WO2025176510A1

WO2025176510A1 - Driver circuit with linear headroom control

Info

Publication number: WO2025176510A1
Application number: PCT/EP2025/053552
Authority: WO
Inventors: Dalibor Cvoric; Karnekumar ARULANDU; Theo Gerrit Zijlman
Original assignee: Signify Holding BV
Current assignee: Signify Holding BV
Priority date: 2024-02-21
Filing date: 2025-02-11
Publication date: 2025-08-28
Anticipated expiration: 2026-08-21

Abstract

The invention relates to a driver circuit for a luminaire or other load, wherein a driver output voltage for the luminaire is supplied by a high-side converter circuit. A headroom control voltage is generated relative to a ground potential of the driver output, whereas the ground level of the high-side converter circuit is floating. A headroom feedback circuit is provided as an interface circuit to transfer the headroom control voltage to the floating ground level of the high-side converter circuit and thereby achieve a linear and stable wide-range linear headroom control characteristic.

Description

Driver circuit with linear headroom control

FIELD OF THE INVENTION

The invention relates to the field of driver circuits for different types of loads, such as - but not limited to - luminaires of lighting systems, for use in various different applications for home, office, retail, street lightings, hospitality and industry.

BACKGROUND OF THE INVENTION

A headroom is the difference between input and output voltages in a series pass regulator. The headroom voltage refers to the actual voltage drop across the regulator, that needs to occur during operation. This is the difference between the amount of input voltage that must be supplied to the regulator, and the specified regulated output voltage.

In driver circuits for various types of loads, headroom control is used to increase efficiency of linear stages. A control signal may be generated to control a supply voltage level (e.g., bus voltage level) of one or more connected loads (such as luminaires). In this way, impact of e.g. forward voltage spread and temperature of the load(s) on driver efficiency can be mitigated. The headroom voltage may fluctuate due to signal modulation (e.g., pulse width modulation (PWM)) and/or sinusoidal mains input. Moreover, load current change or temperature change may cause load voltage change. To maximize efficiency, a minimum required headroom needs to be tracked and controlled. Also, in different modes of operation it can be desirable to adapt the value of the headroom voltage. Therefore, an adaptive headroom voltage control circuitry may be provided to ensure that sufficient voltage is supplied to all load devices (e.g., luminaires) while minimizing residual or headroom voltage to avoid unwanted dissipation of power. By tracking and controlling the minimum required headroom voltage, both efficiency and response of the driver circuit can be improved.

In luminaire drivers (e.g., light emitting diode (LED) drivers), control circuitry may include circuitry for operating one or more strings of light sources (e.g., LEDs) using pulse-width modulation (PWM) to control the brightness of the light sources. Each string may include one or more light sources coupled in series between a supply voltage source and a current controller. The supply voltage source may provide a common supply voltage to the strings. A headroom voltage control circuitry may then sample a headroom voltage for each string of light sources and may raise or lower the supply voltage to maintain a desired headroom voltage.

However, the headroom control signal may need to be transferred from a low- voltage side of the driver to a high-voltage side of the driver. Conventionally, this has been achieved by using an opto-coupler. In practice, such a solution has drawbacks including nonlinear control, limited control range, high feedback gain and/or negative impact on converter stability.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a driver circuit with improved headroom control.

This object is achieved by a driver circuit as claimed in claim 1 and by a luminaire as claimed in claim 12.

According to a first aspect, a driver circuit for driving a load device is provided, the driver circuit comprising: a converter stage for generating a supply voltage for the load device; a switching circuit for controlling power conversion of the converter stage, wherein the switching circuit is located on a high-side portion of the driver circuit, so that a ground potential of the switching circuit is floating; a headroom control circuit for generating a headroom control signal relative to a ground potential of a low-side portion of the driver circuit; and an interfacing feedback circuit configured to shift a level of the headroom control signal and to feed back the level-shifted headroom control signal to the converter stage; wherein the driver circuit is configured to combine the level-shifted headroom control signal with a feedback signal for controlling the supply voltage of the load device to obtain an enhanced feedback signal that is supplied to the switching circuit.

Furthermore, according to a second aspect, a luminaire is provided, which comprises one or more light sources and a driver circuit according to the first aspect.

Accordingly, improved converter stability can be achieved by the proposed driver configuration with interfacing feedback circuit. Converter operation instabilities caused typical solutions with opto-couplers can be prevented. The feedback input of switching circuits is sensitive and its operation can be disrupted/affected by any additional circuitry being connected there, such as an opto-coupler. The proposed driver configuration with interfacing feedback circuit mitigates such instabilities.

