Abstract:
A power stage for light emitting diode (LED)-based light bulbs may include a bipolar junction transistor (BJT). The base of BJT switch may be biased externally and the operation of the BJT may be through a single pin to the emitter of the BJT. A controller integrated circuit (IC) may control the power stage through the main BJT&#39;s emitter pin in an emitter-controlled BJT-based power stage. The emitter-controlled BJT-based power stage may replace the conventional buck-boost power stage topology. For example, the controller may activate and deactivate a switch coupling the BJT&#39;s emitter to ground. A power supply for the controller IC may be charged from a reverse recovery of charge from the BJT, and the reverse recovery controlled by the controller IC.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of priority of U.S. Provisional Patent Application No. 61/824,725 filed on May 17, 2013 to Ramin Zanbaghi et al. entitled “Embedded Auxiliary Chip-Supply Path Using the BJT Switch Reverse Recovery Time in the Power Converter Stages,” which is incorporated by reference herein. This application is related to U.S. Non-provisional patent application Ser. No. ______ filed on May 16, 2014 entitled “Charge Pump-based Drive Circuitry for Bipolar Junction Transistor (BJT)-based Power Supply,” the entire contents of which are specifically incorporated by reference herein without disclaimer. 
     
    
     FIELD OF THE DISCLOSURE 
       [0002]    The instant disclosure relates to power supply circuitry. More specifically, this disclosure relates to power supply circuitry for lighting devices. 
       BACKGROUND 
       [0003]    Alternative lighting devices to replace incandescent light bulbs differ from incandescent light bulbs in the manner that energy is converted to light. Incandescent light bulbs include a metal filament. When electricity is applied to the metal filament, the metal filament heats and glows, radiating light into the surrounding area. The metal filament of conventional incandescent light bulbs generally has no specific power requirements. That is, any voltage and any current may be applied to the metal filament, because the metal filament is a passive device. Although the voltage and current need to be sufficient to heat the metal filament to a glowing state, any other characteristics of the delivered energy to the metal filament do not affect operation of the incandescent light bulb. Thus, conventional line voltages in most residences and commercial buildings are sufficient for operation of the incandescent bulb. 
         [0004]    However, alternative lighting devices, such as compact fluorescent light (CFL) bulbs and light emitting diode (LED)-based bulbs, contain active elements that interact with the energy supply to the light bulb. These alternative devices are desirable for their reduced energy consumption, but the alternative devices have specific requirements for the energy delivered to the bulb. For example, compact fluorescent light (CFL) bulbs often have an electronic ballast designed to convert energy from a line voltage to a very high frequency for application to a gas contained in the CFL bulb, which excites the gas and causes the gas to glow. In another example, light emitting diode (LEDs)-based bulbs include a power stage designed to convert energy from a line voltage to a low voltage for application to a set of semiconductor devices, which excites electrons in the semiconductor devices and causes the semiconductor devices to glow. Thus, to operate either a CFL bulb or LED-based bulb, the line voltage must be converted to an appropriate input level for the lighting device of a CFL bulb or LED-based bulb. Conventionally, a power stage is placed between the lighting device and the line voltage to provide this conversion. Although a necessary component, this power stage increases the cost of the alternate lighting device relative to an incandescent bulb. 
         [0005]    One conventional power stage configuration is the buck-boost power stage.  FIG. 1  is a circuit schematic showing a buck-boost power stage for a light-emitting diode (LED)-based bulb. An input node  102  receives an input voltage, such as line voltage, for a circuit  100 . The input voltage is applied across an inductor  104  under control of a switch  110  coupled to ground. When the switch  110  is activated, current flows from the input node  102  to the ground and charges the inductor  104 . A diode  106  is coupled between the inductor  104  and light emitting diodes (LEDs)  108 . When the switch  110  is deactivated, the inductor  104  discharges into the light emitting diodes (LEDs)  108  through the diode  106 . The energy transferred to the light emitting diodes (LEDs)  108  from the inductor  104  is converted to light by LEDs  108 . 
         [0006]    The conventional power stage configuration of  FIG. 1  provides limited control over the conversion of energy from a source line voltage to the lighting device. The only control available is through operation of the switch  110  by a controller. However, that controller would require a separate power supply or power stage circuit to receive a suitable voltage supply from the line voltage. Additionally, the switch  110  presents an additional expense to the light bulb containing the power stage. Because the switch  110  is coupled to the line voltage, which may be approximately 120-240 Volts RMS with large variations, the switch  110  must be a high voltage switch, which are large, difficult to incorporate into small bulbs, and expensive. 
         [0007]    Shortcomings mentioned here are only representative and are included simply to highlight that a need exists for improved power stages, particularly for lighting devices and consumer-level devices. Embodiments described here address certain shortcomings but not necessarily each and every one described here or known in the art. 
