JPH0670489U - Switching power supply - Google Patents

Abstract

(57) [Abstract] [Purpose] To stabilize the operation of a switching power supply when the load is small. [Configuration] A series circuit of a primary winding 3 of a transformer and a switching element 4 is connected to a power supply. In order to form a PWM pulse for turning on / off the switching element 4,
A comparator 20, an oscillator 21, and an RS flip-flop 22 are provided. The oscillator 21 is a variable period oscillator.
In order to obtain a voltage corresponding to the width of the PWM pulse, a capacitor 31 is connected via a resistor 30 between the control terminal of the switching element 4 and the ground. The voltage of the capacitor 31 controls the cycle of the oscillator 21.

Description

[Detailed description of the device]

[0001]

[Industrial applications]

 The present invention relates to a switching power supply device of a type in which a switching element is on / off controlled by a PWM pulse to adjust a DC voltage.

[0002]

[Prior Art and Problems to be Solved by the Invention]

 The switching element is connected to the DC power supply via the primary winding of the transformer, and the voltage controlled by turning the switching element on and off with the PWM pulse is output to the output stage of the rectifying and smoothing circuit on the secondary side of the transformer. The type of switching power supply that can be obtained is widely used. Control methods for this kind of switching power supply device are roughly classified into a voltage-current combined control method (current mode method) that forms a PWM pulse based on both output voltage detection and switching element current detection, and output voltage detection. There are two types: a voltage control method (voltage mode method) that forms a PWM pulse on the basis of.

As shown in FIG. 7, the conventional switching power supply device of the voltage / current combined control method uses a primary winding 3 having the inductance of a transformer 2 between one end and the other end of a DC power supply 1, for example, A switching element 4 composed of a field effect transistor is connected, a load 8 is connected to a secondary winding 5 of a transformer 2 via a rectifying / smoothing circuit 6 and an output terminal 7, and the switching element 4 is turned on / off by a PWM pulse. Is configured to.

Further, a voltage detection circuit 9 and a resistor 10 as a current detector are provided in order to form the PWM pulse by the voltage / current combined method. The voltage detection circuit 9 has voltage dividing resistors 11 and 12 connected between the output terminal 7 and the ground, a reference voltage source 13, and one input terminal connected to the voltage dividing point of the resistors 11 and 12, and the other. The input terminal of is composed of an error amplifier 14 connected to the reference voltage source 13, and a light emitting diode 15 connected to the error amplifier 14. The light emitting diode 15 outputs an optical signal having an intensity corresponding to the voltage of the output terminal 7. The current detection resistor 10 is connected between the switching element 4 and the other end (ground) of the power source 1 and generates a voltage corresponding to the current flowing through the switching element 4.

A phototransistor 16, a resistor 17, and a capacitor 18 are provided as a combined detection signal forming circuit for combining the voltage detection signal and the current detection signal to form a combined detection signal. That is, the phototransistor 16 and the resistor 17 are connected between the power supply terminal 19 and the right end of the current detecting resistor 10, and the connection midpoint between these is connected to one input terminal of the comparator 20.

The other input terminal of the comparator 20 is connected to a reference voltage source 20a that supplies a reference voltage Vth. Therefore, the comparator 20 generates a comparison output pulse of the combined detection signal Vin and the reference voltage Vth.

The oscillator 21 has a square wave pulse output terminal a and a ground terminal b, and generates a square wave pulse at a constant cycle in order to obtain a PWM pulse.

The RS flip-flop 22 is provided as a PWM pulse forming circuit, the set terminal S is connected to the oscillator 21, the reset terminal R is connected to the comparator 20, and the output terminal Q is connected to the switching element 4. It is connected to the control terminal (gate).

In order to obtain the voltage of the control circuit, a tertiary winding 23 is provided, and a rectifying / smoothing circuit 24 is connected to this, and its output is supplied to the terminal 19. Although the connection of the power source to the comparator 20, the oscillator 21 and the flip-flop 22 is omitted in FIG. 7, these are also connected to the terminal 19.

