Abstract:
A driving circuit for light-emitting diodes includes a power unit providing an input voltage, a voltage regulation unit providing a voltage-increasing loop and a voltage-decreasing loop and having an energy-storing capacitor with an intermediate voltage therebetween, a current control unit generating a driving current for the light-emitting diodes according to the intermediate voltage, and a switching control unit. The switching control unit generates a reference voltage based on the input voltage and compares the reference voltage with a feedback voltage associated with driving current. The switching control unit controls the voltage regulation unit to increase the intermediate voltage through the voltage-increasing loop when the feedback voltage is smaller than the reference voltage. The switching control unit controls the voltage regulation unit to decrease the intermediate voltage through the voltage-decreasing loop when the feedback voltage is larger than the reference voltage.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The technical field relates to a driving circuit for light-emitting diodes (LEDs), and especially to a driving circuit used for LEDs and adapted to input voltages of various magnitudes. 
     2. Description of Related Art 
     In LED lighting system, driving circuit is crucial account for the performance and the cost of overall system, therefore, the design of driving circuit is important issue for the LED lighting system. 
     Linear driver is one of major driving schemes for LED lighting system and has advantages of simple design and immunity to electromagnetic interference (EMI). 
     However, the current linear driver for LED lighting system has demanding requirement for matching between input and output voltages. Namely, the linear driver fails to work when the overall forward voltage of the LED is higher than the input voltage. On the contrary, the transistor switch (generally MOSFET switch or BJT switch) in series with the LED will have large voltage stress when the overall forward voltage of the LED is excessively lower than the input voltage. 
     Preferably, the overall forward voltage of the LED is slightly lower than the input voltage for the LED lighting system worked in high AC input voltages such as 200˜240 volts, whereby the LED lighting system can work normally and the voltage stress for the MOSFET switch or BJT switch can be reduced. Nevertheless, the above high AC-input LED lighting system cannot work in low AC-input application (such as 100˜120 volts) because the input voltage is generally lower than the overall forward voltage of the LED. 
       FIG. 1  shows a related art linear driver for LED lighting system, a bridge rectifier  10 A rectifies an AC input Vac into an input voltage Vin. The LED strings  21 A and  22 A are electrically connected to constant-current circuits  31 A and  32 A, respectively and the constant-current circuits  31 A and  32 A control the current flowing through the LED strings  21 A and  22 A via corresponding transistor switches Q 1A  and Q 2A  in order to drive the LED strings  21 A and  22 A as pure resistive load. 
     However, in above-mentioned related art linear driver for LED lighting system, considerable voltage drop is across the transistor switches Q 1A  and Q 2A  in the constant-current circuits  31 A and  32 A when the input voltage is much higher than the driving voltage V LED  of the LED strings  21 A and  22 A. The power dissipation associated with the voltage drop V Q  is absorbed by the transistor switches Q 1A  and Q 2A . The transistor switches Q 1A  and Q 2A  has risk of damage when the power dissipation thereof is excessively high. On the contrary, the LED strings  21 A and  22 A cannot be turned on when the input voltage Vac is lower than the driving voltage V LED  of the LED strings  21 A and  22 A 
     The commercially available LED lamps are generally classified in terms of different input voltages corresponding to different countries/regions such that the LED lamp can satisfy rated voltage in the selling countries/regions. Namely, the LED lamp with single specification cannot be used for worldwide voltage. It is inconvenient for user to spend extra cost to buy lamp suitable for local region. For circuit designer, more labor and cost are needed to develop LED lamps adapted for the various voltages of different regions. 
     Therefore, it is desirable to provide a driving circuit for LED, which can maintain constant current and voltage operation for the LED module and meet the input/output matching requirement for linear driver as well as render the LED applicable to worldwide voltage. 
