JP2000060117A - Switching power supply unit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To speed up operation of an output on/off circuit and prevent the generation of overshooting of an output voltage by installing a switch commanding on or off of an output voltage, and an MOS type FET switching the voltage level of a control terminal according to the on/off command. SOLUTION: In the case of normal operation, a control voltage Vc which exceeds a threshold is applied to a control terminal C. By the application of a pulse conduction width corresponding to Vc, a switching element 2A of an IC2 for control is switched, an a specified DC output voltage Vo is generated on the secondary winding 4 side of a transformed 1. From a switch 24, a command for turning off the DC output voltage Vo is outputted. An MOS type FET 26 changes over the voltage level of the control terminal C to a voltage lower than the threshold, and turns off the switching element 2A of the IC2 for control. In this case, switching of the FET 26 is quick, and the control voltage Vc of the control terminal C instantaneously becomes lower than the threshold, so that there is no time lag until the control voltage Vc reaches a value than the threshold.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of controlling a switching element, wherein a pulse conduction width of a built-in switching element is varied in accordance with a control voltage applied to a control terminal. The present invention relates to a switching power supply device provided with a control IC for performing switching and an output on / off circuit for turning on or off an output voltage.

[0002]

2. Description of the Related Art In general, a switching power supply of this type uses a control IC in which a switching element and its drive circuit are integrated. For example, as shown in FIG. 2, reference numeral 1 denotes a transformer for insulating a primary side and a secondary side, and 2 denotes a control IC incorporating a switching element 2A and its drive circuit (not shown). The DC input voltage Vi input from between the input terminals + Vin and -Vin is intermittently applied to the primary winding 3 of the transformer 1 by the control IC 2. The voltage induced in the secondary winding 4 of the transformer 1 is rectified by rectifying diodes 6 and 7 constituting a rectifying / smoothing circuit 5, then smoothed by a choke coil 8 and a smoothing capacitor 9, and output terminals + Vout and −Vout. Vo
It is output as a DC output voltage Vo during ut.

On the other hand, as a feedback circuit for stabilizing the DC output voltage Vo, there is provided an output voltage detection circuit 10 for detecting the fluctuation of the DC output voltage Vo. The output voltage detection circuit 10 connects a series circuit 13 of a light emitting diode 11a, which is a light emitting element of a photocoupler 11, and a shunt regulator 12 together with a resistor 14 between output terminals + Vout and -Vout. V
It is configured such that o is divided and applied by resistors 15 and 16. The phototransistor 11b, which is a light receiving element of the photocoupler 11 that outputs a voltage detection signal, is
The operation voltage of the phototransistor 11b is connected to the control terminal of C2, that is, the control terminal C.
The voltage is induced by rectifying and smoothing the voltage induced in the auxiliary winding 17 wound through the rectifier diode 18 and the smoothing capacitor 19. The DC output voltage Vo is determined by the resistors 15, 16
Is applied to the reference of the shunt regulator 12, and the current value flowing into the cathode of the shunt regulator 12 changes according to the difference between the applied voltage and the reference voltage of the shunt regulator 12, and the voltage of the light emitting diode 11a is changed. The amount of light emission also changes. The control IC 2 inputs a change in current flowing into the phototransistor 11b to the control terminal C as a voltage detection signal in accordance with a change in the light emission amount of the light emitting diode 11a, and outputs a DC output signal from the output detection circuit 10 The pulse conduction width of the DC input voltage Vi is controlled so as to keep the voltage Vo constant.

More specifically, when the DC output voltage Vo rises, the control voltage Vc input to the control terminal C increases with the increase in the light emission amount of the light emitting diode 11a.
Will be higher. When the control voltage Vc of the control terminal C increases, the control IC 2 acts to reduce the pulse conduction width of the switching element 2A and decrease the DC output voltage Vo. Conversely, when the DC output voltage Vo drops, the control IC
2 acts to increase the pulse conduction width of the DC input voltage Vi and increase the DC output voltage Vo. When the control voltage Vc inputted to the control terminal C becomes lower than a certain threshold value Vth, the switching element 2A of the control IC 2 is forcibly turned off in order to avoid an excessive rise of the DC output voltage Vo. Then, the operation of the switching power supply is stopped. That is, the control IC 2 changes the pulse conduction width of the built-in switching element 2A according to the control voltage Vc applied to the control terminal C, and turns off the switching element 2A when the control voltage Vc reaches the threshold value Vth. It has the function of

On the other hand, an output on / off circuit 20 for controlling the DC output voltage Vo to be on or off (on / off) while the DC input voltage Vi is being supplied is provided. The output on / off circuit 20 is a PNP transistor that inputs an on / off signal to the control terminal C of the control IC 2
The collector of the PNP transistor 21 is connected between the base of the PNP transistor 21 and the input terminal-Vin.
22, a resistor 23 and a switch 24 serving as a switch are connected in parallel, and the base of the PNP transistor 21 and the input terminal +
A resistor 25 is connected between Vin. The switch 24 for instructing ON / OFF of the DC output voltage Vo includes, for example, a mechanical switch and a transistor.

