Light emitting diode, LED, driver arranged for driving at least one LED, as well as a corresponding LED based lighting device, integrated circuit, IC, and method

Abstract

A Light Emitting Diode, LED, driver arranged for driving at least one LED, said LED driver comprising a rectifier arranged for receiving an Alternating Current, AC, supply voltage and for providing an output Direct Current, DC, voltage to said at least one LED, a voltage regulating branch arranged for controlling an LED voltage provided to said at least one LED, via a supply line, wherein said voltage regulating branch is arranged for receiving said DC voltage from said rectifier, wherein said voltage regulating branch comprises, a capacitor arranged for providing said LED voltage to said at least one LED, a current limiter, connected in series with said capacitor, and arranged for charging said capacitor thereby providing said LED voltage and a feedback circuit arranged for controlling the charging of said capacitor by said current limiter, and thereby controlling the LED voltage provided to said at least one LED, by regulating a minimum residual headroom voltage present on a LED current limiter coupled in series with said at least one LED.

Claims

1. A Light Emitting Diode, LED, driver arranged for driving at least one LED, said LED driver comprising: a rectifier arranged for receiving an Alternating Current, AC, supply voltage and for providing an output Direct Current, DC, voltage to said at least one LED; a voltage regulating branch arranged for controlling an LED voltage provided to said at least one LED, via a supply line, wherein said voltage regulating branch is arranged for receiving said DC voltage from said rectifier, wherein said voltage regulating branch comprises: a capacitor arranged for providing said LED voltage to said at least one LED; a current limiter, connected in series with said capacitor, and arranged for charging said capacitor thereby providing said LED voltage; an LED current limiter, connected in series with said at least one LED, such that a residual headroom voltage is present over said LED current limiter; and a feedback circuit comprising a comparator block arranged for providing an output based on a potential difference between two input terminals of said comparator block, wherein a first of said two input terminals is set to a predefined residual headroom voltage, and wherein a second of said two input terminals is arranged for receiving a value of said residual headroom voltage present on the LED current limiter, and wherein an output of said comparator block is used for controlling said current limiter, wherein the feedback circuit is arranged for controlling the charging of said capacitor by said current limiter, and thereby controlling the LED voltage provided to said at least one LED, by regulating the residual headroom voltage present on the LED current limiter.

2. The LED driver in accordance with claim 1, wherein said feedback circuit further comprises: a minimum voltage detector block, for providing an output based on a potential difference between two input terminals of said minimum voltage detector block, wherein a first of said two input terminals is set to a predefined minimum voltage, and wherein a second of said two input terminals is connected to said LED current limiter, wherein said output of said minimum voltage detector block is connected to said second of said two input terminals of said comparator block.

3. The LED driver in accordance with claim 2, wherein said predefined minimum residual headroom voltage is approximately 1 Volt.

4. A Light Emitting Diode, LED, based lighting device comprising: at least one LED for emitting light; a light emitting diode, LED, driver arranged for driving the at least one LED, said LED driver comprising: a rectifier arranged for receiving an Alternating Current, AC, supply voltage and for providing an output Direct Current, DC, voltage to said at least one LED; a voltage regulating branch arranged for controlling an LED voltage provided to said at least one LED, via a supply line, wherein said voltage regulating branch is arranged for receiving said DC voltage from said rectifier, wherein said voltage regulating branch comprises: a capacitor arranged for providing said LED voltage to said at least one LED; a current limiter, connected in series with said capacitor, and arranged for charging said capacitor thereby providing said LED voltage; an LED current limiter, connected in series with said at least one LED, such that a residual headroom voltage is present over said LED current limiter; and a feedback circuit comprising a comparator block arranged for providing an output based on a potential difference between two input terminals of said comparator block, wherein a first of said two input terminals is set to a predefined residual headroom voltage, and wherein a second of said two input terminals is arranged for receiving a value of said residual headroom voltage present on the LED current limiter, and wherein an output of said comparator block is used for controlling said current limiter, wherein the feedback circuit is arranged for controlling the charging of said capacitor by said current limiter, and thereby controlling the LED voltage provided to said at least one LED, by regulating the residual headroom voltage present on the LED current limiter.

