute value of the negative DC voltage V _O2 is going to be larger than a predetermined value, the negative voltage signal due to the DC voltage V _O2 increases, and the collector current of the transistor Q2 decreases. Then, the bias supplied to the switching transistor Q1 is reduced, so that the collector saturation current is reduced and the ON period in the self-excited oscillation operation is shortened. When the ON period of the switching transistor Q1 is shortened, the voltage generated in the primary winding N1 when the switching transistor Q1 is turned off is reduced. As a result, the DC voltage V _O2 is acted on to be small, and its value is kept constant.

As described above, the self-excited switching power supply having the circuit configuration shown in FIG. 5 has a feature that two types of voltages, DC and AC, can be obtained while the circuit configuration is simple. Therefore,
When used as a power supply circuit for a fluorescent display tube, the AC output voltage is used for heating a filament of the display tube, and the DC output voltage is used for driving a control circuit of the display tube.

[0007]

As described above, as shown in FIG.
1 detects and controls the voltage appearing at the DC output terminal 2 for stabilizing each output voltage. With this direct control, high stability is obtained for the DC output voltage V _O2 . On the other hand, the AC output voltage V _O 3, since the voltage generated in each winding of the transformer T in the stabilizing process of the DC output voltage V _O 2 is controlled, connected to the input voltage V _IN and an output terminal 3a~3c If the applied load is constant, it is kept almost constant (indirect control). However, in the input specifications required for the power supply circuit, it is almost impossible that the input voltage V _IN is constant. For example, when a battery is used as an external power supply, the voltage supplied from the battery varies depending on the amount of battery used and the state of storage. Therefore, it is desired that the power supply circuit can be used for an input voltage V _{IN in the} widest possible range in order to enhance versatility.

However, as can be seen from the input / output characteristics shown in FIG. 6, the circuit shown in FIG. 5 has an inconvenience that the fluctuation of the AC output voltage V _O3 is large with respect to the change of the input voltage V _{IN set} as the use range. There are features. With such comes to supply the deflection is large AC output voltage V _O3 voltage value to the filament of the fluorescent display tube, the display surface luminance or tube extends in particular wear of the filament, i.e. a material effect on the lifetime of the tube. Therefore, in the self-excited switching power supply having the circuit configuration shown in FIG. 5, how to improve the stability of the AC output voltage V _O3 has been a major issue. Accordingly, an object of the present invention is to provide a self-excited switching power supply capable of improving the stability of an output voltage that is not directly controlled with respect to a change in an input voltage.

[0009]

SUMMARY OF THE INVENTION A self-excited switching power supply according to the present invention includes a transformer having a plurality of windings, a transformer connected to a first winding of the transformer, and a second winding of the transformer. A switching element that receives a feedback signal obtained from the voltage and performs a self-excited oscillation operation; and a main current path connected to a control terminal of the switching element. The main current path is connected to the switching element according to a voltage generated in a third winding of the transformer and a reference voltage. A control element for controlling the bias of the switching element, and an input detection unit for obtaining a voltage signal corresponding to the input voltage are provided. Then, the control amount of the bias of the switching element by the control element is corrected by a voltage signal corresponding to the input voltage.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Switching element and transformer
A power supply circuit of a self-excited oscillation system is configured by the connection configuration of the next winding and the feedback winding. Then, one end of the main current path of the control element is connected to the control terminal of the switching element. here,
A bias signal obtained by an input voltage and a control signal corresponding to a voltage generated in a primary winding of a transformer and a reference voltage are input to a control terminal of the control element to constitute a switching power supply. In the first embodiment of the present invention, an input detection unit is provided for such a switching power supply, and a resistance element is connected to the other end of the main current path of the control element. A voltage signal corresponding to an input voltage is applied from an input detection unit to a connection point between the control element and the resistance element. In the second embodiment of the present invention, an input detection unit is provided, and a correction transistor is connected between the control terminal of the control element and the other end of the main current path. A voltage signal corresponding to the input voltage is input to the control terminal of the correction transistor from the input detection unit.