       SUMMARY 
       [0008]    A bipolar junction transistor (BJT) may be used as a switch for controlling a power stage of a lighting device, such as a light-emitting diode (LED)-based light bulb. Bipolar junction transistors (BJTs) may be suitable for high voltage applications, such as for use in the power stage and coupled to a line voltage. Further, bipolar junction transistors (BJTs) are lower cost devices than conventional high voltage field effect transistors (HV FETs). Thus, implementations of power stages having bipolar junction transistor (BJT) switches may be lower cost than power stage implementations having field effect transistor (FET) switches. 
         [0009]    In some embodiments, a bipolar junction transistor (BJT) of a power stage may be controlled by a controller through a single pin. The controller may include circuitry for monitoring voltages and/or currents within the power stage and for providing feedback to the bipolar junction transistor (BJT) through a single pin. The single control pin of the controller may be coupled to an emitter of the bipolar junction transistor (BJT). 
         [0010]    In some embodiments, a controller coupled to the bipolar junction transistor (BJT) controls operation of the power stage with a switch coupled to the emitter of the bipolar junction transistor (BJT) through the single pin. The switch may be toggled on and off by the controller to control delivery of energy to a lighting load, such as from an energizing inductor to a set of light emitting diodes (LEDs). By operating the switch, the controller may define a time period for charging the energizing inductor from the line voltage and discharging the energizing inductor into the set of light emitting diodes (LEDs) to generate light. 
         [0011]    Control through the emitter pin of the bipolar junction transistor (BJT), rather than through a base pin, may allow reverse recovery of charge from the bipolar junction transistor (BJT). In some embodiments, a controller coupled to the bipolar junction transistor (BJT) may receive energy for operation through reverse recovery of current from the bipolar junction transistor (BJT). For example, during a first time period, during which the controller configures the bipolar junction transistor (BJT) for charging the energizing inductor, a base charge may be accumulated at a base of the bipolar junction transistor (BJT). During a second time period, during which the controller configures the bipolar junction transistor (BJT) to allow the energizing inductor to discharge into the set of light emitting diodes (LEDs), the accumulated base charge may be discharged to provide power supply for the controller. 
         [0012]    According to one embodiment, an apparatus may include an integrated circuit (IC) configured to couple to a bipolar junction transistor (BJT) through a single pin that is configured to couple to an emitter of the bipolar junction transistor (BJT). The integrated circuit (IC) may include a switch configured to couple to the emitter of the bipolar junction transistor (BJT) and a controller coupled to the switch and configured to control delivery of power to a load by operating the switch and, optionally, configured to sense a current through the bipolar junction transistor (BJT).. 
         [0013]    In some embodiments, the apparatus may also include a bipolar junction transistor (BJT) including a base, an emitter, and a collector, wherein the emitter is coupled to the integrated circuit (IC); a base drive circuit coupled to the base of the bipolar junction transistor (BJT), wherein the base drive circuit is configured to bias the base of the bipolar junction transistor (BJT) from a power supply node; a current detector coupled to the switch and configured to detect when a current from the emitter of the bipolar junction transistor (BJT) reaches a threshold value, wherein the controller is configured to turn off the switch when the current detector detects the threshold value is reached; a reverse-recovery control circuit configured to be coupled to the emitter of the bipolar junction transistor (BJT) and configured to be coupled to a power supply node, wherein the reverse-recovery control circuit is configured to regulate a discharge current from the base of the bipolar junction transistor (BJT) to the power supply node; a capacitive coupling that is configured to be coupled between the emitter and a collector of the bipolar junction transistor (BJT); and/or a zero current detection block configured to be coupled to the emitter of the bipolar junction transistor (BJT) and configured to detect a ringing at the collector of the bipolar junction transistor (BJT) through the high-pass filter. 
         [0014]    In certain embodiments, the power supply node may be coupled to an external source; the controller may be configured to turn on the switch to direct current to charge an inductor during a first time period, during which a base charge is accumulated at the base of the bipolar junction transistor (BJT) and turn off the switch to begin a reverse recovery of the base charge at the base of the bipolar junction transistor (BJT); the controller may be configured to cause the bipolar junction transistor (BJT) to discharge a base charge from the base of the bipolar junction transistor (BJT) until the bipolar junction transistor (BJT) turns off, after which current from the inductor is directed to a lighting load; the reverse recovery of the charge may be used to charge a chip supply for the integrated circuit (IC by redirecting current from the emitter of the BJT through the IC); the current detector may include a sense resistor that can be coupled to the emitter of the bipolar junction transistor (BJT), a comparator coupled to the sense resistor and wherein the comparator can be coupled to a threshold voltage corresponding to the threshold value, wherein the comparator is configured to output a comparator signal based, at least in part, on a comparison of a voltage at the emitter of the bipolar junction transistor (BJT) and the threshold voltage, and wherein the controller is configured to turn off the switch based, at least in part, on the comparator signal; the reverse recovery control circuit may include a plurality of diodes and a plurality of switches corresponding to the plurality of diodes, each of the plurality of switches being coupled in parallel with one of the plurality of diodes; the controller may be coupled to the current detection block and configured to turn on the switch after the ringing is detected; the controller may be configured to detect a valley of the ringing and turn on the switch approximately at the valley of the ringing; and/or the capacitive coupling may include a high-pass filter (HPF) including a capacitor configured to be coupled to the emitter and the collector of the bipolar junction transistor (BJT) and a resistor configured to be coupled to the emitter of the bipolar junction transistor (BJT). 
