Abstract:
The present invention creates an LED driver in which all feedback signals are derived from a power stage media, and presents an isolated off-line LED driver with an accurate primary side controller only to power one or more LEDs. The present invention further provides an effective off-line LED driver comprising AC current shape controller with a minimum number of components. The present invention further provides a high quality luminous system based on LED drivers with the integrated synthesized optical feedback to compensate for imperfections of the LEDs as sources of light.

Description:
RELATED APPLICATIONS 
     This application claims priority to Provisional Application Ser. No. 60/611,162, filed Sep. 20, 2004, the benefit of the filing date of which is hereby claimed under 35U.S.C. § 119(e). 
    
    
     BACKGROUND 
     1. Field 
     The present invention relates to LED drivers, and more particularly to off-line LED drivers with integrated synthesized digital optical feedback. 
     2. Related Art 
     Capacitive drop off-line LED drivers are known (On Semiconductor Application Note AND8146/D). However, this non-isolated driver has low efficiency, delivers relatively low power, and delivers a constant current to the LED but with no temperature compensation, no dimming arrangements, and no protection for the LED. 
     A few isolated off-line LED drivers are known:
         With line frequency transformer and current regulator, On Semiconductor Application Note AND 8137/D;   Off-line LED driver with NCP1014P100 current mode controller, On Semiconductor Application Note AND8136/D;   White LED luminary Light Control System, U.S. Pat. No. 6,441,558;   LED Driving Circuitry with Light Intensity Feedback to Control Output Light Intensity of an LED, U.S. Pat. No. 6,153,985;   Non-Linear Light-Emitting Load Current Control, U.S. Pat. No. 6,400,102;   Flyback as LED Driver, U.S. Pat. No. 6,304,464;   Power Supply for LED, U.S. Pat. No. 6,557,512; and   Voltage Booster for Enabling the Power Factor Controller of a LED Lamp Upon Low AC or DC Supply, U.S. Pat. No. 6,091,614.       

     These drivers in general are too complicated as they use secondary side signals which have to be coupled with the controller on the primary side across the isolation. 
     For a high quality optical system multiple LED system parameters may be measured, which makes almost impossible the technical task of taking these signals across the safety isolation to feed controllers which reside on the primary side. 
     SUMMARY 
     An off-line LED Driver controls the optical output of a luminous system of variable number of LED by providing electrical energy as a constant DC or PWM voltage. An integrated digital model of the LED, in addition to LED current and forward voltage drop sense, provides feedback to a switch mode power converter configured to maintain a high quality of desired lumen output. The power converter further is structured to have either non-isolated or isolated topology. An isolated structure is implemented either by a two stage power converter or a single stage off-line converter. The power converter contains a controller coupled to primary side signals only. Further, the switch mode power converter forms AC input current to be the same shape as input voltage with high power factor and low THD. To achieve the required light source characteristics, the regulator modulates the duty cycle by keeping the desired LED current proportional to the integral of the LED forward drop voltage taken within an on-time of the primary switch. The system has two modes of operation: a) current mode /DC voltage, and b) PWM mode for deep dimming or extreme temperatures. The driver works both in continuous and discontinuous mode of operation. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates an off-line non-isolated LED driver (power stage) in accordance with the present invention. 
         FIG. 2  illustrates a regulator in accordance with the present invention. 
         FIG. 2   a  illustrates an AC/DC converter with a regulator. 
         FIG. 3   a  illustrates a LED model. 
         FIG. 3   b  illustrates a W/B LED model. 
         FIG. 4  illustrates current waveforms for the LED driver in  FIG. 1  at low frequencies. 
         FIG. 5  illustrates current waveforms for the LED driver in  FIG. 1  at high frequencies. 
         FIG. 6  illustrates a block diagram of a controller in accordance with the present invention. 
         FIG. 7  illustrates a non-isolated off-line LED driver in accordance with the present invention. 
         FIG. 8  illustrates a block diagram of the isolated driver in accordance with the present invention. 
         FIG. 9  illustrates primary side current waveforms. 
         FIG. 10  illustrates an algorithm for V c  calculation in accordance with the present invention. 
         FIG. 11  illustrates a simplified algorithm for V c  calculation in accordance with the present invention. 
         FIG. 12  illustrates an algorithm for a definition of the secondary side average current. 
         FIG. 13  illustrates an off-line LED isolated driver with a single discontinuous power stage. 
         FIG. 14  illustrates an off-line LED isolated driver with double power conversion. 
         FIG. 15  illustrates DC and PWM modes of driving an LED string. 
     
