WO2017139251A1

WO2017139251A1 - Light source driving circuits for triac dimmer

Info

Publication number: WO2017139251A1
Application number: PCT/US2017/016793
Authority: WO
Inventors: Ching-Chuan Kuo; Xueshan Liu; Bingzhou FANG; Tiesheng Yan
Original assignee: O2Micro, Inc.
Priority date: 2015-12-03
Filing date: 2017-02-07
Publication date: 2017-08-17
Also published as: GB201521332D0; GB2543108A

Abstract

A light source driving circuit includes a rectifier operable for rectifying an AC voltage from a TRIAC dimmer and providing a rectified voltage, a power converter coupled to the rectifier and operable for receiving the rectified voltage and providing an output current, a dimmer controller operable for controlling the power converter based on the rectified voltage to adjust the output current, and a light source module coupled to the power converter and powered by the output current. The light source module includes a first light source having a first color, a second light source having a second color, and a current allocation unit coupled to the first light source and the second light source. The current allocation unit is operable for adjusting a current through the first light source and a current through the second light source based on the output current.

Description

LIGHT SOURCE DRIVING CIRCUITS FOR TRIAC DIMMER

BACKGROUND ART

[0001 ] LEDs offer several advantages over traditional light sources such as incandescent lamps. For example, LEDs have low power consumption, high power efficiency and long life. Therefore, there is a trend to replace incandescent lamps with LEDs. LED bulbs have similar shapes and sizes as those of incandescent bulbs. LED light sources and control circuitry are integrated within an LED bulb. Using a conventional on/off switch or a conventional TRIAC dimmer, a user can only control the on/off or brightness level of an LED bulb, but cannot adjust the color of the light. In order to adjust the color, a special dimmer or a remote controller is needed.

DISCLOSURE OF THE INVENTION

[0002] Embodiments in accordance with the present invention provide circuits for driving light sources, e.g., light-emitting diode (LED) light sources.

[0003] In one embodiment, a light source driving circuit includes a rectifier operable for rectifying an AC voltage from a TRIAC dimmer and providing a rectified voltage, a power converter coupled to the rectifier and operable for receiving the rectified voltage and providing an output current, a dimmer controller operable for controlling the power converter based on the rectified voltage to adjust the output current, and a light source module coupled to the power converter and powered by the output current. The light source module includes a first light source having a first color, a second light source having a second color, and a current allocation unit coupled to the first light source and the second light source. The current allocation unit is operable for adjusting a current through the first light source and a current through the second light source based on the output current.

[0004] In another embodiment, a light source driving circuit includes a rectifier operable for rectifying an AC voltage from a TRIAC dimmer and providing a rectified voltage, a power converter coupled to the rectifier and operable for receiving the rectified voltage and providing an output current for a light source module, a dimmer controller operable for controlling the power converter based on the rectified voltage to adjust the output current, and a current path coupled to the TRIAC dimmer through the rectifier. The current path is operable for conducting a latching current and a holding current of the TRIAC dimmer. The current path includes a first resistor, a diode coupled in parallel with the first resistor, a capacitor coupled in series with the first resistor and the diode, and a second resistor coupled in series with the first resistor and the diode.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005] Features and advantages of embodiments of the claimed subject matter will become apparent as the following detailed description proceeds, and upon reference to the drawings, wherein like numerals depict like parts, and in which:

[0006] FIG. 1 shows a light source driving circuit, in accordance with one embodiment of the present invention.

[0007] FIG. 2 shows waveforms associated with the light source driving circuit in FIG. 1 , in accordance with one embodiment of the present invention.

[0008] FIG. 3 shows a structure diagram of the light source module in FIG. 1 , in accordance with one embodiment of the present invention.

[0009] FIG. 4 shows a light source driving circuit, in accordance with one embodiment of the present invention.

[0010] FIG. 5 shows waveforms associated with the light source driving circuit in FIG. 4, in accordance with one embodiment of the present invention.

[001 1 ] FIG. 6 shows a light source driving circuit, in accordance with another embodiment of the present invention.

[0012] FIG. 7 shows a light source driving circuit, in accordance with another embodiment of the present invention.

[0013] FIG. 8 shows a light source driving circuit, in accordance with another embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0014] Reference will now be made in detail to the embodiments of the present invention. While the invention will be described in conjunction with these embodiments, it will be understood that they are not intended to limit the invention to these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the invention as defined by the appended claims.