When the switch 24 is ON, the base potential of the PNP transistor 21 is 0, so that no effect is exerted on the control terminal C of the control IC 2. Therefore, the operation is continued while the switching power supply is on.
Conversely, when the switch 24 is turned off, the constant voltage diode 22
As a result, the base potential of the PNP transistor 21 becomes constant at a certain potential, and the PNP transistor 21 is turned ON.
The control voltage Vc of the control terminal C of the control IC 2 is 0
Becomes Here, as described above, the control IC 2 stops the operation of the switching element 2A when the control voltage Vc input to the control terminal C becomes lower than a certain threshold value Vth. The operation of the device stops. By operating the switch 24 of the output on / off circuit 20, the on / off control of the DC output voltage Vo can be performed while the DC input voltage Vi is still supplied. ing.

Incidentally, a PNP transistor 21 is used in the output on / off circuit 20 of the prior art.
Therefore, the operation speed of the output on / off circuit 20 is slow, and when the switch 24 is turned off, an overshoot occurs in the DC output voltage Vo. That is, as shown in FIG. 4, the switching of the output ON / OFF circuit 20 from ON (ON) to OFF (OFF) is slow, and the control voltage Vc of the control terminal C of the control IC 2 decreases with a certain slope. I do. Due to the delay from when the switch 24 is turned off until the control voltage Vc reaches the threshold value Vth, the control IC 2 temporarily stops the pulse conduction width of the DC input voltage Vi as when the DC output voltage Vo decreases. To increase the DC output voltage Vo. Then, due to the rise of the DC output voltage Vo,
Overshoot occurs.

[0008] Such overshoot may cause a reduction in the life of a device to which power is supplied from the switching power supply device, and in some cases, may cause an adverse effect such as damage to the device. Also, in recent years, the tolerance of input voltage fluctuations of ICs used in computers and the like has become extremely narrow, and switching power supply devices have
When used for C, overshoot of the DC output voltage Vo as described above is a problem that must be avoided by all means.

SUMMARY OF THE INVENTION In view of the above problems, an object of the present invention is to provide a switching power supply device in which the operation speed of an output on / off circuit is increased and the occurrence of output voltage overshoot is prevented.

[0010]

According to the switching power supply of the present invention, the pulse conduction width of a built-in switching element is varied in accordance with a control voltage applied to a control terminal, and when the control voltage reaches a threshold value. A control IC for turning off the switching element, intermittently applying a DC input voltage to the primary winding of the transformer by the control IC,
In a switching power supply device provided with an output on / off circuit for supplying a predetermined output voltage and turning on or off the output voltage, the output on / off circuit includes a switch unit for instructing on or off of the output voltage. And a MOS FET for switching the voltage level of the control terminal in accordance with the operation of the switch means.

According to the above configuration, during normal operation, by applying a control voltage exceeding the threshold value to the control terminal,
The switching element of the control IC is switched with a pulse conduction width corresponding to the control voltage, and a predetermined DC output voltage is generated on the secondary winding side of the transformer. In this state, when the switch means instructs to turn off the DC output voltage, the MOS FET switches the voltage level of the control terminal so as to be lower than the threshold value, and turns off the switching element of the control IC. . At this time, the MOS type F
Since the switching of the ET is fast and the control voltage of the control terminal instantaneously becomes lower than the threshold value, there is no time delay from turning off the switch means until the control voltage reaches the threshold value V or less. Therefore, it is possible to increase the operation speed of the output on / off circuit and prevent the DC output voltage from overshooting.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the switching power supply according to the present invention will be described below with reference to FIG. In FIG. 1, the same portions as those in the circuit diagram of FIG. 2 in the conventional example are denoted by the same reference numerals, and detailed description of the common portions will be omitted.

FIG. 1 is a circuit diagram showing one embodiment of the present invention. In the figure, reference numeral 26 denotes a MOS FET used in place of the conventional PNP transistor 21. The MOS FET 26 is configured to connect a source, a drain, and a gate corresponding to the emitter, collector, and base of the PNP transistor 21 of the conventional example.

Next, the operation of the above configuration will be described. In the output on / off circuit 20, when the switch 24 is on, the gate potential of the MOS FET 26 is 0, so that no action is exerted on the control terminal C of the control IC 2. Therefore, the switching power supply device continues the operation of stabilizing the DC output voltage Vo as described in the conventional example. Conversely, when the switch 24 is turned off, the gate potential of the MOS FET 26 becomes constant at a certain potential by the constant voltage diode 22, and the MOS FET 26 is turned on, so that the control voltage Vc of the control terminal C of the control IC 2 becomes 0. Become. Here, as described in the conventional example, when the control voltage Vc input to the control terminal C becomes lower than a certain threshold value Vth, the control IC 2 stops the operation of the switching element 2A. Also, the operation of the switching power supply stops. As described above, by operating the switch 24 of the output on / off circuit 20, the on / off control of the DC output voltage Vo can be performed while the DC input voltage Vi is still supplied. .

