EP3269207B1

EP3269207B1 - Isolated interface with current transformer

Info

Publication number: EP3269207B1
Application number: EP15884241.9A
Authority: EP
Inventors: Quentin LIN
Original assignee: Tridonic GmbH and Co KG
Current assignee: Tridonic GmbH and Co KG
Priority date: 2015-03-11
Filing date: 2015-03-11
Publication date: 2020-05-06
Anticipated expiration: 2035-03-11
Also published as: EP3269207A4; EP3269207A1; WO2016141558A1

Description

The present invention relates to the field of driving dimmable illuminants and in particular to the field of dimmable LED converters, to luminaries equipped with LED converters and the respective interfaces.
Dimming of an illuminant (light source) offers the capability to adapt the characteristics of light emission to the intentions 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 light in theaters or other architectural installations. Dimming of illuminants and in particular of light emitting diodes (LED) offer 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 plurality of different protocols for dimming a light source, for example by an analogue signal with a DC voltage range from 1V to 10 V (0-10V lighting control), by a standardized interface such as the DALI™ 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 (PFC) or phase cutting, which basically corresponds to a method of pulse width modulation (PWM) applied for power limiting of an AC voltage.
For lighting applications safety requirements concerning insulation of electrical components and entire lighting systems in order to protect against electrical shock have to be maintained. The international electrical commission (IEC) and its member organizations define limits for the electrical potential of any conductor against earth (ground) which electrical circuits have to comply with.
Generally known operating devices providing power supply to illuminants thus often have galvanic isolation between a low-voltage area and a high-voltage area of the device. Publication US 2014/327366 discloses a system comprising an illuminant and dimmable converter, and having galvanic isolation.
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.
Being one of the 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 systems 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 for light sources are known, in which a converter which drives the illuminant as a light emitting element 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 a feedback signal 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 line transmitting the feedback signal. 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) or a high intensity discharge lamp (HDI). 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 by a SELV barrier and supplying a load circuit with the illuminant. 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 include 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 or an optocoupler 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 or 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 an 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) from the interface side receiving a dimming signal to the control circuitry for controlling the switching mode stage at the regulating side.
However, the conventional solution requires transferring the analog dimming signal by means of an optocoupler. If however a dimmer with a dimming interface for a 1V to 10V dimming signal and an AC supply input interface which is only basically insulated against the dimming interface is connected with the conventional dimmable converter, the insulation of the dimmable converter is also impaired. The lighting system including the dimmer, the dimmable converter and the illuminant as whole then violates the requirement of double insulation.
Hence it is an object of the invention to improve the dimmable converter as known from prior art in particular with respect to its combination with available dimmers and compensating for its deficiencies.
The invention solves the above problem by the dimmable converter according to the independent claim.
A dimmable converter for driving an illuminant according to the invention comprises a primary side switched isolated power converter stage. A switch is arranged on a primary side of the isolated power converter stage and a control circuit is configured to control an on/off switching of the switch. The control circuit is arranged either on the primary side or on a secondary side of the isolated power converter stage. Further an interface for being supplied with a dimming signal, wherein the interface is isolated both against the primary side and the secondary side of the isolated power converter stage. The interface comprises an interface control circuit which is supplied with a current signal transmitted from the secondary side of the isolated power converter stage, wherein the current signal indicates a current flow on the secondary side of the isolated power converter stage, and the current signal is transmitted using an isolated signal transmission means. The interface control circuit generates a control signal transmitted to the control circuit of the isolated power converter stage.
The solution provides the advantage of being compliant with different kinds of dimmer providing the dimming information and achieving electric safety independent from the dimmer for the dimmable converter. The inventive approach maintains the dimmable converter's SELV compliant output or double-insulation. This offers additional benefits in luminary design while maintaining safety regulations. The selection range of dimmer units for use with the dimmable converter is broadened without needing to make concessions in the matter of safety.
The proposed solution offers furthermore independence from the circuit topology in the dimmable converter, for example buck topology, LLC resonant, etc.... Even more, the solution offers good linearity for the dimming curve realized by the dimmable converter as will be shown with respect to the embodiment.
The dimmable converter according to a preferred embodiment comprises a further isolated transmission means configured to transmit the control signal to the control means.
A further advantageous embodiment of the dimmable converter is characterized by the isolated transmission means and/or further isolated transmission means including either a transformer and/or an optocoupler.
Furthermore the dimmable converter has the isolated power converter stage including a compensating means configured to compensate non-linear characteristics of the optocoupler and/or aging effects of the optocoupler. The compensating means may form part of the interface, the primary side of the isolated power converter stage or the secondary side of the isolated power converter stage.
The dimmable converter according to an embodiment has the isolated signal transmission means being arranged for transmitting the current signal from the secondary side of the power converter stage to the interface.
In another embodiment of the dimmable converter, the further isolated signal transmission means is arranged for transmitting the control signal from the interface to the primary side of the isolated power converter stage.
The dimmable converter of an embodiment shows the current signal indicating a current flow driving the illuminant. In particular the current signal may indicate a current value of the current flow on the secondary side of the isolated power converter stage driving the illuminant.
In an embodiment of the dimmable converter according to the invention the interface control circuit is configured to further obtain a dimming signal received by the interface.
The dimmable converter according to an embodiment has the control circuit of the isolated power converter stage being configured to regulate the current flow to the illuminant in a feedback loop including at least a part of the primary side of the isolated power converter stage at a preset value, when the control circuit receives no control signal from the interface control circuit.
The problem is further solved by a light (luminary) comprising an illuminant and adapted for replacing a conventional illuminant, wherein the light comprises the dimmable converter according to any one of the embodiments. The conventional illuminant is for example an incandescent or halogen light source. The conventional illuminant may be driven via an existing dimmer switch. Employing the inventive dimmable converter layout enables replacing the conventional illuminant without needing to take the insulation, e.g. only basic insulation of the existing dimmer unit into account. The isolated interface stage of the inventive converter maintains even in this case the required safety level.
The dimmable converter with an isolated interface is explained in more detail with reference to the attached figures, wherein