In the switching power supply device of FIG. 7, during the ON period of the switching element 4, the current flowing through the switching element 4 has a waveform (sawtooth wave) that increases with time due to the inductance of the primary winding 3. . On the other hand, the phototransistor 16 optically coupled to the light emitting diode 15 is brought into a conductive state corresponding to the output voltage, and the resistance value at both ends is low when the output voltage is high and is high when the output voltage is low. A signal Vin obtained by dividing the value obtained by subtracting the voltage of the current detection resistor 10 from the voltage + Vcc of the power supply terminal 19 by the resistor of the phototransistor 16 and the resistor 17 is input to one input terminal of the comparator 20. In the comparator 20, as shown in FIG. 8A, the combined detection signal Vin and the reference voltage Vth are compared, and when the combined detection signal Vin crosses the reference voltage Vth, the comparison output pulse shown in FIG. 8B is generated. To do. This comparison output pulse is used as a signal indicating the end point of the PWM pulse. The oscillator 21 generates a square wave pulse at a constant period as shown in FIG. 8 (C), and the flip-flop 22 is triggered by this pulse to be in the set state and reset by the comparison output pulse in FIG. 8 (B). 8D to form the PWM pulse and send it to the switching element 4. This achieves constant voltage control. Further, it is possible to prevent an overcurrent from flowing through the switching element 4.

In FIG. 8, the waveform when the load 8 in FIG. 7 is small is shown by a solid line, and the waveform when the load 8 is large is shown by a dotted line. As is clear from the comparison between the solid line waveform and the dotted line waveform in FIG. 8A, the amplitude of the sawtooth wave greatly changes depending on the magnitude of the load 8, that is, the magnitude of the current of the switching element 4. When the load 8 is large, the amplitude of the sawtooth wave portion is sufficiently large, so there is little risk of malfunction due to noise, etc. However, when the load 8 is small, the amplitude of the sawtooth wave portion decreases, so malfunction due to noise occurs. The fear of That is, when the load 8 is small, the possibility that noise crosses the reference voltage Vth increases, an erroneous comparison output occurs, and the PWM pulse cannot be obtained normally.

The conventional switching power supply device of the voltage control method is configured as shown in FIG. In FIG. 9, the same parts as those in FIG. 7 are designated by the same reference numerals and the description thereof will be omitted. In FIG. 9, the current detection is irrelevant to the input of the comparator 20, and the input terminal of the comparator 20 has a voltage dividing point between the resistor 25 connected between the power supply terminal 19 and the ground and the phototransistor 16. It is connected. The triangular wave output terminal c of the oscillator 21 is connected to the other input terminal of the comparator 20. An AND gate 26 is provided as a PWM pulse forming circuit, one input terminal of which is connected to the comparator 20, the other input terminal is connected to the square wave output terminal a of the oscillator 21, and the output terminal is a switching element. 4 control terminals. Although not shown, an overcurrent protection circuit is provided.

The oscillator 21 of FIG. 9 generates a triangular wave voltage V3 from the triangular wave output terminal c at a constant cycle as shown in FIG. 10 (A), and a square wave output terminal a to a square wave of FIG. 10 (C). Is generated in the same cycle as the triangular wave voltage V3. The comparator 20 compares the feedback control voltage Vf corresponding to the value obtained by dividing the voltage + Vcc of the power supply terminal 19 by the resistance 25 and the resistance of the phototransistor 16 with the triangular wave voltage V3 as shown in FIG. A comparison output pulse corresponding to a period in which the triangular wave voltage V3 crosses the control voltage Vf is generated as shown in FIG. 10 (B). The AND gate 26 as a PWM pulse forming circuit corresponds to the high level shown in FIG. 10 (D) corresponding to the period when the comparison output pulse of FIG. 10 (B) and the square wave pulse of FIG. 10 (C) simultaneously become high level. The PWM pulse of is generated.

In the switching power supply device of the voltage control method of FIG. 10, the problem of malfunction due to the decrease in the amplitude of the sawtooth wave due to the current detection does not occur. However, there is a problem that the PWM pulse cannot be sufficiently narrowed down at a light load due to the response speed of the comparator 20, the AND gate 26 and the like, the delay of the switching element 4 and the like. That is, if the width of the PWM pulse is narrowed, intermittent oscillation may occur. This kind of problem can be solved by connecting a dummy load, but power loss occurs and efficiency deteriorates.