    
    
     DETAILED DESCRIPTION 
     As illustrated in  FIG. 1 , the present invention shapes average current (or voltage) as LED brightness may require by converting AC line energy using switch  3 , connected with its first terminal to the first terminal of the AC Bridge  2 . The second terminal of the bridge  2  is connected to the first terminal of the magnetic inductor  4 , and its second terminal is connected to the second terminal of the switch  3 . The string of LED  5  is connected to the second terminal of inductor  4  and its first terminal via a preferably Schottky rectifier  12 . The DC ground of the system is connected to the second terminals of the switch  3  and inductor  4 . 
     The block diagram of the controller to drive switch  3  is presented in  FIG. 2 , and current waveforms through switch  3  are illustrated in  FIG. 4  and  FIG. 5 . 
     Current in the inductor  4  is discontinuous, its peak value is as follows: 
     
       
         
           
             
               
                 
                   
                     I 
                     s 
                   
                   = 
                   
                     
                       
                         V 
                         s 
                       
                       * 
                       
                         t 
                         ons 
                       
                     
                     L 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     where 
     I s  is the peak current, 
     t ons  is the on time, 
     L is the inductance, and 
     V s  is the instantaneous voltage of the AC line. 
     Average value of I s  current is: 
                     I   sav     =         V   s     *     t   ons   2         2   ⁢   L   *   T               (   2   )               
where T is the cycle time.
 
     If the conversion frequency is constant, T=const and within the AC line the cycle on-time t ons  is unchanged, then the average current I sav  is:
 
 I   sav   =k*V   m  sin ωt  (3)
 
where
 
     
       
         
           
             K 
             = 
             
               
                 t 
                 ons 
                 2 
               
               
                 2 
                 ⁢ 
                 L 
                 * 
                 T 
               
             
           
         
       
     
     V m —is the amplitude of the AC Voltage. 
     Equation (3) is a law for a regulator to shape a sinusoidal input current and to provide close to unity power factor and close to zero THD. Such a regulator  21  is illustrated in  FIG. 2 . 
     Regulator  21  has two loops: a current mode with an error amplifier  6 , and voltage mode with integrators  7   a . The error amplifier  6  is connected with its negative terminal to the current sense of LED I c . The positive terminal of error amplifier  6  is connected to the LED model  200 , which in one embodiment of the invention has an optional customer set signal for an optical output I ref . In another embodiment of the invention, the customer I ref  signal provides level of LED junction temperature. At this configuration, the model  200  will be a thermal LED model. The model  200  and I ref  signal will determine a set current through LED per required luminous output (or junction temperature) of LED light system. I ref  signal has a user interface to be changed for dimming purposes. Forward voltage sensor of rectified voltage V s  is connected to the input terminal of an integrator  7 . Integrator  7  has a reset switch, enabling integrator  7  to integrate only during on time of the switch  3 . During off time of the switch  3 , the integrator  7  is in the reset status. 
     During the integration the output of integrator  7  is: 
     
       
         
           
             
               
                 
                   
                     V 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     7 
                   
                   = 
                   
                     
                       
                         ∫ 
                         0 
                         ton 
                       
                       ⁢ 
                       
                         VS 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           ⅆ 
                           t 
                         
                       
                     
