Abstract:
LED lamp systems as described herein include a dimmer switch and a bleeder circuit. The bleeder circuit provides a bleeder current to management voltage and to prevent the dimmer switch from turning off prematurely. The bleeder circuit may monitor the AC input voltage outputted by the dimmer switch. When the AC input voltage is less than a first threshold, the bleeder circuit provides a bleeder current. When the AC input voltage is greater than a second threshold, the bleeder circuit adjusts the bleeder current to less than a predetermined level.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application Ser. No. 62/050,704, “Dynamic Bleeder Current Control For LED Dimmer,” filed Sep. 15, 2014, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to driving LED (Light-Emitting Diode) lamps and, more specifically, to adaptively dimming the LED lamps. 
     2. Description of the Related Arts 
     A wide variety of electronics applications now use LED lamps. These applications include architectural lighting, automotive head and tail lights, backlights for liquid crystal display devices, flashlights, and electronic signs. LED lamps have significant advantages compared to conventional lighting sources, such as incandescent lamps and fluorescent lamps. These advantages include high efficiency, good directionality, color stability, high reliability, long life time, small size, and environmental safety. Accordingly, LED lamps have replaced conventional lighting sources in many applications. For example, LED lamps are often used in applications where the brightness of the light source is adjusted, such as in a dimmable lighting system. 
     Dimmable lighting systems often use phase cut dimmer switches that employ a triac device to regulate the power delivered to a lamp by conducting during a certain period of an AC voltage supplied to the triac. To maintain the triac in the conducting state, a minimum holding current needs to be supplied to the triac. However, because LED lamp loads vary widely, triac devices may be unable to operate reliably. Furthermore, the minimum holding current varies widely among triac devices, which may further complicate the design of LED-based dimmable lighting systems. When the current through the triac device is less than a minimum holding current threshold, the triac device resets and pre-maturely turns off. As a result, LED lamps may prematurely turn off when they should be on, which may result in a perceivable light flicker or complete failure in the LED lamp. 
     SUMMARY 
     LED lamp systems as described herein include a dimmer switch and a bleeder circuit. The bleeder circuit provides a bleeder current to prevent the dimmer switch from turning off prematurely. Triac dimmers usually require about 100-200 mA to be turned on during a triggering operating mode. When triggered, triac dimmers enter into a triac conducting operating mode, where a triac dimmer continues to conduct until the current through the triac dimmer drops below a threshold current level (e.g., 5-20 mA). During the conducting operating mode, a triac dimmer may turn off when the current through the triac dimmer drops below the threshold current level, resulting in a perceivable flicker in the LED lamp. The bleeder circuit may monitor the AC input voltage outputted by the dimmer switch. When the AC input voltage is less than a first threshold, the bleeder circuit provides a bleeder current. When the AC input voltage is greater than a second threshold, the bleeder circuit adjusts the bleeder current to less than a predetermined level. 
     The features and advantages described in the specification are not all inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings and specification. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and may not have been selected to delineate or circumscribe the inventive subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The teachings of the present disclosure can be readily understood by considering the following detailed description in conjunction with the accompanying drawings. 
         FIG. 1  is a circuit diagram illustrating an LED lamp system, according to one embodiment. 
         FIG. 2  is a circuit diagram illustrating an LED lamp system, according to one embodiment. 
         FIG. 3A  illustrates example voltage waveforms of the LED lamp system of  FIG. 2 , according to one embodiment. 
         FIG. 3B  illustrates an example control signal waveform of the LED lamp system of  FIG. 2 , according to one embodiment. 
         FIG. 3C  illustrates an example bleeder circuit control signal waveform of the LED lamp system of  FIG. 2 , according to one embodiment. 
         FIG. 4A  illustrates example voltage waveforms of the LED lamp system of  FIG. 2 , according to another embodiment. 
         FIG. 4B  illustrates an example control signal waveform of the LED lamp system of  FIG. 2 , according to another embodiment. 
         FIG. 4C  illustrates example bleeder current waveforms of the LED lamp system of  FIG. 2 , according to another embodiment. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     The Figures (FIG.) and the following description relate to embodiments of the present disclosure by way of illustration only. It should be noted that from the following discussion, alternative embodiments of the structures and methods disclosed herein will be readily recognized as viable alternatives that may be employed without departing from the principles of the present disclosure. 
     Reference will now be made in detail to several embodiments of the present disclosure, examples of which are illustrated in the accompanying figures. It is noted that wherever practicable similar or like reference numbers may be used in the figures and may indicate similar or like functionality. The figures depict embodiments of the present disclosure for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the embodiments of the disclosure described herein. 