Therefore, an object of the present invention is to provide a switching power supply device capable of achieving an abnormal operation at a light load with a simple circuit.

[0015]

[Means for Solving the Problems]

 A first invention for achieving the above object is to connect a transformer having a primary winding and a secondary winding and one end and the other end of a DC power supply through the primary winding. A switching element, a rectifying / smoothing circuit connected to the secondary winding, and a voltage detecting circuit for detecting a voltage at a specific position selected between the DC power source and the output terminal of the rectifying / smoothing circuit. , A current detector for detecting a current flowing through the switching element, and a voltage detected by the voltage detection circuit and a current detector coupled to the voltage detection circuit and the current detector. A combined detection signal forming circuit that forms a combined detection signal that changes according to the magnitude of current, a reference voltage source that generates a reference voltage, a combined detection signal forming circuit and the reference voltage source, The reference voltage is set to the composite detection signal Connected to the comparator and the oscillator, and a variable period oscillator in which the period of generation of the output pulse changes according to the voltage applied to the control terminal, A PWM pulse forming circuit that forms a PWM pulse having a width from the generation time point of the output pulse of the oscillator to the generation time point of the output pulse of the comparator, and supplies the PWM pulse to the control terminal of the switching element; And a control signal forming circuit which is connected to an output terminal of the PWM pulse forming circuit and obtains a voltage corresponding to the width of the PWM pulse and supplies the voltage to the control terminal of the oscillator. A second invention for achieving the above object is to connect a transformer having a primary winding and a secondary winding and one end and the other end of a DC power supply through the primary winding. A switching element, a rectifying / smoothing circuit connected to the secondary winding, and a voltage detecting circuit for detecting a voltage at a specific position selected between the DC power source and the output terminal of the rectifying / smoothing circuit. A voltage control signal forming circuit that is coupled to the voltage detecting circuit and forms a voltage control signal based on the voltage detected by the voltage detecting circuit, and is formed to generate a square wave pulse and a triangular wave in the same cycle. Is connected to a variable period oscillator having a control terminal for controlling the generation cycle of the square wave pulse and the triangular wave, the voltage control signal forming circuit and the triangular wave output terminal of the oscillator, and the voltage control Signal and the triangular wave Of the comparator for generating the comparison output pulse of, and the period in which the output pulse of the comparator and the square wave pulse of the oscillator are simultaneously generated, which is connected to the comparator and the square wave pulse output terminal of the oscillator. Corresponding to the PWM pulse generation circuit, which is connected to the PWM pulse forming circuit that supplies the PWM pulse to the control terminal of the switching element and the output terminal of the PWM pulse forming circuit, and corresponds to the width of the PWM pulse. And a control signal forming circuit for obtaining a voltage and supplying the voltage to the control terminal of the oscillator. The control signal forming circuit is preferably formed by a series circuit of a resistor and a capacitor.

[0016]

[Operation and effect of the device]

 According to the first and second inventions of the present application, since the voltage corresponding to the width of the PWM pulse is obtained by the control signal forming circuit connected to the PWM pulse forming circuit and the oscillator is controlled by this, the square wave is formed at a light load. The period of the wave pulse or square wave pulse and the triangular wave voltage can be lengthened with a simple circuit configuration. According to the first invention, when the period of the square wave pulse becomes long, the period of the PWM pulse also becomes long, the amplitude of the sawtooth wave based on the current detection becomes high, and the noise becomes unnecessary in the comparator. Therefore, stable operation at light load becomes possible. According to claim 3, the control signal can be obtained very easily.

