KR19980031641A - Overcurrent Protection Circuit in Monitors - Google Patents

Abstract

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power supply circuit of a monitor, and more particularly to an overcurrent protection circuit for protecting a circuit element when an overcurrent flows through the power supply line.

The circuit of the present invention operates to reduce the output voltage when the feedback current fed back through the first photocoupler P1 increases and, conversely, to increase the output voltage when the feedback current decreases. In the power supply of the monitor consisting of a supply 110, a transformer 120, diodes and smoothing capacitors (D1 to D3, C1 to C3), and the first photocoupler (P1) for providing a feedback current, Overcurrent detecting means (210) for detecting an overcurrent of a power line provided at a secondary side of the transformer (120); A second photocoupler P2 operated when the overcurrent detecting means detects the overcurrent; And a feedback current controller 230 for controlling the switching mode power supply 110 to sharply lower the output voltage by rapidly increasing the feedback current of the first photocoupler when the second photo coupler is operated.

Therefore, the overcurrent protection circuit according to the present invention has an effect of protecting the circuit device from overcurrent by detecting the occurrence of overcurrent in the power supply circuit of the monitor and rapidly lowering the voltage output by the SMPS.

Description

Circuit for protecting elements from overcurrent in a monitor

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power supply circuit of a monitor, and more particularly to an overcurrent protection circuit for protecting a circuit element when an overcurrent flows through the power supply line.

In general, the power system of the monitor receives AC power, rectifies and induces the secondary side through the transformer to supply various powers required by the circuit, and the DC voltage supplied from the SMPS power supply to receive the B + voltage to the FBT. DC-DC converter for providing a high voltage, screen voltage, control grid voltage and the like required for the CRT.

FIG. 1 is a circuit diagram showing an SMPS power supply of a general monitor. The SMPS power supply is a line filter (L1, L2), XY capacitors (C1, C2, C3), a diode bridge (10), inrush current protection resistor (R2), The power control integrated circuit 11, the switching transistor Q1, the transformer 12, the diode D2, the smoothing capacitor C6, the photo couplers 14a and 14b, the output voltage detector 13, and the like.

That is, in FIG. 1, the DC voltage rectified by the diode bridge 10 is applied to the primary side of the transformer 12, and the DC voltage accumulates energy in the primary winding during the turn-on period of the switching transistor Q1. Energy is transferred to the secondary winding during the turn-off period of the switching transistor Q1, and each secondary winding is induced with the desired voltage by the turns ratio with the primary winding, and the diode D2 and the smoothing capacitor C6 Outputs a predetermined DC voltage.

And if the DC voltage output from the secondary side of the transformer 12 is a relatively low voltage such as 5V, 12V, or 24V, using a regulator integrated circuit (IC) that is readily available on the market such as 7805, 7912, 7824, etc. A stable voltage is provided to each circuit element of the monitor.

On the other hand, in such a monitor power supply circuit, a change in the output voltage is detected by the output voltage detector 13, and the detected voltage is represented by a change in the current flowing through the photocouplers 14a and 14b connected to the output voltage detector 13. This change in current will change the amount of light emitted from the photodiode 14a in the photocoupler. The change in the amount of light in the photocoupler is represented by the change of the collector current in the photo transistor 14b, which is connected to the feedback input terminal Vfb of the power control integrated circuit 11 to stabilize the output voltage. do.

That is, in the normal SMPS, the change of the output voltages Vout1 and Vout2 is fed back to the power supply control integrated circuit device 11 through the photo couplers 14a and 14b, and thus the power supply control IC 11 receives the PWM pulse. The duty ratio is varied to stabilize the output voltage.

However, in such an SMPS power circuit, when a load side is shorted or an abnormality occurs and an overcurrent flows, a circuit element is burned out.

Accordingly, the present invention has been made to solve the above problems, to provide an over-current protection circuit of the monitor to protect the circuit elements by blocking the SMPS output when the over-current flows to the load side of the monitor power supply circuit. There is this.

In order to achieve the above object, the circuit of the present invention reduces the output voltage when the feedback current fed back through the first photocoupler increases and, conversely, increases the output voltage when the feedback current decreases. A switching mode power supply in operation, a transformer for transmitting the output of the switching mode power supply to the secondary side, a diode and a smoothing capacitor connected to the secondary winding of the transformer to rectify and smooth the induced voltage of the secondary winding, and A power supply for a monitor, comprising a first photocoupler for monitoring a secondary side output voltage to provide the feedback current, comprising: overcurrent detecting means for detecting an overcurrent of a power line provided at a secondary side of the transformer; A second photocoupler operated when the overcurrent detecting means detects an overcurrent; And a feedback current control unit configured to control the switching mode power supply to rapidly lower the output voltage by rapidly increasing the feedback current of the first photo coupler when the second photo coupler is operated.

