# Patent

## Title
Two-terminal current controller and related LED lighting device

## Abstract

A two-terminal current controller regulates a first current flowing through a load, which is coupled in parallel with the two-terminal current controller, according to a voltage established across the two-terminal current controller. When the voltage established across the two-terminal current controller does not exceed a first voltage, the two-terminal current controller conducts a second current related to a rectified AC voltage, thereby limiting the first current to zero and regulating the second current according to the load voltage. When the voltage established across the two-terminal current controller is between the first voltage and a second voltage, the two-terminal current controller conducts the second current, thereby limiting the first current to zero and limiting the second current to a constant value larger than zero. When the voltage established across the two-terminal current controller is greater than second voltage, the two-terminal current controller is turned off.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is related to a two-terminal current controller and related LED lighting device, and more particularly, to a two-terminal current controller and related LED lighting device with high power factor.
2. Description of the Prior Art
Compared to traditional incandescent bulbs, light-emitting diodes (LEDs) are advantageous in low power consumption, long lifetime, small size, no warm-up time, fast reaction speed, and the ability to be manufactured as small or array devices. In addition to outdoor displays, traffic signs, and LCD backlight for various electronic devices such as mobile phones, notebook computers or personal digital assistants (PDAs), LEDs are also widely used as indoor/outdoor lighting devices in place of fluorescent of incandescent lamps.
FIG. 1 is a diagram illustrating the voltage-current chart of a light-emitting diode. When the forward-bias voltage of the light-emitting diode is smaller than its barrier voltage Vb, the light-emitting diode functions as an open-circuited device since it only conducts a negligible amount of current. When the forward-bias voltage of the light-emitting diode exceeds its barrier voltage Vb, the light-emitting diode functions as a short-circuited device since its current increases exponentially with the forward-bias voltage. The barrier voltage Vb, whose value is related to the material and doping type of the light-emitting diode, is typically between 1.5 and 3 volts. For most current values, the luminescence of the light-emitting diode is proportional to the current. Therefore, a current source is generally used for driving light-emitting diodes in order to provide uniform luminescence.
FIG. 2 is a diagram of a prior art LED lighting device 500. The LED lighting device 500 includes a power supply circuit 110, a resistor R and a luminescent device 10. The power supply circuit 110 is configured to receive an alternative-current (AC) voltage VS having positive and negative periods and convert the output of the AC voltage VS in the negative period using a bridge rectifier 112, thereby providing a rectified AC voltage VAC, whose value varies periodically with time, for driving the luminescent device 10. The resistor R is coupled in series with the luminescent device 10 for regulating its current ILED. In many applications, multiple light-emitting diodes are required in order to provide sufficient brightness. Since a light-emitting diode is a current-driven device whose luminescence is proportional to its driving current, the luminescent device 10 normally adopts a plurality of light-emitting diodes D1-Dn coupled in series. Assuming that the barrier voltage of all the light-emitting diodes D1-Dn is equal to the ideal value Vb and the rectified AC voltage VAC varies between 0 and VMAX with time, a forward-bias voltage larger than n*Vb is required for turning on the luminescent device 10. Therefore, the energy between 0 and n*Vb can not be used. As the number of the light-emitting diodes D1-Dn increases, a higher forward-bias voltage is required for turning on the luminescent device 10, thereby reducing the effective operational voltage range of the LED lighting device 500; as the number of the light-emitting diodes D1-Dn decreases, the large driving current when VAC=VMAX may impact the reliability of the light-emitting diodes. Therefore, the prior art LED lighting device 500 needs to make compromise between the effective operational voltage range and the reliability. Meanwhile, the current-limiting resistor R also consumes extra power and may thus lower system efficiency.
FIG. 3 is a diagram of another prior art LED lighting device 600. The LED lighting device 600 includes a power supply circuit 110, an inductor L, a capacitor C, a switch SW, and a luminescent device 10. The power supply circuit 110 is configured to receive an AC voltage VS having positive and negative periods and convert the output of the AC voltage VS in the negative period using a bridge rectifier 112, thereby providing a rectified AC voltage VAC, whose value varies periodically with time, for driving the luminescent device 10. The inductor L and the switch SW are coupled in series with the luminescent device 10 for limiting its current ILED. The capacitor C is coupled in parallel with the luminescent device 10 for absorbing voltage ripples of the power supply circuit 110. For the same current-regulating function, the inductor L consumes less energy than the resistor R of the LED lighting device 500. However, the inductor L for regulating current and the capacitor for stabilizing voltage largely reduce the power factor of the LED lighting device 600 and the energy utilization ratio. Therefore, the prior art LED lighting device 600 needs to make compromise between the effective operational voltage range and the brightness.
SUMMARY OF THE INVENTION
An LED lighting device comprising a first luminescent device for providing light according to a first current; a second luminescent device coupled in series to the first luminescent device for providing light according to a second current; a two-terminal current controller coupled in parallel with the first luminescent device and in series to the second luminescent device and configured to regulate the second current according to a voltage established across the first luminescent device. When the voltage established across the first luminescent device does not exceed a first voltage during a rising period of a rectified AC voltage whose value varies periodically with time, the two-terminal current controller is turned on for maintaining the first current at substantially zero and regulating the second current according to the voltage established across the first luminescent device; when the voltage established across the first luminescent device is larger than the first voltage and does not exceed a second voltage during the rising period, the two-terminal current controller is turned on for maintaining the first current at substantially zero and setting the second current to a predetermined value larger than zero; when the voltage established across the first luminescent device is larger than the second voltage during the rising period, the two-terminal current controller is turned off for equalizing the first current and the second current.
The present invention further provides a two-terminal current controller for controlling a first current flowing through a load which is coupled in parallel with the two-terminal current controller. When a voltage established across the load does not exceed a first voltage during a rising period of a rectified AC voltage, the two-terminal current controller operates in a first mode for conducting a second current associated with the rectified AC voltage, thereby maintaining the first current at substantially zero and regulating the second current according to the voltage established across the load; when the voltage established across the load is larger than the first voltage and does not exceed a second voltage during the rising period, the two-terminal current controller operates in a second mode for conducting the second current, thereby maintaining the first current at substantially zero and setting the second current to a predetermined value larger than zero; when the voltage established across the load is larger than the second voltage during the rising period, the two-terminal current controller operates in a third mode in which the two-terminal current controller is turned off for maintaining the second current at substantially zero.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram illustrating the voltage-current chart of a light-emitting diode.
FIG. 2 is a diagram of a prior art LED lighting device.
FIG. 3 is a diagram of another prior art LED lighting device.
FIGS. 4, 7, 10 and 12 are diagram of LED lighting devices according to embodiments of the present invention.
FIGS. 5 and 8 are diagrams illustrating the current-voltage chart of a two-terminal current controller according to the present invention.
FIGS. 6, 9 and 11 are diagrams illustrating the variations in the related current and voltage when operating the LED lighting device of the present invention.
FIG. 13 is a diagram of an illustrated embodiment of the two-terminal current controller.



