JPH0744824B2 - Overcurrent protection circuit for 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 overcurrent protection circuit for protecting a switching power supply device from an overload.

[0002]

2. Description of the Related Art Generally, in a capacitor input type switching power supply device, when switching operation is started from a stopped state at the time of starting the device or canceling a protective operation, an excessive current for charging a chemicon is generated. Momentarily flows through the switching circuit. This large current may cause destruction of the switching element.

Therefore, in order to prevent this phenomenon, an overcurrent protection circuit having a slow start function has been conventionally used in a switching power supply.

FIG. 3 is a circuit diagram showing an example of an overcurrent protection circuit for a switching power supply. The overcurrent protection circuit of the switching power supply shown in this figure is a switching control IC.
10, overcurrent detection comparator IC12, voltage dividing resistors 14 and 16 for determining the switching pulse width at the time of normal switching, voltage dividing resistor 1 for setting the overcurrent detection point
8, 20, slow start capacitor 22, reference voltage created inside switching control IC 10 (hereinafter referred to as V
_REF ) is turned on and off and is supplied to a maximum pulse width control terminal (hereinafter referred to as DT terminal), and a transistor 24 and a base current limiting resistor 26 of the transistor 24.

Inside the switching control IC 10, an oscillator 28 for oscillating a sawtooth voltage and a comparator 3 are provided.
Contains 0. The sawtooth voltage oscillated and output from the oscillator 28 is compared with the DT terminal voltage by the comparator 30, and the output thereof determines the switching pulse width.

When the potential of the DT terminal exceeds a predetermined switching stop voltage, the switching control IC 10
The oscillation pulse width of is always 0, and the switching operation is stopped.

Next, the operation of the apparatus will be described. Figure 4
Shows the state of the DT terminal voltage at the time of overcurrent detection.

The DT terminal voltage during normal operation is the reference voltage V _REF generated inside the switching control IC 10.
Is a potential obtained by dividing the voltage by the voltage dividing resistors 14 and 16, and therefore the switching pulse width is uniquely determined. When the overcurrent detection comparator IC12 detects an overcurrent, the transistor 24 becomes conductive and V _REF is directly applied to the DT terminal. Since the reference voltage V _REF is sufficiently higher than the switching voltage at the DT terminal,
The potential of the DT terminal exceeds the switching stop voltage and the operation stops.

The potential of the DT terminal is, as shown in the expression 100, the resistance value R'of the collector-emitter resistor 14 when the transistor 24 is ON and the slow start capacitor 2.
It rises in the CR discharge curve determined by the capacity C of 2. That is, the DT terminal voltage is V = (V _REF −V _CONST ) ε ^{− (1 / CR ′)} + V _CONST
… 100. However, V: D after a lapse of time t from the overcurrent detection point
T terminal voltage, V _CONST : DT terminal voltage in a steady state.

When the overcurrent detection is released, the base potential of the transistor 24 rises and the transistor 24 turns off. At this time, the DT terminal voltage is C due to the slow start capacitor 22 and the resistor 14 as shown in FIG.
It goes down on the R charge curve.

Similarly, even when the power supply device is started up, the V
_{After REF} is applied to the DT terminal, the DT terminal voltage slows down.
It goes down in the CR charge curve by the start capacitor 22 and the resistor 14.

That is, at the start of the switching operation,
By keeping the pulse width to a minimum and gradually expanding it over time, the inrush current generated in the switching circuit is suppressed.

[0013]

Since the conventional overcurrent protection circuit for the switching power supply is configured as described above, even if the overcurrent is detected and the transistor 24 becomes conductive,
Since the potential of the DT terminal rises due to the CR discharge curve of the collector-emitter resistance R'and the slow start capacitor C when the transistor 24 is ON, as shown in the expression 100, before the switching operation is stopped. , The time Δt shown in FIG. 4 is required. Therefore, especially when the switching frequency is high, the overcurrent phenomenon that occurs instantaneously may not be protected due to the delayed response.

An object of the Invention The present invention has been made in view of the above problems,
An object of the present invention is to provide an overcurrent protection circuit for a switching power supply, which can eliminate the time difference between the time when an overcurrent flows through the circuit and the time when overcurrent protection is applied.

[0015]

In order to achieve the above object, an overcurrent protection circuit for a switching power supply according to the present invention comprises switching control means having a reference voltage output terminal for outputting a reference voltage and a DT terminal, and a reference. It is connected to the voltage output terminal and the DT terminal, and is ON-OF for the DT terminal.
A first switching element that performs F switching, a detection unit that detects an overcurrent, a slow start capacitor that suppresses an overcurrent at startup, and a DT of a switching control unit when the detection unit detects an overcurrent. A second switching element connected to the reference voltage output terminal and the DT terminal so as to sharply increase the terminal voltage; and a second switching element.
A slow start command to configure a voltage divider circuit with the child.
Between the capacitor and the maximum pulse width control terminal
And a resistor, is characterized by increasing the response speed of the overcurrent protection operation narrows the switching pulse width instantaneously by the second switching element.

