Method and apparatus for balance compensation of an electronic transformer

Abstract

The invention relates to a method and apparatus for applying an active imbalance compensation by means of a feed-forward control at a load (e.g., a solid-state luminaire) to create a polarity-dependent load based on a detected imbalance.

Description

[0001] METHOD AND APPARATUS FOR BALANCE COMPENSATION OF AN

[0002] ELECTRONIC TRANSFORMER

[0003] FIELD OF THE INVENTION

[0004] The invention relates to the field of power supplies for lighting systems, such as - but not limited to - solid-state lighting systems, for use in various different applications for home, office, retail, hospitality and industry.

[0005] BACKGROUND OF THE INVENTION

[0006] A luminaire can be any type of lighting unit or lighting fixture which comprises one or more light sources (e.g., visible or non-visible (infrared (IR) or ultraviolet (UV)) light sources) for illumination and / or communication purposes and optionally other internal and / or external parts necessary for proper operation of the lighting, e.g., to distribute the light, to position and protect the light sources and ballast (where applicable), and to connect the luminaires to a power supply.

[0007] Luminaire modules may be used as carriers for light source(s) (e.g., light emitting diode(s) (LED(s))) and may be manufactured as printed circuit boards (PCBs) either from typical PCB materials like FR4, flex-on-rigid or on MCPCB (Metal clad PCB) carriers for enhanced cooling.

[0008] Solid-state lighting (SSL) is a type of lighting that uses semiconductor lightemitting diodes (LEDs), organic light-emitting diodes (OLED), or polymer light-emitting diodes (PLED) as sources of illumination rather than electrical filaments, plasma (used in arc lamps such as fluorescent lamps), or gas. Solid state electroluminescence is used in SSL, as opposed to incandescent bulbs (which use thermal radiation) or fluorescent tubes. Compared to incandescent lighting, SSL creates visible light with reduced heat generation and less energy dissipation.

[0009] Transformers are devices used in lighting systems to help reduce or “stepdown” high voltages to lower voltages. Commonly used in homes with track / monorail or landscape lighting systems, transformers convert the standard residential power supply volage (e.g., 120V or 220V) to a lower voltage (e.g., 12V or 24V) required by the luminaire or lighting system. Electronic transformers are often used in residential settings and comprise an inverter that changes the frequency of the voltage. Typically, the frequency of a home power outlet is changed from 50Hz or 60Hz to a switching frequency in the kHz range (e.g., 30-500kHz). The higher the frequency of the voltage, the smaller the transformer needed to provide the required output.

[0010] However, electronic transformers are usually designed for non-solid-state luminaires (such as halogen luminaires). Using an electronic transformer for supplying power to a solid-state luminaire (e.g., an LED luminaire and in particular an LED luminaire that is radio frequency (RF) controlled) is challenging, as a solid-state luminaire as load may require a full range of power from maximum level down to standby level. For example, when the load is represented by an RF-controlled LED lamp, the power to the load is much lower than what the electronic transformer is designed for.

[0011] Moreover, electronic transformers were initially designed for resistive loads, while solid-state luminaires (such as LED luminaires) that use power converters as input circuit no longer represent a resistive load. By contrast, a non-solid-state halogen luminaire act’s a resistive load, also in the switching frequency range of the electronic transformer (e.g., 40-100kHz).

[0012] Electronic transformer circuits may operate on the principle of a selfoscillating half bridge with one or two capacitors. Such circuits are designed for a resistive loads i.e., the load current is proportional to the input voltage. A non-balanced load (such as a solid-state luminaire with a power converter) may lead to a risk of saturation of the electronic transformer and resultant damage thereof.

[0013] SUMMARY OF THE INVENTION

[0014] It is an object of the present invention to provide a protection for electronic transformers when used for supplying power to non-balanced loads.

[0015] This object is achieved by an apparatus as claimed in claim 1, by a luminaire module as claimed in claim 7, by a lighting system as claimed in claim 8, by a method as claimed in claim 10, and by a computer program product as claimed in claim 12.

[0016] According to a first aspect, an apparatus (e.g., an electronic circuit or (programmable) integrated circuit or programmable controller) is provided for compensating a voltage imbalance of an electronic transformer that supplies power to a power converter of a load device (e.g., luminaire), the apparatus being configured to apply a feed-forward control by measuring a voltage amplitude variation of an input voltage of the power converter and controlling the power converter to increase its input current when the measured voltage amplitude increases and to decrease its input current when the measured voltage amplitude decreases.

