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 receives the mains supply voltage and generates 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.

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 an 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 accomodating the one or more LED light source elements.

A complication exists if a dimmable LED lamp unit 30 is to be coupled to an armature 20 supplied by such mains dimmer, for instance because a dimmed incandescent lamp is to be replaced by a dimmable LED lamp unit. In this dimmable LED lamp unit, the driver 32 would now receive phase-cut mains 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. 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, 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. SUMMARY OF THE INVENTION

A problem exists in the mains dimmer 1. Normally, dimmers are developed and specified for operation with a resistive load having a power higher than a certain minimum value (and lower than a certain maximum value), which minimum value may be in the range from 20 to 60 W. In other words, a dimmer needs to have a load higher than a certain value in order to operate properly. The reason is that the dimmer has a TRIAC which requires a certain minimum output current (HOLD current) in order to remain ON: if the output current momentarily drops belows this HOLD current, the TRIAC switches off and refiring may occur. If the dimmer is dimming, the output voltage (and hence the output current) is zero up to a certain phase angle, and then the TRIAC ignites and the full mains voltage is applied to the resistive load, leading to the nominal current being drawn from the dimmer, even if the power taken from the dimmer on average is reduced. In the case of an incandescent lamp, the temperature of the lamp spiral is lower with dimming, resulting in lower resistance and hence higher current consumption by the lamp, so the HOLD current requirement of the dimmer is satified even further.

An LED lamp providing the same luminous output has a much lower electrical rating, for instance a factor 5 lower, as compared to an incandescent lamp. Thus, a 60 W incandescent lamp will for instance be replaced by a 12 W LED lamp. Thus, the LED driver can not draw the same current from the dimmer as compared to the repplaced incandescent lamp. Either the LED driver draws constant current at a lower level, or the LED driver draws current pulses with a duty cycle determining the average current drawn (and hence determining the average power consumed). Anyway, the current drawn from the dimmer is lower than the TRIAC's HOLD current, and this problem may increase if the lamp is dimmed.

This problem is typically approached by providing the electronics circuitry 32 of the LED lamp unit 30 with a bleeder circuit. The bleeder circuit draws a current additional to the LED current, which represent an additional power load for the dimmer 1 on top of the power needed for driving the LEDs. Basically, this constitutes a waste of power, and this waste is increased when the LED unit is dimmed more. For an LED lamp unit 30 provided with a dedicated mains dimmer, it would be possible to optimize the concept, but in practice the lamp units must be capable of working with all sorts of dimmers. To avoid failure operations of the dimmers, LED lamp units tend to draw more bleeder current than actually necessary. International patent publication WO-2005/115058 discloses an LED lamp unit comprising:

a unit input capable of receiving dimmed mains voltage from a supplying mains dimmer;

a lamp driver having input terminals coupled to the unit input to receive the dimmed mains voltage and capable of converting the received dimmed mains voltage to a dimmed LED current;

at least one LED coupled to output terminals of the lamp driver; and a bleeder circuit coupled to the unit input for drawing a bleeder current from the received mains voltage;

wherein the bleeder circuit is an adaptive bleeder circuit comprising an intelligent control circuit.

In this lamp unit, the driver provides a control signal to the control circuit, and the control circuit controls the bleeder current depending on the amount of current drawn by the driver: the more current the driver draws, the less bleeder current is needed for a correct functioning of the dimmer. However, this device must be adapted to one specific type of dimmer, that is expected to supply the lamp unit, and can not take account of different dimmer characteristics, whether caused by tolerances or by different dimmer types. Further, the device does not guarantee a properly functioning dimmer in combination with a current consumption as low as possible: it may be that the current drawn from the dimmer is still to low, or it may be that the current drawn from the dimmer is unnecessarily high, which can easily be seen to happen if the dimmer is supplying a second load additional to the lamp unit.

An object of the present invention is to provide a solution to the above problems. Particularly, the present invention aims to provide an LED lamp unit of the above type in which the bleeder current is adaptively controlled to be just as high as needed for ensuring proper functioning of the dimmer.

To this end, the LED lamp unit according to the present invention comprises at least one sensor for sensing at least one signal received at the unit input, and for providing an output signal to the control circuit; the control circuit is designed to analyse the sensor output signal to determine a condition of the supplying mains dimmer; and the control circuit is designed to adaptively optimize the bleeder current in relation to the condition of the supplying mains dimmer.