[0017]

First Embodiment Next, a switching power supply device according to an embodiment of the present invention will be described with reference to FIGS. However, in FIGS. 1 to 3, the same parts as those in FIGS. 7 and 8 are designated by the same reference numerals and the description thereof will be omitted. The difference between FIG. 1 and FIG. 7 is that the oscillator 21 has a terminal d for controlling the period of the output waveform of the square wave pulse output terminal a and the triangular wave output terminal c, and this terminal d has a resistor 30 and a capacitor 31. It is connected to the middle point of connection with. The resistor 30 and the capacitor 31 constitute a circuit that forms a control signal Vc corresponding to the pulse width of the PWM pulse, and are connected between the output terminal Q of the flip-flop 22 and the ground and connected to the capacitor 31. The voltage is supplied to the control terminal d of the oscillator 21 as the control signal V c. In FIG. 1, the circuit configuration other than the above is the same as that of FIG. 7.

FIG. 2 shows details of the oscillator 21. The oscillator 21 includes first and second operational amplifiers 32 and 33, an oscillation capacitor 34, a discharge current control transistor 35, a Zener diode 36, a power supply line 37, and nine resistors R1 to R9. It The resistors R1 and R2 are connected between the power supply line 37 and the ground terminal b, and the + input terminal of the first operational amplifier 32 is connected to these voltage dividing points. The resistor R3 is connected between the + input terminal and the output terminal of the first operational amplifier 32. The resistor R 4 is connected between the power supply line 37 and the output terminal of the first operational amplifier 32. The oscillation capacitor 34 is connected between the − input terminal of the first operational amplifier 32 and the ground terminal b. A charging resistor R5 is connected between one end of the capacitor 34 and the output terminal of the operational amplifier 32, and a discharging resistor R6 is connected through the transistor 35. The resistor R7 is connected between one end of the capacitor 34 and the base of the transistor 35. The control terminal d is connected to the + input terminal of the second operational amplifier 33. The-input terminal of this operational amplifier 33 is connected to this output terminal. The output terminal of the second operational amplifier 33 is connected to the base of the transistor 35 via resistors R8 and R9. The square wave pulse output terminal a connected between the connection point of the resistors R8 and R9 of the Zener diode 36 and the ground terminal b is connected to the first operational amplifier 32, and the triangular wave output terminal c is connected to the capacitor 34. ing.

In the oscillator 21 of FIG. 2, when the output of the first operational amplifier 32 is at a high level, the capacitor 34 is charged through the resistor R5, and when the capacitor 34 is charged to a predetermined value, the output of the first operational amplifier 32 is output. It goes low and the discharge current of the capacitor 34 flows through the resistor R6 and the transistor 35. As a result, a square wave pulse is obtained at the output terminal a and a triangular wave is obtained at the output terminal c. When the voltage of the control terminal d changes, the resistance of the transistor 35 changes, the discharge time constant of the capacitor 34 changes, and the periods of the square wave pulse and the triangular wave also change.

In FIG. 3, the waveform shown by the solid line shows the waveform of each part when the load 8 is small, and the waveform shown by the dotted line shows the waveform of each part when the load 8 is large. When the load 8 is small, the width of the P WM pulse becomes narrow, so the control voltage Vc obtained from the capacitor 31 is also low as shown in FIG. 3 (E). As a result, the periods of the square wave pulse and the triangular wave of the oscillator 21 are relatively long as shown in FIGS. On the other hand, when the load 8 is large, the width of the PWM pulse becomes large, so that the control voltage Vc also becomes high as shown by the dotted line in FIG. 3 (E), and the period of the square wave pulse and the triangular wave of the oscillator 21 becomes short. When the output cycle of the oscillator 21 changes as shown in FIGS. 3C and 3F, the PWM pulse cycle also changes as shown in FIG. 3D.

The fact that the cycle of the PWM pulse becomes long at a light load means that when supplying a predetermined electric power to the load 8, a larger current can flow through the switching element 4 than when the cycle of the PWM pulse is short. It means that you can do it. As a result, the amplitude of the gradient current flowing through the switching element 4 can be increased, and the amplitude of the sawtooth wave portion of the voltage-current combined detection signal Vin shown in FIG. 3 (A) is increased, which causes malfunction due to noise. To be prevented.

