ITMI20090020A1 - METHOD AND POWER SUPPLY SYSTEM FOR LED LIGHTING - Google Patents

Description

DESCRIPTION

The present invention relates to a method and an electric power supply system for LED lighting.

LEDs [Light Emitting Diode] are becoming increasingly popular as electrical lighting devices thanks to the high efficiency of transformation from electrical energy into light energy.

LEDs are diodes whose I-V characteristic is comparable to a (low) voltage generator (for example of a few volts) with a small series resistance (in the area useful for the generation of light energy) and therefore require a power supply.

The commonly available sources of electricity (batteries, public electricity distribution network) have I-V characteristics similar to a (low or high) voltage generator.

Therefore, LED lighting systems typically require a dedicated power supply system.

To obtain highly efficient lighting, the power supply system must also be efficient.

To obtain a light source with predetermined chromatic characteristics, for example, red, green and blue LEDs can be used at the same time; in fact, it is known that by suitably combining the colors of the RGB [Red Green Blue] triad, any color can be obtained; to obtain the desired color, the LEDs must be crossed by currents of suitable intensity. To obtain the desired light intensity, LEDs of the same color can be connected in series so as to form LED strings; there will therefore be a string for the red component, one for the green one and one for the blue one; in general, the number of LEDs in each string may be different since the efficiency of the LEDs (ratio between luminous flux in lumens and absorbed power in Watts) is different depending on their color. A solution based on this principle is known for example from patent application EP1791400.

When using different colored LED strings, basically two methods can be followed to achieve high efficiency.

According to the first method, each LED string is powered by a switching type current generator; this solution is very flexible but requires a number of generators equal to the number of strings. To obtain low light intensities, the current in each of the strings can be partialized with PWM [Pulse Width Modulation] techniques.

According to the second method, the LED strings are powered by a single switching voltage generator and the current in each string is set by a corresponding current regulator (connected in series to the string); this regulator can operate continuously or modulating the current with PAM [Pulse Amplitude Modulation] techniques.

To increase efficiency without having to resort to a plurality of switching-type current generators, some solutions have been proposed that substantially aim at reducing the voltage across the LEDs to the bare minimum.

From patent application WO2004 / 021744 a driving circuit for high efficiency LEDs is known; this circuit is equipped with a plurality of outputs connected in parallel to each other, each for a corresponding LED, and provides a linear voltage regulator with a charge pump in series; this solution allows to optimize the efficiency of the feeding section. The charge pump output is connected to the LEDs and to a driving circuit; the driving circuit includes a linear current regulator for each of the outputs and a detector consisting of diodes in 'wired OR' connection, connected to the cathodes of the LEDs, which constitutes an undervoltage detector. This minimum voltage, which must be sufficient for the linear zone operation of the current regulators, is used in a feedback circuit to fix the optimum value of the charge pump output voltage. In practice, the charge pump output voltage is kept at the minimum value necessary for the operation of the current regulators. Since the different LEDs have different voltage drops, only one current generator works at the minimum voltage; the other current generators operate at a working point which implies power dissipation and therefore inefficiency of the solution as a whole.

From patent application WO2007 / 064694 a high efficiency power supply for LED lighting applications is known; this solution provides a voltage regulator whose output is connected to a multiplicity of loads consisting of LED strings (such LED strings generally comprise a different number of LEDs); the solution also includes a current regulator for each LED string; these regulators are also adapted to generate a corresponding error signal when the current regulator is about to enter a non-linear zone; these error signals drive the voltage regulator in such a way that the voltage at the output of the voltage regulator is kept at the minimum value necessary for the current regulation in the LED strings to be guaranteed.

The Applicant has realized that the existing solutions for the power supply of LED lighting systems are not yet fully satisfactory.

The solution known from patent application WO2004 / 021744 achieves a certain efficiency, but, since it supplies a plurality of LEDs, which are not identical to each other, with the same voltage and current, it involves power dissipation; in fact the voltage is kept at the minimum useful value compatibly with all the various different LEDs, but this value represents the optimum value only for one of the loads; moreover, this solution requires a current regulator for each output and a detector with a number of inputs equal to all the outputs.

The solution known from patent application WO2007 / 064694 achieves a certain efficiency, but, since it powers a plurality of LED strings with the same voltage (in particular two in the example of Fig.1), which are generally different from each other, it wastes energy when the strings are characterized by different voltage drops (for example because they are formed by a different number of LEDs); in fact, the voltage is kept at the minimum useful value compatibly with all the various different LED strings, but this value represents the optimum value only for one of the loads; moreover, this solution requires a current regulator and a detector for each output.

The Applicant has realized that a fully satisfactory LED lighting power supply system must be able to regulate both the brightness and the color rendering (in simpler, but imprecise words, the "color"), possibly independently ( in particular, once the color rendering has been chosen, it must remain substantially fixed as the brightness changes), and must maintain its efficiency even when the brightness and / or color rendering changes. The Applicant has therefore set itself the goal of providing an electric power supply solution for LED lighting that overcomes the drawbacks of the prior art and meets the requirements mentioned above.

Furthermore, the Applicant sought a solution that was flexible and therefore easily adapts to various application contexts.

This object has been achieved thanks to the method and system having the characteristics set out in the claims annexed hereto.

The present invention starts from the consideration that very rapid variations in brightness are not perceived by the observer who is substantially sensitive to the average brightness. This is true for both a man and a camera: in the case of humans, the perceptible variations are of the order of ten milliseconds; in the case of a camera or video camera, the slightest perceptible variations depend on the shutter opening time; since the typical minimum opening times range from 1/1000 of a second (1 mS) to 1/4000 of a second (250 uS), it can be said that the minimum perceptible variations are of the order of hundreds of microseconds.

The idea behind the present invention is to distribute cyclically (with an appropriately short chosen period, taking into account the perceptible variations, as will be better clarified below) the electric power supply current between the various loads in such a way that, substantially, only one load at a time is electrically powered and that the quantities of electric charge supplied to the loads for each cycle are regulated.

The distribution of the electric current can be carried out in a simple and very efficient way by means of a distribution circuitry comprising substantially loss-free electronic switches; therefore, if the electrical power supply (input to the distribution circuitry) is generated very efficiently, for example by means of a switching type power supply, the entire lighting system is very efficient.

Furthermore, since the adjustment is carried out based on the charge, the control of the electricity supplied to the loads is precise and the electricity supplied to the loads is independent of the trend of the power supply current entering the distribution circuitry; therefore, the control of brightness and “color” is very precise and the solution is very flexible.

The advantages with respect to known solutions are therefore appreciated both in terms of circuits and in functional terms.

In general, the method according to the present invention serves to electrically power a plurality of loads, in particular three loads, each of said loads essentially consisting of one or more LEDs (connected in series to form a "string of LEDs"), to starting from an electric current; said electric current is distributed cyclically among said loads in such a way that substantially only one load at a time is electrically powered and that the quantities of electric charge supplied to said loads for each cycle are regulated.

