KR19980067992U - In the monitor, the overcurrent limiting circuit - Google Patents

Abstract

The present invention relates to a power supply system of a monitor and includes a current sensing resistor R21 for sensing the magnitude of a current input to the control IC 40 through the bridge diode D1, And a transistor Q21 which is turned on when the control IC 40 is turned on and feeds back the overcurrent input to the control IC 40 to the bridge diode D1. By using the control IC having a limited use range, And at the same time, when the input AC power source is higher than the rated voltage and the overcurrent flows through the control IC, it is possible to prevent the control IC from being damaged due to the overcurrent.

Description

Overcurrent limit circuit in monitor

The present invention relates to a power supply system of a monitor, and more particularly to a circuit for limiting the magnitude of an input current to a control IC switching a power transistor in a monitor using a specific AC voltage as an input signal.

2. Description of the Related Art Generally, a monitor is a device that receives a video signal and a synchronization signal received from a video card of a computer and displays information on a CRT screen. The monitor includes a video system for processing video signals, a deflection system for vertical and horizontal deflection, System and so on.

Here, the power supply system of the above-described monitor plays a role of appropriately supplying the voltage required in each part of the monitor. As a related technology, a small, light and efficient SMPS (Linear Power Supply) Switched Mode Power Supply) is rapidly developing.

1 shows a conventional switching power supply SMPS which includes a DC power supply 10, a transformer 20, a power transistor 30, a control IC 40, an output unit 50, An overvoltage detection unit 60, and a power saving mode control unit 70.

When the commercial AC power source of 100 V or 220 V is applied through the fuse, the line filters L 1 and L 2 and the capacitors C 1, The resonance action removes the noise introduced through the AC power supply and blocks the high frequency noise generated by the operation of the bridge diode D1 and the control IC 40 from going outside through the AC input line.

The AC voltage having passed through the line filters L1 and L2 and the capacitors C1, C2 and C3 is rectified by the bridge diode D1 and smoothed in the smoothing capacitor C4 via the inrush current preventing resistor Rs The DC voltage ( And is then applied to the primary winding of the transformer 20. [

At the same time, the rectified voltage rectified in the bridge diode (D1) The resistor And the power transistor 30 is turned on / off (turned on / off) by a control signal output from the control IC 40. The power supply voltage

That is, the DC voltage ( ) Is applied to the capacitor C5, a DC voltage ( ) Is charged, the supply voltage of the control IC 40 Reaches the starting voltage, the control IC 40 is activated to generate the PWM control signal, and the power transistor 30 is turned on / off by the PWM control signal.

When the power transistor 30 is turned on and off for a certain period of time according to the oscillator period in the control IC 40, an induced electromotive force induced in the secondary winding of the primary winding of the transformer 20 is generated .

Therefore, the transformer 20 generates the alternating-current voltage (" .

By repeating this operation, the alternating-current voltage output from the secondary winding of the transformer 20 Is again supplied with the DC voltage (< - >) through the output unit 50 and the microcomputer power supply unit 80 ).

That is, the AC voltage (? Is connected to the DC voltage (Vdd) by the rectifying diodes D2, D3 and D4 and the smoothing capacitors C6, C7 and C8 ), And supplied to each circuit of the monitor to operate the circuit.

However, the above DC voltage ( Is higher than the reference voltage, breakage of the circuit element due to the overvoltage occurs, shortening the service life of the power supply. Therefore, in order to maintain the output voltage of the output unit 50 at a constant voltage, And an overvoltage detection unit 60 as shown in FIG.

More specifically, the power supply voltage supplied to each circuit of the monitor Is detected by the overvoltage detector 60 composed of the error amplifier 61 and the photocoupler 62 and the detection signal is input to the feedback terminal of the control IC 40. [

That is, the power supply voltage (" Is larger than that in the normal operation, the error amplifier 61 detects the overvoltage and lowers the output-side voltage of the error amplifier 61. At this time, the output side of the error amplifier 61 is connected to the photocoupler 62, and the other end of the photocoupler 62 is connected to the SMPS output terminal So that a larger amount of current flows instantaneously in the light emitting portion of the photocoupler 62 than during normal operation to emit as much light as possible.