         [0015]    According to another embodiment, a method may include configuring an integrated circuit (IC) to control a bipolar junction transistor (BJT) through a single pin that is configured to couple the integrated circuit (IC) to the bipolar junction transistor (BJT); controlling, by the integrated circuit (IC), delivery of power to a load by operating a switch configured to couple to an emitter of the bipolar junction transistor (BJT); and sensing, by the integrated circuit (IC), current through the bipolar junction transistor (BJT) through the single pin. 
         [0016]    In some embodiments, the method may also include coupling the integrated circuit (IC) to the bipolar junction transistor (BJT); biasing a base of the bipolar junction transistor (BJT) with an approximately fixed voltage from a power supply node; turning on the switch to direct current to the load during a first time period, during which a base charge is accumulated at a base of the bipolar junction transistor (BJT); turning off the switch to begin a reverse recovery of the base charge at the base of the bipolar junction transistor (BJT); recovering current from the base charge at the base of the bipolar junction transistor (BJT) to supply a controller; detecting when an emitter current from the emitter of the bipolar junction transistor (BJT) reaches a threshold value; turning off the switch after detecting the emitter current reaches the threshold value; and/or regulating a discharge of the base charge from the base of the bipolar junction transistor (BJT). 
         [0017]    In certain embodiments, turning off the switch may cause the bipolar junction transistor (BJT) to discharge a base charge from the base of the bipolar junction transistor (BJT) until the bipolar junction transistor (BJT) turns off, after which current from the inductor maybe directed to a lighting load; the step of detecting may include comparing a voltage at a sense resistor coupled to the emitter of the bipolar junction transistor (BJT) with a reference voltage; and/or the step of regulating may include shorting out one or more diodes. 
         [0018]    According to a further embodiment, an apparatus may include a lighting load comprising a plurality of light emitting diodes (LEDs); a bipolar junction transistor (BJT) comprising a base, the emitter, and a collector, wherein the collector of the bipolar junction transistor (BJT) is coupled to an input node; and/or an integrated circuit (IC) configured to couple to the bipolar junction transistor (BJT) through a single pin that is configured to couple to an emitter of the bipolar junction transistor (BJT). The integrated circuit (IC) may include a switch configured to couple to the emitter of the bipolar junction transistor (BJT); and/or a controller coupled to the switch and configured to control delivery of power to the lighting load by operating the switch and, optionally, configured to sense a current through the bipolar junction transistor (BJT). 
         [0019]    In some embodiments, the apparatus may also include a rectifier coupled to the input node; a dimmer coupled to the rectifier; a line voltage input node coupled to the dimmer; a current sensor coupled to the switch and configured to detect when a current from the emitter of the bipolar junction transistor (BJT) reaches a threshold value, wherein the controller is configured to operate the switch based on the current sensor detecting the current reaching the threshold value; a reverse recovery control circuit configured to be coupled to the base of the bipolar junction transistor (BJT), wherein the controller is configured to operate the reverse recovery control circuit to regulate a discharge of base current from the base of the bipolar junction transistor (BJT); and/or a zero current detect (ZCD) circuit configured to be coupled to the emitter of the bipolar junction transistor (BJT), wherein the zero current detect (ZCD) circuit is further configured to detect a discharge of an inductor coupled to a collector of the bipolar junction transistor (BJT). 
         [0020]    In certain embodiments, the controller and the switch may be integrated into an integrated circuit (IC), wherein the integrated circuit (IC) controls operation of the bipolar junction transistor (BJT) through a single pin; and/or the controller may be configured to operate the switch based, at least in part, on the detection of the discharge of the inductor by the zero current detect (ZCD) circuit. 
         [0021]    The foregoing has outlined rather broadly certain features and technical advantages of embodiments of the present invention in order that the detailed description that follows may be better understood. Additional features and advantages will be described hereinafter that form the subject of the claims of the invention. It should be appreciated by those having ordinary skill in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same or similar purposes. It should also be realized by those having ordinary skill in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. Additional features will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended to limit the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    For a more complete understanding of the disclosed system and methods, reference is now made to the following descriptions taken in conjunction with the accompanying drawings. 
           [0023]      FIG. 1  is a circuit schematic illustrating a buck-boost power stage for a light-emitting diode (LED)-based bulb in accordance with the prior art. 
           [0024]      FIG. 2  is a circuit schematic illustrating a power stage having an emitter-controlled bipolar junction transistor (BJT) according to one embodiment of the disclosure. 
           [0025]      FIG. 3  is a flow chart illustrating controlling delivery of power to a lighting load with a bipolar junction transistor (BJT) according to one embodiment of the disclosure. 
           [0026]      FIG. 4  is a flow chart illustrating controlling delivery of power to a lighting load with a bipolar junction transistor (BJT) with reverse recovery according to one embodiment of the disclosure. 