The quantities of electric charge supplied to said loads can be adjusted to predetermined or set values; the predetermination can take place for example in the production phase while the setting can take place for example directly by an installer or indirectly by a user through a control unit, for example a logic control unit.

Said electric current can be divided among said loads according to a cycle having a duration, fixed or variable, but in any case less than 40 mS, preferably less than 10 mS and / or preferably greater than 1 mS; this choice is particularly suitable for domestic lighting applications.

Said electric current can be divided between said loads according to a cycle having a duration, fixed or variable, but in any case less than 1 mS, preferably greater than 10 uS, typically of the order of 100 uS; this choice is particularly suitable for photographic or cinematographic lighting applications.

According to the present invention, ratios can be defined between the quantities of electric charge supplied to said loads and said ratios can be kept constant as said electric current varies. Said ratios are predetermined or set; the predetermination can take place for example in the production phase while the setting can take place for example directly by an installer or indirectly by a user through a control unit, for example a logic control unit.

Said electric current to be shared can be short-circuited and not sent to any of said loads when it is less than a predetermined value and / or when it is greater than a predetermined value; this possibility is used to avoid anomalous powering of the LEDs. Said divided electric current can be obtained from a switching type power supply, in particular with current control, more particularly with current control and having output current with wavy or impulsive trend.

The average value of said electric current to be distributed can be adjusted to a predetermined or set value; the predetermination can take place for example in the production phase while the setting can take place for example directly by an installer or indirectly by a user through a control unit, for example a logic control unit.

Said electric current to be divided can be supplied to multiple loads of said plurality only for short periods of time, typically lasting in the range from 100 nS to 1 uS. This overlap is useful in particular to avoid the risk of overvoltages in the phase of current deviation from one load to the next and for the time necessary for the transition, a time which is substantially equal to the switching time (on or off) of a circuit breaker. solid state (for example a MOSFET); if the switching time is 150 nS, it is recommended that the overlap time is, for example, 300 nS; this overlap time is preferably significantly shorter than the period of the current sharing cycle, to avoid significant errors in regulating the charge to the loads.

In general, the power supply system for LED lighting according to the present invention has a plurality of outputs, in particular three outputs, for electrically powering a corresponding plurality of loads; it comprises analog and / or digital circuit means adapted to implement the method according to the present invention.

The system can comprise a control unit made entirely of wired logic (for example by means of an FPGA [Field-Programmable Gate Array]) or completely in programmed logic (for example by means of a microcontroller or microprocessor equipped with a program and data stored internally) or partly in wired logic and partly in programmed logic. Said control unit is adapted to control distribution of electric current between said outputs; if the programmed logic is used, the current distribution takes place according to said program and according to said data.

The system, regardless of the type of logic, can advantageously be realized by integrating all its electrical components in a SOC [System On Chip].

The most suitable realization of the system and the control unit depend on the type of application and the number of pieces to be produced. The system can comprise circuit means adapted to selectively realize an internal short circuit as an alternative to electrically powering said loads; this possibility can be used to avoid anomalous powering of the LEDs or to obtain particular electrical and / or luminous effects.

The system may further comprise a switching-type power supply, in particular with current control, more particularly with current control and having an output current with a wavy or impulsive trend; this means that the current control is based on the average value of the current and not on the instantaneous one. Advantageously, said power supply has no capacitors connected in parallel to its output so that the output voltage can vary rapidly and adapt to the load without major repercussions on the current at the output (and which runs through the load).

From the structural point of view, the power supply system for LED lighting according to the present invention has an input, to receive an electric current as an input, and a plurality of outputs, to electrically power a corresponding plurality of loads, and comprises:

- a diverter circuit having an input coupled to said system input, a plurality of outputs respectively connected to said plurality of system outputs (for loads), and adapted to alternatively connect its input to one of its outputs,

- electric charge meter circuit connected in such a way as to measure the charge supplied to said system outputs, e

- a logic circuit connected in such a way as to receive in input the measurements carried out by said measuring circuit and to supply at the output to said deviating circuit deviation command signals in relation to the received measurements in such a way as to regulate the electrical charges supplied to said outputs for each cycle.

Said diverter circuit can essentially consist of a plurality of MOSFET transistors corresponding to the plurality of outputs of the system.

Said diverter circuit can comprise a further output connected to an electrical short-circuit path; in this case, said diverter circuit is able to connect its input to said further output alternatively or in addition to the connection to one of said plurality of outputs; this further output is internal to the power supply system as well as the short-circuit path. Said diverter circuit typically comprises a further MOSFET transistor for said further output. Said electric charge meter circuit can comprise a resistor (connected to a voltage reference, in particular to ground) and an integrator circuit, preferably a single resistor and a single integrator circuit, both connected to the input of said diverter circuit; in this case, said resistor is connected in such a way as to convert the current at the input of said diverter circuit (and which is passing through one of the loads) into voltage and said integrator circuit is connected in such a way as to integrate the voltage that falls across of said resistor and then calculate the electric charge received at the input from said diverter circuit and then supplied at the output to one of the loads.

Said charge meter circuit can comprise a comparator circuit having a first input and a second input; in this case, said first input is connected to the output of said integrator circuit and said second input receives a reference signal. Said integrator circuit can be equipped with a “reset” input which can be used, for example, activated at each new measurement. Said integrator circuit can be equipped with a "hold" input which, when active, blocks the integration and which can be used for example to temporarily stop a measurement.

Said logic circuit can be coupled at the input to the output of an integrator circuit, preferably through a comparator circuit; in this way, the logic circuit can detect the integrations made by said integrator circuit (in other words the electric charge measurements). Said logic circuit can be connected at the output to control inputs of said diverter circuit; in this case, the control inputs of said diverter circuit are equal to the outputs of said diverter circuit; in this way, said logic circuit can control, among other things, the deviation of the electrical input current on the various outputs (and therefore on the various loads).

Said logic circuit can be coupled at the output to an input of a comparator circuit, preferably through a digital to analog conversion circuit; this connection allows the logic circuit to generate reference signals. These reference signals can be used to regulate, for example, the average value of the current supplied to the system outputs (and therefore to the various loads) and / or the electric charge to be supplied for each cycle to each of the system outputs (and therefore to the various loads. ); in the first case, these reference signals allow to control the brightness of the LED lighting through the logic circuit; in the second case, these reference signals make it possible to control the color of the LED lighting through the logic circuit.

Said logic circuit can be connected at the output to a "reset" input of an integrator circuit; in this way, said logic circuit can determine, for example, the beginning and the end of electrical charge measurements carried out thanks to said integrator circuit.

Said logic circuit can essentially consist of a microprocessor or a microcontroller which can also, but not necessarily, be integrated together with other components in a SOC [System On Chip]; alternatively, the same functions can be realized in wired logic, for example by means of a FPGA [Field-Programmable Gate Array].

The system may further comprise a current measuring circuit connected in such a way as to measure the current supplied to said system outputs (and therefore to the various loads).

