Patent Publication Number: US-8111003-B2

Title: Backlight assembly, display apparatus having the same and control method thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from Korean Patent Applications No. 10-2007-0118074, filed on Nov. 19, 2007 and No. 10-2008-0043136, filed on May 9, 2008 in the Korean Intellectual Property Office, the disclosures of which are incorporated herein by reference. 
     BACKGROUND OF INVENTION 
     1. Field of Invention 
     Apparatuses and methods consistent with the present invention relate to a backlight assembly, a display apparatus having the same and a control method thereof, and more particularly to a backlight assembly with a light source unit to be driven by an inverter, a display apparatus having the same and a control method thereof. 
     2. Description of the Related Art 
     A display apparatus with a liquid crystal display (LCD) panel includes a backlight assembly to emit light in the rear side of the LCD panel, and a backlight driver to drive the backlight assembly. 
     In general, the backlight driver includes an inverter that changes a level of input power and supplies it to the backlight assembly. Further, the inverter gets feedback from the backlight assembly with regard to current flowing in the backlight assembly and adjusts a duty ratio of a drive signal for supplying power on the basis of the feedback. 
     Meanwhile, the duty ratio of the inverter is varied according to the LCD panel, more specifically, according to combination of the LCD panel and the backlight assembly. If the duty ratio is low, there is a problem that heat is generated from a switch and a transformer in the inverter. If the duty ratio is lower than an optimum duty ratio of the maximum efficiency, the power has to increase so as to be transmitted from a primary coil of the transformer to a secondary coil, so that heat is generated due to increase of a peak current. 
     To solve these problems, the display apparatus uses a plurality of switching devices to radiate heat, but production costs increases due to the switching devices and the structure of the inverter becomes complicated. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an aspect of the present invention to provide a backlight assembly improved in heat generation with low production costs, a display apparatus having the same, and a control method thereof. 
     Another aspect of the present invention is to provide a backlight assembly capable of easily optimizing a duty ratio of an inverter, a display apparatus having the same, and a control method thereof. 
     The foregoing and/or other aspects of the present invention can be achieved by providing a backlight assembly with a light source unit, including: a drive signal generator which generates a switching drive signal on the basis of a predetermined drive frequency and a feedback power fed back from the light source unit; a switching unit which outputs a power to the light source unit in response to the switching drive signal; and a controller which decreases the drive frequency if the feedback power is higher than a predetermined critical value. 
     The drive signal generator may generate a triangle wave based on the drive frequency, the controller may include a first switch turned off if the feedback power is higher than the critical value, and a second switch allowing a predetermined current to flow toward the drive signal generator by turning on if the first switch is turned off, and the current input to the drive signal generator may decrease steepness of the triangle wave. 
     The drive signal generator may include a feedback inverting terminal through which the feedback power is inverted and output, and the first switch is connected to the feedback inverting terminal. 
     The controller further include a plurality of voltage division resistors connected between the feedback inverting terminal and the first switch to determine a power level at which the first switch remains turned off. 
     The feedback inverting terminal outputs power in inverse proportion to the feedback power. 
     The controller further includes a capacitor to turn on the second switch after a lapse of predetermined delay time after the first switch is turned off. 
     The controller further includes a resistor to adjust a level of the current input to the drive signal generator. 
     The foregoing and/or other aspects of the present invention can be achieved by providing a backlight assembly with a light source unit, including: a drive signal generator which generates a switching drive signal having a duty ratio variable depending on a feedback power fed back from the light source unit; a switching unit which outputs a power in response to the switching drive signal; and a controller which controls the drive signal generator to increase the duty ratio of the switching drive signal if the feedback power is higher than a predetermined critical value. 
     The drive signal generator may include a capacitor to generate a triangle wave based on a predetermined drive frequency, and the controller may increase the duty ratio of the switching drive signal by decreasing steepness of the triangle wave. 
     The controller may include a first switch turned off if the feedback power is higher than the critical value, and a second switch allowing a predetermined current to flow toward the drive signal generator by turning on if the first switch is turned off, and the current input to the drive signal generator may decrease the steepness of the triangle wave. 
     The foregoing and/or other aspects of the present invention can be achieved by providing a display apparatus including: a display unit which displays an image; a light source unit which illuminates the display unit; and a power supply which supplies power to the display unit and the light source unit, the power supply including a drive signal generator which generates a switching drive signal on the basis of a predetermined drive frequency and a feedback power fed back from the light source unit; a switching unit which outputs the power in response to the switching drive signal; and a controller which determines a duty ratio of the switching drive signal on the basis of the feedback power and decreases the drive frequency if the duty ratio of the switching drive signal is lower than an optimum value. 
