Power converter for interfacing a fluorescent lighting ballast to a light emitting diode lamp
An AC/DC power converter is coupled between a fluorescent ballast circuit and a set of light emitting diodes (LEDs) forming an LED lamp. The power converter converts an AC output from the ballast circuit to a DC current applied to drive operation of the LEDs. The power converter transforms and rectifies the AC output from the ballast circuit to generate a DC output current. An open load protection circuit is coupled to protect the ballast circuit when the LED lamp is not connected. Current control is provided by a transistor having a source/drain conduction path coupled to shunt the DC output current in response to a control signal having a duty cycle generated as a function of a zero-crossing of the AC output and a sensed value of the DC output current applied to the LED lamp.
1. An apparatus, comprising:
a rectifier circuit having input terminals coupled to receive an AC signal and output terminals configured to generate a DC signal;
a diode coupled between one output terminal of the rectifier circuit and a first converter output terminal;
a capacitor coupled between the first converter output terminal and a second converter output terminal;
a switched shunt circuit coupled between output terminals of the rectifier circuit; and
a control circuit configured to control actuation of the switched shunt circuit using a PWM control signal having a duty cycle set dependent on a comparison of a current error signal to a ramp signal,
wherein the current error signal is generated by a circuit which integrates a difference between a sensed current flowing at the first and second converter output terminals and a threshold.
2. The apparatus of claim 1 , further comprising a transformer configured to generate the AC signal from an AC input.
3. The apparatus of claim 1 , wherein the switched shunt circuit comprises a MOSFET.
4. The apparatus of claim 1 , wherein the control circuit comprises:
a zero-crossing detection circuit coupled to sense the AC signal; and
a current sensing circuit coupled to one of the first and second converter output terminals;
wherein the control circuit is configured to switch the switched shunt circuit to a first state responsive to the detected zero-crossing of the AC signal and switch the switched shunt circuit to a second state responsive to the comparison.
5. The apparatus of claim 4 , wherein the first state is closed and the second state is open.
6. The apparatus of claim 4 , wherein the PWM control signal duty cycle is increased if the sensed current exceeds the threshold.
7. The apparatus of claim 4 , wherein the PWM control signal duty cycle is decreased if the sensed current is less than the threshold.
8. The apparatus of claim 1 , further comprising an LED load circuit coupled between the first and second converter output terminals.
9. The apparatus of claim 8 , further comprising a housing configured to contain the rectifier circuit, diode, capacitor, switched shunt circuit, control circuit and LED load circuit.
10. The apparatus of claim 9 , wherein the housing is a lamp bulb.
11. The apparatus of claim 9 , wherein the housing is a lamp fixture, further comprising a fluorescent lighting ballast circuit having AC outputs coupled to the input terminals of the rectifier circuit.
12. The apparatus of claim 1 , wherein the control circuit comprises:
a first circuit configured to sense current flowing at one of the first and second converter output terminals;
a second circuit configured to generate the ramp signal in response to said detected zero-crossing; and
a third circuit configured to set the PWM control signal duty cycle responsive to a comparison of the ramp signal to the current error signal.
13. The apparatus of claim 1 , wherein the control circuit comprises an analog control circuit configured to generate the PWM control signal for application to control actuation of the switched shunt circuit.
14. An apparatus, comprising:
a housing;
a plurality of LEDs located within the housing; and
an AC/DC power converter located within the housing and configured to supply current to the plurality of LEDs;
wherein the AC/DC power converter comprises:
a rectifier circuit having input terminals coupled to receive an AC signal and output terminals configured to generate a DC signal;
a diode coupled between one output terminal of the rectifier circuit and a first converter output terminal coupled to the plurality of LEDs;
a capacitor coupled between the first converter output terminal and a second converter output terminal;
a switched shunt circuit coupled between output terminals of the rectifier circuit; and
a control circuit configured to control actuation of the switched shunt circuit using a PWM control signal having a duty cycle set dependent on a comparison of a current error signal to a ramp signal,
wherein the current error signal is generated by a circuit which integrates a difference between a sensed current flowing at the first and second converter output terminals through the plurality of LEDs and a threshold.
15. The apparatus of claim 14 , wherein the AC/DC power converter further comprises a transformer configured to generate the AC signal from an AC input.
16. The apparatus of claim 14 , wherein the housing is a lamp bulb.
17. The apparatus of claim 14 , wherein the housing is a lamp fixture, further comprising a fluorescent lighting ballast circuit having AC outputs coupled to inputs of the rectifier circuit.
18. The apparatus of claim 1 , further comprising a filter configured to filter the sensed current before integration and wherein the circuit which integrates comprises an amplifier and wherein a corner frequency of the filter matches a breakpoint frequency of the amplifier.
19. The apparatus of claim 1 , wherein the switched shunt circuit comprises a transistor switch directly connected between the output terminals of the rectifier circuit.
20. An apparatus, comprising:
a rectifier circuit having input terminals coupled to receive an AC signal and output terminals configured to generate a DC signal;
a diode coupled between one output terminal of the rectifier circuit and a first converter output terminal;
a capacitor coupled between the first converter output terminal and a second converter output terminal;
a switched shunt circuit coupled between output terminals of the rectifier circuit; and
a control circuit comprising:
a first circuit configured to sense current flowing at one of the first and second converter output terminals, integrate the sensed current and generate a current error signal as a difference between the integrated sensed current and a threshold;
a second circuit configured to generate a ramp signal in response to said detected zero-crossing; and
a third circuit configured to set a duty cycle of a PWM control signal configured to control actuation of the switched shunt circuit responsive to a comparison of the ramp signal to the current error signal.
21. The apparatus of claim 20 , further comprising a transformer configured to generate the AC signal from an AC input.
22. The apparatus of claim 20 , wherein the control circuit comprises:
a zero-crossing detection circuit coupled to sense the AC signal; and
a current sensing circuit coupled to one of the first and second converter output terminals;
wherein the control circuit is configured to switch the switched shunt circuit to a first state responsive to the detected zero-crossing of the AC signal and switch the switched shunt circuit to a second state responsive to the comparison.
23. The apparatus of claim 22 , wherein the first state is closed and the second state is open.
24. The apparatus of claim 22 , wherein the PWM control signal duty cycle is increased if the sensed current exceeds the threshold and wherein the PWM control signal duty cycle is decreased if the sensed current is less than the threshold.
25. The apparatus of claim 20 , further comprising an LED load circuit coupled between the first and second converter output terminals.