Figure 1 depicts a schematic drawing of a first embodiment,
Figure 2 depicts a schematic drawing of a second embodiment, and
Figure 3 shows a circuit diagram of a conventional converter circuit.

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 an embodiment of a dimmable converter is shown. The dimmable converter 1 is without restricting the generality depicted as driving a LED as an illuminant 14. The LED 14 is supplied with a DC current via the output lines V₀ ⁺ and V₀ ^- 15.1, 15.2.
The dimmable LED converter 1 is connected to mains supply with phase line L 3.1, neutral line N 3.2 and protective earth (ground) line PE 3.3.
The dimmable LED converter further receives a dimming signal via dimming signal line 17.1. In the depicted case the dimming signal 17.1 is an analogue signal against ground level 17.2 in the form of a 0V to 10V dimming signal. The dimming signal carries dimming information defining a desired level of light to be emitted by an illuminant 14.
The embodiment separates the dimmable LED converter 1 into three substages which are each electrically isolated against each other. The dimmable converter 1 comprises as a first substage a primary side 4 of the converter stage for receiving a mains supply 3.1, 3.2, 3.3. The second substage is represented by a secondary side 6 of the converter stage for supplying the LED drive current 15.1, 15.2. The third substage of the dimmable LED converter 1 is the interface 8 for receiving the dimming signal 17.1, 17.2.
The first, second and third substages of the dimmable LED converter are each isolated against each other by the isolation barriers (SELV barriers) 18.1, 18.2.
The primary side 4 of the power converter stage includes a mains input unit 2 for receiving the mains supply L, N and protective earth PE. The mains supply transfers the input signal to the mains rectification and filter unit 10 via lines 5.1, 5.2. The mains rectification and filter unit 10 rectifies the received AC voltage, filters undesired signal components and provides a DC signal via lines 7.1, 7.2 to the power conversion means 12. The power conversion means 12 is for example implemented by a buck converter circuit topology.
The power conversion means 12 internally provides galvanic isolation between the input and the output of the power conversion means 12.
The power conversion means 12 outputs for example a pulse width modulated (PWM-) signal 11 to an output rectification and filter unit 12 arranged on the secondary side 6 of the power converter stage. The output rectification and filter unit 12 provides the drive current to the illuminant 14. Further the pulse width (PWM-) signal 11 is transferred via the path over signal lines 11 - 13 - 19 back to the isolated transmission means 16.
The interface 8 of the embodiment may be adapted for every kind of present or future dimming protocol without departing from the invention. The invention can be used in any approach for a main controlling unit dimming (MCU dimming), for wireless dimming protocols, in dimming protocols basing of pulse phase modulation for dimming signal transfer, for analogue dimming signals as depicted in figs. 1 and 2 as well as for digital dimming signals. Typical digital dimming signals may be compliant with a 1V-10V signal according to norm IEC 60929, or a 0V to 10V signal compliant with standard ANSI E1.3 - 2001 (R2006).
The further isolated transmission means 16 can be implemented as a current transformer. A current transformer is a measurement device connected in series and designed to provide a current signal in a secondary coil proportional to the current flowing in its primary coil. A current transformer as an isolated transmission means 16 includes a primary coil and secondary coil, the secondary coil also being referenced as detection winding. The current transformer is basic, double or higher insulated, depending on the application. The detection winding of the current transformer is for example connected to the output winding of the flyback transformer as a power conversion stage, or to a secondary winding of an LLC output. The detection winding may be one or more windings and outputs the current signal which includes information reflecting the current output by the isolated power converting stage.
Current transformers are commonly used for metering purposes and as protective relays. The current transformer employed in the isolated transmission means 16 in the present embodiment serves the purpose of galvanically isolating the secondary side 6 of the power converting stage on one hand and the interface 8 on the other hand, while simultaneously generating and outputting the current signal 19 to the interface control circuit 20 in the interface 8. The current signal indicates a current flow on the secondary side 6 of the power converter stage and carries information on the actual current value provided via lines 15.1, 15.2 to the illuminant 14. The current signal is fed via lines 21, 23 to an interface control circuit 20.
The interface 8 receives as external input a dimming signal via dimming input lines 17.1, 17.2 and feeds the dimming signal to the interface control circuit 20. The interface control circuit 20 is also provided with the current signal via lines 21, 23. Thus the interface control circuit 20 has the necessary signals available to generate a control signal and supply the control signal via line 25 to a further isolated transmission means 22.
The further isolated transmission means 22 provides the control signal to the power conversion control circuit 24.
The arrangement of the further isolated transmission means 22 and the power conversion control circuit 24 with respect to the galvanically isolated first and second substages 4, 6 denotes the key difference between the embodiment depicted in fig. 1 and the embodiment depicted in fig. 2.
