Patent Publication Number: US-7906917-B2

Title: Startup flicker suppression in a dimmable LED power supply

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. provisional application Ser. No. 60/622,553, filed Oct. 27, 2004, which the entire subject matter is incorporated herein by reference. 
    
    
     The present invention relates to power supplies for light emitting diodes (LEDs). More specifically, the present invention relates to dimmable power supplies for light emitting diodes (LEDs) including circuitry to prevent flickering of the light output from the light emitting diodes (LEDs) for low output light levels. 
     LEDs are used as light sources for various applications including lighting in theatres, signal lighting in mobile vehicles such as cars, boats and planes, signage and ambient lighting in homes and offices, and mood lighting in retail shops. Some of these applications require the output light from the LEDs to be adjustable from 1% to 100% of the maximum light output. In some application, such as mood lighting, theatrical lighting or tail lights of a car, the LEDs are turned on at a low light output level. 
     LED power supplies capable of producing pulse width modulated current pulses are required to provide this range of light output. Pulse width modulated power supplies achieve dimming by providing a pulse width modulated signal to a switch in series or parallel with the LED load. Duty cycle control of the pulse width modulated pulses produces an adjustable average LED current and a respective current control to the LED. The peak current or nominal LED current is maintained at a constant value. A fly back converter controlled by the IC, such as an L6561 by ST Micro-electronics, constitutes the main power circuit. A pulse width modulation generation circuit provides the desired duty cycle control of the LED current. The LED power supply must build the LED current quickly, for example in less than 10 msec from startup, since the LED response time is on the order of nano-seconds. The pulses generated by the pulse width modulator lag the output voltage build-up with a resultant voltage build-up to the maximum value before the current feedback is detected. A current overshoot occurs for the first pulses due to the voltage build up. The peak detect delay in the feedback can also lead to an excessive voltage buildup. 
     When maximum light output is requested at startup, the resultant current overshoot is not significant since the output voltage is close to the steady state value. When startup occurs at low light output the overshoot is high, since the steady state voltage is lower than the startup output voltage. This LED current overshoot is significant at lower light levels, such as 1% to 25% of the maximum light output, and flickering is observed. 
     It is desirable to have a power supply, which suppresses the observed flicker when a LED is turned on. In particular, it is desirable to suppress the observed flicker when a LED is turned on to emit a light level under 10% of the maximum light output. 
     One form of the present invention is a method of flicker suppression for an LED. The method includes providing a power supply for supplying current to the LED. The power supply includes a flicker suppressor and the power supply is responsive to a dim command signal. The method further includes receiving the dim command signal at the power supply, switching the current on and limiting the current to maintain LED light output below 110 percent of the LED light output corresponding to the dim command signal. 
     A second form of the present invention is a system of flicker suppression for an LED including a power supply for supplying current to the LED. The power supply includes a flicker suppressor, and is responsive to a dim command signal. The power supply includes means for receiving the dim command signal at the power supply, means for switching the current on and means for limiting the current to maintain LED light output below 110 percent of the LED light output corresponding to the dim command signal. 
     A third form of the present invention includes a power supply for an LED, including a power supply circuit having an output for supplying current to the LED and a flicker suppressor operably connected to the output. The power supply circuit is responsive to a dim command signal. 
    
    
     
       The foregoing form as well as other forms, features and advantages of the present invention will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the present invention rather than limiting, the scope of the present invention being defined by the appended claims and equivalents thereof. 
         FIG. 1  shows a block diagram of a first embodiment of a power supply for an LED in accordance with the present invention; 
         FIG. 2  shows a schematic diagram of a first embodiment of a power supply for an LED in accordance with the present invention; 
         FIG. 3  shows a block diagram of a second embodiment of a power supply for an LED in accordance with the present invention; 
         FIG. 4  shows a schematic diagram of a second embodiment of a power supply for an LED in accordance with the present invention; 
         FIG. 5  shows a block diagram of a third embodiment of a power supply for an LED in accordance with the present invention; 
         FIG. 6  shows a schematic diagram of a third embodiment of a power supply for an LED in accordance with the present invention; 
         FIG. 7  shows a block diagram of a fourth embodiment of a power supply for an LED in accordance with the present invention; and 
     
    
    
     In the power supplies  10 - 13  described in reference to  FIGS. 1-7  flicker suppression is achieved at startup by limiting the current to the LED  26  to maintain LED light below 110 percent of the LED light output corresponding to a dim command signal input to the pulse width modulator  40 . In some embodiments, the current to the LED  26  is limited during power-up of the LED  26 . 
