Patent Publication Number: US-9433046-B2

Title: Driving circuitry for LED lighting with reduced total harmonic distortion

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims the benefit of priority from U.S. Provisional Patent Application Ser. No. 61/435,258, entitled “CURRENT CONDITIONER WITH REDUCED TOTAL HARMONIC DISTORTION” and filed on Jan. 21, 2011, which is hereby incorporated by reference in its entirety for all purposes. 
    
    
     BACKGROUND 
     Lighting circuits that use light emitting diodes (LEDs) to produce illumination typically have higher energy efficiency and longer service life than equivalent incandescent bulbs, fluorescent lamps, or other lighting sources. 
     LEDs, however, conduct current in only one direction, and therefore use direct current (DC) to function. In order to function efficiently when powered by an alternating current (AC) power source, a LED-based lighting circuits includes a rectifier circuit to convert a sinusoidal AC input power signal into a half-wave or a full-wave rectified DC power signal. The rectified sinusoidal signal has a variable value that follows a sinusoidal envelope. Because LEDs (and LED lighting circuits) have a threshold voltage below which the LEDs are powered off and neither conduct current or emit light, a LED (or LED lighting circuit) powered by a rectified sinusoidal signal will in general repeatedly turn on and off depending on whether the instantaneous value of the rectified sinusoidal signal exceeds or not the threshold voltage of the LED. 
     In order to make efficient use of the input power, LED lighting circuits can be designed such that different numbers of LEDs are powered at different times during each cycle. In general, the lighting circuit includes a voltage sensing circuit, for measuring the instantaneous value of the rectified sinusoidal signal, and a microprocessor for determining which LEDs should be powered based on the measured value of the rectified sinusoidal signal. The microprocessor controls a set of digital switches for selectively activating various combinations of LEDs based on the microprocessor&#39;s control. For example, the microprocessor may activate a first set of LEDs at the beginning and end of a cycle, when the instantaneous value of the rectified sinusoidal signal is low, and the microprocessor may activate a series connection of two or more sets of LEDs in the middle of the cycle, when the instantaneous value of the rectified sinusoidal signal is high. 
     The activation and deactivation of the sets of LEDs by the digital switches, however, causes elevated levels of harmonic distortion in the LED lighting circuit and the power lines providing the AC driving signal. In addition, the driving of non-linear LED loads causes power factor distortion in the LED lighting circuit and the power lines providing the AC driving signal. The harmonic and power factor distortions both contribute to decreases in the total efficiency of the LED lighting, as the distortion causes harmonic currents to travel through the power lines providing the AC driving signal. 
     A need therefore exists for driving circuitry for LED lighting applications which produces minimal total harmonic distortion. 
     SUMMARY 
     In one aspect, a conditioning circuit for driving two or more LED groups using a rectified AC input voltage is provided. The circuit includes a first series interconnection of a first light-emitting diode (LED) group, a first transistor, and a first resistor, and a second series interconnection of a second LED group, a second transistor, and a second resistor. The second series interconnection is connected between a drain terminal and a source terminal of the first transistor, and the first and second LED groups are selectively activated by a variable voltage applied across the first series interconnection. The first resistor is coupled between the source terminal and a gate terminal of the first transistor. As a result, the first transistor transitions from a conducting state to a non-conducting state when the variable voltage exceeds a first threshold. In addition, the first and second LED groups have respective threshold voltages, such that the first LED group is activated when the variable voltage exceeds the threshold voltage of the first LED group, and the second LED group is activated when the variable voltage exceeds the sum of the threshold voltages of the first and second LED groups. 
     In another aspect, a second conditioning circuit for driving two or more LED groups using a rectified AC input voltage is provided. The second circuit includes the first and second series interconnections of a LED group, a transistor, and a resistor. In the second circuit, however, the second series interconnection is connected between an anode of the first LED group and a source terminal of the first transistor, and the first and second LED groups are selectively activated by a variable voltage applied across the first series interconnection. The first and second LED groups have respective threshold voltages, such that the first LED group is activated when the variable voltage exceeds the threshold voltage of the first LED group and does not exceed a first threshold at which the first transistor transitions into a non-conducting state, and the second LED group is activated when the variable voltage exceeds the threshold voltage of the second LED groups. 
     It is understood that various configurations of the subject technology will become readily apparent to those skilled in the art from the disclosure, wherein various configurations of the subject technology are shown and described by way of illustration. As will be realized, the subject technology is capable of other and different configurations and its several details are capable of modification in various other respects, all without departing from the scope of the subject technology. Accordingly, the summary, drawings and detailed description are to be regarded as illustrative in nature and not as restrictive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawing figures depict one or more implementations in accord with the present teachings, by way of example only, not by way of limitation. In the figures, like reference numerals refer to the same or similar elements. 
         FIG. 1A  is a schematic diagram showing a conditioning circuit for driving two LED groups using a rectified AC input voltage. 
         FIGS. 1B, 1C, and 1D  respectively are a first voltage timing diagram, a current timing diagram, and a second voltage timing diagram illustratively showing the operation of the conditioning circuit of  FIG. 1A . 
         FIGS. 2A, 2B, 2C, and 2D  are schematic diagrams showing various examples of interconnections of LEDs and of LED groups for use in the conditioning circuit of  FIG. 1A . 
         FIG. 3A  is a schematic diagram showing a modified conditioning circuit for driving two LED groups using a rectified AC input voltage. 
         FIG. 3B  is a current timing diagram illustratively showing the operation of the conditioning circuit of  FIG. 3A . 
