Abstract:
Circuitry for damping transitions in an input current of a power converter includes a transistor with a control terminal coupled to receive an impedance control signal and main terminals coupled to variably impede the input current of the power converter in response to the impedance control signal. An impedance control circuit is coupled across input rails of the power converter to provide the impedance control signal. The impedance control circuit includes a first current conduction path coupling the control terminal to a first of the input rails of the power converter, and a second current conduction path coupling the control terminal to a second of the input rails of the power converter. The second of the input rails of the power converter is coupled to the main terminals of the transistor.

Description:
REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 62/108,028, filed Jan. 26, 2015. 
     
    
     BACKGROUND INFORMATION 
       [0002]    1. Field of the Disclosure 
         [0003]    The present invention relates generally to damper circuits, and more specifically damper circuits for use in power converters. 
         [0004]    2. Background 
         [0005]    Electronic devices use power to operate. Switched mode power converters are commonly used due to their high efficiency, small size and low weight to power many modern electronics. Conventional wall sockets provide a high voltage alternating current. In a switching power converter a high voltage alternating current (ac) input is converted to provide a regulated direct current (dc) output through an energy transfer element. The switched mode power converter control circuit provides output regulation by sensing the output and controlling it using a closed loop. During operation, a switch is utilized to provide the desired output by varying the duty cycle (typically the ratio of the on time of the switch to the total switching period) of the switch in a switched mode power converter (also referred as a switching power supply or a switched mode power supply). 
         [0006]    In one type of dimming for lighting applications, a TRIAC dimmer circuit removes a portion of the ac input voltage to limit the amount of voltage and current supplied to an incandescent lamp. This is known as phase dimming because it is often convenient to designate the position of the missing voltage in terms of a fraction of the period of the ac input voltage measured in degrees. In general, the ac input voltage is a sinusoidal waveform and the period of the ac input voltage is referred to as a full line cycle. As such, half the period of the ac input voltage is referred to as a half line cycle. An entire period has 360 degrees, and a half line cycle has 180 degrees. Typically, the phase angle is a measure of how many degrees (from a reference of zero degrees) of each half line cycle the dimmer circuit removes. As such, removal of half the ac input voltage in a half line cycle by the TRIAC dimmer circuit corresponds to a phase angle of 90 degrees. In another example, removal of a quarter of the ac input voltage in a half line cycle may correspond to a phase angle of 45 degrees. 
         [0007]    Although phase angle dimming works well with incandescent lamps that receive the altered ac line voltage directly, it typically creates problems for light emitting diode (LED) lamps driven by a switching power converter. Conventional regulated switching power converters are typically designed to ignore distortions of the ac input voltage and deliver a constant regulated output until a low input voltage causes them to shut off. As such, conventional regulated switching power converters cannot dim LED lamps. Unless a power converter for an LED lamp is specially designed to recognize and respond to the voltage from a TRIAC dimmer circuit in a desirable way, a TRIAC dimmer can produce unacceptable results such as flickering of the LED lamp. 
         [0008]    Another difficulty in using TRIAC dimming circuits with LED lamps comes from a characteristic of the TRIAC itself. A TRIAC is a semiconductor component that behaves as a controlled ac switch. In other words, it behaves as an open switch to an ac voltage until it receives a trigger signal at a control terminal, which causes the switch to close. The switch remains closed as long as the current through the switch is above a value referred to as the holding current. Most incandescent lamps use more than enough current from the ac power source to allow reliable and consistent operation of a TRIAC. However, the low current used by efficient power converters to drive LED lamps may not provide enough current to keep a TRIAC conducting for the expected portion of the ac line period. Therefore, conventional power converter controller designs rely on a dummy load sometimes called a bleeder circuit, to take enough extra current from the input of the power converter to keep the TRIAC conducting. In addition, the sharply increasing input voltage when the TRIAC fires during each half line cycle causes an inrush input current ringing which may reverse several times during the half line cycle. During these current reversals, the TRIAC may prematurely turn off and cause flickering in the LED lamp. A series resistor damper may then be utilized to slow down the charging of the input capacitor, and dampen the input current ringing and prevent voltage overshoot of the input capacitor. In general, the damper circuit is external from the integrated circuit of the power converter controller and is implemented with a resistor coupled at the input of the power converter. However, use of the damper resistor alone degrades the overall efficiency of the system. 
