WO2012016716A1

WO2012016716A1 - Led dimming control

Info

Publication number: WO2012016716A1
Application number: PCT/EP2011/003965
Authority: WO
Inventors: Ian Wilson
Original assignee: Tridonic Uk Ltd.
Priority date: 2010-08-06
Filing date: 2011-08-08
Publication date: 2012-02-09
Also published as: GB201304407D0; GB2497043A; GB2497043B

Abstract

Method for LED dimming control, where a dimmable LED module is connected to a dimmer, where the dimmable LED module contains a bleeding circuit (6), characterized by a pulsed application of a bleeding current, where the pulsed application of a bleeding current is only applied over parts of a mains cycle or advantageously only over a certain number of mains half cycles, followed by a number of mains half cycles, where no bleeding current is applied.

Description

LED Dimming Control

The present invention relates to a method for LED dimming control, where a dimmable LED module is connected to a dimmer also known as phase fired controller.

Technical area

In order to save energy, today energy saving lamps are often used. With the introduction of efficient light emitting diodes (LED) as potential light sources there is also a demand for energy saving lamps using LED which have the ability to be dimmed. Such systems have to provide the functionality of a dimming interface,

particularly for the use with phase dimmers. There are several types of dimmers generally available. Those employing triac or thyristor devices operate in a very similar fashion. Both act as high-speed switches and in a dimmer are used to control the amount of electrical energy passing to a lamp. They do this by 'chopping' the sinusoidal mains voltage waveform. A trigger or firing pulse dictates at what point the device starts to

conduct. The later the device is fired the later it starts to conduct and hence less power is

transmitted to the load.

State of the art

Various circuit arrangements have been proposed which shall add the functionality of dimming to LED based light sources . One example would be the EP 1016062 Bl, where LEDs can be controlled by means of a digital bus, e.g. by DMX.

The main problem is the compatibility of a common triac dimmer in use with a LED driver circuit and to provide a dimming signal according to the control of the phase dimmer. So it is the main focus of the invention to provide a method and a circuitry to solve this problem. Description of the invention

The present invention proposes an improved solution for dimming LEDs based on a phase cut manipulation of an AC supply voltage of a operating means for one or more LEDs, OLEDs or any other lighting means which are comparable as to their electrical characteristics.

The present invention seeks to provide a further

switching arrangement allowing dimming via normal GLS phase cut dimmers (e.g. by triac or thyristor) of an LED driver with low power losses.

This object is achieved by means of the features of the independent claims. The dependent claims develop further the central idea of the invention.

According to the present invention, there is provided a method for LED dimming control,

where a dimmable LED module is connected to a dimmer, where the dimmable LED module contains a bleeding circuit which acts as a load for the phase dimmer holding

current , characterized by a pulsed application of a bleeding current, where the pulsed application of a bleeding current is only applied over parts of a mains cycle or advantageously only over a certain number of mains half cycles, followed by a number of mains half cycles, where no bleeding current is applied.

The invention also relates to a dimmable LED module, the module being designed for being dimmed using a dimmer controlling ^' a phase cut of an AC supply voltage supplied to the LED module, the module comprising:

a bleeding circuit for selectively acting to draw a bleeding current depending the phase cut present in the AC supply voltage,

characterized by a pulsed application of a bleeding current, where the pulsed application of a bleeding current is only applied over parts of a mains cycle or advantageously only over a certain number of mains half cycles, followed by a number of mains half cycles, where no bleeding current is applied.

Another aspect of the invention relates to a dimmable LED module ,

the module being designed for being dimmed using a dimmer controlling a phase cut of an AC supply voltage supplied to the LED module, the module comprising:

- a bleeding circuit for selectively acting to draw a bleeding current,

- a control circuit selectively activating the bleeding circuit, the control circuit being supplied with a signal indicating the activity of the bleeding circuit, . the control circuit determining, based on the bleeding activity indication signal, a value representing the phase cut present in the AC supply voltage and issuing a control signal as a function of the phase cut value, and - at least one driver circuit being supplied with said control signal and adjusting the power supplied associated LED lighting means.

