Dimmable LED driver and driving method

Abstract

Disclosed are dimmable LED driver circuits and methods. A dimmable LED driver can include: an SCR, an electronic transformer, and a rectifier bridge to convert an AC voltage to a DC voltage signal; a power stage circuit that receives the DC voltage signal, and outputs a constant current to drive an LED load, where the power stage circuit includes first and second power stage circuits, the first power stage circuit receiving the DC voltage signal, and generating a first output voltage to the second power stage; and an input current control circuit that receives an input current of the first power stage circuit and the first output voltage, and generates a first control signal to control the input current as a square wave signal during an on time of the SCR, and the input current is substantially zero during an off time of the SCR.

What is claimed is: 
     
       1. A dimmable light-emitting diode (LED) driver, comprising:
 a) a silicon-controlled rectifier (SCR), an electronic transformer, and a rectifier bridge configured to convert an AC voltage to a DC voltage signal; 
 b) a power stage circuitry configured to receive said DC voltage signal, and to output a constant current to drive an LED load, wherein said power stage circuitry comprises first and second power stage circuits, wherein said first power stage circuit is configured to receive said DC voltage signal, and to generate a first output voltage to said second power stage; 
 c) an input current control circuit configured to receive an input current of said first power stage circuit and said first output voltage, and to generate a first control signal to control said input current as a pseudo square waveform signal, wherein a high frequency saw-tooth waveform is formed from upper portions of said pseudo square waveform signal during an on time of said SCR; 
 d) wherein a peak value of said saw-tooth waveform is configured to satisfy a minimum load current requirement of said electronic transformer, and lower portions of said pseudo square waveform signal are maintained as substantially zero; and 
 e) said second power stage circuit being configured to generate a substantially constant output current to drive said LED load in accordance with said input current. 
 
     
     
       2. The LED driver of  claim 1 , wherein said input current control circuit comprises:
 a) a current control circuit configured to receive said input current and a first reference current, and to generate a current control signal, wherein said first reference current corresponds to said peak value of said pseudo square waveform signal; 
 b) a voltage control circuit configured to receive said first output voltage and a first reference voltage, and to generate a voltage control signal; and 
 c) a logic circuit configured to receive said current control signal and said voltage control signal, and to generate said first control signal. 
 
     
     
       3. The LED driver of  claim 2 , wherein:
 a) said current control circuit comprises a first comparator configured to generate said current control signal by a comparison of said input current against said first reference current; 
 b) said voltage control circuit comprises a hysteresis comparator configured to generate said voltage control signal by a comparison of said first output voltage against an upper threshold and a lower threshold based on said first reference voltage; and 
 c) said logic circuit comprises a first OR-gate and a first flip-flop, wherein said first OR-gate is configured to receive said current control signal and said voltage control signal, and wherein an output of said first OR-gate is coupled to a reset terminal of said first flip-flop, and a set terminal of said first flip-flop is configured to receive a first clock signal. 
 
     
     
       4. The LED driver of  claim 2 , wherein:
 a) said current control circuit comprises a first transconductance operational amplifier, a first compensation circuit, and a second comparator, wherein said first transconductance operational amplifier is configured to receive said input current and said first reference current, and to generate a first error voltage signal after being compensated by said first compensation circuit; 
 b) said second comparator is configured to receive said first error voltage signal and a voltage signal that represents an inductor current in said first power stage circuit, and to generate said current control signal; 
 c) said voltage control circuit comprises a third comparator configured to receive said first output voltage and said first reference voltage, and to generate said voltage control signal; 
 d) said logic circuit comprises a second flip-flop, a third flip-flop, and a first AND-gate, wherein a reset terminal of said second flip-flop is configured to receive said current control signal, a set terminal of said second flip-flop is configured to receive a second clock signal, and said second flip-flop is configured to output a second trigger signal; 
 e) a reset terminal of said third flip-flop is configured to receive said voltage control signal, a set terminal of said third flip-flop is configured to receive an enable trigger signal, and said third flip-flop is configured to output a third trigger signal; and 
 f) said first control signal is output from said first AND-gate based on said second trigger signal and said third trigger signal. 
 