       SUMMARY 
       [0009]    In one implementation, circuitry ( 143 ,  443 ) is for damping transitions in an input current ( 103 ,  403 ) of a power converter. The circuitry comprises a transistor ( 142 ,  442 ) comprising a control terminal coupled to receive an impedance control signal and main terminals coupled to variably impede the input current of the power converter in response to the impedance control signal, an impedance control circuit coupled across the input rails of the power converter to provide the impedance control signal to the control terminal. The impedance control circuit comprises a first current conduction path coupling the control terminal to a first of the input rails of the power converter and a second current conduction path coupling the control terminal to a second of the input rails of the power converter, wherein the second of the input rails of the power converter is coupled to a first of the main terminals of the transistor. 
         [0010]    This and other implementations can include one or more of the following features. The circuitry can further comprise a damping resistance ( 144 ,  444 ) to be coupled between the first input rail of the power converter and an input voltage and the transistor is coupled to variably shunt input current of the power converter across the damping resistance in response to the impedance control signal. The first current conduction path can comprise a resistance ( 132 ,  432 ) between the control terminal and the first of the input rails. The second current conduction path can comprise a resistance ( 140 ,  440 ) between the control terminal and the second of the input rails. The transistor can comprise a voltage-controlled transistor and the impedance control signal can be a voltage applied to the control terminal of the voltage-controlled transistor. 
         [0011]    In some implementations, during a first portion (T 1 ) of a time that the transistor impedes the input current of the power converter, current conduction along both the first current conduction path and the second current conduction path tends to bias the voltage-controlled transistor to impede the input current of the power converter. During a second portion (T 2 ) of the time the first portion of the time transistor impedes the input current of the power converter, the transistor can be driven in the linear mode. 
         [0012]    The circuitry can further comprise a nonparasitic capacitance ( 138 ,  438 ) coupled to store the voltage applied to the control terminal. The circuitry can further comprise a Zener diode ( 136 ,  436 ) coupled to limit a voltage applied to the control terminal of the transistor. 
         [0013]    The circuitry can further comprise a resistance ( 134 ,  434 ) coupled between the control terminal and a second of the main terminals of the transistor. The second of the input rails can be a return rail. 
         [0014]    In one implementation, a device comprises the circuitry of the embodiment and the power converter. The device can further comprise a rectifier ( 106 ) coupled to rectify an ac signal for input into the power converter. The device can further comprise a dimmer circuit ( 104 ) coupled to truncate the ac signal rectified by the rectifier. The circuitry can be coupled to increase the impedance of the input current of the power converter relatively quickly in response to relatively fast increases in the input current and to decrease the impedance of the input current relatively slowly in response to relatively slow decreases in the input current. The circuitry can be coupled to decrease the impedance of the input current by the transistor to an approximately zero steady state impedance (T 3 ). The impedance control circuit can be configured to decrease the impedance of the input current by the transistor to a steady state impedance in less than one half of a line cycle. 
         [0015]    In another implementation, a device is for damping transitions in an input current ( 103 , 403 ) of a power converter. The device can comprise a damping resistance ( 144 ,  444 ) to be coupled between a first input rail of the power converter and an input voltage, a bypass transistor ( 142 ,  442 ) comprising a control terminal coupled to receive a bypass control signal and main terminals coupled to selectively shunt the input current of the power converter across the damping resistance in response to the bypass control signal, and a bypass control circuit coupled across the input rails of the power converter to provide the bypass control signal to the control terminal of the bypass transistor. The bypass control circuit can comprise a nonparasitic capacitance ( 138 ,  438 ) coupled between the control terminal of the bypass transistor and a first of the main terminals of the bypass transistor, a first resistance ( 140 ,  440 ) coupled between the control terminal of the bypass transistor and a second of the main terminals of the bypass transistor, and a current conduction path between the control terminal of the bypass transistor and a second input rail of the power converter. 