     In one embodiment, the power supplies  10 - 13  achieve flicker suppression by limiting the current to the LED  26  to maintain LED light output during power-up below 110 percent of the LED light output corresponding to the dim command signal, so that LED light output is below 110 percent of the LED light output corresponding to a dim command signal input to the pulse width modulator  40  to minimize the overshoot and the undershoot. 
     In another embodiment, the power supplies  10 - 13  achieve flicker suppression by limiting the current to the LED  26  during power-up to maintain LED light output less than or equal to the LED light output corresponding to the dim command signal, so that LED light output is less than or equal to the LED light output corresponding to a dim command signal input to the pulse width modulator  40  to minimize the overshoot and the undershoot. 
     In yet another embodiment, the power supply  10 - 13  achieve flicker suppression by limiting the current to the LED  26  during power-up to maintain LED light output to between 105 and 95 percent of the LED light output corresponding to the dim command signal, so that LED light output is between 105 and 95 percent of the LED light output corresponding to a dim command signal input to the pulse width modulator  40  to minimize the overshoot and the undershoot. 
       FIG. 1  shows a block diagram of a first embodiment of a power supply  10  for an LED  26  in accordance with the present invention. The power supply  10  provides power to an LED  26  and includes a power supply circuit  15  and a flicker suppressor  50 . Power supply circuit  15  includes AC/DC converter  22 , power converter  24 , control circuit  38 , pulse width modulator  40 , pulse width modulator switch  28 , and feedback circuit  29 . Feedback circuit  29  includes current sensor  30 , current amplifier  32 , and peak current detector  34 . The power supply  10  achieves flicker suppression at startup by limiting the current to the LED  26  during power-up so that the LED light output is below 110 percent of the LED light output corresponding to the dim command signal input to the pulse width modulator  40 . 
     The power supply  10  uses current feedback circuit  29  to adjust the power to the LED  26 , the pulse width modulator (PWM)  40  to provide dimming capability for the LED  26  and flicker suppressor  50  to prevent overshoot of the current to the LED  26  during startup of the power supply  10 . Single-phase AC input is provided at block  20  and converted to DC by the AC/DC converter  22  to provide a DC voltage to the power converter  24 . Power converter  24  regulates the power to LED  26  based on a current error generated at the control circuit  38 . The flicker suppressor  50  provides a signal to the control circuit  38  to suppress current overshoot at the LED  26  when pulse width modulator  40  starts to pulse the pulse width modulator switch  28 . In particular, the flicker suppressor  50  prevents flicker due to current overshoot when the output light level from the LED  26  is within 1% to 25% of the maximum output light level. Typically, the flicker due to current overshoot is noticeable when the output light level from the LED  26  is within 1% to 10% of the maximum output light level. 
     The current sensor  30  measures the current flow to the LED  26  and provides a sensed current signal to the current amplifier  32 . The amplified sensed current signal from the current amplifier  32  is provided to the peak current detector  34 . The output signal of the peak current detector  34  is input to the control circuit  38  to provide a feedback signal to the control circuit  38  along with the signal from flicker suppressor  50 . A signal output of the control circuit  38  is input to a gate of a switch within the power converter  24 . 
     The pulse width modulator  40  receives a dim command signal  41  operable to adjust the duty cycle of the pulse width modulator  40 . Typically, the user of the LED  26  provides the dim command signal  41  to the pulse width modulator  40 . In one embodiment, the dim command signal  41  is provided by an automated system, which is operable to adjust an output light level from the LED  26  as a function of time. The pulses output from the pulse width modulator  40  operate to switch the pulse width modulator switch  28 , which is in series with the LED  26 . The output of the power converter  24  is input to the LED  26  and current flows through the LEDs  26  when the pulse width modulator switch  28  is pulsed. In this manner, pulse width modulator  40  switches the current on and off through the LED  26 . 