       FIG. 1  is a circuit diagram illustrating an LED lamp system  100  comprising an alternating current (AC) mains  114 , a dimmer switch  104 , and an LED lamp circuit  102 . The AC mains  114  provides an AC voltage  122  to the LED lamp circuit  102 . The dimmer switch  104  is coupled in series with the AC mains  114  and the LED lamp circuit  102  including an LED string  112 . The LED string  112  includes one or more LEDs. The dimmer switch  104  controls the amount (i.e., intensity) of light output by the LED string  112  by phase modulating the AC mains  114  to provide a regulated AC input voltage to the LED lamp circuit  102 . In one embodiment, the dimmer switch  104  is a phase cut dimmer including a triac device (not shown). A triac device included in the dimmer switch  104  is a bidirectional device that can conduct current in either direction when it is turned on (or triggered). One example of a dimmer switch that includes a triac device is described in U.S. Pat. No. 7,936,132. When the dimmer switch  104  including a triac device is turned on, the dimmer switch  104  continues to conduct until the current through the dimmer switch  104  and the LED string  112  drops below a holding current threshold. 
     The dimmer switch  104  determines the amount of adjustment applied to AC voltage  122  provided by the AC mains  114  based on the value of a dimming input signal  116  applied to the dimmer switch  104 . That is, the AC input voltage outputted by the dimmer switch is generated based on the value of the dimming input signal  116 . In some implementations, the dimming input signal  116  is an analog signal produced by a knob, slider switch, or other suitable electrical or mechanical device capable of providing an adjustment signal with a variable range of adjustment settings. In other implementations, the dimming input signal  116  is a digital signal. The dimmer switch  104  outputs an AC input voltage  118  to the LED lamp circuit  102 . The LED lamp circuit  102  adjusts the light output intensity of the LED string  112  substantially proportionally to the received AC input voltage  118 , exhibiting behavior similar to incandescent lamps. The LED lamp circuit  102  controls the current through the LED string  112  in a regulated manner that provides a smooth transition in light intensity level output of the LED lamp circuit  102  responsive to the dimming input signal  116  without perceivable flicker. 
     The LED lamp circuit  102  comprises a rectifier circuit  106 , a bleeder circuit  108 , a driver circuit  110 , and the LED string  112 . The rectifier circuit  106  receives the AC input voltage  118  and outputs a rectified voltage  120  corresponding to the AC input voltage  118 . The dimming level of the LED string  112  may be adjusted such that the current through the LED string  112  is below the holding current threshold of the triac device of the dimmer switch  104 . In such case, the bleeder circuit  108  ensures the triac device of the dimmer switch  104  to remain conducting while the LED string  112  can be adjusted within a dimming setting. The bleeder circuit  108  turns on to provide a bleeder current when the AC input voltage  118  is below a first threshold voltage. As such, the bleeder circuit  108  provides a current path across the output of the rectifier circuit  106 . The bleeder current provided by the bleeder circuit  108  discharges an input capacitor and provides a low impedance current path to ensure the triac device of the dimmer switch  104  to function properly. The internal timer of the triac device of the dimmer switch  104  can reset properly and charge up at the same time, which prevents dimmer phase jitter from cycle to cycle. In some embodiments, the bleeder circuit  108  provides bleeder current at different levels to reduce thermal loss and to increase the over-all system efficiency. When the AC input voltage  118  exceeds a second threshold voltage, the bleeder circuit  108  reduces the bleeder current. The second threshold voltage is greater than the first threshold voltage. Details of the bleeder circuit  108  will be further described with reference to  FIG. 2 . The driver circuit  110  provides a driving current to the LED string  112 . The driver circuit  110  switches on and off thereby to regulate the driving current through the LED string  112  according to a duty cycle determined based on the rectified voltage  120 . 
       FIG. 2  is a circuit diagram illustrating an LED lamp system  100  including a dimmer switch  104  used in conjunction with an LED lamp circuit  102 . The LED lamp circuit  102  controls dimming of the LED string  112  to achieve the desired dimming based on the dimming input signal  116 . The LED lamp circuit  102  adaptively controls dimming in a manner that reduces or eliminates perceivable flickering of the LED string  112  throughout the dimming range, and causes the LED string  112  brightness to respond quickly and smoothly when the dimmer switch  104  is adjusted. In the illustrated example, the rectifier circuit  106  comprises a diode bridge  202  and a capacitor  204 . The rectifier circuit  106  provides a rectified voltage  120 , which is an unregulated direct current (DC) voltage to the bleeder circuit  108 . The capacitor  204  is coupled in parallel to the output of the diode bridge  202 . The diode bridge  202  generates a rectified voltage  120  based on the AC input voltage  118  outputted by the dimmer switch  104  based on the dimming input signal  116 . The rectified voltage  120  is provided to the capacitor  204 . 
