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Title:
SYSTEM AND METHOD FOR DRIVING LIGHT SOURCE
Document Type and Number:
WIPO Patent Application WO/2017/132147
Kind Code:
A1
Abstract:
A system for driving a light source includes a power converter and control circuitry coupled to the power converter. The power converter converts input power to an output voltage to power the light source. The control circuitry senses the output voltage and senses current of the light source. The control circuitry generates a control signal based on a voltage feedback signal indicative of a combination of said output voltage and said current of said light source, and controls the power converter by the control signal to adjust the output voltage.

Inventors:
CHEN, Jen-chun (11F, No.54 Sec. 4, Minsheng E. Rd, Taipei City 105, 105, TW)
LEE, Sheng-tai (11F, No.54 Sec.4, Minsheng E. Rd, Taipei City 105, 105, TW)
LIN, Yung-lin (2518 Indian Drive, Palo Alto, CA, 94303, US)
Application Number:
US2017/014744
Publication Date:
August 03, 2017
Filing Date:
January 24, 2017
Export Citation:
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Assignee:
O2MICRO, INC. (3118 Patrick Henry Drive, Santa Clara, CA, 95054, US)
International Classes:
H05B37/02; H05B39/04; H05B41/36
Foreign References:
US20150351175A12015-12-03
US20130063047A12013-03-14
US20110181199A12011-07-28
US20110062882A12011-03-17
US20100164403A12010-07-01
US20110080119A12011-04-07
Attorney, Agent or Firm:
COHEN, Lawrence, S. (1401 Westwood Blvd, Suite 240Los Angeles, CA, 90024, US)
Download PDF:
Claims:
CLAIMS

What is claimed is:

1. A light-source driving system, comprising:

a power converter operable for converting an input power to an output voltage to power a light source,

a control circuitry, coupled to said power converter, operable for sensing said output voltage and current of said light source, generating a control signal based on a voltage feedback signal indicative of a combination of said output voltage and said current of said light source, and controlling said power converter by said control signal to adjust said output voltage.

2. The light-source driving system of claim 1, wherein said control circuitry comprises:

a voltage feedback circuit coupled to said power converter; and

a current feedback circuit coupled to said light source,

wherein said voltage feedback circuit is operable for generating said voltage feedback signal based on said output voltage,

wherein said current feedback circuit is operable for generating an adjusting current indicative of a difference between said current of said light source and a target current level,

wherein said current feedback circuit is further operable for adjusting said voltage feedback signal by said adjusting current.

3. The light-source driving system of claim 2, wherein said current feedback circuit generates said adjusting current to increase said voltage feedback signal if said current of said light source is greater than said target current level, and wherein said current feedback circuit generates said adjusting current to decrease said voltage feedback signal if said current of said light source is less than said target current level.

4. The light-source driving system of claim 2 or claim 3, wherein said control circuitry generates said control signal based on a comparison of said voltage feedback signal and a voltage reference signal.

5. The light-source driving system of claim 4, wherein said power converter adjusts said output voltage to a target voltage level if said current of said light source is equal to said target current level, wherein said power converter adjusts said output voltage to be less than said target voltage level if said current of said light source is greater than said target current level, and wherein said power converter adjusts said output voltage to be greater than said target voltage level if said current of said light source is less than said target current level.

6. The light-source driving system of any of claims 2 to 5, wherein said light source comprises a plurality of LED (light-emitting diode) strings, wherein said control circuitry further comprises a current sense and balance circuit coupled to said LED strings, wherein said current sense and balance circuit is operable for balancing current of said LED strings and generating a current feedback signal according to said current of said LED strings, wherein said current feedback circuit is operable for generating said adjusting current according to said current feedback signal.

7. The light-source driving system of claim 6, wherein said current feedback signal indicates an average current level of said current of said LED strings.

8. The light-source driving system of claim 6, wherein said current feedback signal indicates current of any one of said LED strings when said current of said LED strings have been balanced with one other.

9. The light-source driving system of any of claims 6 to 8, wherein said current feedback circuit is operable for generating said adjusting current based on said current feedback signal and a current reference signal indicating said target current level, wherein said current feedback circuit sinks said adjusting current if said current feedback signal is less than said current reference signal, and wherein said current feedback circuit outputs said adjusting current if said current feedback signal is greater than said current reference signal.

10. The light-source driving system of claim 9, wherein said voltage feedback circuit comprises a resistor, wherein said voltage feedback signal represents a voltage across said resistor, wherein said voltage across said resistor increases if said current feedback circuit outputs said adjusting current, wherein said voltage across said resistor decreases if said current feedback circuit sinks said adjusting current.

11. The light-source driving system of any of claims 2 to 10, wherein said control circuitry further comprises:

a shunt regulator, coupled to said voltage feedback circuit, operable for generating a control current based on said voltage feedback signal and a voltage reference signal, wherein said voltage reference signal indicates a target voltage level of said output voltage; and

an optical coupler, coupled between said shunt regulator and said power converter, operable for proving said control signal based on said control current.