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:

Fig. 1 A is a block diagram schematically illustrating one possible example of a wall-mounted mains dimmer;

Fig. IB is a block diagram schematically illustrating another possible example of a wall-mounted mains dimmer;

Fig. 2 schematically illustrates a lamp armature;

Fig. 3A schematically illustrates an LED lamp unit;

Fig. 3B schematically illustrates a preferred physical implementation of an LED lamp unit;

Fig. 4 is a block diagram schematically illustrating an LED lamp unit

according to the present invention;

Figs. 5A and 5B are graphs illustrating some signals in the LED lamp unit according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Figure 4 is a block diagram schematically illustrating an LED lamp unit 100 according to the present invention. This LED lamp unit 100 comprises at least one LED 31 and corresponding LED driver 32 as described above. The LED lamp unit 100 has input terminals 101, 102 for receiving dimmed mains, a net filter 110, and a rectifier 120. The output voltage of the rectifier 120, i.e. rectified dimmed mains, is supplied to the LED driver 32 via a diode 130 and smoothened by a buffer capacitor 140, which will typically be an electrolytic capacitor.

The LED lamp unit 100 comprises an adaptive bleeder circuit 200 connected in parallel to the input of the driver 32, the bleeder circuit comprising a series arrangement of a load (resistor) 210 and a controllable switch 220, here implemented as a FET, connected to the output of the rectifier 120. The adaptive bleeder circuit 200 further comprises a FET driver 230 and a control circuit 300 for controlling the FET driver 230. The FET driver 230 and the control circuit 300 may be integrated into one circuit. The control circuit 300 may be implemented as a microprocessor or the like. The control circuit 300 may be designed for driving the FET 220 directly. The control circuit 300 provides a control signal Sc for the controllable switch 220 or driver 230. The control signal Sc may be a digital signal, i.e. either HIGH or LOW, so that the controllable switch 220 is either conducting or non-conducting. The adaptive bleeder circuit 200 only draws bleeder current I _B when the controllable switch 220 is conducting, the instantaneous magnitude of the bleeder current I _B being determined by the series resistor 210. The control circuit 300 can vary the average bleeder current magnitude by varying the frequency and/or duty cycle of the control signal Sc.

The adaptive bleeder circuit 200 is provided with a current sensor 310 and a voltage sensor 320 for sensing the input current and (rectified) input voltage of the LED lamp unit 100, respectively. The current sensor 310 may be implemented in different ways, but is shown as a resistor in series with the rectifier 120 output. The voltage sensor 320 may be implemented in different ways, but is shown as a resistor in parallel to the rectifier 120 output. The current sensor 310 provides a measuring signal SI to a current input 301 of the control circuit 300. The voltage sensor 320 provides a measuring signal S2 to a voltage input 302 of the control circuit 300. The control circuit 300 is designed to generate its control signal Sc on the basis of the measuring signals SI and S2 received from the current sensor 310 and the voltage sensor 320. More particularly, the measuring signals SI and S2 allow the control circuit 300 to analyse the operational status of the dimmer that provides the dimmed mains at the input terminals 101, 102, as will be explained with reference to figures 5 A and 5B.

Figures 5 A and 5B are graphs illustrating some signals in the LED lamp unit according to the present invention, for an exemplary situation where a mains dimmer

(dimmer 1 in figure 1A) is a leading edge dimmer. In figure 5 A, curve 51 shows the control signal Sc of the control circuit 300. Starting at the beginning of a current half-period, this control signal Sc is high for a certain time period, followed by a burst of high frequency signal pulses, after which the control signal Sc returns to zero for the remainder of the current half-period. Curve 52 shows the current taken from the dimmer (measuring signal SI from current sensor 310), while curve 53 shows the rectified mains voltage (measuring signal S2 from voltage sensor 320). In the case of figure 5A, the burst of high frequency signal pulses is too short, so that the bleeder current is insufficient, hence the dimmer does not provide sufficient power which leads to an oscillation inside the dimmer below the triac hold-current, which is visible in the oscillations 54 in the current and voltage signals. Further, due to charging of the capacitor 140, a very high current peak 55 is observed. Figure 5B is a similar graph, for a situation where the burst of high frequency signal pulses is sufficiently long. Curve 56 shows the control signal Sc of the control circuit 300, curve 57 shows the current taken from the dimmer, and curve 58 shows the rectified mains voltage. It can be seen that the rectified mains voltage has a smooth sinusoidal shape, and that the current signal is also smooth, containing just one peak that indicates the firing of the dimmer's TRIAC: this indicates a well- functioning dimmer.

