LED based lighting application
The present invention relates to a lighting application, the lighting application comprises an LED assembly comprising a serial connection of two or more LED units, each LED unit comprising one or more LEDs, each LED unit being provided with a controllable switch for substantially short-circuiting the LED unit. The lighting application further comprises a control unit for controlling a drive unit and arranged to receive a signal representing a voltage level of the supply voltage, and control the switches in accordance with the signal. The invention further provides for an LED driver that enables to operate a TRIAC based dimmer at an optimal holding current and an LED driver comprising a switchable buffer, e.g. a capacitor.
1. An LED driver for powering an LED assembly comprising at least one LED, the LED driver comprising a converter for converting a periodic input voltage to a supply current for powering the LED assembly, the LED driver further comprising a control unit configured to determine a minimal holding current by, in use, gradually reducing the supply current until a value of the input voltage of the converter substantially reduces to zero and the control unit is further configured to subsequently control the converter to operate at a supply current at least equal to the minimal holding current.
2. The LED driver according to claim 1 wherein the control unit is configured to determine the minimal holding current by performing, in use, the following functions:
1. operating the converter to reduce the supply current,
2. measuring a value of the input voltage at the reduced supply current, and
3. repeating steps 1 and 2 until the supply voltage substantially reduces to zero.
3. The LED driver according to claim 1 wherein the control unit comprises an output terminal for outputting a control signal to the converter for gradually reducing the supply current and an input terminal for receiving a signal representing the value of the input voltage.
4. The LED driver according to claim 1 wherein a supply voltage comprises a TRIAC dimmer output voltage.
5. The LED driver according to claim 1 wherein the LED driver further comprises input terminals for receiving a supply voltage.
6. The LED driver according to claim 5 further comprising an input capacitance arranged between the input terminals.
7. The LED driver according to claim 1 wherein the control unit is arranged to determine a dimming level from a supply voltage.
8. The LED driver according to claim 1 wherein the minimal holding current is maintained during at least an entire period of the input voltage in order to determine a dimming level.
9. The LED driver according to claim 1 wherein the control unit is arranged to determine the minimal holding current at least every 10 periods of the input voltage.
10. The LED driver according to claim 1 wherein a trailing end of a period of the periodic input voltage is applied to determine the minimal holding current.
11. A lighting application comprising the LED driver according to claim 1 , the lighting application further comprising an LED assembly comprising at least one LED, the lighting application further comprising a variable load, in use controlled by the control unit, the variable load being connected in series with the LED assembly.
12. The lighting application according to claim 11 wherein the variable load comprises a switchable resistance.
13. The lighting application according to claim 11 wherein a load characteristic of the variable load is varied during a part of the periodic input voltage of the LED driver while the input voltage is monitored.
14. The lighting application according to claim 13 wherein the part is a trailing part.
15. An LED driver for powering an LED assembly comprising at least one LED, the LED driver comprising a converter for converting a periodic input voltage to a supply current for powering the LED assembly, the converter having input terminals for receiving the periodic input voltage, the LED driver further comprising an input buffer for providing a current to the input terminals and a control unit, the LED driver further comprising a switching element connected between the input terminals and the input buffer for opening and closing a current path from the input buffer to the input terminals and wherein the control unit is further arranged to control the switching element based on an input signal representing the periodic input voltage, the control unit being configured to close the switching element in each period of the periodic input voltage during a time when the periodic input voltage is below a minimum level required to supply the converter.
16. The LED driver according to claim 15 wherein the input buffer comprises a capacitor or capacitor assembly.
17. The LED driver according to claim 15 wherein the switching element comprises a first switch for connecting and disconnection a first terminal of the input buffer to a first terminal of the input terminals.
18. The LED driver according to claim 15 wherein the switching element is further arranged to open and close a current path from the input buffer to a supply terminal, in use, connected to the periodic supply voltage.
19. The LED driver according to claim 15 further comprising a rectifier for rectifying an AC input voltage to obtain the periodic input voltage, the rectifier having AC input terminals for receiving the AC input voltage, the switching element comprising a first switch for connecting a terminal of the input buffer to either a first or a second terminal of the AC input terminals.
20. The LED driver according to claim 19 wherein the switching element comprises a second switch for connecting a further terminal of the input buffer to an input terminal of the converter.
21. The LED driver according to claim 15 , wherein the control unit is arranged to synchronize an on/off duty-cycle of the supply current with the periodic input voltage.
22. The LED driver according to claim 15 , wherein the control unit is arranged to control the LED driver to apply a reduced supply current to the LED assembly when the LED assembly is powered from the input buffer.
23. The LED driver according to claim 15 , wherein the input buffer comprises a capacitor and wherein the switching element enables the periodic supply voltage and a voltage over the capacitor to be added.
24. The LED driver according to claim 15 , further comprising a filter capacitor.
25. The LED driver according to claim 24 wherein the switching element is arranged to interrupt a current path from the input buffer to the filter capacitor thereby disabling, in use, the input buffer to discharge to the filter capacitor.