     The details concerning the operation of the pulse width modulator  40  are described in Application Serial No. PCT IB2003/0059 of Tripathi et al. entitled Power Supply for LEDS filed on Dec. 11, 2003. The application is incorporated by reference herein. 
     Those skilled in the art will appreciate that many configurations of and couplings among the components of power supply  10  are possible. For example, the components can be connected electrically, optically, acoustically, and/or magnetically. Therefore, many embodiments of power supply  10  are possible. 
       FIG. 2  shows a schematic diagram of a first embodiment of a power supply  10  for an LED  26  in accordance with the present invention. The power supply  10  limits current to the LED  26  during power-up by limiting output voltage to the LED  26  during power-up. The power supply  10  pulses a switch Q 1  prior to switching the current to the LED  26  on. The switch Q 1  is responsive to a control signal from a control circuit  38  to control the output voltage to the LED  26 . The power supply  10  monitors the output voltage at the flicker suppressor  50  to generate an output voltage feedback signal, provides the output voltage feedback signal to the control circuit  38  and adjusts the control signal in response to the output voltage feedback signal. Specifically, flicker suppressor  50  injects a feedback signal to control circuit  38  in response to an increase in output voltage. This injected feedback signal decreases the rate of change of output voltage and thereby prevents excessive voltage buildup. Subsequently, the decreasing rate of change of output voltage reduces the flicker suppressor  50  feedback signal. 
     Power supply  10  employs a flyback transformer  25  driven by control circuit  38  to supply power to LED  26 . Power supply  10  includes an EMI filter  21 , an AC/DC converter  22 , a flyback transformer  25  including windings W 1  and W 2 , a control circuit  38 , a feedback circuit  29 , pulse width modulator switch Q 2 , a pulse width modulator (PWM)  40 , resistors R 1 -R 6 , R 10 -R 12 , capacitors C 1 -C 2 , C 4 , C 5 , C 7 , diodes D 1 , D 3 , D 4 , and switch Q 1  and operational amplifier O 1 . Switches Q 1  and Q 2  are n-channel MOSFETs. In an alternative embodiment, other types of transistors, such as an insulated gate bipolar transistor (IGBT) or a bipolar transistor, are used in place of n-channel MOSFET switches Q 1  and Q 2  to adjust the current. 
     Input voltage is supplied to power supply  10  at V in  to EMI filter  21 . The voltage can be an AC input and is typically 50/60 Hertz at 120/230 Vrms. EMI filter  21  blocks electromagnetic interference on the input. AC/DC converter  22  converts the AC output of EMI filter  20  to DC and can be a bridge rectifier. The flyback transformer  25  includes a primary winding W 1  and a secondary winding W 2  operable to power the LED  26 . The flyback transformer  25  is controlled by control circuit  38 , which is a power factor corrector integrated circuit, such as model L6561 manufactured by ST Microelectronics, Inc. The flyback transformer  25  with power factor corrector configuration is widely used to provide isolated fixed voltage DC power sources with high line power factors. Additional windings are operable to provide the necessary control V dd  and zero crossing detection signal, as is well known to those skilled in the art. 
     The control circuit  38  supplies a transformer control signal to adjust the current flow through winding W 1  of flyback transformer  25  to match the LED  26  current demand. The transformer control signal is input to the flyback transformer  25  when control circuit  38  pulses the gate of switch Q 1  through resistor R 12 . Typically, the gate of switch Q 1  is pulsed at about 100 kHz. The pulsed signals from switch Q 1  enable energy transfer through the transformer windings W 1 /W 2  to charge capacitor C 2  and to provide the voltage output (V out ) to the LED  26 . 
     The LED  26  is in parallel across capacitor C 2  and resistor R 1 . The LED  26  is in series with the pulse width modulator switch Q 2 . When the pulse width modulator  40  pulses the gate of pulse width modulator switch Q 2 , current flows through the pulse width modulator switch Q 2  and the LED  26  for the duration of the pulse. The pulse width modulator  40  receives a dim command signal, shown as i dim . The dim command signal adjusts the duty cycle of the pulses to set the LED light output. The dim command signal is input to the pulse width modulator  40  to set the duty cycle as described in the above mentioned Patent Application Serial No. PCT IB2003/0059. 
