1. Field of the Invention
The present invention relates to a backlight module and a current providing circuit thereof, and more particularly to a backlight module of a liquid crystal display (LCD) and a current providing circuit thereof.
2. Description of Related Art
With a progress in computer performance and a rapid development of Internet and multimedia technologies, most image data are transmitted in a digital format rather than in an analog format. Nowadays, flat panel displays including LCDs, organic electroluminescent displays (OLEDs), or plasma display panels (PDPs) which are all developed by combining optoelectronic and semiconductor technologies have gradually replaced conventional CRT displays and have become a mainstream of display devices.
As regards the LCD, a backlight module is required to supply a light source to an LCD panel, for the LCD panel itself is not equipped with a light emitting function. Thereby, images can be displayed on the LCD panel. The light source of the backlight module can be categorized into a cold cathode fluorescence lamp (CCFL) and a light emitting diode (LED). In comparison with the LED, the CCFL characterized by great efficiency and long operational life is extensively adopted by a number of the backlight modules for generating the required light source.
Note that the conventional current providing circuit 110 continuously receives the PWM signal PWM1 having a constant frequency. Hence, as a level of a power source Vcc varies, a conversion efficiency of the switch SW1 is correspondingly changed. Relatively, the power consumption of the conventional current providing circuit 110 is then increased, further resulting in a reduction of the operational life of the conventional backlight module 100 and a deteriorated display quality of the display. As a result, for manufacturers of the backlight modules, one of the major issues with respect to the development of the backlight modules lies in a way to effectively improve the conversion efficiency of the switch SW1 for reducing the power consumption of the current providing circuit.
The present invention is directed to a current providing circuit in which the power consumption thereof is reduced by constantly optimizing a conversion efficiency of a switching unit.
The present invention is further directed to a backlight module in which the operational life of a circuit is extended with use of a current providing circuit characterized by low power consumption.
The present invention provides a current providing circuit including a signal generating unit, a switching unit, a first capacitor, a transformer and an output node. The signal generating unit generates a PWM signal according to a level of a power source. The switching unit determines whether a first signal end and a second signal end of the switching unit are conducted according to the PWM signal received by a control end of the switching unit. Following a conduction or a non-conduction of the first and the second signal ends of the switching unit, the first capacitor charges and discharges through a current path provided by a primary coil of the transformer. Thereby, a secondary coil of the transformer generates a corresponding AC voltage by sensing a current change in the primary coil. Finally, the current providing circuit is able to output the AC voltage through the output node.
Note that a duty cycle of the PWM signal is inversely proportional to the level of the power source according to an embodiment of the present invention. Based on the above, the switching unit controlled by the PWM signal can have a constantly optimized conversion efficiency.
According to an embodiment of the present invention, the signal generating unit includes a voltage controlled oscillator and a PWM circuit. The voltage controlled oscillator is used for generating an oscillation signal whose frequency is proportional to the level of the power source. On the other hand, the PWM circuit is utilized for generating the PWM signal according to the frequency of the oscillation signal. In view of the above, the frequency of the PWM signal is proportional to the level of the power source.
The present invention also provides a backlight module including a light source and a current providing circuit. The current providing circuit includes a signal generating unit, a switching unit, a first capacitor, a transformer and an output node. The signal generating unit generates a PWM signal according to a level of a power source. The switching unit determines whether a first signal end and a second signal end of the switching unit are conducted according to the PWM signal received by a control end of the switching unit. Following a conduction or a non-conduction of the first and the second signal ends of the switching unit, the first capacitor charges and discharges through a current path provided by a primary coil of the transformer. Thereby, a secondary coil of the transformer generates a corresponding AC voltage by sensing a current change in the primary coil. Finally, the current providing circuit is able to output the AC voltage through the output node and to drive the light source with use of the AC voltage.
In the present invention, the conversion efficiency of the switching unit is constantly optimized with use of the signal generating unit, and accordingly the power consumption of the current providing circuit is effectively reduced. Besides, the operational life of the backlight module is correspondingly increased.
In order to make the aforementioned and other objects, features and advantages of the present invention more comprehensible, an embodiment accompanied with figures is described in detail below.
One of the main technical features of the present invention lies in that a conversion efficiency of a switching unit can be constantly optimized with use of a PWM signal whose frequency may be changed along with a variation of a power source Vcc. Thereby, the power consumption of a current providing circuit is reduced, and the operational life of a backlight module is effectively extended. The backlight module and the current providing circuit thereof in the present invention are exemplified hereinafter. However, the following embodiment is not intended to limit the scope of the present invention. Those skilled in the art can make appropriate modifications to the following embodiments without departing from the spirit of the present invention.