         FIG. 4A  is a schematic diagram showing a modified conditioning circuit for driving three LED groups using a rectified AC input voltage. 
         FIG. 4B  is a current timing diagram illustratively showing the operation of the conditioning circuit of  FIG. 4A . 
         FIG. 5A  is a schematic diagram showing a modified conditioning circuit for driving two LED groups using a rectified AC input voltage. 
         FIGS. 5B and 5C  are a current timing diagram and a lighting intensity diagram illustratively showing the operation of the conditioning circuit of  FIG. 5A . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In the following detailed description, numerous specific details are set forth by way of examples in order to provide a thorough understanding of the relevant teachings. However, it should be apparent to those skilled in the art that the present teachings may be practiced without such details. In other instances, well known methods, procedures, components, and/or circuitry have been described at a relatively high-level, without detail, in order to avoid unnecessarily obscuring aspects of the present teachings. 
     Driving circuitry for powering light emitting diode (LED) lights generally rely on digital circuitry to measure the instantaneous value of a driving voltage, on a microprocessor to identify LEDs to activate based on the measured value, and on digital switches to selectively activate the identified LEDs. The digital circuitry, however, reduces the overall efficiency of the LED lighting by causing harmonic distortion and power factor distortion in the LED light and the associated power line. In order to reduce the harmonic distortion and power factor distortion caused by the digital circuitry, a current conditioning circuit is presented for selectively routing current to various LED groups in a LED light. The current conditioning circuit uses analog components and circuitry for operation, and produces minimal harmonic distortion and power factor distortion. 
     The current conditioning circuitry is provided to selectively route current to different LED groups depending on the instantaneous value of an AC input voltage. In a preferred embodiment, the conditioning circuitry includes only analog circuit components and does not include digital components or digital switches for operation. 
     The circuitry relies on depletion-mode metal-oxide-semiconductor field-effect transistor (MOSFET) transistors for operation. In a preferred embodiment, the depletion MOSFET transistors have a high resistance between their drain and source terminals, and switch between conducting and non-conducting states relatively slowly. The depletion-mode MOSFET transistors may conduct current between their drain and source terminals when a voltage V GS  between the gate and source terminals is zero or positive and the MOSFET transistor is operating in the saturation (or active, or conducting) mode (or region, or state). The current through the depletion-mode MOSFET transistor, however, may be restricted if a negative V GS  voltage is applied to the terminals and the MOSFET transistor enters the cutoff (or non-conducting) mode (or region, or state). The MOSFET transistor transitions between the saturation and cutoff modes by operating in the linear or ohmic mode or region, in which the amount of current flowing through the transistor (between the drain and source terminals) is dependent on the voltage between the gate and source terminals V GS . In one example, the depletion MOSFET transistors preferably have an elevated resistance between drain and source (when operating in the linear mode) such that the transistors switch between the saturation and cutoff modes relatively slowly. The depletion MOSFET transistors switch between the saturation and cutoff modes by operating in the linear or ohmic region, thereby providing a smooth and gradual transition between the saturation and cutoff modes. In one example, a depletion-mode MOSFET transistor may have a threshold voltage of −2.6 volts, such that the depletion-mode MOSFET transistor allows substantially no current to pass between the drain and source terminals when the gate-source voltage V GS  is below −2.6 volts. Other values of threshold voltages may alternatively be used. 
       FIG. 1A  is a schematic diagram showing a conditioning circuit  100  for driving two LED groups using a rectified AC input voltage. The conditioning circuit  100  uses analog circuitry to selectively route current to one or both of the LED groups based on the instantaneous value of the AC input voltage. 
     The conditioning circuit  100  receives an AC input voltage from an AC voltage source  101 , such as a power supply, an AC line voltage, or the like. The AC voltage source  101  is coupled in series with a fuse  103 , and the series interconnection of the AC voltage source  101  and the fuse  103  is coupled in parallel with a transient voltage suppressor (TVS)  105  or other surge protection circuitry. The series interconnection of the AC voltage source  101  and the fuse  103  is further coupled in parallel with two input terminals of a voltage rectifier  107 . In one example, the voltage rectifier  107  can include a diode bridge rectifier that provides full-wave rectification of an input sinusoidal AC voltage waveform. In other examples, other types of voltage rectification circuitry can be used. 
     Voltage rectifier  107  functions as a source of variable DC voltage, and produces a rectified voltage V rect  between its two output terminals V +  and V − . The rectified voltage V rect  corresponds to a rectified version of the AC driving voltage. In general, the rectified voltage V rect  is a full-wave rectified DC voltage. The rectified voltage V rect  is used as the input DC voltage for driving the LED groups  109  and  111  of the conditioning circuit  100 . In particular, the rectified voltage V rect  is used as an input voltage for driving two series interconnections of an LED group, a transistor, and a resistor. 
     A first series interconnection of a first LED group  109 , a first n-channel depletion MOSFET transistor  113  (coupled by the drain and source terminals), and a first resistor  117  is coupled between the output terminals V +  and V −  of the voltage rectifier  107 . The first LED group  109  has its anode coupled to the terminal V +  (node n 1 ), and its cathode coupled to the drain terminal of first depletion MOSFET transistor  113  (node n 2 ). The source terminal of transistor  113  is coupled to a first terminal of resistor  117  (node n 3 ), while both the gate terminal of transistor  113  and the second terminal of resistor  117  are coupled to the terminal V −  (node n 4 ) of the voltage rectifier  107 , such that the voltage across the first resistor  117  serves as the biasing voltage V GS  between the gate and source terminals of the first transistor  113 . 
     A second series interconnection of a second LED group  111 , a second re-channel depletion MOSFET transistor  115  (coupled by the drain and source terminals), and a second resistor  119  is coupled between the drain and source terminals of the first transistor  113 . In particular, the anode of second LED group  111  is coupled to node n 2 , while the cathode of the second LED group  111  is coupled at node n 5  to the drain terminal of the second transistor  115 . The source terminal of the second transistor  115  is coupled to a first terminal of the second resistor  119  at node n 6 , while both the gate terminal of the second transistor  115  and the second terminal of the second resistor  119  are coupled to node n 3  and the source terminal of the first transistor  113 . The voltage across the second resistor  119  thereby serves as the biasing voltage V GS  between the gate and source terminals of the second transistor  115 . 
     Each of the first and second LED groups  109  and  111  has a forward voltage (or threshold voltage). The forward voltage generally is a minimum voltage required across the LED group in order for current to flow through the LED group, and/or for light to be emitted by the LED group. The first and second LED groups  109  and  111  may have the same forward voltage (e.g., 50 volts), or the first and second LED groups  109  and  111  may have different forward voltages (e.g., 60 volts and 40 volts, respectively). 
     In operation, in the driving circuitry  100  of  FIG. 1A , one or both of the LED groups  109  and  111  may conduct current depending on whether the forward voltage of one or both of the LED groups  109  and  111  is satisfied. The operation of the LED driving circuitry  100  of  FIG. 1A  will be explained with reference to the voltage timing diagram of  FIG. 1B . 
       FIG. 1B  is a voltage timing diagram showing the rectified voltage V rect  during one cycle. The rectified voltage V rect  may be applied at the output of voltage rectifier  107  to the LED groups  109  and  111 , as shown in driving circuitry  100  of  FIG. 1A . 
     The exemplary cycle of the rectified voltage V rect  shown in  FIG. 1B  begins at time t 0  with the rectified voltage V rect  having a value of 0V (0 volts). The rectified voltage V rect  undergoes a half-sine cycle between times t 0  and t 5 . Between times t 0  and t 1 , the value of the rectified voltage V rect  remains below the forward voltage of the first LED group  109 , and no current flows through the first LED group  109 . As the rectified voltage V rect  reaches a value of V 1 , the forward voltage of the first LED group  109  is reached and current gradually begins to flow through the first LED group  109 . At this time, the first depletion MOSFET transistor  113  is in a conducting state such that the current flowing from the rectifier  107  through the first LED group  109  flows through the MOSFET transistor  113  (from drain to source terminals) and the first resistor  117 . 
     As the rectified voltage V rect  increases in value from V 1  to V 2 , the value of the current flowing through the first LED group  109 , the first depletion MOSFET transistor  113 , and the first resistor  117  increases. The increase in current through the first resistor  117  causes the voltage across the first resistor  117  to increase, and the corresponding reverse voltage between the gate and source terminals of the first depletion MOSFET transistor  113  to increase. As the reverse gate-source voltage increases, however, the first depletion MOSFET transistor  113  begins to transition out of saturation and into the “linear” or “ohmic” mode or region of operation. The first depletion MOSFET transistor  113  may thus begin to shut down and to conduct less current as the value of the rectified voltage V rect  reaches the value V 2 . 
     Meanwhile, as the rectified voltage V rect  reaches the value V 2  (at time t 2 ), the rectified voltage V rect  is reaching or exceeding the sum of the forward voltage of the first and second LED groups  109  and  111 . As a result, the second LED group  111  begins to conduct current, and the current flowing through the first LED group  109  begins to flow through the series interconnection of the second LED group  111 , the second depletion MOSFET transistor  115 , and the second and first resistors  119  and  117 . As V rect  exceeds V 2  and the first depletion MOSFET transistor  109  enters the cutoff mode, most or all of the current flowing through the first LED group  109  flows through the second LED group  111 . 
     Thus, during the first half of the cycle, no current initially flows through either of the first and second LED groups  109  and  111  (period [t 0 , t 1 ]). However, as the value of V rect  reaches or exceeds V 1 , current begins to flow through the first LED group  109  which starts to emit light (period [t 1 , t 2 ]) while the second LED group  111  remains off. Finally, as the value of V rect  reaches or exceeds V 2 , current begins to flow through both the first and second LED groups  109  and  111  which both emit light (period after t 2 ). 
     During the second half of the cycle, the rectified voltage V rect  decreases from a maximum of V max  back to 0 volts. During this period, the second and first LED groups  111  and  109  are sequentially turned off and gradually stop conducting current. In particular, while the value of V rect  remains above V 2 , both the first and second LED groups  109  and  111  remain in the conducting state. However, as the value of V rect  reaches or dips below V 2  (at time t 3 ), V rect  no longer reaches or exceeds the sum of the forward voltage of the first and second LED groups  109  and  111 , and the second LED group  111  begins to turn off and to stop conducting current. At around the same time, the voltage drop across the first resistor drops below the threshold voltage of the first depletion MOSFET transistor  109 , and the first depletion MOSFET transistor  109  enter the linear or ohmic operation mode and begins to conduct current once again. As a result, current flows through the first LED group  109 , the first depletion MOSFET transistor  109 , and the first resistor  117 , and the first LED group  109  thus continues to emit light. As the value of V rect  reaches or dips below V 1  (at time t 4 ), however, V rect  no longer reaches or exceeds the forward voltage of the first LED group  109 , and the first LED group  109  begins to turn off and stop conducting current. As a result, both the first and second LED groups  109  and  111  turn off and stop emitting light during the period [t 4 , t 5 ]. 
