DESCRIPTION

Technical Field

The present invention relates to LED converters, and in particular to PFC-DC/DC converter systems operating under a low-load condition.

Background

Known LED converter systems may employ a DC/DC converter stage for supply of a constant current to an LED load. Such a DC/DC converter may regulate against a ripple of its own supply voltage, which is usually provided by a power factor correction, PFC, converter.

Typically, the DC/DC converter is continuously busy with regulation of the LED current since a supply voltage from a PFC converter practically always exhibits some variation. This in turn ensures a particular level of variation of the LED current, resulting in little effective quantization of light output.

However, under a low-load condition, such as in dimming applications, there may be virtually no ripple on the supply voltage of the DC/DC converter. In response, the feedback control of a switched DC/DC converter may have a low frequency variation of the operation of the switch and thus show a low-frequency modulation behavior, causing an effective quantization of light output to become more noticeable. In other words, a less frequent variation of the LED current may translate into visible light flicker.

Summary of the Invention

The object of the present invention is to provide an LED converter capable of suppressing or at least reducing visible light flicker under low-load conditions. The invention is defined by the appended independent claims. Preferred embodiments are set forth in the dependent claims and in the following description and drawings.

According to a first aspect, an LED converter is provided. The LED converter comprises a power factor correction, PFC, converter providing a feedback-controlled output voltage, and a switched DC/DC converter, isolated or non-isolated, such as e.g. a flyback converter, a buck converter, a boost converter etc. The switched DC/DC converter is supplied by the DC output voltage of the PFC converter and providing a feedback-controlled output current to output terminals of the LED converter. The output terminals are designed for supplying an LED load. The PFC converter further comprises a feedback control unit configured to provide a control signal for operating a switch of the PFC converter such that the output voltage is feedback-controlled to a preset nominal value, and a modulation unit configured to modulate the control signal with a modulation term prior to supplying it to the switch, resulting in a modulated output voltage of the PFC converter. A modulation amplitude of the modulation term depends on an electrical power consumption parameter of the LED load fed to the modulation unit. The modulation unit is further configured to selectively apply the modulation only when the value of the electrical power consumption parameter of the LED load is below a preset threshold value, or apply a higher modulation amplitude at lower values of the electrical power consumption parameter of the LED load compared to the modulation amplitude at higher values thereof.

The electrical power consumption parameter may comprise a mean value of the output current of the DC/DC converter; and the preset threshold value may comprise a fraction of a preset nominal value of the output current of the DC/DC converter.

The electrical power consumption parameter may comprise a peak-to-peak value of the output voltage of the PFC converter; and the preset threshold value may comprise a fraction of a preset maximum peak-to-peak value of the output voltage of the PFC converter.

A modulation frequency of the modulation term may be higher than an inverse of a time constant of a closed-loop voltage control of the PFC converter by at least a preset factor. When the PFC converter is provided with a DC input voltage, the PFC converter may be configured to sweep an ON-time of a duty cycle of the control signal.

When the PFC converter is provided with an AC input voltage, the PFC converter may be configured to increase the ON-time of the duty cycle of the control signal in a temporal vicinity of zero-crossings of the AC input voltage.

According to a second aspect, a lighting system is provided. The lighting system comprises an LED converter according to the first aspect or any of its embodiments, and an LED load configured to be provided with the output current of the LED converter.

According to a third aspect, a method of operating an LED converter is provided. The method comprises providing a feedback-controlled output voltage by providing a control signal for operating a switch of the PFC converter such that the output voltage is feedback-controlled to a preset nominal value. The method further comprises, based on supply of the output voltage, providing a feedback- controlled output current to output terminals of the LED converter being designed for supplying an LED load. The method further comprises modulating the control signal with a modulation term prior to supplying it to the switch, resulting in a modulated output voltage of the PFC converter, wherein a modulation amplitude of the modulation term depends on an electrical power consumption parameter of the LED load fed to the modulation unit. The method further comprises selectively applying the modulation only when the value of the electrical power consumption parameter of the LED load is below a preset threshold value, or applying a higher modulation amplitude at lower values of the electrical power consumption parameter of the LED load compared to the modulation amplitude at higher values thereof.

The method may be performed by an LED converter according to the first aspect or any of its embodiments.