12. The light-source driving system of claim 11, wherein said optical coupler is turned off and said control signal has a first voltage level if said voltage feedback signal is less than said voltage reference signal, wherein said optical coupler is turned on and said control signal has a second voltage level if said voltage feedback signal is greater than said voltage reference signal.

13. The light-source driving system of claim 12, wherein said power converter increases said output voltage if said control signal has said first voltage level, wherein said power converter decreases said output voltage if said control signal has said second voltage level.

14. A method for driving a light source, the method comprising:

converting, by a power converter, an input power to an output voltage to power a light source;

sensing, by a control circuitry, said output voltage and current of said light source; generating, by said control circuitry, a control signal based on a voltage feedback signal indicative of a combination of said output voltage and said current of said light source; and

controlling, by said control circuitry, said power converter by said control signal to adjust said output voltage.

15. The method of claim 14, further comprising:

generating, by a voltage feedback circuit, said voltage feedback signal based on said output voltage;

generating, by a current feedback circuit, an adjusting current based on said current of said light source, wherein said adjusting current is indicative a difference between said current of said light source and a target current level; and

adjusting, by said current feedback circuit, said voltage feedback signal by said adjusting current.

16. The method of claim 15, wherein said adjusting said voltage feedback signal by said adjusting current comprises:

generating, by said current feedback circuit, said adjusting current to increase said voltage feedback signal if said current of said light source is greater than said target current level, and

generating, by said current feedback circuit, said adjusting current to decrease said voltage feedback signal if said current of said light source is less than said target current level.

17. The method of claim 15 or claim 16, wherein said generating said control signal comprises:

comparing said voltage feedback signal with a voltage reference signal which indicates a target voltage level.

18. The method of claim 17, wherein said controlling said power converter by said control signal comprises:

adjusting said output voltage to said target voltage level if said current of said light source is equal to said target current level;

adjusting said output voltage to be less than said target voltage level if said current of said light source is greater than said target current level; and

adjusting said output voltage to be greater than said target voltage level if said current of said light source is less than said target current level.

19. The method of any of claims 15 to 18, wherein said generating said adjusting current based on said current of said light source comprises:

generating a current feedback signal indicating an average current level of current of plurality of LED (light-emitting diode) strings in said light source; and

generating said adjusting current according to said current feedback signal.

20. The method of any of claims 15 to 19, wherein said generating said adjusting current based on said current of said light source comprises:

generating a current feedback signal indicating current of any one LED (light- emitting diode) string of a plurality of LED strings in said light source; and

generating said adjusting current according to said current feedback signal.

Description:
SYSTEM AND METHOD FOR DRIVING LIGHT SOURCE

RELATED APPLICATIONS

[001] This application claims priority to U.S. Provisional Application No. 62/286,752, titled "Light-source Driving Systems," filed on January 25, 2016, which is hereby incorporated by reference in their entirety.

BACKGROUND

[002] FIG. 1 illustrates a block diagram of a conventional light-source driving system 100 for driving a light source 110. The light-source driving system 100 includes a full-bridge rectifier 102, an AC/DC (alternating current to direct current) converter 104, a DC/DC converter 150, a feedback circuit 108, and a controller 1 12.

[003] The rectifier 102 rectifies an AC voltage V AC (e.g., 220VAC, 1 lOVAC, or the like from an electric supply) to provide a rectified power to the AC/DC converter 104. The AC/DC converter 104 converts the rectified power to a DC voltage V D C according a signal 106 from the feedback circuit 108. The signal 106 represents a level of the DC voltage V D C and functions as a feedback to control the AC/DC converter 104 such that the DC voltage V D C is maintained at a constant target voltage level. The DC/DC converter 150 includes an inductor L B , a diode 114, and a switch Qsw- When the switch Qsw is on, an increasing current can flow through the inductor L B from the AC/DC converter 104 to ground, and the inductor L B stores energy. When the switch Qsw is off, the inductor L B releases energy, and the energy released from the inductor L B , together with the power, e.g., the DC voltage V D C, from the AC/DC converter 104, are transformed to an output current Io to flow through the light source 110. The controller 112 can sense the output current Io and generate a control signal 116 to control the switch Qsw according to the output current Io, thereby adjusting the output current IQ to a target current level. [004] Due to non-ideality of circuit components in the controller 112 and/or variation in ambient temperature, the output current Io may change even if the output voltage Vo of the DC/DC converter 150 remains constant. Thus, the controller 112 controls the DC/DC converter 150 to increase or decrease the output voltage Vo, thereby maintaining the output current Io at the target current level. Additionally, light sources having controllable brightness are widely used in many applications. For example, a user may increase or decrease a brightness of the light source 110 by rotating a knob-like switch. The changes in the brightness is caused by increasing or decreasing the output current Io flowing through the light source 110, which can also be caused by increasing or decreasing the output voltage Vo. Thus, when the target current level of the output current Io is changed, e.g., by a user, the controller 112 can control the DC/DC converter 150 to increase or decrease the output voltage Vo, thereby adjusting the output current Io to the new target current level.