           [0027]      FIG. 5  is a circuit schematic illustrating a power stage having an emitter-controlled bipolar junction transistor (BJT) with reverse recovery control and zero current detection according to one embodiment of the disclosure. 
           [0028]      FIG. 6  is a circuit schematic illustrating a reverse recovery control and detect circuit according to one embodiment of the disclosure. 
           [0029]      FIG. 7  are graphs illustrating timing diagrams for operating an emitter-switched bipolar junction transistor (BJT) according to one embodiment of the disclosure. 
           [0030]      FIG. 8  is a flow chart illustrating a method for operating an emitter-controlled bipolar junction transistor (BJT) with reverse recovery control and zero current detection according to one embodiment of the disclosure. 
           [0031]      FIG. 9  is a circuit schematic illustrating an emitter-controlled bipolar junction transistor (BJT) with reverse recovery control according to one embodiment of the disclosure. 
           [0032]      FIG. 10  are graphs illustrating timing diagrams for controlling reverse recovery of an emitter-controlled bipolar junction transistor (BJT) according to one embodiment of the disclosure. 
           [0033]      FIG. 11  is a graph illustrating different supply currents at different input voltages for different resistor values according to one embodiment of the disclosure. 
           [0034]      FIG. 12  is a block diagram illustrating a dimmer system for a light-emitting diode (LED)-based bulb with an emitter-controlled bipolar junction transistor (BJT)-based power stage according to one embodiment of the disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0035]    A bipolar junction transistor (BJT) may control delivery of power to a lighting device, such as light emitting diodes (LEDs). The bipolar junction transistor (BJT) may be coupled to a high voltage source, such as a line voltage, and may control delivery of power to the LEDs. The bipolar junction transistor (BJT) is a low cost device that may reduce the price of alternative light bulbs. In some embodiments, the bipolar junction transistor (BJT) may be controlled through a single pin connection from a controller. For example, a controller may include a switch coupled through a single pin to an emitter of the bipolar junction transistor (BJT). 
         [0036]      FIG. 2  is a circuit schematic illustrating a power stage having an emitter-controlled bipolar junction transistor (BJT) according to one embodiment of the disclosure. A circuit  200  may include a bipolar junction transistor (BJT)  220  having a collector node  222 , an emitter node  224 , and a base node  226 . The collector  222  may be coupled to a high voltage input node  202  and a lighting load  214 , such as a plurality of light emitting diodes (LEDs). An inductor  212  and a diode  216  may be coupled between the high voltage input node  202  and the lighting load  214 . 
         [0037]    The emitter node  224  of the BJT  220  may be coupled to an integrated circuit (IC)  230 , which may include a controller  232 , a switch  234 , and a current detect circuit  236 . The IC  230  may be coupled to the BJT  220  through a single pin  240  to the emitter node  224 . For example, the switch  234  may be coupled in a current path from the emitter node  224  to a ground  206 . The current detect circuit  236  may be coupled between the switch  234  and the ground  206 . The controller  232  may control power transfer from the input node  202  to the lighting load  214  by operating the switch  234  to couple and/or disconnect the emitter node  224  of the BJT  220  to the ground  206 . The current detect circuit  236  may provide feedback to the controller  232  regarding current flowing through the BJT  220  while the switch  234  is turned on to couple the emitter node  224  to the ground  206 . 
         [0038]    The base node  226  of the BJT  220  may be coupled to a supply voltage input node  204  through a base drive circuit  228 . The base drive circuit  228  may be configured to provide a relatively fixed bias voltage to the base node  226  of the BJT  220 , such as during a time period when the switch  234  is switched on. 
         [0039]    The controller  232  may control delivery of power to the lighting load  214 . When the controller  232  turns on the switch  234 , current flows from the high voltage input node  202 , through the inductor  212 , the BJT  220 , the switch  234 , to the ground  206 . During this time period, the inductor  212  is charging from the electromagnetic fields generated by the current. When the controller  232  turns off the switch  234 , current flows from the inductor  212 , through the diode  216 , and through the lighting load  214 . The lighting load  214  is thus powered from the energy stored in the inductor  212 , which was stored during the time period when the controller  232  turned on the switch  234 . The controller  232  may repeat the process of turning on and off the switch  234  to control delivery of energy to the lighting load  214 . Control of delivery of energy from a high voltage source may be possible in the circuit  200  without exposing the IC  230  or the controller  232  to the high voltage source. 
         [0040]    The controller  232  may decide the first duration of time to hold the switch  234  on and the second duration of time to hold the switch  234  off based on feedback from the current detect circuit  236 . For example, the controller  232  may turn off the switch  234  after the current detect circuit  236  detects current exceeding a first current threshold. A level of current detected by the current detect circuit  236  may provide the controller  232  with information regarding a charge level of the inductor  212 . 
