FIELD OF THE INVENTION

The present invention relates in general to the field of lighting. BACKGROUND OF THE INVENTION

For the purpose of illumination, for instance in residential houses, it has been known for a long time to use incandescent lamps that are supplied by a mains circuit; in Europe, the mains circuit typically carries 230 V AC at 50 Hz. A big problem with incandescent lamps is the fact that they convert only a small portion of the available electrical energy into light energy: much energy is consumed and wasted in the form of heat. Thus, there has been and still is a development towards using more efficient lamps, for instance gas discharge lamps but particularly solid state lamps such as LEDs. It even is desirable to replace incandescent lamps by solid state lamps in an existing situation. Solid state light sources such as LEDs need to be driven by an electronic driver, which receive the mains supply voltage and generate an output lamp current. This may be a separate device, but LED lamp units have conveniently been developed in which the LED light source and electronic driver are integrated. Particularly, the present invention relates to such integrated solid state lamps units capable of replacing existing incandescent lamps.

Incandescent lamps have a nominal rating, for instance 60 Watt, 100 Watt, etc, which corresponds to the lamp current when driven by mains voltage, and which corresponds to a certain amount of light emitted by the lamp. Likewise, LEDs have a nominal rating, corresponding to a nominal lamp current. In some circumstances, it is desirable to be able to reduce the amount of light emitted by the same lamp. For this purpose, dimmers have been developed. For the case of LEDs, the electronic driver has a dim function, in which the output lamp current is regulated. Also, specifically with a view to incandescent lamps, electronic mains dimmers have been developed, operating on the basis of phase cutting. Since phase cutting dimmers are commonly known, a discussion thereof will be omitted here. It is noted that such dimmers can be implemented as wall-mounted dimmers, so that a lamp supply voltage would always be a "dimmed" supply voltage. A complication exists if a dimmable LED lamp unit is to be coupled to an outlet supplied by such mains dimmer, for instance because a dimmed incandescent lamp is to be replaced by a dimmable LED lamp unit. In a dimmable LED lamp unit, the driver expects to be supplied by normal mains voltage and to receive a control signal indicating the desired dim level, and the driver outputs a regulated lamp current in response to such control signal. Such driver would now receive phase-cut mains voltage as supply. So on the one hand the electronic LED driver needs to function properly when receiving phase-cut mains voltage as supply. On the other hand, the phase-cut mains voltage contains dim information, in the form of the phase angle, relating to the dim level desired by the user, and the electronic LED dimmer needs to be capable to use this information as user control input, to read this dim information, and to regulate the output current for the LED lamp accordingly. Such dimmers are known per se.

SUMMARY OF THE INVENTION

While electronic dimmers for LED lamps are known that are capable of operating properly when receiving phase-cut mains voltage as supply, as mentioned above, it is a practical problem that there are several types of mains dimmers, and the LED dimmer does not know in advance which mains dimmer it is going to be connected to. Further, even between mains dimmers of the same type, different specimens can have different properties due to manufacture tolerances, and even the properties of one single specimen may vary over time caused for instance by ageing or varying temperature. One important property of a mains dimmer in this respect is the variation range of the phase angle: when the user actuation knob is in the position "minimum", it may very well be that the phase angle has a finite value c MiN larger than zero, and when the user actuation knob is in the position

"maximum", it may very well be that the phase angle has a value c MAx smaller than 180°. Even small variations in these values c MiN and c MAx may lead to noticeable differences in the minimum light output and maximum light output of the LED lamp unit concerned.

The present invention aims to solve this inconvenience.

To this end, a driver for a solid state lamp according to the present invention is capable of operating in a learning mode, in which it will detect the minimum phase angle cpMiN and the maximum phase angle c MAx of the supply voltage, and in which it will adapt its own settings so that its own minimum output power is set when the input voltage has the minimum phase angle cpMiN and its own maximum output power is set when the input voltage has the maximum phase angle c MAx- Further advantageous elaborations are mentioned in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects, features and advantages of the present invention will be further explained by the following description of one or more preferred embodiments with reference to the drawings, in which same reference numerals indicate same or similar parts, and in which:

Figures 1A and IB are block diagrams schematically illustrating examples of an illumination set up with a wall-mounted mains dimmer;

Figure 2 is a diagram schematically illustrating more details of a lamp armature;

Figure 3A is a block diagram schematically illustrating a LED lamp unit; Figure 3B schematically illustrates a possible physical embodiment of a LED lamp unit, capable of replacing an in candescent light bulb;

Figure 4A is a graph schematically illustrating a relationship between phase cutting angle and dim level;