The bleeding activity indication signal may indicate directly or indirectly one of the bleeding current or a voltage across resistive means of the bleeding circuit, such as e.g. a resistive means measuring the combined effect of the supply voltage and the activity of the bleeding circuit . The bleeding circuit may be a circuit separate to the control circuit or a circuit which is integrated part of the control circuit.

The bleeding current may be monitored and thus detected by the control circuit as a bleeding activity indication signal and the control circuit determines the pulse width of the bleeding activity indication signal.

The bleeding activity indication signal may be a pulse signal .

The control circuit may determine the pulse width of the bleeding activity indication signal. The bleeding current may be actively PWM controlled.

The bleeding activity indication signal may be produced for every cycle of the mains voltage or periodically, i.e. not for all mains voltage cycles.

The bleeding circuit may comprise a current source and the bleeding circuit may comprise a timed logic actively controlling a switch of the bleeding circuit.

The control circuit may be designed to obtain, based on the bleeding circuit indication signal, the timing of the zero-crossings of the AC supply voltage as well as the timing of any phase cut of the AC supply voltage.

A further aspect relates to a LED module, wherein LED module comprises a bleeding circuit and a driver circuit controlling the power of the LED lighting means via one or more of :

low frequency pulse modulated control such as PWM control, the pulse modulated pulses having a frequency in the order of twice the frequency of the AC supply voltage, preferably with a frequency between 90 and 140 Hz, the pulse modulated pulses being preferably synchronous with the bleeding activity indication signal, high frequency pulse modulated control such as PWM control, the pulse modulated pulses having a frequency of more than 200Hz, preferably more than 500Hz, and/or

- a control of an amplitude of a DC current through the LED lighting means.

The supply potential of the LED lighting means may be isolated from the supply voltage by isolating means, such as e.g. an opto-coupler , wherein the control circuit may be connected to the potential of the primary side or the secondary side of the isolating means.

The invention also relates to a retrofit LED lamp, comprising a LED module as described above.

A further aspect of the invention relates to a method for operating a dimmable LED module, the module being dimmed using a dimmer controlling the phase of an AC supply voltage supplied to the LED module, such as e.g. a dimmer having a triac, wherein:

- a bleeding circuit of the LED module selectively acting draws a bleeding current,

- a control circuit selectively activating the bleeding circuit, the control circuit is supplied with a signal indicating the activity of the bleeding circuit and issues a control signal depending on the bleeding activity indication signal, and

at least one driver circuit is supplied with said control signal and adjusts the power supplied associated LED lighting means.

The bleeding activity indication signal indicates directly or indirectly one of the bleeding current or a voltage across resistive means of the bleeding circuit, such as e.g. a resistive means measuring the combined effect of the supply voltage and the activity of the bleeding circuit. The bleeding activity indication signal is a pulse signal and the control circuit determines the pulse width of the bleeding activity indication signal. Based on the evaluation of the pulse width, the control circuit may determine the timing of the zero crossings of the AC supply voltage.

The control circuit may synchronize the measurement of any phase cut angle present in the supply voltage based on the timing of the zero crossing.

The bleeding current may be constant, varying or pulsed, especially by actively PW controlling a switch of the bleeding circuit.

The bleeding circuit may be activated passively, or the bleeding circuit controlling actively, e.g. via a switch.

The main reasons to incorporate a bleeding circuit are as follows :

♦ Provide the minimum holding current for the triacs used in leading edge dimmers during periods where the load current is low or not continuous.

♦ Provide damping for components inside both the dimmer & driver circuits to prevent ringing that could cause the triac to switch off when a leading edge dimmer is switched on. ♦ To provide a minimum load current to the dimmer in order that the output voltage of the dimmer and therefore the input signal detected by the driver, accurately follow the phase cut signal.

♦ To enable detection of the phase dimming information necessary to define a given dimming operating point.

