T 0142/17 (Driving of LED-based lighting devices / Signify Holding B.V.) 11-03-2022

Reasons for the Decision

Admissibility (Article 13(2) RPBA 2020)

1. The new request was filed in reaction to the Board's communication under Article 15 RPBA. It addresses the contradiction between the claim wording and the description as to the nature of the measuring means. The notion of "forward driving voltage" has been clarified, as claim 1 specifies that the forward driving voltage corresponds to a voltage causing a current through said LED which is less than half of a lowest driving current provided to said LED by the adjustable electrical power supply in the active mode.

2. Claim 1 was further amended to specify that the measurement voltage source is of a higher precision than the adjustable electrical power supply, thus addressing the misgivings of the Board as to the absence of effect deriving from the claim's wording.

3. Both objections as to lack of clarity, as well as the finding regarding the absence of effect applying over the whole scope of the claim, were raised for the first time in the board's provisional opinion. They constitute exceptional circumstances which justify the admission of the new request, which constitutes a promising attempt at overcoming all outstanding objections.

Added subject-matter (Article 123(2) EPC)

4. Claim 1 stems from a combination of original claims 1 to 6, 10, and 11.

5. The feature of an adjustable electrical power supply for driving the LED in an active mode, has been further specified to recite that it provides a forward driving voltage to the LED. A basis for the amendment is to be found in the passage on page 5, lines 9-17, of the original application where it is stated that the adjustable electrical power supply may comprise a battery or other supply with circuitry for setting a desired driving voltage or driving current for the LED. The board is satisfied that the reference, in the description, to a desired voltage or desired current supports the general reference in the claim to the provision of a forward voltage.

6. Original claims 2 and 3, as well as the original application, consistently refer to two different kinds of measuring means: a voltage source in combination with a current meter, and a current source in combination with a voltage meter (see page 3, lines 3-9; 14-17; 20-22; page 4, lines 5-9; page 6, lines 4-17; page 7, lines 11-22). They are presented as equivalent in that they both allow the determination of a voltage and corresponding current value. The knowledge of both parameters is indicative of the resistance and, therefore, the temperature of the LED junction (page 2, lines 19-26). The information thus obtained is used to control the power delivered to the LED and, thus, its luminous output (page 2, lines 27-30).

7. It is established jurisprudence of the boards of appeal that the claims may be amended in the course of the examination proceedings by introducing features of the description which have been described in relation with specific embodiments of the invention. Both alternatives regarding the measuring means have been disclosed.

8. The passage of the original description on page 7, lines 23-25,

A third embodiment, not shown here, comprises a driving current source that can be set to a measurement current for the measurement phase and with a switch that allows 25 for monitoring the voltage across the LED.

does not in any way suggest that the current meter and the voltage source alone are alternatives, as held the Examining Division. On the contrary, it is consistent with the repeated indications, throughout the description, as to the alternative measuring means which were envisaged. It emphasises that the function achieved by the measurement voltage together with the current meter in the embodiment of Figure 2, as described on page 7, lines 11-19, can be provided, alternatively, by an adjustable driving current source in combination with a voltage meter.

9. Claim 1 has been further amended by incorporating a definition of the forward driving voltage being smaller than the forward driving voltage of the LED in the active mode; and causing a current through said LED which is less than half the lowest driving current in the active mode. The added feature derives from the definition of the voltage to be applied by the measuring means on page 3, lines 24-26. The passage refers explicitly to the alternative of the measuring means comprising a voltage source and a current meter.

10. Claim 1 further recites that the measurement voltage source is non-adjustable, of a higher precision than the adjustable electrical power supply, as at page 4, lines 12-14 of the original description. Said feature is disclosed in combination with the switch means allowing for selective connection to the adjustable power supply means in the active mode or to the measuring means during the measuring phases.

11. The board is satisfied that the features regarding the measurement voltage source on page 4, lines 12-14, which immediately follow the definition in the previous paragraph of the forward driving voltage, allows for a combination of both aspects in the independent claim.

