Multi-channel voltage balancing by power multiplexing

Abstract

The invention relates to an apparatus and method for applying power multiplexing to a power converter device to convert an access voltage (which may be a small portion compared to the total load forward voltage) back to an input. A first stage provides a regulated bus voltage and feedback is provided to regulate the bus voltage to keep it just above a required headroom voltage. Series capacitors are coupled to each load device (e.g., LED string) to compensate for differences between the bus voltage and the forward voltage. Inputs can be activated depending on the status of the load device, while the outputs of the power converter device correspond to the capacitors at the bus. Thereby, power losses can be reduced and the size of power converters can be minimized compared to traditional power converter topologies.

Description

[0001] Multi-channel voltage balancing by power multiplexing

[0002] FIELD OF THE INVENTION

[0003] The invention relates to the field of power supplies for lighting systems, such as - but not limited to - solid-state lighting systems or other loads, for use in various different applications for home, office, retail, hospitality and industry.

[0004] BACKGROUND OF THE INVENTION

[0005] 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.

[0006] 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.

[0007] A driver is required to deliver a highly stable constant current to the luminaire(s) irrespective of variations in the luminaire characteristics or the supply voltage while complying with increasingly stringent regulations covering input power requirements such as power factor and total harmonic distortion.

[0008] Efficiency of electric lighting has improved to such an extent that efficiency of the driver (e.g., LED driver) has become a major aspect. Efficiency improvement is therefore an important topic due to legislation and energy labelling. More and more emphasis has been put on efficient LED products such that cheap inefficient circuits will be phased out in the near future. Efficiency is related to the existing topology, e.g., a constant voltage topology where current limitation is applied by resistance. When looking at efficiency, power losses for current control for LEDs can be ranked from worst to best as follows: resistive, linear, SMPS, SMPS with fractional power conversion. The latter is an SMPS that controls the LED current by partial / fractional power conversion (partial power conversion means that only a part / fraction of the LED power is converted such that the inevitable losses in power converter are also a fraction of the total LED power). This not only improves substantially on efficiency, but also on the compactness of the design such that it can be embedded on a LED strip. There are various topologies that can be applied in order to employ fractional power conversion as a second power stage.

[0009] Prior art solution with fly back converter and turns ratio for the compensating the voltage mismatch between the LED channels have been proposed. Such solutions may be iniquitous for applications in televisions. However, they pose the following challenges for LED lighting applications. Feedback control is usually applied to the dominant power consuming output. In multi-channel LED applications, the dominant power per channels may alter by the setting of color or dimming level. This problem can be overcome by taking the minimum headroom of all three channels into account, but this may lead to higher power losses as unequal loading of channels may results in larger voltage variations between individual channels (this has to do with the leakage inductance of the transformer and unequal loading of the different channels).

[0010] An alternative prior art presents a solution by making independent output voltages by means of second stage power converters. In two-stage LED drivers, a boost-PFC (power factor correction) stage is followed by a constant current driver (e.g., a fly-back converter or a buck converter). For efficiency improvement of the driver, the second stage of the driver may be replaced by a mini converter or a linear current source. However, this poses additional challenges on the control circuit if pulse width modulation (PWM) is required on the LED channels: the frequency response of the mini-converters is insufficient to realize accurate PWM control which will affect the perception of the emitted color of the light source.

[0011] SUMMARY OF THE INVENTION

[0012] It is an object of the present invention to provide improved efficiency of drivers while allowing high bandwidth modulation of the LED channels. This object is achieved by a driver device as claimed in claim 1, by a driver device as claimed in claim 10, by a lighting system as claimed in claim 13, by a method as claimed in claim 14, and by a computer program product as claimed in claim 15.

[0013] According to a first aspect, a driver device that supplies power to at least one load is provided, the driver device comprising: a controller circuit for controlling at least one switching element based on a measured headroom voltage across at least one respective current source that controls an output current of the driver device through the at least one load; and at least one serial capacitor connected in series with the at least one load and the at least one current source in a branch connected in parallel to an input buffer capacitor of the driver device; wherein the controller circuit is configured to control the at least one switching element to transfer energy from the at least one serial capacitor to the input buffer capacitor until the measured headroom voltage has reached a predetermined value.

[0014] Furthermore, according to a second aspect, a driver device comprising a first power conversion stage and a second power conversion stage with at least one driver device of the first aspect.

[0015] Additionally, according to a third aspect, a lighting system comprising one or more driver devices of the second aspect for driving respective luminaires is provided.

