JPS5996825A - Switching overcurrent protecting circuit - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a switching overcurrent protection circuit using a primary current detection method in a switching circuit, and particularly to a switching overcurrent protection circuit that is improved so that the overcurrent protection setting value of the output current is not affected by the input power supply. 0 Regarding Overcurrent Protection Circuit (hereinafter referred to as "overcurrent protection circuit") A switching circuit having this type of overcurrent protection circuit is shown in FIG. 1.

The circuit shown in FIG. 1 consists of a switching transistor (hereinafter referred to as a transistor) Q, a control circuit 1 that controls the pulse width of a primary side peak current IP which is the output current of the transistor Q, a reference voltage VR and a primary side peak current IP. A comparator 2 that detects the current IP and compares it with a current detection voltage VF, a rectifier 4 that rectifies the AC voltage VIN of the input power source 3, and a transistor Q that converts the negative side peak current IP into a desired output voltage are loaded. R2., which rectifies and smoothes the voltage drop due to the primary side peak current 1p in the output transformer 5 connected to the emitter side of the transistor Q and the resistor connected to the emitter side of the transistor Q, and divides the voltage. Zener voltage is obtained by connecting a voltage divider circuit 6 that outputs a current detection voltage VF consisting of a diode D1 and a capacitor C1, a Zener diode ZD, and a resistor R6 in series and connecting it in parallel to the DC voltage VC of the rectifier 4. Vz generating circuit and Zener diode ZD
, a reference voltage V consisting of resistors R, , R, connected in parallel to
A voltage dividing circuit 7 that generates R, a Zener diode ZD,
an output voltage control voltage generation circuit 8 that generates an output voltage control voltage VN, which is composed of a series circuit of a resistor R7 and a resistor R8 connected in parallel to the resistor R7, and a photocoupler PC in which a phototransistor PT is connected in parallel to the resistor R7; An output side rectifying and smoothing circuit 9 consisting of diodes D, , D3 tochiyoke L and a capacitor C2 connected to the secondary side of the output voltage control voltage generating circuit 8 which outputs the output voltage 0 of the switching circuit S
A resistor R and a Zener diode ZD are connected in parallel to the output terminal of the switching circuit S to provide feedback to the
.

This is a switching circuit having an overcurrent protection circuit, which includes a feedback circuit 10 in which a light emitting diode PD of a photocoupler PC and a light emitting diode PD of a photocoupler PC are connected in series.

To explain the operation of the above circuit, overcurrent protection of the output current 10 of the switching circuit S is performed by detecting the voltage VD which is the voltage drop due to the resistor R1 inserted on the emitter side of the transistor Q of the negative side peak current IP. Is going. That is, the voltage VD is rectified and smoothed by a diode D and a capacitor C1, and then divided by a resistor R2 and a resistor R3, and the current detected is compared with the reference voltage VR input to the other input side. The reference voltage VR is the AC voltage VIN of the input power supply 3.
From the DC voltage VC obtained by rectifying the Zener diode z
It is obtained by dividing the Zener voltage Vz obtained by D by resistors R4 and R. Zener voltage Vz is input power supply 3
Since the AC voltage VIN remains constant even if it fluctuates, the reference voltage VR remains constant.

Therefore, the current detection voltage VF changes depending on the magnitude of the output current 1o, but when the output current 10 becomes large and the current detection voltage VF becomes higher than the reference voltage VR, the output of the comparator 2 changes depending on the magnitude of the output current 1o. This narrows the pulse width of the primary side peak current IP, reduces the output current IO, and prevents the output current 1o from becoming excessive.

On the other hand, for fluctuations in the output current to within the rated range, a feedback circuit 10 consisting of a Zener diode zD and a photocoupler PC adjusts the output voltage control voltage V.
In addition to controlling the control circuit 1 by changing N, the output voltage control voltage VN is also changed in response to fluctuations in the AC voltage VN of the input power source 3, and the control circuit 1 performs switching pulse width control to control the output voltage vO. It acts to keep it constant.

In such a conventional overcurrent protection circuit, when a voltage fluctuation occurs in the AC voltage VIN of the input power supply 3 when the output current 10 is constant, the switching pulse width of the transistor Q fluctuates.
- Since the next-side peak current IP fluctuates, the current detection voltage VF, which is the input of the comparator 2, fluctuates. Therefore, the output current l
The overcurrent protection setting value of O changes due to fluctuations in the AC voltage VIN of the input power source 3, which has the disadvantage that accurate overcurrent protection of the output current 1o becomes difficult.

In addition, since the overcurrent set point of the output current 10 fluctuates significantly, the current tolerance of diodes, chokes, etc. connected to the secondary side of the transformer 5 is required to be large, which makes the switching circuit large and uneconomical. There is a drawback.

