Abstract:
A load driving circuit and a multi-load feedback circuit is disclosed. The load driving circuit and the multi-load feedback circuit are adapted to drive a LED module that has a current balancing circuit for balancing the currents flowing through LEDs. The load driving circuit and the multi-load feedback circuit modules the electric power transmitted by the LED driving apparatus to a LED module according to voltage level(s) of current balancing terminals having insufficient voltage in the current balancing circuit, and so the voltage levels of the current balancing terminals are higher than or equal to a preset voltage level, further increasing the efficiency thereof.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    (1) Field of the Invention 
         [0002]    The present invention relates to a load driving circuit and a multi-load feedback circuit; in particular, it relates to a load driving circuit and a multi-load feedback circuit used to drive plural Light Emitting Diode strings. 
         [0003]    (2) Description of the Prior Art 
         [0004]    Refer first to  FIG. 1 , wherein a schematic diagram of a conventional constant current driving apparatus for LEDs is shown. The illustrated LED constant current driving apparatus comprises a current balancing circuit  10 , a LED module  60  and an electrical power supply  70 . The electrical power supply  70  stabilizes the output voltage VOUT through a voltage feedback signal VFB generated by a voltage feedback circuit. The LED module  60  has plural LED strings connected in parallel between the electrical power supply  70  and the current balancing circuit  10 . The current balancing circuit  10  has a current setting resistor  11  as well as a current mirror composed of a transistor  12  and multiple transistors  20 . One terminal of the current setting resistor  11  is coupled to a voltage VCC, and the other terminal thereof coupled to the transistor  12 , thereby allowing a setting current to flow through the transistor  12 . The transistor  20  is one-to-one, individually connected to a corresponding LED strings in the LED module  60 , and mirrors the setting current, thereby allowing the setting current to flow through the LEDs for light emissions. In this way, substantially equal current can flow through each LED in the LED module  60  for substantially emitting same brightness. 
         [0005]    Due to significant differences in threshold voltages between the LEDs, the required driving voltage value to maintain the same current may vary. For example, with a current of 20 mA flowing therethrough, the required driving voltage for one single LED is roughly within a range of 3.4˜3.8V, and each LED string in the LED module  60  has 20 LEDs, the required driving voltage for one LED string is accordingly within a range of roughly 68˜76V, and the difference in the difference of driving voltage between each series of LEDs is endured by the transistor switch  20 . Besides, the transistor switch  20  must operate in the saturation range to mirror current. Therefore, to ensure each LED string to acquire the same current flowing therethrough, the output voltage VOUT provided by the electrical power supply  70  must be higher than the maximum driving voltage, e.g., 80V, thereby ensuring the transistor switch  20  to operate in the saturation range. 
         [0006]    Nevertheless, the driving voltages required by the LED strings is unlikely to be individually confirmed beforehand, so the maximum driving voltage for the LED strings in the LED module  60  may be lower than 76V. As a result, excessive provision of 80V as the driving voltage may contrarily cause reduced illumination efficiency. Furthermore, to prevent LED string from open-circuit due to any LED damage in the LED string, the LED can be connected in parallel to a Zener diode, such that current can be successfully bypass through the Zener diode when the LED is damaged. The breakdown voltage in the Zener diode is set to be higher than the threshold voltage of LED, e.g., 2V., so as to prevent occurrences of erroneous actions in the Zener diode. Under such circumstances, if two LEDs are damaged in the same LED string, thus resulting in approximately 4V increments in the driving voltage of the LED strings, it is possible to lead to significant reduction in the current flowing through the LED strings or even no current. Alternatively, to increase the output voltage VOUT provided by the electrical power supply  70  to keep the amount of current, illumination efficiency may be undesirably lowered. 
       SUMMARY OF THE INVENTION 
       [0007]    In view of that, to ensure stable light emissions for the LED module, the conventional constant voltage driving apparatus for LEDs provides a driving voltage higher than the required voltage, yet the overly high driving voltage may cause lowered efficiency of the LED driving apparatus. The present invention is directed to resolve the efficiency issue of the LED driving apparatus by, in accordance with the voltage level associated with one or more current balancing terminals having insufficient voltage level in the current balancing circuit of the LED driving apparatus, adjusting the electric power required to drive the LED module in the LED driving apparatus, such that the LED driving apparatus is capable of balancing the current flowing through each LED as well as improving efficiency. 