[0016] Further, according to a fourth aspect, a method of controlling a driver device that supplies power to at least one load is provided, the method comprising: controlling at least one switching element based on a measured headroom voltage across at least one respective current source that controls an output current of the driver device through the at least one load; wherein the at least one switching element is controlled to transfer energy from the at least one serial capacitor to the input buffer capacitor until the measured headroom voltage has reached a predetermined value.

[0017] Further, according to a fifth aspect, a computer program product is provided, which comprises code means for producing the steps of the method of the fourth aspect when run on a controller circuit of a driver device.

[0018] Accordingly, higher efficiency of driver devices can be achieved by the proposed fractional power conversion via the serial capacitors, which allows more voltage difference between respective loads (e.g., LED strings). Furthermore, fractional power conversion with linear current source with headroom control enables a more compact design due to higher switching frequency, lower power and lower voltages, and allows fast pulse modulation (e.g., PWM) rise and fall times.

[0019] Additionally, the input bus voltage at the buffer capacitor can be selected to provide backwards compatibility with existing platforms / products.

[0020] Moreover, a simplified and independent control can be achieved by setting the sum of duty cycles of multiplexing switches a predetermined value (e.g., 1).

[0021] As each of the current sources can be controlled independently, a flexible load current amplitude and duty-cycle modulation can be achieved with constant modulation (e.g., PWM) current and amplitude at full resolution. Furthermore, the proposed fractional power conversion can be used for cancelling main ripple generated by high power factor correction of a preceding first power conversion stage.

[0022] According to a first option of any of the first to fifth aspects, the input buffer capacitor may be used to generate a bus voltage for the driver device.

[0023] According to a second option of any of the first to fifth aspects, which may be combined with the first option, the controller circuit may be configured to control a plurality of switching elements based on respective measured headroom voltages across respective current sources of a plurality of multiplexed output channels to transfer energy from a plurality of serial capacitors to the input buffer capacitor (e.g., until at least one of the measured headroom voltages has reached a predetermined value).

[0024] According to a third option of any of the first to fifth aspects, which may be combined with the first or second option, the controller circuit may be configured to control the plurality of switching elements to obtain a predetermined sum (e.g., a value of 1) of duty cycles of the plurality of multiplexed output channels.

[0025] According to a fourth option of any of the first to fifth aspects, which may be combined with any one of the first to fourth options, a transformer may be provided, with at least one first winding to which the energy of the at least one serial capacitor is transferred, and a second winding to which the buffer capacitor is connected.

[0026] According to a fifth option, the transformer of the fourth option may comprises a plurality of first windings which are connected to a plurality of the switching elements to transfer energy from a plurality of the serial capacitors of a plurality of multiplexed output channels to the input buffer capacitor.

[0027] 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 at least one current source may be integrated in the driver device and the driver device may comprise at least one input terminal for supplying modulation signals for modulating the output current of the driver device through the at least one load.

[0028] According to a seventh option of any of the first to fifth aspects, which may be combined with any one of the first to sixth options, the controller circuit may comprise an output for controlling a preceding converter stage to control a difference between a bus voltage supplied by the preceding converter stage and the measured headroom voltage.

[0029] According to an eighth option of any of the first to fifth aspects, which may be combined with any one of the first to seventh options, the controller circuit may comprise a shared serial capacitor that is connected in series to two or more loads.

[0030] According to a ninth option of any of the first to fifth aspects, which may be combined with any one of the first to eighth options, the driver device may be configured to control a plurality of cascaded groups or sections of loads, each connected to a respective one of a plurality of multiplexed output voltages.

[0031] According to a tenth option of any of the first to fifth aspects, which may be combined with any one of the first to eighth options, the driver device system may comprises a plurality of driver devices, each arranged in a respective one of a plurality of cascaded groups or sections of loads connected to a plurality of multiplexed output voltages of the driver devices, wherein the driver device may comprise a controller circuit for supplying the plurality of driver devices with a bus voltage and control data.

[0032] It is noted that the above apparatus may be implemented based on discrete hardware circuitries with discrete hardware components, integrated circuits, or arrangements of integrated modules, or based on signal processing devices or integrated circuits controlled by software routines or programs stored in memories, written on a computer readable media, or downloaded from a network, such as the Internet.

[0033] It shall be understood that the driver device of claim 1, the driver device of claim 10, the lighting system of claim 13, the method of claim 14, and the computer program product of claim 15 may have similar and / or identical preferred embodiments, in particular, as defined in the dependent claims.