     The drive signal generator may generate a triangle wave based on the drive frequency, the controller may include a first switch turned off if the feedback power is higher than the critical value, and a second switch allowing a predetermined current to flow toward the drive signal generator by turning on if the first switch is turned off, and the current input to the drive signal generator may decrease steepness of the triangle wave. 
     The controller further include a plurality of voltage division resistors to determine a power level at which the first switch remains turned off. 
     The controller further includes a resistor to adjust a level of the current input to the drive signal generator. 
     The controller further includes a capacitor to turn on the second switch after a lapse of predetermined delay time after the first switch is turned off. 
     The foregoing and/or other aspects of the present invention can be achieved by providing a method of controlling a backlight assembly with a light source unit, the method including: generating a switching drive signal using a triangle wave based on a predetermined drive frequency; supplying power to the light source unit in response to the switching drive signal; receiving a feedback power from the light source unit; and decreasing the drive frequency if the feedback power is higher than a predetermined critical value. 
     The decreasing the drive frequency may include decreasing steepness of the triangle wave if the feedback power is higher than the critical value. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and/or other aspects of the present invention will become apparent and more readily appreciated from the following description of the exemplary embodiments, taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a control block diagram of a backlight assembly according to a first exemplary embodiment of the present invention; 
         FIG. 2  is a circuit diagram of a drive signal generator and a controller in  FIG. 1 ; 
         FIG. 3  is a circuit diagram for explaining a current change by the controller of  FIG. 1 ; 
         FIG. 4  is a waveform diagram for explaining a duty ratio of the backlight assembly in  FIG. 1 ; 
         FIG. 5  is a flowchart for explaining a control method of the backlight assembly in  FIG. 1 ; and 
         FIG. 6  is a control block diagram of a display apparatus according to a second exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION 
     Below, embodiments of the present invention will be described in detail with reference to accompanying drawings so as to be easily realized by a person having ordinary knowledge in the art. The present invention may be embodied in various forms without being limited to the embodiments set forth herein. Descriptions of well-known parts are omitted for clarity, and like reference numerals refer to like elements throughout. 
       FIG. 1  is a control block diagram of a backlight assembly according to a first exemplary embodiment of the present invention, and  FIG. 2  is a circuit diagram of a drive signal generator and a controller in  FIG. 1 . As shown therein, the backlight assembly includes a light source unit  100 , and a light source driver  200  supplying drive power to the light source unit  100 . In this embodiment, the backlight assembly is placed in a rear part of a liquid crystal display (LCD) panel or a large-sized image display and used as a light source to emit light. 
     The light source unit  100  may include a plurality of optical devices. Further, the light source unit  100  may include a line light source such as a cold cathode fluorescent lamp (CCFL) or a hot cathode fluorescent lamp (HCFL), or a point light source such as a light emitting diode (LED). 
     The light source driver  200  includes a drive signal generator  210 , a switching unit  220 , a transformer  230  and a controller  240 . The light source driver  200  adjusts a level of external input power and supplies it to the light source unit  100 . The light source driver  200  is generally called an inverter. 
     The drive signal generator  210  generates a switching drive signal on the basis of a predetermined drive frequency and a feedback power fed back from the light source unit  100 . The switching drive signal includes a pulse width modulation (PWM) control signal for controlling the switching unit  220  connected to the drive signal generator  210 . The drive signal generator  210  adjusts a duty ratio of the switching drive signal in order to adjust a level of the drive power supplied to the light source unit  100 . In the present embodiment, the backlight assembly may be modularized as an independent device for emitting light to a display panel. In this case, the duty ratio of the switching drive signal may vary according to the light source unit  100  connected to the light source driver  200  or the display panel sharing power supply with the light source unit  100 , or according to combination of the light source unit  100  and the display panel. In other words, even if the backlight assembly to be driven through the switching drive signal having an optimum duty ratio is connected to the display panel, the duty ratio may decrease according to the display panel. As the duty ratio decreases, a peak current increases since power to be output per unit time increases, so that the light source driver  200  may generate heat. The drive signal generator  210  generates the switching drive signal having the duty ratio variable according to control of the controller  240  in order to reduce the heat generation, which will be described below in more detail. 
