KR19990027207U - Overcurrent Protection Circuit in DC-DC Converter of Monitor - Google Patents

Abstract

The present invention detects when the B + overcurrent flows through the B + voltage output line and changes the on time of the PWM control signal so that the circuit element is not damaged by the overcurrent. As related, a circuit of the present invention includes a sensing voltage output unit for outputting a sensing voltage according to the magnitude of the B + current flowing through the B + voltage line; A PWM controller for outputting a PWM control signal whose duty ratio is varied according to the feedback voltage obtained from the FBT and the sensing voltage; A step-down converting unit that is turned on / off by a PWM control signal output from the PWM controller and steps down the input DC voltage to the B + voltage to provide the FBT to the FBT; And a reference voltage output unit configured to apply a reference voltage compared with the sensing voltage to the PWM controller.

Therefore, the present invention has the effect of protecting the circuit element from the B + overcurrent because it detects when an overcurrent flows on the B + voltage line and feeds it back to the PWM controller to reduce the duty ratio of the PWM control signal.

Description

A circuit for protecting DC-DC converter while detected B + overcurrent in a monitor

The present invention relates to a DC-DC converter in a monitor, in particular, to detect when B + overcurrent flows through the B + voltage output line and to vary the on time of the PWM control signal to prevent damage to the circuit elements due to the overcurrent. The present invention relates to an overcurrent protection circuit in a DC-DC converter of a monitor.

In general, a monitor is a device that receives information from a computer video card and a synchronization signal and displays information on a CRT screen. The monitor includes a video system for processing a video signal, a deflection system for vertical and horizontal deflection, and a power supply. It consists of a system.

Here, the power supply system of the monitor serves to properly supply the voltage required in each part of the monitor, the AC power is input and rectified and guided to the secondary side through the transformer to supply various power required in the circuit Provides the main power supply (SMPS), DC-DC converter for receiving the B + voltage to the FBT by receiving the DC voltage provided from the main power supply, and the high voltage, screen voltage, control grid voltage required by the CRT It consists of a high pressure generator (FBT) and the like.

Here, the DC-DC converter is a boost type converter (referred to as a boost up converter) that receives a low DC voltage and outputs a large DC voltage, and a step-down converter that receives a high DC voltage and outputs a low DC voltage ( This is also called a buck converter), and a flyback type converter using a transformer.

As an example of the step-down converter, a circuit for generating a B + voltage in a monitor includes a switching element Q1 for switching a DC input voltage DCin according to a control signal, as shown in FIG. 1, and the switching element Q1. Inductor (L1) for accumulating energy when the current flows through the diode, the diode (D1), the inversion buffer 104, which is turned on when the switching element (Q1) is off to provide a current path for the inductor (L1), And a PWM controller 102 for generating a control signal for turning on / off the switching element Q1, an electrolytic capacitor C1 for removing ripple, and the like.

The PWM controller 102 rectifies and smoothes the voltage induced by the tertiary coil of the FBT 110 in the rectifier diode D2 and the capacitor C2 and receives the input through the split resistors R2 and R3. The high voltage is stabilized by generating a PWM pulse according to the input F / B to drive the inversion buffer 104 to turn on / off the switching element Q1.

The operation of the step-down converter will be described with reference to the waveform diagram shown in FIG. 2.

In FIG. 2, (A) shows a PWM control signal output to node A to turn on / off the switching element Q1, and (B) shows a state in which the PWM control signal has been inverted through an inverting buffer. A signal applied to B is shown, and (C) shows a signal applied to the node C, that is, the gate terminal of the switching element. (D) shows the voltage V _D1 between both ends of the diode D1, (E) shows the current I _L1 flowing through the inductor L1, and (F) shows the switching element Q1. The current I _Q1 flowing through is shown.

As shown in FIG. 2A, when the control signal is 'high', the output of the inverting buffer transitions to a low level as shown in FIG. 2B, and the switching element is turned on so that the node C Input DC voltage is applied. At this time, the voltage between the drain and the source of the switching device drops to almost '0', and the current flowing through the switching device rises as shown in FIG. In addition, a voltage is applied to the diode D1 in the reverse direction, so that the diode D1 is turned off. Therefore, a high voltage is applied to both ends of the diode D1 as shown in FIG.

The difference between the input voltage and the output voltage DCin-B + is applied to the inductor L1 so that the current I _L1 flowing in the inductor L1 gradually increases during the turn-on of the switching device. In this case, the increase rate of the inductor current ( ) Is determined by Equation 1 according to the voltage (V _L ) and inductance (L) applied to the inductor (L1).

On the other hand, when the control signal becomes 'low' and the switching element Q1 is turned off, the voltage V _Q1 appears between the drain and the source of the switching element Q1, and the counter electromotive force is generated in the inductor L, thereby the diode D1. ) Becomes conductive so that a current flowing through the switching element Q1 flows through the diode D1. At this time, since the voltage induced in the inductor L1 is in the opposite direction as when the switching element Q1 is turned on, the current flowing through the inductor L gradually decreases as shown in FIG. In addition, since the switching device Q1 is turned off, the current flowing through the switching device becomes '0' as shown in FIG.

