Patent Publication Number: US-8115409-B2

Title: Driving circuit and method of backlight module

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a driving mechanism of a backlight module, and more particularly, to a luminance-adjusting driving circuit and related method of a backlight module using a hot cathode fluorescent lamp (HCFL). 
     2. Description of the Prior Art 
     For a display apparatus having a backlight module, such as a liquid crystal display (LCD), an appropriate luminance-adjusting mechanism is required for adjusting the luminance of a backlight source due to the considerations of an ambient light intensity and a user&#39;s preferences. 
     When a hot cathode fluorescent lamp (HCFL) serves as the backlight source, a frequency modulation control, an amplitude modulation control, or a pulse width modulation (PWM) control is generally used as the luminance-adjusting method of a driving circuit. A driving circuit for performing the frequency modulation control is easy to design, and is able to adjust the luminance of the backlight source efficiently. However, because of a frequency variation of a control signal of this driving circuit, a design of a front-end filter is difficult due to the electro-magnetic interference (EMI), and magnetic components cannot be optimally applied in the driving circuit. Furthermore, the amplitude modulation control adjusts the luminance by changing a DC current of a resonant circuit, and the design of the driving circuit is more difficult. The PWM control adjusts the luminance by adjusting an enabling period of a switch. Generally, a symmetrical PWM control is used as the PWM control, although the driving circuit of the PWM control is more complex than that of the frequency modulation control, and has a higher power consumption because of switching operations. 
     Please refer to  FIG. 1 .  FIG. 1  is a diagram illustrating a prior art quasi-half-bridge frequency-varied driving circuit  100 . The driving circuit  100  includes a DC current source Vdc, a signal generator  110 , a resonant circuit  120  coupled to the signal generator  110 , a capacitor  140  coupled to the resonant circuit  120  and a backlight source  130 , and two capacitors  160  and  170  coupled to the signal generator  110  and the backlight source  130 . The signal generator  110  is used for generating an alternating current (AC) signal having a variable frequency. The resonant circuit  120  is used for generating an oscillation signal to drive the backlight source  130  according to the AC signal. The capacitor  140  is used to provide an impedance to adjust a current value of the backlight source  130 . The capacitors  160  and  170  are used to generate a DC voltage level. In addition, the signal generator  110  includes two transistors  112  and  114 , and the frequency of the AC signal can be determined by adjusting a frequency of switching on/off the transistors  112  and  114 . The resonant circuit  120  includes an inductor  122  and a capacitor  124 , which is used to convert the AC signal generated from the signal generator  110  to a sinusoidal wave to drive the backlight source  130 . 
     As shown in  FIG. 1 , the capacitor  140  is connected in parallel to the backlight source  130 . When the AC signal generated from the signal generator  110  has a frequency ω, the impedance of the capacitor  140  is (1/ωC f ), where C f  is a capacitance of the capacitor  140 . Then, the current of the backlight source  130  is determined according to a ratio between the impedance of the capacitor  140  and an impedance of the backlight source  130 . When the impedance of the capacitor  140  is greater than the impedance of the backlight source  130 , the backlight source  130  is in the main current path and the backlight source  130  lightens; and when the impedance of the capacitor  140  is less than the impedance of the backlight source  130 , the capacitor  140  is in the main current path and the luminance of the backlight source  130  is degraded or even extinguished. 
     A circuit structure of the above-mentioned luminance-adjusting method is simple, however, the front-end filter will be interfered with by the electro-magnetic wave due to the frequency variation, and the magnetic components cannot be optimally applied in the driving circuit. 
     SUMMARY OF THE INVENTION 
     It is therefore an objective of the present invention to provide a luminance-adjusting driving circuit and related method, which uses an AC signal having a fixed frequency to drive the backlight source, in order to solve the above-mentioned problems. 
     According to one embodiment of the present invention, a driving circuit includes a signal generator, a resonant circuit, a control circuit and an adjusting circuit. The signal generator is utilized for generating an alternating current (AC) signal having a fixed frequency. The resonant circuit is coupled to the signal generator, and is utilized for generating an oscillation signal to drive a backlight source according to the alternating current signal. The control circuit is utilized for providing a control signal. The adjusting circuit is coupled to the control circuit, the resonant circuit and the backlight source, and is utilized for providing an impedance according to the control signal to thereby adjust a current value of the backlight source. 
     According to another embodiment of the present invention, a driving method of a backlight module includes: generating an alternating current (AC) signal having a fixed frequency; generating an oscillation signal to drive a backlight source according to the AC signal; providing a control signal; providing an adjusting circuit and connecting the adjusting circuit to the backlight source; and providing an impedance according to the control signal to thereby adjust a current value of the backlight source. 