     The bleeder circuit  108  comprises a bleeder circuit controller  206 , a bleeder current switch  208 , and a resistor  210 . The bleeder circuit controller  206  regulates the bleeder current switch  208  to provide a bleeder current path across the output of the rectifier circuit  106  when the AC input voltage  118  outputted by the dimmer switch  104  is below a first threshold voltage. The bleeder circuit controller  206  monitors the AC input voltage  118 , detects characteristics of the AC input voltage  118 , and determines when the AC input voltage  118  reaches the first threshold voltage indicating that the AC input voltage  118  is at or near 0 volts (i.e., a zero crossing voltage). The bleeder circuit controller  206  may use one or a combination of digital or analog circuit techniques. In one implementation, the bleeder circuit controller  206  includes a digital sampling circuit (not shown) and a comparator (not shown). The digital sampling circuit samples the AC input voltage  118  at a specified interval or over a specified period of time. The samples are provided to the comparator that compares the value of a specified number of samples to detect whether the AC input voltage  118  is at or near the zero crossing voltage. 
     When the bleeder circuit controller  206  determines that the AC input voltage  118  is at or near the zero crossing voltage, i.e., below the first threshold voltage, the bleeder circuit controller  206  generates a control signal  242  to enable the bleeder circuit  108  by turning on the bleeder current switch  208  thereby to provide a path for the bleeder current through the resistor  210  across the output of the rectifier circuit  106 . The bleeder current switch  208  may be a semiconductor power switch such as a metal oxide field effect transistor (MOSFET) as illustrated, a bipolar junction transistor (BJT), and the alike. As illustrated, the source of the bleeder current switch  208  may be coupled to a terminal of the output of the rectifier circuit  106 , a drain may be coupled to the other terminal of the output of the rectifier circuit  106  via the resistor  210 , and a gate is coupled to the output of the bleeder circuit controller  206 . By determining when the AC input voltage  118  zero crossing occurs, the bleeder circuit controller  206  avoids enabling the bleeder circuit  108  during high dissipative periods and enables the bleeder circuit  108  when the triac of the dimmer switch  104  is in the OFF state. That is, when the AC mains  114  is disconnected from the dimmer switch  104 . 
     The bleeder circuit  108  provides a current path across the output of the rectifier  106  during specified time periods to provide a low impedance current path to ensure the triac device of the dimmer switch  104  operates properly, such as stabilizing the dimmer phase. For example, the bleeder circuit  108  detects when the rectified voltage  120  outputted by the rectifier circuit  106  is at or below a first threshold value during each half cycle of the AC input voltage  118 , at which point it enables the bleeder circuit  108  to provide a bleeder current having a value sufficient to discharge the capacitor  210 . The bleeder circuit  108  may provide a bleeder current at different levels to ensure the triac device of the dimmer switch  104  operates properly and to reduce the thermal loss. For example, a bleeder circuit  108  may provide a high bleeder current at around 250 mA to 300 mA and a low bleeder current at around a half or a quarter of the high current level. While the dimmer switch  104  operates in the conducting state, the bleeder circuit  108  may regulate the amount of the bleeder current supplied to the dimmer switch  104  to ensure the dimmer switch remains in the conducting state. Such a regulation scheme avoids enabling the bleeder circuit  108  when the amount of energy stored in the capacitor  204  in the rectifier circuit  106  is at the maximum during each half cycle of the AC input voltage  118 . This increases the overall system efficiency while ensuring the proper operation of the dimmer switch  104  because the bleeder circuit  108  is disabled during high dissipative operating periods, such as when the power stage is operating in output regulation mode. 
     The bleeder circuit  108  accurately detects the correct timing of the AC input voltage  118  to determine the bleeder current control and avoids enabling the bleeder circuit  108  when the amount of energy stored in the bulk capacitor  204  is at the maximum during each half cycle of the AC input voltage  118 . This increases the overall efficiency of the LED lamp system  100  while ensuring the proper operation of the dimmer switch  104 . 
     The bleeder circuit controller  206  reduces the bleeder current when the AC input voltage  118  is above a second threshold value during each half cycle of the AC input voltage  118 . In one implementation, the bleeder circuit controller  206  disables the bleeder circuit  108  when the AC input voltage  118  is above a second threshold value. That is, when the driver circuit  110  operates, the bleeder circuit  108  is disabled and the bleeder current is reduced to zero. The bleeder circuit controller  206  may receive from the power stage controller  216 , a signal  240  indicating whether the switching cycles of the driver circuit  110  have been enabled. The bleeder circuit controller  206  disables the bleeder circuit  108  by switching off the bleeder current switch  208  when the driver circuit  110  has been enabled. 