According to a fourth aspect, a control unit for an LED converter is provided. The control unit is designed for implementing the method according to the third aspect or any of its embodiments. Brief Description of the Drawings

Further aspects, advantages and objects of the invention will become evident for the skilled reader by means of the following detailed description of the embodiments of the invention, as illustrated in the enclosed drawings.

Fig. 1 illustrates an embodiment of an LED converter according to a first aspect, and an embodiment of a lighting system according to a second aspect;

Fig. 2 illustrates an embodiment of a closed-loop voltage control of the PFC converter; and

Fig. 3 illustrates a flow chart of a method according to a third aspect.

Detailed Descriptions of Embodiments

The invention will now be described with respect to various embodiments. The features of these embodiments may be combined with each other unless specified otherwise.

Fig. 1 illustrates an embodiment of an LED converter 1 according to a first aspect, and an embodiment of a lighting system 1, 15 according to a second aspect.

As used herein, an “LED converter” refers to an electronic power supply that efficiently converts electrical power, and in particular voltage and/or current characteristics, by controlling a transfer of electric power from a power source, such as an AC power grid, via at least one inductive power storage element, such as an inductor, to a power sink formed by at least one LED which is connectable to the electronic power supply. Said control is exercised by driving at least one power electronic switching element of the electronic power supply with an appropriate control signal.

With reference to Fig. 1, the LED converter 1 may comprise a rectifier stage 10, such as a diode bridge, supplied e.g. by an AC mains voltage, and providing full-wave rectification of said AC input/supply voltage. With continued reference to Fig. 1, the LED converter 1 comprises a power factor correction, PFC, converter 11 providing a feedback-controlled output voltage 12, and a switched DC/DC converter 13 supplied by the output voltage 12 of the PFC converter 11 and providing a feedback-controlled output current 14 to output terminals of the LED converter 1, which are designed for supplying an LED load 15.

As used herein, a “power factor” refers to a measure of phase mismatch and/or distortion of an ideally sinusoidal current signal with respect to an (virtually) ideal sinusoidal voltage signal. With respect to electronic power supplies, the power factor denotes a ratio of real power to apparent power. A power factor of less than one indicates that voltage and current signals are not in phase and/or a presence of harmonic distortion.

As used herein, “power factor correction”, PFC, refers to measures for increasing a power factor (close) to 1.

Fig. 1 further illustrates a control unit 16 of the LED converter 1 which is designed for operating the LED converter 1 by implementing a method 3 according to a third aspect or any of its embodiments, which will be explained in more detail in connection with Fig. 3 below. As such, the control unit 16 may be configured to communicate with the various entities of the LED converter 1, for example to receive a feedback signal representing the output voltage 12 of the PFC converter 11 and/or an feedback signal representing the output current 14 of the DC/DC converter 13.

The control unit 16 issues control signals 17, 18 e.g. for operating a switch of the (active) PFC 11, and for operating one or more switches of the switched DC/DC converter 13. The control unit 16 may implement: a feedback algorithm in order to determine e.g. the switching frequency of the switch of the PFC 11 based on the feedback signal representing the PFC output voltage 12, and a feedback algorithm in order to determine e.g. the switching frequency of the at least one switch of the switched DC/DC converter 13 based on the feedback signal representing the output current 14 in order to feedback control the respective values of the feedback signals to a nominal value. The nominal value for the output current 14 may be variable and e.g. supplied by a dimming signal 19 supplied to the control unit 16 in order to achieve a dimming of the light output of the LED load 15. Thus the converter can be put into a state of low load operation.

Those skilled in the art will appreciate that the control unit 16 may partially or completely be incorporated in the PFC converter 11 and/or the DC/DC converter 13, although it is illustrated as a separate entity in Fig. 1 for the sake of clarity. Further, the control unit 16 may be implemented as one integrated control unit, or a separate control units.

A symbol of a voltage meter shown in Fig. 1 at an output terminal of the PFC converter 11 indicates a sensing location of the output voltage 12 of the PFC converter 11. The output voltage 12 may be detected in known manner. For example, the output voltage 12 of the PFC converter 11 may be applied across a voltage/potential divider formed by a high-impedance resistor pair connected in series, and a voltage sensed across a low-side resistor of the divider may serve as an indication of the output voltage 12.

