WO2016145646A1

WO2016145646A1 - Dimming method for led converter

Info

Publication number: WO2016145646A1
Application number: PCT/CN2015/074564
Authority: WO
Inventors: Egbert MAO; Yongchang LONG
Original assignee: Tridonic Gmbh & Co. Kg
Priority date: 2015-03-19
Filing date: 2015-03-19
Publication date: 2016-09-22
Also published as: GB201712379D0; GB2551286A

Abstract

A dimmable LED converter (1) includes at least one terminal for supplying power to at least one LED (22) and a control circuit (44) for controlling a switch of the LED converter (1) in order to modulate the power provided at the at least one terminal. An interface (34) is adapted to receive an analogue dimming signal (11), a dimming signal conversion circuit (38) converts a level of the received analog dimming signal (11) into a parameter of a pulsed signal (39) and an isolated signal transmission means (40) transmits the pulsed signal (39) across an isolation barrier (8) and supplies the transmitted pulsed signal (41) directly or indirectly to the control circuit (44).

Description

Dimming method for a LED converter

The present invention relates to the field of LED converters and in particular to the area of dimmable LED converters, methods of driving dimmable LEDs and the corresponding integrated circuits. In particular the invention is in the field of LED control with dimmable LED converters including integrated dimming signal interfaces.

Dimming of light sources offers the capability to adapt the characteristics of light emission to the needs of a user. For example the light intensity may be adapted or varied according to the settings of a dimming switch or a control unit offering the capability to control the light in theaters or architectural installations. Dimming of illuminants and in particular light emitting diodes (LED) offers also the capability to vary the color temperature of light over a given range by mixing the respectively dimmed light of multiple illuminants.

There exist different methods and a variety of various protocols for dimming a light source, for example by analogue signal with a DC voltage range from 1 to 10 V (0-10V lighting control) , by a standardized interface such as the DALI^TM standard for lighting control (IEC 62386) or any other of a plurality protocols which may be used in conjunction with Ethernet. Another conventional method for dimming a light source is phase control or phase cutting (leading or trailing edge dimming) , which represents a method of low freuency pulse width modulation (PWM) applied for power limiting of AC voltages.

Generally known operating devices for illuminants have galvanic isolation between a low-voltage area and a high-voltage area of the device.

Extra-low voltage (ELV) in electricity supply is one of several means to protect against electrical shock. In an ELV circuit, an electrical potential of any conductor against earth (ground) is not more than either 25 volts RMS (35 volts peak) for alternating current (AC current) , or ripple-free 60 volts for direct current (DC current) under dry conditions.

One of the three defined types of ELV systems being distinguished by their respective safety properties, a SELV system is defined as an electrical system in which the voltage cannot exceed ELV under normal conditions, and under single-fault conditions, including earth faults in other circuits.

The acronym: "SELV" stands for "separated extra-low voltage" in installation standards such as BS 7671 and for "safety extra-low voltage"in appliance standards, e.g. BS EN 60335.

A SELV circuit must have -inter alia-protective-separation, for example double insulation, reinforced insulation or protective screening from all circuits that might carry higher voltages, and simple separation from other SELV systems, and other system with lower voltages PELV systems and from earth (ground) .

Means to achieve the required safety of a SELV circuit is provided by the extra-low voltage, the low risk of accidental contact with a higher voltage, and a lack of a return path through earth (ground) that electric current could take in case of contact with a human body. Hence the design of a SELV circuit typically involves an isolating transformer and respective electrical isolation barriers (SELV barriers) .

In particular two-stage operating devices are known, in which a converter which supplies the illuminant is operated with potential isolation via a control circuit on the mains voltage side (primary side) of the potential isolation. If the converter also includes transmitting feedback signals from the secondary side of the potential isolation, in particular from the illuminant, to the control circuit, potential isolation is likewise required for the feedback. The secondary side of the potential isolation is controlled by a further control circuit, which is isolated from the first-mentioned control circuit, which is connected to the mains voltage potential.