[0022]

Second Embodiment Next, a switching power supply device according to a second embodiment will be described with reference to FIGS. 4 and 5. However, in FIGS. 4 and 5, the same parts as those in FIGS. 1 to 3, 9 and 10 are designated by the same reference numerals and the description thereof will be omitted. FIG. 4 shows the circuit of FIG. 9 with a resistor 30 and a capacitor 31 added. The control signal forming circuit including the resistor 30 and the capacitor 31 is connected between the output terminal of the AND gate 26 and the ground, and functions similarly to FIG. That is, the periods of the square wave pulse and the triangular wave of the oscillator 21 are changed according to the change of the width of the PWM pulse. The oscillator 21 of FIG. 4 has the same configuration as the oscillator 21 of FIG. 2, but the charging time constant is set to be longer than that in the case of being applied to FIG.

The solid line waveform in FIG. 5 shows the waveform of each part of FIG. 4 when the load 8 is small, and the dotted line waveform shows the waveform of each part of FIG. 4 when the load 8 is large. Since the width of the PWM pulse is narrow when the load 8 is small, a relatively low control voltage Vc shown in FIG. 5 (E) is generated from the control signal forming circuit corresponding to the PWM pulse width composed of the resistor 30 and the capacitor 31. It When a low control voltage Vc is applied to the control terminal d of the oscillator 21, the period of the square wave pulse at the output terminal a shown in FIG. 5 (C) and the period of the triangular wave voltage V3 at the output terminal c shown in FIG. 5 (A). Both become longer. Therefore, the cycle of the PWM pulse becomes long as shown in FIG. As a result, when a predetermined output voltage (load voltage) is obtained, the width of the PWM pulse can be made larger than that when the cycle is short, and the occurrence of abnormal oscillation can be prevented. When the load 8 is large, the control voltage Vc becomes higher, the periods of the square wave pulse and the triangular wave of the oscillator 21 become shorter, and the period of the PWM pulse becomes shorter than when the load 8 is small.

[0024]

[Modification]

 The present invention is not limited to the above-mentioned embodiments, and the following modifications are possible, for example.

The oscillator 21 of FIGS. 1 and 4 can be configured as shown in FIG. The oscillator 21 of FIG. 6 has first and second operational amplifiers 41 and 42, diodes 43, 44, 45 and 46, a zener diode 47, resistors 48, 49, 50, 51, 52, 53 and 54, Resistors 48 and 49 composed of 55 and a capacitor 56 are connected between the power supply line 57 and the ground terminal b, and these voltage dividing points are connected to one input terminal of the first operational amplifier 41. A triangular wave generating capacitor 56 is connected between the other input terminal of the first operational amplifier 41 and the ground. The upper end of the capacitor 56 is connected to the power supply line 57 via the resistors 50 and 51, and is also connected to the output terminal of the operational amplifier 41 via the resistor 50 and the diode 44. A series circuit of a resistor 52 and a diode 45 and a resistor 53 are connected between the power supply line 57 and the output terminal of the operational amplifier 41. The square wave pulse output terminal a is connected to the operational amplifier 41. The triangular wave output terminal c is connected to the upper end of the capacitor 56. The control terminal d is connected to one input terminal of the second operational amplifier 42 via the resistors 54 and 55. The Zener diode 47 is connected between the connection point of the resistors 54 and 55 and the ground. The other input terminal of the second operational amplifier 42 is connected to this output terminal. A diode 43 is connected between the voltage dividing point P of the resistors 48 and 49 and the output terminal of the second operational amplifier 42. The diode 46 is connected between the connection point of the resistor 52 and the diode 45 and one input terminal of the second operational amplifier 42. When the output of the first operational amplifier 41 in FIG. 6 is at a high level, the diode 46 is turned on, the + input terminal of the second operational amplifier 42 is also set at a high level, and the diode 43 is kept off, so that the first operational amplifier 41 is turned on. The + input terminal of 41 is kept at a constant voltage. When the output of the first operational amplifier 41 is at a low level, the diode 46 is turned off, and the + input terminal of the second operational amplifier 42 becomes the control voltage Vc of the control terminal d. Since the control voltage Vc and the output voltage of the second operational amplifier 42 at this time are set lower than the power supply voltage + Vcc and the voltage across the resistor 49, the diode 43 is turned on. As a result, the voltage across the resistor 49, that is, the + input voltage of the first op-amp 41 changes according to the change of the control voltage Vc, the charging start point of the capacitor 56 changes, and the charge / discharge cycle of the capacitor 56 and the first The generation cycle of the output pulse of the operational amplifier 41 changes.