Said current meter circuit can comprise a resistor (connected to a voltage reference, in particular to ground) and a comparator circuit, preferably a single resistor and a single comparator circuit, both connected to the input of said diverter circuit; in this case, said resistor is connected in such a way as to convert the current at the input of said diverter circuit (and which is passing through one of the loads) into voltage and said comparator circuit is connected in such a way as to compare the voltage falling across of said resistor with a threshold voltage and verify the exceeding of this (upper or lower) voltage threshold which corresponds, in effect, to an exceeding of a (upper or lower) current threshold. The output of said current meter circuit can be connected to a control input of said diverter circuit; in this case, said current measuring circuit is adapted to output a deviation command signal in relation to the measurements made; in this way, it is possible, for example, to divert the input current to a particular output of said diverter circuit in case of exceeding the threshold (upper or lower); typically, this particular output is connected to an electrical short-circuit path.

The output of said current meter circuit can be connected to an integrator circuit; in this case, said current measuring circuit is adapted to output a "hold" signal to said integrator circuit in relation to the measurements made; in this way, it is possible, for example, to block the integration in case of exceeding the threshold (upper or lower);

The system can further comprise a power supply having a power output connected to the input of said diverter circuit; in this case, if high efficiency is desired, said power supply must be of the switching type with current control.

Said power supply can be of the type having an output current with a wavy or impulsive trend; this means that the current control is based on the average value of the current and not on the instantaneous one. Advantageously, said power supply has no capacitors connected in parallel to its output so that the output voltage can vary rapidly and adapt to the load without major repercussions on the current at the output (and which runs through the load).

Said current control can provide a current meter circuit connected in such a way as to measure the current supplied to the output of said power supply.

Said current meter circuit may include a resistor (connected to a voltage reference, in particular to ground) and an error amplifier circuit.

Said current control can also comprise for example a PWM [Pulse Width Modulation] type modulator.

In this case, said resistor is connected in such a way as to convert the current at the output of said power supply (at the input of said diverter circuit) into voltage and said error amplifier circuit is connected in such a way as to compare the voltage which falls across said resistor with a reference voltage which is proportional to the desired average value of the output current.

The output of said pilot error amplifier circuit for example said PWM modulator so as to obtain an output current of the desired average value.

The system can comprise a filter or a low-pass network connected to said error amplifier in such a way as to filter the voltage that falls across said resistor.

The system may include a control unit having an output for a reference voltage; in this case, said output is connected to said error amplifier circuit preferably through a digital to analog conversion circuit; in this way, through the control unit, you can set the average value of the current at the output of said power supply (and therefore the brightness of the LED lighting).

The system may include a single resistor to make measurements relating to two or more different system functions.

The system can comprise a control unit, preferably a logic control unit, in particular a microprocessor or microcontroller control unit, able to generate internal control signals to determine the color rendering and / or the brightness of the LED lighting.

The system may include user interface means connected to said control unit to affect the lighting.

The system may include at least one sensor, in particular for temperature or brightness or color, connected to said control unit to affect lighting.

The system may include wired ("wired") and / or wireless ("wireless") means of communication connected to said control unit to directly or indirectly affect the lighting.

The present invention will become clearer from the following description to be considered in conjunction with the accompanying drawings in which:

Fig.1 shows a conceptual block diagram of a first power supply system according to the present invention,

Fig. 2 shows a block diagram of a switching power supply usable with or in a power supply system according to the present invention,

Fig. 3 shows a very general block diagram of a second power supply system according to the present invention,

Fig. 4 shows the possible temporal trends of the currents in four points of the diagram of Fig. 1,

Fig. 5 shows the current / luminous flux characteristics of a red LED and a blue LED, and

Fig.6 shows the simplified electrical diagram of a power supply system according to the present invention.

This description and these drawings are exemplary and not limitative; moreover, they are schematic and simplified.

It is worth premising that the present invention typically applies to lighting systems which include LEDs of three different colors (red, green and blue), but can be applied without any difficulty or modification even in the case of a smaller number of colors or more colors (for example, red, green, blue, and white). The instantaneous light power emitted by an LED is proportional to the electric current that flows through it according to a coefficient that takes into account the conversion efficiency, see the formula below.

(F1)

The luminous flux of a light source is defined as the integral (in the emission band of the source) of the product of the light power by the (human) coefficient of visibility.

Fig.5 shows the graph of the luminous flux (in lumens) with respect to the electric current (in mA) for a red LED (RC graph) and for a blue LED (BC graph); a green LED has an intermediate graph between the red LED and the blue LED. As a first approximation, the luminous flux is linearly proportional to the current (both for the red LED and for the blue LED) between 100mA and 1000mA; below 100mA the behavior of these LEDs is typically unpredictable and non-repeatable. As a first approximation, the ratio between the luminous flux of the red LED and the luminous flux of the blue LED is substantially constant and independent of the electric current.

The light energy emitted in a certain time interval [t, t + T] is therefore proportional to the charge that has passed through the LED at the same time, see the formula below.

(F2)

The devices for detecting or recording light signals (eye, camera, video camera, ...) operate on the basis of the light energy received in a certain time interval whose duration will be generically indicated below with "τ"; this duration is of the order of a tenth of a second for humans and a hundredth or thousandth of a second (depending on the exposure time) for cameras and video cameras; if we consider the time interval [t, t + τ], the luminous sensation S that can be associated with this interval is proportional, within certain limits, to the light energy received during this interval, see the formula below.

(F3)

The variations in light energy that occur over intervals of significantly shorter duration than "τ" are not perceived as "variations" as the detection or recording devices carry out an integration process.

As already mentioned, in the case of humans, the perceptible variations are of the order of ten milliseconds; in the case of a camera or video camera, the slightest perceptible variations depend on the shutter opening time; since the typical minimum opening times range from 1/1000 of a second (1 mS) to 1/4000 of a second (250 uS), it can be said that the minimum perceptible variations are of the order of hundreds of microseconds.

Therefore, if a method and an electric power supply system for LEDs operates in such a way as to vary the instantaneous luminous power of the LEDs over time on a time scale "T" significantly lower than "τ", the luminous sensation will not depend on the instantaneous luminous power but only from the average value of the instantaneous light power calculated on the “τ” interval. This is one of the main considerations from which the present invention starts.

According to the following invention, an electric input current is available and a plurality of loads must be electrically powered, each of which essentially consists of one or more LEDs connected in series (so-called "LED strings").

With reference to Fig. 1, the electric current considered is that at an input I10, which will be indicated below as i (I10), and the loads are indicated with 12A, 12B, 12C and are connected respectively to the outputs OA, OB, OC; Fig.1 shows three loads because this is the most typical application example of the present invention; in particular, the load 12A is essentially constituted by the connection in series of a certain number (for example seven) of red LEDs, the load 12B is essentially constituted by the connection in series of a certain number (for example six) of green LEDs, the load 12C essentially consists of the series connection of a certain number (for example five) of blue LEDs.