DETAILED DESCRIPTION
FIG. 4 is a diagram of an LED lighting device 100 according to a first embodiment of the present invention. The LED lighting device 100 includes a power supply circuit 110, a two-terminal current controller 120, and a luminescent device 10. The power supply circuit 110 is configured to receive an AC voltage VS having positive and negative periods and convert the output of the AC voltage VS in the negative period using a bridge rectifier 112, thereby providing a rectified AC voltage VAC, whose value varies periodically with time, for driving the luminescent device 10. The luminescent device 10 may adopt n light-emitting units D1-Dn coupled in series, each of which may include a single light-emitting diode or multiple light-emitting diodes. FIG. 4 depicts the embodiment using a single light-emitting diode in which ILED represents the current passing through the luminescent device 10 and VAK represents the voltage established across the luminescent device 10. The two-terminal current controller 120, coupled in parallel with the luminescent device 10 and the power supply circuit 110, is configured to control the current ILED passing through the luminescent device 10 according to the rectified AC voltage VAC, wherein IAK represents the current passing through the two-terminal current controller 120. In the first embodiment of the present invention, the barrier voltage Vb′ of the two-terminal current controller 120 is much smaller than the overall barrier voltage n*Vb of the luminescent device 10 (assuming the barrier voltage of each light-emitting unit is equal to Vb).
FIGS. 5 and 6 illustrate the operation of the LED lighting device 100, wherein FIG. 5 is a diagram illustrating the current-voltage chart of the two-terminal current controller 120, and FIG. 6 is a diagram illustrating the variations in the related current and voltage when operating the LED lighting device 100. In FIG. 5, the vertical axis represents the current IAK passing through the two-terminal current controller 120, and the horizontal axis represents the voltage VAK established across the two-terminal current controller 120. In the first embodiment of the present invention, the two-terminal current controller 120 operates in a first mode and functions as a voltage-controlled device when 0<VAK<VDROP. In other words, when the voltage VAK exceeds the barrier voltage Vb′ of the two-terminal current controller 120, the current IAK changes with the voltage VAK in a specific manner; the two-terminal current controller 120 operates in a second mode and functions as a constant current source when VDROP<VAK<VOFF—TH. In other words, the current IAK is maintained at a maximum current IMAX instead of changing with the voltage VAK; the two-terminal current controller 120 functions in a third mode and is turned off when VAK>VOFF—TH. In other words, the two-terminal current controller 120 functions as an open-circuited device since the current IAK is suddenly reduced to zero.
FIG. 6 illustrates the waveforms of the voltage VAK, the current IAK and the current ILED. Since the voltage VAK is associated with the rectified AC voltage VAC whose value varies periodically with time, a cycle between t0-t6 is used for illustration, wherein the period between t0-t3 is the rising period of the rectified AC voltage VAC and the period between t4-t6 is the falling period of the rectified AC voltage VAC. Between t0-t1 when the voltage VAK gradually increases, the two-terminal current controller 120 is first turned on, after which the current IAK increases with the voltage VAK in a specific manner and the current ILED is maintained at substantially zero. Between t1-t2 when the voltage VAK is larger than the voltage VDROP, the two-terminal current controller 120 is configured to limit the current IAK to the maximum current IMAX, and the current ILED remains substantially zero since the luminescent device 10 is still turned off. Between t2-t4 when the voltage VAK is larger than the voltage VOFF—TH, the two-terminal current controller 120 is turned off and the current associated with the rectified AC voltage VAC thus flows through the luminescent device 10. Therefore, the current IAK is reduced to zero, and the current ILED changes with the voltage VAK. Between t4-t5 when the voltage VAK drops to a value between the voltage VDROP and the voltage VOFF—TH, the two-terminal current controller 120 is turned on, thereby limiting the current IAK to the maximum current IMAX and maintaining the current ILED at substantially zero. Between t5-t6 when the voltage VAK drops below the voltage VDROP, the current IAK decreases with the voltage VAK in a specific manner.