                     = 
                     Vston 
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     The second integrator  7   c  with the same reset switch activated at off time is connected with its input terminal to the output of the first integrator  7 . And the output of integrator  7   c : 
     
       
         
           
             
               
                 
                   
                     V 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     7 
                     ⁢ 
                     c 
                   
                   = 
                   
                     
                       
                         ∫ 
                         0 
                         ton 
                       
                       ⁢ 
                       
                         VSton 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           ⅆ 
                           t 
                         
                       
                     
                     = 
                     
                       
                         Vston 
                         2 
                       
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     Equation (5) is a mathematical model of converter equation (2). Keeping V7c constant will allow the control of the average input current according to the equation (2). 
     The output of the error amplifier  6  is connected to the first terminal of comparator  8 . Its second terminal is connected to the output of integrator  7   c . The output of the comparator  8  is connected to the reset terminal of latch  10 . The set terminal of the latch  10  is connected to the oscillator  9 . The latch  10  is connected to the switch driver  11 . At the rising edge of the clock  9  the latch  10  is set and switch  3  ( FIG. 1 ) is turned on by the driver  11 . When comparator  8  goes high it resets the latch  10 . The driver  11  turns the switch  3  off. At the next clock of oscillator  9  the switching cycle will resume. 
     The LED driver  101  illustrated in  FIG. 2   a  includes the controller  21  coupled to the converter  102 , which is based on the converter  100 , and further including:
         Primary current sense resistor  103  connected between switch  3  and system ground;   Primary voltage sense resistors  104  and  105 , connected across inductor  4 ;   Filter capacitor  106  across resistor  105 ;   The output filter capacitor  107  connected to the cathode of the rectifier  12  and system ground;   The secondary current sense  108 , connected between a cathode of LED and system ground;   A coupling resistor  109  connecting current sense resistor  108  to the negative input of error amplifier  6  of the regulator  21         

     The present invention creates a practical and effective feedback system using LED models. A variety of known LED models may be used for this purpose.  FIG. 3   a  illustrates an example of a two channel brightness and thermal LED model  200 . In a first channel, the voltage drop across LED is sensed by a sensor V c    202  and current through LED by a sensor  1203 . The voltage sensor  202  is connected to an A/D converter  205 . The current sensor  203  is connected to an A/D converter  206 . The converters  205  and  206  are connected to the digital core  209 . A number N of serially connected LED&#39;s is stored in the digital core  209 . Also stored in the digital core  209  is a tested manufacturing relationship of LED V/I electrical parameter to its optical output ( 208 ). Based on signals from  202 ,  203 ,  210  and  208 , the digital core  209  calculates the optical output. This signal is connected to a D/A converter  212  and a block  214  in which the optical output is modeled by an analog signal. This analog signal is connected to a negative terminal of the error amplifier  216  via switch  215 . The positive terminal of the error amplifier  216  is connected to a customer interface signal I ref , which sets the output brightness in this case. 
     The second channel of the thermal model  200  comprises a sensor S of the ambient temperature (“Ta”)  201  connected to the digital core  209  via an A/D converter  204 . The signals  202 ,  203 ,  210  are also being used to create an analog signal of junction temperature Tj in the block  213 . A power loss in a single LED is calculated by the digital core  209  as: 
     
       
         
           
             
               
                 
                   Pl 
                   = 
                   
                     VcIc 
                     N 
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     A manufacturing parameter of thermal resistance pin to junction Rpj is stored in the block  207  which is connected to the digital core  209 . The digital core  209  calculates the real junction temperature:
 
 Tj=Ta+RpjPl   (7)
 
     The output of the thermal channel of the digital core  209  is connected via D/A  211  to the analog block  213 . The output signal of the analog block  213  is connected to the negative terminal of the error amplifier  216  via switch  215 . The positive terminal of the error amplifier  216  is connected to the customer interface signal I ref , which in this case is a junction temperature set signal. 
     The selection of a brightness or thermal model is done by switch  215 . 
     According to the invention, a non-contact method for creating an optical feedback signal comprises the following steps:
         Storing in the digital form the number of serially connected LEDs N;   Storing in the digital form the manufacturing relationship between V/I electrical signal and optical output in L m ;   Measuring a voltage across serially connected LEDs and converting it into the digital form;   Measuring a current through LEDs and converting it into the digital form;   Calculate V/I point;   Using manufacturing data, calculate optical output;   Converting optical output from digital to analog form;   Comparing calculated optical output with a set signal in an error amplifier; and   Using the error amplifier signal as a set signal in the power converter regulator.       