[0034] It shall further 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.

[0035] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] In the following drawings:

[0037] Fig. 1 shows schematically a block diagram of a multiplexing multi-channel LED driver according to various embodiments;

[0038] Fig. 2 shows schematically a circuit diagram of an exemplary implementation of a multiplexing multi-channel LED driver according to a first embodiment;

[0039] Fig. 3 shows schematically exemplary waveforms of control signals to explain a steady state operation of the multiplexing multi-channel LED driver of the first embodiment;

[0040] Fig. 4 shows schematically a circuit diagram of an exemplary implementation of a multiplexing multi-channel LED driver with shared series capacitor, according to a second embodiment;

[0041] Fig. 5 shows schematically a circuit diagram of a first alternative multichannel LED driver implementation with independent boot converters, according to a third embodiment;

[0042] Fig. 6 shows schematically a circuit diagram of a second alternative multichannel LED driver implementation with multiple secondary transformer windings, according to a third embodiment to a fourth embodiment;

[0043] Fig. 7 shows schematically a block diagram of a multiplexing multi-channel LED driver topology with common converter for several LED strips, according to a fifth embodiment; and

[0044] Fig. 8 shows schematically a block diagram of a multiplexing multi-channel LED driver topology with a local converter per LED strip, according to a sixth embodiment.

[0045] DETAILED DESCRIPTION OF EMBODIMENTS

[0046] 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.

[0047] 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 various kinds of LED drivers or converters of luminaires. In conventional multi-channel driver systems with voltage-driven output, the output current may be limited by resistors and pulse width modulation (PWM). Here, the efficiency per output channel on the second driver stage (excluding the losses in the first stage) can be expressed as where Vfw designates the forward voltage at the channel output, Vbus designates the supplied bus voltage, and ILED designates the output current (load current) through the driven LED load (e.g., LED string).

[0048] For a driver system supplied by a constant bus voltage of 24V and supplying three output channels LED1 to LED3 with respective forward voltages 18V, 17V and 20V, this would result in the following efficiencies per channel:

[0049] Although these efficiencies are relatively low, an improvement by means of a second stage could be expected in the range of few percents (e.g., an efficiency of 85% for the second stage including control functions may be practical).

[0050] An alternative way is to power-convert the delta (i.e., Vbus-Vfw) in the second driver stage (fractional conversion). This can be expressed as: where power losses in the second-stage converter (e.g., fly-back converter) are approximated by (kfeus—VW)(l—flyback )! LED - As mentioned above, the efficiency payback of the second-stage converter can be expected to be about 0.85.

[0051] As only a fraction of the output power to the LED loads is converted here, a higher efficiency can be achieved by fractional power conversion of the second stage, which is listed in the table below:

[0052] Even when considering the efficiency of the second-stage converter to be as low as 50%, the system efficiency of such a second stage outperforms the resistive current limiting scheme as long as the second-stage converter outputs more power than the power dissipated by current balancing resistors of conventional systems (while also taking the auxiliary power consumption of a controller circuit used in the second-stage converter into account). For example, a total efficiency of 50% of the second-stage converter would result in total driver efficiencies of:

[0053] Fig. 1 shows schematically a block diagram of a multiplexing multi-channel LED driver according to various embodiments.

[0054] 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.

[0055] The block diagram of Fig. 1 indicates a generalized structure of the proposed fractional power multiplexing multi-channel LED driver circuit. In later embodiments, more details will be added to explain the operation.

[0056] The driver circuit comprises a first power conversion stage (SI) 103 and a second power conversion stage with a controller circuit (CTRL) 200 for controlling a switched-mode power supply (SMPS) with fly-back converter with n multiplexed inputs and a single output.

[0057] The controller circuit 200 may be configured to implement or control a total of n controllable switches for selecting a power multiplexing input. Optionally, the controller circuit 200 may be configured to set duty cycles of the power multiplexing switches so that the total sum of duty cycles across all switches equals 1.

[0058] More specifically, the controller circuit 200 may be configured to control the input power multiplexing switches based on measured head-room voltages across linear current sources of each output channel.

[0059] In examples, the SMPS fly-back converter of the controller circuit 200 may comprise a transformer with multiple primary windings (input windings) for power input and a single secondary winding (output winding) for outputting a total output power towards a bus voltage 106 which is used for power supply to all output channels. A freewheeling diode may be provided at the secondary side of a transformer of the fly-back converter to prevent voltage peaks.