     When the dim command signal is a low light dim command signal, the duty cycle of pulse width modulator  40  is low. In this state, the LED  26  receives current for a low duty cycle. The pulses from the pulse width modulator  40  are low frequency, typically about 300 Hz. 
     The feedback circuit  29  senses the current through the LED  26 . The feedback circuit  29  includes operational amplifier O 1  and a sensing resistor R 1  in series with LED  26 . A sensed current signal generated across resistor R 1  is provided to the non-inverting input of operational amplifier O 1 . Operational amplifier O 1  is configured as a non-inverting amplifier with resistor R 2  across the inverting input and the output. The inverting input of operational amplifier O 1  is grounded through resistor R 3 . 
     The feedback circuit  29  also includes a peak detect circuit, which includes diode D 3 , capacitor C 7  and resistor R 10  at the output of the operational amplifier O 1 . The anode of diode D 3  is at the output of operational amplifier O 1 . Resistor R 10  and capacitor C 7  are in parallel to each other at the cathode side of the diode D 3 . The current feedback circuit  29  provides a feedback signal to control circuit  38  through resistor R 11 . The feedback signal to control circuit  38  adjusts the transformer control signal to the flyback transformer  25  to match the LED  26  current demand. 
     Without a flicker suppressor circuit  50 , the power supply circuit supplies an overshoot of current to the LED  26  during power-up. The overshoot is due to a lag in the generation of a feedback signal to the control circuit  38 , which causes excessive voltage to build up across the LED  26 . Furthermore, the lag is due to lagging pulses from pulse width modulator  40  and/or the time needed to charge capacitor C 7 . 
     Without the flicker suppressor circuit  50 , the transformer control signal input to the switch Q 1  adjusts the current flow through winding W 1  of flyback transformer  25  to match the LED  26  current demand until the sensed current signal and a referenced current signal are equal at the control circuit  38 . When the sensed current signal and the referenced current signal are equal, the feedback error signal goes to zero. The output voltage builds up across capacitor C 2 , which is parallel to the LED  26 , as the sensed current signal and the referenced current signal are reaching equalization. As pulses to the gate of pulse width modulator switch Q 2  pulse the LED  26 , the current sense voltage across resistor R 1  is not continuous. The capacitor C 7  of the peak detect circuit does not charge to a steady state value until pulse width modulator switch Q 2  is turned on and off for a few cycles, since the time period between each pulse of the gate to pulse width modulator switch Q 2  is relatively long for low LED light output. The control circuit  38  keeps building voltage across output capacitor C 2  as capacitor C 7  charges to its steady state value. 
     This voltage buildup causes the current in the LED  26  to build up to a level that is higher than the LED  26  requires. Once the voltage across capacitor C 7  reaches a peak value corresponding to the peak LED current, the control circuit  38  turns off switch Q 1  causing an undershoot in the LED current. Due to this overshoot and subsequent undershoot of the current to LED  26 , a flicker in the optical output from the LED  26  is observed each time the power supply  10  is turned on for low LED light output. 
     Addition of the flicker suppressor  50  to the power supply  10  prevents overshoot and the resultant flicker during power-up of the power supply  10 . Prior to the LED  26  being turned on by the pulsing of pulse width modulator switch Q 2 , the control circuit  38  begins operation and pulses the gate of switch Q 1  through resistor R 12 . The pulsed signals from switch Q 1  start building output voltage across capacitor C 2 . The derivative of voltage with time (dV/dt) across capacitor C 5  provides an output voltage feedback signal to control circuit  38 . 
     Flicker suppressor  50  includes a capacitor C 5  and a resistor R 6  connected in series between the output voltage and ground. Suppressor circuit  50  generates a flicker suppression feedback signal, which is provided to the control circuit  38  through diode D 4  and resistor R 11 . The output voltage feedback signal is acquired at the connection of the capacitor C 5  and the resistor R 6 . The flicker suppression feedback signal received by control circuit  38  decreases output voltage buildup across capacitor C 2 . Thus, during power-up of the LED  26  with power supply  10 , the output voltage buildup across capacitor C 2  is reduced. The output voltage buildup across capacitor C 2  is thereby maintained below the value of voltage buildup obtained during power-up in a power supply that does not include flicker suppressor  50 . The power supply  10  achieves flicker suppression by limiting the current to the LED  26  during power-up so that the LED light output is below 110 percent of the LED light output corresponding to the dim command signal input to the pulse width modulator  40 . 