       FIG. 1C  is a current timing diagram showing the currents I G1  and I G2  respectively flowing through the first and second LED groups  109  and  111  during one cycle of the rectified voltage V rect . 
     As described in relation to  FIG. 1B , the current I G1  through the first LED group  109  begins flowing around time t 1 , and increases to a first value I 1 . The current I G1  continues to flow through the first LED group  109  from around time t 1  to around time t 4 . Between times t 2  and t 3 , the current I G2  flows through the second LED group  111 , and reaches a second value I 2 . During the time period [t 2 , t 3 ], the current I G1  increases to the value I 2 . 
     In general, electrical parameters of the components of driving circuit  100  can be selected to adjust the functioning of the circuit  100 . For example, the forward voltages of the first and second LED groups  109  and  111  may determine the value of the voltages V 1  and V 2  at which the first and second LED groups are activated. In particular, the voltage V 1  may be substantially equal to the forward voltage of the first LED group, while the voltage V 2  may be substantially equal to the sum of the forward voltages of the first and second LED groups. In one example, the forward voltage of the first LED group may be set to a value of 60V, for example, while the forward voltage of the second LED group may be set to a value of 40V, such that the voltage V 1  is approximately equal to 60V and the voltage V 2  is approximately equal to 100V. In addition, the value of the first resistor  117  may be set such that the first depletion MOSFET transistor  113  enters a non-conducting state when the voltage V rect  reaches a value of V 2 . As such the value of the first resistor  117  may be set based on the threshold voltage of the first depletion MOSFET transistor  113 , the drain-source resistance of the first depletion MOSFET transistor, and the voltages V 1  and V 2 . In one example, the first resistor may have a value of around 31.6 ohms. 
     The conditioning circuitry  100  of  FIG. 1A  can be used to provide dimmable lighting using the first and second LED groups  109  and  111 . The conditioning circuitry can, in particular, provide a variable lighting intensity based on the amplitude of the rectified driving voltage V rect .  FIG. 1D  is a voltage timing diagram showing the effects of a reduced driving voltage amplitude on the LED lighting circuitry  100 . 
     As shown in  FIG. 1D , the amplitude of the driving voltage V rect  has been reduced from a value of V max  to a value of V max ′ at  151 . The amplitude of the driving voltage V rect  may have been reduced through the activation of a potentiometer, a dimmer switch, or other appropriate means. While the amplitude of the driving voltage is reduced, the threshold voltages V 1  and V 2  remain constant as the threshold voltages are set by parameters of the components of the circuit  100 . 
     Because the driving voltage V rect  has a lower amplitude, the driving voltage takes a time [t 0 , t 1 ′] to reach the first threshold voltage V 1  during the first half of each cycle that is longer than the time [t 0 , t 1 ]. Similarly, the driving voltage takes a time [t 0 , t 2 ′] to reach the second threshold voltage V 2  that is longer than the time [t 0 , t 2 ]. Additionally, the lower-amplitude driving voltage reaches the second threshold sooner (at a time t 3 ′, which occurs sooner than the time t 3 ) during the second half of each cycle, and similarly reaches the first threshold sooner (at a time t 4 ′, which occurs sooner than the time t 4 ), during the second half of each cycle. As a result, the time-period [t 1 ′, t 4 ′] during which current flows through the first LED group  109  is substantially reduced with respect to the corresponding time-period [t 1 ] when the input voltage has full amplitude. Similarly, the time-period [t 2 ′, t 3 ′] during which current flows through the second LED group  111  is substantially reduced with respect to the corresponding time-period [t 2 , t 3 ] when the input voltage has full amplitude. Because the lighting intensity produced by each of the first and second LED groups  109  and  111  is dependent on the total amount of current flowing through the LED groups, the shortening of the time-periods during which current flows through each of the LED groups causes the lighting intensity produced by each of the LED groups to be reduced. 
     In addition to providing dimmable lighting, the conditioning circuitry  100  of  FIG. 1A  can be used to provide color-dependent dimmable lighting. In order to provide color-dependent dimmable lighting, the first and second LED groups may include LEDs of different colors, or different combinations of LEDs having different colors. When a full amplitude voltage V rect  is provided, the light output of the conditioning circuitry  100  is provided by both the first and second LED groups, and the color of the light output is determined based on the relative light intensity and the respective color light provided by each of the LED groups. As the amplitude of the voltage V rect  is reduced, however, the light intensity provided by the second LED group will be reduced more rapidly than the light intensity provided by the first LED group. As a result, the light output of the conditioning circuitry  100  will gradually be dominated by the light output (and the color of light) produced by the first LED group. 
     The conditioning circuitry  100  shown in  FIG. 1A  includes first and second LED groups  109  and  111 . Each LED group can be formed of one or more LEDs, or of one or more high-voltage LEDs. In examples in which a LED group includes two or more LEDs (or two or more high-voltage LEDs), the LEDs may be coupled in series and/or in parallel. 
       FIGS. 2A and 2B  show examples of interconnections of LEDs that may be used as LED groups  109  and  111 . In the example of  FIG. 2A , an exemplary LED group (coupled between nodes n 1  and n 2 , such as LED group  109  of  FIG. 1A ) is formed of four sub-groups of LEDs coupled in series, where each sub-group is a parallel interconnection of three LEDs. In the example of  FIG. 2B , an exemplary LED group (coupled between nodes n 2  and n 5 , such as LED group  111  of  FIG. 1A ) is formed of three sub-groups of LEDs coupled in series, where each sub-group is a parallel interconnection of two LEDs. 
