EP3603344B1

EP3603344B1 - Electronic circuit for driving a string of light-emitting diodes

Info

Publication number: EP3603344B1
Application number: EP18717700.1A
Authority: EP
Inventors: Sandro Toffoletto
Original assignee: Cynergi Srl
Current assignee: Cynergi Srl
Priority date: 2017-03-24
Filing date: 2018-03-23
Publication date: 2022-07-20
Anticipated expiration: 2038-03-23
Also published as: WO2018172980A1; IT201700032546A1; MA48993B1; LT3603344T; EP3603344A1; BR112019019975A2; MA48993A; ES2928692T3

Description

TECHNICAL FIELD OF THE INVENTION

The present invention concerns a circuit for driving a string of light-emitting diodes.

PRIOR ART

The use of LEDs (light-emitting diodes) to make lamps for lighting in residential, office and industrial environments is known.
More in particular, the lamp is implemented using a string of LEDs, that is a plurality of LEDs serially connected each other (optionally arranged in groups, each one composed of several LEDs), in such a way that the series provides the luminosity required by the application.
Since the distributed line voltage is an alternating type (typically at 50 Hz) and has high values (typically an effective value of 120 or 220-230 Volts), it is necessary to use appropriate devices to convert the alternating voltage into lower values such to enable the LED string to be driven correctly.
A first solution for driving the LED string is to use an AC-DC converter (for example of the SMPS - switch mode power supply type) which converts alternating voltage into direct voltage with lower values (for example, 60 Volts).
The first solution is efficient from an energy standpoint, but requires the use of transformers and large inductors and capacitors, which thus occupy space and moreover reduce reliability.
A second solution (commonly referred to as AC "direct drive") is to avoid the use of AC-DC converters, i.e. the LED string is driven directly using alternating voltage and the LEDs light up in sequence following the sinusoidal waveform of the alternating voltage, through the use of appropriate control circuits; for example, use can be made of the integrated circuits identified with the codes TPS92410, by Texas Instruments, NSl45020, by ON Semiconductor, lS31LT3170, by ISSI and the Acrich family by Seoul Semiconductor.
The second solution (and in part also the first) poses the problem of flicker in the LED string , i.e. the luminous intensity of the LED string is not constant over time as part of the LEDs of the string pulses following the frequency of the half wave generated by the rectifier with a frequency that is typically double that of the alternating voltage of the electric line: this could induce visual fatigue and loss of concentration for the people present in the environment illuminated by the LED string in the event of long stays, as in the case of work environments.
Solutions for reducing flicker in the LED string in the case of direct AC drive of the LED string are known, for example the ones described in patents US 9232576 and US 6141230 .
The Applicant has observed that the known direct AC drive solutions are capable of only partly reducing flicker in the LED string and, furthermore, have other disadvantages, such as, for example, that of requiring the use of an external reference voltage (generated, for example, by a battery; see US 9232576 ), or using an alternating voltage with a frequency higher than that of the line voltage, or it can function only with low values of the alternating voltage supply (again see US 9232576 ).
US 2013/0162149 A1 discloses a lighting apparatus including an energy storage module for applying power to a LED string during low power intervals.
US 2013/0313984 A1 discloses a circuit for actuating a plurality of LEDs connected in series, the circuit comprising a plurality of electronic switches arranged in parallel with at least part of the plurality of LEDs.

SUMMARY OF THE INVENTION

The present invention relates to an electronic circuit for driving a string of light-emitting diodes as defined in the enclosed claim 1 and by its preferred embodiments described in the dependent claims 2 to 8.
The Applicant has perceived that the electronic driving circuit according to the present invention has the following advantages:

it considerably reduces (even to the extent of eliminating) flicker in the luminous intensity of the LED string ;
it is not limited to operating with particular values of the alternating voltage supply, i.e. it can operate with high (115-230 Volts and even up to 400 Volts), medium and low voltage values;
it is capable of operating correctly with an alternating voltage supply having non-negligible fluctuations in value (up to +/- 20%) relative to the nominal value of the alternating voltage supply;
it is not limited to operating with particular values of the frequency of the alternating voltage supply;
it is not limited to particular values of electric power consumption, which may for example reach 50 W and even higher;
it does not require any balancing of the number of LEDs among the various groups of the string;
it does not require the use of transformers or large inductors and capacitors;
it does not require the use of an external reference voltage;
the luminous flux generated by the LED string is adjustable in intensity (dimming).