Figure 4B is a graph comparable to figure 4A, when the relationship has been optimized according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 A is a block diagram schematically illustrating one possible example of an illumination set up with a wall-mounted mains dimmer 1, and figure IB is a similar block diagram of another example. In both cases, the dimmer 1 receives AC mains (Europe: 230V @ 50Hz) as an input voltage and outputs phase cut AC as an output voltage. A user can control the dimmer 1, for instance by rotating a control knob 2, which will cause the dimmer 1 to change the setting of the phase where the AC voltage is cut. In the example of figure 1 A, there is a mains socket 3 connected to the dimmer output, and a lamp armature 20 is provided with an electrical cord 23 terminating in a connector 24 to be plugged into the mains socket 3. In the set up of figure IB, the lamp armature 20 is connected to the dimmer output directly. In both cases, the armature 20 is supplied through the electronic dimmer 1, i.e. it only receives phase cut AC voltage, indicated as PCACV. Figure 2 illustrates, that the armature 20 may comprise a lamp socket 21 for mechanically receiving and holding a lamp foot 12 of a light bulb 10, and for electrically connecting the lamp foot 12 to wiring 22 of the armature 20.

Figure 3A is a block diagram schematically illustrating an LED lamp unit 30, comprising at least one solid state lamp 31, for instance a LED light source element, and an LED driver 32 having supply input terminals 33, 34 for receiving mains voltage and having output terminals 35, 36 for providing output LED current to the LED light source element 31.

Figure 3B schematically illustrates a preferred physical implementation of an LED lamp unit 30 according to the present invention, comprising a first housing part 37 housing the driver electronics circuitry 32 and designed for cooperating with an armature's lamp socket 21, and a second housing part 38 accommodating the one or more LED light source elements.

When the LED lamp unit 30 is mounted in the armature 20, the LED driver 32 receives phase cut AC voltage PCACV at its supply input terminals 33, 34. This lamp driver 32, while designed for receiving full AC voltage as explained above, should be capable of operating in a dimming mode when receiving the phase-cut AC voltage PCACV. In this dimming mode, the LED driver 32 is designed to provide dimming facility such as to dim the associated LEDs, i.e. it provides a suitable output voltage or current for its associated LEDs in response to the phase angle of the input supply voltage.

Typically, solid state lamps should be driven at a certain nominal lamp current, even when being dimmed, and this is typically implemented by using the so-called duty cycle method: at a relatively high frequency, higher than noticeable by the human eye, the lamp is switched on and off, and the average light output is determined by the ratio of the duration of the on periods to the off periods. Since such duty cycle method is known per se, a further explanation will be omitted here. Further, electronic dimmers capable of the above describe functions are known per se, so a more detailed explanation of design and operation of such dimmers is omitted here.

It is important to realize that the electronic dimmer 32 is designed as a dedicated dimmer for the associated lamp 31, so that its output voltage and/or output current meet the requirements of the specific lamp 31. In the following, it will be assumed that the lamp 31 has a certain nominal rating corresponding to operation at a constant current of a certain nominal current magnitude. Although it would be possible to increase the lamp current somewhat above this nominal current level, this nominal design current rating will be taken as a 100% level and will be indicated as IN. Likewise, the corresponding nominal lamp power will be indicated as PN, and the corresponding light output will be indicated as L _N .

When dimming the lamp 31 , the output voltage and current should be such that the actual light output LA is lower than the nominal light output L . The ratio LA/L will be indicated as the dim level β. Ideally, β can be varied between 0 and 1. In practice, it may be that there is a certain practical minimum dim level PMiN>0and that there is a certain practical maximum dim level PMAX<1 -

It is important to realize that the electronic dimmer 32 is designed to calculate the actual dim level β for the lamp 31 , and, corresponding therewith, the settings for lamp voltage and/or lamp current, on the basis of the dim information in the phase-cut AC input voltage PCACV. This dim information is typically available in the form of a phase angle, which will be indicated as φ, and theoretically this phase angle φ can vary between 0° and 180°, referring to the AC mains period. Thus, the electronic dimmer 21 is provided with a function F to calculate the output dimming level β from the received input information φ. This can be expressed as follows:

P = F(q>)

The function can be provided in several ways. For instance, the function may be provided as a formula, or as a look-up table. The precise details of this function may depend on the characteristics of the lamp L3. In an illustrative example, this function is a linear function. In any case, information defining this function is stored in a memory 39 of the dimmer 32.

Figure 4A is a graph with a curve 41 illustrating a possible relationship F between phase cut angle φ (horizontal axis) and output dimming level β (vertical axis) for an ideal case where the phase cut angle φ can be varied over the complete range from 0° to 180° and where the output dimming level β can be varied over the complete range from 0 to 1. In the illustrative case of a linear function, curve 41 is a straight line from point [0°, 0] to point [180°, 1]. In such case, function F can be written as F(cp) = α·φ, with a = 1/180°. Normally, however, the relationship is not linear, in order to take account of the sensitivity

characteristics of the human eye, as should be clear to a person skilled in the art.

In practice, however, it may be that the mains dimmer is only capable of varying the phase cut angle φ within a range from a minimum value c MiN to a maximum value cpMAx, which values are not known to the LED driver 32. Further, it may be that the LED driver 32 is only capable of varying the dimming level β within a range from a minimum value PMIN to a maximum value PMAX, which values are considered to be fixed and known to the driver.