[0015] Furthermore, in the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be recognized by one of ordinary skill in the art that the present invention may be practiced without these specific details. In other instances, well known methods, procedures, components, and circuits have not been described in detail as not to unnecessarily obscure aspects of the present invention.

[0016] FIG. 1 shows a light source driving circuit 100, in accordance with one embodiment of the present invention. In the example of FIG. 1 , the light source driving circuit 100 include a rectifier 108 coupled to a TRIAC dimmer 104, a dimmer controller 1 10, a power converter 1 12 and a light source module 1 18. The TRIAC dimmer 104 is operable for receiving an AC input voltage V|_N from the power source 102 and for generating an AC voltage V_TR|_Ac- The rectifier 108 is operable for rectifying the AC voltage V_TR|_Ac from the TRIAC dimmer 104 and providing a rectified voltage V_REC- The power converter 1 12 is operable for receiving the rectified voltage V_REc and providing an output current Ι₀υτ to power the light source 1 18. The dimmer controller 1 10 is operable for controlling the power converter 1 12 based on the rectified voltage V_REC to adjust the output current Ι₀υτ, so as to adjust the brightness of the light source module 1 18. In one embodiment, the light source driving circuit 100 further includes a current path 1 14 for conducting a latching current and a holding current of the TRIAC dimmer 104. The current path 1 14 is coupled to the TRIAC dimmer 104 through the rectifier 108.

[0017] In the example of FIG. 2A, the TRIAC dimmer 104 includes a TRIAC 206 coupled between the power source 102 and the rectifier 108. The TRIAC 206 has a first terminal A1 , a second terminal A2 and a gate G. The TRIAC dimmer 104 further includes an adjustable resistor 208 coupled in series with a capacitor 210, and a DIAC 212 having one end coupled to the capacitor 210 and the other end coupled to the gate G of the TRIAC 206. The TRIAC 206 is a bidirectional switch which can conduct current in either direction when it is triggered. The TRIAC 206 can be triggered by a positive or a negative voltage applied to the gate G. The current path 1 14 conducts a latching current for the TRIAC 206 if the TRIAC 206 is triggered, and conducts a holding current for the TRIAC 206 during a conduction period after the TRIAC 206 is triggered. Once triggered, the TRIAC 206 continues to conduct until the current through it drops below a threshold value, i.e., the holding current l_H. In other words, in order to keep the TRIAC 206 conducting, a current flowing through the TRIAC 206 is maintained no less than the holding current l_H after the TRIAC 206 is triggered.

[0018] FIG. 2 shows waveforms associated with the light source driving circuit 100 in FIG. 1 , in accordance with one embodiment of the present invention. More specifically, FIG. 2 shows waveforms of the AC input voltage V|_N, a voltage V_A2-AI between the terminal A1 and terminal A2 of the TRIAC 206, a current I DIAC through the DIAC 212, the AC voltage V_TR|_Ac provided by the TRIAC dimmer 104, and the rectified voltage V_REc from the rectifier 108. In the example of FIG. 2, the AC input voltage V|_N has a sinusoidal waveform. FIG. 2 is described in combination with FIG. 1 .

[0019] Initially (e.g., between time T₀ and ΤΊ), the TRIAC 206 is turned off, and the voltage V_A2-AI between the terminal A1 and terminal A2 increases with the AC input voltage V|_N. At time T the voltage across the capacitor 210 turns on the DIAC 212. The DIAC 212 generates a current pulse applied to the gate G of the TRIAC 206. The TRIAC 206 is turned on in response to the current pulse. As a result, the AC input voltage V|_N passes to the rectifier 108, and a current flows through the TRIAC 206. At time T₂ that is near the end of the first half cycle (e.g., between time T₀ and T₂) of the AC input voltage V|_N, the TRIAC 206 is turned off because the current through the TRIAC 206 falls below the holding current of the TRIAC 206. In the second half cycle (e.g., between time T₂ and T₄) of the input voltage V|_N, the TRIAC 206 is turned on again when the voltage across the capacitor 210 turns on the DIAC 212 (e.g., at time T₃). By increasing or decreasing the resistance of the adjustable resistor 208, the current charging the capacitor 210 is varied such that the DIAC 212 can be conducted at a different phase angle. Consequently, the TRIAC 206 can be turned on with an adjustable conducting angle depending on the resistance of the adjustable resistor 208. The rectifier 108 converts the negative portion of the AC voltage V_TR|_AC to corresponding positive portion to generate the rectified voltage V_REc-

