INCORPORATION BY REFERENCE

[00011 This present disclosure claims the benefit of U.S. Provisional Application No. 61 /525,644, "Startup Circuit for Special TRIAC Applications" filed on August 19, 201 I , which is incorporated herein by reference in its entirety.

BACKGROUN D

|0002| The background description provided herein is for the purpose of general ly presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.

|0003| Many electrical and electronic devices are controlled by dimmers to change output characteristics of the devices. In an example, a dimmer is used to change light output from a lighting device. In another example, a dimmer is used to change rotation speed of a fan. Further, a dimmer can includes a receiver to receive a remote control signal, such that the dimmer is remote controllable. The receiver needs to be powered on even when the dimmer is turned off.

SUMMARY

|0004] Aspects of the disclosure provide a circuit. The circuit incl udes a control circuit and a return path circuit. The control circuit is configured to operate in response to a first, conduction angle of a dimmer coupled to the circuit. The first conduction angle is adjusted to control an output power to a first device. The dimmer has a second conduction angle that is independent of the control of the output power to the first device. The return path circuit is configured to provide a return path to enable providing power to a second device in response to the second conduction angle.

|0005| In an example, the circuit includes a startup circuit configured to enable the control circuit to start operation in response to the first conduction angle Further, the return path circuit is configured to provide the return path to enable providing power to the second device in response to the second conduction angle when the control circuit is not in operation. In an example, the control circuit includes a return path control circuit configured to disable the return path when the control circuit is in operation. The return path control circuit is configured to disable the return path based on at least one of an input voltage to the circuit and an output voltage of the circuit.

|0006| According to an aspect of the disclosure, the return path circuit is configured to provide the return path to enable providing power to the second device in the dimmer when the control circuit is not in operation. In an example, the second device is a remote control receiver.

|0007| In an example, the return path circuit includes a transistor configured to be turned on in response to the second conduction angle when the control circuit is not in operation. I n an example, the return path circuit includes a resistor and a capacitor to determine a turn on time of the transistor.

|0008| Aspects of the disclosure provide an electronic system. The electronic system includes the dimmer and the circuit coupled together.

|0009| Aspects of the disclosure provide a method. The method includes receiving an input that is regulated to have a first conduction angle and a second conduction angle. The first conduction angle is adjusted to control an output power to a first device, and the second conduction angle is independent of the control of the output power to the first device. Further the method includes turning on a return path for the input during the second conduction angle to provide power to a second device when the input provides no output power to the first device.

BRIEF DESCRIPTION OF THE DRAWINGS

|0010] Various embodiments of this disclosure that are proposed as examples wil l be described in detail with reference to the following figures, wherein like numerals reference l ike elements, and wherein:

|001 11 Fig. 1 shows an electronic system 100 according to an embodiment of the disclosure;

[0012| Fig. 2 shows a plot 200 of waveforms according to an embodiment of the disclosure;

|0013| Fig. 3 shows a flowchart outlining a process 300 according to an embodiment of the disclosure;

|0014) Fig. 4 shows a block diagram of a circuit example 410 according to an embodiment of the disclosure;

[0015 j Fig. 5 shows a plot 500 of waveforms for the circuit 41 0 according to an embodiment of the disclosure; |0016| Fig. 6 shows a plot 600 of waveforms for the circuit 4 1 0 according to an embodiment of the disclosure;

|0017| Fig. 7 shows a block diagram of a circuit example 7 1 0 according to an embodiment of the disclosure;

|0018| Fig. 8 shows a plot 800 of waveforms according to an embodiment of the disclosure;

|0019] Fig. 9 shows a block diagram of a circuit example 91 0 according to the embodiment of the disclosure; and

|0020| Fig. 1 0 shows a block diagram of a circuit example 1 0 1 0 according to an embodiment of the disclosure.

DETAI LED DESCRI PTION OF EMBODI MENTS

[00211 Fig. 1 shows an electronic system 1 00 according to an embodiment of the disclosure. The electronic system 1 00 includes a dimmer 1 02, a rectifier 1 03, a circuit 1 1 0, an energy transfer module 1 04, and an output device 1 09. These elements are coupled together as shown in Fig. 1 .

|0022| According to an embodiment of the disclosure, the electronic system 1 00 is suitably coupled to an energy source 1 01 . In the Fig. 1 example, the energy source 1 01 is an alternating current (AC) voltage supply to provide an AC voltage V _A C. such as 1 1 0V AC supply voltage, 220V AC supply voltage, and the like. In an example, the electronic system 1 00 includes a power cord that has been plugged into a wall outlet (not shown) on a power grid. In another example, the electronic system 1 00 is coupled to the energy source 1 0 1 via a switch (not shown). When the switch is switched on, the electronic system 1 00 is coupled to the energy source 1 0 1 .

|0023| According to an aspect of the disclosure, the dimmer 1 02 is configured to control electric energy from the energy source 1 0 1 to the electronic system 1 00, and thus controls output power from the output device 1 09. For example, the dimmer 1 02 is turned on/off to turn on/off the output device 1 09, and a dimming angle of the dimmer 1 02 is adjusted to adjust output power from the output device 1 09.