         [0041]    As described above with reference to  FIG. 2 , the bipolar junction transistor (BJT) may be controlled through a single pin and that BJT controlled to delivery power to a load.  FIG. 3  is a flow chart illustrating controlling delivery of power to a lighting load with a bipolar junction transistor (BJT) according to one embodiment of the disclosure. A method  300  begins at block  302  with controlling a bipolar junction transistor (BJT) through a single pin from a controller. In one embodiment, the single pin is coupled to the emitter of the BJT creating an emitter-switched BJT power stage for a light bulb. At block  304 , the controller controls delivery of power to a lighting load with the bipolar junction transistor (BJT). By using the bipolar junction transistor (BJT) to control delivery of power to the lighting load, the controller may be separated from the high voltage source. For example, as shown above in  FIG. 2 , the high voltage input node  202  is coupled to the BJT  220 . Although the switch  234  is coupled to the BJT  220 , the switch  234  and the controller  232  are not subjected to the high voltages present at the input node  202 . At least some power for the controller  232  may be generated from the BJT  220  through a reverse recovery process. 
         [0042]    While the controller is controlling delivery of energy to the lighting load, the controller may also control a reverse recovery period of the BJT. For example, when the switch is operated by the controller, the controller may also control a reverse recovery time period for the bipolar junction transistor (BJT) to return energy to a power supply. Charge may be stored at a base of the bipolar junction transistor (BJT) while the switch is on to allow the BJT to continue operating for a short duration after a base current supply is turned off. During this short time period while the BJT is conducting, energy may be redirected form the emitter of the BJT to charge a supply voltage. This energy may be used to provide power to the controller. For example, energy may be transferred to the capacitor  908  for supply voltage V DD  from the collector node  922  though the emitter node  924 .  FIG. 4  is a flow chart illustrating controlling delivery of power to a lighting load with a bipolar junction transistor (BJT) with reverse recovery according to one embodiment of the disclosure. A method  400  begins at block  402  with turning on a switch coupled to an emitter of a bipolar junction transistor (BJT) to direct current to an inductor, during which a base of the bipolar junction transistor (BJT) may be charged. At block  404 , the switch is turned off to start a reverse recovery period for returning charge from the base of the bipolar junction transistor (BJT) and to deliver current from the inductor to one or more light emitting diodes (LEDs) of a light bulb. The controller may cycle through blocks  402  and  404  to regulate transfer of energy to the light emitting diodes (LEDs), which may provide power to the light bulb and may regulate a brightness of light output by the light bulb. The controller may cycle through block  402  and  404  at a fast enough frequency that the eye cannot detect any variation in output light from the light bulb. 
         [0043]    Additional circuitry may provide feedback to the controller for regulating energy transfer to the lighting load. For example, a zero current detect (ZCD) circuit and a reverse recovery control and detect circuit may be coupled to the emitter of the bipolar junction transistor (BJT).  FIG. 5  is a circuit schematic illustrating a power stage having an emitter-controlled bipolar junction transistor (BJT) with reverse recovery control and zero current detection according to one embodiment of the disclosure. A circuit  500  may include bipolar junction transistor (BJT)  220  coupled to an integrated circuit (IC)  530  through the emitter node  224 . The IC  530  may include a controller  532 , which may be similar to the controller  232 . The IC  530  may also include zero current detect circuit  540  and reverse recovery control and detect circuit  550 . The controller  532  may use information from the circuits  540  and  550  to determine when to switch on and/or off the switch  234  and regulate energy transfer to the lighting load  214 . 
         [0044]    Information about energy transfer from the inductor  212  to the lighting load  214  may be received by the controller  532  from the zero current detect circuit  540 . In one embodiment, this information may include a calculated time estimating a time when zero current occurs by estimating, indirectly, the time at which the current reaches or reached zero in the inductor. This information about the energy transfer may be used by the controller  532  to determine when to switch on and/off the switch  234 . The zero current detect circuit  540  may be coupled to the ground  206 , a switch  542 , and a resistor  544 . When the switch  542  is switched on, the zero current detect circuit  540  and the resistor  544  may be coupled to the emitter node  224 . A high-pass filter (HPF) may couple the emitter node  224  to the collector node  222 . The zero current detect circuit  540  may thus sense a voltage at the collector node  222  to determine when a current through the inductor  212  and the lighting load  214  reaches zero. For example, the zero current detect circuit  540  may detect a ringing at the collector node  222  and provide feedback to the controller  532  about the presence of the ringing. In one embodiment, the zero current detect circuit  540  may monitor zero crossings of the ringing because the high-pass filter (HPF) acts as a differentiator of the voltage at the collector node  222 . The collector node  222  may ring, such as oscillate between two voltages, when the inductor  212  fully discharges into the lighting load  214  such that there is approximately zero current through the lighting load  214 . The controller  532  may use information about when the inductor  212  is fully discharged to determine when to switch on the switch  234 , which initiates charging of the inductor  212 . 