[005] Thus, the light-source driving system 100 includes an AC/DC converter 104 to convert an AC voltage VAC to a constant DC voltage V D C, and includes a DC/DC converter 150 to convert the DC voltage V D C to an adjustable output voltage Vo. The DC/DC converter 150 increases or decreases the output voltage Vo to adjust the output current Io to a target current level. However, the DC/DC converter 150 consumes power, which increases power consumption and reduces a power efficiency of the light- source driving system 100. The DC/DC converter 150 also occupies a relatively large area of a PCB (printed circuit board), which increases the size of a PCB for the light- source driving system 100. The DC/DC converter 150 also increases the cost of the light-source driving system 100.

SUMMARY

[006] In one embodiment, a system for driving a light source includes a power converter and control circuitry coupled to the power converter. The power converter is configured to convert input power to an output voltage to power the light source. The control circuitry is configured to sense the output voltage and senses current of the light source. The control circuitry is configured to generate a control signal based on a voltage feedback signal indicative of a combination of said output voltage and said current of said light source, and is configured to control the power converter by the control signal to adjust the output voltage.

[007] The current feedback circuit may be configured to generate said adjusting current to increase said voltage feedback signal if said current of said light source is greater than said target current level, and the current feedback circuit may be configured to generate said adjusting current to decrease said voltage feedback signal if said current of said light source is less than said target current level.

[008] The control circuitry may be configured to compare the voltage feedback signal and a voltage reference signal. The control circuitry may be configured to generate the control signal based on a difference between said voltage feedback signal and said voltage reference signal.

[009] The control circuitry may be configured to adjust said output voltage to a target voltage level if said current of said light source is equal to said target current level, and may be configured to adjust said output voltage to be less than said target voltage level if said current of said light source is greater than said target current level. The power converter may be configured to adjust said output voltage to be greater than said target voltage level if said current of said light source is less than said target current level.

[010] The light source may comprise a plurality of LED (light-emitting diode) strings, and said control circuitry may further comprise a current sense and balance circuit coupled to said LED strings. Said current sense and balance circuit may be configured to balance current of said LED strings and may be configured to generate a current feedback signal according to said current of said LED strings. Said current feedback circuit may be configured to generate said adjusting current according to said current feedback signal.

[Oi l] The current sense and balance circuit may be configured to generate the current feedback signal such that the current feedback signal indicates an average current level of said current of said LED strings.

[012] Alternatively, the current sense and balance circuit may be configured to generate the current feedback signal such that the current feedback signal indicates a current of any one of said LED strings when said current of said LED strings have been balanced with one other.

[013] The current feedback circuit may be configured to generate said adjusting current based on the current feedback signal and a current reference signal, the current reference signal indicating the target current level. In particular, the current feedback circuit may be configured to generate said adjusting current based on a difference between an output current and a current reference signal. The current feedback circuit may be configured to sink said adjusting current if said current feedback signal is less than said current reference signal, and may be configured to output said adjusting current if said current feedback signal is greater than said current reference signal.

[014] The voltage feedback circuit may comprise a resistor and the voltage feedback signal may represent a voltage across said resistor.

[015] The control circuitry may be configured to increase the voltage across said resistor if the current level of the output currents is greater than the target current level. The control circuitry may be configured to decrease the voltage across said resistor if said current feedback circuit is less than the current level of the output currents is greater than the target current level. [016] The control circuitry may comprise a shunt regulator, coupled to said voltage feedback circuit. The shunt regulator may be configured to sense said voltage feedback signal and to generate a control current based on said voltage feedback signal and a voltage reference signal, wherein said voltage reference signal indicates a target voltage level of said output voltage.

[017] The control circuitry may further comprise an optical coupler, coupled between said shunt regulator and said power converter, the optical coupler being configured to provide said control signal based on said control current.

[018] The control circuit may be configured such that if said voltage feedback signal is less than said voltage reference signal, the optical coupler is turned off and said control signal may have a first voltage level. The control circuit may be configured such that if said voltage feedback signal is greater than said voltage reference signal, the optical coupler is turned on, and said control signal may have a second voltage level.

[019] The power converter may be configured to increase said output voltage if said control signal has said first voltage level. The power converter may be configured to decrease said output voltage if said control signal has said second voltage level.

[020] In another embodiment, a method for driving a light source includes: converting an input power to an output voltage to power a light source; sensing the output voltage and current of the light source; generating a control signal based on a voltage feedback signal indicative of a combination of the output voltage and the current of the light source; and controlling the power converter by the control signal to adjust the output voltage. BRIEF DESCRIPTION OF THE DRAWINGS

[021] Features and advantages of embodiments of the 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:

[022] FIG. 1 illustrates a block diagram of a conventional light-source driving system.

[023] FIG. 2 A illustrates a block diagram of an example of a light-source driving system, in an embodiment according to the present invention.

[024] FIG. 2B illustrates a circuit diagram of an example of a light-source driving system, in an embodiment according to the present invention.

[025] FIG. 3 illustrates a circuit diagram of examples of a current sense and balance circuit and a feedback circuit in FIG. 2B, in an embodiment according to the present invention.