(0024| Further, according to an embodiment of the disclosure, the electronic system 1 00 includes a component that is turned-on no matter the dimmer 1 02 is turned on or off when the electronic system 100 is coupled to the energy source 1 01 . The dimmer 1 02 is configured to provide electric energy to the always-on component.

[0025| In an example, the dimmer 102 is a remote controllable dimmer that includes a remote control receiver 160. When the electronic system 100 is coupled to the energy source 1 01 , the remote control receiver 1 60 is turned on to l isten to control signals from a remote control component 162 no matter the dimmer 102 is turned on or off.

|0026| In an example, the remote control component 162 is configured to transmit a turn- on control signal. When the remote control receiver 160 receives the turn-on control signal, the dimmer 1 02 is turned on to start providing electric energy to other devices, such as to the output device 1 09 in the electronic system 100. Further, in an example, the remote control component 1 62 is configured to transmit a power adjustment signal. When the remote control receiver 1 60 receives the power adjustment signal, the dimmer 102 adjusts the electric energy provided to the output device 109 according to the received power adjustment signal. Then, in an example, the remote control component 162 is configured to transmit a turn-off control signal. When the remote control receiver 160 receives the turn-off control signal, the dimmer 102 is turned off to stop providing electric energy to the other devices in the electronic system 100, and thus turns off the output device 109 in an example.

10027] It is noted that even when the dimmer 102 is turned off to stop providing electric energy to the output device 109, the remote control receiver 1 60 in the dimmer 102 needs to continue operation to listen to the control signals from the remote control component 1 2. In an embodiment, the dimmer 102 provides the necessary energy to support the remote control receiver 160 even when the dimmer 102 is turned off to stop providing electric energy to the output device 109.

|0028| According to an aspect of the disclosure, the dimmer 102 is a phase angle based dimmer. In an example, the AC voltage supply has a sine wave shape, and the dimmer 1 02 includes a forward-type triode for alternating current (TR1 AC) 164 having an adjustable dimming angle a within [0, π]. Every time the AC voltage V _A c crosses zero, the forward-type TR1 AC 1 64 stops firing charges for a dimming angle a. The dimming angle a is adjusted to turn on/off the dimmer 102 and adj ust the output power of the output device 109. For example, when the dimming angle a is equal to π, the dimmer 102 is turned off; when the dimming angle a is reduced from π, the dimmer 1 02 is turned on; when the dimming angle a is further reduced, the output power of the output device 109 is increased; and when the dimming angle a is zero, the output power of the output device 109 is maximized.

|0029| Further, according to an aspect of the disclosure, the forward-type TRIAC 1 64 additionally fires charges for a time duration that is independent of the dimming angle a to provide electric energy to the always-on component in the electronic system 100, such as the remote control receiver 160.

|0030| Thus, in an example, the forward-type TRIAC 164 has first conduction angles that depend on the dimming angle a, such as [α, π] and [π+α, 2π], and has a second conduction angle that is independent of the dimming angle a, such as a relatively small time during at the beginning of each AC cycle. When a phase of the AC voltage V _A is within a conduction angle, the forward-type TRIAC 164 fires charges, and a TRIAC voltage VTRI _A C follows the AC voltage V _A C; and when the phase of the AC voltage is out of any conduction angle, the TRIAC voltage VT _RIA C output from the forward-type TRIAC 164 is zero.

[0031 ] According to an embodiment of the disclosure, the dimmer 102 includes an energy storing element 161 to store electric energy for the remote control receiver 160. In the Fig. 1 example, the energy storing element 161 is a capacitor CTRI _A C- The capacitor CTRI _A C is configured to store electric energy when the forward-type TRI AC 164 fires charges, and provide the stored electric energy to the remote control receiver 160. In an embodiment, even when the dimmer 102 is turned off that the dimming angle a is π, the forward TRIAC 164 fires charges during the second conduction angle that is independent of the dimming angle a, thus the capacitor CTRI _AC stores and provides electric energy to support the remote control receiver 160 that is always turned on.

|0032| According to an aspect of the disclosure, a low impedance return path is required to enable the dimmer 102 to store electric energy in the energy storing element 161. In an example, the capacitor C TRI _AC has a relatively large capacitance, such as in the order of 10 μΤ, and thus the impedance of the return path needs to be much lower than the impedance of the capacitor CT I _A to enable the capacitor CTRI _AC to store the electric energy.

|0033| According to an aspect of the disclosure, even when the dimmer 102 is turned off to stop providing output power to the output device 109, the electronic system 1 00 provides a low impedance return path to enable the energy storing element 161 in the dimmer 102 to store electric energy. [0034] According to an embodiment of the disclosure, the dimmer 102 is integrated with other components in the electronic system 100. In another embodiment, the dimmer 102 is a separate component, and is suitably coupled with the other components of the electronic system 100. It is noted that the dimmer 102 can include other suitable components, such as a processor (not shown), and the like.

|0035| The rectifier 103 rectifies the received AC voltage to a fixed polarity, such as to be positive. In the Fig. 1 example, the rectifier 103 is a bridge rectifier 103. The bridge rectifier 103 receives the AC voltage, generates a rectified voltage V _R ECT, and provides the rectified voltage V _R ECT to other components of the electronic system 100, such as the circuit 1 10 and the like, to provide electric power to the electronic system 100. An example waveform of the rectified voltage VREC ' ^s shown in Fig. 2.

