Method for driving light source and corresponding device and system

Abstract

A method for driving (10) one or more electrically powered light sources, such as light emitting diode (LED) modules (121, 122, ..., 12n), by applying thereto a pulse width modulation (PWM) signal having a pulse repetition frequency and a duty cycle, the duty cycle being selectively variable, in order to vary the intensity of the light emitted by the one or more light sources (121, 122, ..., 12n). In order to combat the occurrence of light artifacts or TLAs, the method envisages varying the pulse width modulation signal (V PWM ) pulse repetition frequency, to frequency modulate (100, V fm )The pulse width modulation signal (V PWM ), thereby generating a signal with combined FM / PWM modulation.

Description

Technical Field

[0001] The present disclosure relates to lighting devices.

[0002] One or more embodiments may function, for example, in lighting systems that use electrically powered solid-state light sources, such as LED light sources. Background Art

[0003] The rate at which a solid-state lighting (SSL) light source can vary the intensity of the emitted optical radiation is one of the main driving forces for innovation in the field of lighting and lighting applications.

[0004] Associated with the rate at which the intensity of the emitted optical radiation may be varied is whether it is desirable or not to convert the modulation of the drive current directly into a modulation of the emitted luminous flux.

[0005] This light modulation causes changes in the perception of the environment.

[0006] In some applications, such as very specific entertainment, scientific, or industrial applications, this change in perception caused by the modulation of light may be a desired effect.

[0007] For most applications and daily activities, this change could be detrimental and undesirable.

[0008] The general term used to identify these changes in environmental perception is "temporal light artifacts" (TLA): these artifacts can have a significant impact on how the quality of light is perceived. Furthermore, visible light modulation can lead to decreased performance, increased fatigue, and health problems such as epileptic seizures and migraine attacks.

[0009] Different terms exist to describe the different types of temporal artifacts (TLA) that humans can perceive.

[0010] The term "flicker" refers to changes in light that are directly perceptible to an observer.

[0011] The “Stroboscopic Effect” is the effect that can be seen by an observer when a moving or rotating object is illuminated (CIE TN 006:2016).

[0012] Possible causes of modulation of the light emitted by a lighting device that may cause a flickering or stroboscopic effect include:

[0013] AC power supply and its control tool topology combined with light source technology;

[0014] Dimming techniques (adjustment of intensity) applied through the use of external dimmers or integrated light level regulators; and

[0015] Mains voltage fluctuations caused by electrical devices connected to the mains (conducted electromagnetic disturbance) or by electrical devices intentionally used to transmit mains signals.

[0016] Lighting products that exhibit unacceptable flickering effects are considered to be of poor quality.

[0017] For flicker, a measure called short-term flicker severity or P is used. st LM The parameter is derived from the widely used and accepted standardized P for evaluating the effect of voltage fluctuations on flicker. st The measurement (see IEC TR 61547-1) has been standardized at the International Electrotechnical Commission (IEC) level.

[0018] For objective evaluation of the stroboscopic effect, the stroboscopic effect visibility measurement (SVM) is described by the Minkowski metric in the IEC TR63158 standard, namely:

[0019]

[0020] in:

[0021] C i is the relative illumination I i the relative amplitude of the ith Fourier component (trigonometric representation of the Fourier series) (relative to the DC level); and

[0022] T i is the visibility threshold of the stroboscopic effect of a sinusoidal wave at the frequency of the i-th Fourier component.

[0023] The visibility threshold function T(f), also known as the stroboscopic effect contrast threshold function, identifies the relative amplitude of the sinusoidal modulation in addition to the constant illumination level that has only a 50% probability of being visible to an ordinary observer.

[0024] This function is defined in CIE TN 006:2016 by the following equation:

[0025]

[0026] Where f is the frequency in Hertz.

[0027] In CIE TN 006:2016, this visibility threshold function is defined up to 2000 Hz. The reason for this is that, in general lighting applications, the stroboscopic effect is not perceptible for modulation frequencies above 2000 Hz. Therefore, the sum of the spectral components is also limited to below 2000 Hz (see LEC TR 63158).

[0028] However, in the case where the spectrum of the waveform extends beyond 2000 Hz, limiting the sum of the spectrum components to such a frequency range may cause anomalies in the calculation of the SVM value.

[0029] To avoid this anomaly, the definition of the visibility threshold function has been extended to above 2000 Hz in the article "Invited Paper: Modeling Visibility of Temporal Light Artefacts" by Perz, M. et al., published in SID Symposium Digest of Technical Papers, 49.1028-1031.10.1002 / sdtp.12194, as shown below:

[0030]

[0031] The existing traditional stroboscopic effect contrast threshold (stroboscopic visibility threshold or SVT) function and the new stroboscopic effect contrast threshold function are Figure 1 The dashed line (I) and the solid line (II) are respectively represented as functions of the frequency f (in Hz).

[0032] As can be seen in the figure, the existing threshold function (dashed line I) is defined only up to 2000 Hz, while the extended threshold function (solid line II) is defined above 2000 Hz. It can also be seen that for the existing threshold function (dashed line I), the asymptotic value near 2000 Hz becomes a constant close to the value 1.

[0033] This is unphysical behavior and is inconsistent with the fact that above 2000 Hz, the stroboscopic effect is not visible. The extended threshold curve T(f) of the last equation shown above shows a trend that the value of T(f) becomes very high above 2000 Hz, which means that waveforms at these modulation frequencies will not be perceived as stroboscopic effects.

[0034] This is more in line with the actual feeling of the strobe effect.

[0035] The European Commission regulation within the framework of the “Ecodesign Requirements”, which sets out eco-compatible design specifications for light sources and their associated power supplies, envisages a rather low limit value (<0.4) for this SVM parameter, which makes it difficult for constant voltage (CV) SSL systems with pulse width modulation (PWM) dimming to meet the requirements of the regulation, especially when using low dimming levels (e.g. below 10%).

[0036] It should be emphasized that pulse-width modulation (PWM) dimming technology is based on an on-off (full-depth) rectangular waveform. The harmonic content of this modulation typically extends to higher harmonics, thus including frequencies well above the "nominal" (carrier) frequency. The highest extent of these harmonics is related to the rise and fall times of the wavefront. For a PWM pulse frequency of 1 kHz, components exceeding 10 kHz are common.

[0037] The contrast threshold function is given for sinusoidal light, and when many components are present in the analyzed light (according to the Fourier decomposition), the Minkowski norm given by the first equation provided above is used for the SVM parameters.

[0038] Most electronically controlled gadget units (ECGs) at constant voltage perform pulse width modulation (PWM) at a fixed frequency, typically equal to or lower than 1.0 kHz.

[0039] These devices were designed many years ago and are far from complying with the latest regulations on temporal artifacts. The minimum non-modulation frequency that complies with the regulations is actually about 2.5 kHz.

[0040] There are indeed some electronic control units (ECGs) that implement standard pulse width modulation (PWM) at frequencies above 2 kHz. This is the case, for example, with the products from the OSRAM group of companies under the trade names OTi BLE 80 / 220, ..., 240 / 241, ..., 4CH (2.01 kHz) (see osram.com) or the product from MeanWell under the trade name PWM-60-KN (up to 4 kHz).

[0041] It may be noted that this type of ECG may not be able to perform lamp failure detection (eg according to DALI requirements) by eg utilizing pulse shifting techniques.

[0042] Furthermore, it can be noted that approximately 2 kHz is the highest usable frequency that allows for reasonable propagation of the shortest PWM pulses over longer cables without excessive distortion resulting in uneven light distribution. Operating at higher frequencies to meet the new specifications for SVM would hinder the possibility of extending the cable length of this system to within the 20-50 meter range.

[0043] For example, the document of US Patent 2016 / 057823 A1 provides an example of the prior art. Other relevant documents include DE 20 2017 002443 U1, US 2009 / 303161 A1 and US 2007 / 103086 A1. Summary of the Invention

[0044] It is an object of one or more embodiments to help overcome the above-mentioned disadvantages.

[0045] According to one or more embodiments, the above-mentioned objects are achieved thanks to a method having the characteristics mentioned in the following claims.