An illuminant may be for example a light emitting diode (LED) . An operating device may be a LED converter adapted to driving at least one LED.

International application publication WO 2010/051984 A2 discloses an illuminant operating appliance with potential isolation. The operating appliance comprises a first converter, a second converter galvanically isolated from the first converter and supplying a load circuit with the illuminant, in particular a LED or a gas discharge lamp. A first logic circuit for controlling the first converter and a second control circuit controlling the second converter respectively are interconnected by an interface which separates the electric potentials of the first and second control circuits. The electrically isolated interface may be for example an integrated coil, an integrated air coil, an external transformer or a piezo transformer for maintaining the integrity of a SELV barrier.

Conventionally a buck stage is used in order to transfer a dimming signal through a SELV barrier.

Patent application publication EP 1 871 144 A1 discloses a LED driver arrangement including a switching mode stage to generate a drive current for feeding one more LEDs from a DC input. The switching mode stage may for example be implemented by a circuit topology functioning as a buck stage. A current sensor senses the drive current and generates a feedback signal indicative of the intensity of the drive current. The feedback signal is compared with a reference value indicative of a LED drive current corresponding to a LED brightness as requested by a user and a control signal is generated which is able to drive control circuitry in order to generate a drive current corresponding to the requested LED drive current in the switching mode stage. Galvanic isolation between the LED driver on one hand and the current sensor, reference value comparing units co-located with the LEDs is ensured by a respective internal layout of the switching mode stage on one hand, and by feeding the feedback signal via an optocoupler (photocoupler) to the control circuitry for controlling the switching mode stage.

However, the conventional solution requires transferring the analog dimming signal by means of an optocoupler. Moreover the control circuit is to be adapted to generate the PWM signal for driving the LEDs. Both characteristics are necessary to ensure galvanic separation by the SELV barrier, but using the optocoupler for transfer of analog signals does not lead to accurate information transfer.

Hence it is an object of the invention to provide a method capable of improving the dimmable LED converter as known from prior art and compensating for its deficiencies and a corresponding converter.

The invention solves the above problem by the dimmable LED converter, and the corresponding method and integrated circuit according to the independent claims.

The dimmable LED converter comprises at least one terminal for supplying power to at least one LED, a control circuit for controlling a switch of the LED converter in order to modulate the power provided at the at least one terminal. The dimmable LED converter further includes an interface for being supplied with an analogue dimming signal, a dimming signal conversion circuit for converting the level of the analog dimming signal into a parameter of a pulsed signal, and an isolated signal transmission means for transmitting the pulsed signal across an isolation barrier and for supplying the pulsed signal either directly or indirectly to the control circuit.

The dimmable LED converter provides the effect of dispensing with a buck stage and any specific circuit for secondary regulation of a LED supply signal. The need for highly specialized ICs is accordingly reduced and the cost of an implementation is low. For example a common IC such as the type ST HVLED815 can be employed for implementing the invention. The interface for the dimming signal and the dimmable LED converter can be designed with advantages in cost and performance characteristics.

The dimmable LED converter of a preferred embodiment may include the dimming signal conversion circuit for converting the level of the analogue dimming signal into a duty cycle of a pulse width modulated (PWM-) signal as the parameter of the pulsed signal. The duty cycle of a signal is to be understood as the percentage of a period of a periodic signal in which the periodic signal is active (ON) . The period of the periodic signal is a time which is necessary for the periodic signal to complete an ON-and-OFF cycle.

An embodiment of the dimmable LED converter further includes the isolated signal transmission means being configured to supply the pulsed signal indirectly to the control circuit after reconversion of the pulsed signal into a reconverted analogue signal. The reconversion of the pulse signal to a reconverted analogue signal may be performed by filtering the pulse signal and therefore the reconverted analogue signal output by the filter is smoothed.

A further embodiment includes the isolated signal transmission means being configured to supply the pulsed signal (39) directly to the control circuit. The control circuit then is specifically adapted to control a dimmable LED based on a digital signal such as the pulsed signal.