Furthermore, a linear control characteristic can be achieved, where the supply voltage level of the load device linearly follows any changes in the headroom control signal. The transfer function of the interfacing feedback circuit can easily be tuned, e.g., by adjusting values of one or more resistors of the feedback circuit. Thus, the feedback gain can easily be tuned, so that a change of the voltage of the headroom control voltage (e.g., from 0 to 3.3 V) can result in any desired range of the supply voltage level.

Moreover, a wide control range can be obtained, since the complete range of the headroom control signal (e.g., from 0 to 3.3 V) has impact on the value of supply voltage of the load. Small feedback gain:

According to a first option of the first or second aspect, the driver circuit may further comprise a current control circuit configured to generate a control input for the headroom control circuit, that is related to fluctuations of characteristics of the load device.

According to a second option that may be combined with the first option or the first or second aspect, the headroom control circuit may be configured to generate the headroom control signal with a level that is adapted to characteristics of the feedback signal for controlling the supply voltage of the load device.

According to a third option that may be combined with the first or second option or the first or second aspect, the feedback circuit may be configured to transfer the headroom control signal to a high-side level of the high-side portion of the driver circuit by means of an up-conversion circuit.

According to a fourth option that may be combined with any one of the first to third options or the first or second aspect, the feedback circuit may be configured to buffer the headroom control signal relative to the floating ground potential of the high-side portion of the driver.

According to a fifth option that may be combined with the fourth option or the first or second aspect, an output of the switching circuit may be connected to one end of a primary side of a transformer of the converter stage, wherein the other end of the primary side of the transformer is connected to an output of the driver circuit; wherein a buck diode of the converter stage is connected between the floating ground potential of the high-side portion and the ground potential of the low-side portion; wherein a capacitor of the converter stage is connected between the output of the driver circuit and the ground potential of the low-side portion; and wherein the output of the switching circuit is connected to the floating ground potential of the high-side portion.

According to a sixth option that may be combined with the fifth option or the first or second aspect, a secondary side of the transformer may be connected between the floating ground potential of the high-side portion and an auxiliary voltage for setting the enhanced feedback signal via a voltage divider at the secondary side of the transformer.

According to a seventh option that may be combined with the sixth option or the first or second aspect, a third resistor may be provided at an output of the feedback circuit to determine, together with resistors of the voltage divider, a transfer function from a controlled headroom voltage to the enhanced feedback signal at a feedback input of the switching circuit.

According to an eighth option that may be combined with any one of the first to seventh options or the first or second aspect, the feedback circuit may comprise a first resistor and a first capacitor connected as a low-pass filter for filtering and buffering the headroom control signal relative to the ground potential of the low-side portion, and further comprises a diode and a second capacitor to shift the filtered headroom control signal to a level of the high-side portion.

According to a ninth option that may be combined with the eighth option or the first or second aspect, the second capacitor may further be configured to shield a feedback input of the switching circuit.

It shall be understood that the driver circuit of claim 1 and the luminaire of claim 12 may have similar and/or identical preferred embodiments, in particular, as defined in the dependent claims.

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.

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

In the following drawings:

Fig. 1 shows schematically a block diagram of a driver with linear headroom control, according to various embodiments; and Fig. 2 shows schematically an exemplary circuit diagram of a luminaire driver with interfacing headroom feedback circuit for linear headroom control according to an embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

Various embodiments of the present invention are now described, which are applicable to drivers for 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.

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

The following embodiments are directed to luminaires with one or more LED light sources that are controlled by a separate or integrated driver circuit or module. They can be implemented in connection with any type of luminaire module or board and are applicable to various kinds of luminaire drivers or converters of luminaires.

Fig. 1 shows schematically a block diagram of a driver with linear headroom control, according to various embodiments.

An input voltage Vi_n of the driver may be generated by a power supply circuit (not shown) e.g. from a power grid voltage of 110 or 220V at a mains frequency of 50 or 60Hz. The power supply circuit may comprise an electromagnetic interference (EMI) filter for attenuating electromagnetic interference from the power system to limit the noise in the system and lower a risk of malfunctioning of the driver. The filtered AC voltage may then be supplied to a rectifier stage which is an electronic device that converts the filtered AC voltage into the 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.