[026] FIG. 4 illustrates a circuit diagram of examples of an AD/DC converter and a voltage-controlled shunt regulator in FIG. 2B, in an embodiment according to the present invention.

[027] FIG. 5 illustrates a flowchart of a method for driving a light source, in an embodiment according to the present invention.

DETAILED DESCRIPTION

[028] 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.

[029] 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.

[030] In an embodiment according to the present invention, a light-source driving system includes control circuitry capable of controlling an output voltage of an AC/DC converter according to a feedback signal indicative of a combination of the output voltage and an output current (or currents) flowing through a light source. As a result, the light-source driving system can maintain the output current (or currents) at a target current level. In the light-source driving system in an embodiment according to the present invention, a DC/DC converter in a conventional light-source driving system, e.g., the DC/DC converter 150 mentioned in relation to FIG. 1 , can be omitted.

[03 1 ] FIG. 2 A illustrates a block diagram of an example of a light-source driving system 200 for powering a light source 210, in an embodiment according to the present invention. The light-source driving system 200 can include a rectifier 202, e.g., a full- bridge rectifier, a power converter, e.g., an AC/DC converter 204, and control circuitry 220. In the example of FIG. 2, the light source 210 includes multiple LED (light- emitting diode) strings coupled in parallel. However, the invention is not so limited. In another embodiment, the light source 210 can include one LED, multiple LEDs coupled in parallel, or one LED string. The rectifier 202 can receive an AC voltage VAC, e.g., 220 VAC, 1 10 VAC, or the like from an electric supply via a power socket, and provide rectified power, e.g., input power, to the AC/DC converter 204. The AC/DC converter 204 converters the input power to an output voltage VOU T - The output voltage VOU T can power the light source 210. The control circuitry 220 can sense the output voltage VOU T and output currents Ii, I 2 , .. . , I N (or LED currents Ii, I 2 , .. . , I N ) flowing through the LED strings in the light source 210, and generate a control signal 206, e.g., a voltage signal, based on a feedback signal indicative of the output voltage VOU T and the output currents Ii, I 2 , .. . , I N , or indicative of a combination of the output voltage VOU T and the output currents L, I 2 , .. . , I N . The control circuitry 220 can utilize the control signal 206 to control the AC/DC converter 204 to adjust the output voltage VOU T , thereby adjusting the output currents L, I 2 , .. . , I N to a target current level.

[032] An example of a circuit diagram of the control circuitry 220 is shown in FIG. 2B, in an embodiment according to the present invention. As shown in FIG. 2B, the control circuitry 220 includes an optical coupler 208, a voltage feedback circuit 218 such as a voltage divider including resistors Ri and R 2 , and a controller 212. The controller 212 includes a current feedback circuit 224. The voltage feedback circuit 218 is coupled to the AC/DC converter 204 and can generate a voltage feedback signal VS EN based on the output voltage VOU T - The current feedback circuit 224 is coupled to the light source 210 and can generates an adjusting current IA D J F indicative of a difference between the current of the light source 210 and a target current level. The current feedback circuit 224 can adjust the voltage feedback signal by the adjusting current.

[033] In one embodiment, the optical coupler 208 is a component that transfers electrical signals between two isolated circuits by using light. In one embodiment, an equivalent circuit of the optical coupler 208 includes an LED and a phototransistor. A current, e.g., IC MP O shown in FIG. 2B, flowing through the LED causes the LED to emit light. The phototransistor receives the light emitted from the LED and generates an electrical signal, e.g., including a collector current Ic and/or a collector-emitter voltage VC E , accordingly. In one embodiment, the optical coupler 208 can receive a control current, e.g., the current IC MP O from the controller 212, and generate a control signal 206 to control the AC/DC converter 204 according to the control current IC MP O- For example, when the optical coupler 208 is turned off, e.g., the control current IC MP O is zero amperes, the control signal 206 can be at a higher voltage level, e.g., logic high, determined by a bias circuit in the AC/DC converter 204. When a control current IC MP O is generated to turn on the optical coupler 208, the control signal 206 can be dragged down to a lower voltage level, e.g., logic low. Thus, in one embodiment, the optical coupler 208 can generate a control signal 206 to indicate whether a non-zero control current IC MP O has been generated.

[034] In one embodiment, the controller 212 includes current-sense terminals ISEN1, ISEN2, ... , ISENN. The number N is determined by the number of LED strings in the light source 210. In one embodiment, the number N can be an arbitrary positive integer. The controller 212 also includes a voltage-sense terminal VSEN, a control terminal CMPO, and an adjusting terminal ADJF. The controller 212 can sense the output currents Ii, I 2 , .. . , I N via the current-sense terminals ISEN1, ISEN2, ... , ISENN, respectively. The controller 212 can also generate an adjusting current IA D J F according to the output currents Ii, I 2 , .. . , IN, and provide the adjusting current IA D J F to the voltage feedback circuit 218 via the adjusting terminal ADJF and to adjust a voltage feedback signal VS EN (e.g., a voltage) at the voltage feedback circuit 218. In the example of FIG. 2B, the voltage feedback signal VS EN is the voltage across the resistor R 2 . Thus, the voltage feedback signal VS EN can be indicative of the output voltage VOU T and the output currents Ii, I 2 , .. . , I N , or a combination of the output voltage VOU T and the output currents Ii, I 2 , .. . , I N . Additionally, the controller 212 can sense the voltage feedback signal VS E N via the voltage-sense terminal VSEN, generate a control current IC MP O according to the voltage feedback signal VS EN , and provides the control current IC MP O to the optical coupler 208 via the control terminal CMPO.