     SUMMARY OF THE INVENTION 
     The disclosure is directed to a driving circuit for LED to overcome above drawbacks. In one of the exemplary embodiments, the driving circuit for LED comprises a power source for providing an input voltage; a voltage regulator electrically connected to the power source and the LED module, the voltage regulator providing a voltage-increasing loop and a voltage-decreasing loop and having an energy-storing capacitor with an intermediate voltage therebetween; a current control unit electrically connected to the voltage regulator and the LED module, the current control unit generating a driving current for the light-emitting diodes according to the intermediate voltage; and a switching control unit electrically connected to the power source, the voltage regulator and the current control unit, the switching control unit configured to generate a reference voltage based on the input voltage and to compare the reference voltage with a feedback voltage associated with driving current; wherein the switching control unit controls the voltage regulation unit to increase the intermediate voltage through the voltage-increasing loop when the feedback voltage is smaller than the reference voltage, the switching control unit controls the voltage regulation unit to decrease the intermediate voltage through the voltage-decreasing loop when the feedback voltage is larger than the reference voltage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a related art linear driver for LED lighting system. 
         FIG. 2  shows the block diagram of the driving circuit for LED according to the present disclosure. 
         FIG. 3  shows the circuit diagram of the driving circuit according to the first embodiment the present disclosure. 
         FIG. 4  shows the voltage regulator operated in the first operation mode. 
         FIG. 5  shows the voltage regulator operated in the second operation mode. 
         FIG. 6  shows the voltage regulator operated in the third operation mode. 
         FIG. 7  shows the voltage regulator operated in the fourth operation mode. 
         FIG. 8  shows the circuit diagram of the driving circuit according to the second embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In cooperation with the attached drawings, the technical contents and detailed description of the present invention are described thereinafter according to preferable embodiment(s), being not used to limit its executing scope. Any equivalent variation and modification made according to appended claims is all covered by the claims claimed by the present invention. 
       FIG. 2  shows the block diagram of the driving circuit for LED (hereinafter briefed as driving circuit) according to the present disclosure. The driving circuit is used for LED module  40  comprising a plurality of LEDs. The driving circuit mainly comprises a power source  10 , a voltage regulator  30 , a current control unit  50  and a switching control unit  70 . 
     The power source  10  provides an input voltage Vin. The voltage regulator  30  is electrically connected to the power source  10  and the LED module  40  and comprises an energy-storing capacitor Cd. Depending on the input voltage Vin, the voltage regulator  30  generates a charging current Ic for charging energy-storing capacitor Cd. Alternatively, the energy-storing capacitor Cd generates a discharging current Id, depending on the input voltage Vin. The current control unit  50  is electrically connected to the voltage regulator  30  and the LED module  40 , and adapted to generate a driving current I LED  for the LED module  40  when the discharging current Id reaches a current threshold. The current control unit  50  is adapted to maintain constant current driving for the LED module  40 . The switching control unit  70  is electrically connected to the power source  10 , the voltage regulator  30  and the current control unit  50 , and receives a feedback voltage Vfb associated with the driving current I LED  for the LED module  40 . The switching control unit  70  controls the voltage regulator  30  to stop generating the charging current Ic when the feedback voltage Vfb reaches a reference voltage. 
     According to one example, the power source  10  receives an external AC power and employs a bridge rectifier and a filter to respectively rectify and filter the external AC power, thus providing the input voltage Vin. Namely, the input voltage Vin is generated through voltage conversion, rectification and filtering. According to another example, the input voltage Vin may be provided by a DC power source. The voltage regulator  30  is used to regulate the input voltage Vin, namely the AC power source, to maintain constant current and constant voltage driving for the LED module  40 , thus matching the input/output voltage for the LED driving circuit. 
     Moreover, the driving circuit further comprises a feedback unit  60  electrically connected to the current control unit  50  and the switching control unit  70 , and used to convert the driving current I LED  to the feedback voltage Vfb. More particularly, the feedback unit  60  senses the driving current I LED  flowing through the LED module  40  and converts the sensed driving current I LED  to the feedback voltage Vfb. The feedback unit  60  further provides the feedback voltage Vfb to the switching control unit  70  and the switching control unit  70  correspondingly generates a control signal Sc for controlling the voltage regulator  30 . In this manner, the voltage regulator  30  regulates the driving voltage for the LED module  40  such that the requirement for constant current and constant voltage driving, and input/output matching can be met. The description of the driving circuit, especially the voltage regulator  30 , will be detailed later. 
