KR20040089778A - Circuit for Controlling Driving Frequency of Lamp for Back Light - Google Patents

Abstract

PURPOSE: A circuit for controlling a driving frequency of a lamp for a backlight is provided to uniformly control current which flows in the lamp by controlling the driving frequency in an operating area of a resonant frequency generated by the matching of a leakage inductance of a leakage transformer and a capacitance of the lamp. CONSTITUTION: An AC(Alternating Current) generating device(200) is switched according to a driving frequency to generate AC. A sensing device(500) is driven by the AC to sense AC outputted from a lamp(400). A control device(100) compares the sensed AC with a certain reference output current, and generates the driving frequency according to the compared result. A resonant frequency for setting the certain reference output current is generated from a leakage inductance of the AC generating device and a capacitance of the lamp. A driving frequency band is included in a resonant frequency band.

Description

Circuit for Controlling Driving Frequency of Lamp for Back Light

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an inverter for driving a backlight lamp. More particularly, the current is constantly controlled by controlling a driving frequency to maintain a constant brightness of a cold cathode tube (CCFL) and an electrodeless tube (EEFL) lamp. It relates to a drive frequency control circuit of a backlight lamp that can obtain a stable output current.

An inverter is generally used as an AC power source, a fluorescent lamp power source of an automobile, an emergency power source in case of power failure, a backlight power source of a liquid crystal display (LCD) or other display device. For example, the liquid crystal display is used as a screen of a portable computer such as a notebook computer, and a battery is used as a power source. Since the liquid crystal display itself does not generate light, a lamp for a back light is used, and the lamp should be provided with a uniform luminance.

In addition, since a high voltage of several kV is required to drive the lamp, in the circuit for driving the lamp as shown in FIG. 1, after the low DC power provided from the battery is converted into a high voltage AC power, the AC power is applied to the lamp. Is provided.

1 is a circuit diagram showing a driving circuit for driving a conventional backlight lamp.

As shown in FIG. 1, a control unit converts DC power supplied from a DC power supply 10 such as a battery into a DC voltage of a predetermined level by a pulse width modulation control method and outputs the same. (20), an AC oscillator (Multi-Vibrator) 30 that receives the DC voltage output from the control unit 20 and converts it into AC, and outputs the AC oscillator 30 to a high voltage AC power source. Leakage transformer (40) for outputting; And a lamp 50 connected in series with a ballast capacitor which makes a high-voltage alternating current power output from the leakage transformer 40 a stable power source and provides the lamp as a load.

Referring to the operation of the driving circuit for driving the conventional backlight lamp configured as shown in Figure 1 as follows.

The control unit 20 provides a constant DC voltage to the AC oscillator 30 through the pulse width modulation (PWM) control of the power provided from the DC power supply 10. The AC oscillator 30 converts the DC voltage input from the controller 20 into a high-voltage AC power and outputs it to the lamp 50. The lamp 50 emits light when a high voltage AC power is applied from the AC oscillator 30 so that a current flows. At this time, the stabilizing capacitor causes the lamp to operate stably by applying a high voltage starting voltage to the lamp immediately after starting the lamp and applying a partial output voltage to the stabilizing capacitor after starting the lamp.

In addition, the control unit 20 compares the voltage output from the lamp with a predetermined reference voltage, and the pulse width input to the leakage transformer is changed according to the comparison result to control the output voltage of the leakage transformer.

However, it is difficult to limit the conversion range of the driving frequency because the driving circuit configured and operated as described above changes the pulse width in accordance with the comparison result with the reference voltage.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object of the present invention is to drive a circuit by varying the driving frequency in a predetermined resonance frequency band formed by matching a leakage inductance of a leakage transformer and a capacitance which is an equivalent load characteristic of a lamp. To control.

A driving frequency control circuit of a backlight lamp according to an aspect of the present invention for achieving the above object, AC switching means for generating an alternating current is switched according to the driving frequency; Sensing means for sensing an alternating current output from a lamp driven by the alternating current; And control means for comparing the sensed alternating current with a predetermined reference output current and generating the drive frequency according to a comparison result, wherein the resonant frequency for setting the predetermined reference output current is the alternating current generation. Generated from the leakage inductance of the means and the capacitance of the lamp, wherein the drive frequency band is included in the resonant frequency band.