The dimming information may be represented by the duty cycle of the pulsed signal only. The filtering of the pulsed signal to generate an analogue signal results in a suppression of light flash which might be emitted by the illuminant when the illuminant is dimmed or faded.

A further advantageous embodiment of the dimmable LED converter comprises at least an optocoupler in the isolated signal transmission means. The isolated signal transfer means including an optocoupler enables to use only a single stage in the LED converter and to omit a buck stage for transferring the 1V to 10V signal to the primary side by an analogue signal, when the current setting reference is at the primary side. The optocoupler offers a change in current transfer ratio of 0, 2 to 0, 4 (200 ％ to 400 ％ ) for same analogue dimming signal. Thus the optopupler introduces robustness into the signal transfer from the secondary side to the primary side via the analogue signal, as it works with DC signals or slowly varying signals contrary to a transformer. The current transfer ratio (CTR) defines the percentage indicating the ratio of the output current of the optocoupler to the input current of the optocoupler and therefore a parameter similar to a current amplification of a transistor.

An embodiment of the invention relates to a method for controlling a dimmable LED converter, the method comprising steps of supplying power to at least one LED via at least one terminal of the dimmable LED converter, a step of controlling a switch of the LED converter in order to modulate the power provided at the at least one terminal and a step of supplying an interface with an analog dimming signal, a step of converting the level of the analogue dimming signal into an parameter of a pulsed signal, a step of transmitting the pulsed signal across an isolation barrier via an isolated signal transmission means, and a step of supplying the pulsed signal directly or indirectly to the control circuit.

In a preferred embodiment of the method for controlling a dimmable LED converter the parameter of the pulsed signal is a duty cycle of a PWM-signal.

In a further advantageous embodiment of the method for controlling a dimmable LED converter, in the step of supplying the pulsed signal, the pulsed signal is supplied indirectly after reconversion into an analogue signal to the control circuit.

A further aspect of the invention relates to a light, which preferably has an illuminant, in particular one or more LEDs and a LED converter of the type defined above.

A further aspect of the invention relates to an LED lamp, in particular an LED lamp for replacement of a conventional incandescent lamp, which includes a LED converter of the type defined above.

The inventive method and system are explained in more detail with reference to the attached figures, wherein

Figure 1 depicts a general schematic drawing of a dimmable LED converter,

Figure 2 depicts a schematic drawing of a dimmable LED converter according to prior art,

Figure 3 depicts a schematic drawing of a dimmable LED converter according to prior art,

Figure 4 depicts a schematic drawing of a dimmable LED converter according to an embodiment of the invention,

Figure 5 depicts a circuit diagram of a dimmable LED converter according to an embodiment of the invention, and

Figure 6 shows a flowchart depicting method steps of a method according to an embodiment of the invention.

In the figures same numerals denote the same elements and a description of the same elements is not necessarily repeated in the following description of the figures.

In Fig. 1 a dimmable LED converter 1 is shown in a general schematic drawing. The dimmable LED converter 1 includes a mains input means which provides an interface to the mains supply including a phase line 3.1 (L) , a neutral line 3.2 (N) and a protective earth line 3.3 (PE, ground) . The main input means 2 may include an electro-magnetic interference filter (EMI-filter) , additionally or alternatively other filters, a power factor correcting means (PFC means) . A power conversion means 4 receives mains signal 5.1, 5.2 from the mains input means 2. The power conversion means 4 is adapted to generate the drive signal 7.1, 7.2 for the constant current generation means 12 from the mains signal 5.1, 5.2. The power conversion means 4 includes a primary side (input) and a secondary side (output) . The power conversion means 4 is in particular adapted to separate the input signal on the primary side from the output signal on the secondary side by an electrically isolating SELV barrier 8.