     Various other interconnections of LEDs may be used. In another example, a first LED group may be formed of 22 sub-groups of LEDs coupled in series where each sub-group is a parallel interconnection of three LEDs, while a second LED group may be formed of 25 sub-groups of LEDs coupled in series where each sub-group is a parallel interconnection of two LEDs. The LEDs in a single group may be wire bonded to a single semiconductor die, or to multiple interconnected semiconductor dies. 
     In general, the structure of a LED group can be selected so as to provide the LED group with particular electrical parameters. For example, the threshold voltage of the LED group can be increased by coupling more LED sub-groups in series, while the maximum power (or maximum current) rating of the LED group can be increased by coupling more LEDs in parallel within each sub-group. As such, a LED group can be designed to have particular electric parameters, such as having a threshold voltage of 40 V, 50 V, 60 V, 70 V, 120 V, or other appropriate voltage level. Similarly, a LED group can be designed to have a particular power rating, such as a power rating of 2, 7, 12.5, or 16 watts. 
     Each LED group may further be formed of LEDs emitting light of the same or of different colors. For example, a LED group only including LEDs emitting a red light may emit a substantially red light, while a LED group including a mixture of LEDs emitting red light and white light may emit a reddish light. 
     As shown in the exemplary current timing of  FIG. 1C , the maximum amplitude of the currents I G1  and I G2  through the first and second LED groups  109  and  111  is approximately the same. However, because the first LED group  109  conducts current for a longer period of time, the total power output by the first LED group  109  is generally higher than the total power output by the second LED group  111 . In order to avoid over-driving the first LED group  109 , the first and second LED groups  109  and  111  can include different interconnections of LEDs, as described in relation to  FIGS. 2A and 2B  above. In one example, the first LED group  109  may include more LEDs coupled in parallel than the second LED group  111 , so as to reduce the maximum amplitude of current flowing through each LED of the first LED group  109  and thereby reduce the chances of over-driving the first LED group  109 . 
     Alternatively, different numbers of LED groups may be used in the conditioning circuitry  100 .  FIGS. 2C and 2D  show two examples in which conditioning circuitry  100  has been modified to include various numbers of LED groups. 
     For example,  FIG. 2C  shows conditioning circuitry  200  which is substantially similar to the conditioning circuitry  100 . However, in the conditioning circuitry  200  of  FIG. 2C , the first LED lighting group has been replaced by a parallel interconnection of two LED groups  109   a  and  109   b . By providing two LED groups  109   a  and  109   b  coupled in parallel, one-half of the current I G1  will flow through each of the LED groups  109   a  and  109   b . The parallel interconnection of the two LED groups  109   a  and  109   b  can thus reduce the total current flowing through each LED group, and reduce the total power output by each LED group. The parallel interconnection may thus minimize the chances that either of the LED groups  109   a  and  109   b  will suffer from over-driving. 
       FIG. 2D  shows another exemplary conditioning circuit  250  which is substantially similar to conditioning circuit  100 . However, in conditioning circuit  250 , the first LED lighting group has been replaced by a parallel interconnection of three LED groups  109   c ,  109   d , and  109   e . Additionally, the second LED lighting group  111  has been replaced by a parallel interconnection of two LED groups  111   a  and  111   b . As described in relation to  FIG. 2C , the parallel interconnection of two or more LED groups in parallel may reduce the total current flowing through each LED group, and reduce the chances that any LED group will suffer from over-driving. 
       FIG. 3A  shows a schematic diagram of a modified conditioning circuit  300  for driving two LED groups using a rectified AC input voltage. The modified conditioning circuit  300  is substantially similar to the conditioning circuit  100  of  FIG. 1A . However, modified circuit  300  does not include the second depletion MOSFET transistor  115  of circuit  100 . Instead, the cathode of the second LED group  111  is coupled directly to the second resistor  119 . 
     The circuit  300  functions substantially similarly to circuit  100 . As described in relation to  FIGS. 1B and 1C , the first LED group  109  of circuit  300  will conduct current during a first time-period [t 1 , t 4 ], while the second LED group  111  of circuit  300  will conduct current during second time-period [t 2 , t 3 ]. However, because the circuit  300  does not include the depletion MOSFET transistor  115 , the peak current flowing through the first and second LED groups during the time-period [t 2 , t 3 ] is not limited by the conductance of the depletion MOSFET transistor  115 . As a result, the current flowing through the first and second LED groups in circuit  300  may peak with a higher value than in the circuit  100 . The circuit  300  may, however, have lower lighting efficiency than the circuit  100  because more power is dissipated by the second resistor  119 . 
       FIG. 3B  is a current timing showing the currents I G1  and I G2  respectively flowing through the first and second LED groups  109  and  111  of circuit  300  during one cycle. As shown in  FIG. 3B , the current flows through circuit  300  are generally similar to the current flows through circuit  100  and shown in  FIG. 1C . However, the peak amplitudes reached by the currents I G1  and I G2  in circuit  300  (as shown in  FIG. 3B ) are higher than the peak amplitudes reached in circuit  100  (as shown in  FIG. 1C ). 
       FIG. 4A  shows a schematic diagram of a modified circuit  400  for driving three LED groups using a rectified AC input voltage. The modified circuit  400  is substantially similar to the conditioning circuit  100  of  FIG. 1A . However, modified circuit  400  includes a series interconnection of a third LED group  112 , a third depletion MOSFET transistor  116 , and a third resistor  120  coupled between the cathode of the second LED group  111  and the source of the second depletion MOSFET transistor  115 . 
