JP3139699B2 - Switching power supply circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a parallel operation as an active dummy circuit which operates so that a tap voltage of an inverter transformer does not decrease when an output voltage of a main output circuit is variably reduced by an external control signal. The present invention relates to a switching power supply circuit to which a control circuit is added.

[0002]

2. Description of the Related Art As shown in FIG. 5, a conventional switching power supply circuit having a composite output rectifies an input voltage of an AC input power supply 10 by a full-wave rectifier 11 and smoothes the input voltage by a capacitor 12, and then a primary circuit 22. The high frequency switching is performed by the switching element 13, the power is converted by the inverter transformer 14, and the power is converted through the first secondary winding 17 and the second secondary winding 18.
The signal is transmitted to the next side, and is transmitted to the main output circuit 19 and the sub output circuit 20.
Rectifies, smoothes and outputs again.

In order to perform control for stabilizing the output voltage by the switching element 13, a detection circuit 21 detects a variable output voltage of a main output circuit 19, and this is detected by a primary circuit 2.
The control signal is fed back to the PWM control IC 23 to control the duty ratio of the switching element 13 so that the output voltage of the main output circuit 19 is varied and stabilized at the set voltage.

On the other hand, the sub-output circuit 20 generally uses 5 V as a power supply for a microcomputer in many cases, and since the output current is small, a stable fixed voltage is always output by a three-terminal regulator 24 or the like. 3 used here
The terminal regulator 24 requires an input voltage higher than the output voltage by 2 to 3 V in order to stabilize the output voltage, and is called a minimum input-output voltage difference.

In a switching power supply having a composite output, even if the output voltage of the main output circuit 19 is varied by an external signal or the like as a result, the sub output circuit 20 which should output a fixed voltage is supplied to the In order to prevent the output voltage from dropping due to insufficient voltage difference between the input and output of the three-terminal regulator 24 due to the voltage drop, the inverter transformer 1
In this case, the tap voltage of the sub-output circuit 20 is increased to maintain the input voltage of the three-terminal regulator 24 of the sub-output circuit 20.

FIG. 5 shows an example of a circuit in which the external control signal input circuit 43 varies the output voltage of the main output circuit 19 with an external signal, which has been previously proposed by the present applicant. (Japanese Patent Application No. 5-319079). The external control signal input circuit 43 includes control signal input terminals 52, 5
When, for example, a control signal that changes linearly is input to 3, the voltage is divided by the resistors 50 and 49 and applied to the transistor 47, and the voltage source that determines the output voltage is determined by the presence of the reference voltage source 42 composed of a Zener diode. Regardless, the output voltage varies linearly and the output voltage also varies continuously linearly.

[0007]

However, in the conventional circuit, the tap voltage of the sub-output circuit 20 is changed to the main output circuit 19.
When the duty ratio of the switching element 13 is determined by the minimum voltage and the minimum current, the input voltage of the sub-output circuit 20 needs to be higher than a specified value. For example, in FIG. 4, the duty ratio of the switching element 13 which is determined by the minimum voltage and the minimum current of the main output circuit 19 approaches and the input voltage Vi of the sub output circuit 20 changes linearly as indicated by the characteristic line A. When the output voltage Vo of the sub output circuit 20 is 5 V as shown by the characteristic line B when the output voltage Vo decreases toward 0, if the minimum input-output voltage difference Vi−Vo is 2 V, the sub output circuit 20 A total of 7 V is required as the minimum input voltage Vi. If the output voltage of the main output circuit 19 is variably reduced by an external control signal, the input voltage Vi of the sub output circuit 20 becomes 7 V or less at a certain voltage or less, and the output voltage Vo also changes as indicated by the characteristic line C. , 5V or less.

If the tap voltage, that is, the input voltage Vi of the sub-output circuit 20 is raised as shown by a characteristic line a in FIG. As shown by the line b, the main output circuit 19
Output voltage can be extended to a smaller range.
However, if the tap voltage is too high, the main output circuit 1
Switching element 13 determined by maximum voltage and maximum current of 9
In the case of the duty ratio, since the input voltage of the three-terminal regulator 24 for stabilizing the output voltage Vo of the sub-output circuit 20 increases, a large loss occurs due to heat generation in the three-terminal regulator 24 of the sub-output circuit 20. In many cases, the overall conversion efficiency of a composite output switching power supply deteriorates.