Similarly, a symbol of a current meter shown in Fig. 1 at an output terminal of the DC/DC converter

13 indicates a possible sensing location of the output current 14 of the DC/DC converter 13. Of course, the output current 14 may be detected in known manner, too. For example, the output current

14 of the DC/DC converter 13 may be passed through a shunt resistor having a very low but accurately known resistance, and a voltage sensed across the shunt resistor may serve as an indication of the output current 14. Those skilled in the art will appreciate that low-side current sensing as shown in Fig. 1 may be also substituted by high-side current sensing.

In combination with an LED load 15, which is indicated on a right-hand side of Fig. 1 and configured to be provided with the output current 14 of the LED converter 1, a lighting system according to a second aspect may be formed.

Fig. 2 illustrates an embodiment of a closed-loop (feedback) voltage control of the PFC converter 11. As used herein, a “closed-loop control” refers to an arrangement in which a process/system is regulated by a controller having a requisite corrective behavior. A feedback loop ensures that the controller exercises a control action to manipulate a process variable to be the same as a reference variable.

As used herein, a “closed-loop voltage control” refers to closed-loop control of a voltage control process.

The PFC converter 11, i.e., its closed-loop voltage control, is configured to regulate a process variable, namely the output voltage 12 of the PFC converter 11, which is shown on a right-hand side of Fig.

2. To this end, the real (sensed) and feedback value of the process variable is subtracted from a preset nominal value 20 which serves as a reference variable (nominal value) for the output voltage 12, and the resulting control error is passed to a feedback control unit 21 of the PFC converter 11.

The feedback control unit 21 is configured to provide a control signal 22 for operating the switch of the PFC converter 11 such that the output voltage 12 is feedback-controlled to the preset nominal value 20. As such, the switch is part of the regulated process 26. Thus, the feedback control for the PFC results in a control signal 22 determining e.g. the switching frequency of the switch of the PFC.

As used herein, a “switch” refers to an active power electronic switch, such as a MOSFET switch.

A modulation unit 23 of the closed-loop voltage control of Fig. 2 is configured to modulate the control signal 22 with a modulation term 24 prior to supplying it to the switch, resulting in a modulated output voltage 12 of the PFC converter 11. Thus, in control theory terms, the modulation effected by the modulation unit 23 can be seen as a disturbance of the control signal 22.

As used herein and in accordance with Fig. 2, “modulation” of a signal refers to an addition or superposition of a modulation term to/on said signal.

Thereby, the control signal 22 may purposely be superimposed with the modulation term 24 to yield a modulated control signal 25. A modulation amplitude of the modulation term 24 depends on an electrical power consumption parameter of the LED load 15 fed to the modulation unit 23. For example, the electrical power consumption parameter of the LED load 15 may be derived from the output voltage 12 of the PFC converter 11 and/or from the output current 14 of the DC/DC converter 13. More specifically, the electrical power consumption parameter may comprise a peak-to-peak value of the output voltage 12 of the PFC converter 11, and/or a mean value of the output current 14 of the DC/DC converter 13. Those skilled in the art will appreciate that low values of either of these electrical power consumption parameters indicate a low-load condition.

The modulation unit 23 of the closed-loop voltage control of Fig. 2 is configured to apply the modulation in the following ways:

Either, the modulation unit 23 is configured to selectively apply the modulation only when the value of the electrical power consumption parameter of the LED load 15 is below a preset threshold value.

So depending on the circumstances, the preset threshold value may comprise a fraction of a preset maximum peak-to-peak value of the output voltage 12 of the PFC converter 11, and/or a fraction of a preset nominal value of the output current 14 of the DC/DC converter 13.

Based on these preset threshold values, the modulation unit 23 may simply apply or activate the modulation under a low-load condition detected when the applicable electrical power consumption parameter falls below the corresponding preset threshold value, and not apply or deactivate the modulation under a normal load condition detected when the applicable electrical power consumption parameter exceeds the corresponding preset threshold value.

Or, the modulation unit 23 is configured to apply a higher modulation amplitude at lower values of the electrical power consumption parameter of the LED load 15 compared to the modulation amplitude at higher values thereof.