[0016]

In the switching power supply overcurrent protection circuit having the above-described structure, when the detection means detects the overcurrent, the switching pulse width is instantly narrowed by the second switching element and the resistor . This increases the response speed of the overcurrent protection operation. At the same time, the first switching element is turned on, and the reference voltage from the reference voltage output terminal is applied to the DT terminal. Therefore, the voltage at the DT terminal exceeds the switching stop voltage and switching stops. When the overcurrent is no longer detected, the switching pulse width gradually increases and the inrush current is suppressed.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a circuit diagram showing an example of an overcurrent protection circuit for a switching power supply according to the present invention. The overcurrent protection circuit for the switching power supply according to this embodiment has a configuration in which a transistor 32 is added to the conventional example shown in FIG. That is, the transistor 32 sharply increases the DT terminal voltage of the switching control IC 10 when the overcurrent detection comparator IC12 detects an overcurrent.
It is provided by being connected to the reference voltage output terminal and the DT terminal, and functions as a second switching element. Transistor 3
The base of 2 is connected to the output of the overcurrent detection comparator IC12 via the resistor 34, and the collector is connected to the resistor 36.
Is connected to the connection point of the voltage dividing resistors 14 and 16 via.

Next, the operation of this embodiment will be described.

FIG. 2 shows the transistor 2 at the time of overcurrent detection.
4 shows changes in each collector voltage of the transistor 32.

When the overcurrent detection comparator IC12 detects an overcurrent, the comparator IC1 in the "H" state
The two output terminals are in the "L" state, and both the transistor 24 and the transistor 32 are simultaneously turned on.

Then, when the transistor 24 becomes conductive,
Since the electric charge stored in the slow start capacitor 22 is discharged through the collector-emitter resistor R'in the ON state of the transistor 24, the collector voltage of the transistor 24 is V _{REF in} a curve as shown by in FIG. Rise to.

Further, due to the conduction of the transistor 32,
Although the collector voltage of the transistor 32 rises, the collector of the transistor 32 is connected to the capacitor 2 by the resistor 36.
Since it is almost separated from the discharge path of No. 2 , it is not affected by the capacitor 22. Therefore, the potential of the collector of the transistor 32, that is, the DT terminal voltage of the switching control IC 10 rises as shown by in FIG. Simultaneously with the current detection, the potential becomes the same as V _REF .

Assuming that the reference voltage of the switching control IC 10 is V _REF and the DT terminal voltage when the switching operation is stopped is V _{DT (STOP)} , the switching control IC 1
0 is designed to satisfy the relationship of V _REF > V _{DT (STOP)} .

Therefore, by the above-mentioned operation, the switching operation is stopped almost at the same time as the overcurrent detection.

The second switching element of the present invention is not limited to a transistor.

[0027]

As described above, according to the overcurrent protection circuit of the switching power supply of the present invention, when the detection means detects the overcurrent, the second switching element makes the DT terminal voltage of the switching control means steep. Since it is configured to increase the voltage, the response speed when overcurrent is detected can be increased, and the overcurrent protection can be surely performed.

[Brief description of drawings]

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

FIG. 2 is a diagram showing changes in the DT terminal voltage of the control IC during the overcurrent protection operation of the overcurrent protection circuit for the switching power supply according to this embodiment.

FIG. 3 is a diagram showing an example configuration of a conventional overcurrent protection circuit for a switching power supply.

[Fig. 4] D of the control IC at the time of overcurrent protection operation of this conventional example
It is a figure which shows the change of T terminal voltage.

[Explanation of symbols]

 10 Switching Control IC 12 Overcurrent Detection Comparator IC 22 Slow Start Capacitor 24 Transistor 32 Transistor

Claims (1)

[Claims] 1. A switching control means having a reference voltage output terminal for outputting a reference voltage and a maximum pulse width control terminal; and a maximum pulse width control terminal connected to the reference voltage output terminal and the maximum pulse width control terminal. ON
A first switching element that performs -OFF switching, a detection unit that detects an overcurrent, and a slow start capacitor that suppresses an overcurrent at startup are provided, and if the detection unit detects the overcurrent, the first switching is performed. When the element is turned on, the reference voltage from the reference voltage output terminal is applied to the maximum pulse width control terminal, the voltage of the maximum pulse width control terminal exceeds the switching stop voltage, switching is stopped, and overcurrent is no longer detected. In the overcurrent protection circuit of the switching power supply that suppresses the inrush current by gradually widening the switching pulse width, the maximum pulse width control terminal voltage of the switching control means rises sharply when the detection means detects the overcurrent. a second switching element to the reference voltage output terminal and the maximum pulse Provided in connection to the width control terminal, so as to constitute a voltage divider circuit with the second switching element
Slow start capacitor and maximum pulse width controller
An overcurrent protection circuit for a switching power supply , wherein a resistor is provided between the control terminals, and the second switching element instantaneously narrows the switching pulse width to increase the response speed of the overcurrent protection operation.