     
     
       5. The LED driver of  claim 4 , wherein said current control circuit further comprises a first reference current generating circuit configured to receive a reference current source, and to generate said first reference current, and wherein:
 a) said reference current source is configured as said first reference current when said square wave signal of said input current of said first power stage circuit is active; and 
 b) said first reference current is zero when said square wave signal is inactive. 
 
     
     
       6. The LED driver of  claim 1 , further comprising a dimming signal generating circuit configured to generate a dimming signal that represents conducting angle information of said SCR in accordance with time intervals of said saw-tooth waveform of said input current. 
     
     
       7. The LED driver of  claim 6 , further comprising an output current control circuit configured to receive a current signal of said LED load and said dimming signal, and to generate a second control signal configured to control said second power stage circuit to provide said output current signal for said LED load. 
     
     
       8. The LED driver of  claim 1 , wherein said first power stage circuit comprises a boost topology. 
     
     
       9. The LED driver of  claim 1 , wherein said second power stage circuit comprises a buck topology or a boost-buck topology. 
     
     
       10. The LED driver of  claim 6 , wherein said dimming signal generating circuit comprises:
 a) a timer circuit configured to receive said input current and a threshold current, and to generate a second square wave signal that represents conducting angle information of said SCR; and 
 b) an averaging circuit configured to receive said second square wave signal and a second reference current, and to generate said dimming signal that represents said conducting angle information of said SCR by averaging said second reference current in accordance with said second square wave signal. 
 
     
     
       11. The LED driver of  claim 1 , wherein during said on time of said SCR, said first output voltage is indicated as a saw-tooth waveform, and wherein a frequency of said saw-tooth waveform of said first output voltage is less than a frequency of said saw-tooth waveform of said input current. 
     
     
       12. A method of driving a dimmable light-emitting diode (LED), the method comprising:
 a) receiving an AC voltage to obtain a DC voltage signal through a silicon-controlled rectifier (SCR), an electronic transformer, and a rectifier bridge; 
 b) generating an input current and a first output voltage using said DC voltage signal, wherein said input current is generated as a square wave signal during an on time of said SCR, and said input current is maintained as substantially zero during an off time of said SCR; 
 c) generating a dimming signal from said input current; and 
 d) generating a constant output current signal using said first output voltage, an LED load current signal, and said dimming signal. 
 
     
     
       13. The method of  claim 12 , wherein said generating said input current and said first output voltage further comprises:
 a) generating a current control signal using said input current and a first reference current, wherein said first reference current corresponds to a peak value of said square wave signal; 
 b) generating a voltage control signal using said first output voltage and a first reference voltage; and 
 c) generating a first control signal by performing a logic operation on said current control signal and said voltage control signal. 
 
     
     
       14. The method of  claim 13 , further comprising:
 a) generating said current control signal by comparing said input current against said first reference current; 
 b) generating a hysteresis comparison signal by a hysteresis comparator receiving said first output voltage and said first reference voltage, wherein said hysteresis comparison signal is configured as said voltage control signal; 
 c) generating a reset signal after an OR logic operation based on said current control signal and said voltage control signal; and 
 d) generating said first control signal using said reset signal and a first clock signal as a set signal. 
 
     
     
       15. The method of  claim 13 , further comprising:
 a) generating a first error voltage signal using said input current and said first reference current; 
 b) generating said current control signal by comparing said first error voltage signal against a voltage signal that represents an inductor current in a first power stage circuit; 
 c) generating said voltage control signal by comparing said first output voltage against said first reference voltage; 
 d) generating a second trigger signal by using said current control signal as a reset signal and a second clock signal as a set signal; 
 e) generating a third trigger signal by using said voltage control signal as a reset signal and an enable trigger signal as a set signal; and 
 f) generating said first control signal after performing a logic operation on said second trigger signal and said third trigger signal. 
 
     
     
       16. The method of  claim 15 , further comprising receiving a reference current source, wherein:
 a) when said square wave signal that represents said input current is active, said reference current source is configured as said first reference current; and 
 b) when said square wave signal is inactive, said first reference current is zero. 
 
     
     
       17. The method of  claim 15 , wherein said generating said dimming signal further comprises:
 a) generating a second square wave signal that represents conducting angle information of said SCR based on said input current and a threshold current; and 
 b) generating said dimming signal representing said conducting angle information of said SCR by averaging said second reference current according to said second square wave signal.