Patent Abstract:
The present invention provides an LED module and an LED light string using the same. The LED module has an input terminal, an output terminal, a primary LED, a spare LED and a switching module. The switching module controls the spare LED to be switched off while the primary LED is switched on; and controls the spare LED to be switched on while the primary LED is burned out. Hence, the present invention extends a service life of the LED light string using the LED modules in the backlight module.

Full Description:
FIELD OF THE INVENTION 
     The present invention relates to an LED module, and more particularly to an LED module that provides a reliable light source and an LED light string using the LED modules. 
     BACKGROUND OF THE INVENTION 
     Presently light sources of a backlight module are mainly sorted into light emitting diodes (LEDs) and cold cathode fluorescent lamps (CCFLs). Since the light emitting diodes have advantages like low power consumption, using light emitting diodes to replace the cold cathode fluorescent lamps is the main trend of the development of backlight module industry. 
     Based on the concept of modular production, backlight modules, no matter direct-type or edge-type, use a light string having a plurality of light emitting diodes connected in series as the light source. Since a backlight module has to provide a stable and uniform surface light source, once one of the light emitting diodes burned out, the broken one needs to be changed immediately so as to maintain the uniformity of the surface light source. Therefore, maintenance or durability of the light strings of the backlight module is restrained by the working life of each of the light emitting diodes. 
     Hence, it is necessary to provide an LED module and an LED light string using the same to overcome the problems existing in the conventional technology. 
     SUMMARY OF THE INVENTION 
     The invention provides an LED module and an LED light string using the same to overcome the problem of lack of durability in a light string of a backlight module. 
     An LED module comprising: an input terminal, an output terminal, a primary LED, a spare LED and a switching module, wherein the switching module controls the spare LED to be switched off when the primary LED is switched on, and controls the spare LED to be switched on when the primary LED is burned out; and the switching module has a first field-effect transistor, a second field-effect transistor, a first resistor and a second resistor, wherein the source electrode of the first field-effect transistor is connected to the cathode of the primary LED and connected to ground orderly through the first resistor and the second resistor, the drain electrode thereof is connected to the output terminal, and the gate electrode thereof is connected to ground through the second resistor; the source electrode of the second field-effect transistor is connected to the cathode of the spare LED, the drain electrode thereof is connected to the output terminal, and the gate electrode thereof is connected to the gate electrode of the first switch; and the threshold voltage Vth 1  of the first field-effect transistor and the threshold voltage Vth 2  of the second field-effect transistor satisfy the following condition: Vth 1 &gt;−(Vs 1 −Vf)R 1 /(R 1 +R 2 )&gt;Vth 2 &gt;−(Vs 1 −Vf), wherein Vs 1  is an input voltage received by the input terminal, Vf is the forward voltage drop of the primary LED and the spare LED, R 1  is the resistance value of the first resistor, R 2  is the resistance value of the second resistor. 
     An LED module comprising: an input terminal, an output terminal, a primary LED, a spare LED and a switching module, wherein the anode of the primary LED is connected to the input terminal, the cathode thereof is connected to the output terminal through the switching module; the anode of the spare LED is connected to the input terminal, the cathode of the spare LED is connected to the output terminal through the switching module; and the switching module controls the spare LED to be switched off when the primary LED is switched on, and controls the spare LED to be switched on when the primary LED is burned out. 
     In one embodiment of the present invention, the switching module has a first switch and a second switch, wherein the first switch is connected to the cathode of the primary LED and the output terminal, and the second switch is connected to the cathode of the spare LED and the output terminal, wherein when the primary LED is switched on, the first switch is switched on and the second switch is switched off; when the primary LED is burned out, the first switch is switched off and the second switch is switched on. 
     In one embodiment of the present invention, the switching module further has a first resistor and a second resistor, wherein the cathode of the primary LED is connected to ground orderly through the first resistor and the second resistor. 
     In one embodiment of the present invention, the switching module has a first field-effect transistor, a second field-effect transistor, a first resistor and a second resistor, wherein the source electrode of the first field-effect transistor is connected to the cathode of the primary LED and connected to ground orderly through the first resistor and the second resistor, the drain electrode thereof is connected to the output terminal, and the gate electrode thereof is connected to ground through the second resistor; the source electrode of the second field-effect transistor is connected to the cathode of the spare LED, the drain electrode thereof is connected to the output terminal, and the gate electrode thereof is connected to the gate electrode of the first switch. 
     In one embodiment of the present invention, the first field-effect transistor and the second field-effect transistor are p-channel metal-oxide-semiconductor field-effect transistors. 
