Abstract:
An LED drive circuit comprises a power supply unit, a first LED lamp string, a first voltage control unit, a second LED lamp string and a first power transfer unit. The power supply unit outputs a drive power to drive the first LED lamp string. The first voltage control unit gets the drive power and stabilizes and provides the voltage to the first LED lamp string. The first voltage control unit includes a power switch where the drive power generates a power loss. The second LED lamp string and first power transfer unit are coupled in series with the first voltage control unit so that the power loss is transferred and output to drive the second LED lamp string. Thus loss of the drive power is reduced and lighting efficiency of the LED improves.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a circuit and particularly to an LED drive circuit. 
       BACKGROUND OF THE INVENTION 
       [0002]    Conventional LED drive circuits typically are designed by adopting a switch-type power converter. Referring to  FIG. 1 , it employs a semiconductor power switch to switch low frequency AC or DC power signals to high frequency DC drive signals usable for LEDs. Such a high frequency conversion wastes a lot of energy on different converter elements and results in power loss. Moreover, the high frequency switch creates signal interference that requires more elements to suppress. As the prevailing trend of products mainly focuses on energy-saving and miniaturization, the structure of the switch-type power converter cannot fully meet the requirement of LED drive circuits. 
         [0003]    Furthermore, the common voltage regulator circuit is typically used for constant current control that is widely adopted. The voltage regulator must share significant energy with the controlled constant current load to get desired control effect. As a result, it also consumes a lot of power that becomes a great concern for designing the power converter. When in use, in the voltage regulator circuits, the more power consumption of the elements, the higher the temperature. If the voltage regulator circuit is adopted in the LED drive circuits, the greater cooling mechanism is needed. This disobeys the requirements of energy-saving and miniaturization of the products. Thus there are still rooms for improvement. 
       SUMMARY OF THE INVENTION 
       [0004]    The primary object of the present invention is to solve the problems of excessive power loss and easy interference occurring to the conventional LED drive circuits that adopt switch-type power converters. 
         [0005]    Another object of the invention is to solve the problem of the conventional LED drive circuits that adopt a voltage regulator circuit that renders higher temperature caused by too much power loss and results in difficulty in product miniaturization because of the constraint of the cooling mechanism. 
         [0006]    To achieve the foregoing objects, the present invention provides an LED drive circuit comprising a power supply unit, a first LED lamp string, a first voltage control unit, a second LED lamp string and a first power transfer unit. The power supply unit includes an output end to output drive power. The first LED lamp string is connected to the output end to receive the drive power to be driven, and includes a first anode end connected to the output end and a first cathode end. The first voltage control unit is connected to the first cathode end to get the drive power and stabilize and provide the voltage to the first LED lamp string. The first voltage control unit includes a power switch. The drive power generates a power loss on the power switch. The second LED lamp string is connected to the first voltage control unit. The first power transfer unit bridges the second LED lamp string and first voltage control unit. 
         [0007]    The first power transfer unit transfers the power loss generated on the power switch to the second LED lamp string for driving thereof. 
         [0008]    Thus, through the technique set forth above, the invention can provide at least the following advantages: 
         [0009]    1. Through the first voltage control unit, the problems of excessive power loss and easy interference occurring to the conventional switch-type power converter can be averted. 
         [0010]    2. By connecting the first power transfer unit and second LED lamp string to the first voltage control unit, the power loss originally consumed at the power switch is transferred and output to drive the second LED lamp string. This not only resolves the problem of providing an extra cooling mechanism due to the high temperature caused by too much power loss of the power switch, the product also can be miniaturized. With the power loss used for lighting the second LED lamp string, lighting efficiency of the drive circuit is enhanced. Hence it also can utilize energy resources more efficiently and save energy. 
         [0011]    3. By dispensing with the switch-type power converter, the invention can be structured simpler at a lower cost, and also can be operated at a lower temperature and stably. 
         [0012]    The foregoing, as well as additional objects, features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a schematic circuit diagram of a conventional LED drive circuit. 
           [0014]      FIG. 2  is a schematic circuit block diagram of the invention. 
           [0015]      FIG. 3  is a schematic circuit diagram of a first embodiment of the invention. 
           [0016]      FIG. 4  is a schematic circuit diagram of a second embodiment of the invention. 
           [0017]      FIG. 5  is a schematic circuit diagram of a third embodiment of the invention. 
           [0018]      FIG. 6  is a schematic circuit diagram of a fourth embodiment of the invention. 
           [0019]      FIG. 7  is a schematic circuit diagram of a fifth embodiment of the invention. 
           [0020]      FIG. 8  is a schematic circuit diagram of a sixth embodiment of the invention. 
           [0021]      FIG. 9  is a schematic circuit diagram of a seventh embodiment of the invention. 
           [0022]      FIG. 10  is a schematic circuit diagram of an eighth embodiment of the invention. 
           [0023]      FIG. 11  is a schematic circuit diagram of a ninth embodiment of the invention. 
           [0024]      FIG. 12  is a schematic circuit diagram of a tenth embodiment of the invention. 
           [0025]      FIG. 13  is a schematic circuit diagram of an eleventh embodiment of the invention. 