     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating a prior art quasi-half-bridge frequency-varied driving circuit. 
         FIG. 2  is a diagram illustrating a quasi-half-bridge frequency-fixed driving circuit according to a first embodiment of the present invention. 
         FIG. 3  is a diagram illustrating a quasi-half-bridge frequency-fixed driving circuit according to a second embodiment of the present invention. 
         FIG. 4  is a diagram of an equivalent circuit of the transistor serving as a variable resistor. 
         FIG. 5  is a diagram illustrating characteristics of the operations of the transistor shown in  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
     Please refer to  FIG. 2 .  FIG. 2  is a diagram illustrating a quasi-half-bridge frequency-fixed driving circuit  200  according to a first embodiment of the present invention. In this embodiment, the driving circuit  200  includes a DC voltage source Vdc, a signal generator  210 , a resonant circuit  220 , a control circuit  240 , an adjusting circuit  250  coupled to the control circuit  240 , the resonant circuit  220  and a backlight source  230 , and two capacitors  260  and  270  coupled to the signal generator  210  and the backlight source  230 . The signal generator  210  is used to generate an AC signal having a fixed frequency. The resonant circuit  220  is used to generate an oscillation signal to drive the backlight source  230  according to the AC signal. The control circuit  240  is used to generate a control signal. The adjusting circuit  250  is used to provide an impedance according to the control signal to thereby adjust a current value of the backlight source  230 . The capacitors  260  and  270  are used to provide a DC voltage level. In addition, the signal generator  210  includes two transistors  212  and  214 , and the AC signal having the fixed frequency can be generated by switching between the transistors  212  and  214 . The resonant circuit  220  includes an inductor  222  and a capacitor  224 , which is used to convert the AC signal generated from the signal generator  210  into a sinusoidal signal to drive the backlight source  230 . The adjusting circuit  250  includes a bi-directional switch  256  and a capacitor  258 , where the bi-directional switch  256  is implemented by two transistors  252  and  254 . 
     As shown in  FIG. 2 , the capacitor  258  is series-connected to the bi-directional switch  256 , and the capacitor  258  and the bi-directional switch  256  are parallel-connected to the backlight source  230 . When the signal generator  210  generates the AC signal having the frequency ω 1  and the bi-directional switch  256  is enabled (switched on), an impedance of the capacitor  258  is (1/ω 1 C f ), where C f  is a capacitance of the capacitor  258 . In this embodiment, the impedance of the capacitor  258  (1/ω1Cf) is designed to be far less than an impedance of the backlight source  230 . Therefore, when the bi-directional switch  256  is enabled, the adjusting circuit  250  is in a main current path, and the backlight source  230  has a minimum luminance. When the bi-directional switch  256  is disabled (switched off), the backlight source  230  is in the main current path, and the backlight source  230  has a maximum luminance. 
     The prior art frequency-varied driving circuit  100  adjusts the luminance of the backlight source by directly adjusting the current of the backlight source. Compared with the prior art driving circuit  100 , in the embodiment of the present invention, the backlight source  230  only has two possible currents respectively representing the maximum and minimum luminance of the backlight source  230 . Therefore, the luminance-adjusting method of the present invention is to control a ratio between an enabling period and a disabling period of the bi-directional switch  256  by the control circuit  240 , where this ratio is also meant to be a ratio between periods where the backlight source  230  respectively has the maximum and minimum luminance. For example, if a half-maximum luminance of the backlight source  230  is required, the control circuit  240  controls the ratio between the enabling and disabling period to be 1:1, that is, the ratio between periods where the backlight source  230  respectively has the maximum and minimum luminance is also 1:1, and a person can feel this required luminance due to visual fatigue. 
     The driving circuit  200  is similar to the prior art frequency-varied driving circuit shown in  FIG. 1 , and both have simple circuit structures. Because the AC signal generated from the signal generator  210  has the fixed frequency, the driving circuit  200  will not be influenced by electro-magnetic interference, and a design and an application of the magnetic components are more efficient. In addition, because of a frequency limitation of the AC signal generated from the signal generator, the impedance of the capacitor  140  of the frequency-varied driving circuit  100  is limited, causing a limited luminance-adjusting range. The frequency-fixed driving circuit  200  has a wider luminance-adjusting range, however, because the luminance of the backlight source is determined according to the ratio between the enabling and disabling period of the bi-directional switch. 