In the step-down converter, the magnitude of the output voltage (B + voltage) is determined by the magnitude of the input DC voltage DCin and the turn-on time Ton and the turn-off time Toff of the switching device as shown in Equation 2 below.

here, Is the duty ratio of the PWM control signal. As represented by Equation 2, as the turn-on period Ton of the switching element Q1 becomes longer, the output voltage Vo becomes higher, and when it is turned on, the output voltage becomes equal to the input voltage.

However, in the DC-DC converter, the frequency of the PWM control signal is determined by the resistance value of the R _T resistor connected to the R _T terminal and the C _T terminal of the PWM controller and the capacitance of the C _T capacitor, and the turn-on of the PWM control signal is turned on. The period is determined by the magnitude of the feedback voltage. The frequency f = PWM control signal. Becomes

The DC-DC converter of the monitor receives the feedback voltage through the tertiary winding of the transformer and outputs a PWM control signal whose duty ratio is variable according to the feedback voltage, so as not to excessively increase the B + voltage. However, since the current flowing through the B + voltage line is not sensed, an abnormality such as a short-circuit occurs in the transformer. When an overcurrent flows through the B + voltage line, the B + voltage decreases and the duty ratio of the PWM control signal increases, which causes the circuit elements to be damaged. There was a problem such as being.

Therefore, the present invention was devised to solve the problems of the prior art as described above. When overcurrent flows on the B + voltage line, the present invention detects it and feeds it back to the PWM controller so that the duty ratio of the PWM control signal is reduced so that the circuit element from the overcurrent is reduced. It is an object of the present invention to provide an overcurrent protection circuit in a DC-DC converter of a monitor that protects the circuit.

The circuit of the present invention for achieving the above object, the PWM controller for outputting a PWM control signal whose duty ratio is variable according to the feedback voltage obtained from the FBT, and the turn on / off by the PWM control signal output from the PWM controller In the DC-DC converter of the monitor consisting of a step-down converting unit for stepping down the input DC voltage to the B + voltage to provide to the FBT, the sensing voltage according to the magnitude of the B + current flowing through the B + voltage line to the PWM controller A sensing voltage output unit configured to vary a duty ratio of the PWM control signal according to the sensing voltage; And a reference voltage output unit configured to provide a reference voltage compared with the sensing voltage to the PWM controller.

1 is a circuit diagram showing a general DC-DC converter in a monitor;

2 is a waveform diagram of each sub-signal of the DC-DC converter shown in FIG. 1;

3 is a circuit diagram showing a DC-DC converter according to the present invention with an internal circuit diagram of the PWM controller of the monitor,

4 is a waveform diagram of each sub-signal of the DC-DC converter illustrated in FIG. 3.

* Names of symbols according to the main parts of the drawings

100: DC-DC converter 110: FBT

120: horizontal output unit 102,200: PWM controller

104: inverting buffer 210: reference voltage supply

220: sensing voltage supply unit Q1: switching device

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

Figure 3 shows a DC-DC converter of the monitor according to the present invention, which is a sensing voltage output unit 220 for outputting the sensing voltage (Vs1) according to the magnitude of the B + current flowing in the B + voltage line, and from the FBT The PWM controller 200 outputs a PWM control signal whose duty ratio is variable according to the feedback voltage F / B and the sensing voltage Vs1 obtained, and is turned on / off by the PWM control signal output from the PWM controller 200. While stepping down the input DC voltage to B + voltage to provide a step-down converting unit and a reference voltage output unit 210 for applying a reference voltage (Vs2) compared to the sensing voltage (Vs1) to the PWM controller 200. It is composed.

Here, the step-down converting part is applied to the switching element Q1 to which the PWM control signal is applied to the gate terminal and turned on / off and the input DC voltage DCin is applied through the drain terminal, and the source terminal of the switching element Q1. A diode D1, an inductor L1, and a capacitor C1 are connected to each other, and a B + voltage is output at a common contact between the inductor L1 and the capacitor C1.

In addition, the reference voltage output unit 210 may include a first divided resistor R21 to a second divided resistor R22 for dividing the reference voltage Vref output from the PWM controller 200. 220 includes a third divided resistor R23 to a fourth divided resistor R24 to which a B + current flows and thus a voltage is applied.

The operation of the present invention configured as described above is as follows.

When a driving voltage is applied to the PWM controller 200, an internal voltage is generated to generate a reference voltage Vref. This reference voltage flows through the constant current source 31 to the C _T capacitor, where the C _T capacitor is charged. That is, when the V _CT voltage gradually rises and the V _CT voltage becomes higher than the reference voltage Vr, the comparator OP31 outputs a high level comparison signal and the transistor Q31 is turned on to charge the C _T capacitor. The voltage drops to the low level. That is, the signal applied to the V _CT line is a sawtooth signal output while having a frequency according to the time constant of the R _T resistance and the C _T capacitor, as shown in FIG.