5. A method of operating a light emitting diode, LED, driver arranged for driving at least one LED, said LED driver comprising: a rectifier arranged for receiving an Alternating Current, AC, supply voltage and for providing an output Direct Current, DC, voltage to said at least one LED; a voltage regulating branch arranged for controlling an LED voltage provided to said at least one LED, via a supply line, wherein said voltage regulating branch is arranged for receiving said DC voltage from said rectifier, wherein said voltage regulating branch comprises: a capacitor arranged for providing said LED voltage to said at least one LED; a current limiter, connected in series with said capacitor, and arranged for charging said capacitor thereby providing said LED voltage; an LED current limiter, connected in series with said at least one LED, such that a residual headroom voltage is present over said LED current limiter; and a feedback circuit comprising a comparator block arranged for providing an output based on a potential difference between two input terminals of said comparator block, wherein a first of said two input terminals is set to a predefined residual headroom voltage, and wherein a second of said two input terminals is arranged for receiving a value of said residual headroom voltage present on the LED current limiter, and wherein an output of said comparator block is used for controlling said current limiter, wherein the feedback circuit is arranged for controlling the charging of said capacitor by said current limiter, and thereby controlling the LED voltage provided to said at least one LED, by regulating the residual headroom voltage present on the LED current limiter, wherein said method comprises: controlling, by said voltage regulating branch, the LED voltage provided to said at least one LED by regulating the residual headroom voltage present on the LED current limiter coupled in series with said at least one LED; and controlling, by said feedback circuit, the current limiter based on an amplified difference between the residual headroom voltage present on the LED current limiter and the predefined residual headroom voltage.

6. The method in accordance with claim 5, further comprises: regulating, by said voltage regulating branch, the LED voltage provided to said at least one LED by stabilizing the residual headroom voltage to a predefined minimum residual headroom voltage.

7. The method in accordance with claim 6, wherein the predefined minimum residual headroom voltage is approximately 1 Volt.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 discloses a functional block diagram of a typical direct mains linear LED driver in accordance with the prior art;

(2) FIG. 2 discloses a graph illustrating the minimum residual headroom voltage using the typical direct mains linear LED driver in accordance with the prior art;

(3) FIG. 3 discloses an LED driver in accordance with the present disclosure;

(4) FIG. 4 discloses a graph illustrating the minimum residual headroom voltage using an LED driver in accordance with the present disclosure.

DETAILED DESCRIPTION

(5) The drawings are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes.

(6) FIG. 1 discloses a functional block diagram illustrating a typical direct mains linear LED driver 100 in accordance with the prior art.

(7) The LED driver 100 is directly connected to the Alternating Current, AC, mains supply voltage V1. The mains supply voltage may be a 230 Vac or a 110 Vac voltage, or anything alike. The AC mains supply voltage is rectified by utilizing a full wave rectifier D1. A full wave rectifier is arranged to convert both halves of each cycle of the AC mains supply voltage into a pulsating Direct Current, DC, signal.

(8) The DC signal is fed into a capacitor C1 via a diode D2. The capacitor C1 is arranged to provide a DC voltage to at least one LED D4.

(9) It is noted that the capacitor C1 is charged using a linear current limiter B2 that is connected in series with the capacitor C1. It is noted that the charge current provided by the linear current limiter B2 is set once, and is tuned to a particular tuning point. The tuning point is determined for the lowest AC mains supply voltage and the maximum forward voltage of the at least one LED. That is a worst case scenario that is taken into account when determining the charge current of the current limiter B2.

(10) The headroom voltage is present over the LED current limiter B1.

(11) As mentioned before, losses in such a direct mains linear LED driver may, at least for a part, be contributed to the LED current limiter B1 that is placed in series with the at least one LED. The voltage over the LED current limiter B1, and thus also the accompanying loss, is linear proportional to the AC mains supply voltage and the forward voltages of the at least one LED. The losses are linear proportional to the AC mains supply voltage as the charge current of the capacitor is fixed.

(12) It is further noted that the voltage over B1, i.e. the headroom voltage, is not static. The voltage over B1 will fluctuate in line with the fluctuations provided by the AC mains supply voltage.

(13) In accordance with the present disclosure, the minimum residual headroom voltage may be defined as the minimum voltage over B1 over a particular AC mains supply voltage cycle.

(14) FIG. 2 discloses a graph 200 illustrating the minimum residual headroom voltage using the typical direct mains linear LED driver in accordance with the prior art as described above.

(15) The line as indicated with reference numeral 201 illustrates the charging current of the capacitor, i.e. the current provided by the current limiter B2. The line as indicated with reference numeral 202 illustrates the voltage provided by the capacitor C1 to the at least one LED D4.

(16) As shown in FIG. 2, the charging current is pulsating and fixed over time. The capacitor C1 is charged when the charging current is enabled. The charging current, in this particular example, is about 63 mA. The voltage provided by the capacitor C1 is increasing whenever the capacitor is being charged. The voltage provided by the capacitor C1 rises from about 282V to about 324V.

(17) It is further shown that the voltage provided by the capacitor is linearly decreasing whenever there is no charging current present. This is caused by the static current provided by the LED current limiter.

(18) In this particular example, the minimum residual headroom voltage may be determined by subtracting the forward voltage of the at least one LED from the 282V as disclosed in FIG. 2.

(19) The presented LED driver is advantageous over the prior art as it may be able to improve the issue of decreased LED driver efficiency outside the tuning point by stabilizing the minimum residual headroom voltage to a pre-defined value, which may be independent of the AC mains supply voltage and the forward voltage of the at least one LED. The thermal load between the LED current limiter B1 and the charge current of the current limiter is also distributed more equally.

(20) The above is explained in more detail with respect to FIGS. 3 and 4.