[035] Thus, in one embodiment, the controller 212 generates a control current IC MP O to control the AC/DC converter 204 according to the voltage feedback signal VS E N indicative of the output voltage VOU T and the output currents Ii, I 2 , .. . , IN- By way of examples, if a current level (e.g., represented by a current feedback signal S I S EN shown in FIG. 3) of the output currents Ii, I 2 , .. . , I N is equal to a target current level (e.g., represented by a current reference signal SA D J shown in FIG. 3), then the controller 212 does not generate the adjusting current IA D J F (e.g., IA D J F = 0), and therefore the voltage feedback signal VS EN can represent, e.g., be linearly proportional to, the output voltage VOUT, e.g., VSEN = VOUT*R 2 /(RI+R 2 )- The controller 212 can compare the voltage feedback signal VS EN with a voltage reference signal V REF and generate the control current IC MP O according to a difference between the voltage feedback signal VS E N and the voltage reference signal V REF - The control current IC MP O can cause the control signal 206 to control the AC/DC converter 204, thereby increasing or decreasing the output voltage VOU T to reduce the difference between the voltage feedback signal VS EN and the voltage reference signal V REF - Thus, the voltage feedback signal VS E N can be adjusted to the voltage reference signal V REF , and the output voltage VOU T can be adjusted to a target voltage level V T A R G ET determined by the voltage reference signal VREF, e.g., VTARGET = VREF*(RI+R 2 )/ R 2 - If the current level of the output currents Ii, I 2 , .. . , I N is greater than the target current level, then the controller 212 can output an adjusting current IA D J F to flow into the connection node between the resistors Ri and R 2 , thereby increasing the voltage across the resistor R 2 (i.e., the voltage feedback signal VS EN )- The increased voltage feedback signal VS EN can cause the control current IC MP O to control the AC/DC converter 204 such that the AC/DC converter 204 decreases the output voltage VOU T , e.g., to be less than the

abovementioned target voltage level V T A R G ET , thereby decreasing the current level of the output currents Ii, I 2 , .. . , IN. If the current level of the output currents Ii, I 2 , .. . , IN is less than the target current level, then the controller 212 can sink an adjusting current IA D J F from the connection node between the resistors Ri and R 2 , thereby decreasing the voltage across the resistor R 2 (i.e., the voltage feedback signal VS EN )- The decreased voltage feedback signal VS E N can cause the control current IC MP O to control the AC/DC converter 204 such that the AC/DC converter 204 increases the output voltage VOU T , e.g., to be greater than the abovementioned target voltage level V T A R G ET , thereby increasing the current level of the output currents Ii, I 2 , .. . , I N

[036] In other words, in one embodiment, the adjusting terminal ADJF can be an input/output terminal, e.g., a bidirectional terminal, that sinks or outputs an adjusting current IA D J F according a status of the output currents Ii, I 2 , .. . , I N - The adjusting current IA D J F can represent a difference between a current level of the output currents Ii, I 2 , .. . , I N and a target current level, and cause the controller 212 and the AC/DC converter 204 to increase or decrease the output voltage VOU T thereby reducing the difference between the current level of the output currents Ii, I 2 , .. . , I N and the target current level. As a result, the current level of the output currents Ii, I 2 , .. . , I N can be adjusted to the target current level.

[037] In one embodiment, the controller 212 includes a current sense and balance circuit 222, a current feedback circuit 224, and a voltage-controlled shunt regulator 230. The current sense and balance circuit 222 can sense the output currents Ii, I 2 , .. . , IN to determine differences among the output currents Ii, I 2 , .. . , IN, and balance the output currents Ii, I 2 , .. . , I N by reducing their differences. The current sense and balance circuit 222 can also generate a current feedback signal 226 according to the output currents Ii, I 2 , .. . , I N , and provide the current feedback signal 226 to the current feedback circuit 224. In one embodiment, the current feedback signal 226 can represent an average current level of the output currents Ii, I 2 , .. . , I N - In another embodiment, the current feedback signal 226 can represent any one of the output currents Ii, I 2 , .. . , I N when the output currents Ii, I 2 , .. . , I N have been balanced with one another. In one embodiment, "balanced with one another" as used herein means that differences among current levels of the output currents Ii, I 2 , .. . , I N are permissible as long as the differences are relatively small and can be ignored, e.g., within a predetermined small range. The current feedback circuit 224 can receive the current feedback signal 226 and output or sink an adjusting current IA D J F according to the current feedback signal 226. The adjusting current IA D J F can control a voltage feedback signal VS EN at the connection node between the Ri and R 2 in the voltage feedback circuit 218. The shunt regulator 230 can sense the voltage feedback signal VS EN and generate a control current IC MP O according to the voltage feedback signal VS EN and a voltage reference signal V REF -