[0011]

FIG. 1 shows a circuit of a first embodiment of a self-excited switching power supply according to the present invention in which the stability of an output voltage which is not directly controlled with respect to a change in input voltage is high.
In FIG. 1, the same components as those shown in the circuit of FIG. 5 are denoted by the same reference numerals. Hereinafter, the same applies to the description of the other embodiments. The circuit shown in FIG. 1 differs from the circuit shown in FIG. 5 in the following points. That is, the resistor R5 is connected in series between the emitter of the transistor Q2 as a control element and the ground, and the connection point between the resistor R5 and the transistor Q2 is connected to the input terminal 1 via the resistor R4.
Connect to The input detector 4a is provided by the resistors R4 and R5.
Is composed. 1 and FIG. 5 have the same circuit configuration.

In the circuit of FIG. 1 having such a configuration, the self-excited oscillation operation is the same as that of the circuit of FIG. 5, and the output control operation is performed except for the operation and operation of the input detection unit 4a described below. The basic part will be the same. In the circuit of FIG. 1, the potential of the emitter of the transistor Q2 is increased by the input detection unit 4a by the voltage corresponding to the voltage division ratio of the resistors R4 and R5 and the input voltage _VIN . As a result, the threshold voltage of the base of the transistor Q2 increases according to the input voltage V _IN, and the current passing amount (control amount) of the main current path of the transistor Q2 is adjusted according to the input voltage V _IN . As a result, the control amount of the bias of the switching transistor Q1 by the transistor Q2 is corrected according to the input voltage V _IN ,
This correction suppresses the fluctuation of the AC voltage V _O3 .

[0013] That is, considering the case where the input voltage V _IN is higher, the emitter potential of the transistor Q2 in response to the input voltage V _IN rises. The rise in the emitter potential of the transistor Q2 changes the direction in which the current conduction of the transistor Q2 decreases. Then, the bias of the switching transistor Q1 decreases, and the ON period is shortened. As a result, the voltage generated in each winding of the transformer T decreases, and the increase in the AC voltage V ₀₃ obtained from the voltage generated in the secondary winding N2 is suppressed. Therefore, in the circuit shown in FIG. 1, the input voltage V
It is possible to suppress a rise in the AC output voltage _VO3 when _IN is high. As a result of such action, the circuit of FIG. 1, the input voltage V _IN as shown in input-output characteristic diagram of FIG. 2
Of the AC output voltage V _{O3 with} respect to. In addition,
By manipulating the resistance values of the resistors R4 and R5, the characteristic curve shown in FIG. 2 can be adjusted.

FIG. 3 shows a circuit of a second embodiment of the self-excited switching power supply according to the present invention. The circuit shown in FIG. 3 is substantially the same as the circuit shown in FIG.
6, and has a circuit configuration in which the resistor R4 is replaced with an independent input detection unit 4b. Here, the input detection unit 4b has a series circuit of resistors R8 and R9 connected between the input terminal 1 and the ground, a base connected to a connection point of the resistors R8 and R9, and an emitter connected to the resistor R6. Transistor Q3
And a resistor R7 connected between the collector of the transistor Q3 and the input terminal 1. According to the input detection unit 4b and the resistor R6 having such a configuration, the same effect as that of the input detection unit 4a including the resistors R4 and R5 in the circuit of FIG. 1 can be obtained. However, in the input detection unit 4b of FIG. 3, the transistors Q2 and Q3 perform an operation similar to that of the differential amplifier, and the input / output characteristics are slightly different from those of FIG.

FIG. 4 shows a circuit of a third embodiment of the self-excited switching power supply according to the present invention. The circuit shown in FIG. 4 is different from the circuit shown in FIG. 5 in that an input detection unit 4c composed of a series circuit of resistors R11 and R12 is provided between the input terminal 1 and the ground, Connect the main current path between the emitters and connect the base with a resistor R
Correction transistor Q connected to the connection point of R11 and R12
4 is provided. The other circuit configuration of the circuit shown in FIG. 4 is the same as that of FIG. In the circuit of FIG. 4, the input detection unit 4c inputs a signal corresponding to the input voltage V _IN to the base of the correction transistor Q4. The correction transistor Q4 becomes conductive according to the input signal, that is, according to the input voltage V _IN . The bias of the transistor Q2 changes according to the conduction state of the correction transistor Q4. As a result, the control amount of the bias of the switching transistor Q1 by the transistor Q2 is corrected according to the input voltage V _IN .

