Abstract:
The present invention provides an LED current balance apparatus, which utilities capacitances coupled to secondary windings of a transformer to store voltage differences between LED strings. Thereby, the driving voltages applied to LED strings are respectively modulated to make the LED strings flowing through a same driving current. A current regulating apparatus is coupled to all LED strings to stabilize total current of the LED strings at a predetermined current.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    (1) Field of the Invention 
         [0002]    The present invention relates to an LED current balance apparatus, and more particularly relates to an LED current balance apparatus with current-balancing capacitive unit. 
         [0003]    (2) Description of the Prior Art 
         [0004]    Due to process errors of light emitting diodes (LEDs), forward voltages of the LEDs are different. Especially, LED strings with a plural LEDs connected in series have serious voltage difference of total forward voltages. Therefore, when a plural LED strings are driven by the same voltage source, the voltage difference of total forward voltages have to be compensated for balancing LED currents. 
         [0005]      FIG. 1  is a detailed circuit diagram of a current balancing apparatus provided by Samsung Electronics Co., Ltd. A switching mode power supply comprises two switches S 1  and S 2 , a capacitor Ca, a transformer, diodes D 1 -D 12 , balancing capacitors Cb 1 , Cb 2  and Cb 3 , and capacitors C 1 -C 6 . The switching mode power supply converts an input current into an alternating current, which is rectified by the diodes D 1 -D 12  and filtered by the capacitors C 1 -C 6  to light LEDs Ld 1 -Ld 6 . The transformer has a primary coil L 1  and three secondary coils L 21 , L 22 , L 23 , and the balancing capacitors Cb 1 , Cb 2 , Cb 3  are coupled to corresponding secondary coils L 21 , L 22 , L 23  for compensating impedance differences of two LEDS of respective secondary coils. An impedance value of a capacitor is determined by operating frequency and capacitance. The impedance values of the balancing capacitors Cb 1 , Cb 2  and Cb 3  in the current balancing apparatus is set to be much higher than the impedance values of the LEDs Ld 1 -Ld 6 , i.e., an impedance value seen by the balancing capacitor looking toward the diode is negligible compared to an impedance value of the balancing capacitor. Therefore, the same DC current is applied to the LEDs Ld 1 -Ld 6 , and so the current balancing of the LEDs Ld 1 -Ld 6  is accomplished. 
         [0006]    However, the LED driving system still has some problems. Firstly, the currents of the LEDs Ld 1 -Ld 6  cannot be stabilized at an excepted current value. It results from that the balancing capacitors Cb 1 , Cb 2  and Cb 3  have tolerances in capacitance and vary in capacitance with temperature. Moreover, the actual operating frequency of the current balancing apparatus may not be a preset frequency and so affects the amount of the current of the LEDs Ld 1 -Ld 6 . Secondly, the currents of the secondary coils of the transformer are sine waves before being rectified and so the amplitudes of the currents are periodically varied in response to the switching of the switches S 1  and S 2 . Even after the currents are rectified by the capacitors C 1 -C 6 , the currents provided by the secondary coils L 21 , L 22 , L 23  to the flowing through the LEDs Ld 1 -Ld 6  still have large current ripples due to the LEDs Ld 1 -Ld 6  with an end grounded having low impedance values. Thirdly, the LEDs Ld 1 -Ld 6  can be dimmed by cutting off the switches S 1  and S 2  to stop a power conversion of the switching mode power supply. However, when cutting off the switches S 1  and S 2 , the LEDs Ld 1 -Ld 6  still lights for a certain time period due to the energy stored in the transformer, the balancing capacitors Cb 1 , Cb 2  and Cb 3 , and the capacitors C 1 -C 6 , and then stops lighting. Furthermore, during the certain time period, the currents of the LEDs Ld 1 -Ld 6  are decreased with time, which cause color shift. When the switches S 1  and S 2  are conducted again, the LEDs Ld 1  -Ld 6  begin to light after a certain time period of recovering the stored energy of the transformer, the balancing capacitors Cb 1 , Cb 2  and Cb 3 , and the capacitors C 1 -C 6 . Similarly, during the certain time period, the currents of the LEDs Ld 1 -Ld 6  are increased with time, which still cause color shift. Thus, this type of the LED driving system has problems of inaccuracy dimming control and color shift. 
       SUMMARY OF THE INVENTION 
       [0007]    In view of the problems of inaccurate LED current, current ripple and color shift in the conventional arts, the LED current balance apparatus of the present invention uses a current regulating apparatus to stabilize the total current of LED units to make every LED units to be stabilized at an except current value. 