[0017] Furthermore, according to a second aspect, a luminaire device or module is provided that comprises an apparatus of the first aspect, the power converter and at least one light source.

[0018] Furthermore, according to a third aspect, a lighting system comprising an electronic transformer and a plurality of luminaire modules of the second aspect is provided.

[0019] Additionally, according to a fourth aspect, a method of compensating a voltage imbalance of an electronic transformer that supplies power to a power converter of a load device is provided, wherein the method comprises applying a feed-forward control by measuring a voltage amplitude variation of an input voltage of the power converter and controlling the power converter to increase its input current when the measured voltage amplitude increases and to decrease its input current when the measured voltage amplitude decreases.

[0020] Moreover, according to a fifth aspect, a computer program product is provided, which comprises code means for producing the steps of the above method of the fourth aspect when run on a controller device.

[0021] Accordingly, the input voltage to a power converter of load device is measured to detect an imbalance between voltage amplitudes of positive and negative polarities or cycles of an output signal of an electronic transformer. If there is an imbalance, the power converter of the load device can be controlled to draw different amounts of current at positive and negative cycles to correct the imbalance. Thereby, solid-state loads (such as luminaires) can operate in any given power level that is required by a single or a plurality of LED lamps without risking damage of the electronic transformer. Harmful effects caused by a non- resistive load in the electronic transformer can thus be compensated by an active balancing polarity-dependent load.

[0022] Moreover, voltage imbalance caused by repetitive start-ups with absence of minimum load (typically in standby) can be compensated once power is drawn by the load device (e.g., LED luminaire).

[0023] According to a first option of any of the first to fifth aspects, the input voltage of the power converter may be a rectified output voltage of the electronic transformer at a transformer switching frequency, that is amplitude-modulated by a modulation signal at double mains frequency, wherein the apparatus may be configured to measure the voltage amplitude variation by measuring a difference between voltage amplitudes at different halfwaves of the switching frequency.

[0024] According to a second option of any of the first to fifth aspects, which may be combined with the first option, the voltage amplitude variation may be measured at or close to a maximum level of the modulation signal.

[0025] According to a third option of any of the first to fifth aspects, which may be combined with the first or second option, a control signal may be applied to the power converter to control an output of a feedback error amplifier that controls an output driver of the power converter such that the input current drawn by the power converter is varied based on the measured voltage amplitude variation.

[0026] According to a fourth option of any of the first to fifth aspects, which may be combined with any one of the first to third options, the control signal may be used to control a control terminal of a transistor (e.g., transistor Ml of Fig. 4) of the power converter (e.g., the power converter 31 of Fig. 4) to control the input current drawn by the power converter.

[0027] According to a fifth option of any of the first to fifth aspects, which may be combined with any one of the first to fourth options, the apparatus of the first aspect, the power converter and the at least one light source may be integrated on a circuit board of the luminaire module of the second aspect.

[0028] According to a sixth option of any of the first to fifth aspects, which may be combined with any one of the first to fifth options, the apparatuses of the first aspect of the plurality of luminaire modules of the second aspect may be configured to operate together to obtain a balanced total load current of the electronic transformer.

[0029] According to a seventh option of any of the first to fifth aspects, which may be combined with any one of the first to fifth options, the input voltage may be sampled at a sampling frequency that is synchronized with a switching frequency of the electronic transformer, the measured voltage amplitude may be compared with another measured voltage amplitude of a sample that corresponds to another polarity of the input voltage, wherein if the measured input voltage amplitude has decreased, the power converter is controlled to increase its input current, wherein if the measured input voltage amplitude has not changed, the power converter is controlled to maintain its input current, and wherein if the measured input voltage amplitude has increased, the power converter is controlled to increase its current input current.

[0030] It is noted that the above apparatus may be implemented based on discrete hardware circuitries with discrete hardware components, integrated chips, or arrangements of chip modules, or based on signal processing devices or chips controlled by software routines or programs stored in memories, written on a computer readable media, or downloaded from a network, such as the Internet.

[0031] It shall be understood that the apparatus of claim 1, the luminaire module of claim 7, the lighting system of claim 8, the method of claim 10, and the computer program product of claim 12 may have similar and / or identical preferred embodiments, in particular, as defined in the dependent claims.