12. The subject-matter of claim 1 derives directly and unambiguously from the original application documents, as required under Article 123(2) EPC.

Clarity - Article 84 EPC

13. The description has been amended to remove any contradiction with the claims. It explicitly states that the measuring means according to the invention combine a voltage source and a current meter, and that the alternative of a current source and voltage meter does not fall under the definition of the invention.

14. The term "forward driving voltage", in claim 1, has a recognised meaning in connection with diodes. It refers to the voltage that drives the LED, that is, the voltage that allows current to flow through said LED in the direction of conductivity.

15. The definition of the predetermined measuring voltage, as being a voltage in a forward direction that causes a current through said LED which is less than half of a lowest driving current provided to said LED by the adjustable electrical power supply in the active mode, is clear in the context of the invention.

16. While it is acknowledged that the characteristic associating the voltage to the current intensity depends on the temperature of the LED junction, as for example illustrated in Figure 4 of the application, the claim feature is to be construed as a condition applying over the whole range of envisageable temperatures. In other words, the voltage to be selected is to be less than half the lowest current intensity that can be observed in the active mode over the range of expected junction temperatures.

17. The claimed subject-matter is clearly defined.

Inventive step (Article 56 EPC)

18. Document D3 discloses (D3, Figure 1) a lighting device with a LED (diode 1), an adjustable electrical power supply (current source 2) for driving the LED, and a control device providing a control signal to the adjustable electrical power supply for driving the LED (cf. page 4, lines 7-11). The control device consists of measuring means, which combine a voltmeter 4 and a current meter 3, and of control means in the form of a signal processor 5 (page 2, line 24 - page 3, line 7). The measuring means are arranged to measure both the voltage applied through the LED and the electric current flowing therethrough.

19. The control means of D3 further comprise an information (temperature) retrieval means (page 3, line 25 - page 4, line 7; Figure 2) connected to the power supply control means, containing information on the control signal to be applied to the power supply as a function of the measured parameters. In D3, the power supply control means is constructed to retrieve the control signal from the information retrieval means on the basis of the determined current and voltage (page 3, line 25 - page 4, line 7). The electric current flowing through the LED, combined with the measured voltage, is indicative of the temperature and resistance of the LED junction (Figure 2).

20. D3 shares a common purpose with the claimed invention. It relies on the forward bias voltage/current characteristic to provide an indication of the LED junction temperature during a measuring phase (D3, paragraph bridging pages 1 and 2; page 4, lines 7-20; Figure 2). It is a valid starting point on the basis of which the existence of inventive step may be assessed.

21. However, the claimed subject-matter differs from the lighting device known from D3 in more respects than recognised in the impugned decision.

22. The measuring means of D3 do not incorporate any additional measurement voltage source that is used to measure the current at a predetermined measuring voltage. It is stressed, in this respect, that the voltmeter 4 in Figure 1 of D3 cannot be equated with a voltage source, as the Examining Division held. As underlined in the statement of grounds, the current source 2 constitutes the only source of power available in the context of D3. There is also no mention, in D3, of any switching means for selectively connecting the LED to measuring means or to the adjustable power supply. In the context of D3, the measurement is carried out by appropriate control of the sole power supply, by reducing the voltage or current it delivers (Figure 2, page 4, lines 15-20).

23. A further difference between claimed subject-matter and the circuit of D3 is that the measuring voltage is smaller than the forward driving voltage of said LED and is such that it causes the current through the LED which is less than half of the lowest driving current provided to the LED by the adjustable electrical power supply in the active mode. While D3 explicitly mentions a lower voltage or current during the measuring phase as compared to the active mode, it does not provide any concrete information as to how much lower.

24. By using a separate voltage source for measurement, which is designed for better performance when measuring, while the electrical power supply for driving the LED in active mode can be designed for better performance in that mode or for lower cost (cf. published application, page 4, lines 8-14), the claimed lighting device provides more precise measurements as to the voltage and current values and accordingly also as to the resistance or temperature values that are derived therefrom. As a result, the claimed lighting device allows a more precise control of the adjustable power supply in the active mode.