         [0008]    To achieve the aforementioned objective, the present invention provides a multi-load feedback circuit which is adapted to control a load driving circuit to adjust the electric power to drive a plurality of loads connected in parallel. The multi-load feedback circuit according to the present invention comprises a plurality of semiconductor switches. Each semiconductor switch includes a first terminal, a second terminal and a third terminal, wherein the first terminals are coupled to corresponding plurality of the reference voltages, the second terminals are respectively coupled to corresponding loads, and the third terminals are coupled with each other to generate a detection signal according to each conducting state of the plurality of semiconductor switches in the conducting states, for having the load driving circuit to accordingly adjust the electric power to drive the plurality of loads. 
         [0009]    The present invention also provides a load driving circuit for driving plural LED strings connected in parallel. The load driving circuit according to the present invention comprises an electrical power supply, a current balancing circuit and a multi-load feedback circuit. The electrical power supply is coupled to the plural LED strings for driving the plural LED strings. The current balancing circuit includes a plurality of current balancing terminals correspondingly coupled to the plural LED strings for balancing the current flowing through the plural LED strings. The multi-load feedback circuit includes a plurality of semiconductor switches. Each semiconductor switch is respectively coupled to a corresponding current balancing terminal among the plurality of current balancing terminals and is conducted or cut off based on based on the voltage level of the corresponding plurality of current balancing terminals and a reference voltage of the corresponding plurality of the reference voltages. Herein the multi-load feedback circuit generates a detection signal based on the voltage level(s) associated with the current balancing terminal(s) corresponding to semiconductor switch(es) conducted, for having the electrical power supply to adjust the power to drive the plural LED strings according to the detection signal 
         [0010]    Therefore, the driving electrical power provided by the load driving circuit according to the present invention can be set to a lower level and adjusted depending on the electrical power actually required by the LED module, so as to improve the efficiency thereof. 
         [0011]    The aforementioned summary as well as the detailed descriptions set forth hereinafter both aim to further illustrate the scope of the present invention. Other purposes and advantages in relation to the present invention will be construed with reference to the following specifications and appended drawings thereof. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The present invention will now be specified with reference to its preferred embodiment illustrated in the drawings, in which: 
           [0013]      FIG. 1  is a schematic diagram of a conventional constant current driving apparatus for LEDs. 
           [0014]      FIG. 2  is a schematic diagram of the load driving circuit according to the present invention. 
           [0015]      FIG. 3  is a schematic diagram of the multi-load feedback circuit according to a first embodiment of the present invention. 
           [0016]      FIG. 4  is a schematic diagram of the multi-load feedback circuit according to a second embodiment of the present invention. 
           [0017]      FIG. 5  is a schematic diagram of the multi-load feedback circuit according to a third embodiment of the present invention. 
           [0018]      FIG. 6  is a schematic diagram of the multi-load feedback circuit according to a fourth embodiment of the present invention. 
           [0019]      FIG. 7  is a schematic diagram of the multi-load feedback circuit according to a fifth embodiment of the present invention. 
           [0020]      FIG. 7A  is a schematic diagram of the multi-load feedback circuit according to a sixth embodiment of the present invention. 
           [0021]      FIG. 8  is a schematic diagram of the multi-load feedback circuit according to a seventh embodiment of the present invention. 
           [0022]      FIG. 8A  is a schematic diagram of the multi-load feedback circuit according to an eighth embodiment of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0023]    Referring now to  FIG. 2 , wherein a schematic diagram of the load driving circuit according to the present invention is shown. The depicted load driving circuit comprises a multi-load feedback circuit  110 , a current balancing circuit  120  and an electrical power supply  170  for driving a Light Emitting Diode (LED) module  160 . The LED module  160  has plural LED strings connected in parallel, and each LED string has a plurality of LEDs connected in series. The electrical power supply  170  is coupled to the plural LED strings in the LED module  160 , thereby providing an output voltage VOUT to drive the plural LED strings for lighting. The current balancing circuit  120  has a plurality of current balancing terminals DA 1 ˜DAn correspondingly coupled to the plural LED strings for balancing the current flowing through such plural LED strings, such that the current flowing there through becomes approximately equal. The multi-load feedback circuit  110  is coupled to the current balancing terminals DA 1 ˜DAn for generating a feedback signal FB or a detection signal VD based on the voltage levels of the current balancing terminals, thereby allowing the electrical power supply  170  to adjust the electrical power to drive the LED module  160  based on the detection signal VD or the feedback signal FB. In this way, the voltage levels of current balancing terminals DA 1 ˜DAn can be ensured to be above a predetermined level, yet confined not to become excessively high, thus keeping the efficiency of the load driving circuit at a higher level. 