     The drive signal generator  210  includes a microcomputer U 1  with a plurality of terminals as shown in  FIG. 2 . The terminal of the microcomputer U 1  is connected to a plurality of resistors or capacitors. The drive signal generator  210  generates a triangle wave to generate the switching drive signal, and the triangle wave is formed on the basis of a predetermined drive frequency. Generally the triangle wave is generated by changing a waveform of input alternating current (AC) power, and at this time differential/integral circuits with the resistor and the capacitors are used. Among the terminals of the microcomputer U 1 , a terminal connected to a resistor R 201  and a terminal connected to a capacitor C 206 , which are related to generation of the triangle wave, will be called a resistor terminal RT and a capacitor terminal CT, respectively. Further, the microcomputer U 1  includes a feedback terminal F/B to which the feedback power of the light source unit  100  is input, and a feedback inverting terminal ERO from which the feedback power input from the feedback terminal F/B is inverted and then output. Also, the microcomputer U 1  internally includes an operator (not shown) provided between the feedback terminal F/B and the feedback inverting terminal ERO and inverting the input power. If the feedback power increases, the power of which level decreases is output from the feedback inverting terminal ERO via the operator. That is, the power is output from the feedback inverting terminal ERO in inverse proportion to the feedback power input to the feedback terminal F/B. Between the feedback terminal F/B and the feedback inverting terminal ERO is connected a capacitor C 207  for stabilizing the feedback power. The switching drive signal generated in the microcomputer U 1  is input to the switching unit  220  via a first output unit NOUT and a second output unit POUT. 
     The switching unit  220  includes a plurality of metal oxide semiconductor field effect transistors (MOS-FETs), and the connection and the number of MOS-FETs may be changed in various manner. The switching unit  220  is turned on/off by the switching drive signal output from the drive signal generator  210 , and transmits the external input power to the light source unit  100 . The switching unit  220  is turned on and off depending on the duty ratio of the switching drive signal. For example, if the duty ratio increases, a period of time during which the switching unit  220  is turned on increases and thus the amount of power supplied to the light source unit  100  increases. 
     The transformer  230  includes primary and secondary coils (not shown) and boosts up a voltage of the input power. When the duty ratio of the switching drive signal decreases, the amount of power that has to be supplied for a predetermined time is relatively increased as compared with that in the case of the optimum duty ratio, so that the current flowing in the transformer  230 , particularly, in the primary coil increases, thereby generating heat. 
     The controller  240  controls the drive frequency to decreases when the feedback power is higher than a predetermined critical value. In the case that the drive frequency decreases, the amount of current transmitted to the switching unit  220  and the transformer  230  decreases so that the generation of heat decreases. As shown in  FIG. 2 , the controller  240  includes a first switch  241  connected to the feedback inverting terminal ERO, and a second switch  243  connected to the first switch  241 . The first switch  241  and the second switch  243  may be also achieved by the MOS-FET like the switching unit  220 . Further, the controller  240  includes voltage division resistors R 1  and R 2  to determine a power level at which the first switch  241  remains turned off. The voltage division resistors R 1  and R 2  includes a first resistor R 1  connected between the feedback inverting terminal ERO and a control terminal of the first switch  241 , and a second resistor R 2  connected between the control terminal of the first switch  241  and a ground terminal. The first switch  241  is turned on or off depending on resistance ratio of the voltage division resistors R 1  and R 2 . For example, suppose that the first switch  241  is turned on at a voltage of 1.2V and the first and second resistors R 1  and R 2  have the same resistance. In this case, if a voltage output from the feedback inverting terminal ERO is 2.4V or higher, a voltage of 1.2V or higher is applied to the control terminal of the first switch  241  to thereby turn on the first switch  241 . 
     On the other hand, if the voltage output from the feedback inverting terminal ERO is lower than 2.4V, the first switch  241  is turned off. In this embodiment, if the feedback power fed back from the light source unit  100  has a level higher than the critical value, i.e., if the power output through the transformer  23  is higher than a certain amount, it means that the duty ratio of the switching drive signal does not reach the optimum duty ratio. Typically, the optimum duty ratio is set within a range not higher than 50%. Alternatively, the optimum duty ratio may be variously set in consideration of configuration of the switching unit  220 . Thus, the critical value of the feedback power is determined as a value for determining whether the duty ratio of the switching drive signal is lower than the optimum duty ratio. Also, the voltage division resistors R 1  and R 2  may act as a factor on determining the critical value of the feedback power. 
     Input and output terminals of the first switch  241  are connected to a predetermined power source terminal VREF and a ground terminal, respectively, and a third resistor R 3  is connected between the power source terminal VREF and the first switch  241 . The second switch  243  has a control terminal connected between the third resistor R 3  and the input terminal of the first switch  241 , an input terminal connected to the power source terminal VREF, and an output terminal connected to the microcomputer U 1  of the drive signal generator  210 . If the first switch  241  is turned on, the second switch  243  is turned off because power supplied from the power source terminal VREF is consumed in the third resistor R 3 . On the other hand, if the first switch  241  is turned off, the second switch  243  is turned on by power from the power source terminal VREF. As the second switch  243  is turned on, the power from the power source terminal VREF, i.e., current is input to the microcomputer U 1  that generates the switching drive signal via the second switch  243 . 