The power conversion control circuit 24 of fig. 1 controls the power conversion unit 12 such that current driving the illuminant 14 is controlled to a value suitable to generate a desired light intensity to be emitted by the illuminant and indicated by the dimming signal received via inputs 17.1, 17.2 by the dimmable converter.
If the power conversion unit 12 includes a buck converter, this is implemented by the power conversion unit 12 controlling a switch S1 of the buck converter and thereby setting an output current of the power conversion unit 12 accordingly.
The further isolated transmission means 22 is in an embodiment implemented by using an optocoupler (photocoupler, optoisolator). An optocoupler is an optoelectric circuit component that transfers electric signals between two isolated circuit by using light. Optocouplers prevent the transfer of high voltages and thereby avoid affecting a system receiving a signal by maintaining galvanic isolation between input and output of the optocoupler.
A common type of an optocoupler consists of an LED as a source (emitter) of light and a phototransistor in an opaque package. Usually an optocoupler transfers a digital signal (ON/OFF), but some are capable to transfer an analogue signal. The LED, for example a near infrared light-emitting diode, converts an electric input signal into light, a closed optical channel (dielectrical channel) transfers the light to a photosensitive element, the photosensitive element detects the incoming light and either generates electric energy directly, or modulates an electric current flowing from an external power supply by the received incoming light.
The power conversion control circuit 24' of fig. 2 also controls the power conversion unit 12 such that current driving the illuminant 14 is controlled to a value suitable to generate a desired light intensity to be emitted by the illuminant and indicated by the dimming signal received via inputs 17.1, 17.2 by the dimmable converter. However the power conversion control circuit 24' is arranged at the secondary side 6 of the power conversion stage. Accordingly the further transmission means 22' for transferring the control signal received in the interface 8 via line 25 transfers the control signal to the secondary side 6 of the power conversion stage. The power conversion control circuit 24' arranged on the secondary side 6 of the power conversion stage and the power conversion unit 12' may be configured as a secondary side controlled power conversion unit, contrary to the power conversion regulation in the shown in fig. 1.
Both variants of isolated transmission elements, transformers and optocouplers discussed above are efficient in breaking ground loops caused by high or noisy return currents in signal lines. The optocoupler is even suited to transfer DC signals or slowly varying signals.
In an embodiment the control circuit 24, 24' of the isolated power converter stage regulates the current flow to the illuminant 14 at a preset current value in a feedback loop including at least a part of the primary side 4 of the isolated power converter stage, when the control circuit 24, 24' receives no control signal from the interface control circuit 20.
The interface control circuit 20 can be analog, a digital or a hybrid circuit. The interface control circuit 20 is in one embodiment adapted to control the power regulation loop to regulate the current to the illuminant 14. Another embodiment controls the current to the illuminant by the control unit 24, 24'.
Fig. 3 shows an existing solution for the problem solved by present application and starting from the existing solution of fig. 3, additional benefits of the invention are discussed.
Generally for many applications in luminary design the converter requires either SELV output or a double insulation from input to output. If a conventional LED converter includes an interface which is directly connected to the output, meaning there is no galvanic isolation between the interface receiving the dimming signal and the secondary side of the power converting stage. If a basically insulated dimmer unit is connected with such conventional dimmable converter, the entire luminary arrangement including the conventional converter and the only basically insulated dimmer is also only basically insulated. This luminary arrangement may result in a potential danger to a human being coming into contact with the luminary.
The circuit design 28 depicted in fig. 3 offers a solution which is commonly used. A transformer 30 with a fixed operating frequency receives power via a driver input port 31 (X6-a) for communication between a dimmer unit which is not shown and the dimmable LED converter 1 from dimming signal input ports 32 (X6-b, X6-c) to communication ports 33 (X4-a and X4-b). However the circuit shown in fig. 3 suffers from a lack of linearity of the dimming curve, as the RC filter introduces a distortion for transmitting dimming levels which may vary over a large range.
The embodiments of the dimmable converter according to fig. 1 and fig. 2 however dispense with respective RC-filter layout as shown in fig. 3 and therefore the inventive dimmable converter layout offers good linearity over the dimming curve as an additional advantage, while simultaneously double isolation or SELV-output of the dimmable converter 1 is achieved.
The approach according to embodiments of the invention is independent from the actual general circuit topology used in the power converting stage. The approach is equally applicable for converter types such as a half bridge converter, a LLC resonant converter, a flyback converter, a forward converter, a buck converter, a boost converter, a buck-boost converter or else.
All single features of the discussed embodiments described and illustrated in the accompanying figures 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

