Abstract:
Methods and apparati for controlling bleed current (IBLEED) in a driver circuit ( 20 ) for a lighting device ( 23 ). A method embodiment of the present invention comprises the steps of coupling a dimmer ( 21 ) to an input of the driver circuit ( 20 ), and forcing the bleed current (IBLEED) to be inversely proportional to the time-averaged voltage (VLEDP) at said lighting device ( 23 ). The dimmer ( 21 ) consumes power even when the lighting device ( 23 ) is not emitting light.

Description:
RELATED APPLICATION 
     This patent application claims the benefit of commonly owned U.S. provisional patent application 61/442,611 filed Feb. 14, 2011 entitled “Variable Bleed Current for Triac-Dimmed LED Circuits”, and is also related to U.S. patent application Ser. No. 13/110,724 filed May 18, 2011 entitled “Load Driver with Integrated Power Factor Correction”, which two patent applications are hereby incorporated by reference in their entireties into the present patent application. 
    
    
     TECHNICAL FIELD 
     This invention pertains to the field of driver circuits for lighting devices, particularly LEDs, that employ the use of dimmers. 
     BACKGROUND ART 
     The use of high-brightness LEDs (light emitting diodes) in lighting applications is growing rapidly as a result of inherent benefits to LED technology such as long lifetimes, good efficiency, and the ability to use non-toxic materials. However, retrofitting existing applications with LED fixtures often requires compatibility with the large installed base of dimmers, particularly leading-edge triac-based dimmers. Because these dimmers were commonly designed for current levels much higher than those consumed by LED applications, many problems occur with existing LED driver solutions. 
     Triac-based dimmers function by allowing current to pass during a fraction of the half-cycle of the input AC mains voltage. One of the most common types of triac dimmers is the leading-edge type, which initially turns on at some point past the zero-crossing of the AC waveform (in both the upward direction and the downward direction), and then turns off at the next zero-crossing. 
     Most leading-edge triac-based dimmers were designed for use with incandescent light bulbs. In order to turn on and power the bulb, the triac requires a latching current to flow through the load. Subsequently, to maintain the triac&#39;s on state until the next AC zero-crossing, a lesser holding current must be present. This triac behavior matches well with the strongly positive temperature coefficient of incandescent bulbs. When cold and unpowered, an incandescent bulb presents a filament resistance which is a fraction of its value when powered. As current and power dissipation increase, temperature and hence resistance increase greatly. By its nature, the incandescent bulb provides a large latching current at the time of turn on, and maintains a lesser holding current while lit. Since one of the advantages of LED-based incandescent bulb replacements is power efficiency, it naturally draws less current than the hot incandescent bulb, and much less than the cold incandescent bulb. 
     When powered with triac-based dimmers, the performance of traditional LED driver ICs suffers in several ways. First, the driver efficiency generally falls well short of the desired targets. Even with the degraded efficiency due to a bleed of either constant current or constant resistance, many driver solutions fail in terms of gross functionality with digitally-controlled triac-based dimmers, which require low load impedance even in the standby state, when the dimmer is not explicitly powering the driver yet needs to keep standby circuits alive. 
     When trying to address these concerns, existing solutions can grow substantially in size, complexity, and power consumption. These concerns are addressed by the present invention. 
     DISCLOSURE OF INVENTION 
     Methods and apparati for controlling bleed current (IBLEED) in a driver circuit ( 20 ) for a lighting device ( 23 ). A method embodiment of the present invention comprises the steps of coupling a dimmer ( 21 ) to an input of the driver circuit ( 20 ), and forcing the bleed current (IBLEED) to be inversely proportional to the time-averaged voltage (VLEDP) at said lighting device ( 23 ). The dimmer ( 21 ) consumes power even when the lighting device ( 23 ) is not emitting light. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other more detailed and specific objects and features of the present invention are more fully disclosed in the following specification, reference being had to the accompanying drawings, in which: 
         FIG. 1  is a graph of how the driver circuit  20  of the present invention controls the bleed current (IBLEED) as a function of the time-averaged voltage at the lighting device  23 . 
         FIG. 2  is a circuit  20  that effectuates the graph of  FIG. 1 . 
         FIG. 3  is a circuit diagram of an alternative embodiment of the present invention in which a transformer T 1  is used for isolation. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention controls the bleed current IBLEED in the driver circuit  20 , while maintaining excellent efficiency of the driver circuit  20  and accommodating many types of dimmers  21 . Flicker and other unwanted manifestations in the lighting device  23  are avoided. 