         [0024]    Next, refer to  FIG. 3 , wherein a schematic diagram of the multi-load feedback circuit according to a first embodiment of the present invention is shown. The present multi-load feedback circuit  210  comprises a plurality of semiconductor switches  212  and a determining circuit  214 . Each semiconductor switch has a first terminal, a second terminal and a third terminal. The first terminals are coupled to a common reference voltage VREF. The second terminals are individually coupled to the plurality of current balancing terminals DA 1 ˜DAn of the current balancing circuit  220 ; that is, coupled to the plural LED strings in the LED module  160 . The third terminals are coupled with each other and also coupled to the determining circuit  214 , thereby generating a detection signal VD to the determining circuit  214 . 
         [0025]    The current balancing circuit  220  includes a plurality of current balancing units  222 , with each current balancing unit  222  including a transistor switch SW, a resistor R and an error amplifier EA. Each of resistors R generates a current detection signal to the inverse terminal of a corresponding error amplifier EA based on the current flowing through a corresponding current balancing terminal among the current balancing terminals DA 1 ˜DAn. The non-inverse terminals of the error amplifiers EA receive the same current reference signal Vb, and accordingly the error amplifiers EA control the equivalent resistance of the transistor switch SW, such that the voltage level of the current detection signal is equal to the level of the current reference signal Vb. Therefore, the current balancing unit  222  is able to control the current flowing through the LED strings coupled to the current balancing terminals DA 1 ˜DAn. 
         [0026]    In the present embodiment, each semiconductor switch  212  in the multi-load feedback circuit  210  has two Metal-Oxide-Semiconductor Field Effect Transistors (MOSFET&#39;s), in which the drains of the two MOSFET&#39;s are coupled with each other and both the gates thereof are connected to the common reference voltage VREF. One of the sources of the two MOSFET&#39;s is coupled to a corresponding current balancing terminal among the plurality of current balancing terminals DA 1 ˜DAn, while the other one source is coupled to the determining circuit  214 . Additionally, the body diodes of the two MOSFET&#39;s are arranged in an opposite direction, so as to prevent transfers of the current signal or voltage signal via the body diodes of the two MOSFET&#39;s when the two MOSFET&#39;s are both in a cutoff state. The determining circuit  214  includes a comparator, in which the inverse terminal of the comparator receives the detection signal VD and the non-inverse terminal of the comparator receives the common reference voltage VREF; the comparator generates the feedback signal FB from the output terminal. 
         [0027]    When any one of the plurality of current balancing terminals DA 1 ˜DAn has a voltage level lower a predetermined voltage difference than the common reference voltage VREF (i.e., there is a voltage difference higher than the conducting voltage of the semiconductor switch  212 ), the semiconductor switch  212  is in a conducting state, otherwise in a cutoff state. That is, the semiconductor switch  212  is conducted or cutoff based on the voltage level of the corresponding current balancing terminal, and it also determines the level of the detection signal VD based on the voltage level(s) of the current balancing terminal(s) corresponding to the conducted semiconductor switch(es)  212 . In the present embodiment, since the semiconductor switch  212  includes two MOSFET&#39;s, the level of the detection signal VD is determined based on an average value of the voltage levels of the current balancing terminals corresponding to the conductive semiconductor switches  212 , and lower than the common reference voltage VREF by at least a predetermined voltage difference. Meanwhile, the determining circuit  214  outputs a feedback signal FB of high level. The electrical power supply  170  shown in  FIG. 2  increases the electrical power to drive the LED module  160  upon reception of the feedback signal FB of high level. That is, the output voltage V 0  is elevated so as to increase the voltage level at the current balancing terminals DA 1 ˜DAn, until the feedback signal FB turns to low level, thus having the voltage levels at the current balancing terminals DA 1 ˜DAn all to be higher than or equal to the common reference voltage VREF. 
         [0028]    Consequently, the load driving circuit according to the present invention adjusts the electrical power to drive the LED module  160  based on the signal from the multi-load circuit, such that the voltage level at each current balancing terminal is higher than or equal to a predetermined voltage. When the voltage level at the current balancing terminal having the lowest level is higher than or equal to a predetermined level, the load driving circuit no longer increases the electrical power to drive the LED module  160  in order to confine the voltage difference between the current balancing terminal and ground into a limited range, thus keeping higher efficiency of the circuitry. 
