Patent Description:
Switched-mode power converters (SMPCs), such as LLC converters or buck converters, can be used in electronic devices (such as e.g. LED converter) in order to convert electric power with high efficiency. However, high levels of electromagnetic interference (EMI) can occur in SMPCs. These high levels of EMI can be both conducted and radiated.

Therefore, when designing a SMPC, attention should be payed to the reduction of EMI. In order to reduce the EMI, different techniques can be used, for example, filtering, shielding or soft-switching. Moreover, another technique, the so-called spread-spectrum technique, can be used in order to reduce EMI in SMPCs. Moreover, the suppression of peak EMI levels can also be achieved by the modulation of the switching frequency of the converter, since the switching frequency modulation (SFM) can reduce both conducted and radiated EMI.

However, if the SFM is not done properly in the converter, this can cause a visible flicker in the light output.

Especially, the activation /deactivation of the SFM, when done while the SFM modulation signal is high, can lead to a jump in the operation frequency which can lead to a visible jump in the light output of LEDs supplied by such converter.

<CIT> discloses a feedback-controlled switched converter having the features of the preambles of the independent claims.

Thus, it is an objective to provide for an improved switched converter for a LED load which allows to reduce the EMI and/or the visible flicker in the light output of a supplied LED load.

According to a first aspect of the invention, a switched converter is provided. The switched converter comprises at least one (preferably one switch or two switches forming a half bridge, or four switches forming a full bridge) switch, terminals for supplying an LED load, and a control unit being supplied with a feedback signal (generated by a feedback signal generating unit) indicating a load current of the LED load. The control unit is configured to generate an output signal on the basis of the feedback signal, combine the output signal with a periodic modulation signal in order to obtain a control signal, the control signal being configured to set an operation parameter of the at least one switch, and apply the control signal to the at least one switch. Moreover, the switched converter comprises means for enabling/disabling the periodic modulation signal, wherein the means for enabling/disabling is configured to enable/disable the periodic modulation signal only at time periods of the periodic modulation signal in which an amplitude of the periodic modulation signal is lower than an (preset) amplitude threshold.

In a preferred embodiment, the means for enabling/disabling the periodic modulation signal is configured to only enable/disable the periodic modulation signal at first time periods of the periodic modulation signal being centered at zero-crossings of the periodic modulation signal, these first time periods being separated from each other by time periods in which no enabling/disabling of the modulation signal is performed.

If an event triggering the enabling/disabling of the modulation signal occurs during time periods/amplitudes during which the enabling/disabling shall not be performed, means are provided for delaying the execution of the enabling/disabling until the next amplitude/time period starts in which the enabling/disabling is allowed.

An event triggering the enabling can be when the load at the output terminals raises (e.g., by dimming) beyond a preset load threshold.

An event triggering the disabling can be when the load at the output terminals drops (e.g., by dimming) below a preset load threshold.

This provides the advantage that enabling/disabling is only performed in amplitude regions/time periods in which the enabling/disabling does only lead to a low or no visible jump in the light output.

In a preferred embodiment, the switched converter further comprises a signal generator, wherein the signal generator is configured to generate the periodic modulation signal.

In another preferred embodiment, the periodic modulation signal is derived from a signal within the converter, such as, e.g., a ripple in the DC supply voltage of the converter.

This provides the advantage that a visible flicker in the output light of the LED is reduced, preferably, eliminated.

In a preferred embodiment, the means for enabling/disabling the periodic modulation signal is configured to detect a maximum amplitude of the periodic modulation signal and wherein the amplitude threshold is less than <NUM>% of the maximum amplitude of the periodic modulation signal.

This provides the advantage that the enable/disable signal is delayed close to the next zero crossing of the periodic modulation signal and, in such a way, there is no significant instantaneous jump in the output signal provided to the LED load. Therefore, the flicker in the output light is advantageously reduced.

In an alternative embodiment, the means for enabling/disabling the periodic modulation signal is configured to detect a change in a sign of the modulation signal. The change enable/disable signal thus is delayed until the next zero-crossing event of the modulation signal and will be executed upon detection of the zero-crossing.

In particular, the means for enabling/disabling the periodic modulation signal is configured to detect the change in the sign by detecting that the sign of the measured signal or modulation signal changes from positive to negative values, or vice versa, during the crossing of the zero point. Since a digital evaluation of the signals is performed (sensing by an analog-to-digital converter (ADC) and digital processing of the signals) such evaluation is, advantageously, easy to implement.

In a preferred embodiment, the periodic modulation signal is a triangle waveform or a sinusoidal waveform.

This provides the advantage that the suppression of peak EMI levels can be achieved by the modulation of switching frequency by making use of periodic modulating waveforms, such as sine, triangle or saw-tooth, therefore, spreading the spectrum of SMPC voltages and currents.

In a preferred embodiment, wherein the periodic modulation signal is based on an inverted input voltage of the switched converter.

This provides the advantage that a flicker in the output light of the LED is significantly reduced.

In a preferred embodiment, the operation parameter of the switched converter, such as a switching frequency or a peak current value of the switched converter, is configured to determine a power provided at the terminals for supplying the LED load.

In a preferred embodiment, the control unit comprises a proportional integral, PI, regulator.

This provide the advantage that well-known regulators, such as PI regulators, can be used.

In a preferred embodiment, the converter is an LLC converter, a flyback converter or a buck converter.

This provides the advantage that well-known converters can be used, thus facilitating the implementation of this embodiment of the invention.

According to a second aspect, a LED lighting means is provided. The LED lighting means comprises the switched converter of the first aspect and any one of the implementation forms thereof and a LED load connected to output terminals of the switched converter.

According to a third aspect, a method for switching a switching converter for supplying a LED load is provided. The method comprises the following steps: supplying a control unit with a feedback signal indicating a load current of the LED load; generating an output signal on the basis of the feedback signal; combining the output signal with a periodic modulation signal in order to obtain a control signal, the control signal being configured to set an operation parameter of the at least one switch; applying the control signal to at least one switch of the switched converter; and enabling/disabling the periodic modulation signal only at time periods in which an amplitude of the periodic modulation signal is lower than an amplitude threshold at first time periods of the periodic modulation signal being centered at zero-crossings of the periodic modulation signal, these time periods being separated from each other by time periods in which no enabling/disabling of the modulation signal is performed.

In particular, the means for enabling/disabling the periodic modulation signal is configured to detect the change in the sign by detecting that the sign of the measured signal or modulation signal changes from positive to negative values, or vice versa, during the crossing of the zero point.

In a preferred embodiment, the method further comprises the step of generating the periodic modulation signal by a signal generator in the switched converter.

In a preferred embodiment, the method further comprises the step of deriving the periodic modulation signal from a signal within the converter, such as e.g. a ripple in the dc supply voltage of the switched converter.

In a preferred embodiment, the method further comprises the step of detecting a maximum amplitude of the periodic modulation signal, wherein the amplitude threshold is less than <NUM>% of the maximum amplitude of the periodic modulation signal.

The method according to the third aspect and the implementation forms thereof provide the same advantages as the switched converter of the first aspect and the implementation forms thereof.

Aspects of the present invention are described herein in the context of a switched converter.

Now referring to <FIG>, a schematic representation of a LED lighting means <NUM> comprising a switched converter <NUM> is shown according to an embodiment of the invention.

The LED lighting means comprises the switched converter <NUM> and the LED load <NUM>, wherein the LED load <NUM> is connected to the switched converter <NUM> by means of the supplying terminals 102a and 102b.

The switched converter <NUM> comprises at least one switch <NUM>, the terminals 102a, 102b for supplying the LED load <NUM>, and a control unit <NUM> being supplied with a feedback signal indicating a load current of the LED load <NUM>. The control unit <NUM> is configured to generate an output signal on the basis of the feedback signal, combine the output signal with a periodic modulation signal in order to obtain a control signal, the control signal being configured to set an operation parameter of the at least one switch <NUM>, apply the control signal to the at least one switch <NUM>.

Moreover, the switched converter <NUM> comprises an enable/disable means <NUM> for enabling/disabling the periodic modulation signal, wherein the means for enabling/disabling <NUM> is configured to enable/disable the periodic modulation signal only at time periods of the periodic modulation signal in which an amplitude of the periodic modulation signal is lower than an amplitude threshold.

The means for enabling/disabling <NUM> the periodic modulation signal may be configured to only enable/disable the periodic modulation signal at separated time periods of the periodic modulation signal being centered at zero-crossings of the periodic modulation signal.

The means for enabling/disabling <NUM> may be supplied with a signal indicating the current amplitude or the phase of the modulation signal e.g. in order to delay any enabling/disabling action until the next amplitude.

The switched converter <NUM> can be, for example, an LLC resonant converter, a flyback converter or a buck converter.

In general, these switched converters can be feedback controlled, such that the feedback signal is fed to the control unit <NUM>, such as for example a PI regulator, which correspondingly (i.e., applying a control algorithm) issues the output signal on the basis of which, for example, the frequency (or the switch off threshold of a current through the switch) of the switched converter <NUM> can be set. In order to improve the performance of the switched converter <NUM>, it can be foreseen that the output signal of the control algorithm or control unit <NUM> is combined with another signal, i.e. the periodic modulation signal, and the combined frequency signal or control signal is actually used for setting the switching frequency of the switched converter <NUM>.

<FIG> shows a schematic representation of the LED lighting means <NUM> comprising the switched converter <NUM> according to an embodiment of the invention.

In the example shown in <FIG>, the switched converter <NUM> further comprises a signal generator <NUM>, which is configured to generate the periodic modulation signal. In this example, the periodic modulation signal is a triangle waveform. This provides the advantage that the performance of the switched converter <NUM> is improved by reducing the EMI. In fact, by making use of the triangle waveform, the power is spread over a wider frequency range.

In other examples, the periodic modulation signal can be a sinusoidal waveform.

Moreover, in the example shown in <FIG>, the operation parameter of the switched converter <NUM> is a switching frequency of the switched converter <NUM>, and it is configured to determine a power provided at the terminals 102a, 102b for supplying the LED load <NUM>.

Therefore, <FIG> shows the combination of the output signal of control unit <NUM>, in this example, a PI regulator, with a periodic triangular-shaped artificially generated signal, which leads to a correspondingly periodic modulation of the operation frequency of the switched converter <NUM>. This approach is, typically, used to have a modulation of the switching frequency of the switched converter <NUM> (in contrast to a non-varying frequency) in order, thus, to improve (broaden) the EMI spectrum.

Moreover, in this embodiment, the control signal directly sets the switching frequency of the switched converter <NUM>.

In <FIG> an implementation of the switched converter <NUM> is shown in which any ripple in the input voltage is reduced, in case the control unit <NUM> might not be able to compensate for it. To this regard, an inverted signal of the input ripple of the input DC voltage is added to the output signal of the PI regulator, and the, thus, combined signal or control signal is fed to the switched converter <NUM>. This provides the advantage that the power supply rejection ratio (PSRR) is increased.

The output signal of the PI regulator can directly set the switching frequency (<FIG>) or indirectly set the frequency by setting thresholds (<FIG>, in which the peak reference switch-off current through the at least one switch <NUM> of the switched converter <NUM> is set by the PI regulator).

Therefore, the control unit <NUM> can issue a signal setting a switching frequency or can issue a signal setting directly or indirectly a switch-off threshold (in which case the switching frequency might be constant).

<FIG> shows a schematic representation of a modulation signal, a modulated signal, and an enable/disable signal according to prior art.

Both scenarios shown in <FIG> and <FIG> can lead to the problem that in case the artificial modulation of the frequency or switch-on timing of the switched converter <NUM> is selectively switched on/off (for example, only enabled at low loads), at the time of enabling or disabling, the periodic modulation signal does not necessarily have a small or zero amplitude, as shown in <FIG>. In <FIG>, an example of modulation signal (upper panel), of modulated output signal (middle panel), and enable/disable signal (lower panel) are shown according to prior art. As it can be taken from <FIG>, the enabling/disabling performed at the peak of the sinusoidal waveform of the modulation signal leads directly to a jump in the modulated signal and, therefore, operation parameter of the switched converter <NUM> (frequency, switch-on timing, etc.). This is, typically, directly translated in the resulting light output, such that the enabling/disabling of the modulation signal may lead to a visible jump in the light output.

<FIG> shows a schematic representation of a modulation signal, a modulated signal, and an enable/disable signal according to an embodiment of the invention.

In order to solve the problems which were illustrated with reference to <FIG>, according to an embodiment of the invention, the disabling/enabling of this periodic modulation signal can only done at low amplitude of this modulation signal. Ideally, it is the zero crossing of this periodic modulation signal (see <FIG>). In particular, the disabling/enabling is only done in the next following time period of the periodic signal, in which the amplitude of the periodic signal is, for example, less than <NUM>% of the maximum amplitude.

Typically, this leads to a delay (waiting) until the amplitude has decreased to the predefined threshold. This leads, at the end, to a reduction of the visible light output variation upon enabling/disabling of the periodic modulation signal.

The frequency or switching off threshold represent the operation parameter of the switched converter determining the power provided at the output of the switched converter.

Moreover, the disabling/enabling may represent any modification of a parameter of the periodic modulation signal, which could also be, for example, a change of the frequency or amplitude of the periodic modulation signal.

<FIG> shows a schematic representation of a method <NUM> for switching a switched converter <NUM> for supplying a LED load <NUM> according to an embodiment of the invention.

In one embodiment, the enabling/disabling <NUM> the periodic modulation signal only at time periods in which an amplitude of the periodic modulation signal is lower than an amplitude threshold or at first time periods of the periodic modulation signal being centered at zero-crossings of the periodic modulation signal, these first time periods being separated from each other by time periods in which no enabling/disabling of the modulation signal is performed.

Claim 1:
A feedback-controlled switched converter (<NUM>), comprising:
- at least one switch (<NUM>);
- output terminals (102a, 102b) for supplying an LED load (<NUM>) ;
- a control unit (<NUM>) being supplied with a feedback signal indicating a load current of the LED load (<NUM>), wherein the control unit (<NUM>) is configured to:
- generate an output signal on the basis of the feedback signal;
- combine the output signal with a periodic modulation signal in order to obtain a control signal, the control signal being configured to set an operation parameter of the at least one switch (<NUM>);
- apply the control signal to the at least one switch (<NUM>) ;
characterized by
- means (<NUM>) for enabling and disabling the periodic modulation signal,
o wherein the means (<NUM>) for enabling and disabling is configured to perform an enabling or a disabling of the periodic modulation signal only during time periods of the periodic modulation signal in which an amplitude of the periodic modulation signal is lower than an amplitude threshold, or
o wherein the means (<NUM>) for enabling and disabling the periodic modulation signal is configured to perform the enabling or the disabling of the periodic modulation signal only during first time periods of the periodic modulation signal being centered at zero-crossings of the periodic modulation signal, these first time periods being separated by second time periods centered around the peaks of the periodic modulation signal and in which no enabling/disabling of the periodic modulation signal is performed, or
o wherein the means (<NUM>) for enabling and disabling the periodic modulation signal is configured to perform the enabling or the disabling of the periodic modulation signal only as soon as zero-crossing detection means of the feedback-controlled switched converter (<NUM>) indicate a zero-crossing of the modulation signal,
wherein the means (<NUM>) for enabling and disabling the periodic modulation signal is configured to disable the periodic modulation signal when the load at the output terminals (102a, 102b) is low and to enable the periodic modulation signal when the load at the output terminals (102a, 102b) is high,
wherein the means (<NUM>) for enabling and disabling the periodic modulation signal is configured to detect a maximum amplitude of the periodic modulation signal and wherein the amplitude threshold is less than <NUM>% of the maximum amplitude of the periodic modulation signal.