         [0016]    This and other implementations can include one or more of the following features. The current conduction path between the control terminal and the second input rail can comprise a resistance ( 132 ,  432 ). The bypass transistor can comprise a MOSFET. The bypass control circuit can be configured so that nonparasitic capacitance supports voltages to drive the MOSFET in the linear mode. The device can further comprise a Zener diode ( 136 ,  436 ) coupled to limit a voltage applied to the control terminal of the bypass transistor. The device can further comprise a second resistance ( 134 ,  434 ) coupled between the control terminal and a first of the main terminals of the transistor. The first of the input rails can be a return rail. The circuitry can be coupled to decrease shunting of the input current of the power converter relatively quickly in response to relatively fast increases in the input current and to increase shunting of the input current relatively slowly in response to relatively slow decreases in the input current. The circuitry can be coupled to increase shunting of the input current until approximately all of the input current is shunted across the damping resistance (T 3 ). The impedance control circuit can be configured to increase shunting of the input current by the transistor to a steady state impedance in less than one half of a line cycle. 
         [0017]    In another implementation, a power supply comprises the device of any one of the previous implementations, the power converter, and a rectifier ( 106 ) coupled to rectify an ac signal for input into the power converter. 
         [0018]    In another implementation, a circuit is for use in a power converter. The circuit comprises a damping resistance ( 144 ,  444 ) having a first terminal coupled to an output of a rectifier circuit ( 106 ), the damping resistance having a second terminal coupled to a first terminal of an input filter capacitor ( 108 ,  408 ) of the power converter, wherein a second terminal of the input filter capacitor is to be coupled to the output of the rectifier circuit, a transistor ( 142 ,  442 ) having a first main terminal coupled to the first terminal of the damping resistance, the transistor having a second main terminal coupled to the second terminal of the damping resistance, and a control circuit including a first resistance, a second resistance, and a first capacitance. The first resistance ( 132 ,  432 ) is coupled between the control terminal of the transistor and the second terminal of the input filter capacitor. The second resistance ( 140 ,  440 ) is coupled between the control terminal of the transistor and the first terminal of the input filter capacitor. The first capacitance ( 138 ,  438 ) is coupled between the control terminal of the transistor and the first terminal of the damping resistance. 
         [0019]    This and other implementations can include one or more of the following features. The input filter capacitor can be coupled across a primary winding ( 112 ,  412 ) and a switch ( 126 ,  426 ) of the power converter. The switch of the power converter can be coupled to be opened and closed in response to a drive signal ( 129 ,  429 ) to regulate an output of the power converter. The rectifier circuit ( 106 ) can be coupled to receive a dimmer voltage from a dimmer circuit ( 104 ). The dimmer circuit can be coupled to disconnect a fraction of a period of each half line cycle of an ac input voltage from the power converter. The damping resistance can be coupled to reduce a ringing in an input current of the power converter caused by the dimmer circuit when the transistor is set by the control circuit to have a high impedance between the main terminals. The damping resistance can be coupled to be bypassed by the transistor when the transistor is set by the control circuit to have a low impedance between the main terminals. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    Non-limiting and non-exhaustive embodiments of the present invention are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified. 
           [0021]      FIG. 1  is a functional block diagram illustrating an example power converter in accordance with the teachings of the present invention. 
           [0022]      FIG. 2A  is a waveform illustrating an example ac voltage in accordance with the teachings of the present invention. 
           [0023]      FIG. 2B  is a waveform illustrating an example output voltage of a dimmer circuit in accordance with the teachings of the present invention. 
           [0024]      FIG. 2C  is a waveform illustrating an example output of a rectifier circuit in accordance with the teachings of the present invention. 
           [0025]      FIG. 3A  is a waveform illustrating an example of the input current from the output dimmer circuit in accordance with the teachings of the present invention. 
           [0026]      FIG. 3B  illustrates the voltage of V R4  of the damper circuit from  FIG. 1  in accordance with the teachings of the present invention. 
           [0027]      FIG. 4  is a functional block diagram illustrating another example power converter with a damper circuit in accordance with the teachings of the present invention. 
       
    
    
       [0028]    Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention. 
       DETAILED DESCRIPTION 
       [0029]    Examples of a power converter that utilizes a damper circuit are described herein. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one having ordinary skill in the art that the specific detail need not be employed to practice the present invention. In other instances, well-known materials or methods have not been described in detail in order to avoid obscuring the present invention. 