[0060] Furthermore, a single primary winding of the fly-back transformer may be used to generate a supply volage for powering the controller circuit 200 and controlling the input power multiplexing switches. Alternatively, a separate auxiliary winding of the flyback transformer may be used to generate a supply voltage for the controller circuit 200 and to control the input power multiplexing switches.

[0061] The controller circuit 200 may comprise independent digital inputs in parallel or a serial digital input (in case the linear current sources are implemented internally) to control application of individual PWM signals to the output channels.

[0062] Alternatively, the controller circuit 200 may comprise n analog inputs in case the linear current sources are implemented externally.

[0063] Optionally, a controller circuit feedback output 104 may be provided for controlling the output power of the first power conversion stage 103 to minimize the difference between the bus voltage 106 (supplied by the first power conversion stage 103) and a minimal head room of the linear current sources of the output channels.

[0064] Furthermore, series capacitors 207, 307, ... n07 are connected in series with LED loads 201, 301,. . ,n01 for each individual LED PWM output channel such that a dominant part of voltage mismatch between the voltage at the LED loads 201-n01 and the bus voltage 106 is generated across the series capacitors 202-n02 in order to be converted to the secondary side of the transformer of the SMPS fly-back converter.

[0065] In examples, LED PWM channels may (preferably) be independently controlled by the controller circuit 200 with respect to the input power multiplexing circuit. It is important to notice that the duty cycles of the LED PWM output channels are completely independent of each other and from the power multiplexing of the input power multiplexing circuit.

[0066] The first power conversion stage 103 may be supplied with power via supply terminals 101 and 102. More specifically, a power supply AC voltage VAC (e.g., a power grid voltage of 110 or 220V at a mains frequency of 50 or 60Hz) may be supplied to an electromagnetic interference (EMI) filter which is an electronic device that attenuates electromagnetic interference from the power system to limit the noise in the system and lower a risk of malfunctioning of the luminaire driver.

[0067] The filtered AC voltage may then be supplied to a rectifier stage which is an electronic device that converts the transformed AC voltage into a DC voltage by using one or more rectifying elements (e.g., diodes or other valve elements) that allow current to flow in a single direction only. In an example, the rectifier stage may be a full-bridge rectifier stage.

[0068] The rectified DC voltage may then be supplied to a high-voltage startup circuit that is configured to supply a sufficient level of current to a supply voltage terminal of a switching stage at switch-on. The high-voltage startup circuit may be an integrated part of the switching stage.

[0069] Furthermore, switched converters of the first and second power conversion stages of the LED driver may be configured as isolating power converters and may comprise a switching part, magnetics (inductor / transformer), and a converter output part (rectifying part). The switching stages may be configured to control the converter output parts of the first and second power conversion stages via an electronic transformer to generate a desired output power level. The switched converters may be a fly-back converters, forward converters or the like.

[0070] As already mentioned, the first power conversion stage may contain a feedback control input that is controlled by the feedback output 104 of the controller circuit 200 in order to regulate the output voltage (bus voltage) 106 across an output buffer capacitor 107 of the first power conversion stage 103. The controller circuit 200 may provide the feedback output 104 based on a minimum voltage drop across the current sources measured at input connections 204, 304, . . ,n04 serially arranged after to the LED loads 201-n01 in order to ensure that linear current sources have sufficient head room for PWM. Each string of the LED loads 201- nOl is connected to a respective one of the series capacitors 207-n07 which are charged in case the bus voltage 106 of the first power conversion stage 103 across the buffer capacitor 107 is higher than the sum of voltages of each branch / channel containing series capacitor 207-n07, the LED load 201-n01 and the linear current source within the controller circuit 200 of the second power conversion stage.

[0071] Additionally, the controller circuit 200 of the second power conversion stage may comprise a PWM control input 109 for PWM signals, which can be configured to supplied serial data or parallel analog input signals in order to control an LED current in output branches 213, 313, ... nl3.

[0072] A steady state condition of the multiplexing multi-channel LED driver is achieved once the series capacitors 207-n07 have reached a steady state voltage, i.e., the supply current towards these capacitors equals to the source currents of these capacitors 207- n07 towards the LED loads 201-n01 and the minimum headroom voltage across all linear current sources of the controller circuit 200 are achieved.

[0073] Furthermore, the controller circuit 200 of the second conversion stage may comprise a first output terminal connected to the bus voltage 106 and a second output terminal 105 connected to the first power conversion stage, to which the controller circuit 200 can output power drawn from the series capacitors 207-n07 towards the buffer capacitor 107.