     A capacitor (C 1 )  245  is connected between the control terminal of the second switch  242  and a ground terminal. The capacitor  245  delays signal transmission so that the second switch  243  is turned on after a lapse of predetermined delay time after the first switch  241  is turned off. Depending on capacitance of the capacitor  245  and resistance of the resistor R 3  (multiply of the capacitance and the resistance corresponds to a time constant), a point of time to turn on the second switch  243  is determined, and thus a point of time to change the duty ration of the switching drive signal and the drive frequency is determined. 
     Also, the controller  240  further includes a fourth resistor R 4  connected between the output terminal of the second switch  243  and the resistor terminal RT of the microcomputer U 1 . The fourth resistor R 4  causes the level of current input to the microcomputer U 1  to be adjusted, thereby adjusting the decrease of the drive frequency and the increase in the duty ratio of the switching drive signal. 
     Overall, the controller  240  operates as follows. When the feedback power fed back from the light source unit  100  is the critical value or below, it is determined that the switching drive signal is maintained in the optimum duty ratio. Thus, the first switch  241  is turned on. As the first switch  241  is turned on, the power from the power source terminal VREF is not supplied to the microcomputer U 1  of the drive signal generator  210 . 
     On the other hand, when the feedback power is higher than the critical value, the level of the power from the feedback inverting terminal ERO decreases. As the voltage level of the feedback inverting terminal ERO decreases, the first switch  241  is turned off, and the second switch  243  is turned on after the lapse of a certain delay time. When the second switch  243  is turned on, the power from the power source terminal VREF is input to the resistor terminal RT of the microcomputer U 1  via the fourth resistor R 4 . 
       FIG. 3  is a circuit diagram for explaining a current change by the controller of  FIG. 1 , and  FIG. 4  is a waveform diagram for explaining a duty ratio of the backlight assembly in  FIG. 1 .  FIG. 3  illustrates the controller  240  and a partial internal circuit of the microcomputer U 1  connected to the controller  240 , in which a control current I 1  is input from the controller  240  to the resistor terminal RT of the microcomputer U 1 . Other detailed circuits and internal structure of the microcomputer U 1  are publicly known and descriptions thereof will be omitted. 
     The resistor terminal RT and the capacitor terminal CT are connected to each other, and a switch A is arranged between the resistor terminal RT and the capacitor terminal CT to adjust the current there between. Further, an operator B connected to a control terminal of the switch A serves to maintain a constant level of voltage applied to the resistor terminal RT. Since the resistor terminal RT always receives a predetermined voltage of 2V input to a non-inverting terminal of the operator B, the current I flowing in the resistor R 201  is invariable. The current I 2  flowing in the capacitor C 206  is equal to a value obtained by subtracting the control current I 1  from the current I flowing in the resistor R 201 . Thus, when the control current I 1  is output from the controller  240 , the current I 2  flowing in the capacitor C 206  decreases. In other words, if the feedback power is higher than the critical value and thus the second switch  243  is turned on, the current I 2  flowing in the capacitor C 206  decreases. 
     As described above, the capacitor C 206  connected to the capacitor terminal CT and the resistor R 201  connected to the resistor terminal RT participate in forming the triangle wave. The current I 2  flowing in the capacitor C 206  can be expressed by an equation: I 2 =C 206 *(dV/dt). Referring to this Equation, a differential in voltage with respect to a differential in time (dV/dt) decreases when the current I 2  decreases, which means that the steepness of the triangle wave decreases. 
     Referring to  FIG. 4 , the triangle wave oscillates within certain voltage levels V 0 ˜V 1  and has a frequency corresponding to on the drive frequency. In  FIG. 4 , (a) shows the triangle wave and the voltage level ERO I  output from the feedback inverting terminal (ERO) in a first case I where the duty ratio is lower than an optimum value; (b) is the switching drive signal according to (a); (c) shows the triangle wave in a second case II where the control current I 1  is input to the resistor terminal RT of the microcomputer U 1 ; and (d) is the switching drive signal according to (c). 
