Abstract:
A flicker suppression system for a dimmable LED bulb. In one embodiment, the system includes a rectifier circuit having input terminals and output terminals. The rectifier circuit is configured to rectify a line voltage to generate a rectified voltage at its output terminals. A resistor and switch are also included and coupled in series. A switch control circuit is directly coupled between the output terminals and configured to control the switch only as a function of the rectified voltage.

Description:
RELATED APPLICATIONS 
     This application claims the domestic benefit under Title 35 of the United States Code §119(e) of U.S. Provisional Patent Application Ser. No. 61/530,185, entitled “Flickering Suppressor System for LED Light Bulb TRIAC Dimmable,” filed Sep. 1, 2011, and naming Jean Claude Harel as the inventor, which is hereby incorporated by reference in its entirety and for all purposes as if completely and fully set forth herein. 
    
    
     BACKGROUND OF THE INVENTION 
     Dimmers are devices that allow users to adjust the amount of power delivered to light bulbs in various lighting applications (e.g., home, commercial, etc.). Many types of conventional dimmers are often mounted on a wall and have a user interface such as a knob or a slider, which can be manipulated by a user. Typically, the user interface is mechanically coupled to a variable resistor, and as a user manipulates the user interface the resistance of the variable resistor increases or decreases, which in turn increases or decreases the power delivered to the light bulb. 
     SUMMARY OF THE INVENTION 
     A flicker suppression system for a dimmable LED bulb. In one embodiment, the system includes a rectifier circuit having input terminals and output terminals. The rectifier circuit is configured to rectify a line voltage to generate a rectified voltage at its output terminals. A resistor and switch are also included and coupled in series. A switch control circuit is directly coupled between the output terminals and configured to control the switch only as a function of the rectified voltage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention may be better understood in its numerous objects, features, and advantages made apparent to those skilled in the art by referencing the accompanying drawings. 
         FIG. 1  is a schematic and block diagram illustrating an example lighting system. 
         FIG. 2  illustrates waveforms relevant to the operation of the lighting system shown in  FIG. 1 . 
         FIG. 3  is a schematic and block diagram illustrating a lighting system employing a flicker suppression circuit. 
         FIG. 4  is a schematic and block diagram illustrating a lighting system employing a flicker suppression circuit. 
         FIG. 5  is a schematic and block diagram illustrating a lighting system employing a flicker suppression circuit. 
         FIG. 6  shows the lighting system of  FIG. 4  with a more detailed view of an example flicker suppression circuit. 
         FIG. 7  is a schematic diagram illustrating an example of the flicker suppression circuit shown in  FIG. 6 . 
     
    
    
     The use of the same reference symbols in different drawings indicates similar or identical items. 
     DETAILED DESCRIPTION 
     Many lighting systems are powered by an alternating current (AC) source, commonly referred to as “line voltage” (e.g., 120 volts RMS at 60 hertz). A conventional AC dimmer typically receives line voltage as an input, and provides an adjusted output voltage for a light bulb. 
     Conventional dimmers can control power delivered to a light bulb in different ways. Most commonly, adjustments by the user causes the dimmer to adjust the duty cycle of the output (e.g., by chopping out portions of the AC voltage cycles). This technique is sometimes referred to as phase angle control. The most commonly used dimmers of this type employ a triode-for-alternating current (TRIAC), which is an electronic component that can conduct current in either direction when it is triggered (turned on). When employed in a dimmer, the TRIAC chops off rising portions of the AC voltage half cycles (e.g., portions after 0 volt crossings and before peaks) depending on where the user interface (e.g., slider or knob) is set as will be more fully described below. 
       FIG. 1  illustrates a lighting system that employs a TRIAC-based dimmer. The lighting system includes voltage source  100  that delivers a line voltage V 1  to TRIAC-based dimmer  104 , which in turn controls the power delivered to light bulb  102 . Dimmer  104  is one example of one of many types of dimmers that can be employed for controlling power to a light bulb, such as light bulb  102  shown in  FIG. 1 . It should be noted that the term dimmer should not be limited to dimmer  104  shown within  FIG. 1 . 
     Dimmer  104  includes a variable resistor R, the resistance of which can be varied by a user via an interface (not shown) such as a wall-mounted slider. Variable resistor R is coupled in series to resistor  112 , the combination of which is coupled to capacitor  114 . A diode-for-alternating current (DIAC)  116  is coupled between a capacitor  114  and a gate of TRIAC  120 . Light bulb  102  is connected in series with TRIAC  120 . 