[0020] FIG. 3 shows a structure diagram of the light source module 1 18 in FIG. 1 , in accordance with one embodiment of the present invention. FIG. 3 is described in combination with FIG. 1 . The light source module 1 18 includes a first light source 121 and a second light source 122. The first light source 121 can be a first LED string having a first color (e.g., a cold light LED string). The second light source 122 can be a second LED string having a second color (e.g., a warm light LED string). The light source module 1 18 further includes a current allocation unit 120 coupled to the first light source 121 and the second light source 122. The current allocation unit 120 is operable for adjusting a current through the first light source 121 and a current through the second light source 122 based on the output current Ι₀υτ- The current allocation unit 120 includes a sensing unit 302, a control unit 304 and a current regulating unit 306. In one embodiment, the current regulation unit 306 includes a current limiting unit 308. The sensing unit 302 is coupled to the first light source 121 and a second light source 122 and is operable for providing a sensing signal indicating the output current Ι₀υτ- The current regulating unit 306 is coupled to the second light source 122. The control unit 304 is coupled to the sensing unit 302 and the current regulating unit 306, and is operable for controlling the current regulating unit 306 based on the sensing signal to regulate the current through the second light source 122. The current limiting unit 308 is operable for limiting a maximum level of the current through the second light source 122.

[0021 ] FIG. 4 shows a light source driving circuit 400, in accordance with one embodiment of the present invention. FIG . 5 shows waveforms associated with the light source driving circuit 400 in FIG. 4, in accordance with one embodiment of the present invention. FIG . 4 is described in combination with FIG. 5. In FIG . 4, the elements labeled the same as in FIG . 1 have similar functions. The light source driving circuit 400 includes a rectifier 108 coupled to the TRIAC dimmer 104, a dimming controller 1 10, a power converter 1 12, a current path 1 14 and a light source module 1 18.

[0022] In the example of FIG. 4, the current path 1 14 includes a resistor 408 and a diode 406 which are coupled in parallel with each other, and includes a capacitor 404 and a resistor 407 which are coupled in series with the resistor 408 and the diode 406. An anode of the diode 406 is coupled to the capacitor 404. A cathode of the diode 406 is coupled to ground though the resistor 407. When the capacitor 404 is being charged, the diode 406 is conducted, the resistor 408 is shorted. A charging current flows through the capacitor 404, the diode 406 and the resistor 407 to ground. The charging current of the capacitor 404 is not affected by the resistor 408, and therefore the trigger of the TRIAC dimmer 104 is not affected. When the capacitor 404 is being discharged, the diode 406 is reversely biased. A discharging current flows through ground, the resistor 407, the resistor 408 and the capacitor 404. The discharging current is reduced due to the resistor 408. Compared with a conventional current path which does not include the resistor 408 and the diode 406, the current path 1 14 in FIG . 4 includes the resistor 408 and the diode 406 coupled in parallel and therefore the discharging current of the capacitor 404 is reduced while the charging current of the capacitor 404 is not affected. When the capacitor 404 is being discharged, a current flowing into the power converter 1 12 is a sum of the discharging current of the capacitor 404 and the current through the TRIAC dimmer 104. The current flowing into the power converter 1 12 is constant if the current consumed by a load of the circuit is constant. As a result, the decrement of the discharging current of the capacitor 404 results in an increment of the current through the TRIAC dimmer 104. In other words, by using the current path 1 14 shown in FIG. 4, the current through the TRIAC dimmer 104 can be maintained at a relatively high level that is greater than the hold current required by the TRIAC dimmer 104. As such, the flickering of the light source can be avoided.