     Our invention is particularly well-suited for lighting devices  23  that comprise one or several LEDs (light emitting diodes)  33 , but the invention also has applicability where the lighting device  23  comprises one or more fluorescent light bulbs. 
     Dimmer  21  is typically a dimmer comprising a triac, a semiconductor that is a three-terminal device but is bidirectional, i.e., power can flow both ways through its power terminals. This is the most convenient way to dim in the present state-of-the-art. However, the present invention can be used with dimmers  21  other than those comprising triacs. Dimmer  21  is typically operated by a person, but it can also be programmed to automatically dim and brighten lighting device  23  using, for example, a pre-programmed software program. Dimmer  21  is typically rated for between 300 watts and 600 watts of power, whereas in the embodiment where lighting device  23  comprises one or more LEDs  33 , lighting device  23  is typically rated for about 10 watts. 
       FIG. 1  shows a graph that is deemed desirable by the present invention. The graph shows IBLEED as a function of the time-averaged voltage (VLEDP) applied to the lighting device  23 . As used herein, “time-averaged” means averaged (RC low-passed) over many cycles of the input AC waveform. We use time averaging because we want IBLEED to be steady (non-choppy) over time. In other words, we want to average out periodic line fluctuations. In  FIG. 1 , the horizontal bar across the letters VLEDP means “average”. 
     The number of cycles in the time duration over which the averaging takes place must be greater than the frequency of changing the amplitude of dimmer  21 , but less than the input AC frequency at input terminals LINE, NEUT. This AC line frequency is normally 60 Hz in the United States, but in other countries may be some other frequency, such as 50 Hz. The frequency of moving the amplitude of dimmer  21  is usually quite low, because dimmer  21  is normally activated by a human. However, as stated above, dimmer  21  can be activated by an automated means, in which case the amplitude of dimmer  21  can vary more rapidly than by human operation. 
     The graph in  FIG. 1  does not have to be a straight line. For example, it could be curved, but for purposes of illustration, this specification describes the case where IBLEED as a function of voltage is a straight line. 
     We preselect IMAX (the maximum value of IBLEED, and the (BLEED that occurs at 0 volts) to be greater than the standby current (ISTAND) required by whatever dimmer  21  or dimmers  21  we plan to use in conjunction with the driver circuit  20  and lighting device  23 . In general, the lower the quality of the dimmer  21 , the higher its standby current will be, and therefore the higher we need IMAX to be. 
     We preselect VMAX, the voltage where IBLEED is zero, to be less than the maximum input voltage applied to circuit  20 , i.e., the voltage when dimmer  21  is turned up to its maximum amplitude. VMAX is typically less than the voltage VMAXR where the LEDs  33  achieve full brightness. Finally, the third criterion that we satisfy is that the bleed current IINT present at some intermediate voltage VINT where the dimmer  21  needs to latch and hold (stay on) must be sufficient to enable said dimmer  21  to latch and hold, to avoid flicker from the lighting device  23 . 
     An advantage that the graph of  FIG. 1  has over the prior art is that the reduced bleed current IBLEED at the higher voltages allows for a higher IMAX and IBLEED in general at the lower voltages, without the penalty of higher power consumption, since bleed circuit power consumption is the simultaneous product of bleed current and voltage. 
       FIG. 2  shows a circuit that we have designed that effectuates the graph of  FIG. 1 . The AC line voltage applied to dimmer  21  is shown as having two inputs, a line voltage input LINE and a neutral input NEUT. Dimmer  21  typically likes to look into a purely resistive load, while an LED  33  is not a pure resistor. However, this can be compensated for. Dimmer  21  consumes power even when the lighting device  23  is off. This is the origination of the bleed current IBLEED. Even a basic triac dimmer  21  has some residual current. So-called smart dimmers  21  have quite a bit of residual current. The bleed current IBLEED does not contribute to activating the lighting device  23 . 
     An optional EMI (electromagnetic interference) filter  22  can be inserted between dimmer  21  and driver circuit  20 . When used, filter  22  helps to filter out unwanted electromagnetic energy. Rectifier  24  is typically but not necessarily a full bridge rectifier comprising four diodes in a standard bridge configuration. Lighting device  23  is shown as an array  33  of several LEDs connected in series, with an optional capacitor C 3  connected in parallel across array  33 . Capacitor C 3  works in conjunction with smoothing inductor L 1  to smooth the current going into the lighting device  23 , and in particular, going into the LED array  33 , making said current closely resemble a direct current. The output voltage across lighting device  23  is taken at two terminals, LEDP and LEDN, representing positive and negative polarities, respectively. The voltage at LEDP (VLEDP) is a function of how much dimming is being employed, and may or may not vary over a given time interval. 