     Those skilled in the art may use a variety of other LED models to create a non-contact feedback for an LED driver according to this invention. More accurate models may be used also. For example, calculations of the optical output may be used complementary to V/I point junction temperature adjustment. 
     According to another embodiment of the invention the following process is suggested for a non-contact thermal feedback of a LED driver:
         Storing in the digital form a number of serially connected LEDs N;   Storing in the digital form the manufacturing value of a thermal resistance pin to junction;   Measuring a voltage across a string of LEDs and converting it into the digital form;   Measuring a current via LEDs and converting it into the digital form;   Calculating power loss in a single LED by multiplying a measured voltage by current and dividing by the number of LEDs;   Sensing ambient temperature and converting it into the digital form;   Calculating a LED junction temperature by adding ambient temperature to the product of power losses in an LED by thermal resistance pin to the junction;   Converting a junction temperature into an analog signal;   Comparing a calculated junction temperature with a set signal in an error amplifier; and   Using the error amplifier signal as a set signal in the power converter regulator.       

       FIG. 3   b  illustrates a model  300  for thermal feedback based on a non-contact method of determining junction temperature of phosphor-converted white LED, according to a theory published by Prof. Nadarajan Narendran. The feedback model includes a sensor  301  of total radiant energy W connected to a digital core  306  via an A/D converter  303 . A sensor  302  of the radiant energy within the blue emission (B) is connected to the digital core  306  via an A/D converter  304 . A relationship of W/B ratio to the LED junction temperature in the analytical or table forms is stored in the block  307 , connected to the digital core  306 . Based on the W/B ratio, the digital core  306  calculates the junction temperature Tj. The output of the digital core  306  is connected to analog block  309  via a D/A converter  308 . The output of the analog junction temperature block  309  is connected to the negative terminal of the error amplifier  310 . The positive terminal of the error amplifier  310  is connected to a set signal of maximum junction temperature. The output of the error amplifier  310  is connected to the error amplifier of the power converter. 
     The following process is suggested for creating a thermal feedback of LED Driver using the W/B ratio:
         Storing a relationship between the W/B ratio and the junction temperature in the digital form;   Measuring the total radiant energy W of the radiant energy and converting it into the digital form;   Measuring the radiant energy within the blue emission (B) and converting it into the digital form;   Calculating the W/B ratio;   Calculating the junction temperature;   Converting the junction temperature signal into the analog form;   Comparing the calculated junction temperature with a set signal in an error amplifier; and   Using the error amplifier signal as a set signal in the power converter regulator.       