     In one embodiment, a current controller operable to compare the sensed current with a reference current is included in the feedback system  29 . In another embodiment, a current controller and an optocoupler are included in the feedback system  29 . The optocoupler is operable to isolate the DC circuit supplying the LEDs  26  from the AC circuit power supply at the EMI filter  21 , the two circuits being on opposite sides of the transformer windings W 1 /W 2 . The feedback signal from the current controller is operable to drive the optocoupler. 
     The LED  26  can be white or colored LEDs, depending on the application, such as ambient mood lighting or vehicular tail lights. The LEDs  26  can be a number of LEDs connected in series or parallel or a combination of series and parallel circuits as desired. 
       FIG. 3  shows a block diagram of a second embodiment of a power supply  11  for an LED  26  in accordance with the present invention. The power supply  11  supplying LED  26  includes a power supply circuit  15  and a flicker suppressor  70 . Power supply circuit  15  includes AC/DC converter  22 , power converter  24 , control circuit  38 , pulse width modulator  40 , pulse width modulator switch  28 , and feedback circuit  29 . Feedback circuit  29  includes current sensor  30 , current amplifier  32 , and peak current detector  34 . 
     The power supply  11  achieves flicker suppression by limiting the current to the LED  26  during power-up so that the LED light output is below 110 percent of the LED light output corresponding to the dim command signal input to the pulse width modulator  40 . 
     The flicker suppressor  70  clamps the output voltage to a maximum value in the event of excessive voltage buildup during start-up and speeds up the feedback signal generation to suppress flicker. In particular, the flicker suppressor  70  prevents flicker due to current overshoot when the output light level from the LED  26  is within 1% to 25% of the maximum output light level. Typically, the flicker due to current overshoot is noticeable when the output light level from the LED  26  is within 1% to 10% of the maximum output light level. 
       FIG. 3  differs from  FIG. 1  in that the flicker suppressor  70  does not input a signal to the control circuit  38 . The power supply  11  uses current feedback circuit  29  to adjust the power to the LED  26 , the pulse width modulator (PWM)  40  to provide dimming capability for the LED  26  and flicker suppressor  70  to prevent overshoot of the current to the LED  26  during startup of the power supply  11 . Single-phase AC input is provided at block  20  and converted to DC by the AC/DC converter  22  to provide a DC voltage to the power converter  24 . Power converter  24  regulates the power to LED  26  based on the feedback signal representing a current error generated at the current controller  36 . The feedback circuit  29  and pulse width modulator  40  operate as described in reference to  FIG. 1 . 
     The flicker suppressor  70  is turned on after the output voltage reaches a set level during the power-up of the LED  26 . When flicker suppressor  70  turns on, the current flows through flicker suppressor  70  and not the LED  26 . Once steady state is reached, flicker suppressor  70  is turned off and the current flows through the LED  26 . Flicker suppressor  70  is on during the power-up phase in which the LED  26  is otherwise susceptible to a current overshoot. 
     Those skilled in the art will appreciate that many configurations of and couplings among the components of power supply  11  are possible. For example, the components can be connected electrically, optically, acoustically, and/or magnetically. Therefore, many embodiments of power supply  11  are possible. 
       FIG. 4  shows a schematic diagram of the second embodiment of a power supply  11  for an LED  26  in accordance with the present invention. Power supply  11  employs a flyback transformer  25  driven by control circuit  38  to supply power to LED  26 . Power supply  11  includes an EMI filter  21 , an AC/DC converter  22 , a flyback transformer  25  including W 1  and W 2 , a control circuit  38 , a feedback circuit  29 , pulse width modulator switch Q 2 , a pulse width modulator (PWM)  40 , resistors R 1 -R 5 , R 8 , R 10 -R 12 , capacitors C 1 , C 2 , C 4 , C 7 , diodes D 1 , D 3 , switches Q 1  and Q 3 , control block  42  and operational amplifier O 1 . Switches Q 1 , Q 2  and Q 3  are n-channel MOSFETs. In an alternative embodiment, other types of transistors, such as an insulated gate bipolar transistors (IGBT) or bipolar transistors, are used in place of n-channel MOSFETs Q 1 , Q 2  and Q 3  to adjust the current. 