           [0026]      FIG. 14  is a schematic circuit diagram of a twelfth embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0027]    Please refer to  FIG. 2  for a circuit block diagram of the invention. It is an LED drive circuit comprising a power supply unit  10 , a first LED lamp string  20 , a first voltage control unit  30 , a second LED lamp string  40  and a first power transfer unit  50 . The power supply unit  10  includes an output end  11  to output drive power. The first LED lamp string  20  is connected to the output end  11  to receive the drive power to be driven, and includes a first anode end  21  connected to the output end  11  and a first cathode end  22 . The first voltage control unit  30  is connected to the first cathode end  22  to get the drive power and stabilize and provide the voltage to the first LED lamp string  20 . The first voltage control unit  30  includes a control end  32 , a feedback end  33  and a power switch  31  connecting to the control end  32  and feedback end  33 . The drive power generates a power loss on the power switch  31 . The second LED lamp string  40  is connected to the first voltage control unit  30 . The first power transfer unit  50  bridges the second LED lamp string  40  and first voltage control unit  30  to form a serial connection with the second LED lamp string  40 , and also bridges the control end  32  and feedback end  33  with the second LED lamp string  40 . Thereby, the first power transfer unit  50  transfers and outputs the power loss generated on the power switch  31  to the second LED lamp string  40  for driving thereof. 
         [0028]    Please refer to  FIG. 3  for the circuit diagram of a first embodiment of the invention. In this embodiment the first LED lamp string  20  is coupled in parallel with a first capacitor  23  and a first resistor  24  to filter out ripple waves and get a stable DC current. The first anode end  21  is connected to the output end  11  of the power supply unit  10 . The power supply unit  10  includes a full bridge rectifier  12  and an AC power source  13  connected to the full bridge rectifier  12 . The first voltage control unit  30  is connected to the first cathode end  22  of the first LED lamp string  20 . The second LED lamp string  40  is connected to the first voltage control unit  30 , and includes a second anode end  41  and a second cathode end  42 . In this embodiment the second anode end  41  is connected to the first voltage control unit  30  via a rectification diode  45 , and the second LED lamp string  40  also is coupled in parallel with a second capacitor  43  and a second resistor  44 . The first power transfer unit  50  bridges the second cathode end  42  of the second LED lamp string  40  and the first voltage control unit  30 , and has the same circuit structure as the first voltage control unit  30 . 
         [0029]    In this embodiment, the first voltage control unit  30  includes the power switch  31  a and a control circuit  34  connecting to the power switch  31   a.  The power switch  31   a  is an N-type power switch in this embodiment. The control circuit  34  includes a Zener diode  342 , a third resistor  343 , a voltage stabilization element  341   a,  a fourth resistor  344  and a fifth resistor  345 . The Zener. diode  342  is connected to the gate of the power switch  31   a  to provide voltage stabilization protection for the power switch  31   a.  The third resistor  343  bridges the gate of the power switch  31   a  and control end  32 . The voltage stabilization element  341   a  is an NPN transistor here and connected to the gate of the power switch  31   a . The fourth resistor  344  is connected to the emitter of the NPN transistor. The fifth resistor  345  is connected to the gate of the NPN transistor. The AC power source  13  is rectified through the full bridge rectifier  12  to generate a DC voltage supplied to the first LED lamp string  20  for emitting light. The control current of the first voltage control unit  30  is a constant current, hence the first LED lamp string  20  can stably emit light a constant luminosity. The power loss on the power switch  31   a  is transferred to the second LED lamp string  40  and first power transfer unit  50  to drive the second LED lamp string  40 . 
         [0030]    Please refer to  FIG. 4  for the circuit diagram of a second embodiment of the invention. It differs from the first embodiment by replacing the NPN transistor by a three-end voltage regulator to be the voltage stabilization element  341   b . The three-end voltage regulator has a gate connecting to the fifth resistor  345  to provide a stable voltage for the fifth resistor  345 . 
         [0031]    Please refer to  FIG. 5  for the circuit diagram of a third embodiment of the invention. It differs from the first embodiment by replacing the NPN transistor by serial-connected diodes to be the voltage stabilization element  341   c.  The serial-connected diodes provide a stable voltage for the fifth resistor  345  as well. 
         [0032]    Please refer to  FIG. 6  for the circuit diagram of a fourth embodiment of the invention. It differs from the first embodiment by replacing the NPN transistor by a Zener diode to be the voltage stabilization element  341   d.  The Zener diode also can provide a stable voltage for the fifth resistor  345 . 
         [0033]    Please refer to  FIG. 7  for the circuit diagram of a fifth embodiment of the invention. It differs from the first embodiment by replacing the N-type power switch by an NPN transistor to be the power switch  31   b.    