     In one embodiment of the present invention, the threshold voltage Vth 1  of the first field-effect transistor and the threshold voltage Vth 2  of the second field-effect transistor satisfy the following condition: Vth 1 &gt;−(Vs 1 −Vf)R 1 /(R 1 +R 2 )&gt;Vth 2 &gt;−(Vs 1 −Vf), wherein Vs 1  is an input voltage received by the input terminal, Vf is the forward voltage drop of the primary LED and the spare LED, R 1  is the resistance value of the first resistor, R 2  is the resistance value of the second resistor. 
     An LED light string comprising: multiple serial-connected LED module s, wherein each LED module has an input terminal, an output terminal, a primary LED, a spare LED and a switching module, wherein the anode of the primary LED is connected to the input terminal, the cathode thereof is connected to the output terminal through the switching module; the anode of the spare LED is connected to the input terminal, the cathode of the spare LED is connected to the output terminal through the switching module; and the switching module controls the spare LED to be switched off when the primary LED is switched on, and controls the spare LED to be switched on when the primary LED is burned out. 
     In one embodiment of the present invention, the switching module has a first switch and a second switch, wherein the first switch is connected to the cathode of the primary LED and the output terminal, and the second switch is connected to the cathode of the spare LED and the output terminal, wherein when the primary LED is switched on, the first switch is switched on and the second switch is switched off; when the primary LED is burned out, the first switch is switched off and the second switch is switched on. 
     In one embodiment of the present invention, the switching module further has a first resistor and a second resistor, wherein the cathode of the primary LED is connected to ground orderly through the first resistor and the second resistor. 
     In one embodiment of the present invention, the switching module has a first field-effect transistor, a second field-effect transistor, a first resistor and a second resistor, wherein the source electrode of the first field-effect transistor is connected to the cathode of the primary LED and connected to ground orderly through the first resistor and the second resistor, the drain electrode thereof is connected to the output terminal, and the gate electrode thereof is connected to ground through the second resistor; the source electrode of the second field-effect transistor is connected to the cathode of the spare LED, the drain electrode thereof is connected to the output terminal, and the gate electrode thereof is connected to the gate electrode of the first switch. 
     In one embodiment of the present invention, the first field-effect transistor and the second field-effect transistor are p-channel metal-oxide-semiconductor field-effect transistors. 
     In one embodiment of the present invention, the LED modules are orderly assigned as L 1 , L 2 , . . . Li, . . . ,Ln, wherein n≧2, and 1≦i≦n, the threshold voltage Vth 1  of the first field-effect transistor and the threshold voltage Vth 2  of the second field-effect transistor of each LED module Li satisfy the following condition: Vth 1 &gt;−(Vs 1 −i*Vf)R 1 /(R 1 +R 2 )&gt;VTh 2 &gt;−(Vs 1 −i*Vf), wherein Vs 1  is an input voltage received by the input terminal, Vf is the forward voltage drop of the primary LED and the spare LED, R 1  is the resistance value of the first resistor, R 2  is the resistance value of the second resistor, and Vs 1 −(n*Vf)&gt;0. 
     Comparing with the conventional technology, the LED module of the present invention includes a primary LED, a spare LED and a switching module, wherein the switching module controls the spare LED to be switched off while the primary LED is switched on; and controls the spare LED to be switched on while the primary LED is burned out. Therefore, an LED light string using the LED modules can continue to provide a stable light source while one primary LED is burned out, and thereby has better durability. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram of a preferred embodiment of an LED module in accordance with the present invention; and 
         FIG. 2  is a circuit diagram of a preferred embodiment of an LED light string in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The foregoing objects, features and advantages adopted by the present invention can be best understood by referring to the following detailed description of the preferred embodiments and the accompanying drawings. Furthermore, the directional terms described in the present invention, such as upper, lower, front, rear, left, right, inner, outer, side and etc., are only directions referring to the accompanying drawings, so that the used directional terms are used to describe and understand the present invention, but the present invention is not limited thereto. 
     With reference to  FIG. 1 ,  FIG. 1  is a circuit diagram of a preferred embodiment of an LED module in accordance with the present invention. The LED module L has an input terminal (not labeled), an output terminal (not labeled), a primary LED  1 , a spare LED  2  and a switching module S. The input terminal receives an input voltage Vs 1 . The output terminal outputs an output voltage Vs 2 . 
     The anode of the primary LED  1  and the anode of the spare LED  2  are both connected to the input terminal. The forward voltage drop of the primary LED  1  and the spare LED  2  is Vf. 