[0074] The controller circuit 200 of the second power conversion stage may further comprise a data output 110 configured to apply auto-addressing via data bytes of the controller circuit 200 and passing a remaining data stream to a further controller circuit of a subsequent stage e.g. in a cascaded power conversion topology.

[0075] Even through the power multiplexing can be applied independently for all output channels, it can be advantageous to ensure that the sum of duty-cycles of all output channels equals 1 for the sake of minimizing losses, maintaining a resolution over a dimming range of the LED loads 201 to nOl and / or for simplifying the controls (e.g., as the first and second stage converters do not require synchronization).

[0076] To realize the condition that the sum of duty cycles adds up to 1, the following equation can be applied to approximate the duty-cycles D203 to Dno3 of the output channels 203 to n03 for minimum head voltages V204,minto Vno4,min at the connections 204 to n04:

[0077] Other algorithms can be applied, e.g., Dn03=l-D203-D303, to ensure that the sum of duty cycles applied to all output channels 213 to nl3 adds up to 1.

[0078] For example, in case of very high variations in head room voltage between the output channels 213-nl3, cycle skipping may be applied, e.g., by configuring the controller circuit 200 to skip 1 to n PWM power multiplexing cycles for the channel with lowest duty cycle.

[0079] Such mechanisms not only result in power savings, but also improve control design at very low duty-cycle levels. Yet another advantage of setting the sum of duty cycles to 1 is that at lower dimming levels of the LED loads 201 to nOl the control can achieve full resolution for applying power multiplexing, thereby risk of audible noise caused by intermodulation can be minimized.

[0080] Fig. 2 shows schematically a circuit diagram of an exemplary implementation of a multiplexing multi-channel LED driver according to a first embodiment.

[0081] The circuit diagram of Fig. 2 shows more details of a practical implementation of the fractional power multiplexing multi-channel LED driver.

[0082] As it becomes clear from Fig. 2, the circuit has two positions where the voltage drop can be controlled. Namely, one is at the high side i.e., the supply voltage 106 , and the other is at the low side voltage drop across the linear current sources. There are different ways of implementing such current sources, but in essence they all require a sufficient headroom voltage 211 , 311 , ... nl 1 in order to control PWM appropriately. The headroom voltages 211 to nl 1 are supplied to respective input terminals of the controller circuit 200.

[0083] Yet, to convert power and to minimize losses, the controller circuit 200 of the second power conversion stage may apply respective higher frequency control signals 204, 304, . . ,n04 and 205, 305, . . ,n05 to respective anti-series metal oxide semiconductor (MOS) transistors 202, 302, ...n02 and 203, 303, ...n03 (i.e., SMPS and multiplexing switches) for converting the input power through primary winding 1082 and connection 113 to the output winding 1081 of a transformer 108 via a rectifying diode 109. The input power is supplied to the controller circuit 200 via a rectifying diode 110 and a smoothing capacitor 111.

[0084] For improvement of efficiency, an SMPS switch (implemented by MOS transistors 202 to n02) may only be operated when a corresponding multiplexing switch (implemented by MOS transistors 203 to n03) is in a conductive state. If the corresponding multiplexing switch is not in a conductive state, the SMPS switch will not be able to draw current. Therefore, the control signals (gate signals) 204 to n04 and 205 to n05 can be applied in parallel.

[0085] The control of the second power conversion stage can be achieved by independently applying PWM signals to each of the LED output channels 213 to nl3 via PWM inputs 206, 306, . . ,n06 of the current sources without loss of resolution. LED PWM control is decoupled per channel by the common buffer capacitor 107 and the series capacitors per channel 207 to n07.

[0086] The current sources can be implemented by transistors 212, 312, . . ,nl2 with negative feedback signals 209, 309, . . ,n09 obtained by respective emitter resistors 210, 310, . . .nlO. The negative feedback signals 209 to n09 are supplied to respective voltage reference circuits 208, 308, . . ,n08 controlled via the PWM inputs 206 to n06 to obtain a PWM- controlled output current through the LED loads 201 to nOl.

[0087] Optionally, the minimum headroom voltage of the current sources can be taken into account for controlling the output power of the first power conversion stage 103 through the feedback output 104 of the second power conversion stage. E.g., if one of headroom voltages at the output channels 213 to nl3 falls below a predefined threshold, output power of the first power conversion stage 103 is controlled to be increased via feedback output 104.