     Please refer to  FIG. 3 .  FIG. 3  is a diagram illustrating a quasi-half-bridge frequency-fixed driving circuit  300  according to a second embodiment of the present invention. In this embodiment, the driving circuit  300  includes a DC voltage source Vdc, a signal generator  310 , a resonant circuit  320  coupled to the signal generator  310 , a control circuit  340 , an adjusting circuit  350  coupled to the control circuit  340 , the resonant circuit  320  and a backlight source  330 , and two capacitors  360  and  370 . The signal generator  310  is used to generate an AC signal having a fixed frequency. The resonant circuit  320  is used to generate an oscillation signal according to the AC signal to drive the backlight source  330 . The control signal  340  is used to provide a control signal. The adjusting circuit  350  is used to provide an impedance according to the control signal. The capacitors  360  and  370  are used to provide a DC voltage level. In addition, the signal generator  310  includes two transistors  312  and  314 , and the AC signal having the fixed frequency can be determined by switching between the transistors  312  and  314 . The resonant circuit  320  includes an inductor  322  and a capacitor  324 , which is used to convert the AC signal generated from the signal generator  310  into a sinusoidal signal to drive the backlight source  330 . The adjusting circuit  350  includes two transistors  352  and  354  and serves as a bi-directional switch. 
     As shown in  FIG. 3 , the adjusting circuit  350  is the bi-directional switch, and one of two transistors in the bi-directional switch is designed as a variable resistor. Please refer to  FIG. 4 .  FIG. 4  is a diagram of an equivalent circuit of the transistor  352  shown in  FIG. 3 . It is noted that the equivalent circuit of the transistor  352  is for illustrative purposes only, and is not meant to be a limitation of the present invention. As shown in  FIG. 4 , the equivalent circuit of the transistor  352  includes a gate electrode G, a drain electrode D and a source electrode S, a gate resistor Rg, a diode Dg, a resistor Rgd between the gate electrode and drain electrode, a capacitor Cgd between the gate electrode and drain electrode, a capacitor Cgs between the gate electrode and source electrode, and a resistor Rs. The characteristics of the operations of the transistor  352 , which are relationships respectively between time and a voltage Vgs between the gate electrode and the source electrode, a voltage Vds between the drain electrode and the source electrode, and a current In between the drain electrode and the source electrode, are illustrated in  FIG. 5 . First, when the transistor  352  is activated during a period (a) shown in  FIG. 5 , because the voltage Vgs is not greater than a threshold voltage Vth of the transistor  352 , there is no current between the drain electrode and the source electrode, and the voltage Vds remains constant. As the voltage Vgs gradually rises over the threshold voltage Vth (during a period (b) in  FIG. 5 ), the current In is generated. Then, due to a constant current In between the drain electrode and the source electrode, the voltage Vds continues decreasing until it is equal to zero as shown in a period (c) in  FIG. 5 . In addition, because a resistor Rds between the drain electrode and the source electrode is a ratio between the voltage Vds and the current In, the resistor Rds is variable during period (c). Finally, during period (d), the voltage Vds and the current In remains constant. 
     In the frequency-fixed driving circuit  300  shown in  FIG. 3 , when the control circuit  340  disables the transistors  352  and  354 , the adjusting circuit  350  has a very large impedance, and the backlight source  330  is in the main current path. At this time, the backlight source  330  has the maximum luminance. When the control circuit  340  enables the transistors  352  and  354 , the adjusting circuit  350  has a lower impedance, and the adjusting circuit  350  is in the current path, and the backlight source  330  has the minimum luminance. In this embodiment, when the control circuit  340  controls the transistors  352  or  354  to operate as the variable resistor, the current of the backlight source  330  can be determined by a ratio between the impedance of the adjusting circuit  350  and the impedance of the backlight source  330  to thereby control the luminance. 
     The driving circuit  300  is similar to the prior art frequency-varied driving circuit  100  shown in  FIG. 1  and the frequency-fixed driving circuit  200  shown in  FIG. 2 , and all of them have simple circuit structures. In addition, as described in the embodiment shown in  FIG. 2 , the driving circuit  300  will not be influenced by electro-magnetic interference, and the design and the application of the magnetic components are more efficient. Similarly, in the driving circuit  300 , the control circuit  340  controls the impedance of the bi-directional switch (adjusting circuit  350 ), where a range of the impedance of the bi-directional switch is from a value (e.g., 10 micro-ohms) to a nearly unlimited value. Therefore, the frequency-fixed driving circuit  300  has a wider luminance-adjusting range. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention.