Patent Description:
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.

<CIT> discloses a two-stage power supply circuit including a power factor correction (PFC) circuit as a first stage. Based on an improved voltage sensing circuit, a low power consumption is achieved in a standby mode and in a normal steady-state mode, while maintaining a reasonably low overvoltage protection voltage threshold.

<CIT> discloses an LED converter comprising a boost regulator. The boost regulator includes a regulation band control circuit that controls a size of a regulation band of a load voltage based on a load current level.

<CIT> discloses a dimmable current-controlled LED converter.

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.

Preferred embodiments are set forth in the dependent claims and in the following description and 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.

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> illustrates an embodiment of an LED converter <NUM> according to a first aspect, and an embodiment of a lighting system <NUM>, <NUM> 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>, the LED converter <NUM> may comprise a rectifier stage <NUM>, 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>, the LED converter <NUM> comprises a power factor correction, PFC, converter <NUM> providing a feedback-controlled output voltage <NUM>, and a switched DC/DC converter <NUM> supplied by the output voltage <NUM> of the PFC converter <NUM> and providing a feedback-controlled output current <NUM> to output terminals of the LED converter <NUM>, which are designed for supplying an LED load <NUM>.

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 <NUM>.

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

The control unit <NUM> issues control signals <NUM>, <NUM> e.g. for operating a switch of the (active) PFC <NUM>, and for operating one or more switches of the switched DC/DC converter <NUM>. The control unit <NUM> may implement:.

in order to feedback control the respective values of the feedback signals to a nominal value. The nominal value for the output current <NUM> may be variable and e.g. supplied by a dimming signal <NUM> supplied to the control unit <NUM> in order to achieve a dimming of the light output of the LED load <NUM>. Thus the converter can be put into a state of low load operation.

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

A symbol of a voltage meter shown in <FIG> at an output terminal of the PFC converter <NUM> indicates a sensing location of the output voltage <NUM> of the PFC converter <NUM>. The output voltage <NUM> may be detected in known manner. For example, the output voltage <NUM> of the PFC converter <NUM> 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 <NUM>.

Similarly, a symbol of a current meter shown in <FIG> at an output terminal of the DC/DC converter <NUM> indicates a possible sensing location of the output current <NUM> of the DC/DC converter <NUM>. Of course, the output current <NUM> may be detected in known manner, too. For example, the output current <NUM> of the DC/DC converter <NUM> 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 <NUM>. Those skilled in the art will appreciate that low-side current sensing as shown in <FIG> may be also substituted by high-side current sensing.

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

<FIG> illustrates an embodiment of a closed-loop (feedback) voltage control of the PFC converter <NUM>.

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 <NUM>, i.e., its closed-loop voltage control, is configured to regulate a process variable, namely the output voltage <NUM> of the PFC converter <NUM>, which is shown on a right-hand side of <FIG>. To this end, the real (sensed) and feedback value of the process variable is subtracted from a preset nominal value <NUM> which serves as a reference variable (nominal value) for the output voltage <NUM>, and the resulting control error is passed to a feedback control unit <NUM> of the PFC converter <NUM>.

The feedback control unit <NUM> is configured to provide a control signal <NUM> for operating the switch of the PFC converter <NUM> such that the output voltage <NUM> is feedback-controlled to the preset nominal value <NUM>. As such, the switch is part of the regulated process <NUM>. Thus, the feedback control for the PFC results in a control signal <NUM> 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 <NUM> of the closed-loop voltage control of <FIG> is configured to modulate the control signal <NUM> with a modulation term <NUM> prior to supplying it to the switch, resulting in a modulated output voltage <NUM> of the PFC converter <NUM>. Thus, in control theory terms, the modulation effected by the modulation unit <NUM> can be seen as a disturbance of the control signal <NUM>.

As used herein and in accordance with <FIG>, "modulation" of a signal refers to an addition or superposition of a modulation term to/on said signal.

Thereby, the control signal <NUM> may purposely be superimposed with the modulation term <NUM> to yield a modulated control signal <NUM>.

A modulation amplitude of the modulation term <NUM> depends on an electrical power consumption parameter of the LED load <NUM> fed to the modulation unit <NUM>. For example, the electrical power consumption parameter of the LED load <NUM> may be derived from the output voltage <NUM> of the PFC converter <NUM> and/or from the output current <NUM> of the DC/DC converter <NUM>. More specifically, the electrical power consumption parameter may comprise a peak-to-peak value of the output voltage <NUM> of the PFC converter <NUM>, and/or a mean value of the output current <NUM> of the DC/DC converter <NUM>. 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 <NUM> of the closed-loop voltage control of <FIG> is configured to apply the modulation in the following ways:
Either, the modulation unit <NUM> is configured to selectively apply the modulation only when the value of the electrical power consumption parameter of the LED load <NUM> 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 <NUM> of the PFC converter <NUM>, and/or a fraction of a preset nominal value of the output current <NUM> of the DC/DC converter <NUM>.

Based on these preset threshold values, the modulation unit <NUM> 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 <NUM> is configured to apply a higher modulation amplitude at lower values of the electrical power consumption parameter of the LED load <NUM> 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 <NUM> is increased under low-load conditions, and so is a regulation activity of the DC/DC converter <NUM>, which in turn reduces the effective output quantization. That is to say, the added modulation at the input of the DC/DC converter <NUM> 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 <NUM> may suppress visible light flicker.

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

As used herein, a "time constant" τ refers to a parameter characterizing a (speed of) response of a process/system to a step input. As such, the time constant τ 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 τ is inherently related to a cut-off frequency fc = <NUM>/(2πτ), 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 τ or the cut-off frequency fc, the less will the process/system be able to respond.

As a result, the closed-loop voltage control of the PFC converter <NUM> having a time constant τ 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 <NUM>/τ, or - equivalently - modulation frequencies which are significantly higher (e.g., by a preset factor C/(2π) than the cut-off frequency fc = <NUM>/(2πτ).

Thereby, the output voltage <NUM> of the PFC converter <NUM> may purposely be modulated by adding a modulation term having an appropriately chosen modulation frequency.

When the PFC converter <NUM> is provided with a DC input voltage, the PFC converter <NUM> may be configured to sweep an ON-time of a duty cycle of the control signal <NUM>.

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

In the alternative, when the PFC converter <NUM> is provided with an AC input voltage, the PFC converter <NUM> may be configured to increase the ON-time of the duty cycle of the control signal <NUM> 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 <NUM>.

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> illustrates a flow chart of a method <NUM> according to a third aspect.

The method <NUM> is for operating an LED converter <NUM> according to the first aspect, such as the LED converter <NUM> of <FIG> in combination with <FIG>.

The method <NUM> comprises providing <NUM> a feedback-controlled output voltage <NUM> by providing a control signal <NUM> for operating a switch of the PFC converter <NUM> such that the output voltage <NUM> is feedback-controlled to a preset nominal value <NUM>.

The method <NUM> further comprises providing <NUM>, based on supply of the output voltage <NUM>, a feedback-controlled output current <NUM> to output terminals of the LED converter <NUM> being designed for supplying an LED load <NUM>.

The method <NUM> further comprises modulating <NUM> the control signal <NUM> with a modulation term <NUM> prior to supplying it to the switch, resulting in a modulated output voltage <NUM> of the PFC converter <NUM>. A modulation amplitude of the modulation term <NUM> depends on an electrical power consumption parameter of the LED load <NUM> fed to the modulation unit <NUM>.

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

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

Claim 1:
An LED converter (<NUM>), comprising:
a power factor correction, PFC, converter (<NUM>) configured to provide a feedback-controlled output voltage (<NUM>), and
a switched DC/DC converter (<NUM>) supplied by the output voltage (<NUM>) of the PFC converter (<NUM>) and configured to provide a feedback-controlled output current (<NUM>) to output terminals of the LED converter (<NUM>),
the output terminals being designed for supplying an LED load (<NUM>),
the PFC converter (<NUM>) comprising
a feedback control unit (<NUM>) configured to provide a control signal (<NUM>) for operating a switch of the PFC converter (<NUM>) such that the output voltage (<NUM>) is feedback-controlled to a preset nominal value, and
a modulation unit (<NUM>) configured to modulate the control signal (<NUM>) by superposition with a modulation term (<NUM>), having a modulation frequency and a modulation amplitude, prior to supplying it as a modulated control signal (<NUM>) for operating the switch, resulting in a modulated output voltage (<NUM>) of the PFC converter (<NUM>),
wherein the modulation amplitude of the modulation term (<NUM>) depends on an electrical power consumption parameter of the LED load (<NUM>) fed to the modulation unit (<NUM>); and
wherein the modulation unit (<NUM>) is further configured to:
selectively apply the modulation only when the value of the electrical power consumption parameter of the LED load (<NUM>) 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 (<NUM>) compared to the modulation amplitude at higher values thereof; and
wherein the modulation frequency of the modulation term (<NUM>) is higher than an inverse of a time constant τ of a closed-loop voltage control of the PFC converter (<NUM>) by at least a preset factor (C).