[0032] It shall be understood that a preferred embodiment of the invention can also be any combination of the dependent claims or above embodiments with the respective independent claim.

[0033] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

[0034] BRIEF DESCRIPTION OF THE DRAWINGS

[0035] In the following drawings:

[0036] Fig. 1 shows schematically a block diagram of a luminaire system with feedforward control according to various embodiments;

[0037] Fig. 2 shows schematically a waveform diagram of a voltage imbalance between two polarities;

[0038] Fig. 3 shows a flow diagram of an enhanced luminaire driving procedure according to various embodiments; and

[0039] Fig. 4 shows schematically a circuit diagram of an enhanced luminaire with feed-forward network according to an embodiment.

[0040] DETAILED DESCRIPTION OF EMBODIMENTS

[0041] Various embodiments of the present invention are now described, which are applicable to luminaires of a solid-state lighting system, such as semiconductor LEDs, semiconductor lasers, vertical -cavity surface emitting lasers (VCSELs), organic lightemitting diodes (OLED), or polymer light-emitting diodes (PLED) as sources of illumination or light sources in visible or non-visible light spectra.

[0042] It is however pointed out that the present invention can be used for any kind of load device which causes voltage imbalances. More specifically, the following embodiments are directed to LED luminaires. They can be implemented in connection with any type of LED module or board and are applicable to any kind of LED driver or converter of luminaires.

[0043] Fig. 1 shows schematically a block diagram of a luminaire system with feedforward control according to various embodiments.

[0044] It is noted that - throughout the present disclosure - the structure and / or function of blocks or circuit components with identical reference numbers that have been described before are not described again, unless an additional specific functionality is involved. Moreover, only those structural elements and functions are shown, which are useful to understand the embodiments. Other structural elements and functions are omitted for brevity reasons.

[0045] A power supply AC voltage VAC (e.g., a power grid voltage of 110 or 220V at a mains frequency of 50 or 60Hz) is supplied to an electronic transformer (ET) 10 which operates in a frequency range of e.g. 30KHz to 500kHz to transform the power supply AC voltage VAC to a lower voltage in a range of e.g. 12-24V suitable for operating a solid-state load device (SS-LD) 33, such as an LED luminaire. The output waveform of the electronic transformer 10 may be an amplitude-modulated (AM) signal with the transformer switching frequency as carrier frequency and with an amplitude modulation that corresponds to the double mains frequency.

[0046] The transformed AC voltage is then supplied to a rectifier (RECT) 20 which is an electronic device that converts the transformed AC voltage into a DC voltage VDC by using one or more rectifying elements (e.g., diodes or other valve elements) that allow current to flow in a single direction only.

[0047] The DC voltage VDC is supplied to a DC / DC converter 31 configured to generate a drive voltage VLD to be applied to the solid-state load device 33 (e.g., in order to activate and drive a light source (e.g., LED luminaire).

[0048] The DC / DC converter 31 is an electrical power converting device (power converter) that regulates the power to the load device 33 and that may respond to changing needs of the load device 33 by supplying a constant amount of power to the load device 33 as its electrical properties may change with the temperature. Changing the output power as a function may be dominated by an optional thermal protection circuit. Alternatively, component tolerances and temperature coefficients may impact the output power as well. The DC / DC converter 31 is configured to provide the load device 33 with very specific electrical power in order to operate properly. If the load device 33 is an LED and the voltage supplied to the LED and the voltage is lower than required, very little current runs through the diode junction, resulting in low light and poor performance. On the other hand, if the voltage is too high, too much current flows through the LED and it can overheat and be severely damaged or fail completely (thermal runaway). This certainly applies to other kinds of luminaires or other load devices as well. Power regulation through the DC / DC converter 31 may be used to enable a dimming function (required brightness as mentioned later) and / or also a standby operation where limited power is taken from the input.

[0049] The DC / DC converter 31 may be a buck converter (step down conversion), a boost converter (step up conversion) or a buck-boost converter. Based on a predetermined input voltage range for the DC voltage VDC, the DC / DC converter 31 converts the input power to a desired output voltage and / or current to the load device 33.

[0050] In an example, the DC / DC converter 31 may be configured as a controllable current source for providing sufficient current to light one or more light sources of the load device 33 at a required brightness, but to limit the current to prevent damaging the light source(s).