     Voltage is supplied to power supply  11  as described for power supply  10  of  FIG. 2 . The feedback circuit  29  is configured and is operational as described for power supply  10  of  FIG. 2 . When the dim command signal is a low light dim command signal, the duty cycle of pulse width modulator  40  is low. 
     The power supply circuit supplies an overshoot current to the LED  26  without a flicker suppressor circuit  70 . As described above, the overshoot is due to a lag in the generation of a feedback signal to the control circuit  38  as voltage across the LED  26  builds up to excessive levels. The transformer control signal input to the switch Q 1  adjusts the current flow through winding W 1  of flyback transformer  25  to match the LED  26  current demand until the sensed current signal and the referenced current signal are equal at the control circuit  38 . When the sensed current signal and the referenced current signal are equal, the feedback error signal goes to zero. The output voltage builds up across capacitor C 2 , which is parallel to the LED  26 , as the sensed current signal and the referenced current signal are reaching equalization. As pulses to the gate of pulse width modulator switch Q 2  pulse the LED  26 , the current sense voltage across resistor R 1  is not continuous. When the dim command signal is set for a low light level, the capacitor C 7  of the peak detect circuit does not charge to a steady state value until pulse width modulator switch Q 2  has turned on and off for a few cycles. For low LED light output levels, the time between each of the pulses to the gate of pulse width modulator switch Q 2  is relatively long. The control circuit  38  keeps building voltage across output capacitor C 2  as capacitor C 7  charges to its steady state value. 
     This voltage buildup causes the current in the LED  26  to build up to a level that is higher than the LED  26  requires. Once the voltage across capacitor C 7  reaches a steady state value, the control circuit  38  turns off switch Q 1  causing an undershoot in the LED current. Due to this overshoot and resulting undershoot of the current to LED  26 , a flicker in the optical output from the LED  26  is observed each time the power supply  10  is turned on for low LED light output levels. 
     Addition of the flicker suppressor  70  to the power supply  11  prevents overshoot and the resultant flicker during power-up of the power supply  11 . Switch Q 3  is gated by a control block (CB)  42 , which provides a continuous signal. Control block  42  is operable to turn on when the output voltage across capacitor C 2  reaches a set level, which is below the level that would produce a current overshoot in the LED  26 . When switch Q 3  is turned on by the continuous signal from a control block  42 , current flows through resistor R 8  and switch Q 3 . Resistor R 8  and switch Q 3  form a series circuit in parallel across the LED  26 . The value of resistor R 8  is chosen to limit the current through switch Q 3 . This clamps the output voltage to the set level. 
     The feedback circuit  29  receives continuous feedback while switch Q 3  is switched on so the capacitor C 7  starts to charge. As capacitor C 7  starts to charge, a feedback signal is injected into control circuit  38 . The response rate of the control circuit  38  is increased, thereby preventing flicker when switch Q 2  is gated. Once capacitor C 7  is charged to its steady state value, switch Q 3  is turned off allowing the current to flow through the LED  26 . Thus, the power supply  11  achieves flicker suppression by limiting the current to the LED  26  during power-up so that the LED light output is below 110 percent of the LED light output corresponding to the dim command signal input to the pulse width modulator  40 . 
     The control block  42  can be controlled by additional circuitry within the power supply  11  or circuitry external to the power supply  11 , such as circuitry associated with the output voltage level. 
     In one embodiment, flicker suppressor  70  and flicker suppressor  50  are both included in the power supply  11  and each functions as described above. 
       FIG. 5  shows a block diagram of a third embodiment of a power supply  12  for an LED  26  in accordance with the present invention. The power supply  12  providing power to LED  26  includes a power supply circuit  16  and a flicker suppressor  60 . Power supply circuit  16  includes AC/DC converter  22 , power converter  24 , control circuit  38 , pulse width modulator  40 , pulse width modulator switch  28 , and feedback circuit  29 . Feedback circuit  29  includes current sensor  30 , current amplifier  32 , and peak current detector  34 . The power supply  12  achieves flicker suppression by limiting the current to the LED  26  during power-up so that the LED light output is below 110 percent of the LED light output corresponding to the dim command signal input to the pulse width modulator  40 . 