FIG. 7 is a diagram of an LED lighting device 200 according to a second embodiment of the present invention. The LED lighting device 200 includes a power supply circuit 110, a two-terminal current controller 120, and a luminescent device 20. Having similar structures, the first and second embodiments of the present invention differ in the luminescent device 20 and how it is connected to the two-terminal current controller 120. In the second embodiment of the present invention, the luminescent device 20 includes two luminescent elements 21 and 25: the luminescent element 21 is coupled in parallel to the two-terminal current controller 120 and includes m light-emitting units D1-Dm coupled in series, wherein ILED—AK represents the current flowing through the luminescent element 21 and VAK represents the voltage established across the luminescent element 21; the luminescent element 25 is coupled in series to the two-terminal current controller 120 and includes n light-emitting units D1-Dn coupled in series, wherein ILED—AK represents the current flowing through the luminescent element 25 and VLED represents the voltage established across the luminescent element 25. Each light-emitting unit may include a single light-emitting diode or multiple light-emitting diodes. FIG. 7 depicts the embodiment using a single light-emitting diode.
The two-terminal current controller 120 is configured to control the current passing through the luminescent device 20 according to the rectified AC voltage VAC, wherein IAK represents the current passing through the two-terminal current controller 120 and VAK represents the voltage established across the two-terminal current controller 120. In the second embodiment of the present invention, the barrier voltage Vb′ of the two-terminal current controller 120 is far smaller than the overall barrier voltage m*Vb of the luminescent element 21 (assuming the barrier voltage of each luminescent element is equal to Vb).
FIGS. 8 and 9 illustrate the operation of the LED lighting device 200 according to the second embodiment of the present invention, wherein FIG. 8 is a diagram illustrating the current-voltage chart of the two-terminal current controller 120, and FIG. 9 is a diagram illustrating the variations in the related current and voltage when operating the LED lighting device 200. In FIG. 8, the vertical axis represents the current IAK passing through the two-terminal current controller 120, and the horizontal axis represents the voltage VAK established across the two-terminal current controller 120.
During the rising period of the rectified voltage VAC, the two-terminal current controller 120 operates in the first mode and functions as a voltage-controlled device when 0<VAK<VDROP. In other words, when the voltage VAK exceeds the barrier voltage Vb′ of the two-terminal current controller 120, the current IAK changes with the voltage VAK in a specific manner; the two-terminal current controller 120 operates in the second mode and functions as a constant current source when VDROP<VAK<VOFF—TH. In other words, the current IAK is maintained at a maximum current IMAX instead of changing with the voltage VAK; the two-terminal current controller 120 operates in the third mode and is turned off when VAK>VOFF—TH. In other words, the two-terminal current controller 120 functions as an open-circuited device since the current IAK is suddenly reduced to zero.
During the falling period of the rectified voltage VAC, the two-terminal current controller 120 is turned on and operates in the second mode for limiting the current IAK to the maximum current IMAX when VDROP<VAK<VON—TH; the two-terminal current controller 120 operates in the first mode and functions as a voltage-controlled device when 0<VAK<VDROP. In other words, when the voltage VAK exceeds the barrier voltage Vb′ of the two-terminal current controller 120, the current IAK changes with the voltage VAK in a specific manner.
FIG. 9 illustrates the waveforms of the voltage VAC, VAK, VLED and the current IAK, ILED—AK and ILED. Since the rectified AC voltage VAC varies periodically with time, a cycle between t0-t6 is used for illustration, wherein the period between t0-t3 is the rising period of the rectified AC voltage VAC and the period between t4-t6 is the falling period of the rectified AC voltage VAC. Between t0-t1, the voltage VAK established across the two-terminal current controller 120 and the voltage VLED established across the n serially-coupled light-emitting units D1-Dn increase with the rectified AC voltage VAC. Due to smaller barrier voltage, the two-terminal current controller 120 is first turned on, after which the current IAK and the current ILED increase with the voltage VAK in a specific manner and the current ILED—AK is maintained at substantially zero.
Between t1-t2 when the voltage VAK is larger than the voltage VDROP, the two-terminal current controller 120 is configured to limit the current IAK to the maximum current IMAX, and the current ILED remains substantially zero since the luminescent element 21 is still turned off. With VF representing the forward-bias voltage of each light-emitting unit in the luminescent element 25, the value of the voltage VLED may be represented by m*VF. Therefore, the luminescent element 21 is not conducting between t0-t2, and the rectified AC voltage VAC provided by the power supply circuit 110 is applied to the two-terminal current controller 120 and the n light-emitting units in the luminescent element 25, depicted as follows:

VAC=VAK+VLED  (1)

Between t2-t4 when the voltage VAK is larger than the voltage VOFF—TH the two-terminal current controller 120 is turned off and the current associated with the rectified AC voltage VAC thus passes through the luminescent elements 21 and 25. The current IAK is reduced to zero, and the current ILED—AK changes with the voltage VAK. Therefore, when the two-terminal current controller 120 is conducting between t2 and t4, the voltage VAK established across the two-terminal current controller 120 is supplied as the luminescent device 20 performs voltage dividing on the rectified AC voltage VAC, depicted as follows:




  
    
      
        
          V
          AK
        
        =
        
          
            m
            
              m
              +
              n
            
          
          ×
          
            V
            
              A
              ⁢
              
                
              
              ⁢
              C
            
          
        
      
    
    
      
        (
        2
        )
      
    
  




Between t4-t5 when the voltage VAK drops to a value between the voltage VDROP and the voltage VON—TH, the two-terminal current controller 120 is turned on, thereby limiting the current IAK to the maximum current IMAX and maintaining the current ILED—AK at substantially zero. Between t5-t6 when the voltage VAK drops below the voltage VDROP, the current IAK decreases with the voltage VAK in a specific manner. As depicted in FIGS. 7 and 9, the value of the current ILED is the sum of the current ILED—AK and the current IAK. The two-terminal current controller 120 according to the second embodiment of the present invention may increase the effective operational voltage range (such as the output of the rectified AC voltage VAC during t1-t2 and t4-t5), thereby increasing the power factor of the LED luminescence device 200.
In the second embodiment of the present invention, the moment when the two-terminal current controller 120 is switched on or switched off, the voltage VAK and the voltage VLED both encounter a sudden voltage drop ΔVd, which results in a current fluctuation ΔId. The voltage drop ΔVd may be represented as follows:

ΔVd=VON—TH−VOFF—TH  (3)

According to equation (1), prior to t2 at the time when the voltage VAK reaches the voltage VOFF—TH, the rectified AC voltage VAC may be represented as follows:

VAC=VOFF—TH+n*VF  (4)

According to equation (2), prior to t4 at the time when the voltage VAK reaches the voltage VON—TH, the rectified AC voltage VAC may be represented as follows:




  
    
      
        
          V
          AK
        
        =
        
          
            V
            
              ON
              ⁢
              _
              ⁢
              TH
            
          
          =
          
            
              m
              
                m
                +
                n
              
            
            ×
            
              V
              
                A
                ⁢
                
                  
                
                ⁢
                C
              
            
          
        
      
    
    
      
        (
        5
        )
      
    
  




Introducing equation (4) into equation (5) results in:




  
    
      
        
          V
          
            ON
            ⁢
            _
            ⁢
            TH
          
        
        =
        
          
            m
            
              m
              +
              n
            
          
          ×
          
            (
            
              
                V
                
                  OFF
                  ⁢
                  _
                  ⁢
                  TH
                
              
              +
              
                n
                ×
                
                  V
                  F
                
              
            
            )
          
        
      
    
    
      
        (
        6
        )
      
    
  




Introducing equation (6) into equation (3) results in:




  
    
      
        
          V
          d
        
        =
        
          
            
              
                m
                +
                n
              
              
                m
                +
                n
              
            
            ×
            
              V
              F
            
          
          -
          
            
              n
              
                m
                +
                n
              
            
            ×
            
              V
              
                OFF
                ,
                TH
              
            
          
        
      
    
    
      
        (
        7
        )
      
    
  




In actual applications, the value of the voltage VOFF—TH may be determined according to the maximum power dissipation PD—MAX and the maximum output current IMAX of the two-terminal current controller 120, depicted as follows:

PD—MAX=VOFF—TH*IMAX  (8)