In general, the signal generating unit 221 generates a PWM signal PWM2 according to a level of the power source VCC. On the other hand, the switching unit 222 receives the PWM signal PWM2 through the control end TM2 and determines whether the two signal ends TM3 and TM4 of the switching unit 222 are conducted according to the PWM signal PWM2. Following the change of a conducting state between the two signal ends TM3 and TM4 of the switching unit 222, the capacitor C21 charges and discharges through a current path provided by the primary coil 223a of the transformer 223.
For example, as shown in
In detail, since current directions of the currents I1 and I2 passing through the primary coil 223a are opposite to each other, a polarity of the voltage at the first primary coil 223a accordingly varies with time. Thereby, the secondary coil 223b generates a corresponding AC voltage VAC by sensing the current passing through the primary coil 223a. In addition, the current providing circuit 220 outputs the AC voltage VAC through the output node TM1, so as to drive the light source 210 by using the AC voltage VAC.
It should be noted that a duty cycle of the PWM signal PWM2 generated by the signal generating unit 221 is inversely proportional to the level of the power source VCC. For example, as a beginning time is defined as t0, the duty cycle of the PWM signal PWM2 is T1. When the level of the power source Vcc is decreased at a time t1 as time passes by, the duty cycle of the PWM signal PWM2 is immediately changed to T2 by the signal generating unit 221. Here, T2>T1.
Thus, when the level of the power source Vcc is increased as time goes by, the frequency of the PWM signal PWM2 utilized for controlling the switching unit 222 is correspondingly increased. On the contrary, when the level of the power source Vcc is decreased as time goes by, the frequency of the PWM signal PWM2 used for controlling the switching unit 222 is correspondingly decreased. Based on the above, the conversion efficiency of the switching unit 222 is constantly optimized, and accordingly the power consumption of the current providing circuit 220 is effectively reduced. Besides, the operational life of the backlight module 200 is correspondingly increased.
Referring to
The current providing circuit 220 further includes capacitors C22˜C24. The capacitor C22 is coupled between the power source Vcc and the ground end. The capacitor C23 is coupled to the secondary coil 223b and the output node TM1. The capacitor C24 is coupled between the output node TM1 and the ground end. Here, the capacitor C22 filters ripples in the power source Vcc, such that a relatively stable power source Vcc may be received by the current providing circuit 220. On the other hand, the capacitors C23 and C24 are utilized to correct a waveform of the AC voltage VAC, such that the waveform of the AC voltage VAC tends to become a pure sine waveform.
It should be noted that the light source 210 exemplified in the present embodiment is a fluorescent lamp including a CCFL or a flat fluorescent lamp. Besides, in order to make those skilled in the art easily implement the present invention, a detailed description in relation to the signal generating unit 221 is provided hereinafter.
The voltage adjusting unit 310 adjusts the level of the power source VCC with a scaling factor and outputs an adjusted DC voltage VDC to the voltage controlled oscillator 320. Thereby, the voltage controlled oscillator 320 generates an oscillation signal SOC based on a level of the DC voltage VDC, and the frequency of the oscillation signal SOC is proportional to the level of the DC voltage VDC. Moreover, when the voltage adjusting unit 310 operates, the level of the DC voltage VDC is proportional to the level of the power source Vcc. Accordingly, the frequency of the oscillation signal SOC is proportional to the level of the power source Vcc.
On the other hand, the PWM circuit 330 generates the PWM signal PWM2 according to the frequency of the oscillation signal SOC. It should be noted that the frequency of the oscillation signal SOC is proportional to the level of the power source Vcc. Hence, the frequency of the PWM signal PWM2 generated by the PWM circuit 330 is also in proportion to the level of the power source Vcc. In other words, as illustrated in
Here, f0 is a constant, and m is a slope of a line segment 410. Additionally, when the level of the power source Vcc is set as LV41, the frequency of the PWM signal PWM2 is f1. On the other hand, when the level of the power source Vcc is defined as LV42, the frequency of the PWM signal PWM2 is f2.
In light of the foregoing, with use of the signal generating unit of the present invention, the frequency of the PWM signal is proportional to the level of the power source. Thereby, the conversion efficiency of the switching unit controlled by the PWM signal is constantly optimized, and accordingly the power consumption of the current providing circuit is effectively reduced. Besides, the operational life of the backlight module is correspondingly increased.
It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.
This application claims the priority benefit of U.S.A. provisional application Ser. No. 60/914,042, filed on Apr. 26, 2007, all disclosures are incorporated therewith.
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