[0036| Fig. 2 shows a plot 200. of waveforms for the electronic system 100 according to an embodiment of the disclosure. The plot 200 includes a first waveform 210 for the AC supply voltage V.. _\ c, a second waveform 220 for the TR1AC voltage VTRI _A C, and a third waveform 230 for the rectified voltage

100371 As can be seen in Fig. 2, the AC voltage V, _\ c has a sinusoidal waveform, and has a frequency of 50 Hz. The TRIAC voltage VTRI _AC is zero when the phase of the AC voltage VA - IS out of any conduction angle and follows the shape of the AC voltage V _A c when the phase of the AC voltage V _A c is in a conduction angle. The rectified voltage VR CT is rectified from the TRI AC voltage V _TRIA C to have positive polarity.

|0038| Specifically, in the Fig. 2 example, the dimmer 102 has a dimming angle a. Thus, the TRI AC voltage VTRI _AC l> ^as first conduction angles, such as [α, π] and [π+α, 2π], that depend on the dimming angle a and has a second conduction angle, such as [0, β ^" |, that is independent of the dimming angel a.

|0039| In each cycle [0, 2π], when the phase of the AC voltage V _A c is within the second conduction angle [0, β], the AC voltage V _A c is positive, the TRIAC voltage VTRIAC follows the AC voltage V _AC , as shown by 240, and the rectified voltage VKECT is about the same as the TRIAC voltage VTRI _A C, as shown by 250; when the phase of the AC voltage V _A c is within [β, a] or [π, π+α], the TRIAC voltage VT _RIAC output from the forward-type TRIAC dimmer 102 is about zero, and the rectified voltage VRE _C T is about zero; when the phase of the AC voltage V _A c is within [a, 7t], the AC voltage V.«: is positive, the TRI AC voltage VT I _A C follows the AC voltage VAC, and the rectified voltage VRKCT is about the same as the TRIAC voltage VTRIA ; and when the phase of the AC voltage V _AC is within [π+α, 2π], the AC voltage V _AC is negative, the TRI AC voltage VTRI _A fol lows the AC voltage V _AC , and the rectified voltage VRKCT is about negative of the TRIAC voltage V I _A C.

[0040J According to an embodiment of the disclosure, the second conduction angle is relatively small and i ndependent of the dimming angle a. At the beginning of each cycle, the rectified voltage V H;T increases from zero to a peak voltage, and then drops to zero in response to the second conduction angle, as shown by 250.

(00411 The rectified voltage VRKCT is provided to following circuits, such as the circuit 1 10, the energy transfer module 104, and the output device 109, and the like in the electronic system 1 00. n an embodiment, the circuit 1 10 is implemented on a single integrated ci rcuit (1C) chip. In another embodiment, the circuit 1 10 is implemented on multiple IC chips. The circuit 1 10 is suitably coupled with the other components in the electronic system 100. For example, the circuit 1 10 provides control signals to the energy transfer module 104. The energy transfer module 1 04 transfers the provided electric energy by the rectified voltage VR _KC T to the output device 1 09.

I ^' 0042 ^' l In an example, the energy transfer module 1 04 includes a transformer T and a switch S r. The energy transfer module 104 also includes other suitable components, such as a diode D r, a capacitor Or, and the like. The transformer T includes a primary winding coupled with the switch S r and a secondary winding coupled to the output device 1 09. In an embodiment, the circuit 1 10 provides control signals to control the operations of the switch S r to transfer the energy from the primary winding to the secondary winding. In an example, the circuit 1 10 provides pulses having a relatively high frequency, such as in the order of 100 KHz, to control the switch ST. The relatively high frequency pulses enable power factor correction (PFC) for the AC supply.

|0043| The output device 109 can be any suitable device, such as a light bulb, a plurality of light emitting diodes (LEDs), a fan and the like.

|0044| According to an embodiment of the disclosure, the circuit 1 10 includes a return path circuit 140. The return path circuit 140 is configured to provide a low impedance return path when the dimmer 102 is turned off to stop providing electric energy to the output device 1 09. |0045| According to an embodiment of the disclosure, when the dimmer 1 02 is turned on to provide electric energy to the output device 1 09, the electronic system 1 00 has a low impedance return path. For example, when the dimmer 1 02 is turned on, the circuit 1 1 0 is powered up, and provides relatively high frequency pulses to repetitively switch on/off the switch ST- Thus, the transformer T and the switch S r form a return path when the dimmer 1 02 is turned on.

|0046| When the dimmer 1 02 is turned off to stop providing energy to the output device 1 09 (e.g., the dimming angle a being π), the circuit 1 1 0 is powered down and unable to provide the pulses to the switch S r, and the switch ST is in the off state, and breaks the return path formed by the transformer T and the switch ST. The return path circuit 1 40 is configured to provide a low impedance return path to the dimmer 1 02 when the dimmer 1 02 is turned off.