         [0008]    To accomplish the aforementioned and other objects, an exemplary embodiment of the invention provides an LED current balance apparatus, comprising a transformer, two rectifying energy-storage circuits, at least one capacitive unit and a current regulating apparatus. The transformer has a primary winding and a secondary winding, wherein two terminals of the secondary winding are respectively coupled to an LED unit to drive the LED unit lighting. The rectifying energy-storage circuits are respectively coupled to corresponding LED units. Every rectifying energy-storage circuit comprises a rectifying unit and an energy-storage unit. The rectifying unit is coupled to a corresponding terminal of the secondary winding for rectifying an electric power supplied by the secondary winding. The energy-storage unit is coupled to the rectifying unit for storing the rectified electric power. The least one capacitive unit is coupled to a corresponding terminal of the secondary winding for storing a bias voltage. The current regulating apparatus has a common node to be coupled to the LED units and stabilizes a sum of currents flowing through the LED units at a predetermined current. 
         [0009]    It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. In order to make the features and the advantages of the invention comprehensible, exemplary embodiments accompanied with figures are described in detail below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The present invention will now be specified with reference to its preferred embodiment illustrated in the drawings, in which: 
           [0011]      FIG. 1  is a detailed circuit diagram of a current balancing apparatus provided by Samsung Electronics Co., Ltd. 
           [0012]      FIG. 2  is schematic diagram of an LED current balance apparatus according to a first preferred embodiment of the present invention. 
           [0013]      FIG. 3  is schematic diagram of an LED current balance apparatus according to a second preferred embodiment of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0014]    In the following detailed description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the disclosed embodiments. It will be apparent, however, that one or more embodiments may be practiced without these specific details. In other instances, well-known structures and devices are schematically shown in order to simplify the drawings. 
         [0015]      FIG. 2  is schematic diagram of an LED current balance apparatus according to a first preferred embodiment of the present invention. The LED current balance apparatus comprises a switching mode converter circuit  100 , two rectifying energy-storage circuits, a capacitive unit  120  and a current regulating apparatus  150 , for lighting two LED units  130   a  and  130   b.  The switching mode converter circuit  100  comprises a transformer, which converts an input electric power received by a primary winding into an alternating current electric power at a secondary winding. The two rectifying energy-storage circuits respectively comprise a rectifying unit  110   a  and an energy-storage unit Co 1 , and a rectifying unit  110   b  and an energy-storage unit Co 2 , correspondingly coupled to two terminals of the secondary winding to rectify and store the alternating current electric power provided by the secondary winding. The capacitive unit  120  is coupled in series to one terminal of the secondary winding. With the switching of the switching mode converter circuit  100 , current difference between the LED units  130   a  and  130   b  causes the capacitive unit  120  to be charged till it reaches a bias voltage of the capacitive unit  120  that makes the currents of the LED units  130   a  and  130   b  to be equal. Hence, the bias voltage stored in the capacitive unit  120  is in response to the driving voltage difference between the LED units  130   a  and  130   b  when flowing through the same driving current. The current regulating apparatus  150  has a common node coupled to the LED units  130   a  and  130   b  for stabilizing a sum of the currents of the LED units  130   a  and  130   b  at a predetermined current. 
         [0016]    The capacitive unit  120  is capable of compensating the driving voltage difference of the LED units  130   a  and  130   b  with the same driving current. The sum of the currents of the LED units  130   a  and  130   b  is stabilized at the predetermined current; even the current of the secondary winding of the transformer periodically varies. Especially, in the conventional arts, the LED driving system under fixed driving power changes impedance of the LED units when any LED in the LED units is short-circuited or open-circuited. It occurs the current value of the LED units to be changed and further color shift. The current regulating apparatus  150  of the present invention could ensure that the currents of the LED units  130   a  and  130   b  are still stabilized to avoid color shift. 