[0023] In the example of FIG. 4, the power converter 1 12 is a buck-boost converter including a switch 418, a diode 412, a capacitor 410 and a inductor 414. The terminals of the dimmer controller 1 10 include HV, COMP, VDD, FB, GATE, GND and CS. The terminal HV is coupled to the rectifier 108 and receives a signal V_REc' indicating the rectified voltage V_REC. The signal V_REc' can be proportional to the rectified voltage V_REC. In one embodiment, the dimmer controller 1 10 includes a reference signal generating unit (not shown in FIG. 4). The reference signal generating unit generates a reference signal REF (not shown in FIG. 4) based on the signal V_REC'. A voltage of the reference signal REF can indicates the conducting angle of the TRIAC dimmer 104. Refer to FIG. 2. If a user adjusts the TRIAC dimmer 104 and decreases the conducting angle, the conduction time T-_\ is shifted backward, and the voltage of the reference signal REF is decreased. If a user adjusts the TRIAC dimmer 104 and increases the conducting angle, the conduction time T-_\ is shifted forward, and the voltage of the reference signal REF is increased. A person having ordinary skill in the art can understand the reference signal generating unit can be implemented in different ways. In one embodiment, the reference signal generating unit includes an RC filter that filters the signal V_REC' to generate the reference signal REF. In another embodiment, the reference signal generating unit includes an integrator that performs integration of the signal V_REC' to generate the reference signal REF. In yet another embodiment, the reference signal generating unit includes an A/D converter and a D/A converter that digitize and convert the signal V_REC' to generate the reference signal REF. The dimmer controller 1 10 generates a driving signal based on the reference signal REF, and outputs the driving signal through the terminal GATE to control the switch 408, thus to adjust the output current Ι₀υτ from the power converter 1 12. As such, if a user adjusts the TRIAC dimmer 104, the output current Ι₀υτ and the brightness of the light source module 1 18 are adjusted accordingly.

[0024] In the example of FIG. 4, the light source module 1 18 includes a first light source and a second light source. The first light source 121 can be a first LED string LED1 having a first color (e.g., a cold light LED string). The second light source 122 can be a second LED string LED2 having a second color (e.g., a warm light LED string). The light source module 1 18 further includes a current allocation unit coupled to the first LED string LED1 and the second LED string LED2. The current allocation unit is operable for adjusting a current l_LED1 through the first LED string LED1 and a current I_LED2 through the second LED string LED2 based on the output current Ι₀υτ- In the example of FIG. 4, the current allocation unit includes a sensing unit (e.g. , a resistor R44), a control unit (e.g., a transistor Q41) and a current regulating unit (e.g., a transistor Q42 and a resistor R43, wherein the resistor R43 functions as a current limiting unit). The resistor R44 is coupled to the LED strings LED1 and LED2, and provides a sensing signal indicating the output current l_0UT. The transistor Q42 is coupled to the second LED string LED2 and regulates the current l_LED2 through the second LED string LED2. The transistor Q41 is coupled to the resistor R44 and the transistor Q42, and controls the transistor Q42 based on the sensing signal to regulate the current I_LED2 through the second LED string LED2. Neglecting a current through the resistor R43, ILEDI is equal to Ιουτ minus ILED2- Therefore, by adjusting ILED2, the allocation of the current through the LED strings LED1 and LED2 can be adjusted, and thus the current l_LED1 through the first LED string LED1 can be adjusted. The resistor R43 is coupled to the transistor Q42 and is operable for limiting a maximum level of the current through the second LED string LED2. More specifically, as shown in FIG. 4, the resistor R44 is coupled to both LED strings LED1 and LED2. The current l_LED1 through the first LED string LED1 and the current l_LED2 through the second LED string LED2 both flow through the resistor R44. The voltage V_R44 across the resistor R44 is the sensing signal indicating the output current Ι₀υτ- The voltage V_R44 is approximately proportional to the output current Ι₀υτ- The resistor R44 is coupled between the base and the emitter of the transistor Q41 . The conduction status of the transistor Q41 is determined by the voltage V_R44. The collector of the transistor Q42 is coupled to the LED string LED2. The emitter of the transistor Q42 is coupled to the resistor R44. Furthermore, the base of the transistor Q42 and the collector of the transistor Q41 are coupled to a common node. The resistor R43 is coupled between the common node and the anodes of the LED strings LED1 and LED2.

[0025] The operation of the light source driving circuit 400 is described with FIG. 4 and FIG. 5 in an example that a user decreases a conduction angle of the TRIAC dimmer 104. In one embodiment, the forward voltage of the LED string LED1 is greater than the forward voltage of the LED string LED2. The resistance of the resistor R44 is configured in such a way that the voltage V_R44 across the resistor R44 is greater than a threshold V_BE SAT if the conduction angle of the TRIAC dimmer 104 is set to a maximum value. The threshold V_BE SAT is a parameter associated with character of the transistor Q41 . If the base-emitter voltage V_BE of the transistor Q41 is greater than the threshold V_BE_SAT, the transistor Q41 operates in the saturation region. FIG . 5 shows waveforms of the voltage V 44 across the resistor R44, the base current IQ4I__b of the transistor Q41 , the collector current IQ4I_C of the transistor Q41 , the base current IQ42_B of the transistor Q42, the current l_LED1 through the first LED string LED1 and the current II_ED2 through the second LED string LED2.