SUMMARY OF THE INVENTION It is an object of the present invention to provide, with a simple circuit configuration, a circuit capable of maintaining a voltage required for an internal circuit even when an output voltage of a main output circuit is externally reduced. .

[0010]

According to the present invention, an AC input power supply 10 is switched at a high frequency, and an inverter transformer 14 is provided.
In the switching power supply circuit, the power is converted to obtain a stable output voltage at the secondary side main output circuit 19,
When the output voltage of the main output circuit 19 is variably reduced by an external control signal and reduced, the parallel control circuit 54 that detects a decrease in the output voltage and increases a dummy current of the main output circuit 19 is provided by the parallel control circuit 54. This parallel control is connected to the output circuit 19.
The circuit 54 determines that the output voltage of the main output circuit 19 is below a certain value.
Zener diode 61 for detecting
Transistor 5 which is turned off when the turn-off diode 61 is turned off
6 and the main output is turned on when the transistor 56 is turned off.
The current determined by the output voltage of the circuit 19 and the resistor 57 is
The transistor 55 flowing as a dummy current of the circuit 19
It was provided, a switching power supply circuit characterized by comprising Te <br/> allowed act as an active dummy circuit.

[0011]

When the output voltage of the main output circuit becomes equal to or lower than the reference voltage, a dummy current of the main output circuit flows into the parallel control circuit, and the output voltage of the main output circuit increases, and the output voltage of the switching element is increased. Operate to increase the ratio. Then, the tap voltage in the inverter transformer 14 acts to increase, and the input voltage to the three-terminal regulator 24 of the sub output circuit 20 increases, so that a stable output voltage can always be obtained.

[0012]

Embodiments of the present invention will be described below with reference to the drawings. In FIG. 1, an AC input power supply 10, a full-wave rectifier 1
1, a capacitor 12, a primary circuit 22, a main output circuit 19,
The sub output circuit 20 and the detection circuit 21 of the main output circuit 19 are the same as the conventional circuit. In the present invention, a parallel control circuit 54 is connected in parallel to the main output circuit 19 to operate as an active dummy circuit. When the output voltage decreases, the output current of the main output circuit 19 increases, and Is increased, and as a result, the output power is increased.
The purpose is to increase the input voltage of the terminal regulator 24 and stabilize the output voltage.

More specifically, the primary circuit 2
Reference numeral 2 denotes, for example, the case of a separately excited system using the PWM control IC 23, and the primary winding 1 of the inverter transformer 14
And a PWM control IC 23 for controlling the pulse width of the switching element 13.
And a light receiving element 46 of a photocoupler 44 as an insulating means for feeding back an output voltage signal, and an auxiliary winding 16 for supplying power.

The main output circuit 19 includes a diode 3 connected to the first secondary winding 17 of the inverter transformer 14.
1, the output voltage detection circuit 21 and the capacitor 32. The main output circuit 19 is connected in parallel with a parallel control circuit 54 as an active dummy circuit according to the present invention.

The detection circuit 21 includes resistors 35 and 36 for detecting an output voltage, a Zener diode 42 for setting a reference voltage, a transistor 38, a light emitting element 45 of a photocoupler 44 as an insulating means, resistors 40 and 41, and a Zener diode 42. , Resistors 36, 37 and 39.

The external control signal input circuit 43 connected in parallel with the Zener diode 42 has a function as described above.
In the circuit previously proposed by the present applicant, the cathode and anode of a Zener diode 42 for limiting the maximum value of the output voltage are connected to the emitter and collector of the transistor 47, and the base of the transistor 47 and the control signal input terminal are connected. 52, resistors 48 and 50 are connected.
A resistor 49 and a capacitor 51 are connected in parallel with each other between the connection points 8 and 50 and the control signal input terminal 53. The resistors 49 and 50 divide the control voltage, and the capacitor 51 is inserted to remove noise mixed in the control voltage.