         [0017]    The current regulating apparatus  150  comprises a transistor M and a current regulating controller  140 . The transistor M is coupled to the two LED units  130   a  and  130   b . A resistor Rse is coupled to the transistor M to detect a current flowing through the transistor M. The current regulating apparatus  150  regulates a state of the transistor M according to the detected result of the resistor Rse for stabilizing the current flowing through the transistor M at the predetermined current. The current regulating apparatus  150  may further receive a dimming signal DIM to accordingly control the transistor M to be turned on and off. When the transistor M is turned on, the current flowing through the transistor M is stabilized at the predetermined current. When the transistor M is turned off, the current flowing through the transistor M is zero. Furthermore, when the transistor M is turned off, there is no current loop to conduct the currents of the LED units  130   a  and  130   b  and so the LED units  130   a  and  130   b  immediately stop lighting. Thereby, the energy stored in the energy-storage units Co 1  and Co 2 , the capacitive unit  120  and the transformer could be held. When the transistor M is just turned on again, the currents flowing through the LED units  130   a  and  130   b  are immediately stabilized because the energy stored in the energy-storage units Co 1  and Co 2 , the capacitive unit  120  and the transformer are almost saved during the duration of the transistor M turned off. Hence, the LED current balance apparatus of the present invention has the advantages of accurately dimming and color-shift free. 
         [0018]      FIG. 3  is schematic diagram of an LED current balance apparatus according to a second preferred embodiment of the present invention. In the present embodiment, the switching mode converter circuit is an LLC resonant converter circuit. The LLC resonant converter circuit comprises an LLC converting controller  200 , a current detecting circuit  270  and a resonant circuit  280 . The resonant circuit  280  has a primary side and a secondary side. The primary side comprises transistor switches M 1  and M 2 , a primary winding of a transformer T, a resonant capacitance Cr and a resonant inductance Lr. The secondary side comprises a secondary winding of the transformer T, rectifying units  210   a  and  210   b , two capacitive units  220   a  and  220   b  and energy-storage units Co 1  and Co 2 . The primary side of the resonant circuit  280  is coupled to an input power source Vin to transmit an electric power supplied by the input power source Vin to the secondary winding. The rectifying unit  210   a , the capacitive unit  220   a  and the energy-storage unit Co 1  are coupled to one terminal of the secondary winding, and the rectifying unit  210   b , the capacitive unit  220   b  and the energy-storage unit Co 2  are coupled to the other terminal of the secondary winding, for rectifying and filtering an electric power output by the secondary winding. With the switching of the LLC converting controller  200 , current difference between the LED units  130   a  and  130   b  causes the capacitive units  220   a  and  220   b  to be charged till they reach a bias voltages between the capacitive unit  220   a  and  220   b  that make the currents of the LED units  130   a  and  130   b  to be equal. Hence, bias voltages stored in the capacitive unit  220   a  and  220   b  are in response to the driving voltage difference between the LED units  130   a  and  130   b  when flowing through the same driving current. Thereby, the output voltages Vo 1 , Vo 2  are respectively provided to light the LED units  130   a  and  130   b . The current detecting circuit  270  is coupled to a primary side of the resonant circuit  280  to detect a resonant current flowing through the primary side of the resonant circuit  280  and accordingly generates a current detecting signal Ise. 
         [0019]    A current regulating apparatus comprises a transistor M 3  and a current regulating controller  350 . An end of the transistor M 3  serves as a common node CN to be coupled to the LED units  130   a  and  130   b . The other end of the transistor M 3  is coupled to a resistor Rs. Currents of the LED units  130   a  and  130   b  flow together through the resistor Rs to generate a current detecting signal Cs. The current regulating controller  350  comprises a controller circuit  352 , an OR gate  360  and a protection circuit, wherein the protection circuit comprises an over-voltage comparator  354 , an under-voltage comparator  356 , and a timer circuit  365 . The controller circuit  352  generates a current control signal G 1  to regulate a drain-source on resistance of the transistor M 3  according to a current detecting signal Cs and a reference voltage signal Vr, thereby stabilizing a sum of the currents of the LED units  130   a  and  130   b  at a predetermined current. A voltage detection apparatus  305  comprises two detection units Z 1  and Z 2 , a capacitance Cop and resistors R 1 , R 2 , and R 3 . In the present embodiment, the detection units Z 1  and Z 2  are zener diodes, whose negative ends are respectively coupled to the energy-storage units Co 1  and Co 2 , and positive ends are coupled with each other. One end of the resistor R 2  is coupled to a voltage source Vcc, the other end thereof is coupled to one end of the capacitance Cop, and the other end of the capacitance Cop is grounded. The resistor R 3  and the capacitance Cop are connected in parallel. One end of the resistor R 1  is coupled to a connection node of the detection units Z 1  and Z 2 , and the other end thereof is coupled to a connection node of the resistor R 2  and the capacitance Cop to generate a protection signal Spro. 