     In one embodiment, the bleeder circuit  108  provides different levels of bleeder current. For example, during periods when the driver circuit  110  is disabled, the bleeder circuit  108  may provide different levels of bleeder current to properly manage voltage and to reduce thermal loss. As another example, during periods when the driver circuit  110  is enabled, the current through the LED string  112  may still be below the holding current of the dimmer switch  104 . The bleeder circuit  108  may provide a bleeder current to ensure the dimmer switch  104  remains conducting while the driver circuit  110  is enabled. In one implementation, the power stage controller  216  determines whether the regulation threshold is met by determining whether the energy being delivered to the output stage  214  is sufficient to maintain the proper output regulation of the LED string  112 . The power stage controller  216  may measure the current loading of the dimmer switch  104  and compare the measured current to the holding current threshold or a range of threshold values. The regulation threshold value may be specified or dynamically adjusted based on the loading characteristics of the dimmer switch  104  and the LED string  112 . When the bleeder circuit  108  determines that the driver circuit  110  is not operating, and based on an indication to maintain the output regulation, for example, provided by the power stage controller  216 , the bleeder circuit  108  returns to the operating mode as previously described. The power stage controller  216  may generate the indication to maintain the output regulation in response to determining the regulation threshold is not met. 
     The driver circuit  110  provides a driving current to the LED string  112 . The driver circuit  110  comprises a power stage  212  and an output stage  214 . The power stage  212  regulates the amount of energy provided to the output stage  214 , and the output stage  214  supplies the driving current to the LED string  112 . The power stage  212  includes a power stage controller  216 , a power stage switch  218 , and an inductor  220 . The power stage controller  216  may detect the AC input voltage  118  outputted by the dimmer switch  104  and output a control signal  242  to activate or deactivate the power stage switch  218 . For example, in one implementation, the power stage controller  216  may comprise an input coupled to the output of the dimmer switch  104  and measure the AC input voltage  118  outputted by the dimmer switch  104 . When the measured AC input voltage  118  meets a specified threshold voltage level or range, the triac included in the dimmer switch  104  transitions into a conducting state during each half cycle of the AC input voltage  118 . The power stage controller  216  regulates the driving current provided to the LED string  112  by controlling the duty cycle of the power stage switch  218 . The power stage controller  216  generates a control signal  242  in a first state (e.g., ON) to activate the power stage switch  218  based on a determination that the measured AC input meets or exceeds the specified threshold value or range. When the AC input voltage  118  is at the threshold value during each half cycle of the AC voltage  122  of the AC mains  114 , the power stage controller  216  generates a control signal  242  that transitions from the first state (e.g., ON) to a second state (e.g., OFF) to maintain output regulation. On the other hand, when the power stage controller  216  determines that the measured AC input voltage  118  is greater than a threshold indicating that the amount of energy being delivered to the output stage  214  is sufficient to maintain proper output regulation, the power stage controller  216  generates a control signal  242  in the second state (e.g., OFF) to deactivate the power stage switch  218 . The power stage switch  218  may be a semiconductor power switch such as a MOSFET as illustrated, a BJT, and the alike. 
     The output stage  214  comprises a rectifier diode  222  and an output capacitor  224 . The anode of the rectifier diode  222  is coupled to the drain of the power stage switch  218  and the cathode of the rectifier diode  222  is coupled to the positive terminal of the output capacitor  224 . The rectifier diode  222  ensures the current through the LED string  112  flows from the anode of the LED string  112  to the cathode of the LED string  112 . The capacitor  224  is connected in parallel with the LED string  112 , where the anode of the LED string  112  is connected to the positive terminal of the output capacitor  224  and the cathode of the LED string  112  is connected to the negative terminal of the output capacitor  224 . The capacitor  224  maintains the voltage across the LED string  112  is substantially constant. The rectifier diode  222  and the capacitor  224  together ensure reliable operation of the LED string  112 . 