     The switching module is connected to the cathode of the primary LED  1  and the cathode of the spare LED  2 , and controls the spare LED  2  to be switched off when the primary LED  1  is switched on; and controls the spare LED  2  to be switched on and light up when the primary LED  1  is burned out. In detail, the switching module S has a first switch Q 1  and a second switch Q 2 , wherein the first switch Q 1  is connected to the cathode of the primary LED  1  and the output terminal. The first switch Q 1  is switched on as the primary LED  1  is switched on and lights up. When the primary LED  1  is burned out, the first switch Q 1  is switched off due to an open circuit condition. The second switch Q 2  is connected to the cathode of the spare LED  2  and the output terminal, and is switched on while the primary LED  1  is burned out and thereby switches on and lights up the spare LED  2 . 
     In this embodiment, the switching module S further has a first resistor R 1  and a second resistor R 2 . The first switch Q 1  is a first field-effect transistor, preferably a p-channel metal-oxide-semiconductor field-effect transistor (MOSFET), and has a threshold voltage Vth 1 , wherein the source electrode of the first switch Q 1  is connected to the cathode of the primary LED  1 , and also connected to ground orderly through the first resistor  103  and the second resistor  104 . The drain electrode of the first switch Q 1  is connected to the output terminal. The gate electrode of the first switch Q 1  is grounded though the second resistor  104 . The second switch Q 2  is a second field-effect transistor, preferably a p-channel metal-oxide-semiconductor field-effect transistor (MOSFET), and has a threshold voltage Vth 2 . The source electrode of the second switch Q 2  is connected to the cathode of the spare LED  2 . The drain electrode of the second switch Q 2  is connected to the output terminal. The gate electrode of the second switch Q 2  is connected to the gate electrode of the first switch Q 1 . 
     Set the resistance value of the first resistor  103  to be R 1  and the resistance of the second resistor  104  to be R 2 . The threshold voltage Vth 1  of the second switch Q 1  satisfies: Vth 1 &gt;−(Vs 1 −Vf)R 1 /(R 1 +R 2 ); and the threshold voltage Vth 2  of the second switch Q 2  satisfies: −(Vs 1 −Vf)R 1 /(R 1 +R 2 )&gt;Vth 2 &gt;−(Vs 1 −Vf). Assume the node voltage on the source electrode of the first switch Q 1  is Va; the node voltage on the source electrode of the second switch Q 2  is Vb; and the node voltage on the gate electrodes of the first switch Q 1  and the second switch Q 2  is Vc. 
     The control method of the LED module L according to this embodiment is described as follows: 
     When the primary LED  1  is normally working, Va=Vs 1 −Vf, Vc=(Vs 1 −Vf)R 2 /(R 1 +R 2 ). A voltage difference between the gate electrode and source electrode of the first switch Q 1  is Vgs 1 =Vc−Va. Therefore, Vgs 1 =−(Vs 1 −Vf)R 1 /(R 1 +R 2 ). Since Vth 1 &gt;−(Vs 1 −Vf)R 1 /(R 1 +R 2 ), therefore Vgs 1 &lt;Vth 1 , which satisfies a switched-on condition for the first switch Q 1 , the first switch Q 1  is then switched on. 
     In the meantime, assume the spare LED  2  is also switched on, then Vb=Vs 1 −Vf, and the voltage difference between the gate electrode and source electrode of the second switch Q 2  is Vgs 2 =Vc−Vb, therefore, Vgs 2 =−(Vs 1 −Vf)R 1 /(R 1 +R 2 ). Since Vth 2 &lt;−(Vs 1 −Vf)R 1 /(R 1 +R 2 ), therefore Vgs 2 &gt;Vth 2 , which does not satisfy a switched-on condition for the second switch Q 2 , and thereby the assumption fails, and the spare LED  2  should be switched off in the meantime. 
     When the primary LED  1  is burned out, Vc=0, and Vgs 1 =0 for the first switch Q 1 , therefore the first switch Q 1  is switched off. In the meantime, assume that the spare LED  2  is switched on and normally works, then Vb=Vs 1 −Vf and the voltage difference between the gate electrode and source electrode of the second switch Q 2  is Vgs 2 =Vc−Vb=−(Vs 1 −Vf). Since Vth 2 &gt;−(Vs 1 −Vf), we know that Vgs 2 &lt;Vth 2 , which satisfies the switched-on condition for the second switch Q 2 , therefore the spare LED  2  indeed is switched on. 
     When a plurality of the LED modules L are applied to an LED light string, the control method of the LED light string is executed by the switching module S of each LED module. 