A major focus of this application is towards the last of the cited items (to enable detection of the phase dimming information necessary to define a given dimming operating point) . The aim is to use a bleeding system in order to detect the phase dimmer firing angle for both leading and trailing edge dimmer applications. It is advantageous to maintain high driver efficiency by limiting the bleeding power. To provide the necessary bleeding current continuously would result in high power dissipation, large bleeding components and increased ambient temperature around the driver electronics reducing lifetime and reliability.

Further features, advantages and objects of the present invention will become evident from the following detained description of preferred embodiments of the invention, when taken in conjunction with the figures of the enclosed drawings .

Description of the preferred embodiments

As an aid to understanding the invention, a preferred embodiment thereof will now be described, by way of example only and not in any limitative sense, with

reference to the accompanying drawings, in which:

Figure 1 shows a first embodiment of a dimmable ballast for LED lighting means;

Figure 2 shows a second embodiment of a dimmable ballast for LED lighting means; Figure 3 illustrates the output signal of a leading edge dimmer and the corresponding bleeding current detection signal at R_Shunt;

Figure 4 shows a preferred embodiment of a bleeding circuit arrangement of the invention;

Figure 5 shows another embodiment of a bleeding circuit arrangement of the invention.

With reference to Fig. 1 now a first embodiment of a dimmable ballast for LED lighting means will be described. According to this first embodiment a secondary side control (regulation) for the LED power is proposed.

Note that 'primary side' and 'secondary side' relates to the primary side and secondary side, respectively, of an isolating means separating the potential of the LED lighting means from the supply voltage, as will be explained later on in the detail. As can be seen from Fig. 1, a AC supply voltage 1, such as for example a AC mains voltage of a frequency of 50Hz or 60Hz and a RMS voltage of 120V or 230V, is supplied to an input filter 2 of the ballast.

As schematically shown in Fig. 1, the AC supply voltage may be phase-cut, e.g. by a dimmer operated manually by a user, the dimmer comprising a Triac or Thyristor for the phase-cut. The time duration of the phase cut represents a dimming command .

The output signal, i.e. the filtered AC supply voltage of the input filter 2 is then supplied both to a first rectifier 3 and a second rectifier 4.

The first rectifier 3 is provided in order to transmit the power to the LED lighting means 5.

The second rectifier 4 is provided in order to feed a bleeding circuit 6 and a dimming circuit/interface 7.

Note that it is also possible to provide only a single rectifier both for the power transmission to the LED lighting means 5 and for the bleeding circuit 6.

The output of the first rectifier 3, i.e. the filtered and rectified AC supply voltage, can be optionally supplied to

- a valley-fill circuitry 8 (e.g. an active valley fill circuitry, a passive valley fill circuitry or a combined active-passive valley fill) , - an actively switched PFC circuit 9 (wherein a switch of the PFC circuit is controlled by a control circuit receiving at least one input signal) , or

- a filter circuit 10 comprising e.g. an electrolyte capacitor.

The thus processed rectified and filtered supply voltage is then supplied to the LED driver 11 having a DC/DC converter such as for example a flyback converter 12, especially a quasi-resonant flyback converter. Note that other isolated or non-isolated DC/DC converters can be used. The isolation can also be outside the DC/DC converter. The dimmable LED module may also be nonisolated, as it may comprise a non- isolated buck- converter as DC/DC converter.

The LED driver 11 supplies the LED lighting means 5 with a regulated (feedback-controlled) power. As schematically shown in Fig. 1, the LED lighting means 5 can present a plurality of LEDs (or OLEDs, or other lighting means with comparable electrical characteristics) can be connected in series and/or in parallel .

A feedback signal 13 indicating e.g. the current trough the LED lighting means is fed back to the LED driver 11. The LED driver in the shown example, being a flyback converter 12, has a primary side switch 14. By adapting the clocking of the switch 14 the power supplied to the LED lighting means can be controlled such that the measured value of the feedback signal 13 is controlled to be close if not identical to a nominal value .

It may be possible to set the LED via calibration and to accept an open loop method to run the LED . The modification of the electrical power supplied to the LED lighting means could be done by change of amplitude of the LED current or just by burst operation, according to phase dimmer signal derived by the bleeding circuitry.