[0046] One or more embodiments may relate to a corresponding device (eg a so-called electronic control tool or ECG for a lighting system).

[0047] One or more embodiments may relate to a corresponding lighting system.

[0048] The claims form an integral part of the technical teaching provided herein in connection with the described embodiments.

[0049] One or more embodiments may help achieve one or more of the following advantages:

[0050] Ability to achieve SVM values that comply with the latest regulations;

[0051] Reduced maximum and average frequency, reduced pulse width modulation (PWM) pulse distortion related to cable / module length, and improved end-to-end uniformity of light over relatively long modules; and

[0052] Multi-channel current can be measured using pulse shifting techniques, as well as detecting lamp failures, also for low dimming levels (<5%). BRIEF DESCRIPTION OF THE DRAWINGS

[0053] One or more embodiments will now be described, purely by way of non-limiting example, with reference to the accompanying drawings, in which:

[0054] As discussed above Figure 1 ;

[0055] Figure 2 It is a block diagram of the lighting system;

[0056] Figure 3A and Figure 3B A diagram showing possible signals that may be used in the solution described herein;

[0057] Figure 4 Example based on Figure 3A and Figure 3B Possible plots of the Fast Fourier Transform (FFT) of an example standard generated signal;

[0058] Figure 5A and Figure 5B A possible diagram representing signals that may be used according to some embodiments;

[0059] Figure 6 Example based on Figure 5A and Figure 5B Possible plots of the Fast Fourier Transform (FFT) of the standard generated signal exemplified in FIG;

[0060] Figure 7 is a block diagram illustrating a lighting system according to some embodiments; and

[0061] Figure 8 illustrates possible diagrams of signals in embodiments according to the present description. DETAILED DESCRIPTION

[0062] In the following description, various specific details are shown to provide a deeper understanding of various examples of embodiments according to the present disclosure. The embodiments can be obtained without one or more of the specific details, or with other methods, components, materials, etc. In other cases, well-known structures, materials, or operations are not shown or described in detail so as not to obscure aspects of the embodiments.

[0063] References to "an embodiment" or "one embodiment" throughout this specification are intended to indicate that a particular configuration, structure, or feature described with respect to that embodiment is included in at least one embodiment. Therefore, phrases such as "in an embodiment" or "in one embodiment" that may appear at various points in this specification do not necessarily refer to the same embodiment. Furthermore, in one or more embodiments, the particular configurations, structures, or features may be combined in any appropriate manner.

[0064] The terminology / references used herein are provided for convenience only and thus do not limit the scope of protection or the scope of the embodiments.

[0065] Figure 2 A constant voltage (CV) type solid-state lighting (SSL) system is illustrated.

[0066] As shown, such a system may include an electronic power supply (electronic control means or ECG) arranged between a power grid PG (e.g., an alternating current (AC) power source or network) and one or more solid-state lighting modules 121, 122, ..., 12n (e.g., LED lighting modules).

[0067] In the system shown here, the ECG 10 is capable of providing a desired voltage (eg, 12V, 24V, or 48V) to the modules 121 , 122 , . . . , 12 n via the connection line 14 .

[0068] As shown, the ECG 10 can perform additional functions, such as brightness adjustment (dimming), power factor correction, radio frequency interference suppression, and lighting control interfaces (e.g., DALI, BLE, Zigbee).

[0069] The foregoing is obtained according to standards known to those skilled in the art, so that it becomes redundant to provide a more detailed description here.

[0070] For example, the dimming function can be achieved by the pulse width modulation (PWM) technique, for example with a constant frequency (e.g., 250 Hz or 1.0 kHz), possibly reaching very low dimming levels (e.g., 1.0% or even 0.1% relative to full intensity).

[0071] However, in order to obtain an LED system with dimming that avoids the generation of undesirable flicker effects, the previously mentioned new standard (corresponding to a setting that can be defined as human-centric lighting, i.e., lighting centered around people) involves the use of a minimum frequency higher than 2.0 kHz (if the existing Stroboscopic Effect Visibility Measurement, SVM, is applied) or even higher than 2.5 kHz (if the new extended Stroboscopic Visibility Threshold function, SVT, is applied).

[0072] As shown, the connection line 14 that transmits power from the ECG 10 to the modules 121, 122, ..., 12n may comprise a cable, which may be 0.5-50 m (or even higher) in length.

[0073] The one or more modules 121, 122, ..., 12n can each include one or more LED chains or strings and a plurality of parallel-connected electrical units. Each electrical unit (often defined as a "minimum electrical unit," or SEU) can in turn include a plurality of LEDs connected in series and a current regulator for setting a desired current level, which can range from a few milliamperes to several hundred milliamperes.

[0074] If obtained in linear form, the length of each module of this type can be freely defined and customized up to a maximum of 20 meters.

[0075] In so-called DALI-compliant ballasts, in addition to the basic status monitoring, load monitoring can also be added, for example to detect lamp failure, by periodically (for example, at intervals of less than 30 seconds) detecting the current change when each load of the dimmer of the lamp is reduced to a low dimming level (for example, 5%).

[0076] Load changes are monitored by the current sensing circuit, which requires a minimum pulse on-time to enable accurate and repeatable measurements.

[0077] At high modulation frequencies (>2 kHz) and low dimming levels (<10%), such measurements (as exemplified in US Pat. No. 9,986,608 B2) can be dangerous due to excessively short ON times.

[0078] For a solid-state light source, such as an LED, during each pulse of the PWM, the emitted light pulse is proportional to the injected charge (i.e., the time integral of the forward current through the LED); therefore, for a fixed current level, the emitted light increases as the duty cycle of the pulse increases.

[0079] As we all know, the duty cycle D of the pulse width modulation signal refers to the on-time t ON The ratio between the duration of the PWM pulse and the period of the PWM pulse, the latter being the on-time t ON Duration and on-off time t OFF The sum of the durations of

[0080] D=t ON / (t ON +t OFF )

[0081] When using rather long cables and LED modules (with high total current), the distributed parasitic inductances and capacitances can modify the PWM signal. Figure 2 The actual conduction time of the more distant electrical units ( Figure 2 The actual on-time of the CMOS image sensors (those further to the right in the figure) can be significantly reduced. This can lead to considerable end-to-end variation in the level of light emitted by the lighting system.

[0082] This effect is further exacerbated for high frequency PWM signals (>2.0 kHz) and low dimming levels (eg, below 5.0%), since the distortion of the pulse affects the on-time by a fixed amount.

[0083] One or more embodiments may utilize frequency modulation of the PWM signal: for a given dimming level, the duty cycle of the PWM signal remains constant while its frequency is varied, an approach which may be defined as FM-PWM (Frequency Modulated Pulse Width Modulation).

[0084] In one or more embodiments, the modulation frequency and frequency offset of the PWM signal may be set in such a way that a reduction in the sum of harmonics expressed by the aforementioned Minkowski relationship is achieved, thereby being able to remain below regulatory limits.

[0085] The pattern of this procedure is based on the spread of energy over a relatively wide frequency range or spectrum.

[0086] Due to the fact that the exponent of the Minkowski relation used is greater than 2 (it is 3.7), the sum of the spectral components returns a lower value than for the same spectral power in the presence of a fixed (non-modulated) frequency of the PWM signal.

[0087] Although in this connection one does not wish to be bound by a particular analytical method, it has been noted that the modulating waveform can play an important role in reducing the final value that can be defined by the Minkowski relation.

[0088] For example, as discussed below regarding Figure 5A and Figure 5B , it can be noted that using a triangular or sinusoidal modulation profile is beneficial, allowing the identification of customized modulation waveforms that can improve the final results in terms of SVM.

[0089] One possible concept to consider when defining modulation (e.g., non-uniform modulation) is to gain greater advantage from the weighting law by identifying the highest harmonic content of energy, where the strobe visibility threshold is higher (and therefore has a lower weight in the calculation of the SVM value).

[0090] It has been noted that, for example, a frequency deviation of 300 Hz around the center frequency (1700 Hz) leads to very good final SVM values (<0.4), even in the case of uniform (triangular) modulation and in the presence of low dimming levels (below 10%).

[0091] Of course, the values mentioned are only for confirming our idea and are not intended to limit the embodiments.

[0092] In general, it is useful to consider one or more of the following criteria when identifying advantageous solutions for frequency modulation of the PWM signal:

[0093] Keeping the average frequency as low as possible to improve the uniform distribution of light along the system (this is related, on the one hand, to the length of the cable 14 and of the modules 121 , 122 , . . . , 12n);

[0094] Keeping the modulation frequency below a certain value for a sufficient time to measure the load, for example using the pulse offset solution described in US 9 986 608 B2 (cited); this helps to detect possible failures of the lighting modules (e.g. 121, 122, ..., 12n) according to the specifications of the DALI environment;

[0095] Advantageously prevents the generation of harmonics below 100 Hz during FM-PWM: This helps maintain another TLA parameter, namely P st , whose measurement is based on the lower frequency band;

[0096] Try to move the harmonic content with low weighting factors in an equally favorable way; together with the criteria seen previously, this means keeping it in the higher part of the spectrum (1kHz) as much as possible. <f<2kHz)。

[0097] In the following, examples of possible waveforms that can be used to modulate the frequency of the PWM signal are given.

[0098] As a reference example, Figure 3A An example of a modulation waveform V with a triangular envelope fm , the amplitude of the modulating waveform is normalized between 0V and 1V, which provides frequency modulation of the PWM signal, such as Figure 3B As shown in the example in: This is a rectangular wave signal with a duty cycle equal to 10% (its amplitude is also normalized between 0V and 1V), and as a result of the modulation, the frequency of the rectangular wave signal varies in the range of 1700+ / -300 Hz.

[0099] It should be understood that through Figure 3A and Figure 3B The modal example in:

[0100] Figure 3A The modulation signal V fm A "low" value of corresponds to a smaller distance between the pulses of the PWM signal and therefore to Figure 3B The period of the PWM signal is reduced and the frequency is increased; and

[0101] Figure 3A The modulation signal V fm A "high" value of corresponds to a larger distance between the pulses of the PWM signal and therefore to Figure 3B The period of the PWM signal increases and the frequency decreases.

[0102] This selection is of course of a purely exemplary and non-limiting nature.

[0103] Figure 4 The graph of represents one possible resulting FFT. It has been found that employing an FM-PWM technique (actually a spread spectrum modulation technique, in this case with uniform spectrum spreading) helps to reduce the SVM value as desired.

[0104] As examples of some embodiments, Figure 5A The modulation waveform V is shown as an example fm , which has an amplitude normalized between 0 V and 1 V, which provides a frequency modulation of the PWM signal between a minimum value and a maximum value, as Figure 5B As shown in the example.

[0105] Also in this case, this is a rectangular wave signal with a duty cycle equal to 10% (also with an amplitude normalized between 0V and 1V), the frequency of which in this case also varies between maximum and minimum values in the range of 1700+ / -300Hz as a result of the modulation.

[0106] exist Figure 5A and Figure 5B In the example case, the modulation waveform V fm Does not present a symmetrical triangular graph, such as Figure 3A The case where the modulating signal V fm There are (again, this choice is not mandatory) rising and falling edges with constant angle coefficients (this is the same, but with respect to the sign, rising edges are positive and falling edges are negative).

[0107] exist Figure 5A and Figure 5B In the example case, the modulation waveform V fm We have a graph (which can be defined as a "mixed" triangular graph) where:

[0108] The rising edge initially has a first value of the angle coefficient, when reaching a value of approximately 0.3 V (i.e. below the half-amplitude value of 0.5 V), followed by a second value of the angle coefficient which is higher than the first value; i.e. the slope is steeper;

[0109] The falling edge has a symmetrical diagram (the signs are the same for the rising and falling edges) and initially has an angular coefficient corresponding to a steeper slope, followed by an angular coefficient corresponding to a flatter slope, and the same is true when the falling edge reaches a value of approximately 0.3 V (i.e. below the half-amplitude value of 0.5 V, which corresponds to a value of the frequency of the modulated PWM equal to the average value between the minimum and maximum values of the frequency generated by the modulation).