     The modified circuit  400  functions similarly to LED lighting circuit  100 . However, the modified circuit  400  selectively routes current to zero, one, two, or all three of the LED groups depending on the instantaneous value of the rectified driving voltage V rect . The modified circuit  400  may have three voltage thresholds V 1 , V 2 , and V 3  at which different LED groups are activated. In particular, the first LED group  109  may be activated for a period [t 1 , t 4 ] during which the driving voltage V rect  exceeds the first voltage threshold V 1 , the second LED group  111  may be activated for a period [t 2 , t 3 ] during which the driving voltage V rect  exceeds the second voltage threshold V 2 , and the third LED group  112  may be activated for a period [t 21 , t 22 ] during which the driving voltage V rect  exceeds the third voltage threshold V 3 . The voltage thresholds may be such that V 1 &lt;V 2 &lt;V 3 , and the time-periods may be such that [t 21 , t 22 ] forms part of [t 2 , t 3 ], and such that [t 2 , t 3 ] forms part of [t 1 , t 4 ]. 
       FIG. 4B  is a current timing diagram showing the currents I GI , I G2 , and I G3  respectively flowing through the first, second, and third LED groups  109 ,  111 , and  112  during one cycle of operation of the circuit  400 . As shown in  FIG. 4B , the first and second LED groups function substantially similarly to those shown in  FIG. 1C . In particular, according to the timing diagram of  FIG. 4B , a current I G1  flows through the first LED group  109  during the period [t 1 , t 4 ], while a current I G2  flows through the second LED group  111  during the period [t 2 , t 3 ]. However, in the circuit  400 , the current I G3  additionally flows through the third LED group  112  during the period [t 21 , t 22 ]. 
     In circuit  400 , electrical parameters of the components can be selected to adjust the functioning of the circuit  100 . For example, the voltage V 1  may be substantially equal to the forward voltage of the first LED group, while the voltage V 2  may be substantially equal to the sum of the forward voltages of the first and second LED groups and the voltage V 3  may be substantially equal to the sum of the forward voltages of the first, second, and third LED groups. In one example, the forward voltage of the first LED group may be set to a value of 40V, for example, while the forward voltages of the second and third LED group may be set to values of 30V each, such that the voltages V 1 , V 2 , and V 3  are respectively approximately equal to 40V, 70V, and 100V. In addition, the value of the first resistor  117  may be set such that the first depletion MOSFET transistor  113  enters a non-conducting state when the voltage V rect  reaches a value of V 2 , and the value of the second resistor  119  may be set such that the second depletion MOSFET transistor  115  enters a non-conducting state when the voltage V rect  reaches a value of V 3 . 
     While LED lighting circuits have been presented that selectively drive two LED groups  109  and  111  (see  FIG. 1A , circuit  100 ) and that selectively drive three LED groups  109 ,  111 , and  112  (see  FIG. 4A , circuit  400 ), the teachings contained herein can more generally be used to design circuits that drive four or more LED groups. For example, a circuit driving four LED groups may be substantially similar to circuit  400 , but may include an additional series interconnection of a fourth LED group, a fourth depletion MOSFET transistor, and a fourth resistor coupled between the cathode of the third LED group  112  and the source of the third depletion MOSFET transistor  116 . Similarly, a circuit driving five LED groups may be substantially similar to the circuit driving four LED groups, but may include an additional interconnection of a fifth LED group, a fifth depletion MOSFET transistor, and a fifth resistor coupled between the cathode of the fourth LED group and the source of the fourth depletion MOSFET transistor. 
       FIG. 5A  shows a schematic diagram of a modified circuit  500  for driving two LED groups using a rectified AC input voltage. The modified circuit  500  is similar to the conditioning circuit  100  of  FIG. 1A . However, in modified circuit  500 , the first and second LED groups  509  and  511  are coupled in parallel and may therefore be substantially alternately provided with a driving current (instead of being substantially concurrently provided with a driving current, as in circuit  100 ). 
     In particular, in circuit  500 , the first series interconnection of the first LED group  509 , the first depletion MOSFET transistor  513  (coupled by the drain and source terminals), and the first resistor  517  is coupled between the output nodes V +  and V −  of the voltage rectifier  107 . The gate terminal of the first depletion MOSFET transistor  513  is coupled to the node V − . However, the second series interconnection of the second LED group  511 , the second depletion MOSFET transistor  515  (coupled by the drain and source terminals), and the second first resistor  519  is coupled between the output node V +  of the voltage rectifier  107  and the source terminal of the first depletion MOSFET transistor  513 . The gate terminal of the second depletion MOSFET transistor  515  is coupled to the source terminal of the first depletion MOSFET transistor  513 . 
     The functioning of the circuit  500  will be explained with reference to the current timing diagram of  FIG. 5B . As in the case of conditioning circuit  100 , conditioning circuit  500  has first and second voltage thresholds V 1  and V 2 , and the rectified driving voltage V rect  respectively exceeds the first and second thresholds during time-periods [t 1 , t 4 ] and [t 2 , t 3 ] of each cycle. 
     Because the first and second LED groups  509  and  511  are not coupled in series, however, the current I  G1  flowing through the first LED group  509  does not flow through the second LED group  511 , and the current I G2  flowing through the second LED group  511  does not flow through the first LED group  509 . As a result, as the first MOSFET depletion transistor  513  enters and operates in a non-conducting state (period [t 2 , t 3 ]), the current I G1  through the first LED group  509  is reduced or cut-off. As a result, the first LED group  509  turns substantially off (and stops emitting light) during the period [t 2 , t 3 ]. Meanwhile, the second LED group  511  of circuit  500  functions substantially as in circuit  100 . In particular, the second LED group  511  conducts current (and emits light) during the period [t 2 , t 3 ]. 