         [0020]    The over-voltage comparator  354  compares the protection signal Spro with an over-voltage reference voltage Vop, and the under-voltage comparator  356  compares the protection signal Spro with an under-voltage reference voltage Vsh. When the LED units  130   a  and  130   b  operate normally, a storing voltage of the energy-storage unit Co 1  is lower than a breakdown voltage of the detection unit Z 1  and a storing voltage of the energy-storage unit Co 2  is lower than a breakdown voltage of the detection unit Z 2 . At this moment, a level of the protection signal Spro is determined by a voltage dividing ratio of the resistors R 2  and R 3 , and is lower than the over-voltage reference voltage Vop but higher than the under-voltage reference voltage Vsh. Therefore, all the over-voltage comparator  354  and the under-voltage comparator  356  output a low-level signal. When any one of the LED units  130   a  and  130   b  operates abnormally (e.g.: open-circuit), a corresponding storing voltage of the energy-storage units Co 1 , Co 2  (i.e., output voltage Vo 1  or Vo 2 ) would start to be increased to a value higher than a predetermined over-high protection voltage. When the level of the protection signal Spro is higher than a level of the over-voltage reference voltage Vop, the over-voltage comparator  354  generates a high-level signal. Alternatively, when any one of the LED units  130   a  and  130   b  operates abnormally (e.g., short circuit), a corresponding storing voltage of the energy-storage units Co 1 , Co 2  start to be reduced to a value lower than a predetermined over-low protection voltage. When the level of the protection signal Spro is lower than a level of the under-voltage reference voltage Vsh, the under-voltage comparator  354  generates a high-level signal. The timer circuit  365  is coupled to a timing capacitance Ct and set a predetermined time period according to a capacitance value of the timing capacitance Ct. The timer circuit  365  starts to time count when receiving any one of the high-level signals output by the over-voltage comparator  354  and the under-voltage comparator  356  for avoiding erroneous judgment due to the dimming signal DIM or noises. When the over-voltage comparator  354  or the under-voltage comparator  356  generates the high-level signal for a time period longer than the predetermined time period of the timer circuit  365 , the timer circuit  365  outputs a stop conduction signal F 1 . The OR gate  360  is coupled to the timer circuit  365  and receives the dimming signal DIM. The OR gate  360  modulates the current control signal G 1  according to the dimming signal DIM for stopping or conducting the currents flowing through the LED units  130   a  and  130   b . When the timer circuit  365  determines that the LED units  130   a  and  130   b  operate abnormally and then outputs the stop conduction signal F 1 , the controller circuit  352  is triggered to execute a protection process, such as: temporarily turning the transistor M 3  off until the abnormal condition is removed, turning the transistor M 3  off until the current regulating controller  350  is restarted and so on. The timer circuit  365  may output an error notification signal ERR to an external circuit. 
         [0021]    The LLC converting controller  200  generates control signals Sc 1  and Sc 2  to control a power conversion of the resonant circuit  280  according to a feedback signal FB indicative of the voltage of the common node CN to stabilize the voltage of the common node CN at a predetermined voltage. A resonant controller comprises a resonance deviation protection unit  220 , a frequency sweeping unit  240  and a logic control unit  250 . The frequency sweeping unit  240  generates a clock signal CLK and determines maximum and minimum values of the operating frequency of the clock signal CLK when executing a frequency sweeping process according a frequency resistance RT. The clock signal CLK may have a fixed duty cycle of  50 % or less, or an adjustable duty cycle adjusted according to the feedback signal FB. The frequency sweeping unit  240  executes the frequency sweeping process to decrease the operating frequency of the clock signal CLK with time after the LLC converting controller  200  starts. After the frequency sweeping process finishes, the frequency sweeping unit  240  receives the feedback signal FB and accordingly adjusts the operating frequency of the clock signal CLK to stabilize the voltage of the common node CN at the predetermined voltage. 