[0047| In an embodiment, the circuit 1 1 0 includes a startup circuit 1 20 and a control circuit 1 30. The startup circuit 1 20 is configured to startup the circuit 1 1 0 when the dimmer 1 02 is switched from being turned off to being turned on. In an embodiment, after startup, the control circuit 1 30 is enabled to provide pulses to the switch ST, and thus the transformer T and the switch S form a low impedance return path.

|0048| According to an example of the disclosure, the return path circuit 1 40 is coupled to the startup circuit 1 20 to operate based on the operation of the startup circuit 1 20. For example, the return path circuit 1 40 turns on a return path in the circuit 1 1 0 before the startup circuit 1 20 starts up the circuit 1 1 0 and the return path circuit 1 40 turns off the return path in the circuit 1 1 0 to reduce current leakage after the startup circuit 1 20 starts up the circuit 1 1 0.

[0049| In an example, the control circuit 1 30 includes a return path control circuit 1 50 coupled to the return path circuit 1 40. In an example, before startup, the return path circuit 1 40 turns on the return path when control signals from the return path control circuit are not available. After startup, the return path control circuit 1 50 generates control signals to turn off the return path formed by the return path circuit 1 40.

J0050J It is noted that the control circuit 1 30 incl udes various control circuits, such as a control circuit for control ling a depletion mode transistor in the start-up circuit 1 20, a control circuit for controlling the switch ST, the return path control circuit 1 50 for controlling the return path circuit 1 40, and the like. Different control circuits can be enabled to start operation in response an output voltage from the start-up circuit 1 20 at different voltage levels. In an example, the control circuit for controlling the switch ST is configured to operate when the output voltage from the start-up circuit 1 20 is above a relatively high voltage level, such as 1 0V and the like; and the control circuit for controlling the depletion mode transistor in the start-up circuit 1 20 and the return path control circuit 1 50 are configured to operate when the output voltage from the start-up circuit 1 20 is above a relatively low voltage level, such as 4V and the like.

(00511 Fig. 3 shows a flowchart outl ining a process 300 performed by the electronic system 1 00 according to an embodiment of the disclosure. The process starts at S30 1 and proceeds to S3 1 0.

|0052| At S3 1 0, the dimmer 1 02 receives the AC power supply, and adj usts power supply to following circuits according to conduction angles. Specifical ly, in each AC cycle, when the phase of the AC power supply is within a conduction angle, the dimmer 1 02 fires charges, and the output voltage from the dimmer 1 02 follows the voltage of the AC power supply; and when the phase of the AC power supply is not within any conduction angle, the dimmer 1 02 does not fire charges, and the output voltage from the dimmer 1 02 is zero. I n an example, when the dimmer 1 02 is turned on, in each AC cycle, there exists at least a first conduction angle and a second conduction angle. The first conduction angle is related to the dimming angle a of the dimmer 1 02 that determines output power to the output device 1 09. The second conduction angle is independent of the dimming angle a. When the dimmer 1 02 is turned off, the first conduction angle does not exist, and the second conduction angle still exists at the beginning of each AC cycle. The second conduction angle is intended to provide electric energy to certain circuits, such as the remote control receiver 1 60, that need to stay in operation even when the dimmer 1 02 is turned off.

[0053| At S320, the control circuit 1 30 operates in response to the first conduction angle to control output power to a first device, such as the output device 1 09. For example, when the first conduction angle exists in each AC cycle, the start-up circuit 1 20 starts up the circuit 1 1 0 and enables the operation of the control circuit 1 30. The control circuit 1 30 then provides control signals to control the energy transfer module 1 04 to transfer the provided electric energy by the rectified voltage V _RE CT to the output device 1 09.

|0054| At S330, the return path circuit 1 0 provides a return path to enable providing electric energy to a second device, such as the remote control receiver 1 60, in response to the second conduction angles when the dimmer 1 02 is turned off. For example, when the dimmer 1 02 is turned off, the dimming angle is π, the first conduction angle does not exist in an AC cycle. The control circuit 1 30 is not in operation, and no output power is provided to the output device 1 09. Then, the return path circuit 1 40 in the circuit 1 1 0 provides a return path to enable the capacitor CTRI.AC to store electric energy in response to the second conduction angles. The stored electric energy supports the operation of the remote control receiver 1 60. Then, the process proceeds to S399 and terminates.

[0055| Fig. 4 shows a block diagram of a circuit example 41 0 according to an

embodiment of the disclosure. The circuit 4 1 0 can be used in the electronic system 1 00 as the circuit 1 1 0.

|0056| In the Fig. 4 example, the circuit 4 1 0 includes a start-up circuit 420, a return path circuit 440, and a control circuit 430. According to an embodiment of the disclosure, the start-up circuit 420 is configured to start up at least a portion of the circuit 4 1 0, such as the control circuit ^' 430, when the dimmer 1 02 is turned on to provide output power to the output device 1 09. The return path circuit 440 is configured to provide a return path for the dimmer 1 02 when the dimmer 1 02 is turned off, in an example. The control circuit 430 is configured to provide various control signals to internal circuits of the circuit 4 1 0 and external circuits to the circuit 41 0 when the dimmer 1 02 is turned on.