In particular, the modulation amplitude may be an arbitrary strictly monotonic decreasing function with increasing electrical power consumption parameter. Non-limiting examples comprise linearly, polynomially, logarithmically, or negative exponentially decreasing functions with increasing electrical power consumption parameter, with or without further terms such as constants, for example

Thereby, a variation/modulation at the input of the DC/DC converter 13 is increased under low-load conditions, and so is a regulation activity of the DC/DC converter 13, which in turn reduces the effective output quantization. That is to say, the added modulation at the input of the DC/DC converter 13 spreads the quantization noise of the DC/DC output current over a wider frequency range. In other words, a more frequent variation of the LED current 14 may suppress visible light flicker.

A modulation frequency of the modulation term 24 may be higher than an inverse of a time constant of the closed-loop voltage control of the PFC converter 11 by at least a preset factor.

As used herein, a “time constant" r refers to a parameter characterizing a (speed of) response of a process/system to a step input. As such, the time constant r of a closed-loop control is given by design/dimensioning and indicates how long it takes for the process variable to respond to a changing output of the feedback control unit.

The time constant r is inherently related to a cut-off frequency f _c = 1/(2JTT), a well-known boundary in said process/system's frequency response at which an energy flow through the system begins to be reduced rather than passing through. Correspondingly, the faster changes occur in relation to the time constant r or the cut-off frequency f _c , the less will the process/system be able to respond.

As a result, the closed-loop voltage control of the PFC converter 11 having a time constant r will not be able to eliminate modulation frequencies which are significantly higher (e.g., by a preset factor C) than the inverse of the time constant 1/r , or - equivalently - modulation frequencies which are significantly higher (e.g., by a preset factor C/(2JT) than the cut-off frequency f _c = 1/(2JTT).

Thereby, the output voltage 12 of the PFC converter 11 may purposely be modulated by adding a modulation term having an appropriately chosen modulation frequency. When the PFC converter 11 is provided with a DC input voltage, the PFC converter 11 may be configured to sweep an ON-time of a duty cycle of the control signal 22.

Thereby, electromagnetic interference, EMI, may be mitigated during DC operation.

In the alternative, when the PFC converter 11 is provided with an AC input voltage, the PFC converter 11 may be configured to increase the ON-time of the duty cycle of the control signal 22 in a temporal vicinity of zero-crossings of the AC input voltage.

Thereby, total harmonic distortion, THD may be corrected during AC operation.

As used herein, “total harmonic distortion” refers to a measure of harmonic distortion present in a signal, such as a voltage, for example. THD correction refers to measures for decreasing a power factor (close) to 0.

Because THD correction is virtually inactive under a low-load condition, no performance penalty will be faced when simultaneously applying or activating the modulation.

Fig. 3 illustrates a flow chart of a method 3 according to a third aspect.

The method 3 is for operating an LED converter 1 according to the first aspect, such as the LED converter 1 of Fig. 1 in combination with Fig. 2.

The method 3 comprises providing 30 a feedback-controlled output voltage 12 by providing a control signal 22 for operating a switch of the PFC converter 11 such that the output voltage 12 is feedback- controlled to a preset nominal value 20.

The method 3 further comprises providing 31, based on supply of the output voltage 12, a feedback- controlled output current 14 to output terminals of the LED converter 1 being designed for supplying an LED load 15. The method 3 further comprises modulating 32 the control signal 22 with a modulation term 24 prior to supplying it to the switch, resulting in a modulated output voltage 12 of the PFC converter 11. A modulation amplitude of the modulation term 24 depends on an electrical power consumption parameter of the LED load 15 fed to the modulation unit 23.

The method 3 further comprises selectively applying 33 the modulation only when the value of the electrical power consumption parameter of the LED load 15 is below a preset threshold value, or applying 33 a higher modulation amplitude at lower values of the electrical power consumption parameter of the LED load 15 compared to the modulation amplitude at higher values thereof.

The method 3 may be performed by an LED converter 1 according to the first aspect or any of its embodiments.

Accordingly, the advantages mentioned in connection with the various embodiments of the LED converter 1 of the first aspect apply similarly to the corresponding embodiments of the method 3 according to the third aspect.