         [0030]    Referring now to  FIG. 1 , a diagram of an example switching power converter  100  is depicted including ac input voltage V AC    102 , a dimmer circuit  104 , a bridge rectifier circuit  106 , a dimmer voltage V DO    105 , a rectified voltage V RECT    107 , an energy transfer element T 1   115 , a primary winding  112  of the energy transfer element T 1   115 , a secondary winding  114  of the energy transfer element T 1   115 , a switch S 1   126 , an input return  113 , a clamp circuit  110 , an input capacitor C F    108 , a rectifier D 1   116 , an output capacitor C O    118 , an output quantity U O    123 , an output voltage V O    119 , an output current I O    121 , a feedback circuit  122 , a feedback signal U FB    125 , a controller  124 , a drive signal  129 , a current sense signal  127 , and switch current I D    131 . Also illustrated in  FIG. 1  is a load  120  (e.g., one or more light emitting diodes) coupled to the switching power converter  100 . The example switching power converter  100  illustrated in  FIG. 1  is configured generally as a flyback regulator, which is one example of a switching power converter topology that may benefit from the teachings of the present disclosure. However, it is appreciated that other known topologies and configurations of switching power converter regulators may also benefit from the teachings of the present disclosure. 
         [0031]    Further depicted is a thyristor damper circuit  143  coupled between a first and second terminal  170 ,  172  of the rectifier circuit. The thyristor damper circuit  143  includes a first resistor R 1   132 , a second resistor R 2   134 , a third resistor R 3 , a fourth resistor R 4   144 , a second rectifier D 2   126 , a first capacitor C 1   138 , and a transistor Q 1   142 . In one example, transistor Q 1   142  is a MOSFET. The thyristor damper circuit  143  further includes a voltage V R4    146  across the fourth resistor  144 . 
         [0032]    The switching power converter  100  provides output power to the load  120 , such as a light emitting diode (LED) for example, from an unregulated input voltage such as the ac input voltage V AC    102 . The dimmer circuit  104  provides the dimmer voltage V DO    105  in response to the input voltage V AC    102 . The dimmer circuit  104  can be any known dimmer circuit such as a thyristor dimmer circuit or a triode for alternating current (TRIAC) dimmer circuit for example. The bridge rectifier  106  provides the rectified voltage V RECT    107  in response to the dimmer voltage V DO    105 . The bridge rectifier  104  is coupled to the energy transfer element T 1   115 . In some embodiments, the energy transfer element T 1   115  can be a coupled inductor. In other embodiments, the energy transfer element T 1   115  can be a transformer. In the example of  FIG. 1 , the energy transfer element T 1   115  includes two windings, a primary winding  112  and a secondary winding  114 . However, it should be appreciated that the energy transfer element T 1   115  can have more than two windings if desired. The primary winding  112  is coupled to switch S 1   126 , which is further coupled to input return  116 . In one embodiment, the switch S 1   126  can be a transistor such as a metal-oxide-semiconductor field-effect transistor (MOSFET). In another example, controller  124  can be implemented as a monolithic integrated circuit or may be implemented with discrete electrical components or a combination of discrete and integrated components. In addition, the controller  124  and switch S 1   126  can be included in an integrated circuit that is manufactured as either a hybrid or monolithic integrated circuit. An open (i.e., ON) switch may conduct current, while a closed (i.e., OFF) switch cannot conduct current. 
         [0033]    As shown, the clamp circuit  110  is coupled across the primary winding  112  of the energy transfer element T 1   115 . The input capacitor C F    108  can couple across the primary winding  112  and switch S 1   126 . In other words, the input capacitor C F    108  can be coupled to the rectifier  106  and input return  113 . The secondary winding  114  of the energy transfer element T 1   115  is coupled to the rectifier D 1   116 . Although the rectifier D 1   116  is depicted as a diode in this example, the rectifier D 1   116  can be a transistor used as a synchronous rectifier if desired. In this example, the output capacitor C O    118  and the load  120  are coupled to the rectifier D 1   116 . An output is provided to the load  120  and can be provided as either a regulated output voltage V O    119 , regulated output current I O    121 , or a combination thereof. 