It is also an object of the present invention an integrated circuit as defined in the enclosed claim 11.
It is also an object of the present invention a lamp for illuminating environments (for example, of the domestic, industrial or public type) as defined in the enclosed claim 9.

BRIEF DESCRIPTION OF THE DRAWINGS

Further characteristics and advantages of the invention will emerge from the following description of a preferred embodiment and variants thereof, said description being provided by way of example with reference to the attached drawings, wherein:

Figure 1 shows a block diagram of an electronic circuit for driving a string of light-emitting diodes according to a first embodiment of the invention;
Figure 2 shows a block diagram of an electronic circuit for driving a string of light-emitting diodes according to a second embodiment of the invention;
Figure 3 shows a block diagram of an electronic circuit for driving a string of light-emitting diodes according to a third embodiment of the invention;
Figures 4A-4C schematically show a possible trend of some signals of the electronic driving circuit of the first embodiment.

DETAILED DESCRIPTION OF THE INVENTION

Note that in the description below, even if they are appeared in different embodiments of the invention, identical or similar blocks, components or modules are indicated by the same numerical references in the figures.
With reference to Figure 1, it shows an electronic circuit 1 for driving a string 5 of light-emitting diodes according to a first embodiment of the invention.
The term "string of diodes" means a series connection of two or more light-emitting diodes, hereinafter indicated as LEDs.
Preferably, a LED string can be divided into a plurality of segments, each segment comprising the series connection of a plurality of LEDs.
In other words, two or more LEDs connected in series can be grouped in such a way as to form a group of LEDs and thus a LED string can be composed of two or more groups of LEDs.
Furthermore one or more groups (or segments) of LEDs can be in turn composed of the parallel connection of two or more series of LEDs.
The electronic driving circuit 1 comprises:

a rectifier 3;
a current regulator 4;
the string of LEDs 5;
an electronic switch 6;
a capacitor 7;
a bias stage 8;
a Zener diode 10.

The rectifier 3 comprises two input terminals adapted to receive a positive alternating voltage V_AC+ and a negative alternating voltage V_AC- and comprises an output terminal adapted to generate a rectified alternating voltage V_RTF, as a function of the positive and negative alternating voltages V_AC+ and V_AC-.
Preferably, the rectifier 3 is implemented with a full wave diode bridge, as shown in Figure 1.
The current regulator 4 is electrically connected to the rectifier 3 and to the LED string 5.
The current regulator 4 comprises an input terminal It₀ adapted to receive the rectified alternating voltage V_RTF and comprises four input terminals It₁, It₂, It₃, It₄ electrically connected to four respective different voltages of the LED string 5, as will be explained in greater detail below.
The current regulator 4 has the function of regulating the total value of the current l_str flowing across the LED string 5, as will be explained in greater detail below.
Preferably, the current regulator 4 is an integrated circuit by Altoran Chip & Systems (www.altoranCNS.com), identified with the code ACS1404.
The LED string 5 comprises a first terminal connected to the rectified alternating voltage V_RTF and comprises a second terminal connected to the current regulator 4.
It should be observed that, more in general, it is possible to interpose further electronic components between the output of the rectifier 3 and the first terminal of the LED string 5.
In particular, the LED string 5 comprises the series connection of four LEDs 5-1, 5-2, 5-3, 5-4, wherein:

anode a₁ of the first LED 5-1 (i.e. the first terminal of the LED string 5) is connected to the output terminal of the rectifier 3 and thus it is such to receive the rectified alternating voltage V_RTF;
cathode of the first LED 5-1 is connected to anode a₂ of the second LED 5-2;
cathode of the second LED 5-2 is connected to anode a₃ of the third LED 5-3;
cathode of the third LED 5-3 is connected to anode a₄ of the fourth LED 5-4.