[0110] Also in Figure 5A and Figure 5B In the example case, the modulated signal V fm There are (again, this choice is not mandatory) rising and falling edges with symmetrical angular coefficient variations (but with respect to the signs, the rising edge is positive and the falling edge is negative).

[0111] Also in Figure 5A and Figure 5B In the example modal case:

[0112] Figure 5A The modulation signal V fm A "low" value of corresponds to a shorter distance between the pulses of the PWM signal and therefore to Figure 5B The period of the PWM signal is reduced and the frequency is increased; and

[0113] Figure 5A The modulation signal V fm A "high" value of corresponds to a larger distance between the pulses of the PWM signal and therefore to Figure 5B The period of the PWM signal increases and the frequency decreases.

[0114] Of course, also in this case, the above options are of a purely exemplary and non-limiting character.

[0115] This also applies to Figure 5A A steeper or flatter triangular waveform (with a dual slope) is shown.

[0116] The waveform above is an example that simplifies the understanding of the fact that, if Figure 5A and Figure 5B For example, the alternation of two values of the angle coefficient below the half-amplitude value (about 0.5 V) takes into account the fact that where the slope of the modulation signal is steeper (in the rising and falling edges), the modulation signal V fm The changes of θ are faster than they are where the slope of the modulating signal is flatter (here also in the rising and falling edges).

[0117] In this way, the pulse width modulation signal V can be modulated PWM The frequency of the pulse width modulation signal V PWM The pulse repetition frequency is kept between the value at which the modulation is performed and the highest (maximum) frequency value, which lasts longer than the pulse width modulation signal V PWM The pulse repetition frequency remains between the value at which the modulation is performed and the lowest (minimum) frequency value for a period of time.

[0118] This fact can be Figure 5A To understand the example, remember that in the figure, the modulating signal V fmThe "low" value corresponds to an increase in the frequency of the PWM signal, and the modulation signal V fm A "high" value corresponds to a reduction in the frequency of the PWM signal, where the half-amplitude value (e.g., 0.5 V) corresponding to the value of the frequency of the frequency-modulated PWM signal is equal to the average value between the minimum and maximum values of the frequency produced by the modulation.

[0119] exist Figure 5A In this example, the modulated signal V fm The time interval below the half-amplitude value (i.e. the time interval between the value at which the modulation signal frequency falls and the highest (maximum) frequency value) is longer than the modulation signal V fm The time interval above the half-amplitude value (ie the time interval at which the frequency of the modulation signal lies between the value at which the modulation is performed and the lowest (minimum) frequency value) is long.

[0120] The purpose of the foregoing is to shift the harmonic content where the weighting factors are low, trying to keep it in the higher part of the spectrum for as long as possible.

[0121] It has been noted that employing a non-uniform modulation profile distributes the spectrum differently so as to concentrate more energy where it has a lower weight.

[0122] Figure 6 The figure shows that Figure 5A and Figure 5B The FFT that may occur in the application of the example FM-PWM standard (in this case also a non-uniform spread spectrum modulation technique), and Figure 3A and Figure 3B Compared with the uniform spread spectrum modulation in the example, it helps to further reduce the SVM value.

[0123] Figure 7 The block diagram illustrates the overall equivalent of Figure 2 In the lighting system shown in the figure, the modulation signal V is used to change the carrier frequency of the PWM signal. fm The possibility of integrating a function of the frequency modulation of the PWM signal generated by the ECG 10 may result in a variation of the pulse repetition period of the PWM signal even given the same duty cycle value.

[0124] It will be appreciated that the intensity of the emitted luminous flux continues to be primarily determined by the aforementioned duty cycle value, since the frequency modulation of the PWM signal is performed around an average value and within a frequency range (e.g., 1700 Hz + / - 300 Hz) so as not to have a perceptible effect on the time integral of the forward current through the LED.

[0125] For example, the FM-PWM illustrated herein may be implemented by configuring (in a manner known per se to those skilled in the art) the ECG 10 with a voltage controlled oscillator (VCO) function, which may be provided by the modulator circuit 100 (which is Figure 7 , but may be integrated into the ECG 10) generates a signal V fm Driven by frequency modulation.

