Multi-lamp driving circuit

This application claims the benefit of Taiwan application Serial No. 95125543, filed Jul. 12, 2006, the subject matter of which is incorporated herein by reference.

1. Field of the Invention

The invention relates in general to a driving circuit, and more particularly to a multi-lamp driving circuit which generates equal current in each lamp of the backlight module.

2. Description of the Related Art

Referring to FIG. 1 , a schematic diagram of a conventional multi-lamp driving circuit is shown. A conventional multi-lamp driving circuit 10 includes at least a main transformer 110 , a balance transformer 120 , and lamps L( 1 ) and L( 2 ). The main transformer 110 has a secondary coil coupled to one end of the lamp L( 1 ) and one end of the lamp L( 2 ). Each of the lamps L( 1 ) and L( 2 ) has the other end coupled to the balance transformer 120 . The balance transformer 120 has the same winding number in the primary and secondary coils, which are opposite in polarity.

When current of the lamps L( 1 ) and L( 2 ) is different, the primary and secondary coils of the balance transformer can respectively induce voltages of opposite polarity. Owing that the induced voltages have the same amount and opposite polarity, the current of the lamps L( 1 ) and L( 2 ) is approximately the same.

However, due to overlarge volume of the balance transformer, the available area of the printed circuit board (PCB) is reduced and the difficulty of circuit layout is increased. Besides, due to high cost of the balance transformer and requirement of one balance transformer for per two lamps, the production cost for a lamp driving circuit is increased along with the number of lamps.

It is therefore an object of the invention to provide a multi-lamp driving circuit. The multi-lamp circuit has a current/voltage converting circuit for balancing lamp current. Owing that the current/voltage converting circuit is composed of cheap resistors, diodes and transistors, which can be integrated in a chip through a semiconductor process, not only the production cost of the driving circuit can be reduced, but also the PCB available area can be improved.

The invention achieves the above-identified object by providing a multi-lamp driving circuit including a transformer, a first lamp, a second lamp, a current/voltage converting circuit and a voltage/current converting circuit. Transformer includes a secondary coil having a first output terminal and a second output terminal. Each of the first lamp and the second lamp has one end coupled to the first output terminal. The current/voltage converting circuit is for outputting a voltage according to a first current of the first lamp. The voltage/current converting circuit is for correspondingly generating a second current according to the voltage, wherein the second current is substantially equal to current of the second lamp.

The invention achieves the above-identified object by providing a multi-lamp driving circuit including a transformer, a first lamp, a second lamp, a first current/voltage converting circuit, a second current/voltage converting circuit, a first voltage/current converting circuit and a second voltage/current converting circuit. The transformer includes a secondary coil having a first output terminal and a second output terminal. One end of the first lamp is coupled to the first output terminal via the first current/voltage converting circuit, and the other end of the first lamp is coupled to the second output terminal via the second current/voltage converting circuit. One end of the second lamp is coupled to the first output terminal via the first voltage/current converting circuit, and the other end of the second lamp is coupled to the second output terminal via the second voltage/current converting circuit. Each of the first current/voltage converting circuit and the second current/voltage converting circuit outputs a voltage according to a first current of the first lamp, each of the first voltage/current converting circuit and the second voltage/current converting circuit correspondingly generates a second current according to the voltage, and the second current is substantially equal to the current of the second lamp.

Other objects, features, and advantages of the invention will become apparent from the following detailed description of the preferred but non-limiting embodiments. The following description is made with reference to the accompanying drawings.

FIG. 1 (Prior Art) is a schematic diagram of a conventional multi-lamp driving circuit.

FIG. 2 is a block diagram of a multi-lamp driving circuit according to the invention.

FIG. 3 is a circuit diagram of the first kind of multi-lamp driving circuit according to the first embodiment of the invention.

FIG. 4 is a schematic diagram of an AC voltage in a positive half period.

FIG. 5 is a schematic diagram of the AC voltage in a negative half period.