Furthermore, the first input terminal It₁ of the current regulator 4 is connected to the anode a₂ of the second LED 5-2, the second input terminal It₂ of the current regulator 4 is connected to the anode a₃ of the third LED 5-3, the third input terminal It₃ of the current regulator 4 is connected to the anode a₄ of the fourth LED 5-4 and the fourth input terminal It₄ of the current regulator 4 (i.e. the second terminal of the LED string 5) is connected to the cathode c₄ of the fourth LED 5-4.
It may be observed that, more in general, each of the LEDs 5-1, 5-2, 5-3, 5-4 can be a series connection of two or more LEDs, that is, each series connection is a segment of the LED string 5.
Therefore, the component 5-1 represents a first group of LEDs connected in series (i.e. a first segment of the LED string 5), the component 5-2 represents a second group of LEDs connected in series (i.e. a second segment of the LED string 5), the component 5-3 represents a third group of LED connected in series (i.e. a third segment of the LED string 5) and the component 5-4 represents a fourth group of LEDs connected in series (i.e. a fourth segment of the LED string 5).
The current regulator 4 is such to control:

the value of the current flowing from the output of the rectifier 3 through the first LED 5-1, then the current I₁ enters the first input terminal It₁ of the current regulator 4 and flows towards the ground reference voltage;
the value of the current flowing from the output of the rectifier 3 through the series of the first and second LEDs 5-1, 5-2, then the current I₂ enters the second input terminal It₂ of the current regulator 4 and flows towards the ground reference voltage;
the value of the current flowing from the output of the rectifier 3 through the series of the first, second and third LEDs 5-1, 5-2, 5-3, then the current I₃ enters the third input terminal It₃ of the current regulator 4 and flows towards the ground reference voltage;
the value of the current flowing from the output of the rectifier 3 through the series of the first, second, third and fourth LED 5-1, 5-2, 5-3, 5-4, then the current I₄ enters the fourth input terminal It₄ of the current regulator 4 and flows towards the ground reference voltage.

The electronic switch 6 comprises a first terminal It₆, a second terminal Ot₇ and a control terminal It₇ for controlling the opening and closing of the electronic switch 6.
The electronic switch 6 is switchable between an open position and a closed position as a function of the value of a control signal V_g on the control terminal It₇, wherein:

in the open position the electronic switch 6 is equivalent to an open circuit between the first terminal It₆ and the second terminal Ot₇;
in the closed position the electronic switch 6 is substantially equivalent to a short circuit between the first terminal It₆ and the second terminal Ot₇.