     A turning-on period of the switching drive signal is determined depending on the triangle wave and the voltage level of the feedback inverting terminal ERO. The lower the steepness of the triangle wave or the higher the voltage level, the longer the period of time during which a switching drive unit is turned on. When the control current I 1  is input to the resistor terminal RT, the steepness of the triangle wave more decreases than that of the first case I and thus the drive frequency decreases. As the drive frequency decreases, the power supplied to the light source unit  100  decreases, thereby decreasing the feedback power. The decrease in the feedback power corresponds to the increase in the level of the power of the feedback inverting terminal ERO. Therefore, the voltage level ERO II  of the second case II is higher than the voltage level ERO I  of the first case I. Through this causal sequence, the turning-on period d II  of the switching drive signal in the second case II becomes longer than the turning-on period d I  in the first case I. The increase in the turning-on period of the switching unit  220  causes the duty ratio to increase up to the optimum duty ratio, thereby decreasing the peak current. Consequently, the decrease of the drive frequency causes the duty ratio to increase, thereby reducing heat generation from the light source driver  200 . 
       FIG. 5  is a flowchart for explaining a control method of the backlight assembly in  FIG. 1 . Referring to  FIG. 5 , a method of adjusting the drive frequency according to an embodiment of the present invention is as follows. 
     First, the switching drive signal is generated using the triangle wave (S 10 ), and the input power is supplied to the light source unit  100  on the basis of the switching drive signal (S 20 ). 
     Then, the light source unit  100  feeds back the feedback power (S 30 ). The feedback power informs characteristics of the light source unit  100  and the duty ratio of the switching drive signal corresponding to the characteristics of the light source unit  100 . Depending on the feedback power, the first switch  241  and the second switch  243  are decided to be turned on or off. 
     The controller  240  determines whether or not the feedback power is higher than the critical value (S 40 ). The critical value is set as a level of a voltage to be applied to the control terminal of the first switch  241  according to the resistances of the first and second resistors R 1  and R 2 , and the determining is performed as the first switch  241  is turned on. 
     If the feedback power is higher than the critical value and thus the first switch  241  is turned off, the second switch  243  is turned on and thus the control current I 1  is input to the drive signal generator  210 . The control current I 1  decreases the current I 2  flowing in the capacitor C 206  generating the triangle wave, thereby decreasing the steepness of the triangle wave (S 50 ). As the steepness of the triangle wave decreases, the drive frequency decreases, thereby causing the duty ratio to increase. When the duty ratio increases, the peak current to be transferred from the primary coil to the secondary coil of the transformer  230  decreases, thereby reducing the heat generated in the transformer  230 . 
       FIG. 6  is a control block diagram of a display apparatus according to a second exemplary embodiment of the present invention. As shown therein, the display apparatus includes a display unit  300 , a light source unit  100  to illuminate the display unit  300 , and a power supply  400  to supply power to the display unit  300  and the light source unit  100 . 
     The display unit  300  may include a liquid crystal display (LCD) panel that needs a light source to display an image, and receives panel drive power from the power supply  400 . 
     The light source unit  100  and components  410  to  440  of the power supply  400  are substantially equal or similar to those shown in  FIG. 1 , and repetitive descriptions thereof will be avoided as necessary. In the second exemplary embodiment, the power supply  400  may be an integrated power supply that converts alternating current (AC) power received from the outside into a plurality of direct current (DC) power different in a level, and supplies them to the display unit  300  and the light source unit  100 . The power supply  400  may further include a switched-mode power supply (not shown). The power supply  400  optimizes a duty ratio of a switching drive signal in consideration of characteristics of the display unit  300  and the light source unit  100  which are connected to the power supply unit  400  and load. To this end, the power supply  400  determines the duty ratio of the switching drive signal on the basis of feedback power fed back from the light source unit  100 , and decreases a drive frequency if the duty ratio is lower than the optimum value. The decrease of the drive frequency brings the increase of the duty ratio. When the duty ratio increases and thus has the optimum value, it is possible to reduce heat generated by the switching unit  420  and the transformer  430 . 
     Thus, according to the embodiments of the present invention, the drive frequency and the duty ratio are adjusted by incorporating the characteristics of the display unit  300  and the light source  100 , thereby reducing heat generated in the power supply. 
     As described above, the present invention to provide a backlight assembly improved in heat generation with low production costs, a display apparatus having the same, and a control method thereof. 
     Another aspect of the present invention is to provide a backlight assembly capable of easily optimizing a duty ratio of a switching drive signal, a display apparatus having the same, and a control method thereof. 
     Still another aspect of the present invention is to provide a backlight assembly improved in heat generation from a light source driver by easily adjusting a drive frequency, a display apparatus having the same, and a control method thereof. 
     Although a few exemplary embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.