LED arrangement and LED driving method
A lighting circuit uses a tapped linear driver architecture in which there are at least two LED types; a first LED type with a first forward voltage, and a second LED type with a second forward voltage at least double the first forward voltage. The first segment of the tapped linear driver comprises more LEDs of the first type than the second type and the last segment comprises more LEDs of the second type than the first type. This arrangement enables a reduction in the number of LEDs needed, but without significantly impacting on the efficiency of the circuit.
1. A lighting circuit comprising:
an input adapted to receive a time-variable input voltage;
a first plurality of LEDs having a first forward voltage;
a second plurality of LEDs having a second forward voltage, the second forward voltage being at least double the first forward voltage;
a plurality of LED segments comprising at least a first LED segment and a last LED segment, wherein the first LED segment comprises more LEDs of the first plurality of LEDs than the second plurality of LEDs, wherein the last LED segment comprises more LEDs of the second plurality of LEDs than the first plurality of LEDs;
a switch network coupled to the plurality of LED segments,
wherein the switch network, including a plurality of switches elements respectively coupled to the plurality of LED segments, is adapted to:
switch only the first LED segment when an amplitude of the time-variable input voltage is below a first threshold, and
switch the first LED segment and the last LED segment in series when the amplitude of the time-variable input voltage is above a last threshold, such that a total forward voltage of the plurality of LED segments is selected in dependence on the amplitude of the time-variable input voltage.
2. The circuit as claimed in claim 1 , wherein the first LED segment comprises only LEDs of the first plurality of LEDs.
3. The circuit as claimed in claim 1 , and the last LED segment comprises only LEDs of the second plurality of LEDs.
4. The circuit as claimed in claim 1 , wherein the time-variable input voltage comprises a mains input.
5. The circuit as claimed in claim 1 , wherein the first plurality of LEDs comprises a single junction LED, and the second plurality of LEDs comprises a multi junction LED.
6. The circuit as claimed in claim 1 , wherein the first forward voltage is in a range of 3V to 6V and the second forward voltage is in the range 12V to 48V.
7. The circuit as claimed in claim 1 , wherein the forward voltage of the first LED segment is equal to or higher than 0.5 times a peak amplitude of the input voltage.
8. The circuit as claimed in claim 7 , comprising a capacitor circuit or a valley fill circuit between the input and the plurality of LED segments.
9. The circuit as claimed in claim 8 , wherein, in use, the capacitor circuit or the valley fill circuit is adapted to provide a voltage no less than the first threshold when the time-variable input voltage is less than the first threshold, and the first LED segment is adapted to remain permanently on throughout a whole period of the time-variable input voltage.
10. The circuit as claimed in claim 1 , wherein an output lumen of the first LED segment is equal to or greater than 0.5 times a total lumen of the circuit.
11. The circuit as claimed in claim 1 , wherein the forward voltage of the last LED segment is at least 0.25 times a peak amplitude of the input voltage, and at most 0.5 times the peak amplitude of the input voltage.
12. The circuit as claimed in claim 1 , wherein an output lumen of the last LED segment is equal to or less than 0.2 times the total lumen of the circuit.
13. The circuit as claimed in claim 1 , wherein LEDs of the first plurality of LEDs and the second plurality of LEDs have the same output color.
14. A method driving a lighting circuit, comprising:
providing a time-varying input voltage and coupling the time-varying input voltage to a set of LED segments, wherein the set of LED segments includes a first plurality of LEDs having a first forward voltage, and a second plurality of LEDs having a second forward voltage, wherein the set of LED segments includes a first LED segment and a last LED segment, the first LED segment comprising more LEDs of the first plurality of LEDs than the second plurality of LEDs, the last LED segment comprising more LEDs of the second plurality of LEDs than the first plurality of LEDs;
switching only the first LED segment when an amplitude of the time-varying input voltage is below a first threshold; and
switching the first LED segment and the last LED segment in series when the amplitude of the time-varying input voltage is above a last threshold, such that the total forward voltage of the set of LED segments is selected in dependence on the amplitude of the time-varying input voltage.
15. The method as claimed in claim 14 , further comprising the step of smoothing the time-varying input voltage such that the first LED segment remains permanently on in use.