[0026] As shown in FIG . 5, if a user adjusts the TRIAC dimmer 104 to decrease the conduction angle starting from the maximum value (e.g. , between time t₀ and ti), then the current through the resistor R44 decreases, and the voltage V_R44 decreases with time. Before the voltage V_R44 decreases to the threshold V_BE_SAT, e.g. , between time t₀ and t| , the transistor Q41 operates in the saturation region and is fully turned on. Therefore, the base and the emitter of the transistor Q42 are reversely biased such that the transistor Q42 operates in the cut-off region. During this phase, the LED string LED1 which generates cold light is turned on and the LED string LED2 which generates warm light is turned off. As a result, the light source 1 18 generates cold light. If the voltage V_R44 decreases to the threshold V_BE_ SAT, e.g. , at time ti , the transistor Q41 enters the active region. With the increment of the voltage drop between the base and the emitter of the transistor Q42, the transistor Q42 also enters the active region. Both LED strings LED1 and LED2 are turned on. As such, the light source module 1 18 generates a cold and warm mixed light output. From time to t₂, with the decrement of the conduction angle of the TRIAC dimmer 104, the current of the LED string LED1 decreases, the current of the LED string LED2 increases, and the output current Ι₀υτ decreases. The color of the light source module 1 18 transits gradually from cold to warm. If the output current Ι₀υτ decreases to be equal to the current I_LED2 through the LED string LED2 (e.g. , at time t₂), the LED string LED1 is turned off, the LED string LED2 remains on, the light source 1 18 generates warm light. The conduction angle of the TRIAC dimmer 104 continues decreasing until the TRIAC dimmer 104 cannot be triggered (e.g. , at time t₃) , the TRIAC dimmer 104 is turned off and the LED string LED2 is turned off. As can be seen from the waveforms of the current l_LED1 through the LED string LED1 and the current l_LED2 through the LED string LED2 shown in FIG. 5, the overall brightness of the light source module 1 18, which is determined by the total current of the LED strings LED1 and LED2, decreases with the decrement of the conduction angle of the TRIAC dimmer 104.

[0027] In the example of FIG. 4, the resistor R43 acts as the current limiting unit. As can be seen in FIG . 4, the base current l_Q42 __B of the transistor Q42 is proportional to the current through the resistor R43. If the resistance of the resistor R43 is relatively large, then the current through the resistor R43 is relatively small and the base current l_{Q42 B} of the transistor Q42 is also relatively small. According to the characteristic of a transistor, the collector current of the transistor 42 (that is, the current II_ED2 through the LED string LED2) and the base current l_{Q42 B} of the transistor Q42 can be described in below equation,

where β is the common-emitter current gain of the transistor Q42. Therefore, by adjusting the resistance of the resistor R43, the maximum level of the current l_LED2 can be determined.

[0028] FIG. 6 shows a light source driving circuit 600, in accordance with another embodiment of the present invention. Elements labeled the same as in FIG. 4 have similar functions. The transistors in FIG . 4 such as Q41 and Q42 are NPN type bipolar junction transistors. In the example of FIG . 5, the transistors Q61 and Q62 are PNP type bipolar junction transistors. One skilled in the art will appreciate the principle of the light source driving circuit 600 in FIG . 6 is similar as that of the light source driving circuit 400 in FIG. 4.

[0029] FIG. 7 shows a light source driving circuit 700, in accordance with another embodiment of the present invention. Elements labeled the same as in FIG. 4 have similar functions. In the example of FIG . 7, a voltage-dividing unit 702 acts as the sensing unit that provides the sensing signal indicating the output current Ιουτ- The voltage-dividing unit 702 includes resistors R76 and R72 connected in series. A voltage across the resistor R72 acts the sensing signal. More specifically, if the LED string LED1 is on, the current l_LED1 is exponentially related to the voltage across the LED string LED1 . Moreover, the current l_LED1 increases if the output current Ι₀υτ increases. Therefore, the voltage across the resistor R72 can indicate the output current Ι₀υτ-

[0030] FIG. 8 shows a light source driving circuit 800, in accordance with another embodiment of the present invention. Elements labeled the same as in FIG. 4 have similar functions. In the example of FIG. 8, the power converter 1 12 is a flyback converter which includes a transformer 806, a diode 802 and a capacitor 804.