Generally speaking, the Dc/DC converter has at least one control input for modifying the electrical power supplied to the LED lighting means 5.

The potential of the LED lighting means 5 may be galvanically isolated from the AC supply voltage 1. In the shown example, this isolation is actually implemented by means of the transformer 15 of the AC/DC converter 12.

The current control e.g. via the switch 14 of the DC/DC converter 12 is performed as controlled by an output signal 16 of the dimming circuit/interface 7. The dimming circuit/interface 7 thus supplies, via the signal 16, a nominal value for the LED lighting means power.

A control circuit 17 actually drives e.g. the switch 14 of the LED driver 11 depending on the measured feedback signal 13 and the controlled (nominal value) signal 16 of the dimming circuit/interface 7. As schematically shown via the reference numeral 18, the dimming circuit/interface 7 is isolated e.g. by an optocoupler 18 from the potential of the LED lighting means 5 , as in the present example the dimming circuit/interface 7 is not isolated from the AC supply voltage 1. As it may not be necessary to isolate the dimming circuit/interface 7 from the potential of the LED lighting means 5, the dimming circuit/interface 7 could also be directly linked to the secondary side, e.g. the the LED driver 11.

The embodiment of fig. 1 is called a secondary side regulation as the power (or current) control of the LED lighting means 5 is performed on the secondary side of the isolating means 15.

The alternative embodiment of fig.2 implements the concept of a primary side regulation. Again, the AC supply voltage is supplied to an input filter 2 and then to a burst rectifier 3 in a second rectifier 4. The output of the first rectifier 3 is again, in this embodiment, is directly supplied to the LED driver 11. Preferably, also the LED driver 11 has isolating means 15 such as for example the illustrated transformer.

The output of the second rectifier 4 is again both forwarded to a bleeding circuit 6 and a dimming control circuit/interface 7. It is to be understood that the term "interface" refers to the fact that this circuit can receive external dimming signals from a bus, wirelessly (e.g. IR) , etc., which external input is schematically designated to the reference numeral 19 (External control interface) . This obviously also applies to the embodiment of fig. 1.

In the embodiment of fig. 2, as^' a primary side regulation is carried out, the dimming circuit/interface 7 controls the LED driver 11 e.g. by driving the switch 14 on the primary side of the converter 11.

For the primary side regulation (fig. 2) or a secondary side regulation (fig.l) different options to modulate the power of the LED lighting means 5 can be used.

A first example is the modulation of the DC level of the current through the LED lighting means 5.

A second option is a high frequency pulse-modulated control (e.g. PWM control), wherein "high frequency" is to be understood that the resulting current pulses through the LED lighting means 5 have a frequency which is higher than the frequency of the rectified AC input voltage. In the case of an AC mains voltage supply, thus the high frequency pulse-modulated pulses, e.g. PWM pulses, will have a frequency of more than 120 Hz. A third option is pulse-modulated control (e.g. PWM control) of the power dissipated by the LED lighting means 5, wherein "low frequency" is to be understood that the low frequency pulse-modulated pulses of the current through the LED 5 a frequency in the order of the rectified supply voltage, e.g. 100 Hz or 120 Hz. If a primary side regulation is carried out, the dimming circuit/interface 7 may control the LED driver 11 e.g. by driving the switch 14 on the primary side of the converter 11.

The dimming circuit/interface 7 may be also connected to additional sensing means. It may be connected to an External control interface 19. The External control interface 19 could be connected to a PIR sensor, a photocell or additional sensors. Thus the External control interface 19 can be used for a presence detection or ambient light intensity detection. By the combination of the dimming control via the mains by a phase-cut signal and control over the External control interface 19 the dimmable ballast for LED lighting means may be controlled both manually by a user (through the dimmer 1) and automatically by the additional sensing means (through the External control interface 19) . Figure 4 shows a preferred embodiment of the bleeding circuit 6 arrangement of the invention. The bleeding circuit 6 is supplied by the filtered AC supply voltage, which is rectified (diode bridge 3) before being supplied to the bleeding circuit 6. The time logic unit 21 shown in the fig. 4 (and similar in fig. 5) may be part of the control circuit 17.