         [0029]    Refer next to  FIG. 4 , wherein a schematic diagram of the multi-load feedback circuit according to a second embodiment of the present invention is shown. The multi-load feedback circuit  310  comprises a plurality of semiconductor switches  312 , an error amplifier  314 , a resistor  316  and a transistor switch  318 . Each semiconductor switch  312  has a first terminal, a second terminal and a third terminal. The first terminals are coupled to a common reference voltage VREF. The second terminals are individually coupled to the plurality of current balancing terminals DA 1 ˜DAn of the current balancing circuit  320 . The third terminals are coupled with each other and also coupled to the error amplifier  314  thereby generating a detection signal VD to the error amplifier  314 . In the present embodiment, the circuits and operations of the semiconductor switch  312  is identical to which of the semiconductor switch  212  illustrated in  FIG. 3 , descriptions thereof are thus omitted for brevity. 
         [0030]    The most significant difference between the multi-load feedback circuit  310  of the present embodiment and the multi-load feedback circuit  210  shown in  FIG. 3  lies in that the determining circuit  214  is replaced by the error amplifier  314 , the resistor  316  and the transistor switch  318 . The drain of the transistor switch  318  is coupled to a drive voltage VDD, the source of the transistor switch  318  is coupled to the resistor  316  and the non-inverse terminal of the error amplifier  314 , and the gate thereof is coupled to the common reference voltage VREF. Therefore, the transistor switch  318  is maintained in a conducting state and a conducting voltage difference exists between the gate and the source. In other words, the signal received at the non-inverse terminal of the error amplifier  314  has a voltage level that is the common reference voltage VREF minus the conducting voltage difference. The semiconductor switch  312  also has a voltage drop therein when the semiconductor switch  312  is conducted because the level at the corresponding current balancing terminal is lower than the common reference voltage VOUT by a predetermined voltage difference. Consequently, through the placements of the resistor  316  and the transistor switch  318 , it is possible to compensate the voltage drop occurring in the conducted semiconductor switch  312 . Additionally, the error amplifier  314  outputs the feedback signal FB based on the voltage difference between the inverse terminal and the non-inverse terminal so as to have the electrical power supply  170  to adjust the power to drive the LED module  160 , thereby making the voltage levels at the current balancing terminals DA 1 ˜DAn become higher than or equal to (common reference voltage VOUT-conducting voltage difference). 
         [0031]    Subsequently, refer to  FIG. 5 , wherein a schematic diagram of the multi-load feedback circuit according to a third embodiment of the present invention is shown. Compared with the multi-load feedback circuit  212  depicted in  FIG. 3 , each gate of the MOSFETs&#39;, having the sources thereof coupled to the current balancing terminals DA 1 ˜DAn, is coupled to the corresponding current balancing terminal, rather than the common reference voltage VREF, so the MOSFET is maintained in a cutoff state. When the level at the current balancing terminal is lower than the common reference voltage VREF by a predetermined voltage difference thereby causing the corresponding multi-load feedback circuit  412  to be in a conducting state, the signal of the current balancing terminal will be passed to the inverse terminal of the comparator  414  through the body diode of the MOSFET in cutoff and another MOSFET conducted. As a result, the multi-load feedback circuit  412  according to the present embodiment can, as the multi-load feedback circuits illustrated in the previous embodiments, control the load driving circuit to adjust the electrical power to drive the LED module  160  through the feedback signal FB generated by the comparator  414 . Since one of the two MOSFET&#39;s in the multi-load feedback circuit  412  is in a cutoff state all the time that only the feature of diode is demonstrated by the body diode, the current balancing terminal having the lowest voltage level among the current balancing terminals DA 1 ˜DAn dominates the level of the detection signal VD, such that the level of the current balancing terminal having the lowest voltage is higher than or equal to a predetermined voltage level, thus ensuring the levels of all current balancing terminals DA 1 ˜DAn to be higher than or equal to the predetermined voltage level. 