According to equations (7) and (8), the voltage drop ΔVd may be adjusted by changing m and n. For example, for the same amount (m+n) of the light-emitting units in the luminescent device 20, the voltage drop ΔVd may be reduced by choosing a larger value of n, thereby providing a more stable driving current ILED.
FIG. 10 is a diagram of an LED lighting device 300 according to a third embodiment of the present invention. The LED lighting device 300 includes a power supply circuit 110, a plurality of two-terminal current controllers, and a luminescent device 30. Having similar structures, the third embodiment differs from the second embodiment in that the luminescent device 30 includes a plurality of two-terminal current controllers (FIG. 10 depicts 4 two-terminal current controllers 121-124) and luminescent device 30 includes a plurality of luminescent elements (FIG. 10 depicts 5 luminescent elements 21-25). The luminescent elements 21-24, respectively coupled in parallel with the corresponding two-terminal current controllers 121-124, each include a plurality of light-emitting units coupled in series, wherein ILED—AK1-ILED—AK4 respectively represent the currents flowing through the luminescent elements 21-24 and VAK1-VAK4 respectively represent the voltages established across the luminescent element elements 21-24. The luminescent element 25, coupled in series to the two-terminal current controllers 121-124, includes a plurality of light-emitting units coupled in series, wherein ILED represents the current flowing through the luminescent element 25 and VLED represents the voltage established across the luminescent element 25. Each light-emitting unit may include a single light-emitting diode or multiple light-emitting diodes, and FIG. 10 depicts the embodiment using a single light-emitting diode. In the embodiment shown in FIG. 10, the two-terminal current controllers 121-124 are configured to regulate the currents passing through the corresponding luminescent element elements 21-24 according to the voltages VAK1-VAK4 respectively, wherein IAK1-IAK4 respectively represent the currents flowing through the two-terminal current controllers 121-124 and VAK1-VAK4 respectively represent the voltages established across the two-terminal current controllers 121-124. In the third embodiment of the present invention, the barrier voltages of the two-terminal current controllers 121-124 are much smaller than the overall barrier voltages of the corresponding luminescent elements 21-24.
Reference may also be made to FIG. 8 for the current-voltage chart of each two-terminal current controller in the LED lighting device 300. The values of VDROP1-VDROP4, VOFF—TH1-VOFF—TH4 and VON—TH1-VON—TH4 may be determined according to the maximum power dissipation and the maximum output current of the two-terminal current controllers 121-124, as well as the characteristics and the amount of the light-emitting diodes in use. FIG. 11 is a diagram illustrating the operation of the LED lighting device 300 according to the third embodiment of the present invention. Since the rectified AC voltage VAC varies periodically with time, a cycle between t0-t10 is used for illustration, wherein the period between t0-t5 is the rising period of the rectified AC voltage VAC and the period between t5-t10 is the falling period of the rectified AC voltage VAC.
The operation of the LED lighting device 300 during the rising period t0-t5 is hereby explained. Between t0-t1 when the voltages VAK1-VAK4 increase with the rectified voltage VAC, the two-terminal current controllers 121-124 are turned on earlier due to smaller barrier voltages, and the current flows from the power supply circuit 110 to the luminescent element 25 sequentially via the two-terminal current controllers 121-124 (i.e., ILED=IAK1=IAK2=IAK3=IAK4 and ILED—AK1=ILED—AK2=ILED—AK3=ILED—AK4≈0). Between t1-t2 when the voltage VAK1 is larger than the voltage VOFF—TH1 the two-terminal current controller 121 is turned off first, and the current flows from the power supply circuit 110 to the luminescent element 25 sequentially via the luminescent element 21 and the two-terminal current controllers 122-124 (i.e., ILED=ILED—AK1=IAK2=IAK3=IAK4 and IAK1=ILED—AK2=ILED—AK3=ILED—AK4≈0). Between t2-t3 when the voltage VAK2 is larger than the voltage VOFF—TH2 the two-terminal current controller 122 is turned off next, and the current flows from the power supply circuit 110 to the luminescent element 25 sequentially via the luminescent element 21, the luminescent element 22 and the two-terminal current controllers 123-124 (i.e., ILED=ILED—AK1=ILED—AK2=IAK3=IAK4 and IAK1=IAK2=ILED—AK3=ILED—AK4≈0). Between t3-t4 when the voltage VAK3 is larger than the voltage VOFF—TH3, the two-terminal current controller 123 is turned off next, and the current flows from the power supply circuit 110 to the luminescent element 25 sequentially via the luminescent element 21, the luminescent element 22, the luminescent element 23 and the two-terminal current controller 124 (i.e., ILED=ILED—AK1=ILED—AK2=ILED—AK3=IAK4 and IAK1=IAK2=IAK3=ILED—AK4≈0). Between t4-t5 when the voltage VAK4 is larger than the voltage VOFF—TH4, the two-terminal current controller 124 is turned off next, and the current flows from the power supply circuit 110 to the luminescent element 25 sequentially via the luminescent elements 21-24 (i.e., ILED=ILED—AK1=ILED—AK2=ILED—AK3=ILED—AK4 and IAK1=IAK2=IAK3=IAK4≈0). During the falling period t5-t10, when the voltages VAK4-VAK1 sequentially drop below VON—TH4-VON—TH1, respectively, the two-terminal current controllers 124-121 are sequentially turned on at t6-t9, respectively. The operation of the LED lighting device 300 during the falling period t5-t10 is similar to that during the corresponding rising period t0-t6 as previously illustrated.