[0031 ] As described above, the light source driving circuits disclosed in present invention can cooperate with TRIAC dimmers. A user can utilize a conventional TRIAC dimmer to adjust both the brightness and color of the light source, without the need for special dimmer or remote controller. [0032] While the foregoing description and drawings represent embodiments of the present invention, it will be understood that various additions, modifications and substitutions may be made therein without departing from the spirit and scope of the principles of the present invention as defined in the accompanying claims. One skilled in the art will appreciate that the invention may be used with many modifications of form, structure, arrangement, proportions, materials, elements, and components and otherwise, used in the practice of the invention, which are particularly adapted to specific environments and operative requirements without departing from the principles of the present invention. The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims and their legal equivalents, and not limited to the foregoing description.

Claims

CLAIMS What is claimed is:

1. A light source driving circuit comprising:

a rectifier operable for rectifying an AC voltage from a TRIAC dimmer and providing a rectified voltage;

a power converter, coupled to said rectifier, operable for receiving said rectified voltage and providing an output current;

a dimmer controller operable for controlling said power converter based on said rectified voltage to adjust said output current; and

a light source module, coupled to said power converter and powered by said output current, comprising:

a first light source having a first color;

a second light source having a second color; and

a current allocation unit, coupled to said first light source and said second light source, operable for adjusting a current through said first light source and a current through said second light source based on said output current.

2. The light source driving circuit of claim 1 , wherein said first light source comprises a first LED string and said second light source comprises a second LED string, wherein a forward voltage of said first LED string is greater than a forward voltage of said second LED string.

3. The light source driving circuit of claim 1 or claim 2, further comprising,

a current path, coupled to said TRIAC dimmer through said rectifier, operable for conducting a latching current and a holding current of said TRIAC dimmer, said current path comprising:

a first resistor;

a diode coupled in parallel with said first resistor;

a capacitor coupled in series with said first resistor and said diode; and a second resistor coupled in series with said first resistor and said diode.

4. The light source driving circuit of claim 3, wherein when said capacitor is being charged, a charging current flows through said diode and said second resistor, wherein when said capacitor is being discharged, a discharging current flows through said second resistor and said first resistor.

5. The light source driving circuit of any of claims 1 to 4, wherein said current allocation unit comprises:

a sensing unit, coupled to said first light source and said second light source, operable for providing a sensing signal indicating said output current;

a control unit coupled to said sensing unit; and

a current regulating unit coupled to said second light source,

wherein said control unit is operable for controlling said current regulating unit based on said sensing signal to regulate said current through said second light source.

6. The light source driving circuit of claim 5, wherein said current regulating unit further comprises:

a current limiting unit operable for limiting a maximum level of said current through said second light source.

7. The light source driving circuit of claim 5 or claim 6, wherein said sensing unit comprises a third resistor coupled to said first light source and said second light source, wherein said current through said first light source and said current through said second light source both flow through said third resistor, wherein said sensing signal comprises a voltage across said third resistor,

wherein said control unit comprise a first transistor coupled to said third resistor, wherein a conductance status of said first transistor is determined by said voltage across said third resistor,

wherein said current regulating unit comprises a second transistor coupled to said first transistor, wherein said second transistor regulates said current through said second light source under control of said first transistor.

8. The light source driving circuit of claim 7, wherein if said voltage across said third resistor is greater than a first threshold, said first transistor operates in a saturation region, said first light source is turned on, said second transistor is turned off, said second light source is turned off.

9. The light source driving circuit of any of claims 5 to 7, wherein said sensing unit comprises a voltage dividing unit coupled to said first light source in parallel, wherein said voltage dividing unit comprises a fourth resistor and a fifth resistor coupled in series, wherein said sensing signal comprises a voltage drop across said fifth resistor,

wherein said control unit comprises a first transistor coupled to said voltage dividing unit, wherein a conductance status of said first transistor is controlled by said voltage drop across said fifth resistor,

10. The light source driving circuit of claim 9, wherein if said voltage across said fifth resistor is greater than a first threshold, said first transistor operates in a saturation region, said first light source is turned on, said second transistor is turned off, said second light source is turned off.

1 1. A light source driving circuit comprising:

a power converter, coupled to said rectifier, operable for receiving said rectified voltage and providing an output current for a light source module;

a first resistor;

a diode coupled in parallel with said first resistor;

12. The light source driving circuit of claim 1 1 , wherein when said capacitor is being charged, a charging current flows through said diode and said second resistor, wherein when said capacitor is being discharged, a discharging current flows through said second resistor and said first resistor.