         [0034]    The switched mode power converter  100  further comprises circuitry to regulate the output, which is shown as output quantity U O    123 . In general, the regulated output quantity U O    123  is either an output voltage V O    119 , output current I O    121 , or a combination thereof. The feedback circuit  122  is operative to sense the output quantity U O    123  of the switched mode power converter  100  and produces the feedback signal U FB    125  based thereon. In one embodiment, the feedback circuit  122  may sense the output quantity U O    123  from the output of the power converter  100 . In other embodiments, the feedback signal U FB    125  can be derived from sensing one or more quantities on the input side of the transformer that are representative of the output quantity U O    123 . The feedback circuit  122  is coupled to a terminal of the controller  124  such that the controller  124  receives the feedback signal U FB    125 . The controller  124  also includes a terminal for receiving the current sense input signal  127 . The current sense input signal  127  is representative of the switch current I D    131  in the switch S 1   126 . In addition, the switch S 1   126  receives the drive signal  129  from the controller  124 . 
         [0035]    In operation, the switching power converter  100  of  FIG. 1  provides output power to the load  120  from an unregulated input such as the ac input voltage V AC    102 . The ac input voltage V AC    102  is received by the dimmer circuit  104  and provides the dimmer voltage  105  based thereon. The dimmer circuit  104  can be utilized when the load  120  coupled to the power converter  100  is a light emitting diode (LED) array to limit the amount of power delivered to the power converter  100 . As a result, the current delivered to the load of LED arrays is limited and the LED array dims. In one embodiment, the dimmer circuit  104  is a TRIAC dimmer circuit or other suitable switching dimmer circuit. The dimmer circuit  104  disconnects the ac input voltage V AC    102  from the power converter when the ac input voltage V AC    102  crosses zero voltage. After a given amount of time, the dimmer circuit  104  reconnects the ac input voltage V AC    102  with the power converter  100 . Depending on the amount of dimming desired, the dimmer circuit  104  controls the amount of time the ac input voltage V AC    102  is disconnected from the power converter  100 . In general, more dimming corresponds to a longer period of time during which the dimmer circuit  104  disconnects the ac input voltage V AC    102 . For phase dimming applications of LEDs that utilize a TRIAC dimmer circuit, the TRIAC requires a minimum holding current to keep the TRIAC itself from turning off. The controller  124  utilizes the damper circuit  143  to help ensure that the current through the TRIAC does not fall below the holding current of the TRIAC. 
         [0036]    The rectifier circuit  104  provides the rectified voltage V RECT    107  in response to the dimmer voltage  105 . The input capacitor C F    108  filters the high frequency current from the switch S 1   126 . In one example, the input capacitor C F    108  has a capacitance large enough such that a dc voltage is applied to the energy transfer element T 1   115 . However for power converters with power factor correction (PFC), a small input capacitor C F    108  can be utilized to allow the voltage applied to the energy transfer element T 1   115  to substantially follow the rectified voltage V RECT    107 . As such, the value of the input capacitor C F    108  can be chosen such that the voltage on the input capacitor C F    108  reaches substantially zero during each half-line cycle of the input line voltage. Or in other words, the voltage on the input capacitor C F    108  substantially follows the positive magnitude of the ac input voltage V AC    102 . 
         [0037]    The thyristor damper circuit  143  dampens input current I IN    103  to reduce ringing when the dimmer circuit  104  switches on. As noted above, when the dimmer circuit  104  switches on, inrush input current ringing occurs, which may reverse several times during the half line cycle. During these current reversals, the TRIAC of the dimmer circuit  104  may prematurely turn off and cause flickering in the LED lamp. 
         [0038]    For each switching cycle of the dimmer circuit  104 , the thyristor damper circuit  143  dampens the input current I IN    103  for a predetermined time when the dimmer circuit  104  switches on and thereafter ceases to dampen the input current I IN    103  after the predetermined time has lapsed. In one example, a value of the first resistor R 1   132  may be 1 MΩ. In another example, the first resistor R 1   132  may be a value in the range from 500Ω to 1 MΩ. In one example, the value of the second resistor may be 100 kΩ. As such, the thyristor damper circuit  143  reduces ringing when the dimmer circuit  104  switches on. In addition, since the thyristor damper circuit  143  is only on for a portion of the time during which the dimmer circuit  104  is on, embodiments of the present invention may dissipate less power than a conventional damper circuit that dissipates power when an ac voltage is present. 