     The construction and process of creating feedback signals based on  FIGS. 3   a  and  3   b  are applicable to when the LED model is used as a feedback signal for the LED regulation. However, the controller  21  can be configured such that the main feedback signal is LED DC current, then the described above LED models may be used for the adjustment of DC current feedbacks. In these cases, amplifiers  216  or  310  should be removed and direct analog signals  213 ,  214  or  309  could be used for the DC feedback adjustments (for example, adjustment of forward DC current based on real junction temperature to maintain the desired optical output). 
     The regulator  21  in  FIG. 2  is described and presented in the analog form. It should be understood as an architecture, which may be implemented in different ways by those skilled in the art without departing from the spirit and scope of the present invention. For example, the regulator  21  can be implemented in the digital form. If so, then the feedback models  200  and  300  described as analog models should be implemented in the digital form as well. It is conceivable then that D/A converters  211 ,  212  and  308 , as well as analog blocks  213 ,  214  and  309 , should be removed. The error amplifiers  216  and  310 , if functionally needed, should be realized in the digital form. 
     A block diagram of a controller  120  is presented in  FIG. 6 . On top of fundamental functions presented in  FIG. 2 , it includes:
         Soft start circuit  601  connected to the output of integrators  7   a;      Start up circuit  602 , connected to the output of error amplifier  6 ;   OVP circuit C 2   603 , connected to the input logic of the driver  11 ;   Maximum on time limit  604 , connected to the output of integrator  7   a;      LED current limit comparator C 5   605  connected to the LED current sense I c ;   Controller V cc  power on reset comparator  606 ; and   Input peak current limiter comparator C 3   607 , connected to the Input current sense I s .       

     A functional AND logic  608  is connected with its input to the output of latch Q  609  to interface this signal to the driver. Logical signals from LED current limit comparator C 5   605 , enable signal EN, OVP comparator C 2   603 , and power on reset comparator C 4   606  are assembled at the input of AND logic  608 . If any of these signals goes inactive, the AND logic  608  is blocked and the switch  3  ( FIG. 1 ) remains in the off position. 
     A practical off line non-isolated LED system is illustrated in  FIG. 7 . According to this embodiment of the invention, the off-line LED driver  110  comprises the buck-boost converter  100  ( FIG. 1 ) and further includes:
         Input fuse  30 ;   Input EMI filter  31 ;   Gate drive resistor  32 , connected between power switch  3  and controller  120 ;   Primary current sense  33 , connected between power switch  3  and ground;   V cc  precharge resistor  34 , connected between the positive port of rectifier  2  and the V cc  capacitor  36 ;   V cc  protection zener diode  37 , connected across the V cc  capacitor  36 ;   Output voltage sense  39  and  42 , connected to the controller  120 ;   Current sense filter  35 ,  44 , connected between current sense resistor  46  and the controller  120 ;   V cc  supply resistor  40  and diode  41  connected to the anode of rectifier  12 ; and   LED filter  43  connected across LEDs  5  anodes and ground.       

     When the input AC Voltage  1  is applied the V cc  capacitor  36  is charged via resistor  34  and inductor  4 . This is an additional network to precharge the capacitor  36  as ground is connected to the positive rail of the rectified voltage. When controller  120  is turned on, it starts driving the power switch  3 , and voltage builds across output  5 . The V cc  energy then is supplied by the inductor  4  via blocking diode  41  and current limiting resistor  40 . 
     Enable pin EN is being used for enabling/disabling the Driver and for LED dimming via a pulse width modulator (PWM). 
     A block diagram of an isolated LED driving system is illustrated in  FIG. 8 . The first terminal of the AC bridge  2  is connected to the first terminal of switch  3 . A second terminal of switch  3  is connected to the first terminal of the first primary winding of the transformer  48 . The second terminal of the first primary winding of transformer  48  is connected to the second terminal of the bridge  2 . LEDs  5  are connected to the secondary winding of the transformer  48  in the flyback configuration via Schottky rectifier  12 . The second primary winding  48   a  of the transformer  48  is connected to the circuit generating the V c  signal proportional to a V c  voltage across the LEDs  5 . A primary capacitive filter  46  is connected across the output of the bridge  2 , and secondary capacitive filter  49  is connected across LEDs  5 . A current sense circuit  151  is connected in series with the LEDs  5 . 
     The converter  150  in  FIG. 8  will keep up with the law in equation (3), delivering high power factor if input signals to its controller  160  ( FIG. 13 ) are processed to be transmitted over an isolation barrier and to be compliant with the regulator  21  requirements ( FIG. 2 ). 
     Primary and secondary current waveforms for the converter  150  in  FIG. 8  are presented in  FIG. 9 . Here: 
                     Δ   ⁢           ⁢     I     p   ⁢           ⁢   1         =         V   s     *     t   ons         L   m               (   8   )               
where
 
