Open protection circuit for backlight module

An open protection circuit (300) for a backlight module includes a PWM (350) with a control port (351) to disable the PWM. A switch (371) has a gate and a source connected to the control port. A plurality of first input circuits (330) includes first diodes (331), each of which has a first positive terminal connected to the gate and a negative terminal connected to ground. A plurality of second input circuits (340) include input resistors (341) connected to the control port. A plurality of detecting circuits (310, 380) are connected to the fluorescent lamps (311) and the first and second input circuits respectively. Each detecting circuit includes a sampling resistor (313) connected to one of the negative terminals of the first diodes, thereby the control port of the PWM is pulled low when either one of the fluorescent lamps are open-circuit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to protection circuits, and in particular to an open protection circuit for a backlight module that uses fluorescent lamps.

2. General Background

Liquid crystal displays (LCDs) generally have the advantages of lightness in weight, a thin profile, flexible sizing, and low power consumption. For these reasons, LCDs are widely used in products such as laptops, personal digital assistants, mobile phones, and so on.

An LCD typically includes an LCD panel and a backlight module. The backlight module commonly includes fluorescent lamps, and acts as a light source for the LCD panel. Thereby, liquid crystal in the LCD panel can provide images for a display screen of the LCD panel. Conventional backlight modules for LCDs generally utilize pulse width modulators (PWMs) to control working currents of the fluorescent lamps. An open protection circuit is also provided to protect the PWM from damage when one of the fluorescent lamps fails or when an open circuit occurs for some other reason.

FIG. 3is a diagram of a conventional open protection circuit of a backlight module that uses four fluorescent lamps. The fluorescent lamps are typically cold cathode fluorescent lamps (CCFLs). The open protection circuit100includes four detecting circuits110, an input circuit130, a PWM150, and a switch170.

Each of the detecting circuits has an output port112, and is connected to a respective one of the fluorescent lamps111. The input circuit130has four branches which correspond to the four fluorescent lamps111. Each branch input circuit includes a bias resistor132, a filtering capacitor133, and a transistor. The transistors of the four branch circuits are designated1331,1332,1333,1334. The source terminal of the first transistor1331is connected to the drain terminal of the second transistor1332. The source terminal of the second transistor1332is connected to the drain terminal of the third transistor1333. The source terminal of the third transistor1333is connected to the drain terminal of the fourth transistor1334. The source terminal of the fourth transistor1334is connected to ground. The drain terminal of the first transistor1331is utilized an output port of the input circuit130.

The gate terminal of the first transistor1331in the first branch of the input circuit130is connected to ground via a bias resistor132and a filtering capacitor135, which are arranged in parallel. The positive terminal of the diode131in the first branch acts as one of the input ports of the input circuit130. The second, third, and fourth transistors1332˜1334in the other three branches are respectively connected to the other three bias resistors132, filtering capacitors135and diodes131in similar arrangements to that of first branch. The positive terminals of the diodes131in the other three branches act as the other three input ports of the input circuit130, in similar fashion to the diode131in the first branch.

The PWM150has a control port151. The PWM150can be turned off or disabled when a low voltage level is provided to the control port151thereof.

The switch170includes a fifth transistor171and a current restriction resistor172. The source terminal of the fifth transistor171is connected to ground, and the drain terminal of the fifth transistor171is connected to the control port151of the PWM150. The gate terminal of the fifth transistor170and the drain terminal of the first transistor1331are both connected to a 5V power source provided by a pin of the PWM150via the current restriction resistor172.

When the fluorescent lamps111are ignited and function normally, the four output ports112of the detecting circuits110are in a high voltage state. Thereby, the four output ports112enable the first through fourth transistors1331˜1334, and pull the gate terminal of the fifth transistor171into a low voltage state. Simultaneously, the fifth transistor171is disabled. The control port151of the PWM150maintains a high voltage level, which keeps the PWM150functioning.

When any one of the fluorescent lamps111has an open circuit, the output port112of the corresponding detecting circuit110is pulled to a low voltage state, which disables a corresponding one of the four transistors1331˜1334. The gate terminal of the fifth transistor171is then enabled by a high voltage provided by the 5V power source from the PWM150. Simultaneously, the control port151of the PWM150is pulled to a low voltage state, and the PWM150is disabled.