       FIG. 3  shows the circuit diagram of the driving circuit according to the first embodiment the present disclosure. The voltage regulator  30  comprises a first transistor switch Q 1 , a second transistor switch Q 2 , an inductor Ld, and an energy-storing capacitor Cd. The voltage regulator  30  provides a voltage-increasing loop and a voltage-decreasing loop, where the voltage-increasing loop has a first voltage-increasing path and a second voltage-increasing path, and the voltage-decreasing loop has a first voltage-decreasing path and a second voltage-decreasing path. The more detailed description for the voltage-increasing loop and the voltage-decreasing loop will be made later with reference to  FIGS. 4 to 7 . 
     The first transistor switch Q 1  is parallel connected to a first diode D 1 , and the second transistor switch Q 2  is parallel connected to a second diode D 2 , where the first transistor switch Q 1  and the second transistor switch Q 2  are connected at a first common node P A  and the first transistor switch Q 1  and the second transistor switch Q 2  are further connected, respectively through a first capacitor C 1  and a second capacitor C 2  to a second common node P B . 
     The inductor Ld has a first end connected to the first common node P A  and a second end. The energy-storing capacitor Cd has a first end connected to the second end of the inductor Ld and the anode of the LED module  40  and a second end connected to the second common node P B  and the current control unit  50 . An intermediate voltage is defined between the first end and the second end of the energy-storing capacitor Cd. 
     The first transistor switch Q 1  and the second transistor switch Q 2  can be realized by MOSFET or BJT. In the shown embodiment, the first transistor switch Q 1  and the second transistor switch Q 2  are MOSFET switches and the gates of the first transistor switch Q 1  and the second transistor switch Q 2  are connected to each other to receive the control signal Sc from the switching control unit  70  such that the first transistor switch Q 1  and the second transistor switch Q 2  can be controlled by the control signal Sc. 
     The switching control unit  70  mainly comprises a voltage comparator Qpv and a voltage-division resistor network. The voltage-division resistor network comprises a first voltage-division resistor R 1  and a second voltage-division resistor R 2  in series with the first voltage-division resistor R 1 . The input voltage Vin is applied to the series-connected first voltage-division resistor R 1  and second voltage-division resistor R 2  such that a reference voltage, which is a divided voltage of the input voltage Vin, is present across the second voltage-division resistor R 2 . 
     The invert input end of the voltage comparator Qpv receives the feedback voltage Vfb and the non-invert input end of the voltage comparator Qpv receives the reference voltage Vref such that the voltage comparator Qpv compares the feedback voltage Vfb with the reference voltage Vref. The voltage comparator Qpv outputs the control signal Sc of high level when the reference voltage Vref is larger than the feedback voltage Vfb. The voltage comparator Qpv outputs the control signal Sc of low level when the reference voltage Vref is equivalent to or smaller than the feedback voltage Vfb. In this way, the switching control unit  70  controls the first transistor switch Q 1  and the second transistor switch Q 2  of the voltage regulator  30  to regulate the input voltage Vin. Therefore, the LED module  40  can be adapted to input voltages of various magnitudes and maintain constant-voltage and constant-current operation. Moreover, the first transistor switch Q 1  can be an NPN MOSFET while the second transistor switch Q 2  can be a PNP MOSFET. Alternatively, the first transistor switch Q 1  can be a PNP MOSFET while the second transistor switch Q 2  can be an NPN MOSFET. In latter embodiment, the invert input end of the voltage comparator Qpv receives the reference voltage Vref and the non-invert input end of the voltage comparator Qpv receives the feedback voltage Vfb. 