     A DIAC is a diode that activates or turns-on only after its breakover voltage has been reached. When this occurs, there is a decrease in the voltage drop across the diode and, usually, a sharp increase in current through the diode. The diode remains active or “on” until the current through it drops below a value characteristic for the device, called the DIAC holding current. Below this value, the diode switches back to its high-resistance (non-conducting) off state. DIACs are mainly used for triggering (turning on) TRIACs. TRIACs can be triggered by either a positive or a negative current applied to its gate electrode. However, a minimum amount of current (latching current) is required to maintain the TRIAC in the on-state immediately after a TRIAC is triggered. Moreover, a minimum current (holding current) is required to maintain the TRIAC in the on-state, not allowing it to turn off. 
     Typical TRIAC-based dimmers can dim light through phase angle control as mentioned above. Initially, assume TRIAC  120  is turned off so that no power flows to light bulb  102 . As line voltage V 1  increases from zero (at the start of every half wave), capacitor  114  charges. When the voltage on capacitor  114  exceeds the breakover voltage of DIAC  116 , DIAC  116  activates and conducts current from capacitor  114  to the gate of TRIAC  120  and turns it on. The DIAC  116  is active for a short period of time while discharging capacitor  114 . Eventually the voltage across DIAC  116  drops and it deactivates, which terminates the gate current to TRIAC  120 . If current flowing through TRIAC  120  exceeds its latching current when the gate current terminates, the TRIAC will remain on and continue to conduct current to light bulb  102  so long as the TRIAC&#39;s holding current is exceeded. The current to light bulb  102  will eventually fall below the TRIAC&#39;s holding current as the line voltage V 1  drops to zero near the end of the half cycle, at which point TRIAC  120  will turn off. A similar process repeats for the next half cycle. 
       FIG. 2  illustrates wave forms relevant to the operation of dimmer  104 . In particular,  FIG. 2  shows the line voltage V 1 , and the chopped output voltage  202  provided to light bulb  102  as TRIAC  120  turns on and off as described above.  FIG. 2  also shows pulses during which DIAC is activated and conducting current to the gate of TRIAC  120 . TRIAC  120  turns on with each pulse of DIAC  116  as described above. A time constant is formed by capacitor  114  and the series combination of variable resistor R and resistor  112 . The time at which the DIAC pulses occur depends on the time constant, which can be adjusted by changing the resistance of variable resistor R. One of ordinary skill in the art understands that the input wave form can be clipped at different phase angles than that shown within  FIG. 2  by adjusting the resistance of variable resistor R. 
     Light emitting diode (LED) based light bulbs are becoming more popular due to their long service life and high energy efficiency. LED light bulbs can be made interchangeable with other types of light bulbs such as incandescent light bulbs. Some LED light bulbs are made with identical bases so that they are directly interchangeable with incandescent light bulbs. 
     LEDs operate based on substantially DC power sources. In other words, LEDs need to be powered in constant current/constant voltage mode. In  FIG. 1 , light bulb  102  takes form in an LED light bulb that includes a LED converter circuit  106  and LEDs  110 . The LED converter  106  is configured to receive the chopped AC output of dimmer  104  and provide constant current and voltage to the LED  110 . In one embodiment, the LED controller  106  may include a rectifier, a low pass filter, and a DC converter (not shown). In this embodiment, the output of the DC converter tries to provide a stable DC voltage to LED  110   s  as duty cycle changes for the output voltage provided by dimmer  104 . Reducing the duty cycle of the voltage may not achieve light dimming since the LED controller  106  tries to adjust for the missing portion of the input voltage. If dimmer  104  outputs a voltage having a duty cycle of 50% “on” or if dimmer  104  outputs a voltage having a duty cycle of 25% “on” the difference in light output by light bulb  102  may not be perceivable by the human eye. In other words, a user may not notice a perceptible difference in the light output as the user adjusts down the variable resistor R due to the action of the DC converter until under-voltage circuitry (not shown) of controller  106  kicks in, at which point the LED converter  106  turns off and no light is generated by LEDs  110  at all. 
     To resolve this problem, the LED controller  106  may include a circuit (e.g., a microcontroller) that reads the phase angle of the chopped input voltage and adapts the DC converter to reduce the power delivered to LEDs  110 , which results in perceivable dimming. Unfortunately, this means the dimming range for LED light bulb  102  is dependent upon the dynamic range of LED controller  106  and not dimmer  104 . While dimming LED light bulbs, it is not uncommon to reduce the power applied to the LED light bulbs by 99% or more in order to achieve a comparable dimming effect that one would experience when dimming incandescent light bulbs. That means for a 9-watt LED light bulb, when fully dimmed, the power provided by a dimmer (e.g., dimmer  104 ) could be well below 1 watt. 