     With further reference to  FIG. 2 ,  FIG. 2  is a circuit diagram of a preferred embodiment of an LED light string in accordance with the present invention. The LED light string has multiple serial-connected LED modules as shown in  FIG. 1 : L 1 , . . . ,Li, . . . Ln, wherein n≧2, and 1≦i≦n. 
     Each LED module L 1  includes an input terminal (not labeled), an output terminal (not labeled), a primary LED  1 , a spare LED  2  and a switching module S. The input terminal receives an input voltage Vsi. The output terminal outputs a voltage Vs(i+1). 
     Set the forward voltage drop of the primary LED  1  and the spare LED  2  of each LED module L 1  is Vf, and the input voltage received by the first LED module L 1  is Vs 1 . The input voltage of the LED module L 1  will be Vsi=Vs 1 −(i−1)Vf, wherein the input voltage Vs 1  of the first LED module L 1  satisfies a condition of: Vs 1 −(n*Vf)&gt;0. Assume that the resistance value of the first resistor  103  of each LED module L 1  to be R 1 , and the resistance of the second resistor  104  of each LED module L 1  to be R 2 . 
     Set the threshold voltage Vth 1  of the first switch Q 1  of each LED module Li to satisfy: Vth 1 &gt;−(Vsi−Vf)R 1 /(R 1 +R 2 )=−(Vs 1 −i*Vf)R 1 /(R 1 +R 2 ). And set the threshold voltage Vth 2  of the second switch Q 2  of each LED module L 1  to satisfy: −(Vs 1 −i*Vf)=−(Vsi−Vf)&lt;VTh 2 &lt;−(Vs 1 −i*Vf)R 1 /(R 1 +R 2 ). The node voltage on the source electrode of the first switch Q 1  is Vai; the node voltage on the source electrode of the seconde switch Q 2  is Vbi; and the gate electrodes of the first switch Q 1  and the second switch Q 2  is Vci. 
     The control method of the LED light string is described as follows: 
     For each LED module L 1 , when the primary LED  1  is normally working, Vai=Vsi−Vf, and Vci=(Vsi−Vf)R 2 /(R 1 +R 2 ). The voltage difference between the gate electrode and the source electrode of the first switch Q 1  is Vgs 1 =Vci−Vai=−(Vsi−Vf)R 1 /(R 1 +R 2 ). Since Vth 1 &gt;−(Vsi−Vf)R 1 /(R 1 +R 2 ), therefore Vgs 1 &lt;Vth 1 , which satisfies a switched-on condition for the first switch Q 1 , thereby the first switch Q 1  is switched on. 
     In the meantime, assume that the spare LED  2  is also switched on, thereby Vbi=Vsi−Vf. The voltage difference between the gate electrode and the source electrode of the second switch Q 2  is Vgs 2 =Vci−Vbi. Therefore, Vgs 2 =−(Vsi−Vf)R 1 /(R 1 +R 2 )=−(Vs 1 −i*Vf)R 1 /(R 1 +R 2 ). 
     Since Vth 2 &lt;−(Vsi−Vf)R 1 /(R 1 +R 2 ), therefore Vgs 2 &gt;Vth 2 , which does not satisfy a switched-on condition for the second switch Q 2 , thereby the assumption fails and the spare LED  2  in the meantime is switched off. 
     When the primary LED  1  is burned out, Vci=0, and Vgs 1 =0 for the first switch Q 1 , therefore the first switch Q 1  is switched off. In the meantime, assume that the spare LED  2  is switched on and normally works, then Vbi=Vsi−Vf and the voltage difference between the gate electrode and source electrode of the second switch Q 2  is: Vgs 2 =Vci−Vbi=−(Vsi−Vf). From Vth 2 &gt;−(Vsi−Vf), we know that Vgs 2 &lt;Vth 2 , which satisfies the switched-on condition for the second switch Q 2 , therefore the spare LED  2  indeed is in a switched-on status. 
     Comparing with the conventional technology, the LED module L 1  of the LED light string of the present invention includes a primary LED  1 , a spare LED  2  and a switching module S. When the primary LED  1  is normally working, the switching module S controls the spare LED  2  to be switched off; when the primary LED  2  is burned out, the switching module S then controls the spare LED  2  to be switched on. Therefore, the LED light string using the LED modules can continue to provide a stable light source while one primary LED is burned out, and thereby has better durability and relatively reduces repair frequency and cost. 
     The present invention has been described with a preferred embodiment thereof and it is understood that many changes and modifications to the described embodiment can be carried out without departing from the scope and the spirit of the invention that is intended to be limited only by the appended claims.

Technology Classification (CPC): 7