In addition, the alternating current generating means may include switching means for switching voltage driving in accordance with the driving frequency; And a leakage transformer for converting the DC voltage drawn from the switching means into an AC voltage and outputting the same.

The control means increases the output power by lowering the driving frequency when the current sensed by the sensing means is lower than the reference output current, and increases the output frequency by increasing the driving frequency when the current sensed by the sensing means is higher than the reference output current. Decreases.

The lamp is a cold cathode tube lamp or an electrodeless tube lamp.

Therefore, according to the present invention, by controlling the driving frequency to zero current switching (Zero Current Switching) it is possible to maximize the efficiency of the switching element to achieve high efficiency.

1 is a circuit diagram showing a driving circuit for driving a conventional backlight lamp.

2 is a circuit diagram showing an inverter for driving a lamp for a backlight according to the present invention.

3 is a circuit diagram illustrating a load characteristic of the lamp of FIG. 2 and a leakage transformer.

4 is a graph showing experimental data applied to the present invention.

5 is a graph showing frequency vs. power applied to the present invention.

6 is a graph showing the frequency versus lamp current amount applied to the present invention.

<Explanation of symbols for the main parts of the drawings>

100 control means

200: AC generating means

220: switching means

240: leakage transformer

400: lamp

500: detection means

600: DC input voltage

Hereinafter, with reference to the accompanying drawings will be described a preferred embodiment of the present invention.

2 is a circuit diagram showing an inverter for driving a lamp for a backlight according to the present invention.

As shown in FIG. 2, the inverter frequency control circuit includes a control means 100, alternating current generating means 200, a lamp 400, for converting a driving DC voltage from the control means 100 into an AC voltage. The sensing means 500 and the direct current input voltage 600 are comprised.

The lamp 400 may be a cold cathode fluorescent lamp (CCFL) or an external electrode fluorescent lamp (EEFL).

The alternating current generating means 200 includes a switching means 220 in which a field effect transistor (FET) or a transistor is turned on / off when the incoming current is zero, and a leakage transformer. Transformer, 240).

The leakage transformer 240 is used in a high voltage low current output inverter for lighting the cold cathode tube lamp or the electrodeless tube lamp, and has a large leakage inductance (Lt) by itself.

In addition, the lamp 400 connected in series with the leakage transformer 240 is used for the backlight of the LCD, and its load characteristic has a capacitance (C _L ) characteristic. .

FIG. 3 is a circuit diagram illustrating the load characteristics of the lamp of FIG. 2 and the leakage transformer, and is an equivalent circuit diagram illustrating the leakage inductance Lt of the leakage transformer 240 and the capacitance C _{L of the} lamp. In the equivalent circuit diagram, the capacitance C _L and the resistance R are arbitrary equivalent impedances of CCFL and EEFL lamps.

In this case, the resonance frequency may be calculated using the leakage inductance Lt and the capacitance of the lamp C _L as shown in Equation 1 below.

At least one lamp 400 is connected in series with the leakage transformer 240, and two or more lamps 400 are connected in parallel with each other.

The sensing means 500 is connected in series with the lamp 400 to receive the current I output from the lamp 400. The sensing means 500 includes a sensing resistor 510, a diode 520, first and second resistors 530 and 540.

A voltage is applied across the sensing resistor 510 by the current I. The voltage is fed back to the control means 100 through a diode connected in parallel with the sensing resistor.

In other words, the respective currents output from the respective lamps 400 pass through the diodes connected in series with the lamps 400, and then are input to the control means 100 through the first resistor 530. . At this time, the voltage sensed by the control means 100 becomes a voltage applied to the second resistor 540.

The control means 100 receiving the fluctuating voltage measured by the sensing means 500 controls the output voltage by controlling a driving frequency for switching the current flowing through the lamp to a constant current compared to the reference voltage.

The control means 100 also drives a drive frequency for switching the current flowing through the lamp to a constant current by comparing the current sensed by the sensing means 500 with a predetermined reference output current by the relationship between the current and the voltage. You can also control.

4 is a graph showing experimental data applied to the present invention.

Reference numeral (a) is a graph showing the voltage waveform applied to the switching power element when switching.

Reference numeral (b) is the switching control waveform of the switching element, and reference numeral (c) is the primary current waveform of the leakage transformer. When the current waveform of reference number (c) is close to zero, the waveform is adjusted to operate the switching element.