[038] Accordingly, in one embodiment, the control circuitry 220 can control the AC/DC converter 204 to increase or decrease the output voltage VOU T according to the voltage feedback signal VS EN indicative of the output voltage VOU T and the output currents Ii, I 2 , .. . , I N , thereby adjusting the output currents Ii, I 2 , .. . , I N to a target current level. Advantageously, a DC/DC converter in a conventional light-source driving system, e.g., the DC/DC converter 150 mentioned in relation to FIG. 1, can be omitted in the light-source driving system 200. As a result, compared with the conventional light-source driving system, the light-source driving system 200 consumes less power, which reduces its power consumption and increases its power efficiency. The size of a PCB (printed circuit board) for the light-source driving system 200 and the cost for the light-source driving system 200 are also reduced. Additionally, the light- source driving system 200 can include fewer components compared with the conventional light-source driving system, and therefore the light-source driving system 200 can have a higher reliability.

[039] FIG. 3 illustrates a circuit diagram of examples of the current sense and balance circuit 222 and the current feedback circuit 224 in FIG. 2B, in an embodiment according to the present invention. For clarity, some portions of the light-source driving system 200 are not shown in FIG. 3. In FIG. 3, the components labeled SI, S2, ... , SN represent the LED strings SI, S2, ... , SN in FIG. 2B. In one embodiment, the current sense and balance circuit 222 includes a set of switches Ql, Q2, ... , QN and a balance control circuit 332. As shown in FIG. 3, the switches Ql, Q2, ... , QN are coupled in series to the LED strings SI, S2, ... , SN and current sensors, e.g., sense resistors, Rsi, Rs2, . . . , RSN, respectively. The sense resistors Rsi, Rs2, · · · , RSN provides sense signals Vsi, Vs2, · · · , VSN which indicate output currents Ii, I 2 , .. . , IN flowing through the LED strings SI, S2, ... , SN. When a switch Qj (j=l, 2, ... , N) is turned on, a current I j can be generated to flow through the switch Qj and a corresponding current sensor Rs j , and the current sensor Rs j can provide a sense signal, e.g., a voltage Vs j on the current sensor Rs j , to a corresponding current-sense terminal ISENj . In one embodiment, the balance control circuit 332 can generate a driving signal SoR j to alternatively switch on and off the switch Qj. The balance control circuit 332 can also increase the current I j by increasing a duty cycle of the switch Qj, and decrease the current I j by decreasing the duty cycle of the switch Qj. As used herein, "a duty cycle of a switch" represents a ratio of a time period during which the switch is turned on to a time period of a switching cycle of the switch. In one embodiment, the balance control circuit 332 can receive the sense signals V S i, V S2 , V SN , generate driving signals S D RVI, S D RV2, · · · , S D RVN according to the sense signals Vsi, Vs2, . . . , VSN, and balance the output currents Ii, I2, . . . , IN by using the driving signals SDRVI, SDRV2, · · · , SDRVN to control the switches Q 1 , Q2, ... , QN. For example, the balance control circuit 332 may compare the sense signals Vsi, Vs2, . . . , VSN with a balance reference to generate a set of comparison results. The comparison results can cause the balance control circuit 332 to increase an output current I K (K=l, 2, ... , N), e.g., by increasing a duty cycle of the switch QK, if the output current I K is less than the balance reference. The comparison results can also cause the balance control circuit 332 to decrease an output current I L (L=l, 2, ... , N), e.g., by reducing a duty cycle of the switch QL, if the output current I L is greater than the balance reference. The balance reference may be a predetermined reference signal or a reference signal indicative of an average current level of the output currents Ii, I 2 , . . . , I N - For another example, the balance control circuit 332 may identify a maximum current I M A X of the output currents Ii, I 2 , . . . , I N , and identify a minimum current I MI N of the output currents Ii, I2, . . . , IN- If a difference between the maximum current I M A X and the minimum current I MIN is greater than a predetermined threshold, then the balance control circuit 332 may reduce a duty cycle of a switch Ql, Q2, ... , or QN corresponding to the maximum current I M A X , and/or increase a duty cycle of a switch Ql, Q2, ... , or QN corresponding to the minimum current I MIN , thereby reducing the difference between the maximum current I M A X and the minimum current I MIN - Methods for balancing the output currents Ii, I 2 , .. . , I N are not limited to the examples mentioned herein. The balance control circuit 332 may balance the output currents Ii, I 2 , .. . , IN based on another method. Advantageously, the output currents Ii, I 2 , .. . , IN can be balanced with one another and have substantially the same current level. As used herein, "substantially the same current level" means that differences among current levels of the output currents Ii, I 2 , .. . , I N are permissible as long as the differences are relatively small and can be ignored, e.g., within a predetermined small range.