1 is a circuit diagram showing a conventional power supply circuit in a monitor;

2 is a circuit diagram showing a power supply circuit according to the present invention in a monitor.

* Explanation of symbols for main parts of the drawings

10: diode bridge 11: power control integrated circuit (PWM IC)

12,120: transformer 13: output voltage detector

14a and 14b: photo coupler 100: power supply

110: switching mode power supply (SMPS) 200: detection unit

210: overcurrent detection unit 220: photocoupler driving unit

230: feedback current control unit

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

As shown in FIG. 2, the overcurrent protection circuit of the monitor according to the present invention regulates an internal oscillation signal according to a feedback current, rectifies an AC power, and rectifies a voltage whose magnitude is adjusted according to the oscillation signal. Switching mode power supply (SMPS: 110) provided on the primary side of the transformer 120, diodes (D1, D2, D3), smoothing capacitors (C1, C2, C13), first photocoupler (P1-1, A power supply unit 100 composed of P1-2) and the like; It is composed of an overcurrent detector 210, a second photocoupler driver 220, a second photocoupler (P2), a feedback current controller 230, etc. for detecting an overcurrent flowing in a line for transmitting the output of the power supply unit to the load. It is composed of an overcurrent detection unit 200.

Referring to FIG. 2, the switching mode power supply 110 receives and rectifies an AC power and applies the rectified power to the primary side of the transformer 120. The switching mode power supply 110 turns the switching element on and off according to a control signal to turn the primary winding of the transformer 120 on. Switch the current flowing in. At this time, the control signal is determined by the feedback current input through the oscillation circuit and the photocouplers P1-1 and 2 inside the switching mode power supply 110. In the conventional PWM control method, the frequency of the oscillation circuit is constant (eg, After about 20KHz), the output voltage is controlled by adjusting the on / off duty ratio.

In the secondary winding of the transformer 120, the output voltage is induced corresponding to the winding ratio with the primary winding, and the induced voltage is rectified by the diodes D1, D2, and D3, and is then applied to the smoothing capacitors C1, C2, and C13. The ripple is removed and delivered to the load. For example, the B + voltage is connected to the horizontal output unit 400 through the FBT (300).

In addition, the first photocouplers P1-1 and P1-2 may switch the feedback current according to the photodiode P1-1 for monitoring the secondary output voltage of the transformer 120 and the light amount of the photodiode. It is composed of a photo transistor (P1-2) provided to the) to transfer the feedback current while isolating the primary side and the secondary side of the transformer.

The overcurrent detector 210 includes a resistor R21 connected in series to a power supply line, and a transistor Q21 turned on and off according to a voltage falling on the resistor R21, and the second photocoupler driver 220 includes: When the output voltage of the overcurrent detection unit 210 reaches the reference voltage, the zener diode ZD21 conducts and the transistor Q22 turns on when the zener diode ZD21 conducts, thereby turning on the photocoupler P2.

Finally, the feedback current control unit 230 includes a transistor Q23 that is turned on and off according to the output of the photo transistor of the photocoupler P2, and a transistor Q11 that is turned on and off according to the output of the transistor Q23 and the transistor ( Q12).

Subsequently, the operation of the circuit of the present invention configured as described above will be described as follows when the normal current flows and the over current flows.

1.at normal current

Referring to the operation of the overcurrent detector when the normal current flows to the load side, the transistor Q210 is turned off because the voltage across both ends of the overcurrent detection resistor R1 is very low, and thus the voltage applied to the zener diode ZD1 is low. (Q22) is also turned off. When the transistor Q22 is turned off, the photodiode of the second photocoupler P2 also becomes inoperative, and the phototransistor is turned off. Accordingly, the transistor Q23 is also turned off. Therefore, in normal operation of the overcurrent detector, the output becomes 'high'.

Referring to the operation of the power supply circuit in the normal state, since the high is applied to the transistor Q11 from the overcurrent sensing unit 200, the transistor Q11 is turned on, and thus the transistor Q12 is turned off.

Accordingly, current flows through the photodiode P1-1 of the first photocoupler, and thus the phototransistor of the first photocoupler operates to determine the duty ratio of the PWM control signal of the SMPS 110.