FIG. 6 is a circuit diagram of the second kind of multi-lamp driving circuit according to the first embodiment of the invention.

FIG. 7 is a circuit diagram of the third kind of multi-lamp driving circuit according to the first embodiment of the invention.

FIG. 8 is a schematic diagram of lamp arrangement in a backlight module.

FIG. 9 is a circuit diagram of the fourth kind of multi-lamp driving circuit according to the first embodiment of the invention.

FIG. 10 is a circuit diagram of the first kind of multi-lamp driving circuit according to the second embodiment of the invention.

FIG. 11 is a circuit diagram of the second kind of multi-lamp driving circuit according to the second embodiment of the invention.

FIG. 12 is a circuit diagram of the third kind of multi-lamp driving circuit according to the second embodiment of the invention.

FIG. 13 is a circuit diagram of the fourth multi-lamp driving circuit according to the second embodiment of the invention.

FIG. 14 a circuit diagram of the first kind of multi-lamp driving circuit according to the third embodiment of the invention.

FIG. 15 is a circuit diagram of the second kind of multi-lamp driving circuit according to the third embodiment of the invention.

FIG. 16 is a circuit diagram of the third kind of multi-lamp driving circuit according to the third embodiment of the invention.

FIG. 17 is a circuit diagram of the fourth kind of multi-lamp driving circuit according to the third embodiment of the invention.

Referring to FIG. 2 , a block diagram of a multi-lamp driving circuit according to the invention is shown. A multi-lamp driving circuit 20 , for example, applied to a backlight module, includes at least a transformer 210 , a current/voltage converting circuit 220 , a voltage/current converting circuit 230 and lamps L( 1 )˜L(n), wherein n is a positive integer.

The transformer 210 is for outputting an alternating-current (AC) voltage AC to drive the lamps L( 1 )˜L(n). The current/voltage converting circuit 220 is for outputting a voltage V according to current 11 of the lamp L( 1 ), and the voltage/current converting circuit 230 is for correspondingly generating current 12 according to the voltage V, wherein the current 12 is substantially equal to the current of the lamps L( 2 )˜L(n).

Because the current of the lamps L( 2 )˜L(n) is adjusted on the basis of the current 11 , impedance of the lamp L( 1 ) is larger than impedance of the lamps L( 2 )˜L(n) preferably.

The above-mentioned current/voltage converting circuit 220 and the voltage/current converting circuit 230 are composed of cheap resistors, diodes and transistors for reducing production cost of the driving circuit. Besides, the resistors, diodes and transistors of the current/voltage converting circuit 220 and the voltage/current converting circuit 230 can be integrated in a chip through a semiconductor process. Therefore, not only the production cost can be reduced, but also the PCB available area can be improved.

Referring to FIG. 3 , a circuit diagram of the first kind of multi-lamp driving circuit according to the first embodiment of the invention is shown. A multi-lamp driving circuit 20 ( 1 ) includes a transformer 210 , a current/voltage converting circuit 220 ( 1 ), a voltage/current converting circuit 230 ( 1 ), a diode circuit 240 and lamps L( 1 )˜L(n).

The transformer 210 has a primary coil N 1 and a secondary coil N 2 , and the secondary coil N 2 has output terminals S 1 and S 2 . Each of the lamps L( 1 )˜L(n) has a first end coupled to the output terminal S 1 , the lamp L( 1 ) has a second end coupled to the output terminal S 2 via the current/voltage converting circuit 220 ( 1 ), and each of the lamps L( 2 )˜L(n) is coupled to the output terminal S 2 via the voltage/current converting circuit 230 ( 1 ) and the diode circuit 240 .