The rectified voltage Vi_n is supplied to a switching circuit (SW) 10 of the driver. The switching circuit 10 may be part of a converter stage (CONV) 20 of the driver, that may be configured as an isolating or non-isolating power converter and may further comprise magnetics (inductor/transformer), a converter output part (rectifying part) and a controller of the switching stage. In isolating power converters, the switching circuit 10 of the converter stage 20 may be configured to control the converter output part of the converter stage via an electronic transformer to supply a desired power level to a load device (LD) 30, e.g., a luminaire.

The converter stage 20 may be configured as a buck converter, a boost converter, a buck-boost converter, a flyback converter, a half-bridge converter, a full bridge converter, a forward converter, a push-pull converter, a resonant converter, or combinations of these.

In embodiments, the switching circuit 10 of the converter stage 20 may be configured to use one or more switching elements to transform the DC voltage Vi_n into a pulsed waveform. The switching elements may be semiconductor switches, such as metal oxide semiconductor field effect transistors (MOSFETs) or bipolar junction transistors (BJTs).

The switching circuit 10 and the converter stage 20 (e.g., with a (optional) transformer and the converter output part) may be configured to operate as an electrical power converting device that regulates the power to the load device 30 and that may respond to changing needs of the load device 30 by supplying a constant amount of power or current to the load device 30, as its electrical properties (e.g., light source(s) such as LEDs) may change e.g. with temperature. Thereby, the load device 30 can be provided with a very specific electrical power in order to operate properly.

In the driver of Fig. 1, the switching circuit 10 is placed in an upper circuit portion of the driver with respect to the load device 30 and therefore belongs to a high-side portion (power supply side), while the output part of the converter stage 20 belongs to a low- side portion (ground side). Thus, the switching circuit 10 and the upper circuit portion (e.g., a primary side of a transformer) of the converter stage 20 belong to the high side (HS) of the driver, while the output part (e.g., a secondary side of the transformer) of the converter stage 20 and the load device 30 belong to a low side (LS) of the driver.

According to embodiments, headroom control is achieved via feedback from a headroom voltage sensor of a headroom control circuit (HRC) 50. A headroom control signal HRCS (which is associated with the sensed headroom voltage) is generated by the headroom control circuit (HRC) 50 and supplied to the converter stage 20 via an interfacing feedback circuit (FBC) 60 which may be configured to provide a filter and/or buffer function. Thereby, the headroom control signal HRCS is filtered, level-shifted and fed back as a headroom signal VHR to the converter stage 20. The feedback headroom signal VHR is combined at the converter stage 20 with a control signal for controlling a bus voltage (supply voltage) Vbus of the load device 30 to obtain an enhanced feedback signal VFBthat is supplied to the switching circuit 10 for controlling the power conversion of the converter stage 20. In response to the enhanced feedback signal VFB, the switching circuit 10 modifies its switching operation (e.g., switching phase, switching frequency, duty cycle, etc.) in a closed loop circuit to maintain a proper bus voltage with sufficient headroom.

The current through the load device 30 may be controlled by a current control (CC) circuit 40 which generates a control input for the headroom control circuit 50, that is related to fluctuations of characteristics (e.g., forward voltage, resistance etc.) of the load device 30. The current control circuit 40 may be configured as a constant current source, wherein a control voltage (e.g., that is associated with the level of the bus voltage Vbus) generated by the current control circuit 40 may be used as the control input for the headroom control circuit 50.

The headroom control circuit 50 (which may be implemented as passive or active signal converter) is configured to generate the headroom control signal HRCS with a level that is adapted to the characteristics of the control signal generated at the converter stage 20 for controlling the bus voltage Vbus of the driver.

In embodiments, the feedback circuit 60 may be configured to filter and buffer the headroom control signal HRCS at the output of headroom control circuit 50. This ensures that the operation of the headroom control circuit 50 is not affected by high switching frequency and high voltage transitions (dV/dt) of the switching circuit 10 and/or the converter stage 20. Furthermore, the feedback circuit 60 may be configured to transfer the generated headroom control signal HRCS to a high-side level by means of an up-conversion circuit (which may consist of diode(s) and resistor(s)). In examples, the switching circuit 10 and/or the converter stage 20 may measure the bus voltage Vbus during a non-conducting state of the switching circuit 10 and a conduction state of a freewheeling diode of the converter stage 20 (e.g., a back diode of a buck converter). At this moment, the ground levels of the high-side and low-side portions of the driver are nearly on the same level. Moreover, to ensure that that the feedback input of the switching circuit 10 is not exposed to any unwanted high voltage transients (dV/dt) or excessive high voltage levels, the feedback circuit 60 of the driver may be configured to buffer the transferred headroom control signal HRCS relative to the ground potential of the high-side portion of the driver. Fig. 2 shows schematically an exemplary circuit diagram of a luminaire driver with interfacing headroom feedback circuit 60 for linear headroom control according to an embodiment.