The capacitor 7 is interposed between the first terminal of the LED string 5 and the electronic switch 6.
In particular, the capacitor 7 comprises a first terminal connected to the anode a₁ of the first LED 5-1 (and thus connected to the output of the rectifier 3) and thus it is such to have the value of the rectified alternating voltage V_RTF.
The capacitor 7 further comprises a second terminal connected to the first terminal It₆ of the electronic switch 6.
The capacitor 7 has the function of storing electric energy in each period of the rectified alternating voltage V_RTF when the latter has an increasing trend (or is in any case greater than a threshold voltage V_th).
Furthermore, in each period the capacitor 7 has the function of discharging the electric energy stored therein at least in part through the LED string 5, when the rectified alternating voltage V_RTF has a decreasing trend and smaller than the threshold voltage V_th: this allows to reduce the flicker of the luminous intensity of the LED string 5.
Advantageously, the electronic switch 6 is implemented with a transistor 6-1 of the n-channel IGBT (insulated gate bipolar transistor) type, having a collector terminal C which coincides with the first terminal It₆, having an emitter terminal E which coincides with the second terminal Ot₇ and having a gate terminal G which coincides with the control terminal It₇; therefore, in this case the collector terminal C of the IGBT transistor 6-1 is connected to the second terminal of the capacitor 7, the emitter terminal E is connected to the ground reference voltage and the gate terminal G is connected to the output of the bias stage 8.
The electronic switch 6 (in particular, the IGBT transistor 6-1) has the function of switching the capacitor 7 during charging/discharging in an active manner, as will be explained in greater detail below.
Alternatively, the electronic switch 6 is implemented with a p-channel IGBT transistor 6-2, as shown in Figure 3.
It should be observed that in order to implement the electronic switch 6, components other than the IGBT transistor can be used, provided that they are capable of switching between the open and closed conditions with a frequency of around 100 Hz, i.e. a switching period comprised between 1 millisecond and 12 milliseconds.
The bias stage 8 is interposed between the LED string 5 and the electronic switch 6.
In particular, the bias stage 8 comprises an input terminal It₅ adapted to receive a voltage signal V_d3 selected from a voltage internal to the LED string 5 and comprises an output terminal connected to the control terminal It₇ of the electronic switch 6.
The bias stage 8 has the function of generating an appropriate value of the control signal V_g of the control terminal It₇ so as to control the closing and opening of the electronic switch 6 at appropriate time instants in order to charge and discharge the capacitor 7 with the purpose of reducing the flicker of the luminous intensity of the LED string 5, caused mainly by the third and fourth LEDs 5-3, 5-4.
In fact, choosing a voltage value V_d3 selected from a voltage internal to the LED string 5 allows to anticipate (with respect to the known solution) the instant wherein it is activated the discharging of the capacitor 7 through the LED string 5 and this allows a larger number of light-emitting diodes of the string 5 to be maintained in a conduction state, thus reducing flicker of the luminous intensity of the LED string 5.
In particular, the input terminal of the bias stage 8 is connected to the anode a₄ of the fourth LED 5-4 and thus the voltage signal V_d3 is the voltage of the anode a₄ of the fourth LED 5-4 (which is equal to the cathode voltage of the third LED 5-3).
It should be observed, however, that the bias stage 8 is alternatively such to draw the voltage signal V_d3 from voltage values different than the voltage of the anode a₄ of the fourth LED 5-4, such as for example the voltage of the anode a₃ of the third LED 5-3, as shown in Figure 2 in the second embodiment.
More in general, the bias stage 8 is such to draw a voltage value V_d3 selected from a voltage internal to the LED string 5 so as to generate, as a function of the selected internal voltage value, a value of a control voltage signal V_g that is less than the value of the rectified alternating voltage V_RTF, provided that said selected voltage value V_d3 is such to generate the control voltage V_g controlling the closing of the electronic switch 6 so as to allow a sufficient charging at the appropriate time of the capacitor 7, whose charge is subsequently used to power the LED string 5 when the value of the rectified alternating voltage V_RTF is not sufficient to power all of the LEDs of the string 5: in this way the flicker of the luminous intensity generated by the LED string 5 is reduced.
For example, in case of alternating voltage with nominal values of 220 Volts, the selected value of the voltage V_d3 of the LED string 5 is equal to 50 Volts and the value of the control voltage V_g is comprised between 0 and 5 Volts: when V_g= 5 Volts the electronic switch 6 is closed and the capacitor 7 is charged, whereas when V_g< 5 Volts the electronic switch 6 is open and the capacitor 7 is discharged at least in part through the LED string 5.
The bias stage 8 is implemented with a voltage divider comprising a first resistor 8-1 and a second resistor 8-2 serially connected each other, wherein the first resistor 8-1 is connected between the voltage V_d3 selected from the LED string 5 and the control terminal It₇ of the electronic switch 6 and wherein the second resistor 8-2 is connected between the control terminal It₇ of the electronic switch 6 and the ground reference voltage.
When the rectified alternating voltage V_RTF has an increasing trend (or is in any case greater than the threshold voltage V_th), the electronic switch 6 is closed and the capacitor 7 is charged; when, by contrast, the rectified alternating voltage V_RTF has a decreasing trend and is less than the threshold voltage V_th, the electronic switch 6 opens and the capacitor 7 is discharged.
The discharge phase of the capacitor 7 allows at least part of the energy stored therein to be discharged through the LED string 5 and this allows to compensate the reduction of the voltage at the ends of the LEDs string 5, thus considerably reducing (even to the extent of eliminating) the flicker of the luminous intensity of the LEDs string 5.
The time instant wherein the discharge phase of the capacitor 7 occurs will vary depending on which voltage signal V_d3 is selected from the LED string 5; therefore it is possible to anticipate or postpone the activation of the capacitor 7 discharge (and thus modify the entity of the reduction of the flicker of the luminous intensity of the LED string 5) by changing the selected voltage signal V_d3 and thus changing the value of the selected voltage.
Advantageously, the electronic driving circuit 1 further comprises a discharge circuit 9 connected to the capacitor 7 (for example, interposed between the first terminal of the capacitor 7 and the ground reference voltage) and having the function of further contributing to the discharging of the capacitor 7.
Preferably, the discharge circuit 9 is implemented with a resistor 9-1 having a first terminal connected to the first terminal of the capacitor 7 (and thus connected to the output of the rectifier 3) and having a second terminal connected to the ground reference voltage.
With reference to Figures 4A-4C, they show the trend in the voltage and current signals of the electronic driving circuit 1 of the first embodiment, wherein an n-channel IGBT transistor 6.1 is used.
In particular:

Figure 4A shows the trend in the rectified alternating voltage V_RTF, the control voltage V_g of the gate terminal of the IGBT transistor 6.1 and the selected voltage V_d3;
Figure 4B shows the trend of the voltage drop ΔVC at the ends of the capacitor 7 and the voltage V_c of the collector terminal of the IGBT transistor 6.1;
Figure 4C shows the trend of the current I₁ flowing through the first LED 5-1, the current I₂ flowing through the second LED 5-2, the current I₃ flowing through the third LED 5-3 and the current I₄ flowing through the fourth LED 5-4.

It is possible to observe that the behaviour of the electronic driving circuit 1 periodically repeats itself equally, wherein each period is defined by a wave of the rectified alternating voltage V_RTF.
For the purpose of explaining the invention, three periods of time ΔT1, ΔT2, ΔT3 are shown, comprised between the instants to and t10, t10 and t20, t20 and t30, respectively.
In the time period ΔT1 (and similarly in ΔT2 and ΔT3) it is possible to observe the following behaviour:

during a first phase comprised between the instants t0 and t3 wherein the rectified alternating voltage V_RTF has a substantially increasing trend from the null value to values less than the threshold value V_th, the value of the gate voltage V_g of the IGBT transistor 6-1 has a first value such to maintain the IGBT transistor 6-1 open: during this first phase the capacitor 7 is discharged at least in part through the LED string 5 and thus the voltage drop ΔVC at the ends of the capacitor 7 has a decreasing trend;
during a second central phase (following the first phase) comprised between the instants t3 and t7 wherein the rectified alternating voltage V_RTF has a substantially increasing trend from the threshold value V_th to the maximum value and then has a decreasing trend from the maximum value to the threshold value V_th, the value of the gate voltage V_g of the IGBT transistor 6-1 has a second value such to maintain the IGBT transistor 6-1 closed: during this second phase the capacitor 7 is charged and thus the voltage drop ΔVC at the ends of the capacitor 7 has first an increasing trend and then a decreasing trend;
during a third phase (following the second phase) comprised between the instants t7 and t10 wherein the rectified alternating voltage V_RTF has a substantially decreasing trend from the threshold value V_th to the null value, the value of the gate voltage V_g of the IGBT transistor 6-1 has a first value such to maintain the IGBT transistor 6-1 open: during this third phase the capacitor 7 is discharged at least in part through the LED string 5 and thus the voltage drop ΔVC at the ends of the capacitor 7 has a decreasing trend;
the value of the current I₁ and I₂ flowing through the first and second LEDs 5-1, 5-2 is constant and thus the luminous intensity of the first and second LEDs 5-1, 5-2 is constant;
the value of the current l₃ flowing through the third LED 5-3 is equal to the maximum value I_3,max most of the time and thus the luminous intensity of the third LED 5-3 is substantially constant;
the value of the current I₄ flowing through the fourth LED 5-4 is equal to the maximum value I_4,max most of the time and thus the luminous intensity of the fourth LED 5-4 is substantially constant;
the total current I_str flowing through the LED string 5 has values that are always uniform and thus the luminous intensity of the LED string 5 is substantially constant, since any small fluctuations that might be present are compensated by the intrinsic hysteresis of the LEDs,thus the flicker of the luminous intensity of the LED string 5 is substantially null.