The current/voltage circuit 220 ( 1 ) includes diodes D 1 and D 2 and impedance Z 1 , such as a resistor. A positive end of the diode D 1 and a negative end of the diode D 2 are coupled to the second end of the lamp L( 1 ), a negative end of the diode D 1 and a positive end of the diode D 2 are coupled to a first end of the impedance Z 1 , and a second end of the impedance Z 1 and the output terminal S 2 are coupled to a ground voltage. The negative end of the diode D 2 and positive end of the diode D 2 can be coupled to a feedback circuit so as to stabilize the current 11 of the lamp L( 1 ).

The diode circuit 240 includes diodes D 3 and D 4 , and a positive end of the diode D 3 and a negative end of the diode D 4 are coupled to the second ends of the lamps L( 2 )˜L(n).

The diode circuit 240 is coupled to the output terminal S 2 via the voltage/current converting circuit 230 ( 1 ). The voltage/current converting circuit 230 ( 1 ) includes transistors Q 1 and Q 2 , and impedance Z 3 and Z 4 , wherein the impedance Z 3 and Z 4 is substantially the same.

The transistors Q 1 and Q 2 are bipolar junction transistors (BJT) for instance. The transistor Q 1 can be a NPN transistor and the transistor Q 2 can be a PNP transistor. The bases (B) of the transistors Q 1 and Q 2 are coupled to the positive end of the diode D 1 and the negative end of the diode D 2 . The collector (C) of the transistor Q 1 is coupled to the negative end of the diode D 3 while the collector of the transistor Q 2 is coupled to the positive end of the diode D 4 . The emitters (E) of the transistors Q 1 and Q 4 are respectively coupled to the first ends of the impedance Z 3 and Z 4 . The impedance Z 3 and Z 4 is, such as, a resistor, and the second ends of the impedance Z 3 and Z 4 are coupled to the output terminal S 2 .

Owing that the impedance Z 1 , Z 3 and Z 4 is substantially the same and the forward bias voltages V D1 and V D2 of the diodes D 1 and D 2 are substantially equal to the base-to-emitter voltages V BE1 and V BE2 of the transistors Q 1 and Q 2 , the current of the lamps L( 1 )˜L(n) is substantially the same.

Referring to FIG. 4 , a schematic diagram of an AC voltage in a positive half period is shown. For example, when the transformer 210 outputs an AC voltage in a positive half period, the transistor Q 1 is turned on and the current I 1 (+) flows by the lamp L( 1 ), and flows to the impedance Z 1 via the diode D 1 . The transistor Q 1 has the base voltage V B1 =V D1 +I 1 (+)×Z 1 =V BE1 +I 2 (+)×Z 3 , and the voltage drop V D1 of the diode D 1 is substantially equal to the base-to-emitter voltage V BE1 of the transistor Q 1 , so the current 12 (+) is substantially equal to the current I 1 (+) of the lamp L( 1 ). In other words, no matter how the current I 1 (+) would be changed, the current I 2 (+) is the same as the current I 1 (+). Therefore, even if the lamps L( 1 )˜L(n) may be different in characteristics, the current of the lamps L( 1 )˜L(n) can be maintained in balance, thereby evening luminance distribution of the backlight module.

Referring to FIG. 5 , a schematic diagram of the AC voltage in a negative half period is shown. Conversely, when the transformer 210 outputs the AC voltage in a negative half period, the transistor Q 2 is turned on and the current I 1 (−) flows by the impedance Z 1 , and flows to the lamp L( 1 ) via the diode D 2 . The transistor Q 2 has the base voltage V B2 =V B1 =V D2 +I 1 (−)×Z 1 =V EB2 +I 2 (−)×Z 3 , and the voltage drop V D2 of the diode D 2 is substantially equal to the base-to-emitter voltage V EB2 of the transistor Q 2 , so the current I 2 (−) is substantially equal to the current I 1 ( 1 ) of the lamp L( 1 ). In other words, no matter how the current I 1 (−) would be changed, the current I 2 (−) is the same as the current I 1 (−). Therefore, even if the lamps L( 1 )˜L(n) may be different in characteristics, the current of the lamps L( 1 )˜L(n) can be maintained in balance, thereby evening luminance distribution of the backlight module.