At least one transistor1331˜1334and many passive electronic components are required for each one of the fluorescent lamps111in order to implement the open protection circuit100. That is, the open protection circuit100is complicated and costly. This problem is even more pronounced in the case of a backlight module that has more than four fluorescent lamps111. Hence, there is a need for a simpler and inexpensive open protection circuit for a backlight module.

SUMMARY

Embodiments of the invention provide an open protection circuit for a backlight module that uses fluorescent lamps.

One embodiment of the invention provides an open protection circuit including a pulse width modulator (PWM) having a control port, a switch, a first and a second input circuits, and a first and a second detecting circuits. The control port is configured to disable the pulse width modulator according to a predetermined signal. The switch has a gate terminal and a drain terminal connected to the control port. The first input circuit includes a first diode with a first positive terminal connected to the gate terminal and a first negative terminal connected to ground. The second input circuit includes a first input resistor connected to the control port at one end. The first detecting circuit includes a first sampling resistor connected to a first fluorescent lamp and the first negative terminal at one end and connected to ground at the other. The second detecting circuit includes a second sampling resistor connected to a second fluorescent lamp and the control port via the first input resistor at one end and connected to ground at the other, thereby the control port of the PWM pulled to ground, disabling the PWM when either one of the first and second fluorescent lamps are open-circuit.

Furthermore, the open protection circuit provided in the above first embodiment can further include a third and a fourth input circuits, and a third and a fourth detecting circuits. The third input circuit includes a second diode with a second positive terminal connected to the gate terminal of the switch and a second negative terminal connected to ground. The fourth input circuit includes a second input resistor connected to the control port at one end. The third detecting circuit includes a third sampling resistor connected to a third fluorescent lamp and the second negative terminal at one end and connected to ground at the other. The fourth detecting circuit includes a fourth sampling resistor connected to a fourth fluorescent lamp and the control port of the PWM via the second input resistor at one end and connected to ground at the other, thereby the control port of the PWM pulled to ground, disabling the PWM when either one of the third and fourth fluorescent lamps are open-circuit.

The switch is a transistor, or a (positive-negative-positive) PNP type transistor as an example. The open protection circuit further includes a current restriction resistor connected to the gate terminal of the transistor and an external power source. The first input circuit includes a first capacitor disposed between the first negative terminal of the first diode and ground, and the third input circuit includes a second capacitor disposed between the second negative terminal of the second diode and ground. The exemplary capacitances of the first capacitor and the second capacitor are 0.22 μF. The exemplary resistances of the first input resistor and the second input resistor are 3.9KΩ. The resistances of the first, second, third, and fourth sampling resistors are 560 Ω.

Another embodiment of the invention provides an open protection circuit including a PWM having a control port. The control port is configured to disable the pulse width modulator according to a predetermined signal. A switch has a gate terminal and a drain terminal connected to the control port. A plurality of first input circuits include first diodes, and each of the first diodes has a first positive terminal connected to the gate terminal and a first negative terminal connected to ground. A plurality of second input circuits include first input resistors, and each of first input resistors is connected to the control port at one end. A plurality of first detecting circuit and second detecting circuit are provided and connected to the fluorescent lamps. Each of the first detecting circuit includes a first sampling resistor connected to one of the fluorescent lamps and one of the first negative terminals of the first diodes at one end and connected to ground at the other. Each of the second detecting circuit includes a second sampling resistor connected to one of the other fluorescent lamps and the control port via one of the first input resistors of the second input circuits at one end and connected to ground at the other, thereby the control port of the PWM can be pulled to ground, disabling the PWM when either one of the fluorescent lamps are open-circuit.

Furthermore, backlight modules incorporating the above exemplary open protection circuits are also provided. A detailed description of various embodiments is given below with reference to the accompanying drawings.

DETAILED DESCRIPTION

FIG. 1shows a diagram of an open protection circuit200for a backlight module that uses fluorescent lamps, in accordance with a first embodiment of the present invention. The backlight module utilizes two fluorescent lamps211and221. The fluorescent lamps211and221cooperatively serve as a light source for a liquid crystal display (LCD), and can for example be cold cathode fluorescent lamps (CCFLs) or hot cathode fluorescent lamps (HCFLs). The open protection circuit200includes a pulse width modulator (PWM)250, a switch270, a first and a second input circuits230,240, and a first and a second detecting circuits210,220.