Instead of connecting the voltage detection circuit 9 to the output terminal 7 in FIGS. 1 and 4, it may be configured to detect the voltage of the power supply 1 (input voltage) or the voltage of the transformer 2.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a switching power supply device according to a first embodiment.

FIG. 2 is a circuit diagram showing the oscillator of FIG.

FIG. 3 is a waveform diagram showing states of points A to F in FIG.

FIG. 4 is a circuit diagram showing a switching power supply device according to a second embodiment.

5 is a waveform diagram showing the states of points A to E in FIG.

FIG. 6 is a circuit diagram showing a modified example of the oscillator.

FIG. 7 is a circuit diagram showing a conventional switching power supply device.

8 is a waveform diagram showing the states of points A to D in FIG.

FIG. 9 is a circuit diagram showing another conventional switching power supply device.

10 is a waveform diagram showing the states of points A to D in FIG.

[Explanation of symbols]

 4 Switching element 20 Comparator 21 Oscillator 22 Flip-flop

Claims (3)

[Scope of utility model registration request] 1. A transformer having a primary winding and a secondary winding, a switching element connected between one end and the other end of a DC power supply via the primary winding, and the secondary winding. A rectifying / smoothing circuit connected to the rectifying / smoothing circuit, a voltage detecting circuit for detecting a voltage at a specific position selected between the DC power supply and an output terminal of the rectifying / smoothing circuit, and a current flowing through the switching element. A current detector for, a combined detection signal that is coupled to the voltage detection circuit and the current detector, and that changes according to the magnitude of the voltage detected by the voltage detection circuit and the current detected by the current detector. A combined detection signal forming circuit for forming, a reference voltage source for generating a reference voltage, connected to the combined detection signal forming circuit and the reference voltage source, and outputs an output pulse when the combined detection signal crosses the reference voltage. Occurrence A comparator, a variable period oscillator whose output pulse generation period changes according to the voltage applied to the control terminal, and the comparator and the oscillator, which are connected from the oscillator output pulse generation point to the Forming a PWM pulse having a width up to the time of generation of the output pulse of the comparator,
A PWM pulse forming circuit that supplies M pulses to the control terminal of the switching element, and an output terminal of the PWM pulse forming circuit, which obtains a voltage corresponding to the width of the PWM pulse and supplies the voltage to the control terminal of the oscillator. And a switching power supply device including a control signal forming circuit. 2. A transformer having a primary winding and a secondary winding, a switching element connected between one end and the other end of a DC power supply via the primary winding, and the secondary winding. A voltage detection circuit that detects a voltage at a specific position selected from between the DC power supply and the output terminal of the rectification and smoothing circuit; and a voltage detection circuit that is coupled to the voltage detection circuit. A voltage control signal forming circuit that forms a voltage control signal based on the voltage detected by the circuit, and is configured to generate a square wave pulse and a triangular wave in the same cycle, and controls the generation cycle of the square wave pulse and the triangular wave. A variable period oscillator having a control terminal for controlling the voltage control signal forming circuit and the triangular wave output terminal of the oscillator, and generating a comparison output pulse of the voltage control signal and the triangular wave. Connected to the comparator and the square wave pulse output terminal of the oscillator to generate a PWM pulse corresponding to a period in which the output pulse of the comparator and the square wave pulse of the oscillator are simultaneously generated. A PWM pulse forming circuit that supplies this PWM pulse to a control terminal of the switching element; and an output terminal of the PWM pulse forming circuit, which obtains a voltage corresponding to the width of the PWM pulse to obtain the control terminal of the oscillator. A switching power supply device comprising a control signal forming circuit for supplying to the. 3. The control signal forming circuit is a series circuit of a resistor and a capacitor connected between the PWM pulse forming circuit and the ground, and uses the voltage of the capacitor as a control signal. The switching power supply device according to 1 or 2.