In the example of figure 4 a digital signal generated by a time logic unit 21 can switch Tl in order to apply a bleeding current set via the V_BE of T2 at typically

between 20 mA up to 30mA. The time logic unit 21 (e.g. a microcontroller or ASIC) controls the bleeding application and senses the resulting current inside

Rshunt (by the input pin Input) .

In a preferred embodiment of the invention the shown bleeding circuit is a part of a LED driver circuit, e.g. as a dimmable ballast for LED lighting means, i.e. as part of a LED lamp for use with normal GLS phase dimmers, preferably phase cut dimmers such as leading edge and trailing edge dimmers, e.g. using a triac or a thyristor. The bleeding circuit is connected to the mains input of the LED driver circuit. For instance it can be arranged after the mains filter 2 of the LED driver circuit or in parallel to the mains rectifier 3 of the LED driver circuit. The bleeding circuit 6 may comprise a constant current source (Tl, T2) .

One feature of the invention is the pulsed application of a high enough bleeding current, in order to reach the dimmer holding & latching current, but the pulsed

application of a high enough bleeding current may be only applied periodic, meaning it is only applied over some mains half cycles to keep the dissipation under control . This means, that the periodic application of a high enough bleeding current takes only place for a certain number of mains half cycles, followed by a number of mains half cycles, where no bleeding current is applied. In other words, the bleeding circuit 6 is used in a kind of burst mode. This burst mode is characterized by an periodic application of a high enough bleeding current only for a certain number of mains half cycles, followed by a number of mains half cycles, where no bleeding current is applied. Preferably the number of mains cycles, where the bleeding - current is applied, is lower than the number of mains cycles, where no bleeding current is applied. Bleeding control can be done by a time logic unit 21 (control unit U6) , preferably by means of a

microcontroller or an ASIC can also be used.

By the use of the periodic application of a high enough bleeding current an improved detection of both leading & lagging edge dimmer signals can be achieved and through this achievement a larger support of the dimmer (triac) can be reached. As an intelligence is required to differentiate between the different bleeding circuit current timing signals, the time logic unit 21 (control unit U6) , preferably a microcontroller, can be used in order to monitor the mains input (for detection the dimming signal provided by the dimmer, e.g. triac) and to control the periodic application of a high enough bleeding current.

The detection of the dimmer signal is on the bleeding current signal (over Rshunt) .

The invention can offer an improved mains detection and dimming linearity due to the given circuit intelligence. As there is a time logic unit 21 (e.g. a microcontroller) available, a digital or analogue dimming signal can be automatically produced via the time logic unit 21 (e.g. the microcontroller) . The invention enables very high input pulse currents at some phase angles .

A delay in adjustment of LED current may appear because of lower sampling time.

The time logic unit 21 as an output which preferably through a digital signal, can switch a transistor Tl in order to activate a bleeding current flowing through a first resistor R_series the transistor Tl (when switched on) and a measurement shunt R_Shunt ·

Via the base-emitter voltage V_BE of a second transistor T₂ the amplitude of the bleeding current can be controlled, e.g. to a range of between 10-50mA, preferably 20-30mA.

The time logic unit 21 can be e.g. a microcontroller, an ASIC, or an hybrid thereof. The time logic unit 21 senses the resulting bleeding current at an input pin of the time logic unit 21.

Typically, the bleeding current is activated by the timed logic unit for periods in which the input voltage (supply voltage) is low and e.g. has an amplitude of less than 30Vpk. Thus, the timed logic unit, once synchronized with the AC supply voltage, enables the bleeding circuit in these low voltage periods (around the zero crossings) and then detects the activity of the enabled bleeding circuit. Therefore, the current pulses will be detected at R_Sh_Un near the zero crossings of the AC supply voltage, and furthermore different bleeding current pulses will be sensed by R_Shunt at the input of the time logic unit when the dimmer (triac) in the manually operated dimmer switches on.

