Electronic ballast-compatible lighting driver for light-emitting diode lamp
A lighting driver includes a rectifier having an input connected to receive AC electrical power from an electronic ballast and having an output connected to supply a current to a plurality of light emitting diodes (LEDs) arranged in series with each other in a string, and a switching device disposed at the output of the rectifier and configured to receive a switching control signal and in response thereto to execute a switching operation to modulate an amount of power supplied to the plurality of LEDs so as to cause an average of the power supplied to the plurality of LEDs to be equal to a target power level. A current sensor may be provided to sense the current, and a controller configured to control the switching operation of the switching device in response to the sensed current.
1. An apparatus comprising a light emitting diode tube (TLED) lamp, the TLED lamp comprising:
a tube having at least one electrical connector configured to be installed in a fluorescent light fixture;
a plurality of light emitting diodes (LEDs) arranged in series with each other in a string and disposed inside the tube; and
a lighting driver disposed inside the tube and connected to the at least one electrical connector and being configured to supply power to the plurality of LEDs, the lighting driver comprising:
a rectifier having an input connected to receive AC electrical power from the at least one electrical connector and having an output connected to supply power to the plurality of LEDs;
a switching device connected to the output of the rectifier and configured to be opened and closed to modulate an LED voltage from the plurality of LEDs that appears across the output of the rectifier;
a current sensor configured to sense an LED current passing through the string of LEDs; and
a controller configured to control a switching operation of the switching device in response to the sensed LED current.
2. The apparatus of claim 1 , wherein the controller is configured to execute an algorithm to compare the sensed LED current to a reference value and to control at least one of a duty cycle and a switching rate of a switching control signal provided to the switching device so as to drive the sensed LED current to equal the reference value.
3. The apparatus of claim 2 , further comprising:
a second current sensor configured to sense an output current of the rectifier; and
a zero crossing detector connected to an output of the second current sensor and configured to determine approximate times when zero crossings occur in an AC input current of the received AC electrical power and to provide a signal to the controller that indicates the approximate times of the zero crossings in the AC input current,
wherein the controller is configured to synchronize pulses of the switching control signal with the approximate times of the zero crossings in the AC input current.
4. The apparatus of claim 1 , wherein the controller is configured to execute an algorithm to:
compare the sensed LED current to a reference value to produce a comparison result;
proportionally integrate the comparison result to determine a pulse duration; and
modulate a switching control signal provided to the switching device with the pulse duration so as to drive the sensed LED current to equal the reference value.
5. The apparatus of claim 1 , further comprising a voltage sensor configured to sense the LED voltage, wherein the controller is configured to execute an algorithm to calculate from the sensed LED current and the sensed LED voltage an average LED power supplied to the LEDs, to compare the average LED power to a reference value, and to control at least one of a duty cycle and a switching rate of a switching control signal provided to the switching device so as to drive the average LED power to equal the reference value.
6. The apparatus of claim 5 , further comprising:
a second current sensor configured to sense an output current of the rectifier; and
a zero crossing detector connected to an output of the second current sensor and configured to determine approximate times when zero crossings occur in an AC input current of the received AC electrical power and to provide a signal to the controller that indicates the approximate times of the zero crossings in the AC input current,
wherein the controller is configured to synchronize pulses of the switching control signal with the approximate times of the zero crossings in the AC input current.
7. The apparatus of claim 1 , further comprising a voltage sensor configured to sense an LED voltage across the LEDs, wherein the controller is configured to execute an algorithm to:
determine an LED power supplied to the LEDs based on the sensed LED current and the sensed LED voltage;
compare the determined LED power to a reference value to produce a comparison result;
proportionally integrate the comparison result to determine a pulse duration; and
modulate a switching control signal provided to the switching device with the pulse duration so as to drive the sensed LED current to equal the reference value.
8. The apparatus of claim 1 , further comprising an electronic ballast connected to supply the AC electrical power to the at least one electrical connector.
9. The apparatus of claim 1 , further comprising a second switching device connected to the output of the rectifier and configured to be opened and closed to modulate the LED voltage appearing across the output of the rectifier from the plurality of LEDs, wherein the switching device and second switching device are opened together and closed together so as to cause a first subset of the LEDs in the string to be in parallel with a second subset of the LEDs in the string across the output of the rectifier when both switches are closed and to cause a first subset of the LEDs in the string to be in series with a second subset of the LEDs in the string across the output of the rectifier when both switches are opened.
10. The apparatus of claim 1 , further comprising a switch driver configured to provide a switching control signal to the switching device, wherein the switching control signal has a periodic switching rate and a duty cycle, and wherein the controller controls the duty cycle of the switching control signal so as to cause an average power supplied to the LEDs to equal a reference value.
11. The apparatus of claim 1 , wherein the switching device is connected in parallel across a first subset of the LEDs comprising less than all of the LEDs that are in series with each other.
12. The apparatus of claim 1 , wherein the LEDs in the string include a first subset of LEDs and a second subset of LEDs, wherein the apparatus further includes a bridging diode connected in series in between the first subset of LEDs and the second subset of LEDs in the string, and wherein the apparatus further comprises a second switching device, wherein the switching device is connected in parallel with a series combination of the first subset of LEDs and the bridging diode, and wherein the second switching device is connected in parallel with a series combination of the second subset of LEDs and the bridging diode.
13. A method of driving a plurality of light emitting diodes (LEDs), the method comprising:
receiving a supply of AC electrical power from at least one electrical connector;
rectifying an AC input voltage of the supplied AC electrical power and supplying an LED current to the LEDs;
sensing the LED current; and
controlling a switching operation of a switching device in response to the sensed LED current to modulate an amount of power supplied to the plurality of LEDs by switching between at least a first non-zero power level and a second non-zero power level so as to cause an average of the power supplied to the plurality of LEDs to be equal to a target power level.
14. The method of claim 13 , further comprising comparing the sensed LED current to a reference value, and controlling at least one of a duty cycle and a switching rate of the switching operation so as to drive the sensed LED current to equal a reference value.
15. The method of claim 13 , further comprising determining approximate times when zero crossings occur in an AC input current of the AC electrical power, and synchronizing the switching operation with the approximate times of the zero crossings in the AC input current.
16. A device, comprising:
a rectifier having an input connected to receive AC electrical power from an electronic ballast and having an output connected to supply a current to a plurality of light emitting diodes (LEDs) arranged in series with each other in a string; and
a switching device disposed at the output of the rectifier and configured to receive a switching control signal and in response thereto to execute a switching operation to modulate an amount of power supplied to the plurality of LEDs by switching between at least a first non-zero power level and a second non-zero power level so as to cause an average of the power supplied to the plurality of LEDs to be equal to a target power level.
17. The device of claim 16 , further comprising:
a current sensor configured to sense the current, and
a controller configured to control the switching operation of the switching device in response to the sensed current.
18. The device of claim 17 , further comprising a voltage sensor configured to sense an LED voltage from the plurality of LEDs that appears across the output of the rectifier, wherein the controller is further configured to control the switching operation of the switching device in response to the sensed LED voltage.
19. The device of claim 16 , further comprising the plurality of LEDs.
20. The device of claim 19 , wherein the LEDs in the string include a first subset of LEDs and a second subset of LEDs, wherein the apparatus further includes a bridging diode connected in series in between the first subset of LEDs and the second subset of LEDs in the string, and wherein the apparatus further comprises a second switching device, wherein the switching device is connected in parallel with a series combination of the first subset of LEDs and the bridging diode, and wherein the second switching device is connected in parallel with a series combination of the second subset of LEDs and the bridging diode.