[040] As mentioned above, the balance control circuit 332 can sense the output currents Ii, I 2 , .. . , IN, e.g., by receiving the sense signals Vsi, Vs 2 , .. . , VSN, in one embodiment. Additionally, the balance control circuit 332 can provide a current feedback signal S I S EN , indicative of a current level of the output currents Ii, I 2 , .. . , I N , to the current feedback circuit 224. In one embodiment, the current feedback signal S I S EN represents an average current level of the output currents Ii, I 2 , .. . , I N . In another embodiment, the current feedback signal S I S EN can represent any one of the output currents , I 2 , .. . , IN when the output currents , I 2 , .. . , IN have been balanced with one another. The current feedback circuit 224 generates the adjusting current IA D J F according to the current feedback signal S I S EN - In one embodiment, the current feedback circuit 224 includes an OTA (operational transconductance amplifier) having a non-inverting input terminal coupled to the balance control circuit 332 to receive the current feedback signal S I S EN , e.g., a voltage signal, and having an inverting input terminal to receive a current reference signal SA D J, e.g., a voltage signal, from a reference signal generator (not shown). In one embodiment, the current reference signal SA D J represents a target current level of the output currents , I 2 , .. . , I N . Thus, if the current feedback signal S I S EN is greater than the current reference signal SA D J, e.g., indicating that a current level of the output currents Ii, I 2 , .. . , I N is greater than the target current level, then the current feedback circuit 224, e.g., OTA, can output the adjusting current IA D J F - If the current feedback signal S I S EN is less than the current reference signal SA D J, e.g., indicating that a current level of the output currents Ii, I 2 , .. . , IN is less than the target current level, then the current feedback circuit 224, e.g., OTA, can sink the adjusting current IA D J F -

[041] In one embodiment, the current feedback circuit 224 may further include one or more circuit components (not shown in FIG. 3) coupled to the OTA or coupled between the OTA and the voltage feedback circuit 218 (in FIG. 2B). The one or more circuit components can cooperate with the OTA to generate the adjusting current IA D J F - Although FIG. 3 shows that the current feedback circuit 224 includes an OTA to generate the adjusting current IA D J F , the invention is not so limited. In another embodiment, the current feedback circuit 224 may generate the adjusting current IA D J F without using an OTA. In other words, the current feedback circuit 224 can have a different circuit structure, and in such a circuit structure, the current feedback circuit 224 is capable of outputting an adjusting current IA D J F if the current feedback signal S I S EN is greater than the current reference signal SA D J, and sinking an adjusting current IA D J F if the current feedback signal S I S E N is less than the current reference signal SA D J-

[042] FIG. 4 illustrates a circuit diagram of examples of the AC/DC converter 204 and the shunt regulator 230 in FIG. 2B, in an embodiment according to the present invention. For clarity, some portions of the light-source driving system 200 are not shown in FIG. 4. As shown in FIG. 4, the AC/DC converter 204 includes a PWM (pulse-width modulation) signal generator 442 and a switching-mode transformer circuitry 434. The PWM signal generator 442 can generate a PWM signal to control the transformer circuitry 434 such that the transformer circuitry 434 converters an AC voltage VAC to an output voltage VQU T - [043] More specifically, in one embodiment, the transformer circuitry 434 can be any type of converter that is capable of converting an input signal, e.g., the AC voltage VAC, to an output signal, e.g., the output voltage VOU T , and controlling the output signal to a preset target level. For example, the transformer circuitry 434 can have a flyback converter topology, a forward converter topology, a push-pull converter topology, a center-tapped transformer topology, or the like. In one embodiment, the transformer circuitry 434 includes a primary winding circuit 436 (e.g., including one or more primary windings), a switch circuit 440 (e.g., including a switch, a half-bridge circuit, a full-bridge circuit, or the like) coupled to the primary winding circuit 436, and a secondary winding circuit 438 (e.g., including one or more secondary windings) magnetically coupled to the primary winding circuit 436. In one embodiment, the transformer circuitry 434 is in a flyback transformer topology. When the PWM is in a first state, e.g., logic high (or logic low), the switch circuit 440 can enable power transfer from the power source VAC to the primary winding circuit 436, a primary current I P can be generated to flow through the primary winding circuit 436, and the magnetic core of the transformer circuitry 434 can store magnetic energy. When the PWM is in a second state, e.g., logic low (or logic high), the switch circuit 440 can disconnect the primary winding circuit 436 from the power source VAC, the magnetic core of the transformer circuitry 434 can release magnetic energy to the secondary winding circuit 438, and a secondary current ¾ can be generated to flow through the secondary winding circuit 438 to the light source 210 (shown in FIG. 2B). In another embodiment, the transformer circuitry 434 is in a center-tapped transformer topology. When the PWM is in a first state, e.g., logic high (or logic low), the switch circuit 440 can enable power transfer from the power source VAC to the primary winding circuit 436, a primary current I P can be generated to flow through the primary winding circuit 436, which can cause a secondary current ¾ to be generated to flow through the secondary winding circuit 438 to the light source 210 (shown in FIG. 2B). The primary current I P and the secondary current ¾ can increase, and the magnetic core of the transformer circuitry 434 can store magnetic energy. When the PWM is in a second state, e.g., logic low (or logic high), the switch circuit 440 can disconnect the primary winding circuit 436 from the power source V AC , the magnetic core of the transformer circuitry 434 can release magnetic energy to the secondary winding circuit 438, and the secondary current ¾ can decrease. The PWM signal generator 442 can generate the PWM signal to control the switch circuit 440 thereby alternately enabling and disabling the power transfer from the power source V AC to the primary winding circuit 436. The PWM signal generator 442 can increase the output voltage V OUT by increasing a duty cycle of the switch circuit 440, and decrease the output voltage V OUT by decreasing the duty cycle of the switch circuit 440. As used herein, "a duty cycle of the switch circuit 440" represents a ratio of a time period during which the primary winding circuit 436 receives power from the power source V AC to a time period of an alternation cycle of the switch circuit 440.