This invention was made in view of the drawbacks of the conventional circuits mentioned above, and is used as a control input of a control circuit for controlling the switching pulse width of a switching transistor to maintain the output voltage of the switching circuit at a constant voltage. By dividing the output voltage control voltage to be applied using a resistor and inputting it as a reference voltage to the comparator of the overcurrent protection circuit, the reference voltage is changed in response to voltage fluctuations of the input power supply, and the overcurrent protection set value is changed. An object of the present invention is to provide a switching overcurrent protection circuit that is not affected by voltage fluctuations of an input power supply.

Hereinafter, this invention will be explained in detail with reference to an embodiment shown in FIG. Note that the same reference numerals as in FIG. 1 indicate the same or corresponding parts, and redundant explanation will be omitted.

The basic difference between the circuit shown in FIG. 2 and the circuit shown in FIG. The output voltage control voltage VN is divided by a resistor R1.5R11 and applied as a reference voltage VR.

Therefore, the operation of the circuit shown in FIG. 2 is such that when the AC voltage VIN of the input power supply 3 fluctuates, the output voltage Vo of the switching circuit S' is controlled to a constant voltage (the DC voltage VC also fluctuates). ZD, , resistance R,
, a voltage is detected by a feedback circuit 10 consisting of a photocoupler PC, and this is fed back to change the output voltage control voltage VN by a parallel circuit of a resistor R1 and a phototransistor P1' of the photocoupler PC. , the switching pulse width is controlled via the base of transistor Q.

Due to fluctuations in pulse width control, the negative side peak current ip fluctuates, the voltage VD across the resistor R1 also fluctuates, and the current detection voltage VF fluctuates.

Further, when the input AC voltage VIN fluctuates, the output voltage control voltage VN also fluctuates, so the reference voltage V'R input to the comparator 2 also fluctuates.

For example, when the output current 10 is a constant current, if the AC voltage VIN is low, the switching pulse width of the transistor Q becomes wide, the -side peak current IP becomes low, and the current detection IN of the comparator 2 becomes When the output voltage control voltage VN is low, the output voltage control voltage VN becomes low so as to widen the switching pulse width, and the reference voltage VR input to the comparator 2 is set low.

Furthermore, when the AC voltage VIN is high, the switching pulse width becomes narrow and the current detection voltage VF also becomes high, but since the output voltage control voltage VN becomes high, the reference voltage VR is also set high.

In this way, a change in the switching pulse width of the primary side peak current IP due to a change in the AC voltage VIN of the human power source 3 changes the current detection voltage VF, but the reference voltage VR also corresponds to the change in the input AC voltage VIN. The overcurrent set point can then be changed in a constant manner to achieve the initial objective.

As described above, according to the present invention, the set value of the overcurrent detection voltage is corrected in response to the pulse width change of the primary side peak current of the output transformer due to the fluctuation of the input power supply voltage. Because of this configuration, the overcurrent protection setting value of the output current of the switching circuit is not affected by voltage fluctuations of the input power supply, and stable and accurate overcurrent protection operation is performed, making it possible to use switching power supplies such as switching regulators with higher reliability. In addition, the rectifier circuit on the secondary side can be miniaturized.

[Brief explanation of the drawing]

FIG. 1 is an electric circuit diagram showing a conventional overcurrent protection circuit, and FIG. 2 is an electric circuit diagram showing an embodiment of the present invention. ■0・・・・・・Output voltage VN・・・・・・Output voltage control voltage Q・・・・・・
...Switching transistor IP...
・・・Next side peak current VF・・・・・Current detection voltage V′R・・・・・・・Reference voltage 10−・・・・・・・Output current S゛......Switching circuit (11693
-P) Procedural amendment written August 17, 1980, Mr. Kazuo Wakasugi, Commissioner of the Japan Patent Office 1, Indication of the case 1981 Patent Application No. 20445
4 No. 2, Name of the invention Switching overcurrent protection circuit 3, Relationship with the person making the amendment Applicant address (residence) Serial number (name) (230) Stanley Electric Co., Ltd. Representative 5, Date of amendment order Showa year month day (voluntary)
6. Number of inventions increased by amendment 8. Contents of amendment (1) - 1-VRJ on page 6, line 18 of the specification is amended to i VR'J. (2) J V/Rj on page 7, line 16 of the specification is corrected to [V R'J. (3) On page 8, lines 5, 9, and 13, “V
RJ is corrected to ``VR'j. (4) JV'RJ on page 9, line 15 of the same page is corrected to ``V'.
Correct as R'J.

Claims (1)

[Claims] In a constant voltage output type switching circuit that feeds back the output voltage to generate an output voltage control voltage to control the pulse width of the primary side peak current flowing through the switching transistor, a current detection voltage corresponding to the negative side peak current. and a reference voltage obtained by dividing the output voltage control voltage to limit the output current of the switching circuit.