FIG. 12 is a diagram illustrating an LED lighting device 400 according to a fourth embodiment of the present invention. The LED lighting device 400 includes a power supply circuit 410, a two-terminal current controller 120, and a luminescent device 10. Having similar structures, the first and fourth embodiments of the present invention differ in the power supply circuits. In the first embodiment of the present invention, the power supply circuit 110 is configured to rectify the AC voltage VS (such as 110-220V main) using the bridge rectifier 112, thereby providing the rectified AC voltage VAC whose value varies periodically with time. In the fourth embodiment of the present invention, the power supply circuit 410 is configured to receive any AC voltage VS, perform voltage conversion using an AC-AC converter 412, and rectify the converted AC voltage VS using the bridge rectifier 112, thereby providing the rectified AC voltage VAC whose value varies periodically with time. References may be also be made to FIGS. 5 and 6 for illustrating the operation of the LED lighting device 400. Similarly, the second and third embodiments of the present invention may also use the power supply circuit 410 for providing the rectified AC voltage VAC.
FIG. 13 is a diagram of an illustrated embodiment of the two-terminal current controller 120. In this embodiment, the two-terminal current controller 120 includes a switch QN, a control circuit 50, a current-detecting circuit 60, and a voltage-detecting circuit 70. The switch QN may include a field effect transistor (FET), a bipolar junction transistor (BJT) or other devices having similar function. In FIG. 13, an N-type metal-oxide-semiconductor (NMOS) transistor is used for illustration. With the gate coupled to the control circuit 50 for receiving a turn-on voltage Vg, the drain-to-source voltage, the gate-to-source voltage and the threshold voltage of the switch QN are represented by VDS, VGS and VTH, respectively. When the switch QN operates in the linear region, its drain current is mainly determined by the drain-to-source voltage VDS; when the switch QN operates in the saturation region, its drain current is only related to the gate-to-source voltage VGS.
During the rising period of the rectified AC voltage VAC, the drain-to-source voltage VDS of the switch QN increases with the voltage VAK. When the voltage VAK does not exceed VDROP, the drain-to-source voltage VDS is smaller than the difference between the gate-to-source voltage VGS and the threshold voltage VTH (VDS<VGS−VTH). The turn-on voltage Vg from the control circuit 50 provides a bias condition VGS>VTH which allows the switch QN to operate in the linear region where the drain current is mainly determined by the drain-to-source voltage VDS. In other words, the two-terminal current controller 120 is configured to provide the current IAK and voltage VAK whose relationship corresponds to the I-V characteristic of the switch QN when operating in the linear region.
During the rising period of the rectified AC voltage VAC when the voltage VAK falls between VDROP and VOFF—TH, the drain-to-source voltage VDS is larger than the difference between the gate-to-source voltage VGS and the threshold voltage VTH (VDS>VGS−VTH). The turn-on voltage Vg from the control circuit 50 provides a bias condition VGS>VTH which allows the switch QN to operate in the saturation region where the drain current is only related to the gate-to-source voltage VGS and the current IAK no longer varies with the voltage VAK. In the present invention, the current-detecting circuit 60 is configured to detect the current flowing through the switch QN and determine whether the corresponding voltage VAK exceeds VDROP. In the embodiment depicted in FIG. 13, the current-detecting circuit 60 includes a resistor R and a comparator CP1. The resistor R is used for providing a feedback voltage VFB which is associated with the current passing the switch QN. The comparator CP1 is configured to output a corresponding control signal S1 to the control circuit 50 according to the relationship between the feedback voltage VFB and a reference voltage VREF. If VFB>VREF, the control circuit 50 maintains the gate-to-source voltage VGS to a predetermined value which is larger than the threshold voltage VTH, thereby limiting the current IAK to IMAX.
The voltage-detecting circuit 70 includes a logic circuit 72, a voltage edge-detecting circuit 74, and two comparators CP2 and CP3. The comparator CP2 is configured to determine the relationship between the voltages VAK and VON—TH, while the comparator CP3 is configured to determine the relationship between the voltages VAK and VOFF—TH. Meanwhile, when the voltages VAK is between VOFF—TH and VON—TH, the voltage edge-detecting circuit 74 is configured to determine whether the rectified AC voltage VAC is during the rising period or during the falling period. Based on the results of the voltage edge-detecting circuit 74 and the comparators CP2 and CP3, the logic circuit 72 outputs a corresponding control signal S2 to the control circuit 50. During the rising period of the rectified AC voltage VAC when the voltage VAK is between VOFF—TH and VON—TH, the control circuit 50 keeps the turn-on voltage Vg smaller than the threshold voltage VON—TH according to the control signal S2, thereby turning off the switch QN and maintaining the current IAK at zero. During the falling period of the rectified AC voltage VAC when the voltage VAK is between VON—TH and VOFF—TH, the control circuit 50 keeps the turn-on voltage Vg larger than the threshold voltage VGN—TH according to the control signal S2, thereby operating the switch QN in the saturation region and maintaining the current IAK at IMAX.
In the LED lighting devices 100, 200, 300 and 400 of the present invention, the number of the two-terminal current controllers 120-124, the number and configuration of the luminescent elements 21-25, and the type of the power supply circuits 110 and 410 may be determined according to different applications. FIGS. 4, 7, 10 and 12 are merely for illustrative purpose and do not limit the scope of the present invention. Also, the two-terminal current controller 120 depicted in FIG. 13 is an embodiment of the present invention and may be substituted by devices which are able to provide characteristics as shown in FIGS. 5, 6, 8, 9 and 11.
The LED lighting device of the present invention regulates the current flowing through the serially-coupled light-emitting diodes and controls the number of the turned-on light-emitting diodes using a two-terminal current controller. Some of the light-emitting diodes may be conducted before the rectified AC voltage reaches the overall barrier voltage of all light-emitting diodes for improving the power factor. Therefore, the present invention may provide lighting devices having large effective operational voltage range and high brightness.
Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention.

