SCR dimming circuit and method

1. A silicon-controlled rectifier (SCR) dimming circuit, comprising:

a) an SCR rectifying circuit having an SCR element configured to receive an AC power supply, and to generate a lack-phase AC voltage;

b) a rectifier bridge coupled to said SCR rectifying circuit, wherein said rectifier bridge is configured to convert said lack-phase AC voltage to a lack-phase DC voltage, wherein said lack-phase DC voltage is configured to be filtered through a filter capacitor to generate a smooth DC voltage;

c) a conduction phase angle signal generator coupled to said rectifier bridge, wherein said conduction phase angle signal generator is configured to receive said lack-phase DC voltage, and to generate a controlling signal that indicates a conduction phase angle range of said SCR element; and

d) a dimming signal generator coupled to said conduction phase angle signal generator and being configured to compare said controlling signal and a slope reference signal to output a dimming signal, said dimming signal generator having a dimming phase angle range from a predetermined start-up phase angle to a cut-off phase angle, wherein said cut-off phase angle is determined by said filter capacitor and said AC power supply,

e) wherein said slope reference signal is configured to indicate an average value at each phase angle of said AC power supply in a half cycle having a first reference corresponding to said start-up phase angle and a second reference corresponding to said cut-off phase angle,

f) wherein when said controlling signal is equal to said first reference, a light-emitting diode (LED) load is configured to be controlled by said dimming signal to operate in a maximum luminance,

g) wherein when said controlling signal is equal to said second reference, said LED load is configured to be controlled by said dimming signal to operate in a minimum luminance, and

h) wherein when said controlling signal is between said first reference and said second reference, said LED load is configured to be controlled by said dimming signal to change linearly between said maximum luminance and said minimum luminance.

2. The SCR dimming circuit of claim 1 , wherein said start-up phase angle comprises a phase angle between 0° and 90°, and said cut-off phase angle is about 90°.

3. The SCR dimming circuit of claim 1 , wherein said conduction phase angle signal generator comprises:

a) a phase angle detection circuit coupled to said rectifier bridge, wherein said phase angle detection circuit is configured to receive said lack-phase DC voltage, and to generate a square wave signal; and

b) a controlling signal generator coupled to said phase angle detection circuit, wherein said controlling signal generator is configured to filter said square wave signal, and to generate a smooth controlling signal that indicates said conduction phase angle range of said SCR element.

4. The SCR dimming circuit of claim 3 , wherein said phase angle detection circuit comprises a clamping circuit having a zenera diode and a transistor, wherein:

a) a breakdown voltage of said zener diode is arranged as a value of said AC power supply corresponding to said start-up phase angle;

b) an anode of said zener diode is coupled to a second output terminal of said rectifier bridge, and a cathode of said zener diode is coupled through a first resistor to a first output terminal of said rectifier bridge; and

c) a controlling terminal of said transistor is coupled to said cathode of said zener diode, a first terminal of said transistor is coupled to said first output terminal of said rectifier bridge, and a second terminal of said transistor is coupled to ground through a second resistor, wherein said clamping circuit is configured to generate said square wave signal that indicates said conduction phase angle range of said SCR element.

5. The SCR dimming circuit of claim 3 , wherein said controlling signal generator comprises a first inverter and a first filter circuit having a resistor and a capacitor, wherein:

a) a first terminal of said inverter is configured to receive said square wave signal;

b) said resistor and said capacitor are coupled in series between a second terminal of said inverter and a terminal of said rectifier bridge; and

c) a voltage at a common node of said resistor and said capacitor is configured to be used as said controlling signal.

6. The SCR dimming circuit of claim 1 , wherein said dimming signal generator comprises:

a) a slope reference signal generator configured to generate said slope reference signal; and

b) a compare and processing circuit configured to compare said controlling signal through a first input terminal and said slope reference signal through a second input terminal, and to generate said dimming signal.

7. A silicon-controlled rectifier (SCR) dimming method for regulating luminance of a light-emitting diode (LED) load, the method comprising:

a) converting an external AC power supply to a lack-phase AC voltage;

b) rectifying said lack-phase AC voltage to generate a lack-phase DC voltage;

c) converting said lack-phase DC voltage to a controlling signal that indicates a conduction phase angle range of an SCR element;

d) comparing said controlling signal and a slope reference signal to generate a dimming signal;

e) controlling, by said dimming signal, said LED load to operate in a maximum luminance when said controlling signal is equal to a first reference of said slope reference signal;

f) controlling, by said dimming signal, said LED load to operate in a minimum luminance when said controlling signal is equal to a second reference of said slope reference signal; and

g) controlling, by said dimming signal, said LED load to change linearly when said controlling signal is between said first reference and said second reference of said slope reference signal.

8. The method of claim 7 , further comprising controlling, by said dimming signal, said LED load to operate in a fixed luminance when said controlling signal is less than said second reference of said slope reference signal.

9. The method of claim 7 , further comprising:

a) clamping said lack-phase DC voltage to generate a square wave signal that indicates said conduction phase angle range of said SCR element; and

b) filtering said square wave signal based on a predetermined reference to generate a smooth voltage signal that indicates said conduction phase angle range of said SCR element.

10. An LED driver, comprising:

a) said SCR dimming circuit of claim 1 ; and

b) a power stage and a driving circuit, wherein said power stage and said driving circuit are configured to receive said DC voltage generated by said filter capacitor and said dimming signal to drive and to regulate luminance of said LED load.