     In the embodiment shown in  FIG. 3 , the switching control unit  70  is realized by an LED driver IC. The feedback unit  60  is realized by an optical coupler, which detects the driving current I LED  for the LED module  40  and then generates corresponding feedback voltage Vfb. The feedback voltage Vfb is sent to the voltage comparator Qpv to compare with the reference voltage Vref. The current control unit  50  comprises a voltage-stabilization diode Z D , and a voltage-stabilization resistor Rv. The voltage-stabilization diode Z D  is, for example, a Zener diode to stabilize the source of the switch Qd. More particularly, the voltage-stabilization diode Z D  can provide a stabilization voltage of 2.5V. The current control unit  50  is electrically connected to the intermediate voltage through a resistor Rd such that the driving current I LED  for the LED module  40  is relevant to the intermediate voltage. More particularly, when the LED module  40  is in normal lighting operation, the voltage-stabilization diode ZD, and the voltage-stabilization resistor Rv can be used to provide constant driving current I LED  for the LED module  40 , which is the ratio between the stabilization voltage (such as 2.5V) and the voltage-stabilization resistor Rv. 
       FIGS. 4 to 7  show the circuit diagrams for the voltage regulator  30  in different operation modes. To simplify description, the first transistor switch Q 1  is an NPN MOSFET while the second transistor switch Q 2  is a PNP MOSFET. Moreover, in  FIGS. 4 to 7  the symbol Vin indicates an equivalent voltage of the AC input Vac after rectifying and filtering, and the output control signal Sc of the switching control unit  70  directly controls the gates of the first transistor switch Q 1  and the second transistor switch Q 2 . 
       FIG. 4  shows the voltage regulator  30  operated in the first mode, where the AC input Vac starts to activate the driving circuit and no current flows through the LED module  40  due to non-conduction of the LED module  40 . At this time, the input voltage Vin charges the inductor Ld and the energy-storing capacitor Cd as well as the resistor Rd (shown in  FIG. 3 ) such that a discharging current Id through the resistor Rd increases. 
     At this time, the feedback voltage Vfb sent to the voltage comparator Qpv is smaller than the reference voltage Vref (voltage division of Vin by the first voltage-division resistor R 1  and the second voltage-division resistor R 2 ) due to zero driving current I LED  through the LED module  40 . Therefore, the voltage comparator Qpv output high-level control signal Sc to turn on the first transistor switch Q 1  and turn off the second transistor switch Q 2 . 
     As shown in  FIG. 4 , in this operation mode, the driving circuit provides a first voltage-increasing path Lp 1  including, in sequence, the input voltage Vin, the first transistor switch Q 1 , the inductor Ld, the energy-storing capacitor Cd, the capacitor C 2  and then back to the input voltage Vin. In this mode, the inductor Ld and the energy-storing capacitor Cd are in energy-storing operation and the voltage V LED  of the LED module  40  increases gradually due to the continual charging of the inductor Ld and the energy-storing capacitor Cd. 
     Moreover, the voltage comparator Qpv continues outputting high-level control signal Sc to turn on the first transistor switch Q 1  and turn off the second transistor switch Q 2  when the feedback voltage Vfb is still smaller than the reference voltage Vref, thus still increasing the discharging current Id. 
     As shown in  FIG. 5 , the driving circuit is operated in the second operation mode. More particularly, the LED module  40  starts to turn on and the driving current I LED  for the LED module  40  starts to increase when the discharging current Id such increases that the driving voltage (the product of the discharging current Id and the resistor Rd) is large enough to turn on the switch Qd in serial connection with the LED module  40  (shown in  FIG. 3 ). 
     At this time, the LED module  40  is normally driven for lighting and the voltage comparator Qpv outputs the control signal Sc to turn off both the first transistor switch Q 1  and the second transistor switch Q 2  such that the inductor Ld and the energy-storing capacitor Cd are in energy-releasing operation. As shown in  FIG. 5 , in this operation mode, the driving circuit provides a second voltage-increasing path Lp 2  including, in sequence, the inductor Ld, the energy-storing capacitor Cd, the capacitor C 2 , the second diode D 2  and then back to the inductor Ld. 