       FIG. 5  differs from  FIG. 1  in that the flicker suppressor  60  is in series with the LED  26 . The power supply  12  uses current feedback circuit  29  to adjust the power to the LED  26 , the pulse width modulator (PWM)  40  to provide dimming capability for the LED  26  and flicker suppressor  60  to prevent overshoot of the current to the LED  26  during startup of the power supply  12 . Single-phase AC input is provided at block  20  and converted to DC by the AC/DC converter  22  to provide a DC voltage to the power converter  24 . Power converter  24  regulates the power to LED  26  based on the feedback signal representing a current error generated at the current controller  38 . The feedback circuit  29  and pulse width modulator  40  operate as described in reference to  FIG. 1 . The flicker suppressor  60  absorbs some of the output power during the power-up of the LED  26  and thus limits the voltage to the LED  26 . This is accomplished by providing a temporary increased resistance in series with the LED  26  during the power-up and by removing the increased resistance during steady state. 
     The flicker suppressor  60  prevents flicker due to current overshoot when the output light level from the LED  26  is within 1% to 25% of the maximum output light level. 
     Typically, the flicker due to current overshoot is noticeable when the output light level from the LED  26  within 1% to 10% of the maximum output light level. 
     Those skilled in the art will appreciate that many configurations of and couplings among the components of power supply  12  are possible. For example, the components can be connected electrically, optically, acoustically, and/or magnetically. Therefore, many embodiments of power supply  12  are possible. 
       FIG. 6  shows a schematic diagram of the third embodiment of a power supply  12  for an LED  26  in accordance with the present invention. Power supply  12  employs a flyback transformer  25  driven by control circuit  38  to supply power to LED  26 . Power supply  12  includes an EMI filter  21 , an AC/DC converter  22 , a flyback transformer  25  including W 1  and W 2 , a control circuit  38 , a feedback circuit  29 , pulse width modulator switch Q 2 , a pulse width modulator (PWM)  40 , resistors R 1 -R 5 , R 7 , R 10 -R 12 , capacitors C 1 , C 2 , C 4 , C 7 , diodes D 1  and D 3 , switches Q 1  and S 7  and operational amplifier O 1 . In the example of  FIG. 6 , switches Q 1  and Q 2  are n-channel MOSFETs. Switch S 7  may be an n-channel MOSFETs, which is open when power-up of the LED  26  begins and which is closed after power-up of the LED  26  is completed. In an alternative embodiment, other types of transistors, such as an insulated gate bipolar transistors (IGBT) or bipolar transistors, are used in place of n-channel MOSFETs Q 1 , Q 2  and S 7  to adjust the current. 
     The flicker suppressor  60  includes the resistor R 7  and switch S 7 . Resistor R 7  is in series with the LED  26  and is in parallel across switch S 7 . In operation, the flicker suppressor  60  increases the resistance in series with the LED  26  during power-up to limit the current to the LED  26  to maintain the LED light output to less than or equal to the LED light output which corresponds to the dim command signal. Voltage is supplied to power supply  12  as described for power supply  10  of  FIG. 2 . The feedback circuit  29  is configured and is operational as described for power supply  10  of  FIG. 2 . 
     The output pulses of pulse width modulator  40  have a duty cycle related to the dim command signal input to pulse width modulator  40  as described in the description of power supply  10  in  FIG. 2 . The output pulses of pulse width modulator  40  are provided to the gate of pulse width modulator switch Q 2 . During each pulse, current flows through the serially connected LED  26  and pulse width modulator switch Q 2 . When the dim command signal is a low light dim command signal, the duty cycle of pulse width modulator  40  is low. 
     During power-up of LED  26 , the switch S 7  in series with LED  26  is maintained in an open position and the gate of pulse width modulator switch Q 2  is pulsed by the pulse width modulator  40 . The current flows through resistor R 7  since switch S 7  is open. The voltage drop across resistor R 7  reduces the voltage across the LED  26  to a level that prevents a current overshoot above the reference current. After power-up of the LED  26 , the switch S 7  is closed. The current then flows through the switch S 7  with little or no resistance. This prevents the losses across resistor R 7  during steady state operation. In one embodiment, the resistance of resistor R 7  is about 10 ohms. The switch S 7  can be controlled by additional circuitry within the power supply  12  or circuitry external to the power supply  12 , such as circuitry associated with the dim command signal or an on command signal. 