The luminaire driver generates a bus voltage Vbus supplied to a luminaire 30 which may consist of a series of LEDs.

Headroom control is achieved by a buck converter stage with a high-side buck converter switching integrated circuit (B-IC) 10 for high-efficiency design, to which the input voltage Vin is applied and which is connected to high-side ground (GND-H). Furthermore, an output of the buck converter switching circuit 10 is connected to one end of a primary side of a transformer of the buck converter stage, while the other end of the primary side of the transformer is connected to the driver output. A buck diode of the buck converter stage is connected between high-side ground and low-side ground (GND-L) and a capacitor of the buck converter stage is connected between the output of the luminaire driver and the low-side ground. The output of the buck converter switching circuit 10 is connected to the high-side ground.

The secondary side of the transformer is connected between the high-side ground and an auxiliary voltage V_aux used to properly set a feedback voltage VFB generated by a series connection of resistors R4 and R5 that act as voltage divider for the secondary voltage of the transformer. The voltage across the resistor R5 is fed back to a feedback input of the buck converter switching circuit 10 for controlling the bus voltage Vbus at the output of the luminaire driver.

It is noted that, in other embodiments, the secondary winding (i.e., auxiliary winding) can be replaced by a circuit where the resistor R4 is connected to the output (Vbus) of the driver circuit at a reduced efficiency. Then, the transformer is replaced by an inductance that reflects the primary side of the transformer.

The buck or step-down converter stage is a DC-to-DC converter which decreases voltage, while increasing current, from its input (Vin) to its output (Vbus). It is a class of switched-mode power supply. Switching converters (such as buck converters) provide much greater power efficiency as linear regulators which are simpler circuits that dissipate power as heat, but do not step-up the output current.

The luminaire current ILED is controlled by a current control circuit 40 which is a linear stage that generates a feedback signal supplied to a headroom control circuit 50 which uses the feedback signal to generate a headroom control signal HRCS. The headroom control signal HRCS is supplied to the feedback circuit 60 for obtaining a control loop for the headroom control. The current control circuit 40 as well as the headroom control circuit 50 are connected to low-side ground. Thus, the luminaire current ILED is controlled by a linear stage and the headroom control signal HRCS is generated relative to the low-side ground potential.

According to the embodiment of Fig. 2, the bus voltage Vbus is generated by the buck converter stage, wherein the buck converter switching circuit 10 is located on a high-side portion of the luminaire driver, so that the ground potential (high-side ground) of the buck converter switching circuit 10 is floating. The bus voltage is controlled by the feedback signal (feedback voltage VFB) relative to the high-side ground.

The proposed feedback circuit 60 for headroom control in the embodiment of Fig. 2 comprises a first resistor R1 and a first capacitor Cl connected as a low-pass filter for filtering and buffering the received headroom control signal HRCS relative to the low-side ground. Furthermore, a diode D2 and a second resistor R2 are provided to transfer the filtered headroom control signal to the high-side portion when the buck diode of the buck converter stage is conducting. The characteristics of the diode D2 are selected to sufficiently protect the low-voltage circuitry of headroom control on the low-side portion of the luminaire driver from the high voltage (e.g., mains voltage) at the high-side portion of the luminaire driver.

Additionally, the feedback circuit 60 comprises a second capacitor C2 for shifting/up-converting the headroom control signal HRCS to the high-side level of the high side portion and for shielding the feedback input (e.g., pin) of the buck converter switching circuit 10 from high transient voltages (dv/dt) and high voltages, to thereby ensure proper operation of buck converter closed-loop control.

An additional third resistor R3 may be provided at the output of the feedback circuit 60 to determine, together with the resistors R5 and R4, the transfer function (voltage divider ratio) from the controlled headroom voltage to the feedback voltage VFB at the feedback input of the buck converter switching circuit 10. The feedback voltage VFB is level- shifted to a high-side controller of the buck converter switching circuit 10 using the diode D2 which is only conductive when the (freewheel) buck diode is conductive. Thus, a properly dimensioned third resistor R3, together with the resistors R4 and R5, leads to a control of the bus voltage Vbus within a desired predefined range. The voltage divider R4/R5 sets/limits the maximum bus voltage Vbus which can be lowered by the headroom control signal HRCS (i.e., an extra current through R3 will increase the feedback voltage VFB and thereby reduce the bus voltage Vbus). Without any feedback from the headroom control circuit 50, the maximum bus voltage Vbus will be generated at the output of the driver circuit. To summarize, a driver circuit for a luminaire or other load has been described, wherein a driver output voltage for the luminaire is supplied by a high-side converter circuit. A headroom control voltage is generated relative to a ground potential of the driver output, whereas the ground level of the high-side converter circuit is floating. A headroom feedback circuit is provided as an interface circuit to transfer the headroom control voltage to the floating ground level of the high-side converter circuit and thereby achieve a linear and stable wide-range linear headroom control characteristic.