The parallel control circuit 54 as the active dummy circuit according to the present invention includes transistors 55 and 56,
Zener diode 61 and resistors 57, 58, 59,
60.

The auxiliary output circuit 20 includes a diode 2 connected to the second secondary winding 18 of the inverter transformer 14.
5, resistor 26, capacitor 27, three-terminal regulator 2
4. Consisting of a capacitor 28 and sub-output terminals 29 and 30.

The operation of the above configuration will be described. The operation for stabilizing the output voltage is as described above. Here, when the output voltage of the main output circuit 19 falls below the threshold voltage of the Zener diode 61, the transistor 5
6 is turned off, and the transistor 55 is replaced with a resistor 58
Is turned on by forward bias.

Since the transistor 55 is connected in series with the resistor 57 and is arranged in parallel with the main output circuit 19, the transistor 55 outputs a current determined by the output voltage of the main output circuit 19 and the resistance value of the resistor 57. It operates as a dummy current of the circuit 19 to increase the output voltage of the main output circuit 19 to increase the duty ratio of the switching element 13. Then, the tap voltage of the second secondary winding 18 of the sub-output circuit 20 in the inverter transformer 14 acts to increase, and the input voltage to the three-terminal regulator 24 of the sub-output circuit 20 increases, and the output voltage is always stable. An output voltage is obtained.

That is, in FIG. 3, the main output circuit 19
When the output voltage of the main output circuit 19 reaches the threshold voltage point D of the Zener diode 61 even if the tap voltage Vi of the sub-output circuit 20 decreases due to the decrease of the output voltage of
The parallel control circuit 54 supplies a dummy current to the main output circuit 1
The duty ratio of the switching element 13 is controlled so that the slope of the output voltage 9 decreases. Therefore, even if the output voltage of main output circuit 19 is reduced by an external control signal, the reduction of input voltage Vi of sub output circuit 20 is reduced,
The output voltage Vo of the sub output circuit 20 holds a constant value as shown by the characteristic line B.

It should be noted that the slope of the drop in the tap voltage Vi of the sub output circuit 20 due to the drop in the output voltage of the main output circuit 19 is:
If the characteristic line is smaller than the characteristic line A as shown by the dashed line characteristic A 'in FIG. 3, the threshold voltage point of the Zener diode 61 may be set slightly higher as D'.

The Zener diode 61 is necessary for accurately setting the operating point voltage of the parallel control circuit 54 as an active dummy circuit. However, for a relatively rough setting, the Zener diode 61 is omitted. A voltage obtained by dividing the output voltage of the main output circuit 19 by the resistors 59 and 60 is applied to the transistor 61. And this voltage is
When the voltage between the base electrode and the emitter electrode of the transistor 56 (Vbe = 0.7 V) or less, the transistor 56
Is turned off, and the transistor 55 is turned on by being forward biased by the resistor 58. The subsequent operation is performed by the Zener diode 6
This is the same as when there is one.

In the first embodiment of the present invention shown in FIG. 1, the case where the primary circuit 22 is of the separately excited type using the PWM control IC 23 has been described. However, the present invention is not limited to this example. 2, the auxiliary winding 1 is connected to the base of the switching element 13 connected to the primary winding 15 as shown in FIG.
6, a capacitor 63, a resistor 64, a diode 6
5 and the resistor 62 and the light receiving element 46
Can be applied even in a self-excited system such as a so-called ringing choke converter.

[0025]

According to the present invention, the AC input power supply 10 is switched at a high frequency, and the power is converted by the inverter transformer 14, and the stable output voltage is supplied to the secondary side main output circuit 19 and / or one or more sub output circuits 20. When the output voltage of the main output circuit 19 is variably reduced by an external control signal to reduce the output voltage, a decrease in the output voltage is detected and the dummy current of the main output circuit 19 is increased. Control circuit 54 for the main output circuit 19
Connected to the parallel control circuit 54 to operate as a dummy current of the main output circuit 19 to increase the output voltage of the main output circuit 19 to increase the duty ratio of the switching element 13. Acting to increase the voltage, the input voltage to the three-terminal regulator 24 and the like of the sub output circuit 20 increases, and a stable output voltage can always be obtained.