         [0045]    Information about reverse recovery of the BJT  220  may be received by the controller  532  from the reverse recovery control and detect circuit  550 . This information may include a base current value and may be used by the controller  532  to determine when to switch on and/or off the switch  234 . The controller  532  may also regulate a level of the base current during the reverse recovery period. One embodiment of a reverse recovery control and detect circuit  550  is shown in  FIG. 6 .  FIG. 6  is a circuit schematic illustrating a reverse recovery control and detect circuit according to one embodiment of the disclosure. The circuit  550  may include a current detect circuit  652 , such as a sense resistor, for detecting a level of the base current during reverse recovery of the BJT  220 . The circuit  550  may also include diodes  654 A-N coupled in series with the current detect circuit  652 . The diodes  654 A-N may be coupled in parallel with corresponding switches  656 A-N. The switches  656 A-N may be switched on to short out some of the diodes  654 A-N and effectively set a number of diodes, N, in the circuit  550 . The switches  656 A-N may thus be operated by the controller  532  to set a base current during reverse recovery. 
         [0046]    During regulation of the energy transfer to the lighting load  214  by the BJT  220  and the controller  532 , a reverse recovery period of the BJT may be used to generate a power supply for the controller  532 . For example, during the first time period, when the switch  234  is on, a current supplied to the base node  226  may be approximately: 
         [0000]    
       
         
           
             
               
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         [0000]    where V DD  is a voltage at input node  204 , V BE  is a voltage between the base node  226  and the emitter node  224 , R b  is a resistance in the base drive circuit  228 , and V D1,th  is a threshold for turning on a forward-biased diode in the base drive circuit  228 . After the switch  234 , the reverse recovery period for the BJT  220  may start and a current supplied from the base node  226  may be approximately: 
         [0000]    
       
         
           
             
               
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         [0000]    where V D3,th  is a threshold voltage for turning a reverse-biased diode in the base drive circuit  228 , and N is a number of diodes in the forward-biased direction. The number of diodes, N, may be controlled to adjust a current supplied from the base node  226 . A higher number of diodes, N, increases the current supplied from the base node  226 . Because a fixed charge exists on the base node  226 , a higher number of diodes, N, will decrease a time duration of the reverse recovery period. That is, the charge stored at the base node  226  will be discharged faster. 
         [0047]    Referring back to  FIG. 5 , in one embodiment, the base drive circuit  228  of circuit  500  may include a forward-biased diode  514  in series with a resistor  512 . The diode  514  and resistor  512  may be coupled in parallel with a series connection of a reverse-biased diode  516  and a resistor  518 . Depending on a voltage at the base node  226 , current may flow through the resistor  512  and the diode  514  or through the resistor  518  and the diode  516 . 
         [0048]    In one embodiment, the current detect circuit  236  may include a sense resistor  536  coupled between the switch  234  and the ground  206 . The circuit  236  may also include a comparator  534  for comparing a voltage across the sense resistor  536  with a threshold voltage, V TH , and provide a result of the comparison to the controller  532 . 
         [0049]    The controller  532  may receive feedback from the current detect circuit  236 , the zero current detect circuit  540 , and the reverse recovery control and detect circuit  550 . The controller  532  may use feedback from these inputs to determine when to switch on or off the switch  234  and the switch  542 . The controller  532  may output a V PLS,T1  signal to control the switch  532  and a V PLS,T2  signal to control the switch  542 . 
         [0050]    One method of operation of the circuit  500  under control of the controller  532  is shown through timing diagrams in  FIG. 7 .  FIG. 7  are graphs illustrating a timing diagram for operating an emitter-switched bipolar junction transistor (BJT) according to one embodiment of the disclosure.  FIG. 7  includes graphs  712 - 724 . Graph  712  illustrates a V PLS,T1  signal generated by the controller  532  for operating the switch  234 . Graph  714  illustrates a current, I E , at the emitter node  224 . Graph  716  illustrates a current, I sense , in the current detect circuit  236 . Graph  718  illustrates a current, I RR , in the reverse recovery control and detect circuit  550 . Graph  720  illustrates a V PLS,T2  signal generated by the controller  532  for operating the switch  542 . Graph  722  illustrates a voltage, V C , at the collector node  222 . Graph  724  illustrates a voltage, V E , at the emitter node  224 . 
         [0051]    During a first time period  702 , T 1 , the controller  532  may turn on the switch  234  with a high V PLS     —     T1  signal of graph  712 . While the V PLS     —     T1  signal is high, current passes from the input node  202  through the BJT  220  and through the emitter node  224 . As shown in graph  714 , the current through the emitter node  224 , I E , ramps up over a portion of the time period  702 . While, the V PLS     —     T1  signal of graph  712  is high, current also flows through the switch  234  and through the current detect circuit  236  as current I sense . During the first time period  702 , the current I sense  of graph  716  is approximately equal to the current I E  of graph  714 . The voltage at the emitter node  224 , V E , is shown in graph  724 , and a corresponding voltage for the collector node  222 , V c , is shown in graph  722 . During the first time period  702 , the emitter voltage, V E , may be equal to a voltage, V sns , across the sense resistor  536 . While the V PLS     —     T1  signal is high, the controller  532  may hold the V PLS     —     T2  signal low to turn off the switch  542 . 
