Abstract:
Control methods and apparatuses providing a holding current for a TRIAC dimmer are disclosed. A control method is suitable for a power controller powered by an operation power source. A high-voltage device in the power controller is connected between the operation power source and an input power source, from which the high-voltage device drains a conduction current. An operation voltage of the operation power source is detected, and, when it is below a first reference, the conduction current is forwarded to charge the operation power source. A detected voltage representing an input voltage of the input power source is provided, and if the detected voltage is below a second reference, the conduction current is forwarded to a ground line instead of charging the operation power source.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to and the benefit of Taiwan Application Series Number 103119532 filed on Jun. 5, 2014, which is incorporated by reference in its entirety. 
       BACKGROUND 
       [0002]    The present disclosure relates generally to control methods and apparatuses relevant to TRIACs, more specially to control methods and apparatuses for providing the holding current that a TRIAC needs for proper operation. 
         [0003]    TRIAC dimmers are designed for resistive loads such as incandescent or halogen lights, and they have been significantly installed in the United States and worldwide. Unfortunately, these phase-controlled dimmers are not readily compatible with LEDs since LEDs do not appear as a resistive load. Therefore LED-based solutions using traditional LED drivers will not perform as expected with TRIAC wall dimmers. 
         [0004]    A TRIAC dimmer blocks or cuts off a portion of the waveform of an alternating-current (AC) voltage power source, so as to reduce the power transferred through and to dim the light source that the TRIAC drives. Therefore, TRIAC dimmers are also named as phase-cut dimmers.  FIGS. 1 and 2  demonstrate waveforms generated by a leading-edge TRIAC dimmer and a trailing-edge TRIAC dimmer, respectively. Waveform  100  represents the voltage of an AC outlet connected to a power grid. The shadowed areas  110  mean the portions of the waveform  100  that a TRIAC dimmer bypasses to a load. A cutoff time T CUTOFF  refers to a period of time when a TRIAC dimmer blocks the AC voltage of the power grid; and a conduction time T CONDUCTION  refers to a period of time when a TRIAC dimmer just bypasses the AC voltage to a load. 
         [0005]    As shown in  FIGS. 1 and 2 , the voltage across a load during a cutoff time T CUTOFF  is almost 0V. Nevertheless, a load must drain a minimum amount of current during a cutoff time T CUTOFF  to keep a TRIAC dimmer work properly, and this current is called holding current in the art. When the holding current becomes zero, a TRIAC dimmer restarts or resets. The holding currents for different TRIAC dimmers differ, depending on their designs and specifications. 
         [0006]    It is a challenge for power converter manufactures to design a LED driver capable of providing a holding current during the cutoff time T CUTOFF . 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    Non-limiting and non-exhaustive embodiments of the present invention are described with reference to the following drawings. In the drawings, like reference numerals refer to like parts throughout the various figures unless otherwise specified. These drawings are not necessarily drawn to scale. Likewise, the relative sizes of elements illustrated by the drawings may differ from the relative sizes depicted. 
           [0008]    The invention can be more fully understood by the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
           [0009]      FIGS. 1 and 2  demonstrate waveforms generated by a leading-edge TRIAC dimmer and a trailing-edge TRIAC dimmer, respectively; 
           [0010]      FIG. 3  demonstrates a power converter according to embodiments of the invention; 
           [0011]      FIG. 4  demonstrates several circuit blocks relevant to the high-voltage pin HV; and 
           [0012]      FIG. 5  shows another power converter according to embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    An embodiment of the invention provides a power converter controlled under a power controller to convert an input power source and to generate an output power source. An AC voltage from a mains supply is inputted to a TRIAC dimmer, whose output is for generating the input power source. The power controller is equipped with a high-voltage startup circuit, which has an embedded high-voltage device for draining, during a startup procedure, a conduction current from the input power source, to charge an operation power source powering the power controller itself. The power controller can detect an input voltage of the input power source. When the input voltage is below a threshold, an occurrence of a cutoff time T CUTOFF  when a TRIAC dimmer blocks is determined, and accordingly the conduction current through the high-voltage device is stopped from charging the operation power source, but is instead released to a ground line, and acts as the holding current required for proper operation of the TRIAC dimmer. 
         [0014]    To accommodate a TRIAC dimmer that requires a minimum holding current larger than the maximum conduction current that the high-voltage device can conduct, an embodiment of the invention discloses a discharge circuit, which is triggered by the occurrence of the conduction current to provide a discharge current. The discharge current also drains from the input power source to the ground line. During a cutoff time T CUTOFF , the discharge current and the conduction current together act as the holding current for the TRIAC dimmer. 