     ΔI p1  is the change of the primary current, and 
     L m  is the magnetizing inductance of the transformer; and 
                     Δ   ⁢           ⁢     I     p   ⁢           ⁢   2         =       N   *     V   c     ⁢           ⁢     t   rs         L   m               (   9   )               
where
 
     ΔI p2  is the change of the secondary current, 
     N is the transformer ratio, 
     V c  is the output voltage, and 
     t rs  is the reset time of the transformer. 
     Finding L m  from equation (4) and substituting it in equation (5), an expression for V c  follows: 
     
       
         
           
             
               
                 
                   Vc 
                   = 
                   
                     
                       Δ 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         I 
                         
                           p 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           2 
                         
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         V 
                         s 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         t 
                         ons 
                       
                     
                     
                       
                         Nt 
                         rs 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       Δ 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         I 
                         
                           p 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           1 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
     The process for finding the secondary feedback signal V c  on the primary side is illustrated in the flow chart in  FIG. 10 . This algorithm applies for both steady state and transients for discontinuous as well as continuous modes of operation and comprises the following steps:
         Starting cycle s, via step  1001 ;   Turning on a switch, via step  1002 ;   Acquiring V s , t ons , ΔI p , via step  1003 ;   Turning off the switch, via step  1004 ;   Acquiring N, t rs , ΔI p2 , via step  1005 ;   Calculating V c  per equation (10), via step  1006 ; and   Starting a new cycle, via step  1007 .       

     A simplified algorithm can be suggested for a steady state when NΔI p1 =ΔI p2   
     
       
         
           
             
               
                 
                   Vc 
                   = 
                   
                     
                       
                         V 
                         s 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         t 
                         ons 
                       
                     
                     
                       t 
                       rs 
                     
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
           
         
       
     
     The simplified process is illustrated in the flow chart of  FIG. 11  and comprises the steps of:
         Starting cycle s, via step  1101 ;   Turning on a switch, via step  1102 ;   Acquiring V, and t ons , via step  1103 ;   Turning off the switch, via step  1104 ;   Acquiring t rs , via step  1105 ;   Calculating V c , via step  1106 ; and   Starting a new cycle, via step  1107 .       

     The secondary average current I c  for a discontinuous mode can be also found on the primary side: 
     
       
         
           
             
               
                 
                   Ic 
                   = 
                   
                     
                       
                         NI 
                         
                           p 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           1 
                         
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         t 
                         rs 
                       
                     
                     
                       2 
                       ⁢ 
                       T 
                     
                   
                 
               
               
                 
                   ( 
                   12 
                   ) 
                 
               
             
           
         
       
     
     The subsequent process to define secondary current is presented in  FIG. 12  and comprises the following steps in addition to the steps in  FIGS. 10 and 11 :
         Acquiring at off time I p1 , t rs , and T, via step  1201 ; and   Calculating I c  per equation (12), via step  1202 .       

     In another embodiment of the invention, an implementation of the off-line LED driver based on primary control algorithms as illustrated in FIGS.  10 , 11 , and  12  is illustrated in  FIG. 13  The system in  FIG. 3  is quite simple and provides a high quality luminous system. The off-line LED driver  130  is based on the isolated converter  150  illustrated in  FIG. 8  and further comprises:
         Input fuse  30  connected between AC line  1  and input terminal of bridge  2 ;   Current sense resistor  33  connected in series with the switch  3 ;   Voltage sense resistive divider  52 ,  53  connected across the switch  3 ;   V cc  capacitor  36  connected via rectifier  41  to the second primary winding  48   a  of the transformer  48 ;   V cc  protection zener diode  37 , connected across V cc  capacitor  36 ;   Controller  160 , including functions of the processes in  FIGS. 10 ,  11 , and  12 , and connected with its terminals to the gate resistor  32 , current sense resistor  38 , V cc  capacitor  36 , voltage sensor  52 ,  53 , and feedback signal S.       