1 and 3 described above changes the potential of the emitter of the transistor Q2 and corrects the control amount, whereas the circuit of FIG. 4 changes the bias of the base of the transistor Q2. Is used to correct the control amount. Although the action point of signal correction is different depending on whether it is the emitter or the base of the transistor Q2, the circuit parts of the input detection unit 4c and the correction transistor Q4 in FIG. 4 are the input detection unit 4a in FIG. The detection portion 4b and the circuit portion of the resistor R6 have similar actions to the transistor Q2. Therefore, the same effect as the circuits of FIGS. 1 and 3 can be obtained by the circuit having the configuration shown in FIG.

[0017]

As described above, in the self-excited switching power supply according to the present invention, a signal corresponding to the input voltage is obtained in the input detecting section, and the signal corresponding to the input voltage biases the switching element by the control element. It is characterized by changing. This makes it possible to provide a self-excited switching power supply that has high stability with respect to the input voltage of the output voltage that is not directly controlled. Further, by changing the value of the resistance element constituting the input detection unit, there is an effect that the input / output characteristics can be easily adjusted.

[Brief description of the drawings]

FIG. 1 shows a first embodiment of a self-excited switching power supply according to the present invention.
FIG.

FIG. 2 is an input / output characteristic diagram of a regulated uncontrolled output voltage of the circuit of FIG. 1;

FIG. 3 shows a second embodiment of the self-excited switching power supply according to the present invention.
FIG.

FIG. 4 shows a third embodiment of the self-excited switching power supply according to the present invention.
FIG.

FIG. 5: Conventionally monitors a voltage generated in a specific winding,
FIG. 3 is a circuit diagram of an example of a self-excited switching power supply that controls an output voltage of the power supply by controlling the switching power supply.

6 is an input / output characteristic diagram of an uncontrolled output voltage of the circuit of FIG.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Input terminal 2 Output terminal (DC) 3a-3c Output terminal (AC) 4a-4c Input detection part Q1 Switching transistor as a switching element Q2 Transistor as a control element Q3 transistor Q4 Correction transistor T transformer N1 Primary winding ( N2 secondary winding (fourth winding) N3 feedback winding (second winding) DZ1 constant voltage diode for providing reference voltage R6 first resistance element R5 first R3 second resistance element

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H02M 7/48 H02M 3/155 H02M 7/537 H05B 41/24

Claims (5)

(57) [Claims] 1. A transformer having a plurality of windings including a primary winding and a secondary winding for obtaining an AC output, the transformer being connected in series to a first winding of the transformer, and a second winding of the transformer. A switching element that receives a feedback signal obtained by a voltage generated in the winding and performs a self-excited oscillation operation; a main current path is connected to a control terminal of the switching element;
A control element for controlling a bias of the switching element according to a voltage generated in a third winding of the transformer and a reference voltage, and an input detection unit for obtaining a voltage signal corresponding to an input voltage. A self-excited switching power supply, wherein a bias control amount of the switching element is corrected by a voltage signal corresponding to the input voltage. 2. The method according to claim 1, wherein a first resistance element is connected in series to a main current path of the control element, and a voltage signal corresponding to the input voltage is applied to a connection point between the control element and the resistance element. The self-excited switching power supply according to claim 1. 3. The input detection section has a series circuit of second and third resistance elements, and the first resistance element and the third resistance element are shared by one resistance element. The self-excited switching power supply according to claim 2. 4. A correction transistor is connected between a control terminal of the control element and a main current path, and a voltage signal corresponding to the input voltage is applied to a control terminal of the correction transistor. Item 1. A self-excited switching power supply according to Item 1. 5. A multi-output by obtaining a first output voltage from a voltage generated in a third winding of the transformer and obtaining a second output voltage from a voltage generated in a fourth winding of the transformer. The self-excited switching power supply according to any one of claims 1 to 4, wherein the switching power supply is a type.