The PWM250has a control port251. The control port251disables the PWM250when the control port251is pulled to a low voltage state. The switch270includes a transistor271and a current restriction resistor272. A gate terminal of the transistor271is connected to a 5V DC power source provided by a pin of the PWM250via the current restriction resistor272. A source terminal of the transistor271is connected to the control port251of the PWM250, and a drain terminal of the transistor271is connected to ground.

The first input circuit230includes a diode231and a filtering capacitor232. A positive terminal of the diode231is connected to the gate terminal of the transistor271, and to the power pin of the PWM250via the current restriction resistor272. A negative terminal of the diode271is connected to ground via the filtering capacitor232, which can filter out high frequency noise. A first input port233of the first input circuit230is connected to the negative terminal of the diode231.

The first detecting circuit210is connected to one terminal of a fluorescent lamp211, and includes a sampling resistor213and two rectification diodes (not labeled). One end of the sampling resistor213is connected to ground. An opposite end of the sampling resistor213is connected to the fluorescent lamp211via a negative terminal of one of the rectification diodes. An output port212of the first detecting circuit210is connected to the negative terminal of the same one rectification diode, and is also connected to the input port233of the first input circuit230.

The second input circuit240includes an input resistor241. The input resistor241is connected to the control port251, the source terminal of the transistor271, and ground via a capacitor (not labeled).

The second detecting circuit220is connected to one terminal of a fluorescent lamp221, and has a similar structure to that of the first detecting circuit210. The second detecting circuit220includes a sampling resistor223and two rectification diodes (not labeled). One end of the sampling resistor223is connected to ground, and an opposite end of the sampling resistor223is connected to the fluorescent lamp221via a negative terminal of one of the rectification diodes. An output port222of the second detecting circuit220is connected to the negative terminal of the same one rectification diode, and is also connected to the input resistor241of the second input circuit240.

In this exemplary embodiment, the PWM250is an OZ9910G PWM control chip. The transistor271is a PNP type transistor. The current restriction resistor272has a resistance of about 10KΩ. The diode231of the first input circuit230and the unlabeled rectification diodes in the first and second detecting circuits210,220are SN4148 diodes. The filtering capacitor has a capacitance of about 0.022 μF. The input resistor241of the second input circuit240has a resistance of about 3.9KΩ. The sampling resistors213,223in the first and second detecting circuits210,220each have a resistance of about 560 Ω.

When the fluorescent lamps211,221are ignited and function normally, the output ports212,222of the first and second detecting circuits210,220each output a high voltage of about 3.6V. The 3.6V high voltage provided by the output port212disables the transistor271of the switch270. The 3.6V high voltage provided by the output port222maintains the control port251of the PWM250in a high voltage state, which keeps the PWM250functioning.

When the fluorescent lamp211connected to the first detecting circuit210fails or has an open circuit, the output port212is pulled to a low voltage state of about 0.067V. Simultaneously, the transistor271is enabled. The control port251of the PWM250is pulled to a low voltage state, which disables the PWM250. Thus the PWM250is protected from high voltage currents or impulses.

When the fluorescent lamp221connected to the second detecting circuit220fails or has an open circuit, the output port222is pulled to a low voltage state of about 0.067V. Simultaneously, the control port251of the PWM250is directly pulled to a low voltage state, which disables the PWM250. Thus the PWM250is protected from high voltage currents or impulses.

Unlike a conventional open protection circuit such as the open protection circuit100, the structure of the open protection circuit200is relatively simple. Thus, the cost of the open protection circuit200can be reduced.

FIG. 2is a diagram of an open protection circuit300for a backlight module that uses fluorescent lamps, in accordance with a second exemplary embodiment of the present invention. The open protection circuit300has a similar structure to that of the first embodiment, and can be applied to a backlight module that has four or more fluorescent lamps. In order to simplify the drawing and following description of the open protection circuit300, only four fluorescent lamps311,321,381, and391are shown inFIG. 2and described below.

The open protection circuit300includes a PWM350, a switch370, a first and a second input circuits330,340, and a first, a second, a third, and a fourth detecting circuits310,320,380, and390.