[0051] It is to be noted that two requirements that may need to be met in the electronic transformer 10 are that the voltage-time product across a magnetizing inductor (within the electronic transformer) needs to be zero and that the current-time product at the primary self-oscillating capacitive half bridge needs to be, low, almost zero. If these requirements are not met, this can lead to a permanent damage of the electronic transformer 10.

[0052] A violation of at least one of the above two requirements can be detected as an imbalance in the capacitive half-bridge, i.e., the voltage amplitude of one polarity will be higher than the voltage amplitude of the other polarity as shown in Fig. 2 below.

[0053] According to the following embodiments, a feed-forward control solution for preventing harmful imbalances in the electronic transformer 10 is provided.

[0054] More specifically, the imbalance of the electronic transformer 10 is compensated by drawing a higher current from the electronic transformer 10 if the voltage amplitude of a polarity is increased (higher) and a lower current if the voltage amplitude of a polarity is decreased (lower). The purpose of such a control is to draw an absolute current (i.e., average current drawn over a plurality of cycles of the transformer switching frequency) that is equal to the required current to power the load device 33.

[0055] In an embodiment, a feed forward circuit (FF-CTRL) 32 is adapted to influence a control loop at the DC / DC converter 31 by applying a feed-forward control signal FF to the DC / DC converter 31 in such a way that the input current of the DC / DC converter 31 follows the input DC voltage VDC in a positive correlation (e.g., the DC / DC converter 31 acts as a resistive load) in the switching frequency range of the electronic transformer 10 (e.g., 30KHz-500kHz).

[0056] In examples, the feed-forward control circuit 32 may be implemented by a software-controlled processing device (e.g., controller) or may be implemented as an application-specific integrated circuit (ASIC) or a field-programmable gate-array (FPGA).

[0057] The feed-forward circuit 32 may comprise a detection circuit or function configured to measure an amplitude variation (imbalance) of the modulated rectified voltage VDC at the output of the rectifier 20, i.e., the difference between the positive amplitude and the rectified negative amplitude of the switching frequency after rectification. In an example, this can be achieved by measuring a difference of successive voltage amplitudes of the near rectangular rectified voltage VDC at a given time within the mains cycle (50Hz-60Hz). Practically many problems are noticed at the start-up which indeed is near the maximum level, but the proposed feed-forward control may be applied throughout the full mains cycle as long as the electronic transformer 10 is oscillating (thus not near zero crossing of mains). Also, the impact of imbalance may worsen, as momentary input volage becomes higher. In a more specific example, a currently measured sample of the amplitude of the rectified voltage VDC at the input of the DC / DC converter 31 is compared with a stored or latched previous sample of the amplitude of a previous half-wave of the switching frequency.

[0058] Additionally, the feed-forward circuit 32 may comprise a compensation circuit or function that interacts with the DC / DC converter 31 (e.g., with a controller of the DC / DC converter 31) through the feed-forward control signal FF such that the higher voltage amplitude of the first polarity draws a higher current, and the lower voltage amplitude of the second polarity draws a lower current.

[0059] In an example, the feed-forward control may be adjusted so that the average input current to the DC / DC converter 31 over multiple cycles is constant, while the drawn input current per half cycle may vary. The voltage amplitude may be sensed / measured with a capacitive divider circuit. If the measured voltage amplitude increases, the drawn input current will also increase, so that the drawn input current can be linked to and thus follows the input voltage amplitude.

[0060] Fig. 2 shows schematically a waveform diagram of a voltage imbalance between two polarities at the output of the electronic transformer 10. The voltage imbalance may have been caused by an imbalance in a capacitive half-bridge of the electronic transformer 10. The resulting amplitude difference is noticeable at the output of the electronic transformer 10 i.e., one polarity of the switching frequency will have a higher amplitude compared to the other polarity.

[0061] The exemplary waveform of Fig. 2 shows a small window of a length of 20ps between 4.960ms and 4.980ms around a peak (amplitude) of the 50Hz sinusoidal AM modulation signal. The maximum voltage of the positive polarity of the switching frequency ranges between +12 V and +14V, while the maximum voltage of the negative polarity of the switching frequency ranges between -20V and -21V. This results in a voltage imbalance in the range between 7 V and 8 V at the input of the DC / DC converter 31.