During the phase cut period of the AC supply voltage, the supply voltage amplitude will be low and the only current drawn through R_Shunt is the quiescence current necessary to supply (hold) the dimmer electronics, i.e. the electronics in the manually operated dimmer outside the LED lighting ballast. Note that in the shown example the bleeding activity is sensed by measuring the bleeding current, as especially timings and/or widths of the bleeding current pulses. The width serves as a discrimination criterion for pulses having differing causes. The bleeding current may detected by the control circuit (e.g. by the timed logic unit 21) as a bleeding activity indication signal and the control circuit determines the pulse width of the bleeding activity indication signal. For instance, the current through the R_Shunt may be sensed as the bleeding activity indication signal.

The activity of the bleeding circuit, however,

alternatively or even additionally can also be sensed on the mains voltage line. Fig. 3 shows an illustration of the trailing edge dimmer output signal (upper view, Vin) and the activation period indicated in the lower view. As mentioned before, a key feature of the invention is the pulsed application of a high enough bleeding current, where advantageously the pulsed application of a high enough bleeding current is only applied over enough mains half cycle to keep the dissipation under control. The pulsed application of a bleeding current may be over parts of a mains cycle or only over a certain number of mains half cycles, followed by a number of mains half cycles, where no bleeding current is applied. For example the periodic application of a bleeding

current may take place only for two mains half cycles, followed by a number of mains half cycles, where no bleeding current is applied. During the period of activation of the bleeding circuit 6 the holding current (IH) can be drawn by the dimmer, and the LED driver 11 can draw power (IL) from the AC supply 1. It shall be noted, that the bleeding circuit 6 can be activated repeatedly during the period of activation, the bleeding circuit 6 has not to be activated over the full period of activation, as pulsed or repeated activation of the bleeding circuit 6 may further reduce the power dissipation as described earlier. The activation of the bleeding circuit 6 does not mean, that the bleeding circuit 6 draws automatically a

bleeding current if activated, as the activity of the bleeding circuit 6 may depend as well from other

parameters, e.g. the amplitude of the mains voltage or the status of the dimmer. One example is that the

bleeding circuit 6 is acting to draw a bleeding current when the supply voltage amplitude is below a threshold value and the bleeding circuit 6 is activated by the time logic unit 21.

A trailing or leading edged detection algorithm can be divided in the LED ballast deriving the timing of the phase cut from the current pulse information by looking at the pulse width or the timing of the pulses to compute the timing of the zero crosses of the AC supply voltage as well as the operating frequency of the AC supply voltage. The narrower current pulses indicate the position of the phase cut.

This detection of the broad current pulses (bleeding current) and the narrow pulses (type of the phase cut by the triac or thyristor in the dimmer) , respectively, can be performed by the timed logic unit 21.

Thus, using the current information from the bleeding circuit 6 the phase cut timing of the dimmer can be detected. Thus, the detected phase cut can be used as a dim control information and can be "translated" in different manners (see above: modulation of DC current through the LED lighting means, high frequency pulse- modulated or low frequency pulse-modulated) . In the simplest manner, e.g. the low frequency pulse- modulated control of the LED lighting means 5, the operation of the DC/DC converter 11 is stopped during the detected phase cut.

In case a trailing edged detection algorithm shall be implemented there is the problem that when the dimmer switches off its MOSFET or equivalent there is often insufficient load on the dimmer to enable its output to follow the phase cut information and therefore bleeding current pulses will not occur if the voltage does not fall below 30 Vpk . In order to accurately detect the phase timing information the bleeding current can be activated using an active or adaptive method. An example would be that the bleeding current is activated continuously for one or several cycles of the AC supply voltage to enable the timing to be detected either via the voltage at the measurement shunt R_Shunt (when the dimmer switch is off) or via the supply input voltage signal which can be detected at the output of the bridge rectifier 3. The repetition rate for the bleeding current activation should be sufficient to quickly detect when the dimmer is altered but low enough to keep dissipation inside the bleeding circuit 6, 6' at an acceptable level.