         [0022]    The resonance deviation protection unit  220  detects the current detecting signal Ise in response to a phase of the clock signal CLK and determines that the resonant circuit  280  enters the state of resonance deviation (i.e., entering the region of zero current switching from the region of zero voltage switching) when a level of the current detecting signal Ise is lower than a resonance deviation determination level Vzvs. The resonance deviation protection unit  220  generates a corresponding protection signal according to an indicative signal SSFP representing an operating mode of the LLC converting controller  200  when the resonant circuit  280  enters the state of resonance deviation. The operating modes represented by the indicative signal SSFP have a start-up mode and a normal operating mode. A timing of the start-up mode is a preset period after the resonant controller starting and the frequency sweeping unit  240  starts to execute the frequency sweep process during the period. The start-up mode and the frequency sweeping process may not be end simultaneously. The ends of the start-up mode and the frequency sweeping process depend on the actual applications and it is not affect the function of the LLC converting controller  200  of the present invention. The logic control unit  250  generates the control signals Sc 1  and Sc 2  according to the clock signal CLK to switch the transistor switches M 1  and M 2  and control the power conversion of the resonant circuit  280 . The logic control unit  250  executes a protection process in response to the protection signal generated by the resonance deviation protection unit  220  to avoid the resonant circuit  280  operating in the region of zero current switching. 
         [0023]    The resonance deviation protection unit  220  comprises a falling-edge trigger  202 , a D-type flip-flop  204 , AND gates  206  and  208 , a counter  210 , an OR gate  212 , a restart protection circuit  214  and a counting latch protection circuit  216 . The falling-edge trigger  202  receives the current detecting signal Ise and the resonance deviation determination level Vzvs. The falling-edge trigger  202  generates a high-level signal when the level of the current detecting signal Ise is lower than the resonance deviation determination level Vzvs. For avoiding an erroneous judgment due to switching noise of the transistor switches M 1  and M 2 , the falling-edge trigger  202  may block operating within a preset period after the transistor switch M 1  is turned off by receiving the control signal Sc 1 . An input terminal D of the D-type flip-flop  204  is coupled to an output terminal of the falling-edge trigger  202  and the D-type flip-flop  204  receives the clock signal CLK at a trigger terminal CAR. The D-type flip-flop  204  stops detecting a signal generated by the falling-edge trigger  202  to avoid erroneous judgment when the clock signal CLK is at low level. The D-type flip-flop  204  detects the signal generated by the falling-edge trigger  202  when the clock signal CLK is at high level. The clock signal CLK may be replaced with one of the control signals Sc 1  and Sc 2  inputted to the D-type flip-flop  204 . In general, the phases among the control signals Sc 1  and Sc 2  and the clock signal CLK may have difference, but these may not influence the determination of resonance deviation. When the clock signal CLK is at high level, the falling-edge trigger  202  outputs a high-level signal and the D-type flip-flop  204  also outputs a high-level signal at an output terminal Q. The AND gates  206  and  208  are coupled to the output terminal Q of the D-type flip-flop  204  and respectively receive the indicative signal SSFP and an inverted indicative signal SSFN, wherein the indicative signal SSFP and the inverted indicative signal SSFN are opposite signals. 
         [0024]    The indicative signal SSFP is at high level when the LLC converting controller  200  operates at start-up mode. At this time, the AND gate  206  outputs a high level signal if the D-type flip-flop  204  outputs a high-level signal. Besides, the inverted indicative signal SSFN is at low level, the AND gate  208  is blocked and does not operate. The counter  210  receives the indicative signal SSFP and counts a number of high-level signals generated by the AND gate  206  when the indicative signal SSFP is at high level (i.e., when the LLC converting controller  200  operates at start-up mode). At the start-up mode, the operating frequency of the LLC converting controller  200  is controlled by the frequency sweeping unit  240 , and independent of the loading. It may easily result in that the level of the current detecting signal Ise is lower than the resonance deviation determination level Vzvs. For avoiding the foregoing condition being erroneously determined as a resonance deviation, the counter  210  counts the number and generates a high-level signal when the number reaches a preset value. In the present embodiment, the counter  210  may be replaced with a time counter. The time counter determines whether the number reaches the preset value or not within a preset period for better determination result. When the resonant controller  200  enters the normal operating mode from the start-up mode, the indicative signal SSFP is at low level to block the function of the AND gate  206 . The inverted indicative signal SSFN is at high level at this time. If the D-type flip-flop  204  outputs the high-level signal, the AND gate  208  also outputs a high-level signal. The OR gate  212  is coupled to the counter  210  and the AND gate  208 . When any one of the counter  210  and the AND gate  208  outputs the high-level signal, the OR gate  212  outputs a high-level signal to the restart protection circuit  214 . At this time, the restart protection circuit  214  outputs a restart protection signal Ar. The counting latch protection circuit  216  is coupled to the restart protection circuit  214  to count a number of the restart protection signal Ar generated by the restart protection circuit  214 . The counting latch protection circuit  216  generates a latch protection signal Latch when the number of the restart protection signal Ar reaches the preset value which is one or an integer greater than one. In the present embodiment, the counting latch protection circuit  216  also receives the indicative signal SSFP to provide different latch protection determinations in response to an operating mode of the LLC converting controller  200 , such as the start-up mode or the normal operating mode. For example, the counting latch protection circuit  216  does not execute the latch protection when the LLC converting controller  200  operates at start-up mode; i.e., the counting latch protection circuit  216  does not generate the latch protection signal Latch at the start-up mode. Alternatively, the counting latch protection circuit  216  determines whether generating the latch protection signal Latch or not according to different preset values in response to that the LLC converting controller  200  is at start-up mode or normal operating mode. 