In fig. 1 a control means 6 is shown which receives from a dimming signal receiving means 10 the received dimming signal 16. The dimming signal 11 received by the dimming signal receiving means 10 may be any dimming signal of the format 1 to 10V or any other dimming signal. The dimming signal may for example be a dimming signal with a voltage level between 1 V and +10 V or a voltage level between 1 V and -10 V, the voltage respectively measured in relation to a ground potential 13 (GND) . The dimming control means 6 converts the received dimming signal 16 into a dimming control signal 9 for controlling the power conversion means 8. The dimming control signal 9 is thereby fed to the power conversion means 4 on the primary side of the SELV barrier 8.

The power conversion means 9 outputs the drive signal 7.1, 7.2 on its secondary side including the dimming information received via the dimming signal 9 to a constant current generation means 12 which generates and outputs a constant current signals V₀ ⁺ and V₀ ^- with levels according to the received dimming information via its output terminals 14.1, 14.2 to an illuminant not shown in fig. 1.

The illuminant driven via the constant current signals V₀ ⁺ and V₀ ^- by the dimmable LED converter 1 may include one or more LEDs.

In fig. 2 in the upper portion an overview of a LED converter 20 according to prior art is presented. The upper portion shows a LED converter 20 which drives an illuminant 21. The illuminant 21 may include a plurality of LEDs 22 arranged in series and/or in parallel connection. The LED converter 20 drives the illuminant 21 with a current I. The level of the current I 24 may vary in accordance with a dimming signal between 1V and 10V. In particular as shown in fig. 2, upper portion, an operational amplifier 25 controlling the level of the output current I 24 is trimmed by a reference voltage derived from the dimming signal 1 to 10 V 11. However to achieve this dimming (fading) of the output current I 24, a secondary regulation on the secondary side of the LED converter 20 is required.

The lower portion of fig. 2 illustrates an additional buck stage 26 which is provided with the dimming signal 11 and which generates a PWM modulated current I 24 for driving the illuminant 21 from an input voltage V_IN based on the dimming signal 11. The additional buck stage 26 is required in order to avoid the transfer of the dimming signal 11 over the SELV barrier 8 which would interfere with the isolation requirements.

A further prior art solution is shown in fig. 3. A dimmable LED converter 27 is fed with mains supply AC voltage 29 and connected to earth on it primary side. The dimmable LED converter 27 comprises a first stage acting as a booster converter 30 and a second stage which functions as a flyback converter 31 (buck converter) . The second stage outputs a PWM signal at the output 32 for driving an illuminant 21. The solution presented in fig. 3is disadvantageous as an isolated optocoupler is required to transfer the analogue dimming signal 11 over the SELV barrier 8 from the secondary side of the dimmable LED converter to the control IC 33 being arranged on the primary side of the SELV barrier 8.

In fig. 4 a schematic drawing of a dimmable LED converter according to an embodiment of the invention is shown.

An analogue dimming signal 11 is received at a terminal from a dimming signal generating means 34. The dimming signal 11 carries dimming information. This dimming information may be included in a voltage level of the analogue dimming signal 11. The dimming information is an information about a light intensity of one or more LED 22.

A constant current source 36 generates a constant current signal 37 to the terminal. At the terminal a voltage signal from the analogue dimming signal 11 and the constant current signal is generated. The analogue-to-PWM-conversion means 38 functions as a waveform shaping circuit and generates a PWM signal 39 from the voltage signal. The PWM signal 39 is characterized by including the dimming information contained in the analogue dimming signal 1 to 10 V 11 in at least one of the parameters of the PWM signal 39.

The parameter of the PWM signal 39 which contains the dimming information is in a preferred embodiment of the invention a duty cycle of the PWM signal 39. Using a PWM signal 39 with the duty cycle transmitting the dimming information is advantageous, as the characteristic current transfer ratio (CTR) of an optocoupler 40 introduces a significant robustness when transferring an analogue signal from its input to its output. Hence it is advantageous to transfer the dimming information being modulated in the duty cycle over the optocoupler 40.

The optocoupler 40 is used to transfer the dimming information within the duty cycle of the PWM signal 39 over the SELV barrier 8 isolating the primary side of the dimmable ELD converter 1 from a secondary side of the dimmable LED converter 1.