With reference to Fig.1, according to the present invention, the current i (I10) is distributed cyclically (with a suitable rhythm) among the various loads 12A, 12B, 12C so that, substantially, only one load at a time is powered electrically; this division is carried out by block 11, a diverter circuit, suitably piloted by the remaining circuitry; in Fig.4, the time trends of the input current i (I10) (Fig.4A), of the current at the output OA and flowing in the load 12A (Fig.4B), of the current at the OB output are shown and which flows in the load 12B (Fig.4C), of the current at the output OC and which flows in the load 12C (Fig.4D); the current i (I10) is supplied for a time t1 to the load OA, then for a time t2 to the load OB, then for a time t3 to the load OC, then for a time t4 to the load OA, then for a time t5 to the load OB, then for a time t6 at the load OC, then for a time t7 at the load OA, then for a time t8 at the load OB, then for a time t9 at the load OC, and so on.

In general, t1 ≠ t2 ≠ t3 ≠ t4 ≠ t5 ≠ t6 ≠ t7 ≠ t8 ≠ t9 ≠ ..., if any of the durations "t1 + t2 + t3", "t2 + t3 + t4", "t3 + t4 + t5" , "T4 + t5 + t6", "t5 + t6 + t7", "t6 + t7 + t8", "t7 + t8 + t9" (which correspond to the duration of a cycle, however this is considered), ... less than “τ”, the luminous sensation deriving from the LEDs of the three loads will be linearly proportional to the average value of the instantaneous luminous power emitted by the LEDs calculated on the interval “τ”; the overall brightness will depend on the sum of the average luminous powers emitted by the three loads; while the color rendering will depend on the ratios of the average luminous powers emitted by the three loads.

If the lighting system is used only for domestic lighting (and therefore the reference device is the eye), the cycle duration must be less than 40 mS and preferably between 10 mS and 1 mS; if the lighting system is used with cameras and / or video cameras, it is advisable to choose a cycle duration of less than 1 mS, preferably greater than 10 uS, typically of the order of 100 uS so that it is suitable for the usual times of exposure of these devices (which can reach 1/4000 of a second).

If the input current is constant, the considerations made so far are sufficient to design the power supply system of the LED lighting system. In this case, one can operate with t1 = t4 = t7, t2 = t5 = t8, t3 = t6 = t9, ..., the values of "t1", "t2", "t3" are chosen so that "t1 + t2 + t3 "is less than a" τ "(suitable for the type of lighting of interest) and so that the ratios" t1 / t2 "," t1 / t3 "," t2 / t3 "are such as to provide the color rendering desired, and if it chooses the value from the input (constant) current in order to obtain the desired brightness. Thanks to the current supply and the distribution, a high efficiency is obtained since there is no waste of energy linked to the fact of supplying loads with non-optimal voltages; of course, it is necessary to have a source of continuous and substantially constant electrical current which is intrinsically efficient. In general, the hypothesis that the input current is constant is not valid; in fact, switching power supplies (very efficient) are often used for LED lighting systems as sources of electrical current; their output current (in input to the distribution circuitry), if it is not filtered, is wavy and may have, for example, the trend shown in Fig.4A. It is not uncommon for the current ripples to be such as to reach zero or values close to zero (for example when the power supply operates in "discontinuous mode"); in this case, the current has an impulsive trend and the impulses can have, for example, a triangular or trapezoidal shape. The present invention is applicable in both cases.

Therefore, according to the present invention, the current is distributed cyclically among the various loads in such a way that the quantities of electric charge supplied to the loads for each cycle are regulated; in other words, the electrical variable used to determine the distribution is the charge supplied to the loads. It starts by supplying (i.e. supplying the input current) a first load, the charge supplied to this load is measured until it reaches a reference level (Q1) associated with the first load, then the power supply of the first load is interrupted and the power supply of the second load is started (the input current is diverted from the first load to the second load) until the charge supplied to the second load reaches a reference level (Q2) associated with the second load, finally the power supply is interrupted of the second load and the feeding of the third load is started (the input current is diverted from the second load to the third load) until the charge supplied to the third load reaches a reference level (Q3) associated with the third load; this distribution cycle is repeated.

The distribution mechanism will be described below with the aid of mathematical formulas.

Let Q1, Q2 and Q3 be the electrical charges transferred to the loads in the intervals t1, t2 and t3. Furthermore, let io (t) be the output current provided by the source of electrical current (in particular from the switching power supply) and which will generally consist of a continuous component Io and a series of components at various frequencies.

Q1, Q2 and Q3 are an integral of a current and therefore have the size of electric charge.

Let us consider the currents i1 (t), i2 (t) and i3 (t) which flow respectively through the three loads.

Considering a breakdown cycle, the current i1 is different from zero in the interval t1, and is substantially zero outside that interval; similar considerations apply to the currents i2 and i3.

Due to how the distribution mechanism is constructed, it can be assumed that these currents have period Tr = t1 + t2 + t3.

The average value i1av of the current i1 (t), calculated in the period Tr, is valid

Similarly, for the average values i2av and i3av of the currents i2 and i3 holds

The system ensures, regardless of the trend of the current I (t) in the interval Tr, that the ratio between the average values of i1, i2 and i3 is dependent only on Q1, Q2 and Q3, which are values set in the system. In particular, the ratio between the average values in Tr of the currents i1, i2 and i3 and the average value of the current io in Tr are respectively Q1 / (Q1 + Q2 + Q3) Q2 / (Q1 + Q2 + Q3) Q3 / ( Q1 + Q2 + Q3) By setting the reference levels of the distribution system to the values Q1, Q2 and Q3 and the current supplied by the electric current source (in particular by the switching power supply) to the average value Io, the following average values for the currents I1, I2 and I3

I1 = Io * Q1 / Q I2 = Io * Q2 / Q I3 = Io * Q3 / Q where Q = Q1 + Q2 + Q3.

In general, it can be assumed that in addition to the average component of the current, there may be unwanted components present on the mains voltage or generated by the distribution mechanism. Such a situation can be analyzed as follows. In steady state, the value of each current in the loads can be expressed, in the presence of a disturbance signal that modulates the wavy or impulsive current output to the source in amplitude, as explained below.

We assume that the switching frequency Fc = 1 / Tc of the current source power supply is much greater than the frequency Fr = 1 / Tr of the distribution cycle and that the frequency f = ω / 2π of the modulating signal (of amplitude k) is very less than Fr. Let us consider, without losing generality, the average values of the currents for each period Tr. Since the variation of the average current in the interval Tr is negligible for the assumptions made, it can be written

we have that

and therefore we have that

Analogous expressions are obtained for the other currents i2 and i3.

The above formula expresses what has already been said previously, that is, that the currents that power the LEDs, thanks to the distribution mechanism, have an average value equal to the desired value (in the case of the current i1 at the value Q1 / Q * Io) and have a unwanted signal superimposed (but not significant if the pulsation ω is sufficiently high with respect to the integration time "τ" of the detection or recording devices) multiplied by Q1 / Q.

Instead, the effects of the distribution mechanism are analyzed below.

In the presence of current transients caused by the distribution mechanism (with commutation) of the current itself on different loads, there is a behavior that can be schematized as described below.