On the other hand, the B + voltage output from the DC-DC converter is applied to the FBT, and the voltage fed back from the tertiary winding of the FBT is input to the feedback terminal of the PWM controller 102. At this time, when the high voltage of the FBT becomes higher than the set voltage and the feedback voltage F / B rises, the error amplifier 32 outputs the V _F voltage having a high potential, and the high voltage becomes lower than the set voltage so that the feedback voltage F / B becomes high. When falling, the error amplifier 32 outputs a low potential V _F voltage.

The reference voltage and the sensing voltage are applied to the error amplifier 33. That is, the reference voltage Vref of the PWM controller 200 is divided by the first voltage divider R21 and the second voltage divider R22 and applied to the error amplifier 33 as a reference voltage, and at the same time, the B + current. The voltage drop generated while flowing to the fourth voltage divider R24 is applied to the error amplifier 33 as a sensing voltage.

At this time, when the B + current is normal, the reference voltage is greater than the sensing voltage, but when the B + overcurrent flows because the B + current is larger than the normal value, the first voltage dividing resistor R21 to the fourth voltage dividing resistor R24 so that the sensing voltage becomes larger than the reference voltage. Adjust the resistance value. Here, when the B + current flows normally, the output value of the error amplifier 33 is low, and when the B + overcurrent flows, the output value of the error amplifier 33 is high.

The V _F voltage determined by the output values of the error amplifier 32 fed back with the B + voltage and the error amplifier 33 fed back with the B + current is compared with the sawtooth waveform V _CT voltage at the comparator OP33. ) has a waveform such as (C) of the V _CT voltage V when _F is greater than the voltage output signal of the high level V _CT voltage V _F, when less than the voltage to output a low level signal is also the node b is 4 is do.

On the other hand, the comparator OP32 compares the sawtooth waveform V _CT voltage with the limiting voltage V _L and outputs a high level signal in a section in which the V _CT voltage is greater than the limiting voltage V _L. A waveform like (B) is applied, which limits the maximum duty ratio of the PWM control signal when the V _F voltage is at zero potential. That is, a waveform having a constant duty ratio is always applied to the node a as shown in FIG. 4B, and a waveform having a variable duty ratio is applied to the node b as shown in FIG. 4C. At this time, the duty ratio of the waveform applied to the node b is varies depending on the potential of the voltage V _F, if V _F when the voltage is above zero the duty ratio of the waveform applied to the node b is 1.

At this time, the waveform applied to the node a and the waveform applied to the node b are ORed at the OR gate 34, and a waveform having a small duty ratio is output from the OR gate 34. That is, when the duty ratio of the waveform applied to the node b is smaller than the duty ratio of the waveform applied to the node a as shown in FIG. 4, the OR gate 34 moves to the node b as shown in FIG. 4D. Outputs the same waveform as the applied waveform. On the other hand, if the duty ratio of the waveform applied to the node b is greater than the duty ratio of the waveform applied to the node a, the OR gate 34 outputs the same waveform as the waveform applied to the node a, wherein the duty ratio of the output signal is maximum It is a value.

As described above, in the present design, the B + voltage and the B + current are fed back to the PWM controller 200 so that a PWM control signal having a variable duty ratio is output. That is, when B + overcurrent flows, the sensing voltage is increased to output the V _F voltage having a high potential through the error amplifier 33, and the duty ratio and the oar gate 34 are applied to the node b by the V _F voltage having the high potential. ), The duty ratio of the PWM output signal becomes small. The B + voltage output from the DC-DC converter is expressed by Equation 2, and as the duty ratio of the PWM output signal decreases, the B + voltage output from the DC-DC converter also decreases.

As described above, the circuit of the present invention detects when an overcurrent flows through the B + voltage line and feeds it back to the PWM controller to reduce the duty ratio of the PWM control signal, thereby protecting the circuit element from the B + overcurrent. have.

Claims (1)

A PWM controller for outputting a PWM control signal whose duty ratio is variable according to a feedback voltage obtained from the FBT, and turning on / off by the PWM control signal output from the PWM controller, stepping down the input DC voltage to the B + voltage and providing the FBT to the FBT; In the DC-DC converter of the monitor consisting of a step-down converting unit, A sensing voltage output unit configured to output a sensing voltage according to the magnitude of the B + current flowing through the B + voltage line to the PWM controller so that the duty ratio of the PWM control signal varies according to the sensing voltage; And The over-current protection circuit of the DC-DC converter of the monitor, characterized in that consisting of a reference voltage output unit for providing a reference voltage compared to the sensing voltage to the PWM controller.