|0057J In the Fig. 4 example, the start-up circuit 420 includes a transistor l coupled with a diode D l and a resistor R 2 to charge a capacitor COUT- In an embodiment, the transistor l is a depletion mode transistor, such as an N-type depletion mode metal-oxide- semiconductor-field-effect-transistor (MOSFET) that has a negative threshold voltage, such as (- 3V), configured to be conductive when control voltages are not available. For example, during an initial power receiving stage (e.g., at the time when the dimmer 1 02 is switched from being turned off to being turned on), because the gate-to-sou i ce and the gate-to-drain voltages of the N- type depletion mode MOSFET M l are about zero and are larger than the negative threshold voltage, thus an N-type conductive channel exists between the source and drain of the N-type depletion mode MOSFET M 1 even without a gate control voltage. The N-type depletion mode MOSFET M I allows an inrush current to enter the circuit 4 1 0 and charge the capacitor COUT- Further, when the circuit 41 0 enters the normal operation mode, the control circuit 430 provides control signals to turn on/off the N-type depletion mode OSFET l to charge the capacitor COUT and maintain the voltage on the capacitor COUT.

|0058| In the Fig. 4 example, the return path circuit 440 includes two transistors M2 and M3 and a resistor l . The resistor Rl and M3 are coupled together to receive a control signal from the control circuit 430 and to control a gate voltage of the transistor M2. In an example, the transistor M2 and the transistor M3 are N-type enhance mode MOSFETs that have positive threshold voltage.

[0059| During operation, in an example, when the dimmer 1 02 is turned off, the rectified voltage VRI;CT is unable to charge the capacitor COUT to an output voltage level to enable the operation of the control circuit 430, and thus the control circuit 430 does not provide a control signal to the transistor M3. Thus, the transistor M3 is turned off. Then, the output voltage VQUT controls the gate voltage of the transistor M2 via the resistor Rl . For example, when the output voltage VQUT >s larger than the threshold voltage of the transistor M2, such as larger than 3 V, the transistor M2 is turned on. In an example, the transistor M2 is suitably designed to have a low impedance when it is turned on. When the transistor M2 is turned on, the transistor M2 forms a low impedance return path to ground, and conducts a bleeding current IIH.F.I-.DI-.R to the ground. When the output voltage VOUT is smaller than the threshold voltage of the transistor M2, the transistor M2 is turned off.

|0060| In the Fig. 4 example, the control circuit 430 incl udes a gate control circuit 43 I and a return path control circuit 450. In an embodiment, the gate control circuit 4 1 is configured to control the gate terminal of the transistor M l when the control circuit 430 is i n operation. I n an example, when the dimmer 102 is turned on, the start-up circuit 420 charges the capacitor COUT to above certain voltage level enable the operation of the control circuit 430. It is noted that different portions of the control circuit 430 can be enabled to operate at different voltage levels, in an example, when the output voltage VOUT on the capacitor COUT is above 4V, the gate control circuit 43 1 is operative. Then, the gate control circuit 431 detects the output voltage VOU on the capacitor COUT, and turns on/off the transistor M 1 based on the detected output voltage VQUT ' ⁿ order to maintain the output voltage VOUT on the capacitor COUT For example, when the gate control circuit 43 1 detects that the output voltage VOUT on the capacitor COUT drops to a lower limit of a desired range, the gate control circuit 43 I turns on the transistor 1 to charge the capacitor COUT; when the gate control circuit 43 1 detects that the output voltage VOUT on the capacitor Cou r increases to an upper limit of the desired range, the gate control circuit 43 1 turns off the transistor 1 to stop charging the capacitor Cou It is noted that when the dimmer 102 is turned off, the output voltage VOUT on the capacitor COUT is lower than the voltage level, such as 4V, that can enable the operation of the gate control circuit 431 , and the gate control circuit 43 1 is unable to provide the gate control signal to the transistor M l .

|006l | In another example, the control circuit 430 includes a switch control portion (not shown) configured to provide pulses to, for example, the switch ST in Fig. 1 . The switch control portion is configured to provide the pulses when the output voltage VOUT on the capacitor Cou r is above 10V, for example. When the dimmer 102 is turned off, the output voltage VOUT on the capacitor COUT is lower than the voltage level, such as 10V, to enable the switch control portion of the control circuit 430, then the control circuit 430 does not provide pulses to the switch ST.

|0062| The return path control circuit 450 is configured to control the return path circuit 440 when the control circuit 430 is enabled to operate. In an example, when the dimmer 1 02 is turned on, the start-up circuit 420 charges the capacitor Cou r to above certain voltage level, such as above 10V to enable the operation of the control circuit 430. I n an embodiment, the control circuit 430 provides control signals to external circuits to form a return path that is out of the circuit 41 0. Further, the return path control circuit 450 controls the return path circuit 440 to turn off the return path within the circuit 410 to reduce the power leakage in an example.

[0063] According to an aspect of the disclosure, the return path control circuit 450 is configured to sense the rectified voltage VRKCT and the output voltage VOUT, and controls the return path circuit 440 based on the rectified voltage VRK -r and the output voltage VOUT.