         [0039]    At a first period during a half line cycle of operation, the dimmer circuit  104  switches on and reconnects the ac voltage V AC    102 . The peak of the inrush current occurs at a first time period associated with the moment that the dimmer circuit  104  switches on. At this time, transistor Q 1   142  is currently turned OFF and therefore impedes the input current of the power converter. The capacitor C 1   138  begins to charge. In one example, the value of the capacitor C 1   138  is a range from 1 nF to 10 nF. The amount that capacitor C 1   138  charges to is set by the first resistor R 1   132  and the second resistor  134 . The first resistor R 1   132  and the second resistor R 2   134  form a voltage divider. 
         [0040]    At a second time period during the half line cycle, the capacitor C 1   138  is at a voltage that is greater than the gate source voltage V GS  of transistor Q 1   142 . The second rectifier D 2   136  protects transistor Q 1   142  from exceeding a gate source voltage V GS . Capacitor C 1   138  provides a control signal to the transistor Q 1   142 . Transistor Q 1   142  operates in linear mode and impedes the input current of the power converter with a slope that is dictated by the third resistor R 3   140 . In one example, the value of the third resistor R 3  10 kΩ. The transistor Q 1   142  prevents the input current I IN    103  from falling below a current threshold known as the holding current of the TRIAC to prevent flickering of the LED. 
         [0041]    At a third time period during the half line cycle that occurs near the end of the conduction angle, transistor Q 1   142  is turned off. When the dimmer circuit  104  switches off, the input current I IN    103  reduces, which in turns disables the thyristor damper circuit  143 . The thyristor damper circuit  143  may then be enabled again when the dimmer circuit  104  switches on for the next half-line cycle. 
         [0042]    The switching power converter  100  utilizes the energy transfer element T 1   115  to transfer voltage between the primary  112  and the secondary  114  windings. The clamp circuit  118  is coupled to the primary winding  110  to limit the maximum voltage on the switch S 1   126 . Switch S 1   126  is opened and closed in response to the drive signal  129 . It is generally understood that a switch that is closed may conduct current and is considered on, while a switch that is open cannot conduct current and is considered off. In operation, the switching of the switch S 1   126  produces a pulsating current at the rectifier D 1   116 . The current in the rectifier D 1   116  is filtered by the output capacitor C O    118  to produce a substantially constant output voltage V O    119 , output current I O    121 , or a combination of the two at the load  120 . In some embodiments, the load  120  is an LED array. 
         [0043]    The feedback circuit  122  senses the output quantity U O    123  of the power converter  100  to provide the feedback signal U FB    125  to the controller  124 . The feedback signal U FB    125  may be a voltage signal or a current signal and provides information regarding the output quantity U O    123  to the controller  124 . In addition, the controller  124  receives the current sense input signal  127  which relays the switch current I D    131  in the switch S 1   126 . The switch current I D    131  may be sensed in a variety of ways, such for example the voltage across a discrete resistor or the voltage across a transistor when the transistor is conducting. 
         [0044]    The controller  124  outputs a drive signal  129  to operate the switch S 1   126  in response to various system inputs to substantially regulate the output quantity U O    123  to the desired value. In one embodiment, the drive signal  129  may be a rectangular pulse waveform with varying lengths of logic high and logic low sections, with the logic high value corresponding to a closed switch and a logic low corresponding to an open switch. In another embodiment, the drive signal  129  may be comprised of substantially fixed-length logic high (or ON) pulses and regulated by varying the number of ON pulses per number of oscillator cycles. 
         [0045]      FIGS. 2A-2C  illustrate example waveforms of an ac input voltage  202 , a dimmer output voltage  205 , and a rectified input voltage  207 , in accordance with the teachings of the present disclosure. Ac input voltage  202 , dimmer output voltage  205 , and rectified input voltage  207  are possible representations of ac input voltage  102 , dimmer output voltage  105 , and rectified input voltage  107 , respectively, of  FIG. 1 . 
         [0046]    As shown in  FIG. 2A , ac input voltage V AC    202  is generally a sinusoidal waveform with a period denoted as a full line cycle T AC    248 . A full line cycle T AC    248  of ac input voltage V AC    202  is denoted as the length of time between every other zero-crossing. When expressed as an angular displacement instead of time, a full line cycle spans 360 degrees, with 180 degrees between zero crossings. 