[0126] In one or more embodiments, the modulator circuit 100 can be obtained in the form of a programmable circuit, and the modulator circuit 100 can generate different frequency modulation signals V fm , the frequency modulated signal V fm For example, you can choose according to different applications and usage requirements.

[0127] At this point, it should be understood that the blocks 10 and 100 shown here as distinct elements for simplicity of illustration may be obtained as a single entity, for example as a programmable digital machine (microcontroller) capable of utilizing peripherals (timers) to obtain the described operations.

[0128] It is also understood that the CV SSL system (see Figure 2 and Figure 7 ) are provided purely by way of non-limiting examples of embodiments: although they have been described with particular attention to lighting systems of this nature, one or more embodiments are advantageously applicable to different types of lighting systems, such as constant current (CC) systems.

[0129] At this point, Figure 8 The diagram of illustrates a possible frequency diagram FB of a signal subjected to FM-PWM in the aspects discussed here, and a possible diagram of the corresponding Fast Fourier Transform (FFT), which for example results in an SVM value equal to 0.399.

[0130] Thus, a method similar to the one exemplified here might include:

[0131] By applying a pulse width modulation signal (e.g., V PWM ), to drive (e.g., 10) the at least one electrically powered light source (e.g., 121, 122, ..., 12n), wherein the duty cycle is selectively variable so as to vary the intensity of light emitted by the at least one electrically powered light source; and

[0132] Frequency modulation (e.g., 100, Vfm )The pulse width modulated signal.

[0133] The methods illustrated herein may therefore include employing hybrid FM / PWM modulation, substantially similar to spread spectrum techniques applied to the PWM signal.

[0134] In the method exemplified here, this higher frequency value may be around 2 kHz (eg 2 kHz-2.1 kHz), which on the one hand makes it possible to avoid end-to-end type illumination unevenness and on the other hand facilitates the detection of faults.

[0135] In a method similar to that exemplified herein, the aforementioned frequency modulation between the lower frequency value and the higher frequency value (e.g., 100, V fm ) can occur with uneven frequency variations.

[0136] Figure 5A This type of solution is exemplified in Figure 1, where the pulse repetition frequency can be seen to vary with the following factor (here the rising and falling edges have opposite signs):

[0137] a first rate of change within a first frequency range between the lower frequency value and the upper frequency value; and

[0138] a second rate of change within a second frequency range between the lower frequency value and the higher frequency value,

[0139] The first change rate is different from the second change rate, and the first frequency range is different from the second frequency range.

[0140] certainly, Figure 5A The "dashed line" variation pattern shown is only one possible example of a pattern of non-uniform frequency variation.

[0141] In one or more embodiments, this change may occur as a regular change represented by a different curve, which may also be differentiable, such as a hyperbola or a parabola, or may be a curve defined by a list of points, all of which may be obtained through experimentation or simulation.

[0142] In the method exemplified herein, the above-mentioned specific value may be approximately 1700 Hz.

[0143] The method exemplified herein may include modulating the frequency of the pulse width modulated signal to maintain the pulse repetition frequency of the pulse width modulated signal between the specific value and the higher frequency value (i.e., within the higher frequency range: see Figure 5A , the modulated signal V fmThe time interval below half amplitude, i.e. the time interval during which the frequency of the modulation signal lies between the average value and the highest or maximum frequency value for performing the modulation) is longer than the time during which the pulse repetition frequency of the pulse width modulation signal remains between said specific value and said lower frequency value (i.e. in the lower frequency range: see Figure 5A , the modulated signal V fm The time interval above the half-amplitude value, ie the time interval during which the frequency of the modulation signal lies between the average value and the highest or maximum frequency value at which the modulation is performed) is long.

[0144] The driver circuit illustrated herein (e.g., so-called ECG 10) can be configured to apply a pulse width modulated signal having a pulse repetition frequency and a duty cycle to at least one electrically powered light source, the duty cycle being selectively variable so as to vary the intensity of light emitted by the at least one electrically powered light source.