According to an alternative embodiment, a pulsed signal, e.g. a PWM signal, with high frequency can be applied to the bleeding switch (fig. 4) in order to limit the power dissipation. This can be tracked across the voltage time period to detect the phase cut position. According to this invention, a bleeding circuit

activation can be used for one cycle of the AC supply voltage and the average voltage can be used to predict the dimmer information. This can e.g. be performed every 10 cycles to limit the power dissipation in the bleeding circuitry .

The activation of the bleeding circuit 6 could in addition be depending on the current out of the output of the first rectifier 3, i.e. flowing into the filter circuit 10 comprising an capacitor. The bleeding current would only be enabled when the current into the filter circuit 10 (or the actively switched PFC circuit 9 or the valley fill circuitry 8) would be below a given threshold.

Fig. 5 shows a further possible implementation of a bleeding circuit 6' according to the present invention. According to this embodiment the timed logic unit 21' activates the bleeding current outputting a PWM signal as one example for a pulse-modulated signal, which is filtered through a RC filter circuitry (CI, R2) to control e.g. a MOSFET switch Ml to the conducting state.

Again, resulting bleeding current is measured using a measurement shunt R_Shunt and is supplied to an input pin of the timed logic unit 21' . Inside figure 2 the bleeding current can be controlled actively using a pulse-modulated signal, e.g. PWM signal, and RC filter to control a MOSFET Ml inside its active region.

The examples of fig. 4 and fig. 5 show two possible implementations but many others would be possible as well.

A typical application would be to apply the bleeding current whenever the input voltage to the circuit is low (<30Vpk) . In that way it is possible to detect pulses at Rshunt near the zero cross of the mains (as shown in Figure 3.) and also when the triac switches on.. Inside the phase cut period the mains supply will be low and the only current drawn though Rshunt would be the quiescent current necessary to supply the dimmer electronics.

A possible leading edge detection algorithm can be the following: When using a leading edge dimmer it is usually enough to derive the timing of the phase cut from the pulse information by looking at the width or the pulses to derive the zero cross and operating frequency while the narrow pulses show the position of the phase cut. The logic block (timed logic unit 21) can then apply a control signal to the feedback or feed forward circuit in order to set the required load current according to a dimming curve .

A possible trailing edge detection algorithm can be the following: inside this case when the dimmer switches off its MOSFET or equivalent there is often insufficient load on the dimmer to enable its output to follow the phase cut information and therefore pulses will not occur if the voltage does not fall below 30Vpk. In order to accurately detect the phase timing information the bleeding current can be applied inside an active or adaptive method. An example could be that the bleeding is applied pulsed or continuously for one or several cycles of mains to enable the timing to be detected either via the low Rshunt voltages (when dimmers switch is off) or via the mains input voltage signal which can be detected at the output of the bridge rectifier.

As already mentioned, a periodic application of the bleeding current is possible. The repetition rate for this bleeding current should be sufficient to quickly detect when the dimmer is altered but low enough to keep dissipation inside the bleeding circuit at an acceptable level .

An advantageous method could be to apply a pulse- modulated signal such as a PWM signal at high frequency to the bleeding switch in order to limit dissipation.

This can be tracked across the mains period to detect the phase cut position. At that stage it could be applied in one position only to limit power and only look for dimmer changes .

In addition or as an alternative the bleeding current could be applied for one cycle and the average voltage could be used to predict dimmer information. This could occur every 10 cycles to limit the power inside the bleeding circuit. Different algorithms for phase detection using an active bleeding circuit can be applied.

Figures 1 and 2 show how the bleeding circuit can be employed in a converter and used to derive dimming information for control blocks on primary or secondary side of converter, as described above.