Accordingly, the light receiving portion of the photocoupler 62 receives the light, converts the optical signal into an electrical signal, and outputs the electrical signal to the feedback terminal of the control IC 40, The input current rapidly increases, and the control IC 40 generates the PWM control signal, and the ON time of the power transistor 30 is shortened by the PWM control signal.

As the ON time of the power transistor 30 becomes shorter, the induced electromotive force (i.e., the induced electromotive force) induced in the primary winding of the transformer 20 ), And the power supply voltage (" ) Is maintained at a constant voltage and supplied to each circuit of the monitor.

In addition, the power supply circuit of the monitor includes a power saving mode control unit 70. The power saving mode control unit 70 is a mode in which when the user uses the PC and does not turn off the power and vacates the place for a while or does not use the PC for a long time, The microcomputer 71 built in the PC recognizes the power saving signal and outputs a power saving signal to lower the power consumption to 30 W or less.

The power saving signal is controlled by software in the microcomputer 71 of the PC. If there is no input on the keyboard for a predetermined time (usually 1 or 2 minutes), the power saving signal is automatically output from the microcomputer 71, It indicates high level in normal mode and low level in suspend mode.

When the power saving mode control unit 70 receives a high level power saving signal from the microcomputer 71, the first transistor Q1 is turned on and the power supply voltage of 100 V is supplied to the first transistor Q1 and flows to the emitter terminal to be grounded.

At this time, the collector terminal of the first transistor Q1 and the base terminal of the second transistor Q2 are connected in parallel, so that the current passing through the Zener diode D5 flows only toward the first transistor Q1, The second transistor Q2 does not flow toward the transistor Q2, so that the second transistor Q2 maintains the turn-off state.

However, since the first transistor Q1 is turned off when a low level power saving signal is input, the voltage passing through the Zener diode D5 does not flow toward the first transistor Q1, .

Accordingly, the second transistor Q2 is turned on and the 100V power supply voltage is inputted to the collector terminal of the second transistor Q2 through the diode D6 and flows to the emitter terminal to be grounded.

At this time, more current flows to the photocoupler 61 located between the output unit 50 and the collector terminal of the second transistor Q2 than in the normal mode, and the overcurrent flows to the feedback unit of the control IC 40 ).

Accordingly, the feedback stage of the control IC 40 The control IC 40 outputs a PWM control signal having a short on time so that the on time of the power transistor 30 is also shortened so that the transformer 20 has a higher voltage than the rated voltage A small voltage is output.

While the power saving signal of the low level is outputted from the microcomputer 71, the process is repeated and falls to a certain power saving voltage level, that is, 30V or less.

The power supply circuit of the monitor includes a microcomputer power supply unit 80 and an initial power supply unit 90 that supply power to the microcomputer 71 that controls all operations of the monitor. The power supply voltage is supplied to the microcomputer 71 in a steady state, and the initial power supply unit 90 allows the microcomputer to quickly operate when the power is turned on.

First, the microcomputer power supply unit 80 will be described in detail. The microcomputer power supply unit 80 includes a rectifier D4, (Normally 8 V) is supplied to the power supply terminal of the microcomputer 71 through the regulator 81. The regulator 81 is maintained at 5 V even when the input power is unstable (usually 8 V to 15 V) So that the microcomputer 71 always supplies an accurate 5V power supply voltage.

However, since the rated voltage (8V) is not immediately supplied to the microcomputer 71 from the rectifying means D4 and C8 when the power is turned on, but the output voltage gradually increases to reach 8V, In order to supply the power supply voltage to the microcomputer 71, a high power supply voltage ( : Typically 100 V) to the regulator 81 through the third transistor Q3.

At this time, since the third transistor Q3 is operated by the power saving signal of the microcomputer 71, that is, the power source voltage (5V) is not supplied to the microcomputer 71 when the power is turned on, The first transistor Q1 is turned off and the second transistor Q2 is turned on so that the power supply voltage ) Flows to the ground through the second transistor Q2 through the diode D6 so that a low level signal is applied to the base end of the third transistor Q3, Is supplied to the regulator 81 through the third transistor Q3.

Accordingly, when the power supply voltage ) Than the power supply voltage ( Is supplied to the regulator 81 more quickly so that the regulator 81 operates quickly to supply the power source 5V to the microcomputer 71. [

Since the switching power supply circuit of the monitor constructed as described above is designed to use various AC voltages as input signals, the bridge diode D1 and the control IC 40 must have a wide range of use, .