A system comprising an illuminant (14) and a dimmable converter (1) for driving the illuminant (14), the dimmable converter (1) comprising:
• a power conversion control unit (24, 24'),

• an isolated power conversion unit (12, 12') comprising a switch arranged on a primary side of the isolated power conversion unit (12, 12'),

• a first transmission means (16),

• a second isolated transmission means (22),

• an interface control circuit (20),

• an output rectification and filter unit (10),

• wherein a primary side of the isolated power conversion unit (12, 12') is configured to receive a direct current and is further configured to convert the direct current into a first output current provided on a secondary side of the isolated power conversion unit (12, 12') and is further configured to send the first output current to the output rectification and filter unit (12),

• wherein the output rectification and filter unit (10) is configured to rectify and filter the first output current, thereby arriving at a second output current, the output rectification and filter unit (10) is further configured to send the second output current to the illuminant (14), characterized in that
the first transmission means is an isolated transmission means and the output rectification and filter unit (10) is further configured to send the first output current as an input signal of the first isolated transmission means (16) to a primary side of the first isolated transmission means (16) and,
• wherein the primary side of the first isolated transmission means (16) is configured to receive the first output current from the output rectification and filter unit (10) and the first isolated transmission means (16) is configured to convert the first output current into a third output current provided on a secondary side of the first isolated transmission means (16) and the primary side of the first isolated transmission means (16) is further configured to send the first output current back to the secondary side of the isolated power conversion unit (12,12'),

• wherein the interface control circuit (20) is configured to receive the third output current, to receive a dimming signal and to generate a first control signal based on the third output current and the dimming signal,

• wherein the second isolated transmission means (22) is configured to receive the first control signal from the interface control circuit (20) and convert the first control signal into a second control signal,

• wherein the power conversion control unit (24, 24') is configured to control a switching of the switch based on the second control signal, thereby setting the first output current to a value suitable to generate a desired light intensity to be emitted by the illuminant (14) and indicated by the dimming signal.
The system according to claim 1,
characterized in
that the first and second isolated transmission means (16, 22, 22') include either a transformer and/or an optocoupler.
The system according to claim 1,
characterized in that
the isolated power conversion unit (12, 12') comprises compensating means configured to compensate non-linear characteristics of the optocoupler and/or aging effects of the optocoupler.
The system according to any one of claims 1 to 3,
characterized in
that the second output current indicates a current flow on the secondary side of the isolated power conversion unit (12, 12') for driving the illuminant (14).