Therefore, no matter the transformer 210 outputs the AC voltage in the positive half period or negative half period, the current of each lamp is substantially the same and a good current balance effect can be provided.

Referring to FIG. 6 , a circuit diagram of the second kind of multi-lamp driving circuit according to the first embodiment of the invention is shown. A multi-lamp driving circuit 20 ( 2 ) is different from the multi-lamp driving circuit 20 ( 1 ) in the current/voltage converting circuit 220 ( 2 ). The current/voltage converting circuit 220 ( 2 ) includes diodes D 1 and D 2 , and impedance Z 1 and Z 2 , wherein the impedance Z 1 and Z 2 is substantially the same. The positive end of the diode D 1 and negative end of the diode D 2 are still coupled to the second end of the lamp L( 1 ), but the negative end of the diode D 1 and positive end of the diode D 2 are respectively coupled to the first ends of the impedance Z 1 and 72 , and the second ends of the impedance Z 1 and Z 2 and output terminal S 2 are coupled to a ground voltage.

Because the diodes D 1 and D 2 are respectively coupled in series to the impedance Z 1 and Z 2 , when the current of the lamps L( 1 )˜L(n) is different in the positive and negative half periods, the current waveform can be improved to be equal in the positive and negative half periods by slightly adjusting the impedance Z 1 and Z 2 .

Referring to FIG. 7 , a circuit diagram of the third kind of multi-lamp driving circuit according to the first embodiment of the invention is shown. A multi-lamp driving circuit 20 ( 3 ) is different in the voltage/current converting circuit 230 ( 2 ) from the multi-lamp driving circuit 20 ( 1 ). The voltage/current converting circuit 230 ( 2 ) includes transistors Q 1 and Q 2 and impedance Z 3 . The collectors of the transistors Q 1 and Q 2 are still coupled to the negative end of the diode D 3 and positive end of the diode D 4 respectively, but the emitters of the transistors Q 1 and Q 2 are both coupled to the first end of the impedance Z 3 , and the second end of the impedance Z 3 and output terminal S 2 are coupled to the ground voltage.

Because the transistors Q 1 and Q 2 of the voltage/current converting circuit 230 ( 2 ) use common impedance, the number of impedance used in the voltage/current converting circuit 230 ( 2 ) is smaller than that used in the voltage/current converting circuit 230 ( 1 ), which further helps to reduce production cost.

Referring to FIG. 8 , a schematic diagram of lamp arrangement in a backlight module is shown. Generally, due to air thermal convention, the temperature of lamps at the bottom of the backlight module is lower than that of lamps at other positions. Due to such structure limitation, the impedance of the lamp at the bottom of backlight module is larger than that of other lamps. Therefore, the lamp L( 1 ) is preferably chosen as the lamp located at the bottom of backlight module and current of other lamps is adjusted on the basis of the current I 1 of the lamp L( 1 ).

Referring to FIG. 9 , a circuit diagram of the fourth kind of multi-lamp driving circuit according to the first embodiment of the invention is shown. Like the connecting configuration of the multi-lamp driving circuit 20 ( 4 ), impedance Z 5 is directly coupled in series to the lamp L( 1 ) such that the resistance of the lamp L( 1 ) and impedance Z 5 is larger than that of the lamps L( 2 )˜L(n). Then, the current of lamps L( 2 )˜L(n) is adjusted according to the current I 1 of the lamp L( 1 ).

The multi-lamp driving circuit 20 ( 4 ) uses an extra impedance Z 5 coupled in series to the lamp L( 1 ) in FIG. 3 , but the invention is not limited to the driving circuit of FIG. 9 . The impedance Z 5 can also be coupled in series to the lamp L( 1 ) of FIG. 6 or FIG. 7 such that the resistance of the lamp L( 1 ) and impedance Z 5 is larger than that of the lamps L( 2 )˜L(n).