[044] In the example of FIG. 4, the shunt regulator 230 includes an error amplifier 446, e.g., an operational amplifier, having a non-inverting input terminal coupled to the voltage-sense terminal VSEN to receive the voltage feedback signal V SE N, and an inverting input terminal to receive a voltage reference signal V REF from a reference signal generator (not shown). The voltage reference signal V REF can represent a target voltage level of the output voltage VOUT when the output currents Ii, I 2 , .. . , IN have been adjusted to a target current level, e.g., I ADJF = 0. The shunt regulator 230 also includes a switch 448, e.g., a MOSFET (metal-oxide-semiconductor field-effect transistor) having a control terminal, e.g., a gate terminal, coupled to the error amplifier 446 and controlled by the output of the error amplifier 446. The switch 448 is also coupled to the control terminal CMPO, and can provide a control current I CMPO under the control of the voltage feedback signal V SE N- For example, if the voltage feedback signal V SE N is less than the voltage reference signal V REF , then the error amplifier 446 turns off the switch 448, and no current flows through the optical coupler 208. If the voltage feedback signal V SE N is greater than the voltage reference signal V REF , then the error amplifier 446 turns on the switch 448, and the control current IC MP O is generated to flow through the optical coupler 208 and the switch 448 to ground, and the optical coupler 208 is turned on.

[045] In one embodiment, when the voltage feedback signal VS EN is less than the voltage reference signal V REF , the optical coupler 208 is turned off, and a status (e.g., a voltage level) of the signal line 428 is determined by a bias circuit 444 in the AC/DC converter 204. For example, the bias circuit 444 may provide a first voltage level, e.g., a logic-high voltage level, to the signal line 428 when the optical coupler 208 is turned off. Thus a voltage level of the control signal 206 can be controlled to the first voltage level, e.g., the logic-high voltage level. When the voltage feedback signal VS EN is greater than the voltage reference signal V REF , the optical coupler 208 is turned on, and a voltage level of the control signal 206 can be controlled to a second voltage level, e.g., dragged down to a logic-low voltage level by the optical coupler 208. Thus, in one embodiment, the status of the signal line 428 can be at a first voltage level, e.g., logic high, when the voltage feedback signal VS EN is less than the voltage reference signal V REF , and at a second voltage level, e.g., logic low, when the voltage feedback signal VS EN is greater than the voltage reference signal V REF - In one embodiment, the first voltage level of the control signal 206 can control the PWM signal generator 442 to increase the output voltage VOU T , and the second voltage level of the control signal 206 can control the PWM signal generator 442 to decrease the output voltage VOU T - AS a result, the voltage feedback signal VS EN can be adjusted to the voltage reference signal V REF , and the output voltage VOU T can be adjusted to a voltage level that maintains the output currents Ii, I 2 , .. . , I N at a target current level. As described above, the control current IC MP O is generated based on a comparison of the voltage feedback signal VS EN and the voltage reference signal V REF - The optical coupler 208 provides the control signal 206 based on the control current IC MP O- The AC/DC converter 204 is controlled by the control signal 206 to adjust the output voltage VQU T - [046] Advantageously, in one embodiment, the optical coupler 208 is a component that transfers electrical signals between two isolated circuits by using light. Thus, the optical coupler 208 can prevent the AC voltage VAC, e.g., a high voltage, from affecting the AC/DC converter 204 receiving feedback from the shunt regulator 230.

[047] FIG. 5 illustrates a flowchart of a method for driving a light source, in an embodiment according to the present invention. FIG. 5 is described in combination with FIG. 2A, FIG. 2B, FIG. 3 and FIG. 4.

[048] In block 502, a power converter, e.g., the AC/DC converter 204 in FIG. 2 A and FIG. 2B, converters input power, e.g., power output from the rectifier 202, to an output voltage VOU T to power a light source 210.

[049] In block 504, a control circuitry 220 senses the output voltage and senses the output current (e.g., the current of the light source 210).

[050] In block 506, the control circuitry 220 generates a control signal 206 based on a voltage feedback signal VS EN indicative of a combination of the output voltage and the current of the light source 210.

[051] In block 508, the power converter is controlled by the control signal 206 to adjust the output voltage.

[052] 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. 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, and not limited to the foregoing description.