Let us suppose, for simplicity, that the division mechanism operates between only two LED strings, called "string1" and "string2", that the voltage drop on string1 crossed by the current i1 is equal to V1 (for example 60 V), that the voltage drop on string2 crossed by current i2 is equal to V2 (for example 80 V).

When the current passes from string1 to string2, a negative current transient is generated, while when the current passes from string2 to string1, a positive current transient is generated; the shape and duration of these transients depend on the characteristics of the current source.

In the presence of the transients caused by a distribution mechanism operating at frequency Fr, since the transients have a period Tr, the current I (t) (and this applies to the currents i1, i2 and i3) can therefore

where the contribution due to transients developed in series.

Since the integral in Tr of the components other than Io is zero, we have

The same considerations apply to the currents i1, i2 and i3.

Therefore, transients have been shown to cause unwanted (but may be negligible) components at frequency Fr (and higher) but do not cause any error with regard to continuous components.

The operating frequency of the divider must be chosen sufficiently high and it must be taken into account that this frequency can vary with the variation of the current produced by the current source (unless the thresholds Q1, Q2, Q3 are changed).

However, it should be noted that, even in the presence of undesired undulations, the system ensures precisely the chromatic characteristics set.

In the example of realization of Fig.1 which is a power supply system indicated as a whole with reference 10, the charges supplied to the various loads are measured through a single circuitry connected to input I10; this circuitry comprises an integrator circuit 18 which integrates the current flowing through the input I10 and which provides the measured charge value at the output; the reference levels are represented by block 15 which outputs one of the reference levels; block 17 is a comparator circuit that compares the value of the measured charge with the reference level and outputs a logic value corresponding to the outcome of this comparison; this result is “negative” if the value of the measured charge is lower than the reference level and it is “positive” if the value of the measured charge is equal to or greater than the reference level; block 16 is a logic circuitry that receives, among other things, the result of the comparison at its input and provides, among other things, a load switching command and a "reset" command (R); as soon as the result of the comparison is "positive", the logic circuitry 16 sends the load switching command to a block 14 and the "reset" command (R) to the integrator circuit 18; when the integrator circuit 18 receives a "reset" command, a new measurement begins; block 14 is a selector circuit which, when it receives a load switching command, generates at its output a deviation command for the diverter circuit 11 and a selection command for block 15; the diverter circuit 11 electrically connects the input I10 to one of its outputs OA, OB, OC (the meaning of the internal output OD will be explained later) in relation to the deviation command received; block 15 outputs a reference level corresponding to the selection command received.

In the following we will provide a mathematical treatment that confirms the full applicability of the method according to the present invention even in the case of input current with constant polarity, but not of constant value. In the following, with "i" the index of the "load" will be indicated, with "j" the index of the distribution "cycle", with "G (i)" the gain of the integrator circuit when it operates on the current flowing through the load "i" (in the case of the diagram in Fig.1 the gain of the integrator circuit 18 is constant and independent of the load), with "L (i)" the reference level associated with the load "i", with d (i , j) the time duration between "reset" of the integrator circuit and reaching the reference level L (i) when the current flows in the load "i" - this duration can be called "duration of the integration interval" or " integration"; since the input current has considerable variations, it is preferable, to be rigorous, to take into account that this is different from cycle to cycle; with reference to Fig. 4, d (1,1) = t1, d (2,1) = t2, d (3,1) = t3, d (1,2) = t4, d (2,2) = t5, d (3.2) = t6, d (1.3) = t7, d (2.3) = t8, d (3.3) = t9.

From formula F2 we will have that:

where [t, t + d (i, j)] indicates the time interval relating to the load "i" of cycle "j";

we will also have that:

According to the most typical implementations of the present invention, formula F5 can be simplified by considering that only one of the two parameters L and G varies from load to load; in the implementation of Fig.1, only L, i.e. the reference level, varies from load to load and G remains constant.

Therefore, we can indicate the ratio L (i) / G (i) with (L / G) (i) and therefore we will have that:

and finally that:

It should be noted that although d (i, j) generally depends on the cycle index "j", Eopt (i, [t, t + d (i, j)]) does not depend on the cycle index "j ”, But only from the load index“ i ”due to the fact that the light energy derives from an integration operation over time of the instantaneous light power.

We will therefore have that:

We must therefore make sure that T (1, j) + T (2, j) +… T (n, j) << τ for any value of “j”.

Indicating with "n" the number of loads to be powered and with "m" the number of complete cycles that take place over time "τ" we will have that:

where “δ” indicates the duration of the part of the uncompleted cycle included in the time “τ”.

Since the duration of the uncompleted part of the cycle is less than the duration of a complete cycle, we will have δ << τ.

Since, in the hypotheses considered above, for each interval [t, t + d (i, j)] the load "i" emits the same light energy Eopt (i), we will have that in the interval [t, t + τ] the following formula will apply for the energy emitted by the load "i":

and for the average power in this interval the following formula will apply:

or even the following formula:

Popt (i) is therefore determined by the factor m * k (i) * (L / G (i)) with, at most, a maximum relative error equal to 1 / m, and since for the assumptions made, m >> 1 , such an error can be made very small and therefore neglected. In general, if we consider the sequence of consecutive "p" time intervals of duration "τ", [t + p * τ, t + p * τ + τ], we have that "m" depends on "p" (that is the number of cycles completed over time "τ" is variable); this is due to the fact that both the instantaneous value and the average value of the input current can vary over time.

However, if it is assumed that the average value of the input current is constant or varies very slowly (time scale TT >> τ), then the average value of the input current (calculated in the time τ) can be considered constant for a relatively long sequence of consecutive time intervals of duration "τ" and therefore also the transferred charge Q [t + p * τ, t + p * τ + τ] in these time intervals can be considered constant, i.e. independent of "p", and can be denoted by Q (τ).

In general, Q (i, j) must indicate the charge transferred to the load “i” during the cycle “j”; but, in the above hypotheses, this charge does not depend on "j", but only on "i" and therefore can be indicated with Q (i). We will then have that:

where “q” indicates the charge transferred in an incomplete part of a cycle.

For the assumptions made, it will be q << (Q (1) + Q (2) +… Q (n)) <Q (τ).

As Q (τ) varies, m and therefore Popt (i) will vary in a linearly proportional way.

The invariance of the color rendering corresponds to the invariance of all the Popt (i1) / Popt (i2) power ratios.

Each of these power ratios varies around nominal by more than one factor

Since m is always >> 1, the color rendering is well controlled and can be kept substantially constant as the total brightness varies (and the average value of the input current).

Although the control is discontinuous in nature, it is perceived as a continuous process, if the discontinuities are on a much shorter time scale than that characteristic of light signal detection or recording devices (eye, camera, video camera, ...); this result is obtained through a correct dimensioning of the system parameters.

From what has been said, it is clear that the method according to the present invention can be applied regardless of the trend of the input current.