[0088] The proposed driver topology may also be operated without controlling the feedback output 104, i.e., the output voltage of the first power conversion stage 103 may be fixed (e.g., 24V or any other voltage higher than the highest voltage required to power a channel). The duty cycles of the multiplexing channels 213 to nl3 may be set to be inversely proportional to the headroom voltages of that specific channels. E.g., the output channel with the highest headroom voltage has the lowest duty cycle. Likewise, the lowest headroom channel has the highest duty-cycle.

[0089] In examples, the bandwidth of the control loop for the power multiplexing circuit of the second power conversion stage to be at least an order of magnitude higher than the bandwidth of the control loop of the first power conversion stage 103 to limit the interaction between the two control loops. Practically, this may be the case, as high-power- factor drivers and / or power supplies have a low-bandwidth response to meet requirements concerning total harmonic distortion and power factor. Fig. 3 shows schematically exemplary waveforms of control signals to explain a steady state operation of the fractional multiplexing multi-channel LED driver of the first embodiment shown in Fig. 2.

[0090] In this example, all duty cycles (three are depicted in Fig. 3) of the multiplexer signals 204 to n04 are set to be equal suggesting that the driver circuit has reached steady state condition and all head room voltages 211 to nl 1 are nearly equal. The voltages for the three different LED strings 201, 301 and nOl are practically chosen to be 18V, 17V and 20V, respectively.

[0091] The graphs of the power signals of Fig. 3 explain the steady state operation of the fractional power converter for power multiplexing in multi-channel LED driver applications. The common supply voltage for such an application at the output of the first power conversion stage 103 may be set to 24V (which may be advantageous for backwards compatibility).

[0092] As can be gathered from Fig. 3, a primary current I1082 is multiplexed over the output channels (three channels are shown in Fig. 3) in sense of time division multiplexing. In the first phase, the controller circuit 200 sets control signal 204 of the multiplexer switch 202 into a conductive state so that fly-back primary SMPS switch 203 is controlled via control signal 205 to apply the PWM for fly-back conversion of the first output channel 213. All other switches are set to non-conducting state. Likewise, the second (control signals 304, 305) to n-th phases (control signals n04, n05) are sequentially processed such that power is withdrawn from the respective one of the series capacitors 207 to n07 and pumped into the buffer capacitor 107 at the input of the second power conversion stage.

[0093] The slope and peak of the primary winding current I1082 depends on the voltages across the serial capacitors 207 to n07 and the average LED current. I.e., the higher the capacitor voltage, the steeper the slope, the shorter the duty cycle and the higher the peak of the primary winding current I1082. The lower the capacitor voltage, the flatter the slope, and the lower the peak of the primary winding current Iio82.Thus, voltage drop and LED current determine the peak current, slope and duty cycle for an appropriately chosen inductance.

[0094] Furthermore, a relaxation winding current through the output winding 1081 is represented as Iio8i which supplies all multiplexed power back into the buffer / input capacitor 107 during a non-conductive state of the SMPS switches 203 to n03.

[0095] The magnitude of the winding currents depends power that needs to be converted per channel i.e., the access volage and average current per channel. Fig. 4 shows schematically a circuit diagram of an exemplary implementation of a multiplexing multi-channel LED driver with shared series capacitor according to a second embodiment.

[0096] In the second embodiment, the multi-channel LED driver is configured so that output channels nl3 and p 13 with equal or almost equal forward voltage can make use of the same series capacitor n07, e.g., they are connected to the same series capacitor n07. The other components / signals p08 to p013 of the capacitor-sharing output channel p 13 correspond at least in their function to the respective components of the other output channels 213 to nl3.

[0097] Thus, the LED output channels nl3 and p 13 with almost equal forward voltages can be operated with the single series capacitor n07 in order to save cost and space. As the forward voltage of the output channel p 13 of an LED load pOl is almost equal to the forward voltage of the output channel nl3 of the LED load nOl, the power losses due to voltage mismatch between the two output channels nOl and pOl are negligible (i.e., not worth the effort of adding an additional power multiplexing branch). In examples, such configurations may be applicable in LED strips that contain blue LEDs with phosphor conversion to warm white and cool white LEDs, where similar forward voltages are nearly identical.

[0098] Fig. 5 shows schematically a circuit diagram of a first alternative multichannel LED driver implementation with independent boost converters, according to a third embodiment.