     Capacitor C 1 , coupled between the positive output of rectifier  24  and ground, serves to filter out high frequency noise. The negative output of rectifier  24  is grounded. The bleed current control circuit  25  has an input voltage VIN, which is measured between resistors R 2  and R 6 . VIN is a fixed fraction of VLEDP. 
     The negative terminal of operational amplifier (op amp)  28  is coupled to the negative output terminal of rectifier  24  via resistors R 1  and R 2 . A reference voltage VREF is applied to the positive input terminal of op amp  28 . A fixed control voltage VCC (the supply voltage to circuit  25 ) is applied to the control terminal of op amp  28 . A first low dropout voltage regulator (LDO)  26  is coupled between the control terminal of op amp  28  and LEDP. The output terminal of op amp  28  is applied to a first terminal of a bipolar transistor or FET  29 . When a FET is used, the output terminal of op amp  28  is applied to the gate of FET  29 . In that case, the drain of FET  29  is applied through resistor R 3  to the negative input terminal of op amp  28 , and the source of FET  29  is coupled to a first terminal of a second low dropout voltage regulator (LDO)  27 , referred to as terminal BLD in  FIG. 2 . IBLEED is measured at this terminal. 
     The second terminal of LDO  27  is coupled to LEDP and the negative output terminal of rectifier  24 . For each LDO  26 ,  27 , there is a relatively high voltage at its upper terminal and a relatively low fixed voltage at its lower terminal. Capacitor C 2  is coupled between the negative input terminal of op amp  28 , which is called terminal CBLD (capacitor bleed) in  FIG. 2 , and a terminal denominated RBLD (resistor bleed), which is coupled to the drain of FET  29 . Capacitor C 2  establishes the time constant over which the aforesaid time-averaging takes place. 
     A resistor R 4  is coupled between RBLD and ground. R 4  establishes IMAX. Power supply  30  provides power to control circuit  25 . Op amp  28 , transistor  29 , resistors R 1  and R 3 , and power supply  30  can be implemented in an integrated circuit  25 . 
     Power supply  30  is typically a switch-mode (switching) power supply, since this type of power supply is smaller and more efficient than a conventional power supply. Voltage VCC is applied to the input power terminal of power supply  30 . The SW (switch) terminal of power supply  30  is coupled to SW 1 , which can be a bipolar transistor or an FET.  FIG. 2  shows the example where SW 1  is an FET, in which case the SW terminal of power supply  30  is coupled to the gate of FET SW 1 . In this example, the ISNS (current sense) terminal of power supply  30  is coupled to the drain of FET SW 1 , and, via resistor R 5 , to ground. Resistor R 5  regulates the current that is supplied to the LEDs  33 . R 5  is the sense resistor for SW 1 . The source of FET SW 1  is coupled to a first terminal of diode D 1 , which may be a Schottky diode, and via inductor L 1  to LEDN. The second terminal of diode D 1  is coupled to LEDP. 
     Resistor R 2  is coupled between the negative output of rectifier  24  and, via resistor R 1 , to the negative input terminal of op amp  28 . The resistive bridge comprising resistors R 2  and R 6  serves to set VIN at a point that is optimal for the components within circuit  25 , to stabilize VIN, and to establish VMAX. 
     Resistor R 3 , in conjunction with resistor R 1 , establishes the slope of the  FIG. 1  curve, by the following equation:
 
 V ( RBLD )= R 3*( V IN− V REF)/ R 1.
 
       FIG. 3  illustrates an alternative isolated embodiment of the present invention, in which a transformer T 1  takes the place of power inductor L 1 . The  FIG. 3  embodiment is suitable for higher-end performance lighting applications, where electrical isolation is needed or desired, e.g., for reasons of safety. In  FIG. 3 , SW 1  is located on the rectifier  24  side of transformer T 1 , while diode D 1  is located on the load  23  side of transformer T 1 . 
     The above description is included to illustrate the operation of the preferred embodiments, and is not meant to limit the scope of the invention. The scope of the invention is to be limited only by the following claims. From the above discussion, many variations will be apparent to one skilled in the art that would yet be encompassed by the spirit and scope of the present invention.