The color rendering provided by the LEDs connected to the lighting system, i.e. the loads, depends on how the electric current is divided between the loads (in addition to the characteristics of the loads themselves: the number, color and power of the various LEDs), more precisely how the charge is divided between the various loads.

In the case of the circuit of Fig.1, the color rendering essentially depends on the reference levels provided by block 15; if these reference levels remain constant, the color rendering remains substantially constant even when the average current at input I10 varies (there may be small variations due to the fact that the characteristics of the LEDs are not perfectly linear and linearly proportional to each other as is note in Fig. 5), while if they are changed the color rendering also changes; in this way, it is possible to adjust the color rendering of the lighting; the arrow C1 entering block 15 in Fig.1 indicates the possibility of modifying the reference levels during the operation of the circuit. When choosing the reference levels and sizing the integrator circuit, the time constraints explained above must be kept in mind.

Depending on the applications of the present invention, the color rendering is predetermined in the production phase or is set by the installer or the end user, for example by means of user interface; in the case, for example, of the circuit of Fig.1, the quantities of charge supplied to the various loads at each cycle are predetermined or set.

The brightness adjustment of the LED lighting can be carried out independently of the color rendering adjustment and, preferably, upstream of the color rendering adjustment circuitry. For this purpose, for example, upstream of the circuit 10 of Fig.1, a switching-type power supply can be connected such as the power supply shown in a very schematic way in Fig.2 and indicated as a whole with the reference 20; the set of the two circuits constitutes a power supply system according to the present invention (more complex and sophisticated) and is shown in Fig.3; the circuit of Fig.3 also includes a control unit 30, in particular a logic control unit, which will be discussed later.

The power supply 20 has an input I20 and an output O20, and comprises a power converter circuit 21 having a main input connected to the input I20, a control input and a main output connected to the output O20, a regulator circuit 22 having a main input connected to the output O20, a control input and a main output connected to the control input of the power converter circuit 21, and a block 23 which outputs a reference level to the regulator circuit 22. The current at the the output of the circuit 20 has a regulated value which depends on the reference level supplied by the block 23 to the circuit 22; since it is a switching type power supply, since it is of the type with internal current control, and since there are no filters upstream of its output O20, the current at the output will be wavy or impulsive, periodic and regulated only in terms of average value. Depending on the applications of the present invention, the brightness is predetermined in the production phase or is set by the installer or by the end user, for example by means of user interface. The arrow C3 entering block 23 in Fig. 2 indicates the possibility of modifying the reference level during operation of the circuit.

As mentioned, in principle, according to the present invention, only one load at a time is powered through the input current. However, electrical circuits are unable to make instantaneous switching. To avoid the risk that, for short periods of time, the input current is not supplied to any load and that therefore high voltages are generated (internal to the power supply system according to the present invention), it is preferable that for short periods of time the input current is supplied to at least two loads (these time periods can be included in the range from 100 nS to 1 uS): for example if the input current must be supplied in a first time interval to a first load and in a second time interval to a second load, for a short period of time between the two time intervals the input current is supplied both to the first load and to the second load and is divided between the two loads according to to the dynamic characteristics of the switches (MOSFET) during switching on and off.

Depending on the source of electric current, the value of its output current (at the input to the distribution circuitry) can be considerably reduced during certain periods of time and, sometimes, reach zero (this is the case of switching power supplies when they work in "discontinuous mode"). Moreover, the flux-current characteristic of an LED can be considered linear only if the current flowing through it is greater than a minimum value; below this minimum value, the behavior of the LED is unpredictable and non-repeatable. In Fig.4, this minimum value has been indicated with the reference “ith”; in this specific example, the input current i (I10) is wavy, but its undulations do not go below the minimum value "ith".

To overcome this problem, the present invention proposes to short-circuit the input current if its value is lower than a certain value, in particular for all the time in which this current is lower than this value.

This teaching allows us not to waste energy, at least in a significant way. In fact, the input current is typically obtained from a switching power supply and this has reduced losses when its output is short-circuited, of course if an appropriate topology is used and if an appropriate sizing is carried out. From the circuit point of view, this teaching can be carried out by means of a current measuring circuit connected in such a way as to measure the current supplied to the outputs of the power supply system and circuit means capable of selectively realizing an internal short circuit of the input current in alternative to powering the loads electrically; the current measuring circuit is able to detect when this current is lower than a certain current threshold and is connected to said circuit means in such a way that the internal short circuit occurs when such current is lower than said current threshold. In the example of Fig.1, the current meter circuit (which includes a comparator) corresponds to block 19; arrow C2 at the input of block 19 indicates the possibility of modifying the current threshold during operation of the circuit; the output of block 19 is connected to an input of the logic circuitry 16 which in relation to the current measurements received from block 19 (for example "current below threshold") can generate a particular load switching command for the selector circuit 14 ; when the selector circuit 14 receives the particular load switching command, it generates at the output a particular deviation command for the diverter circuit 11; when the diverter circuit 11 receives the particular load switching command, it electrically connects the input I10 to an "internal" output OD which is connected to an internal short-circuit electrical path 13. Advantageously, the logic circuitry 16, in relation to the current measurements received by the block 19 (for example "current below threshold"), can generate a "hold" command (H) for the integrator circuit 18; when the integrator circuit 18 receives a hold command (H) it (temporarily) interrupts the measurement; in fact, when the input current is small and is not sent to any load, it does not contribute to generating light energy and therefore should not be counted for the purposes of adjusting the color rendering.

If the current flowing through an LED is higher than a maximum value, the LED can be damaged.

To overcome this problem, the present invention proposes to short-circuit the input current if its value is higher than a certain value, in particular for all the time in which this current is higher than this value; this solution can also be used in addition to a possible current limiting circuitry inside the electric current source of the lighting system. This teaching allows us not to waste energy, at least in a significant way. In fact, the input current is typically obtained from a switching power supply and this has reduced losses when its output is short-circuited, of course if an appropriate topology is used and if an appropriate sizing is carried out. In this situation, however, it must be said that a certain part of the energy must necessarily be dissipated in order to bring the input current below the maximum tolerable value of the various LEDs and that a fast current limiter is essential, typically at the primary. From the circuit point of view, this teaching can be carried out by means of a current measuring circuit connected in such a way as to measure the current supplied to the outputs of the power supply system and circuit means capable of selectively realizing an internal short circuit of the input current in alternative to powering the loads electrically; the current measuring circuit is able to detect when said current is lower than a certain current threshold and is connected to said circuit means in such a way that the internal short circuit occurs when said current is higher than said current threshold.

It is therefore understood that it may be appropriate to realize the internal short-circuit both in the case of a current lower than a lower current threshold and in the case of a current greater than a higher current threshold; this functionality can be advantageously integrated into the same circuitry. In the example of Fig.1, this result can be obtained by making sure that the current meter circuit 19 signals to the logic circuitry 16 both the exceeding of the lower threshold and the exceeding of the upper threshold; in this case, arrow C2 may indicate the possibility of modifying both current thresholds.

The system of Fig. 3 comprises an electronic control unit; in particular it is a microprocessor or microcontroller control unit.