         [0015]      FIG. 3  demonstrates a power converter  900  according to embodiments of the invention. The power converter  900  has a flyback topology, but this invention is not limited to. For example, this invention might be suitable for converters with buck, booster, or buck-booster topologies. 
         [0016]    Shown in  FIG. 3 , an AC voltage V AC  might come from an outlet connected to mains supply, having 100 VAC, 110 VAC, or 220 VAC in magnitude, and oscillating at 50 Hz or 60 Hz. A TRIAC dimmer  602 , which might be a leading-edge one or a trailing-edge one, stands for blocking a portion of the waveform of the AC voltage V AC  to generate an AC input power source V TRIAC . For instance, the AC voltage V AC  has the waveform  100  shown in  FIG. 1  or  2 , and the AC input power source V TRIAC  has the waveform outlining the shadowed portion  110 . Abridge rectifier  604  provides full-wave rectification to the AC input power source V TRIAC , and outputs a line voltage V LINE  at a DC power line LINE and a ground voltage at a ground line. 
         [0017]    Transformer  606  has a primary winding PRM, a secondary winding SEC and an auxiliary winding AUX. In one embodiment, power controller  608  is a pulse-width modulator in the form of a monolithic integrated circuit. During a normal operation, the power required for the operation of power controller  608  is provided by an operation power source V CC . Power controller  608  drives pin DRV to provide PWM signal V DRV , so as to turn ON or OFF power switch  610  alternatively and to control the primary-winding current I PRM  passing through the primary winding PRM. When power switch  610  is turned ON, performing a short circuit, DC line voltage V LINE  energizes the transformer  606 . When power switch  610  is turned OFF, performing an open circuit, the transformer  606  de-energizes. During de-energizing, the secondary winding releases current to build output power source V OUT  for powering a load  612 , and the auxiliary winding AUX releases current to build the operation power source V CC . 
         [0018]    The power controller  608  has a zero-current detection pin ZCD, connected to which is a voltage divider including resistors  619  and  621 . This voltage divider is connected in parallel with the auxiliary winding AUX. By clamping the voltage of zero-current detection pin ZCD at about the ground voltage when the power switch  610  is ON, the current out from zero-current detection pin ZCD to the auxiliary winding AUX is about in proportion to the line voltage V LINE  of the DC power line LINE, which is about the same as the peak voltage V TRIAC-PEAK  of the AC input power source V TRIAC  in some embodiments. Accordingly, the power controller  608  is capable of sensing the peak voltage V TRIAC-PEAK  via the help of zero-current detection pin ZCD. During de-energizing, the voltage at the zero-current detection pin ZCD is about in proportion to the voltage of the output power source V OUT , and after the complete of de-energizing, the voltage at the zero-current detection pin ZCD starts oscillating. In one embodiment, the power controller  608  senses the voltage of the output power source V OUT  and/or the complete of the de-energizing, via zero-current detection pin ZCD. 
         [0019]    The power controller  608  has a high-voltage pin HV. Connected between the high-voltage pin HV and the AC input power source V TRIAC  are two rectifying diodes and a current-limiting resistor  614 . During a startup procedure when an operation voltage of the operation power source V CC  is too low, power controller  608  drains a conduction current I CON  from AC input power source V TRIAC , via rectifying diodes, current-limiting resistor  614 , high-voltage pin HV, in order to charge operation power source V CC  and to increase the operation voltage. The current-limiting resistor  614  limits the magnitude of the conduction current I CON . This charging could stop when the operation voltage exceeds a predetermined lower limit, and the startup procedure could stop as well. Later on, the operation voltage of the operation power source V CC  could be sustained by the de-energizing of the transformer through the auxiliary winding AUX. During the startup procedure, PWM signal V DRV  constantly turns OFF power switch  610 . 
         [0020]      FIG. 4  demonstrates several circuit blocks relevant to the high-voltage pin HV. During the startup procedure, resistance detector  806  could output a predetermined current out of zero-current detection pin ZCD, so that the joint voltage at the joint between resistors  619  and  621  (of  FIG. 3 ) is substantially in proportion to the effective resistance of the resistors  619  and  621  in parallel. Based on the joint voltage, the resistance detector  806  could make a corresponding record, in a register for example, according to which a setting voltage V SET  is provided during a normal operation. The power controller  608  could detect the amplitude of AC input power source V TRIAC  via high-voltage pin HV during the normal operation. If that amplitude is determined to be too low, or below the setting voltage V SET  for example, then the power controller  608  deems it as an indication that TRIAC dimmer  602  is currently blocking the waveform of the AC voltage V AC  and the present moment is within a cutoff time T CUTOFF . Therefore, the power controller  608  drains from the input power source V TRIAC , through the rectifying diodes, resistor  614  and pin high-voltage pin HV, a conduction current I CON , which, instead of charging the operation power source V CC , goes and releases to the ground line. This conduction I CON  acts as the holding current that the TRIAC dimmer  602  needs for holding the cutoff time T CUTOFF . 
