JP3002628B2 - 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 switching power supply for electronic equipment requiring a DC power supply.

[0002]

2. Description of the Related Art Heretofore, as this type of switching power supply, there has been known an RCC (ringing choke coil converter) type of frequency fluctuation type as shown in FIG. In this switching power supply, a high-frequency oscillation circuit 6 is connected to a smoothing capacitor 5 of a DC power supply 13 which is obtained by connecting a full-wave rectifier diode bridge 4 and a smoothing capacitor 5 to an AC power supply 1 via a switch 2 and a noise filter circuit 3, A diode 7 is connected to the output terminal of the high-frequency oscillation circuit 6,
A rectification / constant voltage circuit 10 including a smoothing capacitor 8 and a stabilizing circuit 9 is connected, and a load 12 such as an electronic device is connected to an output terminal of the rectified / constant voltage circuit 10 via a capacitor 11.

The high-frequency oscillation circuit 6 includes an output transformer 1
7, an NPN transistor 18 as a semiconductor switching element connected in series to a primary winding 17p of the output transformer 17, and a transistor connected between the positive voltage output terminal of the driving winding 17b and the base of the transistor 18 A transistor drive circuit 19 is provided as a switching element drive circuit that performs on / off control of the transistor 18.

The high-frequency oscillation circuit 6 includes a control NPN transistor 21 connected to the drive winding 17b of the output transformer 17, a positive voltage output terminal of the drive winding 17b when an overcurrent flows through the load 12, and Overcurrent protection main circuit 22 connected to the base of NPN transistor 21
It has.

The overcurrent protection main circuit 22 is specifically constructed by connecting a diode 23, a resistor 24, and a light receiving element of a photocoupler 25 connected in the illustrated polarity in series. The light receiving element of the photocoupler 25 is driven in accordance with on / off control based on output information on the secondary side of the light emitting element of the photocoupler 25.

Further, the high frequency oscillation circuit 6 includes an overcurrent protection auxiliary circuit 2 connected in parallel to the overcurrent protection main circuit 22.
6. A capacitor 27 is connected between the base and the emitter of the control NPN transistor 21.

The overcurrent protection auxiliary circuit 26 is configured so that the overcurrent protection main circuit 22 transmits output information on the secondary side to the photocoupler 2.
5, the overcurrent protection main circuit 22 alone does not have enough current and the control NPN transistor 21 may not be turned on, and is provided for the purpose of compensating for this. Specifically, the overcurrent protection auxiliary circuit 22
Is a Zener diode 2 connected to the resistor 28 with the polarity shown.
9 are connected in series.

Specifically, the transistor drive circuit 19 includes a drive winding 17b of the output transformer 17, and a diode 32 connected to the positive voltage output terminal of the drive winding 17b via a capacitor 31 in parallel with the polarity shown in FIG. , And a pair of resistors 33 and 34 connected in series.

In such a switching power supply, when the switch 2 is turned on, the voltage from the full-wave rectifier diode bridge 4 is applied to the high-frequency oscillation circuit 6, and the output from the high-frequency oscillation circuit 6 is supplied to the rectification constant voltage circuit 10. Rectified, stabilized and applied to the load 12.

When the overcurrent flows through the load 12 connected to the secondary side of the output transformer 17, the overcurrent protection auxiliary circuit 22 is connected to the positive voltage output terminal of the drive winding 17b based on an external control signal. The connection to the base of the control NPN transistor 21 is established.

On the other hand, the overcurrent protection auxiliary circuit 22
When the terminal voltage of the output transformer 17 becomes higher than a predetermined level, conduction between the positive voltage output terminal of the drive winding 17b and the base of the control NPN transistor 21 is performed based on an external control signal. I have.

[0012]

However, since such a switching power supply employs the RCC method, when the output load current is large, the oscillation frequency from the high-frequency oscillation circuit 6 decreases and the control NPN transistor 21 The voltage applied between the base and the emitter also becomes a relatively large voltage (at the time of rated load) as shown in FIG. 4A. However, when the output load current is small, the oscillation frequency increases and the control NPN
The voltage applied between the base and the emitter of the transistor 21 also has the property of being relatively small, as shown in FIG.

