Controlling current flowing through LEDs in a LED lighting fixture

1. An alternating current (“AC”)-powered light emitting diode (“LED”) driver for driving one or more arrays of series-connected LEDs, comprising:

a first transistor comprising a first collector-emitter path connected in series with at least a first LED of a first array of the one or more arrays of series-connected LEDs;

a second transistor configured to selectively activate the first transistor based on a level of current through the first array of series-connected LEDs;

a second array of the one or more arrays of series-connected LEDs, wherein the second array of series-connected LEDs comprises a second turn-on voltage and is connected in parallel to the first array of series-connected LEDs;

a first current limiting circuit comprising the first transistor and the second transistor, wherein the first current limiting circuit is coupled to the first array of series-connected LEDs, wherein the first current limiting circuit is configured to maintain a first maximum current flowing through the first array of series-connected LEDs, wherein the first maximum current corresponds to a first voltage, wherein the first voltage is greater than the first turn-on voltage; and

a second current limiting circuit comprising a third transistor and a fourth transistor, wherein the second current limiting circuit is coupled to the second array of series-connected LEDs, wherein the second current limiting circuit is configured to maintain a second maximum current flowing through the second array of series-connected LEDs, wherein the second maximum current corresponds to a second voltage, wherein the second voltage is greater than a second turn-on voltage of the second array,

wherein the first array of series-connected LEDs has a first turn-on voltage, and

wherein the first array of series-connected LEDs and the second array of series-connected LEDs are connected to a positive voltage output of a full-wave rectifier, wherein the full-wave rectifier is configured to receive AC power from an AC power source and output a positive voltage representation of the AC power to the first array of series-connected LEDs and to the second array of series-connected LEDs.

2. The AC-powered LED driver of claim 1 , further comprising:

a current limiting resistor connected in series with the first array of series-connected LEDs,

wherein the first collector-emitter path of the first transistor is connected in parallel with the current limiting resistor and is configured to at least partially bypass the current limiting resistor when activated.

3. The AC-powered LED driver of claim 2 , further comprising a current sensing resistor disposed between the current limiting resistor and ground for providing a bias voltage to a base of the second transistor.

4. The AC-powered LED driver of claim 1 , wherein the second transistor comprises a second collector-emitter path disposed between a base of the first transistor and ground such that when the second transistor is activated the first transistor is deactivated.

5. The AC-powered LED driver of claim 1 , wherein the first collector-emitter path of the first transistor is further connected in parallel with a second LED of the first array of series-connected LEDs, wherein the first LED and the second LED are different LEDs.

6. The AC-powered LED driver of claim 1 , further comprising:

a third transistor comprising a third collector-emitter path connected in parallel with the first transistor and in series with at least the first LED of the first array of series-connected LEDs,

wherein the second transistor is further configured to selectively activate the third transistor based on the level of current through the first array of series-connected LEDs.

7. The AC-powered LED driver of claim 1 , wherein the first maximum current and the second maximum current are determined based on one or more parameters of the LED lighting fixture, wherein the one or more parameters of the LED lighting fixture are selected from a group consisting of maintaining a high dimming range, reducing flickering, improving total harmonic distortion, improving efficiency, reducing power consumption, and improving flicker immunity and performance.

8. The AC-powered LED driver of claim 1 , wherein the second turn-on voltage is less than the first turn-on voltage, and wherein the second array of series-connected LEDs has substantially no current flowing therethrough when the positive voltage output from the full-wave bridge rectifier is greater than the first turn-on voltage.

9. The AC-powered LED driver of claim 8 , wherein the second maximum current flows through the second array of series-connected LEDs between when the positive voltage output from the full-wave bridge rectifier drops below the first turn-on voltage and when the positive output voltage drops below the second turn-on voltage.

10. The AC-powered LED driver of claim 8 , wherein each array of the one or more arrays of series-connected LEDs is dimmable.

11. A method for controlling a light emitting diode (LED) lighting circuit, the method comprising:

applying a first voltage to a first array of series-connected LEDs and a second array of series-connected LEDs of the LED lighting circuit, wherein the first voltage exceeds a first threshold voltage, wherein the first threshold voltage turns on the first array of series-connected LEDs;

applying, subsequent to applying the first voltage, an increased voltage to the first array of series-connected LEDs and the second array of series-connected LEDs, wherein the increased voltage exceeds a second threshold voltage, wherein the second threshold voltage turns on the second array of series-connected LED and turns off the first array of series-connected LEDs;

applying, subsequent to applying the increased voltage, a decreased voltage to the first array of series-connected LEDs and the second array of series-connected LEDs, wherein the decreased voltage is greater than the first threshold voltage and less than the second threshold voltage, wherein the decreased voltage turns off the second array of series-connected LED and turns on the first array of series-connected LEDs; and

applying, subsequent to applying the decreased voltage, a second voltage to the first array of series-connected LEDs and the second array of series-connected LEDs, wherein the second voltage is less than the first threshold voltage, wherein the second voltage turns off the first array of series-connected LEDs.

12. The method of claim 11 , wherein the first threshold voltage is controlled using a first transistor comprising a collector-emitter path coupled in series with the first array of series-connected LEDs, wherein the first transistor is activated and deactivated using a second transistor electrically coupled to the first transistor.

13. The method of claim 11 , wherein the LED lighting circuit is dimmable.

14. The method of claim 11 , wherein the first threshold voltage and the second threshold voltage are set to maintain a high dimming range.

15. The method of claim 11 , wherein the first threshold voltage and the second threshold voltage are set to reduce flicker effects.

16. The method of claim 11 , wherein the first threshold voltage and the second threshold voltage are set to improve total harmonic distortion.

17. An alternating current (“AC”)-powered light emitting diode (“LED”) driver for driving one or more arrays of series-connected LEDs, comprising:

a plurality of LED paths connected in parallel with each other, wherein each LED path of the plurality of LED paths comprises one or more arrays of series-connected LEDs;

a plurality of first transistors, wherein each first transistor of the plurality of first transistors comprises a first collector-emitter path connected in series with a LED path of the plurality of LED paths; and

a second transistor configured to selectively activate each first transistor of the plurality of first transistors based on a level of current through the respective LED path of the plurality of LED paths,

wherein each of the first array of series-connected LEDs has a first turn-on voltage, further comprising: a plurality of current limiting resistors, wherein each current limiting resistor of the plurality of current limiting resistors is connected in series with a respective LED path of the plurality LED paths, wherein the first collector-emitter path of each of the plurality of first transistors is connected in parallel with a respective current limiting resistor and is configured to, when activated, at least partially bypass the respective current limiting resistor.

18. The AC-powered LED driver of claim 17 , wherein the second transistor comprises a base-emitter voltage that is reduced as temperature increases to generate a reduced BE voltage, wherein the reduced BE voltage reduces a current flowing through the first array of series-connected LEDs.