[0064| In the Fig. 4 example, the return path control circuit 450 includes a rectified voltage sensing circuit 45 1 . The rectified voltage sensing circuit 451 includes resistors R3 and R4, and a first comparator OA 1 . The resistors R3 and R4 form a voltage divider to sense the rectified voltage VRKCT, and to generate a sensed rectified voltage VRKCT_SENSE- The first comparator OA 1 is configured to compare the sensed rectified voltage V _K |¾n S NSK with a reference voltage VRK . It is noted that, in an example, the reference voltage V EI.- is generated by the control circuit 430.

|0065| Further, the return path control circuit 450 includes an output voltage sensing circuit 452. The output voltage sensing circuit 452 includes resistors R5, R6 and R7 and a second comparator OA2. The resistors R5, R6 and R7 form a voltage divider with a switchable ratio to sense the output voltage VOUT, and to generate a sensed output voltage VOUT_SENSE- The second comparator OA2 is configured to compare the sensed output voltage VOUTJSENSK with the reference voltage VREF-

100661 I n the Fig. 4 example, the output of the first comparator OA 1 and output of the second comparator OA2 are combined to control the return path circuit 440.

|0067| According to an aspect of the disclosure, the return path control circuit 450 is configured to control the return path circuit 440 to turn off the return path when the rectified voltage VRKCT is larger than the peak voltage in the second conduction angle. In an example, the second conduction angle is generally a short period at the beginning of an AC cycle that the AC voltage increases from zero to the peak voltage and then drops to zero (e.g., 250 in Fig. 2). A resistance ratio of the resistors R3 and R4 are suitably determined that when the rectified voltage KC T ' ^s larger than the peak voltage of the second conduction angle, the sensed rectified voltage VRKCT SENSE ' larger than the reference voltage VREF- Thus, when the rectified voltage VRECT is larger than the peak voltage, the output of the first comparator OA 1 is " 1 ", and the transistor M 3 in the return path circuit 440 is turned on to pull down the gate voltage of the transistor 2, and thus the transistor M2 is turned off and the return path within the circuit 4 1 0 is shut off.

[0068] It is noted that the rectified voltage sensing circuit 45 1 is not sensitive to low conduction angles. Specifically, when the dimmer 1 02 is turned on to provide relatively small output power to the output device 1 09, the rectified voltage VRKCT during the first conduction angles can be lower than the peak voltage of the second conduction angle. Thus, the sensed rectified voltage VRKCT SENSE can be lower than the reference voltage VRE , and the output of the first comparator OA 1 is "0" .

[0069] In an embodiment, even when the dimming angle is large and the first conduction angles are low, the rectified voltage VRECT is able to charge the capacitor COUT to have a relatively large output voltage VOUT- Then, the output sensing circuit 452 controls the return path circuit 440 to turn off the return path in the circuit 41 0. Specifically, when the sensed output voltage VOUT SENSE is larger than the reference voltage, the output of the second comparator OA2 is " 1 ", and the transistor 3 in the return path circuit 440 is turned on to pul l off the gate voltage of the transistor M2 in order to shut off the return path in the circuit 4 1 0.

[0070] According to another aspect of the disclosure, the output sensing circuit 452 is configured to use two thresholds for the output voltage VOUT to control the return path in the return path circuit 440. In an example, the voltage divider is configured to have a relatively large ratio to sense the output voltage ν _0ϋΤ when the output voltage V ₀ i.rr is below a voltage level that enables the operation of the control circuit 430. For example, at default, the sensed output voltage VOUT SENSE is at P2. Thus, the output sensing circuit 452 uses a relatively small threshold for the output voltage VOU . Further, the voltage divider is configured to have a relatively small ratio to sense the output voltage VOUT when the output voltage VOU is above the voltage level that enables the operation of the control circuit 430. For example, the sensed output voltage VOUT_SENSE > ^{s at} P ' when the control circuit 430 is enabled. In an example, the sensed output voltage VOUT_.SI-.NSE is switched based on a FC-LATCH signal generated by the control circuit 430. In an example, when the capacitor Cou r is charged that the output voltage Vou r is above a certain level, such as i 5V, for the first time, the FC-LATCH signal is latched. The FC-LATCH signal is used to change the thresholds to control the return path in the return path circuit 440.

|0071 ] In an example, when the dimmer 102 is turned off, the output sensing circuit 452 uses the relatively small threshold. In addition, the output voltage VOUT is below the voltage level to enable the operation of the control circuit 430, and thus the control circuit 430 is unable to turn on the transistor M3. Then, the transistor M2 is turned on to form the return path in the circuit 41 0. In an example, the return path enables providing electric energy to the always-on component, such as the remote control receiver 160, in the dimmer 102.

[0072 | Further, in the example, when the dimmer 102 is switched from being turned off to being turned on, the rectified voltage VRK T charges the capacitor Cou r. When the output voltage VOUT on the COUT is above the level to enable the operation of the control circuit 430, the control circuit 430 starts operating. The control circuit 430 generates the reference voltage V ^' KEI.-. When the output voltage VOLT is above 1 5V for the first time, the FC-LATCH signal is latched and is used to switch the sensed output voltage VOUTJSENSE to PI , and the output sensing circuit 452 uses a relatively large threshold for the output voltage VOUT. Then, when the output voltage VOU is larger than the relatively large threshold, the second comparator O A2 outputs " I " to turn on the transistor M3 to pull down the gate voltage of the transistor M2 and turn off the transistor 2.