[0062] After rectification, both voltage amplitudes will have the same (positive) polarity but different amplitudes. Hence, the voltage imbalance (e.g., 7-8V) can be detected by simply measuring the difference between (successive) voltage levels (rectified amplitudes) at different halfwaves of the switching frequency e.g. at or close to the maximum level (amplitude) of the rectified sinusoidal modulation signal at mains frequency (i.e., 50 or 60Hz).

[0063] According to various embodiments, the feed-forward control circuit 32 of Fig. 1 may be configured to apply the procedure shown in Fig. 3 below. To achieve this, the feedforward circuit 32 may comprise or may be implemented by a processing circuit (e.g., processor or controller) that is controlled by instructions of one or more software routines (stored in or accessible by or loaded into a memory of the feed-forward control circuit 32) to perform steps or operations of the following procedure.

[0064] Fig. 3 shows a flow diagram of an enhanced luminaire driving procedure according to various embodiments.

[0065] In a first step S310, the rectified input voltage amplitude or level VDC at the input of DC / DC converter 31 is sampled and measured. The sampling frequency may be synchronized with the switching frequency of the electronic transformer 10.

[0066] Then, in step S302 it is checked whether the measured input voltage amplitude or level VDC has changed in comparison to the last sample or measurement that corresponds to the rectified other polarity (i.e., other halfwave of the switching frequency).

[0067] If the procedure determines in step S302 that the measured input voltage amplitude or level VDC has decreased (VDC<), the procedure branches to step S304 where the DC / DC converter 31 is controlled (e.g., by the feed-forward control signal FF) to decrease its input current (IIN<). Otherwise, if the procedure determines in step S302 that the measured input voltage amplitude or level VDC has not changed (VDC=), the procedure branches to step S303 where the DC / DC converter 31 is controlled (e.g., by the feed-forward control signal FF) to maintain its current input current (IIN=), e.g., no control at all.

[0068] Otherwise, if the procedure determines in step S302 that the measured input voltage amplitude or level VDC has increased (VDC>), the procedure branches to step S305 where the DC / DC converter 31 is controlled (e.g., by the feed-forward control signal FF) to increase its current input current (IIN>).

[0069] Thereby, the DC / DC converter 31 can be controlled to act as resistive load where its input current correlates (e.g., in a linear manner) with its input voltage.

[0070] In the following, a more detailed implementation example of the proposed feed-forward control is explained.

[0071] Fig. 4 shows schematically a circuit diagram of an enhanced luminaire 40 with feed-forward network according to an embodiment (e.g., an LED luminaire which may be one of a plurality of such enhanced luminaires of a lighting system).

[0072] In the embodiment of Fig. 4, the enhanced luminaire 40 may be luminaire module with a circuit board on which a compensation circuitry configured to apply a current compensation for removing or at least mitigating harmful voltage imbalance at the output of an electronic transformer 10.

[0073] The electronic transformer 10 may be connected to a phase conductor L and neutral conductor (N) of a power grid. The electronic transformer 10 outputs an AC voltage (e.g., 12VAC) with a switching frequency of 30kHz-500kHz and an envelope (AM modulation) with double of mains frequency (50Hz or 60Hz), that is supplied to an input of the enhanced luminaire 40.

[0074] The enhanced luminaire (40) comprises (e.g., on its circuit board) a rectifier stage 42 (e.g., a bridge rectifier, which may be configured as an active rectifier) is used to rectify the AC voltage after low-pass filtering by a serial coil LI of a filter stage 41. Then, the filtered and rectified voltage Vrect is supplied to a DC / DC converter 31 which powers an LED load 48 by applying an output voltage Vout and corresponding output current. An optional additional second stage 46 may be provided after the DC / DC converter 31 e.g. on the circuit board (if required by the LED load 48). The drawn input power of the DC / DC converter 31 may contain double of mains frequency (50Hz / 60Hz) power fluctuations that may cause stroboscopic and flicker at the light output. The purpose of the second stage 46 can be to smoothen the light output for improved quality of light (minimal flicker and stroboscopic light effects). Here, stroboscopic is caused by mains frequency, and flicker is caused by 'variations in startup' of the electronic transformer 10.