     Without the voltage limitation provided by the flicker suppressor  60 , the voltage across the LED  26 , would reach levels that would cause the LED light output to exceed the LED light output corresponding to the dim command signal. Thus, the power supply  12  achieves flicker suppression by limiting the current to the LED  26  during power-up so that the LED light output is below 110 percent of the LED light output corresponding to the dim command signal input to the pulse width modulator  40 . 
     The flicker suppressors as described above can be used in combination within a single power supply. In one embodiment, flicker suppressor  60  of  FIG. 5  and flicker suppressor  50  of  FIG. 1  are both included in the power supply and each functions as described above. In one embodiment, flicker suppressor  60  and flicker suppressor  70  of  FIG. 3  are both included in the power supply and each functions as described above. In one embodiment, flicker suppressor  60 , flicker suppressor  50  and flicker suppressor  70  are all included in the power supply and each function as described above. 
       FIG. 7  shows a block diagram of a fourth embodiment of a power supply  13  for an LED  26  in accordance with the present invention. While the power supplies  10 ,  11  and  12  of  FIGS. 1-6  are current controlled voltage source output power converters, power supply  13  of  FIG. 7  shows a current source output power converter for an exemplary DC-DC power converter. The power supply  13  supplying LED  26  includes a power supply circuit  17  and a flicker suppressor  80 . Power supply circuit  17  includes DC/DC converter  23 , control circuit  39 , pulse width modulator  40 , pulse width modulator switch  28 , and feedback circuit  31 . Feedback circuit  31  includes current sensor  30  and current amplifier  32 . 
     The power supply  13  achieves flicker suppression by limiting the current to the LED  26  during power-up so that the LED light output is below 110 percent of the LED light output corresponding to the dim command signal input to the pulse width modulator  40 . 
     In power supply  13 , the DC input  21  is provided to DC/DC power converter  23 . DC/DC power converter  23  regulates the power to LED  26  based on a feedback signal representing a current error generated by the control circuit  39 . 
     The flicker suppressor  80  is operably connected in parallel with the pulse width modulator switch  28  and with the LED  26 . The flicker suppressor  80  prevents overshoot of the current to the LED  26  during startup of the power supply  10  by providing an additional current path across the LED  26  during power-up when the voltage output is greater than a set limit. In particular, the flicker suppressor  80  prevents flicker due to current overshoot when the output light level from the LED  26  is within 1% to 25% of the maximum output light level. Typically, the flicker due to current overshoot is noticeable when the output light level from the LED  26  is within 1% to 10% of the maximum output light level. 
     The feedback signal is generated by feedback circuit  31  and directed to control circuit  39 . The current sensor  30  measures the current flow to the LED  26  and provides a sensed current signal to the current amplifier  32 . The amplified sensed current signal is input to the control circuit  39  as a feedback signal. The control circuit  39  generates a control signal, which is input to the DC/DC power converter  23 . 
     The pulse width modulator (PWM)  40  provides dimming capability for the LED  26 . The pulse width modulator  40  receives a dim command signal  41  operable to adjust the duty cycle of the pulse width modulator  40 . The pulses output from the pulse width modulator  40  operate to switch the pulse width modulator switch  28 , which is in parallel with the LED  26 . 
     Those skilled in the art will appreciate that many configurations of and couplings among the components of power supply  13  are possible. For example, the components can be connected electrically, optically, acoustically, and/or magnetically. 
     It is important to note that  FIGS. 1-7  illustrate specific applications and embodiments of the present invention, and is not intended to limit the scope of the present disclosure or claims to that, which is presented therein. Upon reading the specification and reviewing the drawings hereof, it will become immediately obvious to those skilled in the art that myriad other embodiments of the present invention are possible, and that such embodiments are contemplated and fall within the scope of the presently claimed invention. 
     While the embodiments of the invention disclosed herein are presently considered to be preferred, various changes and modifications can be made without departing from the spirit and scope of the invention. The scope of the invention is indicated in the appended claims, and all changes that come within the meaning and range of equivalents are intended to be embraced therein.