The transferred PWM signal 41 is then on the priamry side of the converter fed to a filter means 42. The filter means 42 filters the PWM signal 41 and forms an analogue signal 43 from the PWM signal 41. The analogue signal 43 is fed to a control means 44 in order to generate an output current 45 being controlled by the original dimming information regenerated by filtering the PWM signal 41. The output current signal 45 is used to supply the illuminant 21, in particular the at least one LED 22 of the illuminant 21.

The output signal 45 may in one embodiment be used to pull down the level at a control pin of the LED driver circuit. Hence a cost effective solution may be implemented by using an integrated circuit such as a primary sensing offline LED driver circuit. An exemplary embodiment may use for this purpose an IC of the type HVLED815PF enabling a high power factor with primary sensing regulation (PSR) . A specialized IC is therefore not needed and the cost of the implementation is advantageously reduced.

Furthermore the inventive solution renders a second regulation redundant and therefore simplifies circuit design and the number of required circuit elements leading to reduced circuit complexity and manufacturing cost. The space requirements are also reduced and therefore the inventive solution is particularly advantageous for use in drivers for bulb replacement lamps. A specialized buck stage is also not required in the proposed solution to the problem.

Fig. 5 provides a circuit diagram of a dimmable LED converter according to an embodiment of the invention. For the function of the subassemblies reference the description fig. 4 is made.

A dimming signal 11 is received at a terminal. The constant current generation means 36 provides a constant current to the 1 to 10 V terminal to generate a constant voltage signal depending on the externally supplied dimming signal 11. The constant voltage signal is supplied to a operational amplifier X3 46 of the dimming signal conversion circuit 38 which changes the waveform from an analogue signal to a PWM signal 39 which comprises the dimming information in a parameter. In the depicted embodiment the parameter of the pulsed signal is the duty cycle which carries the dimming information. The pulsed signal 39 is passed on to the optocoupler 40 and in the optocoupler 40 transferred over the isolation barrier 8 (SELV barrier) not shown in fig. 5. The filter means 42 includes circuitry to reconvert the transferred pulsed signal 41 to an analogue signal 43. The analogue signal 43 is then transferred to a control means 44. The reconverted analogue signal 43 contains the dimming information in its analogue signal level and is used to control the light intensity of the LED 22 or the illuminant 21.

An alternative embodiment of the invention omits the filter means 42 and feeds the pulsed signal to a suitably adapted control means 44 for controlling the illuminant 21.

The control means 45 may comprise a primary side regulator which is provided with the reconverted analogue signal. The reconverted analogue signal is then used in a power conversion stage in a switched isolated converter with a switch on its primary side for generating a drive current for the illuminant 21 according to the dimming information of the dimming signal 11.

The pulsed signal 41 may in a further embodiment be supplied as a digital signal to a primary side regulator. The primary side regulator in this case is adapted to receive a digitally encoded signal and to process the digitally encoded signal in order to generate a corresponding drive signal.

Fig. 6 provides a flowchart depicting method steps of a method for controlling a dimmable LED converter according to an embodiment of the invention. The method comprises a first step S1 of supplying power to at least one LED via at least one terminal. In subsequent step S2 a control means controls switching of the converter in order to modulate the power provided at the at least one terminal. The modulation of the power results in a respective light intensity being emitted by the at least one LED.

In a step S3 an interface is supplied with an analogue dimming signal. The analogue dimming signal is provided to a dimming signal conversion circuit for converting the level of the analogue dimming signal into a parameter of a pulsed signal in step S4. The parameter of the pulsed signal may be a duty cycle of the pulsed signal. The pulsed signal may be a PWM–signal carrying the dimming information of the dimming signal in the duty cycle of the PWM-signal.

In step S4 following to step S3, the t the pulsed signal is transmitted across an isolation barrier 8 via an isolated signal transmission means 46. The isolation transmitting means 46 may be an optocoupler 46 in an advantageous embodiment of the inventive method. The optocoupler 46 is particularly adapted to transfer a signal over an isolation barrier (SELV barrier) 8.