[0075] The voltage amplitude of the rectified voltage Vrect at the input of a DC / DC converter 31 is sensed via an RC low-pass filter 43 comprising a resistor RF and a capacitor CF to filter out higher disturbance frequencies (i.e., frequencies higher than the switching frequency of the electronic transformer 10 which may have been introduced through filtering and / or wiring). As a result, the voltage across the capacitor CF will correspond to and thus track the amplitudes of the rectified voltage Vrect of the two polarities of the AC output voltage of the electronic transformer 10. This sensed signal is passed on to a capacitive divider 44 comprising a first divider capacitor Cdivi and a second divider capacitor Cdiv2, such that a proportional smaller portion of the voltage level variation of the rectified voltage Vrect is applied to a comparison input Comp of a feed-forward controller 47 via a compensation network 45 comprising a compensation resistor Rc and a compensation capacitor Cc. The compensation circuitry may be designed as a part of the feed forward control circuit (e.g., feed-forward control circuit 32 of Fig. 1) and serves to stabilize the control loop. The components Rc and Cc may be connected to the Comp input and thus to an output of an error amplifier. Any change to this pin directly impacts the DC / DC converter behavior. Therefore, the signal applied to the Comp input is based on the input of the DC / DC converter 31 and is processed as a feed-forward (thus not reactive). In power electronics, the Comp pin is commonly known as 'output of the error amplifier' of the integrated circuit (IC) of the feedforward controller 47.

[0076] For exemplary discrete designs according to the embodiment, the sensed voltage portion obtained after the capacitive divider 44 and the compensation network 45 is injected into an input port or pin of the feed-forward controller 47 (which may be a programmable controller device). Additionally, the output voltage Vout of the DC / DC converter 31 may be sensed via an additional sensing input (S) of the feed-forward controller 47 to allow for an additional feedback loop for controlling the output voltage Vout in a standby operation where no power is applied to the LED load 48, but a stable voltage is required for keeping the feed-forward controller 47 powered. In operation, i.e., when the LED load 48 is powered, the feed-forward controller 47 requires an input volage within a specific range (e.g., beyond the forward voltage of the LEDs but not beyond a maximum input voltage of the second stage 46 (e.g., 35V)) in order to function correctly.

[0077] The feed-forward controller 47 is configured or programmed to control the DC / DC converter 31 via a feed-forward control signal to increase the input current of the DC / DC converter 31 so as to draw a higher current amplitude from the electronic transformer 10 if an increase in the sensed voltage portion at the Comp input is measured, and vice versa.

[0078] Since the output level of the DC / DC converter 31 is not solely determined by the related feedback loop, an over-voltage protection circuit may be provided to avoid an excessive output voltage Vout or a no-load condition. In an example, the feed forward control signal may be used to control an output of the feedback error amplifier in a way that directly influences the output level. The error amplifier may be configured to control an output driver of the DC / DC converter 31 in the following manner. When the output voltage Vout is higher than a required value, the error amplifier increases the duty-cycle of a control signal applied to a control terminal (e.g., gate) of a metal oxide semiconductor field effect (MOS-FET) transistor Ml which increases the input current of the DC / DC converter 31 and when Vout is lower than the required value, it lowers the duty-cycle of the transistor Ml which causes the input current to reduce the input current of the DC / DC converter 31.

[0079] In the example of Fig. 4, the feed-forward control signal, which may be a duty-cycle signal (e.g., pulse-width modulated signal) at a switching frequency of least double the maximum switching frequency of the electronic converter 10, is applied to the control terminal of the transistor (Ml of the output driver to control the input current drawn by the DC / DC converter 31.

[0080] In this way, the feed-forward controller 47 follows the input voltage (i.e., rectified voltage Vrect) of the DC / DC converter 31 and controls the input current drawn by the DC / DC converter 31 e.g. by influencing the duty-cycle of the MOSFET control signal to mimic the behavior of a resistive load (such as halogen load of a halogen lamp). More specifically, the feed-forward controller 47 controls a duty cycle of the transistor Ml (e.g., via a pulse width modulator) in order to create a resistive load behavior of the DC / DC converter 31.

[0081] If more LED luminaires or other load devices are connected to the same electronic converter 10, these luminaires may all have a feed forward control function as described above, so that they are controlled demand / draw more input current in case their input voltage rises, and vice versa.