25. Moreover, the claimed device allows measurements at measuring voltages below the forward driving voltage of said LED, thus permitting the measurement of resistance or related quantities in the small-signal domain, that is, in conditions where self-heating of the junction is small, thus giving a high calibration accuracy without the need for high speed measurement circuitry (cf. published application, page 3, lines 27-34). A further effect appears to be that the resistance of the LED junction is much higher than in the active mode (page 3, lines 32-34), thus further contributing to more precise measurements, since the resistance to be measurement is proportionally increased compared to the other resistances in the measuring circuit.

26. The objective problem to be solved starting from document D3 consists thus in providing more accurate control of the lighting circuit.

27. D3 acknowledges that measurements carried out with high drive currents may lead to inaccurate results (cf. page 4, lines 11-20). In D3, the solution to this problem consists in temporarily adjusting the forward current or voltage to a lower level for the purpose of measurement. The solution envisaged in D3 relies on the same effect of limiting the self-heating of the diode junction as the claimed invention.

28. There is, however, no hint in D3 to further improve the accuracy of said measures by using separate measuring means comprising a non-adjustable voltage source of a higher precision than the adjustable power supply means and switching between said measuring means and adjustable power supply.

29. In the impugned decision, reference was made to D2, which was considered to disclose the claimed solution with switching between active and measurement modes with one LED array being switched off (see impugned decision, point 5.7).

30. While it is acknowledged that D2 shares a common purpose with the claimed invention in that it proposes to stabilise the output of a group of LED arrays by appropriate control of an adjustable power supply, it does not disclose or even hint at the claimed solution.

31. In order to stabilise both the intensity and colour temperature provided by a plurality of red, green and blue LED arrays, a control mechanism is provided in D2 that compensates for operating temperatures, age, and manufacturing variations affecting the lighting device (page 2, lines 3-13).

32. The compensation process is carried out on the basis of measurements obtained from an optical sensor and a temperature sensor. The optical sensor (a photo-diode) delivers an indication of the light intensity resulting from the total light output generated by the three LED arrays and the ambient light. The temperature sensor includes a thermistor, or a thermopile, or any silicon-based sensor. It is configured to measure the case temperature on the basis of which the junction temperature of the LEDs is estimated (page 5, lines 4-11).

33. There is no suggestion in D2 to switch between an adjustable power supply and measuring means consisting of a voltage source and a current meter in combination in order to carry out precise measurements of the junction temperature (or resistance) of a LED. The passage referred to by the Examining Division on page 4, line 27 - page 5, line 3, merely describes how the optical sensor is controlled in order to measure the output of one single LED light source (red, green or blue). In a first step, the total output of light generated by the three light sources and ambient light is measured. In a second step, a LED source (in fact the LED array producing a certain colour) is switched off and the resulting total output is measured. It reflects the contribution of the two other Led arrays and ambient light. The difference between the two measurements is then calculated. It yields the light output from the LED light source array which is switched off.

34. The mere reference to some switching operation, in the passage referred above, is not sufficient to establish that the skilled person would have envisaged switching between two power sources.

35. As to the temperature measuring means of D2, not only do they not correspond to the definition of the measuring means in claim 1, but they also do not form part of a circuit that is switched on during the measurement phases, as required by the claim.

36. The Board cannot identify any hint in D2 suggesting that a more precise control can be achieved by providing measuring means as defined in claim 1.

37. While it is acknowledged that the skilled person in the field of diodes undoubtedly knows about switches, it is stressed that the claimed invention is not limited to the sole provision of switching means. General knowledge is not sufficient, in itself, to suggest that the objective problem of the invention can be solved by the provision of dedicated measuring means as defined in claim 1.

38. For these reasons, the subject-matter of claim 1 does not result in an obvious manner from the prior art. It is thus inventive in the sense of Article 56 EPC.