|0073 | When the dimmer 102 is switched from being turned on to being turned off, the rectified voltage VKECT stays low, and the output voltage VOUT starts dropping. Because the threshold voltage is relatively high, the output voltage V ₀ UT drops below the threshold voltage in a relatively short time, and the output of the second comparator OA2 switches from " 1 " to "0" in a relatively short time. The output of the first comparator OA.1 is also "0" due to the low rectified VRECT. Then, the transistor M3 is turned off in a relatively short time, and the transistor M2 is turned on in a relatively short time.

| 0074| Fig. 5 shows a plot 500 of waveforms for the circuit 410 when the dimmer 102 is turned off according to an embodiment of the disclosure. The plot 500 includes a first waveform 510 for the rectified voltage VRECT, a second waveform 520 for the output voltage V ₀ UT, a third waveform 530 for the drain current IDRAIN of the transistor M l , and a fourth waveform 540 for the bleeding current I BLEEDER of the transistor M2.

|0075 | According to an embodiment, at beginning of each AC cycle, the dimmer 1 02 has a conduction angle that is independent of the state of the dimmer 102. The conduction angle allows the dimmer 102 to fire charges to provide electric energy to the always-on component, such as the remote control receiver 160, even when the dimmer 102 has been turned off.

[0076] During the conduction angle at the beginning of each AC cycle, the rectified voltage VR CT follows the AC supply to increase from zero to the peak voltage and then drop to zero, as shown by 51 1 in Fig. 5.

| 0077 | Because the rectified VRECT is non-zero within the conduction angle, the startup circuit 420 charges the capacitor Cou r and increases the output voltage Vou r during the conduction angle. Because when the output voltage Vou r is below a level to enable the operation of the control circuit 430, the control circuit 430 is not able to provide the control signal to the transistor M3. Thus, the transistor M3 is turned off. When the output voltage VQUT is above the threshold voltage of the transistor 2, such as about 3V, the transistor 2 is turned on to form the return path to ground. The return path conducts the bleeding current IBLEEDER that is about same as the drain current IDRAIN- The return path enables the dimmer 102 to provide electric energy to the always-on component. The return path also discharges the buildup on the capacitor Cou r, and thus reduces the output voltage VOUT When the output voltage VQUT drops below the threshold of the transistor M2, the transistor 2 is turned off, and the bleeding current IBLEEDER drops to about zero.

|0078| Fig. 6 shows a plot 600 of waveforms for the circuit 410 when the dimmer 102 is switched from being turned on to being turned off according to an embodiment of the disclosure. The plot 600 includes a first waveform 61 0 for the rectified voltage VK C I , a second waveform 620 for the output voltage VQUT, a third waveform 630 for the drain current IDR _A IN of the transistor Ml , and a fourth waveform 640 for the bleeding current IBU-KDKR of the transistor M2.

|0079| In the Fig. 6 example, at about 0.05 seconds, the dimmer 1 02 is switched from being turned on to being turned off. According to an embodiment, when the dimmer 1 02 is turned on, the dimmer 102 regulates the output according to a first conduction angle that depends on the dimming angle of the dimmer 102, and a second conduction angle at the beginning of each AC cycle that is independent of the dimming angle. When the dimmer 102 is turned off, the first conduction angle does not exist, and the second conduction angle still exists at the beginning of each AC cycle.

[ 0080] As can be seen from the first waveform 610, before the dimmer 102 is switched off, during the first conduction angle and the second conduction angle, the rectified voltage VRECT follows the absolute value of the AC supply voltage.

[0081 J Before the dimmer 102 is switched off, the control circuit 430 is in operation. As can be seen from the second waveform 620 and the second waveform 630, the gate control circuit 431 controls the transistor l to turn on/off to let the rectified voltage VRECT charge the capacitor Com , and maintain the output voltage VQUT in a desired range, such as within [ 1 I V, 1 5V] range.

| 0082] Before the dimmer 102 is switched off, the return path control circuit 450 detects that the dimmer 102 is on, and control the return path circuit 440 to turn off the return path in the circuit 410. For example, the rectified voltage sensing circuit 451 detects the voltage level of the rectified voltage Vmcv and the output voltage sensing circuit 452 detects the output voltage Vou to determine the dimmer 102 is stil l on. As can be seen from the fourth waveform 640, no bleeding current passes the transistor M2 before the dimmer 102 is switched off.

[0083] When the dimmer 102 is switched off, the first conduction angle does not exists, the rectified voltage Vi is only non-zero during the second conduction angle (at the beginning of each AC cycle). The rectified voltage V _K E T can no longer charge the capacitor COUT to maintain the output voltage Vou r, and thus the output voltage VQUT drops to relatively low level, such as 2V. The control circuit 430 is no longer in operation, and cannot provide the control signal to turn on the transistor M3. Further, during the second conduction angle, the output voltage Vou r increases due to the non-zero rectified voltage VRECT. When the output voltage Vow is larger than the threshold voltage of the transistor M2, the transistor M2 is turned on to form the return path.

|0084| Fig. 7 shows a block diagram of a circuit example 710 according to an embodiment of the disclosure. The circuit example 7 10 utilizes certain components that are identical or equivalent to those used in the circuit 410; the description of these components has been provided above and wil l be omitted here for clarity purposes. In this embodiment, the control circuit 730 does not include a return path control circuit to control the return path circuit 740, and the return path circuit 740 is self-controlled.