     The switch mode converter  130  in  FIG. 13  is running in discontinues mode. A single stage power factor corrected converter has a natural limit of processed power to about 100-120 W. If a LED light system requires more power, then a two stage system will be a better fit. Such a system  140  is presented in  FIG. 14 . The two stage system  140  has a combined controller  170  comprising two parts: a voltage source with power factor correction; and a current regulator based on a synthesized optical feedback similar to controller  160  ( FIG. 13 ). The switches Q 1   3  and Q 2   55  may run at arbitrary frequencies. For EMI purposes, their synchronization may be considered. The voltage level of the voltage controller may be set permanent, or may be adjusted by a required secondary current. 
     The off-line driver  140  is based on the converter  150  ( FIG. 8 ) and further comprises:
         Input fuse  30  connected between an AC line and the input terminal of the bridge  2 ;   First switch  3  with its first terminal connected to the positive terminal of the bridge  2  and the second terminal connected to the first terminal of the current sense resistor  33 , and the control terminal connected to the gate resistor  32 ;   Current sense resistor  33  with its second terminal connected to the system ground;   Second switch  55  with its first terminal connected to the second terminal of current sense resistor  56 , with its second terminal connected to the first terminal of the primary winding of transformer  48 , and with its control terminal connected to the gate resistor  57 ;   Current sense resistor  57  with its first terminal connected to the system ground;   Power inductor  4  with its first terminal connected to the system ground and with its second terminal connected to the negative terminal of the bridge  2 ;   Blocking diode  12  with its anode connected to the negative terminal of the bridge  2  and its cathode connected to the second terminal of the primary winding of the transformer  48 ;   First stage capacitive filter  54  connected between the cathode of the blocking diode  12  and the system ground;   First stage voltage sensor  58  and  59  connected across the capacitor  54 ;   V cc  capacitor  38  connected between the V cc  pin of the controller  170  and the system ground;   V cc  energy supply from the first stage filter comprising the blocking diode  41  connected with its anode to the positive rail of filter  54  and its cathode to the current limiting resistor  60 , where the resistor  60  is connected with its second terminal to the V cc  pin of the controller  170 ;   Precharging resistor  34 , connected to the positive terminal of the bridge  2  and positive terminal of filter  54 ; and   Controller  170  connected with its first output to the gate resistor  32  and its second output to the gate resistor  57 , to the first current sense resistor  33  and second current sense resistor  56 , to the input voltage sensor  52 ,  53  to the first stage voltage sensor  58 ,  59 , and to the feedback signal S.       

     In  FIG. 2 , the controller&#39;s  21  performance is demonstrated in DC mode. When the required LED current is approaching extreme values, the controller  21  is switched into PWM mode (see  FIG. 15 ). In the PWM mode, the duty cycle is selected such that the junction temperature of the LED will not exceed manufacturing limits. The following process is suggested:
         The required system interface LED current is monitored;   The junction temperature of LED is sensed or generated with a no-contact method;   The junction temperature is monitored;   If the required LED current is less than a fixed number (for example 10%), driver is run in the PWM mode for higher accuracy;   If required LED current is more than a fixed number (for example 10%), and junction temperature is less than specified limit, the driver is run in the DC mode; and   If the upper limit of the junction temperature is reached, then the controller  21  is turned into a junction temperature regulator with the LED supplied by the PWM mode of operation of the power stage,       

     Foregoing described embodiments of the invention are provided as illustrations and descriptions. They are not intended to limit the invention to precise form described. In particular, it is contemplated that functional implementation of invention described herein may be implemented equivalently in hardware, software, firmware, and/or other available functional components or building blocks, and that networks may be wired, wireless, or a combination of wired and wireless. Other variations and embodiments are possible in light of above teachings, and it is thus intended that the scope of invention not be limited by this Detailed Description, but rather by Claims following.