Therefore, it is possible to limit the use range of the bridge diode D1 and the control IC 40 to limit the monitor cost. This is because most homes and offices use only a specific AC voltage (220 V) It is not necessary to use a device having a wide use range.

However, when a high voltage is applied to the monitor in a state where elements having limited use ranges are used to implement a monitor having a low price as described above, an overcurrent flows to the control IC 40, so that the control IC is damaged .

SUMMARY OF THE INVENTION The present invention has been made in order to solve the above problems, and it is an object of the present invention to provide an overcurrent limiting circuit for a monitor which protects a control IC from an overcurrent by limiting an overcurrent input to a control IC, The purpose is to provide.

According to an aspect of the present invention, there is provided an overcurrent limiting circuit for rectifying an overcurrent limiting circuit in a bridge diode, receiving a specific AC power, switching a power transistor according to a control signal of the control IC, A power supply circuit of a monitor for outputting a voltage, comprising: a current sense resistor for sensing a magnitude of a current input to a control IC through the bridge diode; and an overcurrent To a bridge diode.

1 is a circuit diagram showing a general SMPS,

2 is a circuit diagram showing a part of a power supply system of a monitor to which an overcurrent limiting circuit according to the present invention is applied.

* Name of designator according to main part of the drawing

1: Overcurrent limiting circuit R21: Current sensing resistor

Q21:

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 2 shows a DC power supply unit 10 and a control IC 40 to which the overcurrent limiting circuit 1 according to the present invention is applied. The DC power supply unit 10 receives a specific AC power and rectifies it, And the control IC 40 receives the direct current power through the start resistor Rg and switches the power transistor 30.

Here, the overcurrent limiting circuit 1 is formed at both ends of the start resistor Rg. The overcurrent limiting circuit 1 includes a current sensing resistor R21 for sensing the magnitude of a current input to the control IC 40 through the bridge diode D1 And a transistor Q21 which is turned on when the overcurrent is sensed by the current sense resistor R21 and feeds back the overcurrent input to the control IC 40 to the bridge diode D1.

Next, the operation and effect of the present invention will be described.

First, the current input to the control IC 40 is divided into the overcurrent limiting circuit 1 and the start resistance Rg at a point a. After passing through the overcurrent limiting circuit 1, .

At this time, when the magnitude of the current input to the control IC 40 is equal to or lower than the rated current, the voltage drop in the current sensing resistor R21 becomes 0.7 V or less. That is, since the voltage applied between the base and the emitter of the transistor Q21 is lower than the conduction voltage, the transistor Q21 is turned off and the current from the point "a" is fed back to the bridge diode D1.

Further, the current divided by the start resistance Rg from the point "a" is input to the control IC 40, and the control IC 40 switches the power transistor 30 while normally operating.

However, when the magnitude of the current input to the control IC 40 is an overcurrent, the voltage drop in the current sensing resistor Rg becomes 0.7 V or more, and the voltage drop across the base- So that the transistor Q21 is turned on.

When the transistor Q21 is turned on, the current flowing from the point 'a' to the start resistance Rg flows from the point 'b' to the collector-emitter terminal of the transistor Q21.

That is, when the current input to the control IC 40 is an overcurrent, some of the overcurrents are fed back to the bridge diode D1 through the current sensing resistor Rg at point "a" The control IC 40 stops driving the power transistor 30 so that the power transistor 30 is maintained in the turned off state and the DC voltage is applied to the transformer 20 via the transistor Q21, It is not supplied.

As described above, the present invention can realize an inexpensive monitor using a control IC having a limited range of use, and at the same time, when the input AC power source is higher than the rated voltage and the overcurrent flows through the control IC, There is an effect that breakage of the control IC can be prevented.

Claims (1)

A power source of a monitor which outputs a desired alternating voltage to the secondary side of the transformer 20 while the power transistor 30 is switched according to a control signal of the control IC 40, In the circuit, A current sensing resistor R21 for sensing the magnitude of the current input to the control IC 40 through the bridge diode D1, And a transistor (Q21) which is turned on when the overcurrent is sensed by the current sense resistor (R21) and feeds back the overcurrent input to the control IC to the bridge diode.