According to the present invention, the LED driver 32 is capable of operating in a learning mode. Entering into this learning mode is done on the basis of a specific command code contained in the supply voltage received by the driver 32. Thus, according to the present invention, during normal operation, the driver 32 is designed to monitor the supply voltage received at its input terminals 33, 34 to detect the specific command code, and to enter its learning mode on detection of the specific command code. This monitoring may be done continuously or at predetermined time intervals. Suitably, the specific command code is a code that can be generated by a user through actuation of the mains dimmer 1. In order to prevent the driver 32 from erroneously entering its learning mode, said command actuation should preferably be of a type not normally done during normal use. In a possible implementation, the command actuation involves a rapid sequence of alternating maximum and minimum settings of the phase angle φ, for instance 3 cycles within 10 seconds.

The driver 32 may be designed to remain in its learning mode for a predetermined length of time, for instance 15 seconds since entering the learning mode. Alternatively, the driver 32 may be designed to exit its learning mode on detection of another specific command code in the supply voltage received at its input terminals 33, 34. In a conveniently simple embodiment, such command code involves the phase angle φ remaining constant for a predetermined length of time, for instance 15 seconds.

While operating in its learning mode, the driver 32 continues to monitor the supply voltage received at its input terminals 33, 34, and particularly to monitor the momentary value of the phase angle φ. The driver 32 detects the lowest value assumed by the phase angle φ, which will be stored in a first memory location as minimum cpMiN, and detects the highest value assumed by the phase angle φ, which will be stored in a second memory location as maximum c MAx- The driver 32 continuously compares the detected momentary value of the phase angle φ with the value c MiN in the first memory location, and replaces this value by said momentary value if lower. Likewise, the driver 32 continuously compares the detected momentary value of the phase angle φ with the value c MAx in the second memory location, and replaces this value by said momentary value if higher.

When exiting the learning mode, the driver 32 adapts the information in said memory 39 defining said function F, in such a way that F(cp _M iN) = PMIN and F(cp _M Ax) = PMAX- This optimized situation is illustrated by curve 42 in figure 4B. For sake of clarity, β _Μ ΐΝ is shown as being larger than 0 while PMAX is shown as being smaller than 1 , but it is noted that this is not intended to restrict the invention as it may be that PMIN = 0 and/or PMAX = 1 ·

In an exemplary implementation, the function F is a linear function defined by two coefficients C I and C2 according to the formula β = Cl -φ + C2. It should be clear to a person skilled in the art that in such case the two coefficients CI and C2 can be calculated as

CI = (PMAX-PMIN)/( ⁽ PMAX- ⁽ PMIN) and C2 = PMIN - Cl -cpMiN

If the memory 39 contains a look-up table, the new entries in the look-up table may be calculated using the above function F.

Normally, as mentioned before, the function F is a more complicated function, but it any case it should be clear to a person skilled in the art how such function can be scaled such as to obtain F(cp _M iN) = PMIN and F(cp _M Ax) = PMAX-

It is to be noted that the memory 39 preferably is a non- volatile memory, so that the proper function F matching the current mains dimmer 1 is maintained even when the armature 20 is disconnected, or when the mains power fails.

Summarizing, the present invention provides a driver 32 for a solid state lamp 31 receives phase-cut AV supply voltage PCACV. The lamp is operated at a dimmed light output L _A on the basis of the phase angle φ of the phase-cut AV supply voltage PCACV. The driver comprises a memory 39 containing information defining a function F.

In a normal mode, the driver monitors the supply voltage and calculates the momentary value of the dim factor β from the momentary value of the phase angle φ according to a formula

P = Fcp

In a learning mode, the driver detects the lowest value c MiN and the highest value c MAx assumed by the phase angle φ, and updates the said information in said memory 39 such that the dim factor will have its minimum value and maximum value PMIN, PMAX corresponding to the minimum value and maximum value cpMiN, ΦΜΑΧ of the phase angle φ, respectively.

As a result, the lamp driver always provides minimum output power when the mains voltage has the minimum phase angle c MiN and always provides maximum output power when the mains voltage has the maximum phase angle cpMAx-

While the invention has been illustrated and described in detail in the drawings and foregoing description, it should be clear to a person skilled in the art that such illustration and description are to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiments; rather, several variations and modifications are possible within the protective scope of the invention as defined in the appending claims. For instance, the dimmer may be associated with the lamp armature.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. 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. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

In the above, the present invention has been explained with reference to block diagrams, which illustrate functional blocks of the device according to the present invention. It is to be understood that one or more of these functional blocks may be implemented in hardware, where the function of such functional block is performed by individual hardware components, but it is also possible that one or more of these functional blocks are

implemented in software, so that the function of such functional block is performed by one or more program lines of a computer program or a programmable device such as a

microprocessor, microcontroller, digital signal processor, etc.