At this time, when the secondary voltage of the transformer 120 increases, the current flowing through the photodiode of the first photocoupler increases, and thus the amount of light transferred to the phototransistor P1-2 increases, and thus the phototransistor P1-2 is increased. The feedback current flowing through it increases. As such, when the output voltage increases and the feedback current increases, the PWM control circuit inside the SMPS 110 reduces the duty ratio of the PWM control signal to lower the output voltage.

Therefore, in normal operation, the power supply circuit increases the feedback current of the photocoupler P1 when the output voltage is increased, and the SMPS 110 lowers the output voltage. When the output voltage is decreased, the feedback current is reduced, the photocoupler P1 is reduced. Since the feedback current is lowered, the SMPS 110 increases the output voltage, so that the output voltage is always controlled.

However, when an overcurrent flows to the load side of the B + voltage due to a short circuit or an abnormal reason on the load side, the overcurrent sensing unit 200 according to the present invention operates as described below, thereby lowering the output voltage of the SMPS power circuit. Will be protected.

2. Over current operation

When an abnormality occurs in the FBT 300 or the horizontal circuit side 400 that is a load of the B + voltage, and an overcurrent flows in the B + line, the voltage between both ends of the overcurrent detection resistor R21 is increased to turn on the transistor Q21. When the transistor Q21 is turned on, current flows through the resistor R22 and the capacitor C22, and when the charging voltage of the capacitor C22 reaches the reference voltage of the zener diode ZD21, the zener diode ZD21 is conducted. The transistor Q22 is turned on by the driving current passed low through the resistor R23 and the capacitor C23.

When the transistor Q22 is turned on, the photodiode of the second photocoupler P2 is turned on. Accordingly, the phototransistor is turned on, and the voltage divided by the resistors R27 and R26 is applied to the base of the transistor Q23. Transistor Q23 is turned on. Therefore, when the overcurrent flows, the overcurrent detecting unit 200 outputs 'low'.

When the overcurrent detector 200 outputs 'low', the transistor Q11 is turned off accordingly, and thus the transistor Q12 is turned on.

When the transistor Q12 is turned on, the current flows through the photodiode P1-1 of the first photocoupler, so that the amount of light increases greatly, and the feedback current flowing through the phototransistor P1-2 rapidly increases.

Therefore, when the feedback current increases sharply, the SMPS 110 abruptly decreases the output voltage. Accordingly, the voltages induced to the secondary side of the transformer are drastically lowered to protect the circuit device from overcurrent.

As described above, the overcurrent protection circuit according to the present invention detects when a short occurs or an abnormality occurs in the load side of the power supply circuit of the monitor and detects this when the overcurrent flows, thereby rapidly lowering the voltage outputted by the SMPS to prevent the circuit device from being overcurrent. There is a protective effect.

Claims (4)

A switching mode power supply 110 operable to reduce the output voltage when the feedback current fed back through the first photocoupler P1 increases and to increase the output voltage when the feedback current decreases, Transformer 120 for transmitting the output of the switching mode power supply to the secondary side, diode and smoothing capacitor (D1 to D3, C1 to 1) connected to the secondary winding of the transformer to rectify and smooth the induced voltage of the secondary winding. C3) and a power supply device for a monitor configured with the first photocoupler P1 for monitoring the secondary output voltage to provide the feedback current. Overcurrent detecting means (210) for detecting an overcurrent of a power line provided at a secondary side of the transformer (120); A second photo coupler (P2) operated when the overcurrent detecting means detects an overcurrent; And In the monitor is provided with a feedback current control unit 230 for controlling the switching mode power supply 110 to sharply lower the output voltage by increasing the feedback current of the first photo coupler when the second photo coupler is operated. Overcurrent Protection Circuit. 2. The zener diode ZD21 of claim 1, wherein the overcurrent protection circuit is turned on when the output voltage of the overcurrent detector 210 reaches a reference voltage, and is turned on when the zener diode ZD21 is conducted. The over-current protection circuit of the monitor characterized by further equipped with the 2nd photocoupler drive part 220 comprised by transistor (Q22) which turns on (P2). The method of claim 1, wherein the overcurrent detecting unit 210 is composed of a resistor (R21) connected in series to the power line, and a transistor (Q21) turned on and off in accordance with the voltage falling on the resistor (R21). Overcurrent protection circuit in the monitor. The method of claim 1, wherein the feedback current control unit 230 is a transistor (Q23), which is turned on and off in accordance with the output of the phototransistor of the second photocoupler (P2) and, according to the output of the transistor (Q23) An overcurrent protection circuit for a monitor, comprising a transistor (Q11) and a transistor (Q12).