## Claims

1. A two-terminal current controller for controlling a first current flowing through a load which is coupled in parallel with the two-terminal current controller, wherein:
when a voltage established across the two-terminal current controller does not exceed a first voltage during a rising period of a rectified AC voltage, the two-terminal current controller operates in a first mode for conducting a second current associated with the rectified AC voltage, thereby maintaining the first current at substantially zero and regulating the second current according to the voltage established across the two-terminal current controller;
when the voltage established across the two-terminal current controller is larger than the first voltage and does not exceed a second voltage during the rising period, the two-terminal current controller operates in a second mode for conducting the second current, thereby maintaining the first current at substantially zero and setting the second current to a predetermined value larger than zero; and
when the voltage established across the two-terminal current controller is larger than the second voltage during the rising period, the two-terminal current controller operates in a third mode in which the two-terminal current controller is turned off for maintaining the second current at substantially zero.

2. The two-terminal current controller of claim 1, wherein the two-terminal current controller functions as a voltage-controlled device when operating in the first mode in which the voltage established across the two-terminal current controller does not exceed the first voltage, functions as a current source when operating in the second mode in which the voltage established across the two-terminal current controller is larger than the first voltage and does not exceed the second voltage, and functions as an open-circuited device when operating in the third mode in which the voltage established across the two-terminal current controller is larger than the second voltage.

3. The two-terminal current controller of claim 1, wherein when the voltage established across the two-terminal current controller is larger than the first voltage and does not exceed a third voltage during a falling period of the rectified AC voltage, the two-terminal current controller operates in the second mode for maintaining the first current at substantially zero and setting the second current to the predetermined value, and the third voltage is larger than the second voltage.

4. The two-terminal current controller of claim 3 further comprising:
a switch configured to conduct the second current according to a turn-on voltage;
a control circuit configured to provide the turn-on voltage according to a first control signal and a second control signal;
a current-detecting circuit configured to determine whether the voltage established across the two-terminal current controller is larger than the first voltage according to the second current, thereby providing the first control signal accordingly; and
a voltage-detecting circuit configured to determine relationships between the voltage established across the two-terminal current controller and the second voltage, thereby providing the second control signal accordingly.

5. The two-terminal current controller of claim 4, wherein:
when the current-detecting circuit determines that the voltage established across the two-terminal current controller does not exceed the first voltage, the switch regulates the second current according to the turn-on voltage; and
when the current-detecting circuit determines that the voltage established across the two-terminal current controller is larger than the first voltage, the switch limits the second current to the predetermined value according to the turn-on voltage.

6. The two-terminal current controller of claim 4, wherein:
when the voltage-detecting circuit determines that the voltage established across the two-terminal current controller is larger than the first voltage and does not exceed the second voltage during the rising period, the switch limits the second current to the predetermined value according to the turn-on voltage and maintains the first current at substantially zero; and
when the voltage-detecting circuit determines that the voltage established across the two-terminal current controller is larger than the first voltage and does not exceed the third voltage which is larger than the second voltage during the falling period, the switch limits the second current to the predetermined value according to the turn-on voltage and maintains the first current at substantially zero.