(21) FIG. 3 discloses an LED driver 300 in accordance with the present disclosure.

(22) One of the ideas of the present disclosure is to stabilize the minimum residual headroom voltage of the LED current limiter B1 by modulating the charge current of the capacitor C1 via a current limiter G.

(23) The minimum residual headroom voltage may be defined as the minimum voltage across the LED current limiter B1 during one cycle of the AC mains supply voltage. In practice, the LED current limiter may need about 1V minimum headroom voltage to operate in the linear region and to regulate to the pre-set LED current that is required by the at least one LED's. However, any minimum headroom voltage may be set in accordance with the present disclosure.

(24) The LED driver 300 shown in FIG. 3 has multiple new aspects that are elaborated in more detail here below.

(25) The LED driver 300 comprises a voltage regulating branch arranged for controlling an LED voltage provided to said at least one LED, via a supply line, wherein said voltage regulating branch is arranged for receiving said DC voltage from said rectifier.

(26) The supply line may be considered as the line between the capacitor C1 and the at least one LED D4.

(27) The voltage regulating branch comprises the capacitor C1, the current limiter G and a feedback circuit.

(28) The current limiter G is controllable in that the charging current of C1 may be set. The charging current may thus not be fixed, but may depend on a control signal that it receives. The current limiter G is connected in series with the capacitor C1.

(29) The feedback circuit is arranged for controlling the charging of said capacitor C1 by said current limiter G, and thereby controlling the LED voltage provided to said at least one LED, by regulating a minimum residual headroom voltage present on an LED current limiter coupled in series with said at least one LED.

(30) In this particular scenario, the feedback circuit comprises a comparator block as well as a minimum voltage detector block, both of which are explained in more detail here below.

(31) The minimum voltage detector block is used for providing an output based on a potential difference between two input terminals of said second minimum voltage detector block, wherein a first of said two input terminals is set to a predefined minimum voltage, and wherein a second of said two input terminals is connected to said LED current limiter coupled in series with said at least one LED, wherein said output of said minimum voltage detector block is connected to said second of said two input terminals of said comparator block.

(32) The minimum voltage detector block may be implemented using an OpAmp U2. Here, the capacitor C4 may be charged to the minimum voltage across the LED current limiter B1, i.e. the minimum residual headroom voltage. A first terminal is connected to the capacitor C4, i.e. the minimum voltage across the LED current limiter B1 and the other, i.e. second, terminal is connected to the LED current limiter. It is noted that the residual headroom voltage is present on the LED current limiter. As such, the OpAmp U2 is used for detecting the minimum voltage across the LED current limiter B1.

(33) The output of the minimum voltage detector block is provided to the comparator block.

(34) The comparator block may be arranged for providing an output based on a potential difference between two input terminals of said comparator block, wherein a first of said two input terminals is set to a predefined minimum residual headroom voltage, and wherein a second of said two input terminals is arranged for receiving a minimum value of said residual headroom voltage present on said LED current limiter coupled in series with said at least one LED, and wherein an output of said comparator block is used for controlling said current limiter.

(35) The comparator block may be implemented using an OpAmp U1. Here, the output of OpAmp U2 is connected to the second of the two input terminal of the OpAmp U1. The OpAmp U1 may also be considered as a noise amplifier. The output of the OpAmp U1 is used as a control input for controlling the charge current of the current limiter G, which charge current may be proportional to the output voltage of the OpAmp U1.

(36) FIG. 4 discloses a graph illustrating the minimum residual headroom voltage using an LED driver in accordance with the present disclosure.

(37) The graph 310 may be compared to the graph shown in FIG. 2, i.e. as indicated with reference numeral 200.

(38) The same conditions apply in both cases, i.e. the same AC mains supply voltage and the same forward voltage of the at least one LED.

(39) Difference is that the graph having reference numeral 310 is based on the LED driver in accordance with the present disclosure, while the graph having reference numeral 200 is based on the LED driver in accordance with the prior art.

(40) In FIG. 4, line indicated with reference numeral 312 is the voltage provided by the capacitor C1 and the line indicated with reference numeral 311 is the charging current provided by the current limiter G.

(41) Here, it is shown that the charging current deviates from the charging current as shown in FIG. 2.

(42) Such a deviation in charging current leads to a difference in the minimum headroom voltage present on the LED current limiter. That is, the line having reference numeral 312 drops to about 272V instead of about 282V as shown in FIG. 10. In essence, the minimum headroom voltage is reduced by about 10V in this particular case.

(43) The presented LED driver is beneficial for multiple reasons. One of the reasons is that the LED driver reduces the overall losses occurring in the LED current limiter by reducing the minimum headroom voltage that is present over the LED current limiter. The presented LED driver accomplishes this by tuning, regulating, or controlling the charge current of C1 based on the minimum headroom voltage over the LED current limiter.

(44) In the claims, the word comprising does not exclude other elements or steps, and the indefinite article a or an does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope thereof.