The following describes the operating state of the present invention configured as described above.

When the amount of current output from the at least one lamp 400 is small, the voltage applied to the resistor R connected in series with the corresponding lamp 400 of the sensing means 500 is small. Therefore, the amount of current fed back to the control means 100 becomes small.

The control means 100 having received the fluctuating current amount measured by the sensing means 500 controls the drive frequency introduced into the switching means to adjust the current flowing through the lamp to a constant current compared with a predetermined reference output current, thereby outputting the output current. To control.

As shown in FIG. 3, the control means 100 controls the driving frequency fo in an operating area set based on the resonance frequency fr. When the driving frequency fo is decreased in the operating region of the driving frequency fo, the power Wattage becomes large, thereby increasing the amount of current applied to the lamp 400.

If the amount of current output from the at least one lamp 400 is greater than the predetermined reference output current, the voltage applied to the resistor R connected in series with the corresponding lamp 400 among the sensing means 500 becomes large, thereby controlling the control means ( The amount of current fed back to the display 100 becomes large.

The control means 100 which receives the variable current amount measured by the sensing means 500 outputs by controlling the driving frequency introduced into the switching means to adjust the current flowing through the lamp to a constant current compared to the predetermined reference output current. To control the current.

5 is a graph showing the resonant frequency vs. Wattage applied to the present invention.

A graph showing a relationship between a resonance frequency and an output resulting from matching of a leakage transformer and a load of a lamp, and a driving frequency band to be controlled by the controller is limited to an operating area of the resonance frequency.

The first resonant frequency on the graph is the resonant frequency applied to the present invention, and the second and third consecutive resonant frequencies are due to the influence of harmonics.

6 is a graph showing the driving frequency versus the amount of current in the lamp applied to the present invention.

This shows the relationship between the output and the drive frequency in the operating region of the resonance frequency of FIG. 5 as the relationship between the amount of current flowing through the lamp and the drive frequency.

As shown, when the amount of current flowing through the lamp decreases, the driving frequency output from the control means decreases, and when the amount of current flowing through the lamp increases, the drive frequency output from the control means increases so that the variation of the input of the switching unit is increased. Make sure the output is constant with the lamp.

As described in detail above, according to the driving frequency control circuit of the backlight lamp of the present invention, by controlling the driving frequency in the operating region of the resonant frequency generated by matching the leakage inductance of the leakage transformer and the capacitance which is an electrical characteristic of the lamp. Since the current flowing through the lamp can be controlled constantly, the brightness can be stabilized or the brightness can be adjusted, and excessive current can be prevented to maintain the lamp for a long time.

In other words, when the leakage transformer is driven according to the present invention, a high output can be obtained by the resonance action caused by the leakage inductance and the capacitance.

In addition, by realizing zero current switching (ZCS) in which the operating range of the driving frequency is controlled to drive in a predetermined band based on the resonance frequency, the efficiency of switching elements such as FETs and transistors is maximized. The heat loss of the switching element and the loss of the inverter can be minimized.

Embodiment of the present invention is only one embodiment, of course, many modifications and variations of the components of the present invention without departing from the gist of the present invention, of course, the present invention is not limited to the embodiment. .

Claims (4)

Alternating current generating means for switching according to a driving frequency to generate an alternating current; Sensing means for sensing an alternating current output from a lamp driven by the alternating current; And And a control means for comparing the sensed alternating current with a predetermined reference output current and generating the drive frequency according to the comparison result. The resonance frequency for setting the predetermined reference output current is generated from the leakage inductance of the alternating current generating means and the capacitance of the lamp, And the driving frequency band is included in the resonance frequency band. The method of claim 1, wherein the alternating current generating means Switching means for switching voltage driving in accordance with the driving frequency; And And a leakage transformer for converting the DC voltage drawn from the switching means into an AC voltage and outputting the same. The method of claim 1, wherein the control means increases the output power by lowering the driving frequency when the current sensed by the sensing means is lower than the reference output current, and drives when the current sensed by the sensing means is higher than the reference output current. A driving frequency control circuit for a backlight lamp, characterized by reducing the output power by raising the frequency. 4. The drive frequency control circuit according to any one of claims 1 to 3, wherein the lamp is a cold cathode tube lamp or an electrodeless tube lamp.