[0099] In the third embodiment, each of the output channels 213 to nl3 contains independent boost converters, so that the control function can be simplified. The boost converters are implemented by MOS transistor multiplex switches 203, 303, . . ,n03 that are controlled by respective control signals 205, 305, . . ,n05 to transfer energy from the serial capacitors 207 to n07 via respective diodes 214, 314, . . ,nl4 to the buffer capacitor 107. The control signals 205 to n05 may be generated (e.g., based on the headroom voltages 211 to nl 1) by the controller circuit 200 (not shown in Fig. 5) which also generates the feedback signal 104 as explained in the previous embodiments.

[0100] It is noted that in the third embodiment, the transformer 108 is not required.

[0101] It is worth noting that in the third embodiment the current sources with their respective transistors 121 to nl2 are floating on the potential of the respective series capacitors 207 to n07. Thereby, all LED loads 201 to nOl can be configured with one common forward and return path in order to limit the number of interconnections in the product design. Furthermore, in the third embodiment, the voltage reference circuits 208 to n08 of the current sources may comprise a level shifting circuit or element to convert the incoming ground referenced PWM signal 206 to n06 to the floating current source. As the levels of floating voltages are very low, such level shifting can be implemented very effectively, i.e., efficient with fast response.

[0102] Fig. 6 shows schematically a circuit diagram of a second alternative multichannel LED driver implementation with multiple secondary transformer windings, according to a fourth embodiment.

[0103] In the fourth embodiment, a transformer with multiple primary windings 1082, 1083, . . . 108n and a single (secondary) output winding 1081 is applied to omit the need of the power multiplexing topology with anti-serial MOS transistors 202 to n02 and 203 to n03, as shown in Fig. 2. The energy stored in the respective serial capacitors 207 to n07 is now transferred under control of the MOS transistor switches 203 to n03 based on respective control signals 205 to n05 via the respective primary windings 1082 to 108m and the single output winding 1081 and the rectifying diode 214 to the buffer capacitor 107. Moreover, to prevent conduction of body diodes of the MOS transistor switches 203 to n03, series diodes 215 to nl 5 have been added.

[0104] Similar to the third embodiment, the control signals 205 to n05 may be generated (e.g., based on the headroom voltages 211 to nl 1) by the controller circuit 200 (not shown in Fig. 6) which also generates the feedback signal 104 as explained in the previous embodiments.

[0105] It is worth noting that the winding ratios of the transformer of the fourth embodiment may be linked to the voltage differences between the output channels 213 to nl3 (LED branches). Thereby, a risk of undesired current conduction through respective body diodes of the MOSFET transistor switches 203 to n03 can be prevented.

[0106] Fig. 7 shows schematically a block diagram of a multiplexing multi-channel LED driver topology with first converter stage (AC / DC) and common second converter stage (DC / DC), corresponding to the second power conversion stage of the above embodiments, for several sections or groups of LED strips, according to a fifth embodiment.

[0107] The common second converter stage is configured to output individual output voltages (forward voltages) VI, V2 and V3.1 to V3.3 for five output channels to which cascaded sections or groups of respective five LED loads each may be connected in parallel.

[0108] As indicated in Fig. 7, the second converter stage may comprise an antenna element for wireless control of the LED loads with respect to on / off, intensity and / or color. The output voltages V3.1 to V3.3 may be substantially equal to allow connection to a single shared serial capacitor, as shown in Fig. 4.

[0109] The fifth embodiment allows outputting individual forward voltages for complete LED strip configurations.

[0110] The solution of the fifth embodiment may be logical from cost and design point of view, as the DC / DC converter and controller circuit of the second converter stage are placed outside the LED strip and thereby allow backwards compatibility with existing luminaire products.

[0111] However, efficiency may be further optimized by applying the DC / DC driver device of the second converter stage per section or group of LED strips such that voltage variations between sections or groups of LED strips and / or voltage drops over connecting cable lengths can be compensated by the local second DC / DC converter stages.

[0112] Fig. 8 shows schematically a block diagram of a multiplexing multi-channel LED driver topology with a plurality of locally arranged second DC / DC driver devices per LED load (e.g., LED strip), according to a sixth embodiment.

[0113] The controller circuit (CTRL) of the second converter stage may be configured to supply bus voltage and control data to each of the local second DC / DC driver devices.

[0114] If then the control data can be applied serially, losses in the LED loads (e.g., LED strips) can be further reduced, as the entire LED load can be powered / controlled by two common conductors rather than one common and individual conductors per output channel. For example, individual conductors for separate output channels of colors red, green, amber, warm white, cold white may be designed almost equally while not all conductors are supplying the maximum current at the same time. By using common conductors, the maximum current passes through the same single supply and return conductor regardless of which color is selected.