Since there is no need to increase the tap voltage in the inverter transformer 14, even when the duty ratio of the switching element 13 is determined by the maximum voltage and the maximum current of the main output circuit 19, heat generated by the three-terminal regulator 24 and the like is generated. No loss occurs, and the overall conversion efficiency of the switching power supply is improved.

[Brief description of the drawings]

FIG. 1 is an electric circuit diagram showing a first embodiment of a switching power supply circuit according to the present invention.

FIG. 2 is an electric circuit diagram showing a second embodiment of the switching power supply circuit according to the present invention.

FIG. 3 is a diagram showing characteristic lines according to the switching power supply circuit of the present invention.

FIG. 4 is a diagram showing characteristic lines of a conventional switching power supply circuit.

FIG. 5 is an electric circuit diagram showing a conventional switching power supply circuit.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 10 ... AC input power supply, 11 ... Full-wave rectifier, 12 ... Capacitor, 13 ... Switching element, 14 ... Inverter transformer, 15 ... Primary winding, 16 ... Auxiliary winding, 17 ... First secondary winding, 18 ... second secondary winding, 19 ... main output circuit,
20 ... sub output circuit, 21 ... detection circuit, 22 ... primary circuit,
23 ... PWM control IC, 24 ... 3-terminal regulator,
25 ... diode, 26 ... resistor, 27, 28 ... capacitor, 29, 30 ... sub output terminal, 31 ... diode, 32
... capacitors, 33, 34 ... main output terminals, 35, 36,
37: resistor, 38: transistor, 39, 40, 41 ...
Resistance: 42 Zener diode 43: External control signal input circuit 44: Photocoupler as insulating means 45:
Light emitting element, 46: light receiving element, 47: transistor, 4
8, 49, 50 ... resistor, 51 ... capacitor, 52, 53
... a control signal input terminal, 54 ... a parallel control circuit as an active dummy circuit, 55, 56 ... transistors, 57,
58, 59, 60 ... resistance, 61 ... Zener diode,
62: resistor, 63: capacitor, 64: resistor, 65: diode.

Claims (4)

(57) [Claims] 1. A switching power supply circuit which performs high-frequency switching of an AC input power supply 10 and converts the power by an inverter transformer 14 to obtain a stable output voltage at a secondary-side main output circuit 19. When the output voltage of the
A parallel control circuit 54 for detecting a decrease in the output voltage and increasing the dummy current of the main output circuit 19 is connected to the main output circuit 19, and the parallel control circuit 54
9 to detect when the output voltage of 9 goes below a certain value
The diode 61 and the turning off of the Zener diode 61
A transistor 56 which is turned off sometimes and a transistor 5
6 when it is off, the output voltage and resistance of the main output circuit 19
The current determined by 57 is a dummy current of the main output circuit 19
And a transistor 55 that flows
A switching power supply circuit characterized by acting as a dummy circuit . 2. An AC input power supply 10 is subjected to high-frequency switching by a switching element 13 of a primary circuit 22, converted into electric power by an inverter transformer 14, and converted to a secondary-side main output circuit 1.
When the output voltage of the main output circuit 19 is variably reduced by an external control signal in a composite output switching power supply circuit in which a stable output voltage is obtained at 9 and at least one sub output circuit 20, And a parallel control circuit 54 for detecting a decrease in the output voltage and increasing the dummy current of the main output circuit 19, is connected to the main output circuit 19.
The parallel control circuit 54 of FIG.
Zener diode 61 that detects when the voltage falls below a certain value
When the Zener diode 61 is turned off.
When the transistor 56 is turned off,
And the voltage determined by the output voltage of the main output circuit 19 and the resistor 57.
Current flowing as a dummy current of the main output circuit 19
And an active dummy circuit.
A switching power supply circuit characterized in that the switching power supply circuit operates . 3. The switching power supply circuit according to claim 2, wherein the primary circuit has a separately excited system in which the switching element of the primary circuit is switched at a high frequency by a PWM control IC. 4. The switching power supply circuit according to claim 2, wherein the primary circuit comprises a ringing choke converter whose switching element performs high-frequency switching in a self-excited manner.