     Electrical parameters for circuit  500  can be selected to adjust the functioning of the circuit. For example, the forward voltages of the first and second LED groups  509  and  511  may determine the value of the voltages V 1  and V 2  at which the first and second LED groups are activated. In particular, the voltage V 1  may be substantially equal to the forward voltage of the first LED group, while the voltage V 2  may be substantially equal to the forward voltage of the second LED group. In one example, the forward voltage of the first LED group may be set to a value of 60V, for example, while the forward voltage of the second LED group may be set to a value of 100V, such that the voltage V 1  is approximately equal to 60V and the voltage V 2  is approximately equal to 100V. In addition, the value of the first resistor  117  may be set such that the first depletion MOSFET transistor  113  enters a non-conducting state when the voltage V rect  reaches a value of V 2 . As such the value of the first resistor  117  may be set based on the threshold voltage of the first depletion MOSFET transistor  513 , the drain-source resistance of the first depletion MOSFET transistor  513 , and the voltages V 1  and V 2 . 
     The functioning of LED lighting circuit  500  may present an advantage in terms of providing a constant lighting intensity even in situations in which a driving voltage amplitude is variable. As described in relation to  FIG. 1D , as the amplitude of the rectified voltage V rect  decreases, the length of the periods [t 1 , t 4 ] and [t 2 , t 3 ] during which the first and second LED groups emit light correspondingly decreases. As a result, the total lighting intensity produced by the LED groups is reduced. The LED lighting circuit  500 , however, may provide a relatively constant lighting intensity even as the amplitude of the rectified voltage V rect  undergoes small variations. 
       FIG. 5C  shows a first diagram showing the relative lighting intensity of the first and second LED groups G 1  and G 2  according to the amplitude of the driving voltage V rect . The lighting intensity is normalized, for each LED group, to a value of 100% for a driving voltage amplitude of 120V. As the amplitude of the driving voltage decreases below 120V, the lighting intensity of the second LED group G 2  gradually decreases below 100%. However, as the amplitude of the driving voltage decreases below 120V, the lighting intensity of the first LED group G 1  initially increases before decreasing for low driving voltage amplitudes. As a result, the total lighting intensity produced by the LED circuitry (i.e., the total lighting intensity provided by the combination of the first and second LED groups G 1 +G 2 ) remains relatively constant for a range of amplitudes of input voltage (e.g., the range of amplitudes [120V, 100V], in the example of  FIG. 5C ), before decreasing for low driving voltage amplitudes. The LED lighting circuitry  500  may therefore advantageously be used to provide a constant lighting intensity in the face of a variable power supply amplitude, while nonetheless enabling the lighting intensity to be dimmed at lower power supply amplitudes. For example, the LED lighting circuit  500  can provide a constant lighting intensity even when variations in supply amplitude caused by transients on a power line occur. 
     The various modifications to the conditioning circuit  100  described herein can be applied to the conditioning circuit  500 . For example, the conditioning circuit  500  can include various interconnections of LEDs and of LED groups, such as the serial and parallel interconnections of LEDs and of LED groups described herein in relation to  FIGS. 2A-2D . In another example, the second transistor  515  may optionally be removed from the conditioning circuit  500 , and the cathode of the second LED group  511  coupled to the first terminal of the resistor  519 . In yet another example, additional series interconnections of an LED group, a depletion MOSFET transistor, and a resistor may be included in the conditioning circuit  500 . For instance, a third series interconnection of a third LED group, a third depletion MOSFET transistor, and a third resistor can be coupled between the anode of the first LED group  509  and the source of the second depletion MOSFET transistor  515 . The gate terminal of the third depletion MOSFET transistor would then be coupled to the source of the second depletion MOSFET transistor  515 . Similarly, a fourth series interconnection of a fourth LED group, a fourth depletion MOSFET transistor, and a fourth resistor can be coupled between the anode of the first LED group  509  and the source of the third depletion MOSFET transistor. The gate terminal of the fourth depletion MOSFET transistor would then be coupled to the source of the third depletion MOSFET transistor. 
     The conditioning circuits shown and described in this application, including the conditioning circuit  100 ,  200 ,  250 ,  300 ,  400 , and  500  shown in the figures, and the various modifications to conditioning circuits described in the application, are configured to drive LED lighting circuits with reduced or minimal total harmonic distortion. By using analog circuitry which gradually and selectively routes current to various LED groups, the conditioning circuits provide a high lighting efficiency by driving one, two, or more LED groups based on the instantaneous value of the driving voltage. 
     Furthermore, by using depletion MOSFET transistors with elevated drain-source resistances r ds , the depletion MOSFET transistors transition between the saturation and cutoff modes relatively slowly. As such, by ensuring that the transistors gradually switch between conducting and non-conducting states, the switching on and off of the LED groups and transistors follows substantially sinusoidal contours. As a result, the circuitry produces little harmonic distortion as the LED groups are gradually activated and deactivated. In addition, the first and second (or more) LED groups control current through each other: the forward voltage level of the second LED group influences the current flow through the first LED group, and the forward voltage level of the first LED group influences the current flow through the second LED group. As a result, the circuitry is self-controlling through the interactions between the multiple LED groups and multiple MOSFET transistors. 