Therefore, even if the overcurrent protection auxiliary circuit 26 is provided, as the output load current decreases, the oscillation frequency increases and the response speed of the control NPN transistor 21 exceeds the limit. There is. In such a case, as shown in FIG.
, The voltage applied between the base and the emitter drops at 0.4V before it rises to the drive voltage of the control NPN transistor 21, for example, 0.6V.
This causes a problem that the control NPN transistor 21 does not turn on, and the overcurrent protection auxiliary circuit 26 does not normally operate.

Further, since the RCC method generally uses power transmission, the oscillation frequency from the high-frequency oscillation circuit 6 increases as the input voltage increases, even at a rated load.
Therefore, a difference occurs in the peak current flowing through the load 12 depending on the magnitude of the voltage. Specifically, for example, as shown in FIG. 5, the difference A between the peak current input of the characteristic graph x1 shown by the solid line when the input voltage is large and the peak current of the characteristic graph x2 shown by the dotted line when the input voltage is small is Will occur. As a result, there has been a problem that the deviation of the overcurrent detection point is increased depending on the magnitude of the input voltage.

Therefore, the present invention is to provide a switching power supply capable of operating the overcurrent protection auxiliary circuit normally even under a light load and reducing a shift of an overcurrent detection point due to a change in input voltage. It is assumed that.

[0016]

SUMMARY OF THE INVENTION The present invention comprises a DC power supply,
An output transformer connected to the DC power supply, a semiconductor switching element connected to a primary winding of the output transformer,
A switching element drive circuit that includes a drive winding of an output transformer and turns on and off a semiconductor switching element;
A control transistor for controlling the semiconductor switching element to be turned off by an on operation, and a secondary side connected between a base of the control transistor and a positive voltage output terminal of a drive winding and connected to a secondary winding of an output transformer. An overcurrent protection main circuit that conducts when an overcurrent flows through the circuit, and an overcurrent protection auxiliary circuit that is connected in parallel to the overcurrent protection main circuit and that conducts when the terminal voltage of the output transformer exceeds a predetermined level. ,
It is provided with a series circuit of a capacitor and a resistor connected between the base and the emitter of the control transistor.

[0017]

In the present invention having such a structure, when the switch 2 is turned on, the voltage from the full-wave rectifier diode bridge 4 is applied to the high-frequency oscillation circuit 6, and the output from the high-frequency oscillation circuit 6 is a rectified constant voltage. The current is rectified by the circuit 10, stabilized, and applied to the load 12.

In this case, even when the output load is small, the peak of the base-emitter voltage of the control transistor increases by the voltage applied to the resistor 31 when an overcurrent flows through the load. The driving voltage of the transistor is exceeded, and the control transistor is turned on. Then, the on / off driving of the semiconductor switching element is stopped.

Further, even when the output load is small, when the overcurrent flows through the load, the voltage is increased by the voltage applied to the resistor 31 and stored in the capacitor. It is almost the same as that.

[0020]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a circuit diagram showing the configuration of this embodiment.
The basic circuit configuration is the same as the circuit shown in FIG. 3 except that a series circuit of a capacitor 27 and a resistor 31 is connected between the base and the emitter of the control NPN transistor 21 shown in FIG.

In this embodiment having such a configuration, when the switch 2 is turned on, the voltage from the full-wave rectifier diode bridge 4 is applied to the high-frequency oscillation circuit 6, and the output from the high-frequency oscillation circuit 6 is rectified. Rectified by the voltage circuit 10,
It is stabilized and applied to the load 12.

When the overcurrent flows through the load 12 connected to the secondary side of the output transformer 17, the overcurrent protection auxiliary circuit 26 operates based on the light emitting element of the photocoupler 25 that operates in response to the overcurrent. It turns on and conducts between the positive voltage output terminal of the drive winding 17b and the base of the control NPN transistor 21.