[0145] Such a driver circuit may be configured for frequency modulating a pulse width modulated signal by varying the pulse repetition frequency of the pulse width modulated signal around a certain (average) value between a lower frequency value and an upper frequency value using the methods exemplified herein.

[0146] The driver circuit exemplified herein may include a frequency modulator (e.g., 100) configured to generate a plurality of different frequency modulation signals for modulating the pulse width modulation signal by varying a pulse repetition frequency of the pulse width modulation signal around the specific value between the lower frequency value and the higher frequency value.

[0147] The lighting system exemplified here may include:

[0148] The driver circuit illustrated here; and

[0149] At least one electrically powered light source is coupled to the driver circuit (eg, via a line or cable 14) to apply thereto the pulse width modulated signal having a pulse repetition frequency that varies about a specific value between a lower frequency value and an upper frequency value.

[0150] In the lighting systems exemplified herein, the at least one electrically powered light source may comprise a solid-state light source, optionally an LED light source.

[0151] The details of construction and of the embodiments may vary, even significantly, with respect to what is shown here purely by way of non-limiting example, without prejudice to the underlying principle and without departing from the scope of protection defined by the appended claims.

[0152] LIST OF REFERENCE SIGNS

[0153] I (Existing) Contrast Threshold

[0154] II (New) Contrast Threshold

[0155] PG grid

[0156] 10 Driver circuit (electronic control tool unit)

[0157] 100 Modulator

[0158] 121,122,...12n LED modules

[0159] 14 connecting wires (cables)

[0160] V fm Frequency modulated signal

[0161] V PWM PWM signal

[0162] FB Frequency Modulated Pulse Width Modulation (FM-PWM) signal

[0163] FFT Fast Fourier Transform

Claims

1. A method for driving at least one electric light source, comprising: driving the at least one electrically powered light source by applying a pulse width modulated signal having a pulse repetition frequency and a duty cycle to the at least one electrically powered light source, the method comprising selectively varying the duty cycle of the pulse width modulated signal to vary the intensity of light emitted by the at least one electrically powered light source; and frequency modulating the pulse width modulated signal by varying a pulse repetition frequency of the pulse width modulated signal about a specific value between a lower frequency value and an upper frequency value; wherein the method comprises frequency modulating the pulse width modulated signal by varying a pulse repetition frequency of the pulse width modulated signal between a lower frequency value and a higher frequency value with a non-uniform frequency variation such that the pulse repetition frequency of the pulse width modulated signal remains between the specific value and the higher frequency value for a longer period than the pulse repetition frequency of the pulse width modulated signal remains between the specific value and the lower frequency value.

2. The method of claim 1, wherein the higher frequency value is 2 kHz.

3. The method according to any one of the preceding claims, wherein the specific value is 1700 Hz. 4 . The method according to claim 1 , wherein the specific value is an average value of the higher frequency value and the lower frequency value.

5. A driver circuit configured to apply a pulse width modulated signal having a pulse repetition frequency and a duty cycle to at least one electrically powered light source, wherein the driver circuit is configured to selectively change the duty cycle of the pulse width modulated signal to change the intensity of light emitted by the at least one electrically powered light source, wherein the driver circuit is configured to frequency modulate the pulse width modulated signal by changing the pulse repetition frequency of the pulse width modulated signal around a specific value between a lower frequency value and a higher frequency value using the method of any one of claims 1 to claim 4.

6. The driver circuit according to claim 5 , comprising a frequency modulator configured to generate a plurality of different frequency modulation signals to frequency modulate the pulse width modulation signal to change a pulse repetition frequency of the pulse width modulation signal around the specific value between the lower frequency value and the higher frequency value.

7. A lighting system comprising: The driver circuit according to claim 5 or claim 6; and At least one electrically powered light source is coupled to the driver circuit to apply the pulse width modulated signal to the driver circuit, wherein a pulse repetition frequency of the pulse width modulated signal varies about a specific value between a lower frequency value and an upper frequency value.

8. The lighting system of claim 7, wherein the at least one electrically powered light source comprises a solid state light source.

9. The lighting system of claim 8, wherein the at least one electrically powered light source comprises an LED light source.

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