[0115] To summarize, apparatus and methods have been described for applying power multiplexing to a power converter device to convert an access voltage (which may be a small portion compared to the total load forward voltage) back to an input. A first stage provides a regulated bus voltage and feedback is provided to regulate the bus voltage to keep it just above a required headroom voltage. Series capacitors are coupled to each load device (e.g., LED string) to compensate for differences between the bus voltage and the forward voltage. Inputs can be activated depending on the status of the load device, while the outputs of the power converter device correspond to the capacitors at the bus. Thereby, power losses can be reduced and the size of power converters can be minimized compared to traditional power converter topologies.

[0116] 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 embodiments can be applied in connection with any type of loads, any types of DC / DC converters and any number of output channels (including a single output channel). Moreover, the function of power multiplexing can be implemented by various different multiplexer types known to the skilled person.

[0117] More specifically, elements and components mentioned in the above embodiments may be embedded in one or more integrated circuits (ICs), e.g., application specific ICs (ASICs) or programmable logic arrays (PLAs) or the like.

[0118] The above embodiments may be implemented in various high-efficiency products such as office luminaires, outdoor lighting, LED strips, color-tuneable spots or the like.

[0119] 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.

[0120] 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. The described procedures of the controller circuit 200 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. A driver device adapted to supply power to at least one load (201 to nOl), the driver device comprising: at least one switching element (203 to n03); at least one current source; an input buffer capacitor (107) a controller circuit (200) for controlling the at least one switching element (203 to n03) based on a measured headroom voltage across at least one respective current source that controls an output current of the driver device through the at least one load (201 to nOl); and at least one serial capacitor (207 to n07) connected in series with the at least one load (201 to nOl) and the at least one current source in a branch connected in parallel to the input buffer capacitor (107); wherein the controller circuit (200) is configured to control the at least one switching element (203 to n03) to transfer energy from the at least one serial capacitor (207 to n07) to the input buffer capacitor (107) until the measured headroom voltage has reached a predetermined value.

2. The driver device of claim 1, wherein the input buffer capacitor (107) is used to generate a bus voltage for the driver device.

3. The driver device of claim 1 or 2, wherein the controller circuit (200) is configured to control a plurality of switching elements (203 to n03) based on respective measured headroom voltages across respective current sources of a plurality of multiplexed output channels (213 to n03) to transfer energy from a plurality of serial capacitors (207 to n07) to the input buffer capacitor (107).

4. The driver device of claim 3, wherein the controller circuit (200) is configured to control the plurality of switching elements (203 to n03) to obtain a predetermined sum, inparticular a value of 1, of duty cycles of the plurality of multiplexed output channels (213 to nl3).

5. The driver device of any one of the preceding claims, further comprising a transformer (108) with at least one first winding (1082) to which the energy of the at least one serial capacitor (207 to n07) is transferred, and a second winding (1081) to which the buffer capacitor (107) is connected.

6. The driver device of claim 5, wherein the transformer (108) comprises a plurality of first windings (1082 to 108n) which are connected to a plurality of the switching elements (203 to n03) to transfer energy from a plurality of the serial capacitors (207 to n07) of a plurality of multiplexed output channels (213 to nl3) to the input buffer capacitor (107).

7. The driver device of any one of the preceding claims, wherein the at least one current source is integrated in the driver device and the driver device comprises at least one input terminal for supplying modulation signals for modulating the output current of the driver device through the at least one load (201 to nOl).

8. The driver device of any one of the preceding claims, wherein the controller circuit (200) comprises an output (104) for controlling a preceding converter stage (103) to control a difference between a bus voltage supplied by the preceding converter stage (103) and the measured headroom voltage.

9. The driver device of any one of the preceding claims, wherein the controller circuit (200) comprises a shared serial capacitor (n07) that is connected in series to two or more loads (nOl, pOl).

10. A lighting system comprising one or more driver devices of any one of the preceding claims and the at least one load (201 to nOl).

11. A method of controlling a driver device according to any of the claims 1 to 9, the method comprising:controlling at least one switching element (203 to n03) based on a measured headroom voltage across at least one respective current source that controls an output current of the driver device through the at least one load (201 to nOl); wherein the at least one switching element (203 to n03) is controlled to transfer energy from the at least one serial capacitor (207 to n07) to the input buffer capacitor (107) until the measured headroom voltage has reached a predetermined value.

12. A computer program product comprising code means for producing the steps of claim 11 when run on a controller circuit (200) of a driver device.

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