Along with market competition, in order to reduce a number of connectors, one end of the lamp is coupled to a metal structure of the backlight module for grounding. The second embodiment is provided below accompanied with FIG. 10˜FIG . 14 to illustrate application of the driving circuit of the invention to such a backlight module.

Referring to FIG. 10 , a circuit diagram of the first kind of multi-lamp driving circuit according to the second embodiment of the invention is shown. Different from the multi-lamp driving circuit 20 ( 1 ), in the multi-lamp driving circuit 20 ( 5 ), the first ends of the lamps L( 1 )˜L(n) and the output terminal S 2 are coupled to the ground voltage, and the second ends of the impedance Z 1 , Z 3 and Z 4 are coupled to the output terminal S 1 .

Based on a similar operation principle of the multi-lamp driving circuit 20 ( 1 ), when the transistor Q 1 is turned on, the voltage drop across the impedance Z 3 is the same as that of the impedance Z 1 , and when the transistor Q 2 is turned on, the voltage drop across the impedance Z 4 is the same as that of the impedance Z 1 . Therefore, the current of the lamps L( 1 )˜L(n) is substantially the same, thereby achieving a good current balance effect.

Referring to FIG. 11 , a circuit diagram of the second kind of multi-lamp driving circuit according to the second embodiment of the invention is shown. Different from the multi-lamp driving circuit 20 ( 2 ), in the multi-lamp driving circuit 20 ( 6 ), the first ends of the lamps L( 1 )˜L(n) and the output terminal S 2 are coupled to the ground voltage, and the second ends of the impedance Z 1 ˜Z 4 are coupled to the output terminal S 1 .

Based on a similar operation principle of the multi-lamp driving circuit 20 ( 2 ), when the current of the lamps L( 1 )˜L(n) is different in the positive and negative half periods, the current waveform can be improved to be equal in both the positive and negative half periods by slightly adjusting the impedance Z 1 and Z 2 .

Referring to FIG. 12 , a circuit diagram of the third kind of multi-lamp driving circuit according to the second embodiment of the invention is shown. Different from the multi-lamp driving circuit 20 ( 3 ), in the multi-lamp driving circuit 20 ( 7 ), the first ends of the lamps L( 1 )˜L(n) and the output terminal S 2 are coupled to the ground voltage, and the second ends of the impedance Z 1 and Z 3 are coupled to the output terminal S 1 .

Based on a similar operation principle of the multi-lamp driving circuit 20 ( 3 ), because the transistors Q 1 and Q 2 of the voltage/current converting circuit 230 ( 2 ) use the common impedance Z 3 , the impedance number of the voltage/current converting circuit 230 ( 2 ) is smaller than that of the voltage/current converting circuit 230 ( 1 ), which further reduces the production cost.

Referring to FIG. 13 , a circuit diagram of the fourth kind of multi-lamp driving circuit according to the second embodiment of the invention is shown. Different from the multi-lamp driving circuit 20 ( 4 ), in the multi-lamp driving circuit 20 ( 8 ), one end of each lamp L( 1 )˜L(n) and the output terminal S 2 are coupled to the ground voltage, and the other ends of the impedance Z 1 , Z 3 and Z 4 are coupled to the output terminal S 1 .

The multi-lamp driving circuit 20 ( 8 ) uses an extra impedance Z 5 coupled in series to the lamp L( 1 ) of FIG. 10 . However, the invention is not limited to the driving circuit of FIG. 13 . The impedance Z 5 can be also coupled in series to the lamp L( 1 ) of FIG. 11 or FIG. 12 such that the resistance of the lamp L( 1 ) and impedance Z 5 is larger than that of the lamps L( 2 )˜L(n).

Along with an increasing size of a display panel, the length of lamps is increased, which results in different luminance at two ends of a lamp. In order to avoid this situation, the third embodiment mentioned below accompanied with FIGS. 14˜17 illustrates that the current/voltage driving circuit and voltage/current converting circuit are respectively disposed at two sides of the lamp to even luminance at two ends of the lamp.