         [0047]    Now referring to dimmer output voltage  205  of  FIG. 2B , the half line cycle  250  of the ac input voltage V AC    202  is denoted as the length of time between consecutive zero-crossings. The phase angle Φ  252  is measured as how many degrees (from a reference of zero degrees) the dimmer circuit  102  disconnects the input voltage V AC    202 . For leading edge dimming, dimmer circuit  102  disconnects the ac input voltage V AC    202  from power converter  100  when the ac input voltage V AC    202  substantially crosses zero voltage. After a given amount of time, the dimmer circuit  102  reconnects ac input voltage V AC    202  with power converter  100  and the dimmer output voltage V DO    205  substantially follows the ac input voltage V AC    202 . In other words, the dimmer circuit  102  removes a portion of the ac input voltage  202  to provide the dimmer output voltage V DO    205  thus limiting the amount of power supplied to a load (such as an LED lamp). 
         [0048]      FIG. 2C  illustrates that at the beginning of each half line cycle  250 , rectified input voltage V IN    207  is substantially equal to zero, corresponding to the time that the dimmer circuit  102  disconnects the ac input voltage V AC    202  from the power converter. When the dimmer circuit  102  reconnects the ac input voltage V AC    202  to the power converter, the rectified input voltage V IN    207  substantially follows the positive magnitude of the dimmer output voltage V DO    205  and the ac input voltage V AC    202  (i.e., V IN =|V DO |). As shown, dimmer output voltage V DO    205  sharply changes from zero to substantially follow the ac input voltage V AC    202  when dimmer circuit  102  reconnects the ac input voltage  202 . 
         [0049]      FIG. 3A  illustrates a waveform of the output dimmer voltage V DO    305  and the associated input current I IN    303  during a half line cycle.  FIG. 3B  illustrates the voltage of R 4  V R4    346  of the damper circuit from  FIG. 1 . 
         [0050]    When the dimmer circuit is turned on, an inrush of input current I IN    303  occurs. At a time period T 1 , the transistor Q 1   142  from  FIG. 1  impedes the input current of the power converter, current conduction along both the first current conduction path and the second current conduction path tends to bias the voltage-controlled transistor Q 1  to impede the input current of the power converter, and the voltage across V R4    346  rises to a constant value. Transistor Q 1   142  is turned OFF during this time period. At a second time period T 2 , the capacitor C 1   138  is sufficiently charged such that transistor Q 1   142  operates in linear mode. The slope of V R4    346  is set by the third resistor R 3 . At the third time period T 3  that occurs near the end of the half line cycle, the voltage V R4    346  is at a value substantially zero such that the transistor Q 1   142  turns OFF. 
         [0051]      FIG. 4  illustrates another power converter  400  similar to  FIG. 1  that further includes a bias winding  423 . In operation, the bias winding  423  produces a bias voltage V B    417  that is responsive to the output voltage V O    419  when the output diode D 1   416  coupled to secondary winding  414  conducts. The feedback signal U FB    425  is representative of the output voltage V O    419  during at least a portion of an OFF time of switch S 1   426 . During the on-time of the switch S 1   427 , the bias winding  423  produces a bias voltage V B    417  in is response to the input voltage V RECT    407 . 
         [0052]    It is appreciated that many variations are possible in the use of a bias winding to sense an output voltage V O    419  and for providing sensing while also providing power to a controller with galvanic isolation. For example, a bias winding may apply a rectifier and a capacitor similar to rectifier D 1   416  and capacitor C O    418 , respectively, to produce a dc bias voltage while providing an ac feedback signal from the anode of the rectifier. As such, additional passive components such as resistors may be used on the bias winding  423  to scale the voltage from the winding to a value that is more suitable to be received by controller  424 . 
         [0053]    The above description of illustrated examples of the present invention, including what is described in the Abstract, are not intended to be exhaustive or to be limitation to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible without departing from the broader spirit and scope of the present invention. Indeed, it is appreciated that the specific example voltages, currents, frequencies, power range values, times, etc., are provided for explanation purposes and that other values may also be employed in other embodiments and examples in accordance with the teachings of the present invention. 
         [0054]    These modifications can be made to examples of the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims. Rather, the scope is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation. The present specification and figures are accordingly to be regarded as illustrative rather than restrictive.