         [0032]    Next, refer to  FIG. 6 , wherein a schematic diagram of the multi-load feedback circuit according to a fourth embodiment of the present invention is shown. The multi-load feedback circuit  510  comprises a plurality of semiconductor switches  512 . Each semiconductor switch  512  has an N-type transistor switch whose gate is coupled to the common reference voltage VREF. one of the source and the drain thereof is coupled to a corresponding current balancing terminal among the current balancing terminals DA 1 ˜DAn of the current balancing circuit  520 , and the other one being coupled with each other in order to generate a detection signal VD, while the base thereof coupled to ground. Due to the base being grounded, it ensures that the reverse biased body diode of the N-type transistor switch is cut off. Hence, the plurality of semiconductor switches  512  transfer the voltage levels of the current balancing terminals DA 1 ˜DAn to the detection signal VD only when the voltage levels at the corresponding current balancing terminals DA 1 ˜DAn lower than the common reference voltage VREF by a predetermined voltage difference. The level of detection signal VD is determined based on an average value of the levels at the current balancing terminals corresponding the conducted semiconductor switches  512 , as the embodiment shown in  FIG. 3 . At this moment, the electrical power supply  170  increases the electrical power to drive the LED module  160  in accordance with the detection signal VD thereby gradually elevating the levels at the current balancing terminals DA 1 ˜DAn, until all of the semiconductor switches  512  are in a cutoff state. 
         [0033]    Furthermore, the multi-load feedback circuit according to the present invention may operate conjunctively with the current balancing circuit formed by the plurality of current balancing units  222  shown in  FIG. 3 , and may also alternatively cooperate with the current balancing circuit  520  formed by a current mirror circuit or other circuits capable of balancing current. In  FIG. 6 , the current mirror circuit has multiple transistor switches with gates and sources thereof being mutually connected, wherein the current I generated by a current source is mirrored and thus flows through each transistor switch, such that the current balancing terminals DA 1 ˜DAn formed by the drains of the transistor switches have the equal current flowing therethrough. 
         [0034]    The multi-load feedback circuit can not only use MOSFET to generate a detection signal or a feedback signal as mentioned in the above embodiment, but also use the bipolar junction transistor to be the detecting component for detecting the voltages of the current balancing terminals. Wherein, one of the emitter and the base of the bipolar junction transistor is coupled to a common reference voltage, and the other of it is coupled to a corresponding current balancing terminal. Accordingly, when the different voltage between each current balancing terminal and the common reference voltage reaches the forward bias voltage, such that the bipolar junction transistor is in the conducting state, the voltage level at each current balancing terminal can be transmitted through the conducting bipolar junction transistor, so as to reach the function as the above embodiment. 
         [0035]    Refer now to  FIG. 7 , wherein a schematic diagram of the multi-load feedback circuit according to a fifth embodiment of the present invention is shown. Compared with the embodiment depicted in  FIG. 6 , the multi-load feedback circuit  610  comprises a plurality of semiconductor switches  612 . Each semiconductor switch  612  is formed by a PNP bipolar junction transistor and a resistor. The emitters of the bipolar junction transistors are coupled to the common reference voltage VREF, the bases of the bipolar junction transistors are coupled to the corresponding current balancing terminals DA 1 ˜DAn in the current balancing circuit  620  through the resistor, and the collectors of the bipolar junction transistors are connected with each other. When the level at the current balancing terminal having the lowest level among the current balancing terminals DA 1 ˜DAn is lower than the common reference voltage VREF by a predetermined voltage difference, the corresponding bipolar junction transistor becomes conductive and the level at current balancing terminal having the lowest voltage level dominates the level in the detection signal VD. 
         [0036]    In the present embodiment, the current balancing circuit may receive a dimming signal DIM and accordingly determines whether the currents flowing through the current balancing terminals DA 1 ˜DAn or not. At this point, due to such a signal, variations in the levels at the current balancing terminals DA 1 ˜DAn may occur, so the detection signal VD can be filtered through a filter circuit  616  in order to filter the noises, due to dimming, out of the detection signal VD and transmitted to a determining circuit  614 . Thereby, the determining circuit  614  outputs a feedback signal FB according to a determining reference voltage Vr and the detection signal VD and the load driving circuit adjusts the provided electrical power in accordance with the feedback signal FB. Wherein, the voltage level of the determining reference voltage Vr and the common reference voltage VREF may be the same or not. 
         [0037]    In addition, the common reference voltage VREF which is received by each semiconductor switch  612  may be replaced by different reference voltages VREF 1 ˜VREFn. Refer to  FIG. 7A , wherein a schematic diagram of the multi-load feedback circuit according to a sixth embodiment of the present invention is shown. In the present embodiment, the multi-load feedback circuit  610  comprises a plurality of semiconductor switches  612 . 