Since the PNP transistor 21 is used in the output on / off circuit 20 of the prior art, the operation speed of the output on / off circuit 20 is low, and the DC output voltage Vo overshoots when the switch 24 is turned off. However, in the present invention, since the MOS type FET 26 is used, the operation speed of the output on / off circuit 20 is very high,
The overshoot of the DC output voltage Vo when the switch 24 is turned off can be prevented. That is, as shown in FIG. 3, the switching of the MOSFET 26 from on to off is fast, and the control voltage Vc of the control terminal C of the control IC 2 is controlled.
Instantaneously drops to 0, so there is no delay in the time from when the switch 24 is turned off until the control voltage Vc reaches the threshold value Vth or less. Therefore, unlike the above conventional example, the DC output voltage Vo does not temporarily increase.
No overshoot occurs. The operations of the switching power supply other than the output on / off circuit 20 are as described in the conventional example.

As described above, in this embodiment, the pulse conduction width of the built-in switching element 2A is varied according to the control voltage Vc applied to the control terminal C, and when the control voltage Vc reaches the threshold value Vth. , Switching element 2A
And a control IC 2 for turning off the DC output voltage Vo. The control IC 2 applies a DC input voltage Vi to the primary winding 3 of the transformer 1 intermittently to supply a predetermined DC output voltage Vo. In a switching power supply device having an output on / off circuit 20 for turning on / off Vo,
The output on / off circuit 20 includes a switch 24 as switch means for instructing on / off of the DC output voltage Vo, and a MOS FET 26 for switching the voltage level of the control terminal C in accordance with the operation of the switch 24. I have.

According to the above configuration, during normal operation, the control voltage Vc exceeding the threshold value Vth is applied to the control terminal C.
Is applied, the switching element 2A of the control IC 2 is switched with a pulse conduction width corresponding to the control voltage Vc, and a predetermined DC output voltage Vo is generated on the secondary winding 4 side of the transformer 1. In this state, the DC output voltage Vo
Is turned off by the switch 24, the MOS FET 26 switches the voltage level of the control terminal C so as to be lower than the threshold value Vth, and turns off the switching element 2A of the control IC 2. At this time, the switching of the MOS FET 26 is fast, and the control voltage Vc of the control terminal C instantaneously becomes lower than the threshold value Vth. Therefore, from turning off the switch 24 until the control voltage Vc reaches the threshold value Vth or less. There is no time delay. Therefore, it is possible to increase the operation speed of the output on / off circuit 20 and prevent the DC output voltage Vo from overshooting.

It should be noted that the present invention is not limited to the above embodiment, and various modifications can be made within the scope of the present invention. For example, the switching power supply device may be configured by a flyback system or the like instead of the forward system.

[0019]

According to the switching power supply of the present invention, the pulse conduction width of the built-in switching element is varied in accordance with the control voltage applied to the control terminal, and when the control voltage reaches a threshold value, the switching element is switched. And a control IC for turning off the output voltage. The control IC intermittently applies a DC input voltage to the primary winding of the transformer to supply a predetermined output voltage and turn the output voltage on or off. In a switching power supply device provided with an on / off circuit, the output on / off circuit includes a switch for instructing on or off of the output voltage, and a voltage level of the control terminal according to an operation of the switch. And a switching MOS-type FET.
It is possible to increase the operating speed of the OFF circuit and prevent the DC output voltage from overshooting.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a switching power supply device showing an embodiment of the present invention.

FIG. 2 is a circuit diagram of a switching power supply device showing a conventional example.

FIG. 3 is a waveform chart of each part in the embodiment of the present invention.

FIG. 4 is a waveform diagram of each section in a conventional example.

[Explanation of symbols]

 Reference Signs List 1 transformer 2 control IC 2A switching element 3 primary winding 20 output on / off circuit 24 switch (switch means) 26 MOS type FET C control terminal (control terminal)

Claims (1)

[Claims] 1. According to a control voltage applied to a control terminal,
A control IC for turning off the switching element when the pulse conduction width of the built-in switching element is variable and the control voltage reaches a threshold value is provided. And a switching power supply device that includes an output on / off circuit that supplies a predetermined output voltage and turns on or off the output voltage, wherein the output on / off circuit includes: A switching power supply device comprising: switch means for instructing ON or OFF; and a MOS FET for switching a voltage level of the control terminal according to the operation of the switch means.