The inventive bleeding circuit is able to provide a bleeding current which can improve the performance of a driver in regards to dimming. The so called bleeding current can be constant, variable or pulsed, and must be able to assist the driver to reach the dimmer (leading or trailing edge) minimum specified currents (holding and latching) . In addition the bleeding current has to serve as a load to damp oscillations of the dimmer and the driver circuit of the LED, mainly on the switching on and off of the semiconductors of the dimmer (e.g. by waveform chopping) .

The bleeding circuit can provide a damping effect to damp oscillation of leading or trailing edge dimmers and driver circuit of the LED and thus improve the dimming performance .

The bleeding circuit can enable a correct measurement of the dimming phase angle which is applied by leading or trailing edge dimmers through the application of a

current to the input. This bleeding current may be

applied over parts of the mains cycles or even over one or more cycles of mains when the measurement could then be taken. The described procedure could either be applied periodically or at every cycle of mains.

The bleeding circuit can be formed by timed current source which can be applied in order to provide a

bleeding current as above described. The bleeding circuit shall apply a bleeding current to the input of the LED driver to enable proper phase dimming and timing

detection. The bleeding circuit may comprise a current source and may be passively activated (see fig. 4) , or the bleeding circuit may comprise a timed logic actively controlling a switch Ml of the bleeding circuit (see fig. 5) . The bleeding circuit can be able to apply a regulated input current to the input of a LED driver having as reference only a shunt resistor. The current can be applied via a timed logic unit and the bleeding circuit can potentially be able to regulate the bleeding current inside a range of levels which are required for the application .

The timed logic unit such as a microcontroller or ASIC can be used to detect the phase cut information and translate this into a suitable dimming signal.

The invention gives the ability of flexible dimming characteristics including detection of zero crossings (null points) and dimmer range according to the

adjustment angle and end stop values according to a suitable transfer function which may be implemented in the timed logic unit.

Claims

Claims :

1. Method for LED dimming control, where a dimmable LED module is connected to a dimmer, where the dimmable LED module contains a bleeding circuit (6) , characterized by a pulsed application of a bleeding current, where the pulsed application of a bleeding current is only applied over parts of a mains cycle.

2. Method according to claim 1, characterized that the dimmer signal is detected during the pulsed application of a bleeding current and is used as a dimming

information for the LED driver circuit (11) contained in the dimmable LED module .

3. Method according to claim 2, characterized that dimming information is also used as dimming information for the LED driver circuit during the period where no bleeding current is applied.

4. Method according to claim 2 or 3 , characterized that dimming information is at least temporary stored in a memory .

5. Method according to claim one of the claims 2 to 4 , characterized that dimming information is used for an adjustment of the LED current.

6. A dimmable LED module,

a bleeding circuit (6) for selectively acting to draw a bleeding current depending the phase cut present in the AC supply voltage,

characterized by a pulsed application of a bleeding current, where the periodic application of a bleeding current is only applied over parts of a mains cycle.

7. The LED module of claim 6,

characterized that the dimmer signal is detected during the pulsed application of a bleeding current and is used as a dimming information for the LED driver circuit (11) .

8. The LED module of any of the claims 6 or 7 ,

wherein the bleeding circuit is a circuit separate to the control circuit or a circuit which is integrated part of the control circuit .

9. The LED module of any of the claims 6, 7 or 8 ,

wherein the pulsed application of the bleeding current is only applied over a certain number of mains half cycles, followed by. a number of mains half cycles, where no bleeding current is applied.

10. The LED module of any of the claims 6, 7, 8 or 9 , wherein the pulsed application of the bleeding current is detected by the control circuit as a bleeding activity indication signal and the control circuit determines the pulse width of the bleeding activity indication signal.

11. The LED module of any of the claims 6 to 10,

wherein the bleeding current is actively PWM controlled.

12. The LED module of any of the claims 6 to 11,

wherein the bleeding circuit comprises a current source and is passively activated, or the bleeding circuit comprises a timed logic actively controlling a switch of the bleeding circuit.

13. The LED module of any of the claims 6 to 12,

wherein the control circuit is designed to obtain, based on the bleeding activity indication signal, the timing of the zero-crossings of the AC supply voltage as well as the timing of any phase cut of the AC supply voltage.