     As a user continues to reduce power provided to LED light bulb  102  via dimmer  104 , the current flow through TRIAC  120  will fall accordingly. At some point, the current may fall below the TRIAC&#39;s holding current, or there will not be enough current to latch TRIAC  120  on when current is injected into the gate as DIAC  116  pulses. When this occurs, TRIAC  120  may suddenly turn off or not trigger at all, and the power delivered to LED light bulb  102  is interrupted. For the next few cycles, the input current to TRIAC  120  might be higher, which allows converter  106  to restart and power up LEDs  110 . This condition creates “flickering” or an instability that results in a rapid turn on and turn off of LEDs  110 . 
     Flickering can be reduced or eliminated by maintaining the current flow above a minimum when TRIAC  120  is triggered by a pulse from DIAC  116 . Current in the TRIAC can be maintained above the minimum if a load such as a resistor is permanently coupled between the outputs of dimmer  104  and thus in parallel with light bulb  102 . However, this solution leads to low efficiency due to the power loses in the permanently coupled resistor, particularly when full power is being delivered to light bulb  102 . 
     Alternatively, a load such as a resistor can be selectively coupled in parallel with light bulb  102  when needed.  FIGS. 3-5  illustrate alternative solutions in which a flicker suppressor circuit selectively couples a load such as a resistor in parallel with light bulb  102 . The selectively coupled resistor can operate to maintain a minimum TRIAC current when needed, which should reduce or eliminate flickering. 
       FIGS. 3-5  illustrate lighting systems each of which contain a flicker suppressor circuit (FSC). In one embodiment, the FSC circuits in each of the Figures are identical to each other. In an alternative embodiment the FSC circuits may be different from each other. In  FIG. 3 , the FSC is contained within dimmer  302 , and in  FIG. 4 , the FSC is between dimmer  104  and light bulb  102 . It is noted that dimmer  302  shown in  FIG. 3  is nearly identical to dimmer  104  shown in  FIG. 1 , except for the addition of FSC  304 . In  FIG. 5 , LED converter circuit  502  may be similar to the LED converter  106  circuit shown in  FIG. 3  or  4 , except for the addition of FSC  304 . In one embodiment, the FSC  304  in LED converter circuit  502  may be identical to FSCs  304  shown within  FIGS. 3 and 4 . In an alternative embodiment, the FSC  304  in  FIG. 5  may be different. For example, the FSC  304  in  FIG. 5  may share a bridge rectifier (more fully described below) with other components of LED convertor circuit  502 . 
     The FSC  304  can maintain a minimum current in TRIAC  120  by inserting a bleeder resistor (not shown in  FIGS. 3-5 ) in parallel with LED light bulb  106  when needed. When not needed, the FSC disconnects the bleeder resistor so that current does not flow there through. In one embodiment, the FSC can maintain current in TRIAC  120  by inserting the bleeder resistor just before TRIAC  120  is activated by a pulse from DIAC  116 . The FSC can maintain the bleeder resistor in parallel with light bulb  102  for a short time after the DIAC pulse ends to insure that TRIAC latching current is meet. The bleeder resistor should be sized so that a minimum current (e.g., current that is greater than the latching or holding current for TRIAC  120 ) flows through it. This will insure proper latching of TRIAC  120  for the remaining portion of the voltage cycle and maintain the power provided to the LED light bulb  106 , which in turn should preclude flickering. In one embodiment, the FSC is a stand-alone system, which means that it operates independently of LED converter  106  shown in  FIG. 3  or  4  or the other electronics contained in LED converter  502 . In other words, FSC does not rely on a control signal from of LED converter  106  shown in  FIG. 3  or  4  or the other electronics contained in LED converter  502  when selectively inserting a bleeder resistor in between the outputs of dimmer  104  or  302 . 
       FIG. 6  illustrates an example FSC that can be employed within any of the lighting systems shown in  FIGS. 3-5 . More particularly, the FSC includes a bridge rectifier circuit  600 . A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction. The process is known as rectification. Inputs to bridge rectifier  600  are coupled to conductors  108 R and  108 L as shown. The FSC further includes a bleeder resistor  602  and switch  604 . Lastly, the FSC includes a switch control circuit  606  that controls switch  604 . 
     In one embodiment, the switch control circuit  606  closes switch  604  just before a gate pulse is provided to TRIAC  120  via DIAC  116 . The switch control circuit  606  continues to maintain switch  604  in the closed position for a short time period after the gate pulse completes. While switch  604  is closed the minimal current needed to turn on or maintain TRIAC  120  in the on state can flow through bleeder resistor  602 . Once TRIAC  120  turns on, switch control circuit  606  opens switch  604 . 