         [0052]    When the controller  532  detects the current I sense  of graph  716  reaches a certain value, the controller  532  may turn off the switch  234 . The controller  532  may turn off the switch  234  by switching the V PLS     —     T1  signal of graph  712  to low during a second time period  704 , T 1 ′. During the second time period  704 , current at the emitter node  224  may continue to increase as shown in graph  714 . Furthermore, a reverse recovery current may be generated from the base node  226  of the BJT  220 , which may pass through the reverse recovery control and detect circuit  550 . Graph  718  illustrates this reverse recovery current, I RR , during the second time period  704 . 
         [0053]    During the first two time periods  702  and  704 , current passing through the inductor  212  causes the inductor  212  to store energy. The controller  532  may then determine to transfer the energy from the inductor  212  to the lighting load  214  to generate light in a light bulb. In one embodiment, the energy transfer to the lighting load begins when the BJT  220  turns off after all base charge is discharged. When the controller  532  determines to begin a third time period  706 , the controller  532  switches the V PLS     —     T2  signal of graph  720  to high to turn on the switch  542 . When the switch  542  is turned on, the zero current detect circuit  540  is coupled to the emitter node  224  for monitoring the energy transfer to the lighting load  214 . The circuit  540  may monitor energy transfer to the lighting load  214 . 
         [0054]    In one embodiment, the circuit  540  may be configured to detect ringing to determine when energy transfer to be lighting load  214  is nearing completion or is completed. When the inductor  212  is nearly or completely discharged, a collector voltage, V c , begins ringing as shown in graph  722  at time  732 . Likewise, the emitter voltage, V E , experiences similar ringing as shown in graph  724  at time  732 , although out of phase from the collector voltage, V C . With the switch  542  turned on by the high V PLS     —     T2  signal of graph  720 , the zero current detect circuit  540  may detect the ringing at time  732  and provide information to the controller  532 . The ringing at time  732  may occur when inductor  212  is discharged causing the voltage across diode  216  to reach zero. 
         [0055]    The controller  532  may determine to end the third time period  706  and repeat the cycle of charging and discharging the inductor  212  through the time periods  702 ,  704 , and  706 . The controller  532  begins a new first time period  702  by switching the V PLS     —     T2  signal to low and switching the V PLS     —     T1  signal to high. In one embodiment, the controller  532  may determine a second valley of the collector voltage, V C , at time  734  and switch the V PLS     —     T1  signal at time  734 . The controller  532  may process information received from the zero current detect circuit  540  to determine a timing of the second valley of the collector voltage, V C . For example, the controller  532  may predict the timing of the second valley of the collector voltage, V C , by adding a 90 degree phase shift to the emitter voltage, V E , sensed by the zero current detect circuit  540 . 
         [0056]    A method executed by the controller  532  for controlling delivery of energy to a lighting load as shown in the graphs of  FIG. 7  is shown in a flow chart in  FIG. 8 .  FIG. 8  is a flow chart illustrating a method for operating an emitter-controlled bipolar junction transistor (BJT) with reverse recovery control and zero current detection according to one embodiment of the disclosure. A method  800  begins at block  802  with turning on a switch coupled to an emitter of a bipolar junction transistor (BJT). At block  804 , the controller monitors a current through the emitter switch turned on in block  802 . At block  806 , the controller determines whether the monitored current of block  804  reaches or exceeds a threshold level. If not, current monitoring at block  804  continues. When the threshold level of block  806  is reached, the controller turns off the emitter switch at block  808 . 
         [0057]    At block  810 , the controller delays for a wait time while base charge is reverse recovered from the BJT. At block  812 , the controller determines if the reverse recovery current reaches a second threshold. If not, the controller continues to delay at block  810 . When the threshold level of block  812  is reached, at block  814  the controller may turn on a switch to couple a zero current detect (ZCD) circuit to the emitter of the BJT. 
         [0058]    At block  816 , the controller may monitor the zero current detect (ZCD) circuit to determine when the inductor coupled to a lighting load is nearly or completely discharged. For example, at block  818  the controller may determine whether a ringing is detected at a terminal of the BJT. If not, the controller continues to monitor the zero current detect (ZCD) circuit at block  816 . If ringing is detected at block  818 , then the controller may detect a zero crossing of the ringing, such as a second valley of the ringing, and turn off the zero current detect (ZCD) circuit switch at the zero crossing at block  822 . The method  800  may then return to block  802  to continue another cycle. 
         [0059]    As described above in the circuits of  FIG. 5  and  FIG. 6 , a reverse recovery time may be controlled by coupling additional diodes in series to adjust a value of the reverse recovery current. In another embodiment, a reverse recovery time may be controlled through a variable resistor coupled to a base of the bipolar junction transistor (BJT). A circuit for implementing this embodiment is shown in  FIG. 9 .  FIG. 9  is a circuit schematic illustrating an emitter-controlled bipolar junction transistor (BJT) with reverse recovery control according to one embodiment of the disclosure. A circuit  900  includes an input node  902  for receiving a high voltage, such as a line voltage. The input voltage may pass through rectifier  904  to an inductor  912 . The inductor  912  may store energy from the input voltage and discharge energy into a lighting load  914  through a diode  916  under control of a controller  932 . 