         [0034]    Please refer to  FIG. 8  for the circuit diagram of a sixth embodiment of the invention. It differs from the fifth embodiment by including an integration circuit in the control circuit  34  of the first voltage control unit  30  and replacing the NPN transistor in the integration circuit by an operational amplifier to be the voltage stabilization element  341   e.  The operational amplifier is connected to the base of the power switch  31   b,  the cathode of the Zener diode and a third capacitor  346 . The third capacitor  346  has another end connecting to the inverse end of the operational amplifier and a sixth resistor  347  connecting to the fifth resistor  345 . The sixth resistor  347  and the anode of the Zener diode  342  are connected to the feedback end  33 . The non-inverse ends of the operational amplifier are connected to a seventh resistor  348  connecting to a DC voltage end and an eighth resistor  349  connecting to a ground end. 
         [0035]    Please refer to  FIG. 9  for the circuit diagram of a seventh embodiment of the invention. It differs from the first embodiment by using a transistor  51  as the first power transfer unit  50  which is a current limit resistor, thus has a circuit structure different from the first voltage control unit  30 . 
         [0036]    Please refer to  FIG. 10  for the circuit diagram of an eighth embodiment of the invention. It differs from the first embodiment by replacing the N-type power switch  31  by a P-type power switch to be the power switch  31   c.  The structure of the control circuit  34  can be changed according to the characteristics of the power switch  31   c  being used. In addition, NPN transistor or PNP transistor also can be adopted. 
         [0037]    Please refer to  FIG. 11  for the circuit diagram of a ninth embodiment of the invention. It differs from the first embodiment by including a second voltage control unit  60  and a second power transfer unit  70  in the drive circuit that are coupled in parallel with the first voltage control unit  30  and first power transfer unit  50 . Thus, when the power loss cannot be fully transferred and output from the first power transfer unit  50  to the second LED lamp string  40 , the second power transfer unit  70  can aid to output the residual power loss to the second LED lamp string  40 . It is to be noted that in this embodiment a third voltage control unit and a third power transfer unit, a fourth voltage control unit and a fourth power transfer unit and so on that are coupled in parallel with the second LED lamp string  40  also can be provided according to the amount of the power loss without limitation. 
         [0038]    Please refer to  FIG. 12  for the circuit diagram of a tenth embodiment of the invention. It differs from the first embodiment on the power supply unit  10  which is merely an AC power source  13 . Hence, the drive circuit, according to the characteristics of AC power output from the AC power source  13 , is coupled in series with the first LED lamp string  20 , first voltage control unit  30 , second LED lamp string  40  and a first circuit  80  the same as the first power transfer unit  50  to form an AC LED drive circuit 
         [0039]    Please refer to  FIG. 13  for the circuit diagram of an eleventh embodiment of the invention. It differs from the first embodiment on the power supply unit  10  which is merely a DC power source  14 . Hence, the drive circuit, according to the characteristics of DC power output from the DC power source  14 , is coupled in parallel with the first LED lamp string  20 , first voltage control unit  30 , second LED lamp string  40  and a second circuit  90  the same as the first power transfer unit  50  to form a DC LED drive circuit. 
         [0040]    Please refer to  FIG. 14  for the circuit diagram of a twelfth embodiment of the invention. It differs from the first embodiment by further including a first expansion unit  100  which has a circuit structure the same as that of the second LED lamp string  40  and first power transfer unit  50 . In this embodiment, the first power transfer unit  50  is structured the same as the first voltage control unit  30  by having a first control end  52  and a first feedback end  53  corresponding respectively to the control end  32  and feedback end  33 . The expansion unit  100  bridges the first control end  52  and first feedback end  53 . Thus, when a first power switch  54  of the first power transfer unit  50  generates a first power loss, the first power loss is further transferred to the first expansion unit  100  to reduce the heating temperature of the first power switch  54  and also drive an LED lamp string  101  to emit light. Through the first expansion unit  100 , total transfer efficiency can be enhanced. It is to be noted that a second expansion unit having the same structure as the first expansion unit  100  can be provided to couple with the first expansion unit  100 , and a third expansion unit having the same structure as the second expansion unit also can be provided to couple with the second expansion unit, and so on to get improved transfer efficiency. 
         [0041]    As a conclusion, the invention provides at least the following advantages: 
         [0042]    1. Through the first voltage control unit, the problems of excessive power loss and easy interference occurring to the conventional switch-type power converter can be avoided. 
         [0043]    2. By coupling the first power transfer unit and second LED lamp string with the first voltage control unit, the power loss generated on the power switch of the first voltage control unit is transferred and output to drive the second LED lamp string. Thus not only the problem of adding an extra cooling mechanism due to the high temperature caused by the power loss can be averted, the power loss also can be used for lighting the second LED lamp string to improve lighting performance of the drive circuit, thereby utilize energy resources more efficiently and save energy. 
         [0044]    3. The invention does not require the switch-type power converter, thus can be simply structured at a lower cost to make, and also can be operated in a lower temperature and has improved stability. 
         [0045]    4. Through the second voltage control unit and second power transfer unit, the amount of power loss can be regulated to get improved output transfer effect. 
         [0046]    5. The invention further can effectively use the power loss to generate light through the first expansion unit to achieve improved transfer efficiency. 
         [0047]    While the preferred embodiments of the invention have been set forth for the purpose of disclosure, they are not the limitation of the invention, modifications of the disclosed embodiments of the 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 invention.