Referring to FIG. 14 , a circuit diagram of the first kind of multi-lamp driving circuit according to the third embodiment of the invention is shown. Different from the multi-lamp driving circuit 20 ( 5 ), in the multi-lamp driving circuit 20 ( 9 ), the current/voltage converting circuits 220 ( 1 ) are disposed at two sides of the lamp L( 1 ). The second end of the lamp L( 1 ) is coupled to the output terminal S 1 via the current/voltage converting circuit 220 ( 1 ) and the first end of the lamp L( 1 ) is coupled to the output terminal S 2 via the current/voltage converting circuit 220 ( 1 ). The voltage/current converting circuits 230 ( 1 ) are respectively disposed at two sides of the lamps L( 1 )˜L(n). The second ends of the lamps L( 2 )˜L(n) are coupled to the output terminal S 1 via the diode circuit 240 and voltage/current converting circuit 230 ( 1 ), and the first ends of the lamps L( 2 )˜L(n) are coupled to the output terminal S 2 via the voltage/current converting circuit 230 ( 1 ). The diodes, transistors and impedance of the current/voltage converting circuits 220 ( 1 ), voltage/current converting circuits 230 ( 1 ) and diode circuit 240 have the same connecting configuration as the above circuits, which can be seen from the figure, so any detail is not necessary to be given here.

Based a similar operation principle mentioned above, when the transistor Q 1 is turned on, the voltage drop across the impedance Z 3 is the same as that of the impedance Z 1 , and when the transistor Q 2 is turned on, the voltage drop across the impedance Z 4 is the same as that of the impedance Z 1 . Therefore, the current of the lamps L( 1 )˜L(n) is substantially the same, thereby achieving a good current balance effect.

Referring to FIG. 15 , a circuit diagram of the second kind of multi-lamp driving circuit according to the third embodiment of the invention is shown. Different from the multi-lamp driving circuit 20 ( 6 ), in the multi-lamp driving circuit 20 ( 10 ), the current/voltage converting circuits 220 ( 2 ) are respectively disposed at two sides of the lamp L( 1 ). The second end of the lamp L( 1 ) is coupled to the output terminal S 1 via the current/voltage converting circuit 220 ( 2 ) and the first end of the lamp L( 1 ) is coupled to the output terminal S 2 via the current/voltage converting circuit 220 ( 2 ). The voltage/current converting circuits 230 ( 1 ) are respectively disposed at two sides of the lamps L( 1 )˜L(n). The second ends of the lamps L( 2 )˜L(n) are coupled to the output terminal S 1 via the diode circuit 240 and voltage/current converting circuit 230 ( 1 ), and the first ends of the lamps L( 2 )˜L(n) are coupled to the output terminal S 2 via the voltage/current converting circuits 230 ( 1 ). The diodes, transistors and impedance of the current/voltage converting circuits 220 ( 2 ), the voltage/current converting circuits 230 ( 1 ) and the diode circuit 240 have the same connecting configuration as the above circuits, which can be seen from the figure, so any detail is not necessary to be given here.

Based on a similar operation principle of the multi-lamp driving circuit 20 ( 6 ), when the current of the lamps L( 1 )˜L(n) is different in positive and negative half periods, the current waveform can be improved to be equal in both the positive and negative half periods by slightly adjusting the impedance Z 1 and Z 2 .