At this time, for example, when the current flowing through the light receiving element of the photocoupler 25 is small, the control NPN transistor 21 cannot be turned on, but the voltage across the overcurrent protection main circuit 22 becomes higher than a predetermined voltage. If this is the case, conduction is established between the base of the control NPN transistor 21 and the control NPN transistor 21 is turned on to stop the on / off operation of the transistor 18.

As a result, the current flowing through the load 12 also decreases. When the voltage decreases to some extent, the control NPN transistor 21 turns off, and the on / off operation of the transistor 18 starts. Thus, the current flowing through the load 12 is adjusted, and stable power supply is possible.

In this case, even if the output load current is small and the oscillation frequency from the high-frequency oscillation circuit 6 is large, the base voltage of the control NPN transistor 21 is equivalent to the voltage between the resistors 31.
Since the voltage applied between the emitters increases, the drive voltage of the control NPN transistor 21 can be secured.

Thus, even when the output load current is small, the operation performance of the overcurrent protection auxiliary circuit 26 can be further improved. Specifically, the accuracy of the adjustable range when the output load current flows can be improved from ± 5% of the related art to ± 1%.

On the other hand, in this case, even with the rated load,
When the input voltage is large, the oscillation frequency from the high-frequency oscillation circuit 6 is large. On the other hand, when the input voltage is small, the oscillation frequency is small, so that the current flowing through the control NPN transistor 21 is small. Since only the power is supplied to the capacitor 27 as a power source, even when the input voltage is small, as shown in FIG. The characteristic graph y2 indicated by the dotted line
Is almost the same as the peak current.

As described above, the difference in the output load current caused by the magnitude of the input voltage can be minimized, so that the deviation of the overcurrent detection point due to the difference in the magnitude of the input voltage can be reduced. As a result, the stability of the output voltage can be improved.

In this embodiment, the case where the NPN transistor 21 is used as the semiconductor switching element has been described. However, the present invention is not limited to this, and an FET (field effect transistor) is used as the semiconductor switching element. It may be something.

[0031]

As described above, according to the present invention, the overcurrent protection auxiliary circuit can operate normally even under a light load, and the shift of the overcurrent detection point due to the fluctuation of the input voltage can be reduced. It is possible to provide a switching power supply that can perform the switching.

[Brief description of the drawings]

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

FIG. 2 is a diagram showing a relationship between an output voltage and an output current in the embodiment.

FIG. 3 is a circuit configuration diagram of a conventional switching power supply.

FIG. 4 shows the driving voltage and the base voltage of the transistor shown in FIG.
The figure which shows the relationship with the voltage between emitters.

FIG. 5 is a diagram illustrating a relationship between an output voltage and an output current of the switching power supply illustrated in FIG. 3;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 13 ... DC power supply 17 ... Output transformer 17p ... Primary winding 17a ... Secondary winding 17b ... Drive winding 18 ... Transistor 19 ... Transistor drive circuit 21 ... Control transistor 22 ... Overcurrent protection main circuit 26 ... Overcurrent protection Auxiliary circuit 27 ... Capacitor 31 ... Resistance

Claims (1)

(57) [Claims] An output transformer connected to the DC power supply; a semiconductor switching element connected to a primary winding of the output transformer; and a drive winding of the output transformer. On, a switching element drive circuit for driving off, a control transistor for controlling off the semiconductor switching element by an on operation, and a connection between the base of the control transistor and a positive voltage output terminal of the drive winding; An overcurrent protection main circuit that conducts when an overcurrent flows in a secondary circuit connected to a secondary winding of the output transformer; and an overcurrent protection main circuit that is connected in parallel to the overcurrent protection main circuit, so that the terminal voltage of the output transformer is An overcurrent protection auxiliary circuit that conducts when the voltage exceeds a predetermined level,
A switching power supply comprising a series circuit of a capacitor and a resistor connected between a base and an emitter of the control transistor.