         [0060]    The controller  932  may control transfer of energy to and from the inductor  912  by operating a switch coupled to an emitter node  924  of a bipolar junction transistor (BJT)  920 . The controller  932  may also control a variable resistor  936  in a base drive circuit  928  coupled to a base node  926  of the bipolar junction transistor (BJT)  920 . By increasing or decreasing a resistance of the variable resistor  936 , the controller may decrease or increase, respectively, a discharge current of base from the bipolar junction transistor (BJT)  920 . The reverse recovery time period may be increased when the controller  932  increases the resistance. The reverse recovery time period may be decreased when the controller  932  decreases the resistance. 
         [0061]    The effects of changing the resistance of the variable resistor  936  are shown in  FIG. 10 .  FIG. 10  are graphs illustrating timing diagrams for controlling reverse recovery of an emitter-controlled bipolar junction transistor (BJT) according to one embodiment of the disclosure. A graph  1012  illustrates a signal V PLS  generated by the controller  932  for operating the switch  934 . The V PLS  signal may be high during a time period  1002 , T 1 , and switched low for a time period  1004 , T 1 ′, and  1006 , T 2 . A duration of the time period  1004  may be adjusted by varying the resistance of the variable resistor  936 . The T 1 +T 1 ′ time may be fixed for a fixed output power to the lighting load  214 . Thus, current through the inductor  912  may reach the same peak value regardless of the selected variable resistance. The variable resistance, by controlling a duration of the reverse recovery time period T 1 ′, may vary an amount of energy harvested from the base node  926  of the bipolar junction transistor (BJT)  920 . The amount of energy harvested for a power supply, such as stored in capacitor  908 , may increase as shown in graph  1014 . Graph  1014  illustrates a current  942  to the output node  906 . The energy harvested during time period T 1 ′ may be used to provide a power supply to the controller  932 . For example, the reverse current may charge the capacitor  908 , which is coupled to power supply node  906  and to the controller  932 . The changing supply current to the power supply node  906  as a function of the resistance of the variable resistor  936  is shown in  FIG. 11 .  FIG. 11  is a graph illustrating different supply currents at different input voltages for different resistor values for variable resistor  936  according to one embodiment of the disclosure. For example, lines  1112 ,  1114 ,  1116 ,  1118 ,  1120 , and  1122  correspond to resistor values of 0, 110, 250, 500, 750, and 1000 Ohms. 
         [0062]    The controller and variable resistance load device described above may be integrated into a dimmer circuit to provide dimmer compatibility, such as with lighting devices.  FIG. 12  is a block diagram illustrating a dimmer system for a light-emitting diode (LED)-based bulb with an emitter-controlled bipolar junction transistor (BJT)-based power stage according to one embodiment of the disclosure. A system  1200  may include a dimmer compatibility circuit  1208  with a variable resistance device  1208   a  and a control integrated circuit (IC)  1208   b . The dimmer compatibility circuit  1208  may couple an input stage having a dimmer  1204  and a rectifier  1206  with an output stage  1210 , which may include light emitting diodes (LEDs). The system  1200  may receive input from an AC mains line  1202 . The output stage  1210  may include a power stage based on a bipolar junction transistor (BJT) as described above. For example, the output stage  1210  may include an emitter-switched bipolar junction transistor (BJT) in the configurations of  FIG. 2 ,  FIG. 5 , or  FIG. 9 . 
         [0063]    If implemented in firmware and/or software, the functions described above, such as with respect to  FIG. 3 ,  FIG. 4 , and  FIG. 8  may be stored as one or more instructions or code on a computer-readable medium. Examples include non-transitory computer-readable media encoded with a data structure and computer-readable media encoded with a computer program. Computer-readable media includes physical computer storage media. A storage medium may be any available medium that can be accessed by a computer. By way of example, and not limitation, such computer-readable media can comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium that can be used to store desired program code in the form of instructions or data structures and that can be accessed by a computer. Disk and disc includes compact discs (CD), laser discs, optical discs, digital versatile discs (DVD), floppy disks and blu-ray discs. Generally, disks reproduce data magnetically, and discs reproduce data optically. Combinations of the above should also be included within the scope of computer-readable media. 
         [0064]    In addition to storage on computer readable medium, instructions and/or data may be provided as signals on transmission media included in a communication apparatus. For example, a communication apparatus may include a transceiver having signals indicative of instructions and data. The instructions and data are configured to cause one or more processors to implement the functions outlined in the claims. 
         [0065]    Although the present disclosure and certain representative advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the disclosure as defined by the appended claims. For example, although signals generated by a controller are described throughout as “high” or “low,” the signals may be inverted such that “low” signals turn on a switch and “high” signals turn off a switch. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the present disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.