Referring to FIG. 16 , a circuit diagram of the third kind of multi-lamp driving circuit according to the third embodiment of the invention is shown. Different from the multi-lamp driving circuit 20 ( 7 ), in the multi-lamp driving circuit 20 ( 11 ), the current/voltage converting circuits 220 ( 1 ) are respectively disposed at two sides of the lamp L( 1 ). The second end of the lamp L( 1 ) is coupled to the output terminal S 1 via the current/voltage converting circuit 220 ( 1 ) and the first end of the lamp L( 1 ) is coupled to the output terminal S 2 via the current/voltage converting circuit 220 ( 1 ). The voltage/current converting circuits 230 ( 2 ) are respectively disposed at two sides of the lamps L( 1 )˜L(n). The second ends of the lamps L( 2 )˜L(n) are coupled to the output terminal S 1 via the diode circuit 240 and voltage/current converting circuit 230 ( 2 ), and the first ends of the lamps L( 2 )˜L(n) are coupled to the output terminal S 2 via the voltage/current converting circuits 230 ( 2 ). The diodes, transistors and impedance of the current/voltage converting circuits 220 ( 1 ), the voltage/current converting circuits 230 ( 1 ) and the diode circuit 240 have the same connecting configuration as the above circuits, which can be seen from the figure, so any detail is not necessary to be given here.

Based on a similar operation principle of the multi-lamp driving circuit 20 ( 7 ), because the transistors Q 1 and Q 2 of the voltage/current converting circuit 230 ( 2 ) use the common impedance Z 3 , the impedance number of the voltage/current converting circuit 230 ( 2 ) is smaller than that of the voltage/current converting circuit 230 ( 1 ), which further reduces the production cost.

Referring to FIG. 17 , a circuit diagram of the fourth kind of multi-lamp driving circuit according to the third embodiment of the invention is shown. Different from the multi-lamp driving circuit 20 ( 8 ), in the multi-lamp driving circuit 20 ( 12 ), the current/voltage converting circuits 220 ( 1 ) are respectively disposed at two sides of the lamp L( 1 ). The second end of the lamp L( 1 ) is coupled to the output terminal S 1 via the current/voltage converting circuit 220 ( 1 ) and the first end of the lamp L( 1 ) is coupled to the output terminal S 2 via the current/voltage converting circuit 220 ( 1 ). The voltage/current converting circuits 230 ( 1 ) are respectively disposed at two sides of the lamps L( 1 )˜L(n). The second ends of the lamps L( 2 )˜L(n) are coupled to the output terminal S 1 via the diode circuit 240 and voltage/current converting circuit 230 ( 1 ), and the first ends of the lamps L( 2 )˜L(n) are coupled to the output terminal S 2 via the voltage/current converting circuits 230 ( 1 ). The diodes, transistors and impedance of the current/voltage converting circuits 220 ( 1 ), voltage/current converting circuits 230 ( 1 ) and diode circuit 240 have the same connecting configuration as the above circuits, which can be seen from the figure, so any detail is not necessary to be given here.

The multi-lamp driving circuit 20 ( 12 ) uses an extra impedance Z 5 coupled in series to the lamp L( 1 ) of FIG. 14 . However, the invention is not limited to the driving circuit of FIG. 17 . The impedance Z 5 can be also coupled in series to the lamp L( 1 ) of FIG. 15 or FIG. 16 such that the resistance of the lamp L( 1 ) and impedance Z 5 is larger than that of the lamps L( 2 )˜L(n).

The above-mentioned first, second and third embodiments use a transformer together with the current/voltage converting circuit and voltage/current converting circuit to balance the current in each lamp. Because the current in each lamp is substantially the same, the luminance distribution of the backlight module can be better uniformed. Besides, the multi-lamp driving circuit of the invention only needs a transformer to drive a number of lamps, which reduces the number of transformers to be used. Therefore, production cost for the driving circuit can be reduced to increase market competitiveness.

The multi-lamp driving circuit disclosed in the above-mentioned embodiments of the invention can balance the current of each lamp by using a current/voltage converting circuit and a voltage/current converting circuit. Owing that the current/voltage converting circuit and voltage/current converting circuit are composed of resistors, diodes and transistors, the production cost for the driving circuit can be greatly reduced. In addition, the above-mentioned resistors, diodes and transistors can be integrated in a chip to increase the available area of a PCB.

While the invention has been described by way of example and in terms of a preferred embodiment, it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.