         [0025]    The logic control unit  250  executes a restart process when receiving the restart protection signal Ar. First, the LLC converting controller  200  stops generating the control signals Sc 1  and Sc 2  and so the energy stored in the resonant circuit  280  is decreased. Then, the logic control unit  250  generates a restart signal RE to make the LLC converting controller  200  enter the starting mode again. When the frequency sweeping unit  240  receives the restart signal RE, the frequency sweeping unit  240  re-executes the frequency sweeping process and so the operating frequency of the clock signal CLK is recovered to a higher frequency and then decreased with time. The logic control unit  250  enters a latch protection state to stop outputting the control signals Sc 1  and Sc 2  when receiving the latch protection signal Latch. Therefore, the resonant circuit  280  is stopped receiving the electric power in the primary side until the LLC converting controller  200  is restarted. 
         [0026]    In addition, when the logic control unit  250  receives the error notification signal ERR generated by the current regulating controller  350 , the logic control unit  250  stops outputting the control signals Sc 1  and Sc 2  for avoiding electric shock to users or components damage due to abnormality of the LED units  130   a  and  130   b.    
         [0027]    By means of stabilizing the voltage of the common node CN at the predetermined voltage, the present embodiment further reduces a power consumption of the transistor M 3  to a lower power consumption value, which almost not varying with the driving voltages of the LED units  130   a  and  130   b . That is, the LED current balance apparatus of the present invention could operate with high efficiency and without effects of the driving voltages of the LED units  130   a  and  130   b  varying due to operation temperature increasing and partial LEDs being short circuited. Moreover, all the currents of the LED units  130   a  and  130   b  flows through the transistor M 3  and so it is hard to determine the individual conditions of the LED units  130   a  and  130   b  by the current flowing through the transistor M 3 , especially when a number of LED units is increased. The voltage detection apparatus  305  of the present invention is coupled to the energy-storage units Co 1  and Co 2 . Therefore, the protection signal Spro is changed only when the storing voltages thereof is over high or over low only due to that the LED units  130   a  and  130   b  operate abnormally, thereby avoiding the mentioned-above problem. 
         [0028]    The current balancing apparatus disclosed by Samsung which has to consider a balance of the capacitance value of the balancing capacitor and the operating frequency to achieve current balancing. The limitation in selection of the capacitance value and the set precision of the operating frequency significantly affect the stability and equalization of the current. That also seriously pushes the difficulty up in Mass production. In contrast, the current regulating apparatus of the present invention, the selection in capacitance value of the capacitive unit has more flexible and further an equivalence resistance is very large due to one terminal of the LED coupled to the common node CN. Consequently, the present invention provides the current regulating apparatus with high current stability and equalization. 
         [0029]    Of course, the current regulating apparatus of the present invention is applicable to other converter circuit having transformer, such as half-bridge converter circuit, full-bridge converter circuit, and push-pull converter circuit. 
         [0030]    Compared with the boost converter in the conventional arts, the LED current balance apparatus of the present invention employs the characteristic of the transformer to provide a higher conversion ratio and a request of lower voltage withstanding in transistor switch of the primary side, and so the heat and cost of the system are reduced. Moreover, the present invention uses only one current regulating controller to achieve current balancing two LED units with much lower current ripple, accurate dimming and color-shift free. Furthermore, the voltage detection apparatus of the present invention could provide immediate protection when any one of the LED units operates abnormally. 
         [0031]    While the preferred embodiments of the present invention have been set forth for the purpose of disclosure, modifications of the disclosed embodiments of the present invention as well as other embodiments thereof may occur to those skilled in the art. Accordingly, the appended claims are intended to cover all embodiments which do not depart from the spirit and scope of the present invention.