         [0038]    Each semiconductor switches  612  comprises a PNP bipolar junction transistor and a diode. The emitters of the bipolar junction transistors are coupled to the different reference voltages VREF 1 ˜VREFn correspondingly, the collectors of the bipolar junction transistors are connected with each other. When the voltages of the current balancing terminals DA 1 ˜DAn are abnormally raised, e.g.: the current balancing circuit  620  is stopped the current by the dimming signal DIM or the multi-load feedback circuit is in the abnormal state, a reverse bias voltage may be generated between the base and the collector of each bipolar junction transistor or between the base and the emitter thereof. When the reverse bias voltage is too high and over the withstand voltage of the bipolar junction transistor, the bipolar junction transistor may be breakdown. Therefore, in the present embodiment, the diodes are coupled between the bases of each bipolar junction transistors and the current balancing terminals DA 1 ˜DAn correspondingly to avoid the plurality of semiconductor switches  612  being damaged because of the weaker withstand voltage. Compared with  FIG. 7 , the common reference voltage VREF is replaced by a plurality of the reference voltages VREF 1 ˜VREFn. The plurality of the reference voltages VREF 1 ˜VREFn are coupled to the corresponding emitters of the bipolar junction transistor in the plurality of the semiconductor switches  612 . Beside from that, the plurality of the reference voltages VREF 1 ˜VREFn may be set based on the corresponding LED strings (i.e., to which the corresponding current balancing terminals DA 1 ˜DAn are coupled.) Such that the plurality of the reference voltages VREF 1 ˜VREFn may be all equal, partly equal, or all different. When the level at the current balancing terminal having the lowest level among the corresponding current balancing terminals DA 1 ˜DAn of the bipolar junction transistors is lower than the corresponding reference voltage by a predetermined voltage difference, the corresponding bipolar junction transistor becomes conductive and the level of the detection signal VD is adjusted according to the voltage level of the corresponding current balancing terminal. Furthermore, the determining reference voltage Vr is higher than any of the plurality of the reference voltages VREF 1 ˜VREFn. In other words, the determined level of the feedback signal FB and the determined level of each current balancing terminal, the level to conduct the corresponding semiconductor switch, are set by the system, so as to reduce the restriction of the circuit and increase the flexibility in use. 
         [0039]    Next, refer to  FIG. 8 , wherein a schematic diagram of the multi-load feedback circuit according to a seventh embodiment of the present invention is shown. In the present embodiment, the multi-load feedback circuit  710  comprises a plurality of semiconductor switches  712 , Each semiconductor switch  712  is formed by a NPN bipolar junction transistor and a resistor. The bases of the bipolar junction transistors are coupled to the common reference voltage VREF, the emitters of the bipolar junction transistors are coupled to the corresponding current balancing terminals DA 1 ˜DAn in the current balancing circuit  620  through the resistor, and the collectors of the bipolar junction transistors are connected with each other. When the level at the current balancing terminal having the lowest level among the current balancing terminals DA 1 ˜DAn is lower than the common reference voltage VREF by a predetermined voltage difference, the corresponding bipolar junction transistor becomes conductive and the level at current balancing terminal having the lowest voltage level dominates the level in the detection signal VD. 
         [0040]    In addition, the common reference voltage VREF can also be replaced by the plurality of the reference voltages VREF 1 ˜VREFn. Refer to  FIG. 8A , wherein a schematic diagram of the multi-load feedback circuit according to an eighth embodiment of the present invention is shown. In the present embodiment, the multi-load feedback circuit  710  comprises a plurality of semiconductor switches  712  and each semiconductor switch  712  comprises a NPN bipolar junction transistor, a resistor and two diodes. The first diode is respectively coupled between a corresponding bipolar junction transistor and a corresponding reference voltage, and the second diode is respectively coupled to a collector of the corresponding bipolar junction transistor. The emitters of the bipolar junction transistors are correspondingly coupled to the current balancing terminals DA 1 ˜DAn in the current balancing circuit  720 . When the voltages of the current balancing terminals DA 1 ˜DAn are abnormal raised, e.g.: the current balancing circuit  720  is stopped the current by the dimming signal DIM or the multi-load feedback circuit is in the abnormal state, a reverse bias voltage may be generated between the emitter and the base of each bipolar junction transistor or between the emitter and the collector thereof. Therefore, in the present embodiment, the diode and resistor are coupled in serial between a base of the corresponding bipolar junction transistor and the corresponding reference voltage of the reference voltages VREF 1 ˜VREFn to avoid the plurality of semiconductor switches  712  being damaged because of the weaker withstand voltage. 
         [0041]    As the above description, the invention completely complies with the patentability requirements: novelty, non-obviousness, and utility. It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the invention without departing from the scope or spirit of the invention. In view of the foregoing descriptions, it is intended that the invention covers modifications and variations of this invention if they fall within the scope of the following claims and their equivalents.