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 linear headroom control circuit can be applied in connection with any type of load.

Furthermore, the driver circuit of the above embodiments with at least one of the switching circuit 10, the converter stage 20, the current control circuit 40, the headroom control circuit 50 and the feedback circuit 60 may be integrated in a circuit board or module of the luminaire 30 or another load.

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.

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.

Claims

CLAIMS:

1. A driver circuit for driving a load device (30), the driver circuit comprising: a converter stage (20) for generating a supply voltage for the load device (30); a switching circuit (10) for controlling power conversion of the converter stage (20), wherein the switching circuit (10) is located on a high-side portion of the driver circuit, so that a ground potential of the switching circuit (10) is floating; a headroom control circuit (50) for generating a headroom control signal (HRCS) relative to a ground potential of a low-side portion of the driver circuit; and an interfacing feedback circuit (60) configured to shift a level of the headroom control signal (HRCS) and to feed back the level-shifted headroom control signal (HRCS) to the converter stage (20); wherein the driver circuit is configured to combine the level-shifted headroom control signal (HRCS) with a feedback signal for controlling the supply voltage of the load device (30) to obtain an enhanced feedback signal (VFB) that is supplied to the switching circuit (10).

2. The driver circuit of claim 1, further comprising a current control circuit (40) configured to generate a control input for the headroom control circuit (50), that is related to fluctuations of characteristics of the load device (30).

3. The driver circuit according to claim 1 or 2, wherein the headroom control circuit (50) is configured to generate the headroom control signal (HRCS) with a level that is adapted to characteristics of the feedback signal for controlling the supply voltage of the load device (30).

4. The driver circuit according to any one of claims 1 to 3, wherein the feedback circuit (60) is configured to transfer the headroom control signal (HRCS) to a high-side level of the high-side portion of the driver circuit by means of an up-conversion circuit.

5. The driver circuit according to any one of the preceding claims, wherein the feedback circuit (60) is configured to buffer the headroom control signal (HRCS) relative to the floating ground potential of the high-side portion of the driver.

6. The driver circuit according to any one of the preceding claims, wherein the switching circuit (10) is a buck converter circuit.

7. The driver circuit according to claim 6, wherein an output of the switching circuit (10) is connected to one end of a primary side of a transformer of the converter stage (20), wherein the other end of the primary side of the transformer is connected to an output of the driver circuit; wherein a buck diode of the converter stage (20) is connected between the floating ground potential of the high-side portion and the ground potential of the low-side portion; wherein a capacitor of the converter stage (20) is connected between the output of the driver circuit and the ground potential of the low-side portion; and wherein the output of the switching circuit (10) is connected to the floating ground potential of the high-side portion.

8. The driver circuit according to claim 7, wherein a secondary side of the transformer is connected between the floating ground potential of the high-side portion and an auxiliary voltage (V_aux) for setting the enhanced feedback signal (VFB) via a voltage divider at the secondary side of the transformer.

9. The driver circuit according to claim 8, wherein a third resistor (R3) is provided at an output of the feedback circuit (60) to determine, together with resistors (R4, R5) of the voltage divider, a transfer function from a controlled headroom voltage to the enhanced feedback signal (VFB) at a feedback input of the switching circuit (10).

10. The driver circuit according to any one of the preceding claims, wherein the feedback circuit (60) comprises a first resistor (Rl) and a first capacitor (Cl) connected as a low-pass filter for filtering and buffering the headroom control signal (HRCS) relative to the ground potential of the low-side portion, and further comprises a diode (D2) and a second capacitor (C2) to shift the filtered headroom control signal to a level of the high-side portion.

11. The driver circuit according to claim 10, wherein the second capacitor (C2) is configured to shield a feedback input of the switching circuit (10).

12. A luminaire (30) comprising one or more light sources and a driver circuit according to any one of claims 1 to 11.