[0082] Furthermore, if the DC / DC converter 31 operates in a burst mode asynchronous with the other luminaires on the same electronic transformer 10, the voltage imbalance compensation network (feed-forward control) of all luminaires may be configured to operate together to obtain a balanced total AC load current and overcome damage or malfunction of the electronic transformer 10. To summarize, a method and apparatus have been described for applying an active imbalance compensation by means of a feed-forward control at a load (e.g., a solid- state luminaire) to create a polarity-dependent load based on a detected imbalance.

[0083] While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiments concerning solid-state luminaires (e.g., LED luminaires). The proposed feed-forward balance control can be applied in connection with any type of load that leads to a voltage imbalance at the output of an electronic transformer.

[0084] Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single processor or other unit may fulfil the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. The foregoing description details certain embodiments of the invention. It will be appreciated, however, that no matter how detailed the foregoing appears in the text, the invention may be practiced in many ways, and is therefore not limited to the embodiments disclosed. It should be noted that the use of particular terminology when describing certain features or aspects of the invention should not be taken to imply that the terminology is being re-defined herein to be restricted to include any specific characteristics of the features or aspects of the invention with which that terminology is associated.

[0085] A single unit or device may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

[0086] The described procedures like the one indicated in Fig. 3 can be implemented as program code means of a computer program and / or as dedicated hardware of the receiver devices or transceiver devices, respectively. The computer program may be stored and / or distributed on a suitable medium, such as an optical storage medium or a solid-state medium, supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems.

Claims

CLAIMS:

1. An apparatus (32; 47) for compensating a voltage imbalance of an electronic transformer (10) that supplies power to a power converter (31) of a load device (33), the apparatus being configured to apply a feed-forward control by measuring a voltage amplitude variation of an input voltage of the power converter (31) and controlling the power converter (31) to increase its input current when the measured voltage amplitude increases and to decrease its input current when the measured voltage amplitude decreases, wherein the input voltage of the power converter (31) is a rectified output voltage of the electronic transformer (10) at a transformer switching frequency, that is amplitude-modulated by a modulation signal at double mains frequency, and wherein the apparatus (32; 47) is configured to measure the voltage amplitude variation by measuring a difference between voltage amplitudes at different halfwaves of the switching frequency.

2. The apparatus (32; 47) of claim 1, wherein the apparatus (32; 47) is configured to measure the voltage amplitude variation at or close to a maximum level of the modulation signal.

3. A luminaire module (40) comprising an apparatus (32; 47) as claimed in any one of claims 1 or 2, the power converter (31) and at least one light source (48).

4. The luminaire module (40) of any one of the claim 3, wherein the apparatus (32; 47) is configured to supply a control signal (FF) to the power converter (31) to control an output of a feed-back error amplifier that controls an output driver of the power converter (31) such that the input current drawn by the power converter (31) is varied based on the measured voltage amplitude variation.

5. The luminaire module (40) of claim 4, wherein the apparatus (47) is configured to apply the control signal (FF) to a control terminal of a transistor (Ml) of the power converter (31) to control the input current drawn by the power converter (31).

6. The luminaire module (40) of claim 5, wherein the apparatus (32; 47), the power converter (31) and the at least one light source (48) are integrated on a circuit board of the luminaire module (40).

7. A lighting system comprising an electronic transformer (10) and a plurality of luminaire modules (40) as claimed in claim 5 or 6.

8. The lighting system of claim 7, wherein the apparatuses (32; 47) of the plurality of luminaire modules (40) are configured to operate together to obtain a balanced total load current of the electronic transformer 10.

9. A method of compensating a voltage imbalance of an electronic transformer (10) that supplies power to a power converter (31) of a load device (33), wherein the method comprises applying a feed-forward control by measuring (S301) a voltage amplitude variation of an input voltage of the power converter (31) and controlling (S303 -S305) the power converter (31) to increase its input current when the measured voltage amplitude increases and to decrease its input current when the measured voltage amplitude decreases.

10. The method of claim 9, further comprising: sampling the input voltage at a sampling frequency that is synchronized with a switching frequency of the electronic transformer (10); comparing (S302) the measured voltage amplitude with another measured voltage amplitude of a sample that corresponds to another polarity of the input voltage; if the measured input voltage amplitude has decreased, controlling (S304) the power converter (31) to increase its input current; if the measured input voltage amplitude has not changed, controlling (S303) the power converter (31) to maintain its input current; and if the measured input voltage amplitude has increased, controlling (S305) the power converter (31) to increase its current input current.

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