     In one aspect, the term “field effect transistor (FET)” may refer to any of a variety of multi-terminal transistors generally operating on the principals of controlling an electric field to control the shape and hence the conductivity of a channel of one type of charge carrier in a semiconductor material, including, but not limited to a metal oxide semiconductor field effect transistor (MOSFET), a junction FET (JFET), a metal semiconductor FET (MESFET), a high electron mobility transistor (HEMT), a modulation doped FET (MODFET), an insulated gate bipolar transistor (IGBT), a fast reverse epitaxial diode FET (FREDFET), and an ion-sensitive FET (ISFET). 
     In one aspect, the terms “base,” “emitter,” and “collector” may refer to three terminals of a transistor and may refer to a base, an emitter and a collector of a bipolar junction transistor or may refer to a gate, a source, and a drain of a field effect transistor, respectively, and vice versa. In another aspect, the terms “gate,” “source,” and “drain” may refer to “base,” “emitter,” and “collector” of a transistor, respectively, and vice versa. 
     Unless otherwise mentioned, various configurations described in the present disclosure may be implemented on a Silicon, Silicon-Germanium (SiGe), Gallium Arsenide (GaAs), Indium Phosphide (InP) or Indium Gallium Phosphide (InGaP) substrate, or any other suitable substrate. 
     A reference to an element in the singular is not intended to mean “one and only one” unless specifically so stated, but rather “one or more.” For example, a resistor may refer to one or more resistors, a voltage may refer to one or more voltages, a current may refer to one or more currents, and a signal may refer to differential voltage signals. 
     The word “exemplary” is used herein to mean “serving as an example or illustration.” Any aspect or design described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other aspects or designs. In one aspect, various alternative configurations and operations described herein may be considered to be at least equivalent. 
     A phrase such as an “example” or an “aspect” does not imply that such example or aspect is essential to the subject technology or that such aspect applies to all configurations of the subject technology. A disclosure relating to an example or an aspect may apply to all configurations, or one or more configurations. An aspect may provide one or more examples. A phrase such as an aspect may refer to one or more aspects and vice versa. A phrase such as an “embodiment” does not imply that such embodiment is essential to the subject technology or that such embodiment applies to all configurations of the subject technology. A disclosure relating to an embodiment may apply to all embodiments, or one or more embodiments. An embodiment may provide one or more examples. A phrase such as an embodiment may refer to one or more embodiments and vice versa. A phrase such as a “configuration” does not imply that such configuration is essential to the subject technology or that such configuration applies to all configurations of the subject technology. A disclosure relating to a configuration may apply to all configurations, or one or more configurations. A configuration may provide one or more examples. A phrase such a configuration may refer to one or more configurations and vice versa. 
     In one aspect of the disclosure, when actions or functions are described as being performed by an item (e.g., routing, lighting, emitting, driving, flowing, generating, activating, turning on or off, selecting, controlling, transmitting, sending, or any other action or function), it is understood that such actions or functions may be performed by the item directly or indirectly. In one aspect, when a module is described as performing an action, the module may be understood to perform the action directly. In one aspect, when a module is described as performing an action, the module may be understood to perform the action indirectly, for example, by facilitating, enabling or causing such an action. 
     In one aspect, unless otherwise stated, all measurements, values, ratings, positions, magnitudes, sizes, and other specifications that are set forth in this specification, including in the claims that follow, are approximate, not exact. In one aspect, they are intended to have a reasonable range that is consistent with the functions to which they relate and with what is customary in the art to which they pertain. 
     In one aspect, the term “coupled”, “connected”, “interconnected”, or the like may refer to being directly coupled, connected, or interconnected (e.g., directly electrically coupled, connected, or interconnected). In another aspect, the term “coupled”, “connected”, “interconnected”, or the like may refer to being indirectly coupled, connected, or interconnected (e.g., indirectly electrically coupled, connected, or interconnected). 
     The disclosure is provided to enable any person skilled in the art to practice the various aspects described herein. The disclosure provides various examples of the subject technology, and the subject technology is not limited to these examples. Various modifications to these aspects will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other aspects. 
     All structural and functional equivalents to the elements of the various aspects described throughout this disclosure that are known or later come to be known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the claims. Moreover, nothing disclosed herein is intended to be dedicated to the public regardless of whether such disclosure is explicitly recited in the claims. No claim element is to be construed under the provisions of 35 U.S.C. §112, sixth paragraph, unless the element is expressly recited using the phrase “means for” or, in the case of a method claim, the element is recited using the phrase “step for.” Furthermore, to the extent that the term “include,” “have,” or the like is used, such term is intended to be inclusive in a manner similar to the term “comprise” as “comprise” is interpreted when employed as a transitional word in a claim. 
     The Title, Background, Summary, Brief Description of the Drawings and Abstract of the disclosure are hereby incorporated into the disclosure and are provided as illustrative examples of the disclosure, not as restrictive descriptions. It is submitted with the understanding that they will not be used to limit the scope or meaning of the claims. In addition, in the Detailed Description, it can be seen that the description provides illustrative examples and the various features are grouped together in various embodiments for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed subject matter requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter lies in less than all features of a single disclosed configuration or operation. The following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separately claimed subject matter. 
     The claims are not intended to be limited to the aspects described herein, but is to be accorded the full scope consistent with the language claims and to encompass all legal equivalents. Notwithstanding, none of the claims are intended to embrace subject matter that fails to satisfy the requirement of 35 U.S.C. §101, 102, or 103, nor should they be interpreted in such a way. Any unintended embracement of such subject matter is hereby disclaimed.