     As shown in  FIG. 6 , the driving circuit is operated in the third operation mode. More particularly, during the continual lighting of the LED module  40 , the feedback unit  60  may sense an excessive driving current I LED , which induces an excessive voltage V LED . At this time, the feedback voltage Vfb sent to the voltage comparator Qpv is larger than the reference voltage Vref and the voltage comparator Qpv generates low-level control signal Sc to turn off the first transistor switch Q 1  and turn on the second transistor switch Q 2 . The LED module  40  in this mode is not conducted due to excessive voltage V LED . 
     As shown in  FIG. 6 , in this operation mode, the driving circuit provides a first voltage-decreasing path Lp 3  including, in sequence, the input voltage Vin, the capacitor C 1 , the energy-storing capacitor Cd, the inductor Ld, the second transistor switch Q 2  and then back to the input voltage Vin. In this mode, the voltage regulator  30  controls the voltage V LED  to decrease such that the LED module  40  can be operated in constant driving voltage. 
     As shown in  FIG. 7 , the driving circuit is operated in the fourth operation mode. More particularly, the voltage V LED  is such decreased that the LED module  40  has normal lighting operation. The voltage comparator Qpv generates the control signal Sc to turn off both the first transistor switch Q 1  and the second transistor switch Q 2 . As shown in  FIG. 7 , in this operation mode, the driving circuit provides a second voltage-decreasing path Lp 4  including, in sequence, the energy-storing capacitor Cd, the inductor Ld, the first diode D 1 , the capacitor C 1 , and then back to the energy-storing capacitor Cd. 
     The switching control unit  70  can use schemes other than those shown in  FIGS. 4 to 7  to control the voltage regulator  30 . The other implementation ways can be exemplified as follows: 
     (1) The first transistor switch Q 1  is a PNP MOSFET while the second transistor switch Q 2  is an NPN MOSFET, and vice versa, while both switches are controlled by the control signal Sc of the switching control unit  70 . 
     (2) Both of the first transistor switch Q 1  and the second transistor switch Q 2  can be the same type of MOSFET, namely PNP MOSFET or NPN MOSFET, while the switching control unit  70  generates control signals of opposite levels to respectively control the first transistor switch Q 1  and the second transistor switch Q 2 . 
     (3) Both of the first transistor switch Q 1  and the second transistor switch Q 2  can be the same type of MOSFET, namely PNP MOSFET or NPN MOSFET, while the switching control unit  70  generates a control signal used with a level inverter to control the first transistor switch Q 1  and the second transistor switch Q 2  with two signals of opposite levels. 
       FIG. 8  shows the circuit diagram of the driving circuit according to the second embodiment of the present disclosure. The second embodiment is similar to the first embodiment except that the second embodiment has a plurality of LED modules  40  and each of the LED modules  40  can keep constant-current and constant-voltage operation. In this embodiment, the transistor switch Qd used by the current control unit  50  is realized by bipolar transistor. Similarly, the voltage-stabilization diode Z D  also provide a stabilization voltage of 2.5V and cooperates with the voltage-stabilization resistor Rv to provide a constant LED driving current I LED . The operation of the driving circuit shown in  FIG. 8  is similar to that of  FIG. 3  and the detailed description thereof is omitted here for brevity. 
     To sum up, the driving circuit for LED of the present disclosure has following advantages: 
     1. The voltage regulator  30  can advantageously adjust the input voltage to maintain constant voltage and constant current driving for the LED module, thus solving the matching issue between input and output of linear driving circuit. 
     2. The voltage regulator  30  can adapt the driving circuit of the present disclosure to various input voltages such that the LED lamp with the driving circuit can be used with worldwide voltage. 
     3. The user can save the extra cost for LED lamp and the LED driver designer can save R&amp;D labor and cost. 
     As the skilled person will appreciate, various changes and modifications can be made to the described embodiment. It is intended to include all such variations, modifications and equivalents which fall within the scope of the present invention, as defined in the accompanying claims.