According to the present invention, the control circuit 300 is designed to adjust parameters of the control signal Sc. This adjustment is preferably done continuously (i.e. in each half-period of the mains voltage), or on a regular basis, for instance once per second or once per minute or once per hour, or on start-up, or after detecting a change in the dim level. The parameter to be adjusted may be pulse frequency or boost duration or pulse width or duty cycle, or any combination of two or more of these and possible other parameters. In an adjustment mode, the control circuit 300 may change the parameter(s) to be adjusted such as to reduce the bleeder current drawn from the mains dimmer, while monitoring the measuring signals SI and S2, until a detoriation in operation is detected, for instance resulting in high current peaks and/or oscillation phenomena and/or increase in the number of current peaks and/or increase in the number of voltage peaks (figure 5 A). As soon as this is detected, the control circuit 300 stops the reduction of the bleeder current magnitude, or even increases the (average) bleeder current slightly, just enough to restore normal operation (figure 5B). In such a way, it is assured that operation is always normal with the bleeder current always being as low as possible.

Specifically, if the control circuit 300 finds that the supplying mains dimmer 1 is not operating properly, it may increase the bleeder current by a first amount. If the control circuit 300 finds that the supplying mains dimmer 1 is operating properly, it may decrease the bleeder current by a second amount. The first amount is preferably higher than the second amount.

Specifically, the control circuit 300 may drive the controllable switch 220 in such a way such as to approach a work point where the bleeder current ¾ is minimal while the operation of a dimmer providing the dimmed mains voltage is free from failure and anomalies.

Specifically, the control circuit 300 may vary one or more parameters of the controllable switch 220, analyse the influence of the parameter variation on the operation of a dimmer providing the dimmed mains voltage, and select an operational value for the varied parameters to be equal to the parameter value corresponding to the lowest value bleeder current where measuring signals SI, S2 indicated correct dimmer operation. In a preferred embodiment, where the control circuit controls the controllable switch 220 with a control signal Sc comprising a burst of control pulses, the parameter varied and optimized by the control circuit 300 is burst duration and/or pulse frequency and/or pulse duty cycle.

Specifically, the control circuit 300 may analyse the received measuring signals SI, S2 to determine a sharp peak in the current and/or in the voltage received at the unit input, to count the number of such detected peaks in each half of the mains period, and to determine that the supplying mains dimmer is in a correct operating condition if the number of such detected peaks is equal to a predetermined number or that the supplying mains dimmer is in an incorrect operating condition if the number of such detected peaks is unequal to said predetermined number. For the current, the correct number of peaks is one, while the correct number of peaks for the voltage is equal to zero. Consequently, analysing the input current has the advantage that it is possible to detect a non- functioning dimmer by finding that the current signal does not show one peak. However, it is noted that in stead of analysing for peaks it is also possible to analyse for steep flanks or transients (the time-derivative is higher than a threshold level), and it can be seen that the voltage curve 58 in case of proper functioning shows one steep step coinciding with the one current peak, indicating firing of the TRIAC.

It is noted that the measuring signals SI and S2 may be sampled, converted to a digital signal, and analyzed by a software routine, for instance for detecting peaks and zero- crossings, such as to detect oscillations or any other signs of incorrect dimmer behaviour.

Thus, the present invention succeeds in providing an LED lamp unit with a bleeder circuit in which the bleeder current is adaptively optimized to the supplying dimmer and the set dim-level. As a result, the supplying dimmer functions properly while

simultaneously the amount of power loss is minimized. Further, the lamp unit is better compatible with more types of dimmer. And, last but not least, the power efficiency of the lamp unit is increased.

Summarizing, the present invention provides an LED lamp unit 100 comprising:

an LED lamp unit 100 comprises:

a unit input 101, 102 capable of receiving dimmed mains voltage PCACV from a supplying mains dimmer 1 ; a lamp driver 32 having input terminals 33, 34 coupled to the unit input to receive the dimmed mains voltage and capable of converting the received dimmed mains voltage to a dimmed LED current;

at least one LED 31 coupled to output terminals 35, 36 of the lamp driver 32; and a bleeder circuit 200 coupled to the unit input for drawing a bleeder current ½ from the received mains voltage.

The bleeder circuit 200 is an adaptive bleeder circuit comprising an intelligent control circuit 300, capable of determining a condition of the supplying mains dimmer.

The control circuit 300 adaptively optimizes the bleeder current in relation to the condition of the supplying mains dimmer.

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, a rectifier 120 is not essential to the invention.

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.