The PWM350has a control port351. The control port351disables the PWM350when the control port351is pulled to a low voltage state. The switch370includes a transistor371and a current restriction resistor372. A gate terminal of the transistor371is connected to a 5V DC power source provided by a pin of the PWM350via the current restriction resistor372. A source terminal of the transistor371is connected to the control port351of the PWM350, and a drain terminal of the transistor371is connected to ground.

The first input circuit330is separated into two branches, and includes two diodes331and two filtering capacitors332. In each branch of the first input circuit330, a positive terminal of the diode331is connected to the gate terminal of the transistor371, and to the power pin of the PWM350via the current restriction resistor372. A negative terminal of each diode331is connected to ground via the corresponding filtering capacitor332, which can filter out high frequency noise. Furthermore, the first input circuit330has two input ports333,333′ connected with the negative terminals of the diodes331in each branch respectively.

The second input circuit340includes two input resistors341,342separated into two branches. The input resistors341,342are each connected to the control port351, the source terminal of the transistor371, and ground via a capacitor (not labeled).

The first detecting circuit310is connected to one terminal of a fluorescent lamp311, and includes a sampling resistor313and two rectification diodes (not labeled). One end of the sampling resistor313is connected to ground. The opposite end of the sampling resistor313is connected to the fluorescent lamp311via a negative terminal of one of the rectification diodes. An output port312of the first detecting circuit310is connected to the negative terminal of the same one rectification diode, and is also connected to the input port333of the first input circuit330.

The second detecting circuit320is connected to one terminal of a fluorescent lamp321, and has a similar structure to that of the first detecting circuit310. The second detecting circuit320includes a sampling resistor323and two rectification diodes (not labeled). One end of the sampling resistor323is connected to ground. The opposite end of the sampling resistor313is connected to the fluorescent lamp321via a negative terminal of one of the rectification diodes. An output port322of the second detecting circuit320is connected to the negative terminal of the same one rectification diode, and is also connected to the input port333′ of the first input circuit330.

The third detecting circuit380is connected to one terminal of a fluorescent lamp381, and has a similar structure to that of the first detecting circuit310. The third detecting circuit380includes a sampling resistor383, and two rectification diodes (not labeled). One end of the sampling resistor383is connected to ground. The opposite end of the sampling resistor383is connected to the fluorescent lamp381via a negative terminal of one of the rectification diodes. An output port382of the third detecting circuit380is connected to the negative terminal of the same one rectification diode, and is also connected to the input resistor341of the second input circuit340.

The fourth detecting circuit390is connected to one terminal of a fluorescent lamp391, and has a similar structure to that of the first detecting circuit310. The fourth detecting circuit390includes a sampling resistor393, and two rectification diodes (not labeled). One end of the sampling resistor393is connected to ground. The opposite end of the sampling resistor393is connected to the fluorescent lamp391via a negative terminal of one of the rectification diodes. An output port392of the fourth detecting circuit390is connected to the negative terminal of the same one rectification diode, and is also connected to the input resistor342of the second input circuit340.

When the fluorescent lamps311,321,381,391are ignited and function normally, the output ports312,322,382,392of the first through fourth detecting circuits310,320,380,390each output a high voltage of about 3.6V. The 3.6V high voltage provided by the output ports312and322disables the transistor371of the switch370. The 3.6V high voltage provided by the output ports382and392maintains the control port351of the PWM350in a high voltage state, which keeps the PWM350functioning.

When either of the fluorescent lamps311,321connected to the first and second detecting circuits310,320fails or has an open circuit, the corresponding output port312or322is pulled to a low voltage state of about 0.067V. Simultaneously, the transistor371is enabled. The control port351of the PWM350is pulled to a low voltage state, which disables the PWM350. Thus the PWM350is protected from high voltage currents or impulses.

When either of the fluorescent lamps381,391connected to the third and fourth detecting circuit380,390fails or has an open circuit, the corresponding output port382or392is pulled to a low voltage state of about 0.067V. Simultaneously, the control port351of the PWM350is directly pulled to a low voltage state, which disables the PWM350. Thus the PWM350is protected from high voltage currents or impulses.

The open protection circuit300can protect the PWM350without the need for additional transistors. Thus the structure of the open protection circuit300is relatively simple, and the cost of the open protection circuit300can be reduced.