       FIG. 7  illustrates in an example embodiment of the FSC shown in  FIG. 6 . Rectifier  600  contains diodes arranged in a bridge to convert the AC input to DC output, which is provided to the series combination of bleeder resistor  602  and switch  604 . In one embodiment, switch  604  takes form in an integrated gate bipolar transistor (IGBT). The IGBT combines a FET  712  for the control input, and a bipolar power transistor  714  for the switching mechanism. The switch control circuit  606  takes form in MOSFET  702  having a drain coupled to a gate of FET  712 . The switch control circuit  606  includes a pair of resistors  704  and  706 , and a capacitor  710 . Lastly, capacitor  710  is coupled in parallel with a zener diode  716  and MOSFET  702  as shown. Zener diode  716  is coupled between the gate of MOSFET  702  and common node c. A zener diode is a special kind of diode which allows current or flow in the forward direction in the same manner as an ideal diode, but will also permit it to flow in the reverse direction when the voltage is above a certain value known as the zener breakdown voltage. 
     Bleeder resistor  602  is inserted in parallel with the LED light bulb  102  when switch  604  is activated or turned on. Switch  604  is controlled by MOSFET  702 , which in turn is controlled by the voltage at common node c. To illustrate, assume MOSFET  702  is off or inactive (e.g., the gate to source voltage Vgs of MOSFET  702  is below its threshold voltage Vt). With MOSFET  702  off, switch  704  is closed, which couples bleeder resistor  602  in parallel with light bulb  106  via bridge  600 . Also, assume TRIAC  120  is initially inactive or turned off, and as a result no current flows therethrough or through bleeder resistor  602 . On the other hand, because switch  604  is closed prior to activation of TRIAC  120 , TRIAC  120  can draw current through bleeder resistor  602  when TRIAC  120  is first activated. 
     As line voltage V 1  increases, capacitor  114  integrates charge, which in turn increases the voltage across DIAC  116 . Eventually, this voltage exceeds the DIAC breakover voltage, at which point DIAC  116  conducts current to the gate of TRIAC  120  during the pulse as described above. With the pulse, current begins flowing through bleeder resistor  602  and TRIAC  120 . Bleeder resistor  602  should be sized so that the current flowing TRIAC  120  exceeds a latching current when the DIAC pulse ends. 
     The voltage at common node c within switch circuit  606  continues to increase. More particularly, as current flows through resistor  704  and into capacitor  710 , the voltage on capacitor  710  increases. Eventually, as the voltage at common node c exceeds the zener breakdown voltage of zener diode  716 , the voltage at the gate of MOSFET  702  increases. Once the gate voltage of MOSFET  702  exceeds its threshold voltage Vt, MOSFET  702  activates, which in turn deactivates switch  604  by virtue of deactivating FET  712 . With switch  604  off, current no longer flows through bleeder resistor  602 . However, at that point, current flow through TRIAC  120  should exceed its latching current, and TRIAC  120  turns on. 
     It is noted that current can flow through resistors  704  and  706  within switch control circuit  606  when MOSFET  702  is turned on. However, if resistor  704  is sufficiently sized (e.g., 560 k ohms) relative to resistor  706 , the power consumed by the combination of resistors  704  and  706  should be minimal. It is also noted that capacitor  710  is provided to increase the speed at which MOSFET  702  will activate by virtue of current flow from capacitor  710  to the gate of MOSFET  702  via zener  716 , since resistor  704 , which has a high resistance, restricts the flow of current to common node c, and thus to the gate of MOSFET  702  via zener diode  716 . 
     As the line voltage V 1  begins to drop toward the zero crossing and begin the next half-phase cycle, the voltage at the common node c also begins to drop. When the voltage at node c falls below the zener breakdown voltage of zener diode  716 , MOSFET  702  will deactivate once its gate voltage drops below Vt. When MOSFET  702  deactivates, the voltage at the gate of FET  712  raises and eventually activates FET  712 , which in turn closes switch  604  and places bleeder resistor  602  in parallel with the light bulb  106  once again. It is noted that current may flow through bleeder resistor  602  before line voltage V 1  reaches the zero crossing. However, since line voltage V 1  is reducing toward zero, this current flow through bleed resistor  602  is small, which in turn minimizes the power consumption of bleeder resistor  602  prior to the zero crossing. Further, as the line voltage V 1  reduces toward zero, eventually the current flow through TRIAC  120  falls below its holding current and TRIAC  120  will turn off as a result. Once turned off, current will no longer flow through bleeder resistor  602 . However, switch  604  remains on by virtue of the voltage remaining across capacitor  710 . The time during which switch  604  is closed thus inserting bleeder resistor  602  in parallel with light bulb  106  can be controlled by the size of resistor  706 . As the resistance of resistor  706  increases, the time during which switch  604  remains closed increases, and vice versa. 
     Although the present invention has been described in connection with several embodiments, the invention is not intended to be limited to the specific forms set forth herein. On the contrary, it is intended to cover such alternatives, modifications, and equivalents as can be reasonably included within the scope of the invention as defined by the appended claims.