         [0021]    The effective resistance of resistors  619  and  621  in parallel determines the setting voltage V SET , equivalently determining the criterion that the power controller  60  uses to differentiate a cutoff time T CUTOFF  from a conduction time T CONDUCTION . 
         [0022]      FIG. 3  optionally includes a discharge circuit  616 , in order to provide a higher holding current. The discharge circuit  616  has a PNP BJT  618  and a resistor  620 . The base and the emitter of the BJT  618  are coupled to the high-voltage pin HV and the AC input power source V TRIAC , while the resistor  614  is connected between the base and the emitter. The collector is connected to the ground line through resistor  620 . When the conduction current I CON  occurs, it substantially passes through the base of BJT  618 . Due to the amplification provided by the BJT  618 , collector current I C , larger than the conduction current I CON , occurs when the conduction current I CON  happens, and similar with the conduction current I CON , it also drains from the AC input power source V TRIAC  to the ground line. So the combination of the collector current I C  and the conduction current I CON  could perform as a larger holding current for a TRIAC dimmer. When the conduction current I CON  is about 0, the BJT  618  is switched to be OFF, the collector current I C  is also about 0, so the discharge circuit  616  performs as a high-impedance circuit to isolate the AC input power source V TRIAC  from the ground line. 
         [0023]    Please refer to  FIG. 4 . A regulator  802  acts as a high-voltage startup control circuit, to detect the operation power source V CC  and control the high-voltage device  804  for providing the conduction current I CON . The high-voltage device  804  could be deemed as a controllable current source and is capable of sustaining a high voltage, more than 240V for example, occurring at the high-voltage pin HV. For instance, the regulator  802  is configured to turn ON the high-voltage device  804  for providing a conduction current I CON  of 4 mA when the operation voltage of the operation power source V CC  is below 10V. The regulator  802  could be configured to turn OFF the high-voltage device  804  when the operation voltage exceeds 16V, and the conduction current I CON  is about 0 mA as a result. 
         [0024]    Via the zero-current detection pin ZCD, a resistance detector  806  detects the joint voltage at the joint between the resistors  619  and  621  during a startup procedure, to generate a setting voltage V SET . During a normal operation, the voltage V JOINT  at the joint between resistors  807  and  809  in  FIG. 4  could be in proportion to the amplitude of the AC input power source V TRIAC . A phase detector  808  compares the setting voltage V SET  with the voltage V JOINT  to determine whether the present moment is within a cutoff time T CUTOFF . For example, if the voltage V JOINT  exceeds the setting voltage V SET , the present moment is determined to belong to a conduction time T CONDUCTION , so the phase detector  808  asserts enable signal EN to control de-multiplexer  810 , which accordingly forwards the conduction current I CON , if any, to charge the operation power source V CC . If, in the opposite, the setting voltage V SET  exceeds the voltage V JOINT , the present moment is determined to belong to a cutoff time T CUTOFF , and the phase detector  808  makes the high-voltage device  804  drain the conduction current I CON  and the de-multiplexer  810  forward the conduction current I CON  directly to the ground line. Therefore, resistors  807  and  809 , and the phase detector  808  perform together as a cutoff-time detection circuit for detecting the occurrence of a cutoff time T CUTOFF . 
         [0025]    In some embodiments, the high-voltage device  804  performs two major functions: 1) providing the current to charge the operation power source V CC  during a startup procedure; and 2) providing the holding current required by a TRIAC dimmer  602  during a cutoff time T CUTOFF . 
         [0026]    In some embodiments, only if the phase detector  808  determines the present moment is within a cutoff time T CUTOFF , or the operation voltage of the operation power source V CC  is below 16V, then the high-voltage device  804  is turned ON to provide a conduction current I CON  of 4 mA. Otherwise, the high-voltage device  804  is turned OFF and the conduction current I CON  is kept as 0 mA. 
         [0027]      FIG. 5  shows another power converter  600  according to embodiments of the invention. Unlike the power converter  900  in  FIG. 3 , resistor  614  and discharge circuit  616  in  FIG. 5  are connected to the DC power line LINE for receiving the line voltage V LINE . The line voltage V LINE  in  FIG. 5  preferably follows with the absolute value of the voltage of the AC input power source V TRIAC  all the time. In other words, the line voltage V LINE  in  FIG. 5  should present the absolute value of the voltage of the AC input power source V TRIAC . Detail of the power converter  600  in  FIG. 5  is omitted hereinafter because it is self-explanatory in view of the aforementioned teaching regarding to the power converter  900  in  FIG. 3 . 
         [0028]    While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.