       FIGS. 3A through 3C  illustrate example waveforms of the LED lamp system  100  of  FIG. 2 .  FIG. 3A  shows voltage waveforms of the LED lamp system  100  of  FIG. 2 . Waveform  302  is the AC input voltage  118  outputted by the dimmer switch  104  and waveform  304  is the AC voltage  122  supplied by the AC mains  114 . Waveform  304  (dotted line) is superimposed on the waveform  302 . As illustrated, the AC input voltage  118  includes a first portion  302   a  where the AC input voltage  118  is zero and a second portion  302   b  where the AC input voltage  118  is non-zero. The first portion and the second portion alternate.  FIG. 3B  illustrates an example waveform representing a control signal  242  generated by the power stage controller  216  of the LED lamp system  100  of  FIG. 2 . As shown in  FIG. 3B , the power stage controller  216  generates a control signal  242  when the AC input voltage  118  meets or exceeds the specified threshold value V TH1  or range at time t 1 . The control signal  242  cycles between ON and OFF states to switch on and off the power stage switch  218 . The power stage controller  216  continues to generate a control signal  242  that cycles between ON and OFF states until a regulation threshold (i.e., whether the energy being delivered to the output stage  214  is sufficient to maintain the proper output regulation of the LED string  112 ) is met as previously described with respect to  FIG. 2 . 
       FIG. 3C  illustrates an example waveform representing a control signal  242  generated by the bleeder circuit controller  206  of the LED lamp system  100  of  FIG. 2 . As shown in  FIG. 3C , the bleeder circuit controller  206  monitors the waveform  302  of the AC input voltage  118  and enables the bleeder circuit  108  when the AC input voltage  118  is less than the threshold value V TH1 . As illustrated, during the period (t 0 -t 1 ) corresponding to the first portion  302   a  of the AC input voltage  118 , the voltage level of the AC input voltage  118  is less than the first threshold value V TH1  and the bleeder circuit  108  is enabled to provide a bleeder current. The bleeder circuit controller  206  disables the bleeder circuit  108 , at time t 1 , when the voltage level of the AC input voltage  118  is greater than the threshold value V TH2 . As illustrated, during the period (t 1 -t 3 ) corresponding to the second portion  302   b  of the AC input voltage  118  when the voltage level of the AC input voltage  118  is non-zero, the bleeder circuit  108  is disabled. The bleeder circuit  108  is not enabled during high dissipative periods. As illustrated, the bleeder circuit  108  is disabled even during the period (t 2 -t 3 ) when the switching of the power stage switch  218  is disabled, and enabled at or near the zero crossing voltage of the AC input voltage  118  when the dimmer switch  104  is turned off and the AC mains  114  is disconnected from the rectifier circuit  106 . 
       FIGS. 4A-4C  illustrate example waveforms of the LED lamp system  100  of  FIG. 2  according to another embodiment.  FIGS. 4A and 4B  are equivalent to  FIGS. 3A and 3B , respectively. As illustrated, the AC input voltage  118  includes a first portion  402   a  where the AC input voltage  118  is zero and a second portion  402   b  where the AC input voltage  118  is non-zero. The first portion and the second portion alternate.  FIG. 4C  illustrates an example bleeder current waveform provided by the bleeder circuit  108  of the LED lamp system  100  of  FIG. 2 . As shown in  FIG. 4C , the bleeder circuit generates a bleeder current having different output levels. During the period (t 0 -t 1 ) corresponding to the first portion  402   a  of the AC input voltage  118 , the voltage level of the AC input voltage  118  is less than the first threshold value V TH1  and the bleeder circuit is enabled to provide a bleeder current to discharge the capacitor included in the rectifier circuit. The driver circuit  110  is enabled, at time t 1 , when the voltage level of the AC input voltage  118  is greater than the threshold value V TH2 . During the period (t 1 -t 3 ) corresponding to the second portion  402   b  of the AC input voltage  118  when the voltage level of the AC input voltage  118  is non-zero, the bleeder current is reduced. For example, as illustrated, during the time period (t 1 -t 2 ), the bleeder current circuit  110  generates a bleeder current at a low level to ensure the triac included in the dimmer switch  104  remains in the conducting state while the power stage  212  switching cycles are enabled. The low level of the bleeder current is set based on the holding current threshold of the dimmer switch  104  and the driving current through the LED string  112 . During the time period (t 2 -t 3 ), the bleeder current is reduced to approximately 0 A and the driver circuit  110  disables the switching cycles. 
     Upon reading this disclosure, those of skill in the art will appreciate still additional alternative designs for controlling dimming of an LED lamp using an adaptive bleeder current control. Thus, while particular embodiments and applications of the present disclosure have been illustrated and described, it is to be understood that the disclosure is not limited to the precise construction and components disclosed herein and that various modifications, changes and variations which will be apparent to those skilled in the art may be made in the arrangement, operation and details of the method and apparatus of the present disclosure disclosed herein without departing from the spirit and scope of the disclosure.