[ 0085| The return path circuit 740 includes transistors M2 and M3, resistors R , R3 and R4 and a capacitor Cl . These elements are coupled together as shown in Fig. 7. The resistors R l and R3 and the capacitor C l form an RC circuit to determine a turn-on time of the transistor 2. According to an embodiment of the disclosure, the turn on time T can be expressed by Eq. I :

Rl X i?3

T " Rl + Rs ^Cl Eq. 1

|0086| During operation, in an example, when the dimmer 1 02 is turned on, the output voltage Vow is maintained at a relatively high level, such as above 10V. The resistance ratio of the resistors R l and R3 are suitably determined that the gate voltage of the transistor M3 is above its threshold, thus the transistor 3 is turned on to pull down the gate voltage of the transistor 2, thus the transistor M2 is turned off.

|0087 | When the dimmer 102 is turned off, the output voltage Vow drops. When the output voltage Vow drops to a level that the gate voltage of the transistor 3 is below its threshold, the transistor 3 is turned off. The resistor R4 pulls up the gate voltage of the transistor M2 to a relatively high level to turn on the transistor 2. In an example, the transistor 2 stays on for about the turn on time T, and then the gate voltage of the transistor 2 is below its threshold voltage and the transistor M2 is turned off.

[0088| It is noted that the circuit 710 can be suitably modified. For example, the resistor R l can be connected to node 721 or can be connected to node 722.

|0089| Fig. 8 shows a plot 800 of waveforms for the circuit 71 0 when the dimmer 102 is switched from being turned on to being turned off according to an embodiment of the disclosure. The plot 800 includes a first waveform 81 0 for the rectified voltage VRKCT, a second waveform 820 for the output voltage V ₀ u r, a third waveform 830 for the drain current IDR _A IN of the transistor 1 , and a fourth waveform 840 for the bleeding current ΙΜ„Ι·Ι;Ι:>ΕΚ of the transistor 2.

|0090| In the Fig. 8 example, at about 0.03 seconds, the dimmer 1 02 is switched from being turned on to being turned off. According to an embodiment, before the dimmer 1 02 is switched off, the dimmer 1 02 regulates the output according to a first conduction angle that depends on the dimming angle of the dimmer 1 02, and a second conduction angle that is independent of the dimming angle. After the dimmer 1 02 is switched off, the first conduction angle does not exist, and the second conduction angle sti ll exists at the beginning of each AC cycle.

|009 I I As can be seen from the first waveform 8 1 0, before the dimmer 1 02 is switched off, during the first conduction angle and the second conduction angle, the rectified voltage VRKCT follows the absolute value of the AC supply voltage.

[0092| Before the dimmer 1 02 is switched off, the control circuit 730 is in operation. As can be seen from the second waveform 820 and the third waveform 830, the gate control circuit 73 1 controls the transistor M l to turn on/off to let the rectified voltage VRI T charge the capacitor COUT, and maintain the output voltage VQUT in a desired range, such as within [ 1 I V, 1 5V] range.

|0093| Before the dimmer 1 02 is switched off, because the output voltage Vou r is relatively high, and thus the gate voltage of the transistor M3 is larger than its threshold. The transistor M3 is turned on to pull down the gate voltage of the transistor M2. As can be seen from the fourth waveform 840, no bleeding current passes the transistor M2 before the dimmer 1 02 is switched off.

[0094] When the dimmer 1 02 is switched off, the first conduction angle does not exists, the rectified voltage VRHCT is only non-zero during the second conduction angle (at the beginning of each AC cycle). The rectified voltage VRE I can no longer charge the capacitor COUT to maintain the output voltage VOUT, and thus the output voltage VQUT drops to relatively low level, such as below 1 0. Thus, during the second conduction angle, the output voltage V ₀ UT increases due to the non-zero rectified voltage VRECT, and then drops. When the output voltage Vou r is relatively large, the transistor M3 is turned on and thus the transistor M2 is turned off. When the output voltage VQUT drops to a level that the transistor M3 is turned off, the transistor M2 is turned on for the turn-on time T to form the return path. [0095] Fig. 9 shows a block diagram of a circuit example 910 according to the embodiment of the disclosure. The circuit example 91 0 also utilizes certain components that are identical or equivalent to those used in the circuit 71 0; the description of these components has been provided above and will be omitted here for clarity purposes. However, in this embodiment, the resistor R l is coupled to the rectified voltage VRECT instead of the V ₀ UT.

| 0096| Fig. 10 shows block diagram of a circuit example 1010 according to an embodiment of the disclosure. The circuit 1 010 operates simi larly to the circuit 710 and the circuit 910. The circuit 1010 also utilizes certain components that are identical or equivalent to those used in circuit 7 10 and circuit 910; the description of these components has been provided avove and will be omitted here for clarity purposes. However, in this embodiment, a resistor R 1 _A is coupled to the rectified voltage VKECT, an another resistor R I _B is coupled to the output voltage VQUT-

[0097] While aspects of the present disclosure have been described in conjunction with the specific embodiments thereof that are proposed as examples, alternatives, modifications, and variations to the examples may be made. Accordingly, embodiments as set forth herein are intended to be illustrative and not limiting. There are changes that may be made without departing from the scope of the claims set forth below.