The unit 30 is equipped with a program and data (all stored internally) and is adapted to control the circuitry 10 (as regards the current distribution) and the circuitry 20 (as regards the current regulation) on the basis of such program and such data; in particular, it sends to the circuitry 10 (arrow C1) the reference levels for adjusting the color rendering and to the circuitry 20 (arrow C3) the reference level for adjusting the brightness; in addition, if provided, it can send to the circuitry 10 (arrow C2 dashed) the lower current threshold level and / or the upper current threshold level for the activation of the short circuit path.

The system of Fig.3 could further comprise user interface means connected to unit 30 to affect lighting, in particular brightness and / or color rendering; such means could be, for example, one or more buttons (mechanical or touch) and / or one or more knobs.

The system of Fig.3 could further comprise at least one sensor, in particular for temperature or brightness or color, connected to unit 30 to affect lighting, in particular brightness and / or color rendering. Since the power LEDs have a behavior that depends significantly on the temperature of their junction, it can be advantageous to provide a temperature sensor positioned close to the LEDs, acquire the temperature value and accordingly change the reference level for adjusting the brightness and / o one or more of the reference levels for adjusting the color rendering; in this way, errors due to temperature are compensated. In applications where accuracy of brightness and / or color rendering is required (for example photographic), it may be advantageous to provide a brightness sensor and / or a color rendering sensor, acquire the measurements of the sensor or sensors and change the level accordingly. reference level for adjusting the brightness and / or one or more of the reference levels for adjusting the color rendering.

The system of Fig.3 could further comprise means of communication connected to unit 30 to directly or indirectly affect the lighting; this feature can be useful, in particular, for complex lighting systems or in a "home automation" or "building automation" system. Such resources may for example include wired or wireless interfaces and, optionally, computer programs; by way of example among the interfaces we can remember the serial interfaces (RS-232, USB, ...), the Ethernet interfaces, the WiFi interfaces, the Bluetooth or ZigBee interfaces, the telephone interfaces (line modem or GSM modem and / or GPRS). According to this teaching, the power supply system (and therefore the lighting) can be controlled from a short distance (for example through an interface point) and / or at a medium distance (for example through a LAN [Local Area Network]) and / or over a long distance. distance (e.g. by phone or internet). It should be noted that the means of communication could be used not only to receive brightness and / or color rendering control commands, but also to transmit lighting monitoring information.

From what has been described, it is understood that by means of a diverter circuit it is possible to divert an input current by choosing between a plurality of loads (each of which essentially consists of the series connection of a certain number of LEDs) and also a short circuit path. Previously, it was expected that the short circuit path was connected to the current input as an alternative to the loads in case of too low or too high input current. However, the technical teaching related to the use of a short circuit path goes beyond this specific purpose; in fact, thanks to this path it is possible to obtain particular lighting effects such as, for example, switching off and / or sudden switching on of the lighting or adjusting the brightness of the lighting. Any of the effects described above can be obtained thanks to the deviator circuit of Fig.1 (which corresponds to a plurality of transistors in Fig.6) in order to send it suitable deviation commands. The circuit of Fig.6, indicated as a whole with 600, is a power supply system according to the present invention and can be considered a circuit embodiment of the block diagram of Fig.3. In reality, according to this embodiment, the blocks 10 and 20 of Fig.3 are partially integrated with each other and the block 30 of Fig.3 corresponds largely (though not totally) to the block 630 of Fig.6. In Fig.6 the points of the circuit corresponding to the inputs I10 and I20 and to the outputs O20, OA, OB, OC of the block diagram of Fig.3 are highlighted. The circuit 600 of Fig. 6 includes: a bridge of diodes 601, a correction circuit of the "power factor" 602, a smoothing capacitor 603 of the input voltage, a transformer 604, a diode 605 for protection against inversion of polarity, a resistor 606, four power transistors of the type MOSFET 607, 608, 609, 610, a comparator 611, a threshold voltage generator 612, a logic gate of the OR type 613, a controlled switch 614, a resistor 615 , a comparator 616, a capacitor 617, a controlled switch 618, a comparator 619, a monostable circuit 620, a power transistor of the MOSFET type 621, a resistor 622, a PWM modulator [Pulse-Width Modulation] 623 with also the function of current limiting, a low-pass filter 624, a comparator 625, a compensation network 626, two digital-to-analog converters 627 and 628 in particular of the four-bit "R-2R network" type, a control logic unit 630.

The circuit 600 of Fig. 6 includes:

- a diverter circuit essentially consisting of components 607, 608, 609, 610,

- charge meter circuit essentially consisting of components 606, 615, 616, 617, 618, 619,

- a logic circuit essentially consisting of component 630.

The diverter circuit has an input I10 coupled to the input I20 of the system, a plurality of outputs OA, OB, OC respectively connected to a plurality of outputs of the system, and is adapted to alternatively connect its input I10 to its outputs OA, OB , OC;

The charge meter circuit is connected in such a way as to measure the charge supplied to said outputs OA, OB, OC.

The logic circuit is connected in such a way as to receive in input the measurements carried out by the charge meter circuit and to supply at the output to the deviator circuit deviation command signals in relation to the received measurements.

The diverter circuit comprises a further output OD connected to an electrical short-circuit path, and is adapted to connect its input I10 to this output OD alternatively or in addition to the connection to one of the outputs OA, OB, OC; there is an additional MOSFET 610 transistor for connecting the OD output.

The charge meter circuit comprises a resistor 606 (connected to a voltage reference, in particular to ground) and an integrator circuit (which essentially consists of components 615, 616, 617); in the case of Fig. 6, there is a single resistor and a single integrator circuit, both connected to input I10 of the diverter circuit. The integrator circuit is connected in such a way as to integrate the voltage that falls across resistor 606.

The charge meter circuit further comprises a comparator circuit 619 having a first input and a second input; the first input is connected to the output of the integrator circuit 616 and the second input receives a reference signal (from the control logic unit 630 through the converter 628).

The integrator circuit is equipped with a "reset" input which corresponds to the control input of switch 618.

The integrator circuit is also equipped with a "hold" input which corresponds to the control input of switch 614.

The logic circuit 630 is input coupled to the output of the integrator circuit 616, preferably through a comparator circuit 619. The logic circuit 630 is connected at the output to control inputs of the diverter circuit (gate terminals of transistors 607, 608, 609, 610); these control inputs are equal in number to the outputs OA, OB, OC, OD of the diverter circuit.

The logic circuit 630 is output coupled to an input of a comparator circuit, preferably through a digital to analog conversion circuit; this characteristic applies both to the comparator 619 through the converter 628 and to the comparator 625 through the converter 627.

The logic circuit 630 is indirectly connected to the "reset" input of an integrator circuit (through components 619 and 620); alternatively, this link could be direct.

The logic circuit 630 essentially consists of a microprocessor or a microcontroller; there are, of course, memories for programs and data.