The optocoupler 46 provides at its secondary side the transferred pulsed signal 41 to a control circuit 44 in step S5. The pulsed signal 41 may be in one aspect of the inventive method be supplied directly to the control circuit 46.

In another aspect of the method the pulsed signal 41 is reconverted into an analogue signal 45 after transfer to the secondary side of the optocoupler 46. The reconverted analogue signal may contain for example the dimming information in an amplitude level of the reconverted analogue signal.

In summary, according to the present inventive dimmable LED converter, the corresponding method and integrated circuit a most efficient solution is presented.

By way of example, the invention allows to design a dimmable LED converter 1 for a LED 22 to be constructed in a very compact manner. Because of its compact design and the potential isolation provided, this dimmable LED 1 converter can be integrated well in an illuminant, a LED lamp, particularly a LED lamp for replacing a conventional incandescent lamp or an energy-saving lamp based on a fluorescent lamp. Given this application due to the compact design, the dimmable LED converter 1 can be arranged in the lamp cap of the LED lighting means or illuminant 21.

All single features of the discussed embodiments described and illustrated in the accompanying figures just by way of example, may also be combined in an appropriate and advantageous manner without prejudice to the underlying principles of the invention as defined in the appending claims.

Claims

Dimmable LED converter, comprising:

at least one terminal for supplying power to at least one LED (22) ,

a control circuit (44) for controlling a switch of the LED converter in order to modulate the power provided at the at least one terminal,

an interface (34) for being suppl ied with an analog dimming signal (11) ,

a dimming signal conversion circuit (38) for converting the level of the analogue dimming signal (11) into a parameter of a pulsed signal (39) , , and

an isolated signal transmission means (40) for transmitting the pulsed signal (39) across an isolation barrier (8) and for supplying the transmitted pulsed signal (41) directly or indirectly, to the control circuit (44) .
Dimmable LED converter according to claim 1,

characterized in

that the dimming signal conversion circuit (40) is configured to convert the level of the analogue dimming signal (11) into a duty cycle of a PWM-signal as the parameter of the pulsed signal (39) .
Dimmable LED converter according to any one of claims 1 and 2,

characterized in

that the dimmable LED converter comprises a filter means (42) configured to reconvert he transmitted pulsed signal (41) into a reconverted analogue signal (43) , and

that the isolated signal transmission means (40) is configured to supply the pulsed signal (39) indirectly after reconversion into a reconverted analogue signal (43) to the control circuit (44) .
Dimmable LED converter according to any one of claims 1 to 3,

characterized in

that the isolated signal transmission means (40) is configured to supply the pulsed signal (39) directly to the control circuit (44) .
Dimmable LED converter according to any one of claims 1 to 4,

characterized in

that the isolated signal transmission means (40) is an optocoupler.
Method for controlling a dimmable LED converter (1) , the method comprising steps of

supplying power to at least one LED (22) via at least one terminal (S1) ,

controlling a switch of the dimmable LED converter (1) in order to modulate the power provided at the at least one terminal (S2) ,

supplying an interface means (34) with an analogue dimming signal (11) ,

converting a level of the analogue dimming signal (11) into an parameter of a pulsed signal (39) (S3) ,

transmitting the pulsed signal (39) across an isolation barrier (8) via an isolated signal transmission means (40) (S4) , and

supplying the transmitted pulsed signal (41) directly or indirectly to the control circuit (44) (S5) .
Method for controlling a dimmable LED converter according to claim 6,

characterized in

that the parameter of the pulsed signal (39) is a duty cycle of a PWM-signal.
Method for controlling a dimmable LED converter according to any one of claims 6 and 7,

characterized in

that in the step of supplying the pulsed signal (S5) , the transmitted pulsed signal (41) is supplied indirectly after reconversion into an reconverted analogue signal (43) to the control circuit (44) .
Method for controlling a dimmable LED converter according to any one of claims 6 to 8,

characterized in

that in the step of supplying the pulsed signal (S5) , the transmitted pulsed signal (41) is supplied directly to the control circuit (44) .