The circuit 600 further comprises a current measuring circuit essentially consisting of components 606, 611, 612; it is connected in such a way as to measure the current supplied to the outputs OA, OC, OC. The current meter circuit includes a resistor 606 (connected to a voltage reference, in particular to ground) and a comparator circuit 611, preferably a single resistor and a single comparator circuit, both connected to the input I10 of the diverter circuit; the comparator circuit is connected in such a way as to compare the voltage falling across the resistor 606 with a threshold voltage supplied by the generator 612.

The output of the current meter circuit is connected (indirectly) to an input of the diverter circuit (the gate terminal of transistor 610); the current measuring circuit is adapted to output a deviation command signal in relation to the measurements made. Furthermore, the output of the current meter circuit is connected (indirectly) to the integrator circuit (to the control input of the circuit breaker 614), and is adapted to supply a "hold" signal to the integrator circuit in relation to the measurements. carried out. The OR gate 613, which is connected to an output of the logic circuit 630, allows the logic circuit 630 to control the transistor 610.

The circuit 600 further includes a switching-type power supply connected upstream of the I10 input of the diverter circuit; this power supply is of the type with current control and with non-filtered output (O20) - there is in fact no capacitor connected to its output; in particular, it is a “flyback” AC-DC converter with constant average output current (alternative embodiments may include other converter topologies); essentially it consists of components 601, 603, 604, 605, 606, 621, 622, 623, 625. The power supply current control provides a current measuring circuit connected in such a way as to measure the current supplied to the output O20 the power supply itself; it comprises a resistor 606 (connected to a voltage reference, in particular to ground) and an error amplifier circuit 625, and is connected in such a way as to compare the voltage falling across the resistor 606 with a reference voltage ( coming from the logic control unit 630 through the converter 627 and which is proportional to the desired average value of the current at the output of the power supply). There is also a low-pass filter 624 connected in such a way as to filter the voltage that falls across the resistor 606 before it is supplied to the 625 error amplifier circuit.

In the circuit 600, advantageously, a single resistor, that is the resistor 606, allows to carry out measurements relating to two or more different functions of the system; in this example, it is used for charge measurements for the diverter as well as for current measurements for both the diverter and the power supply.

The correction circuit of the "power factor" 602, connected just downstream of the diode bridge 601 and just upstream of the power converter circuit, is not strictly essential, but may be required by electrical regulations and allows for a more efficient use of electricity; the same can be said for a filter (not shown in Fig. 6) against disturbances generated by the power supply system.

We do not dwell here on the structure and operation of power converters, which are well known; it is just worth clarifying that the resistor 622 serves to detect the current on the primary of the transformer 604 and therefore, among other things, to provide protection against over-currents by means of the modulator 623; the compensation network 626 is used to appropriately adjust the feedback loop and the low-pass filter 624 is used to ensure that the adjustment is made based on the average current supplied at the output and not on the instantaneous current.

Since the 627 and 628 converters are four-bit, it is possible to adjust the brightness to 16 different levels and the color rendering to 4096 different levels (16 levels for each of the three colors); this is certainly more than sufficient for any domestic application and is achieved by means of simple (and cheap) and fast converters.

The loads can be chosen very freely; according to one of the many examples, a first load connected to the output OA can consist of six green LEDs with a characteristic such as that shown in Fig. 5 (luminous flux of 40 lumens at 350 mA), a second load connected to the output OB can consist of seven red LEDs with a characteristic like the one shown in Fig. 5 (luminous flux of 80 lumens at 350 mA), a third load connected to the OC output can consist of five blue LEDs with a characteristic like the one shown in Fig. 5 (luminous flux of 20 lumens at 350 mA).

Whatever the powered load, the yield of the power supply system is substantially equal to the yield of the input power supply which, in the case of a switching power supply, is 80-90%. To be precise, it is necessary to take into account a small loss due to the internal switching of the system, in particular to the switching of the transistors of the diverter circuit; for this reason, it is advisable not to increase the current deviation rate too much (if not necessary for the specific lighting application) even if the circuit according to the present invention could operate at very high rates. Among other things, the method according to the present invention makes it possible to use a period of distribution of the current lower, equal to and even higher than the switching period of the switching power supply while maintaining good accuracy; according to a typical application example, the breakdown frequency (not strictly constant) is about one tenth of the switching frequency (constant and equal for example to 50-100 KHz).

This 80-90% yield is much higher than that obtained when optimizing the power supply to just one of several loads, which easily drops below 70%.

Claims (12)

CLAIMS 1. Method for electrically powering a plurality of loads (12A, 12B, 12C), in particular three loads, each of said loads essentially consisting of one or more LEDs, starting from an electric current (i (I10)), characterized by the fact that said electric current (i (I10)) is distributed cyclically among said loads (12A, 12B, 12C) in such a way that substantially only one load at a time is electrically powered and that the quantities of electric charge supplied to said loads ( 12A, 12B, 12C) for each cycle are regulated. Method according to claim 1, wherein the quantities of electric charge supplied to said loads (12A, 12B, 12C) are adjusted to predetermined or set values (C1). Method according to claim 1 or 2, wherein said electric current (i (I10)) is divided among said loads (12A, 12B, 12C) according to a cycle having duration (t1 + t2 + t3; t4 + t5 + t6 ; t7 + t8 + 79), fixed or variable, but in any case less than 40 mS, preferably less than 10 mS and / or preferably greater than 1 mS. 4. Method according to claim 1 or 2, wherein said electric current is divided among said loads according to a cycle having a duration (t1 + t2 + t3; t4 + t5 + t6; t7 + t8 + 79), fixed or variable, but in any case lower than 1 mS, preferably higher than 10uS, typically of the order of 100uS. Method according to any one of the preceding claims, in which ratios are defined between the quantities of electric charge supplied to said loads (12A, 12B, 12C) and said ratios are kept constant as said electric current varies (i (I10)) . 6. Method according to claim 5, wherein said ratios are predetermined or set (C1). Method according to any one of the preceding claims, wherein said electric current (i (I10)) is short-circuited (13) and not sent to any of said loads (12A, 12B, 12C) when it is less than a predetermined value (ith ) and / or when it is greater than a predetermined value. Method according to any one of the preceding claims, wherein said electric current (i (I10)) is obtained from a switching power supply (20), in particular with current control, more particularly with current control and having current output with wavy or impulsive trend. Method according to any one of the preceding claims, wherein the average value of said electric current (i (I10)) is adjusted to a predetermined or set value (C3). 10. Power supply system for LED lighting, having a plurality of outputs (OA, OB, OC), in particular three outputs, for electrically powering a corresponding plurality of loads (12A, 12B, 12C), characterized in that it comprises means analog and / or digital circuits adapted to implement the method according to any one of the preceding claims. 11. System according to claim 10, characterized in that it comprises circuit means (11, 13) adapted to selectively realize an internal short circuit as an alternative to electrically power said loads (12A, 12B, 12C). System according to claim 10 or 11, characterized by the fact that it further comprises a switching type power supply (20), in particular with current control, more particularly with current control and having an output current with a wavy or impulsive trend .