The electronic driving circuit 1 or 100 can be implemented with an integrated circuit.
Alternatively, the electronic driving circuit 1 or 100 can be realised on one or more printed circuit boards.
With reference to Figure 2, it shows an electronic circuit 50 for driving a string 4 of light-emitting diodes according to a second embodiment of the invention.
The electronic driving circuit 50 of Figure 2 differs from the electronic driving circuit 1 of Figure 1 in that the input of the bias stage 8 is the voltage signal V_d2 drawn from the anode a₃ of the third LED 5-3; therefore, the value of said voltage signal V_d2 controls the opening and the closing of the electronic switch 6 (in particular, it controls the cut-off and conduction state of the IGBT transistor 6-1).
The previous considerations related to the electronic driving circuit 1 may be similarly applied to the electronic driving circuit 50, therefore, the latter, too, is capable of considerably reducing (even to the extent of eliminating) flicker of the luminous intensity of the LED string 5.
With reference to Figure 3, it shows an electronic circuit 100 for driving a string 5 of light-emitting diodes according to a third embodiment of the invention.
The electronic driving circuit 100 of Figure 3 differs from the electronic driving circuit 1 of Figure 1 in that:

there is an electronic switch 106 implemented with a transistor 6-2 of the p-channel IGBT type (instead of an n-channel IGBT transistor 6-1), wherein the IGBT transistor 6-2 has a collector terminal C connected to the rectified alternating voltage V_RTF, an emitter terminal E connected to a first terminal of the capacitor 7 through a resistor 8-4 and a gate terminal G connected to the anode a₄ of the fourth LED 5-4 by means of the voltage divider 108;
the capacitor 7 is connected between the IGBT transistor 6-2 and the ground reference voltage (instead of between the IGBT transistor 6-1 and the rectified alternating voltage V_RTF);
the bias stage 108 comprises an input terminal connected to the anode a₄ of the fourth LED 5-4, a first output terminal connected to the gate terminal G of the IGBT transistor 6-2 and a second output terminal connected to the emitter terminal E of the IGBT transistor 6-2;
there is a resistor 8-4 having a first terminal connected to the electronic switch 106 (in particular, to the emitter terminal E of the IGBT transistor 6-2) and to the bias stage 108 and having a second terminal connected to the first terminal of the capacitor 7.

Preferably, the bias stage 108 is implemented with a voltage divider comprising the resistors 8-1, 8-2, 8-3, wherein:

the resistor 8-1 is connected between the anode a₄ of the fourth LED 5-4 and the control terminal of the electronic switch 106;
the resistor 8-2 is interposed between the resistor 8-1 and the ground reference voltage;
the resistor 8-3 has a first terminal connected to the common terminal between the resistors 8-1, 8-2 (and thus connected to the control terminal of the electronic switch 106) and a second terminal connected to the electronic switch 106 (in particular, to the emitter terminal E of the IGBT transistor 6-2) and the resistor 8-4.

The previous considerations related to the electronic driving circuit 1 may be similarly applied to the electronic driving circuit 100; therefore, the latter, too, is capable of considerably reducing (even to the extent of eliminating) flicker of the luminous intensity of the LED string 5.
It should be observed that the input terminal of the bias stage 108 can be connected to other voltage values selected from the LED string 5, such as, for example, the voltage of the anode a₃ of the third LED 5-3, similarly to what was illustrated previously in relation to the description of Figure 2.
Advantageously, the electronic driving circuit 1 further comprises a disconnecting stage having the function of disconnecting the LED string 5 from the alternating voltage supply V_AC+ so as to interrupt parasitic currents which can activate the LEDs also with a null power signal.
In particular, when the value of the alternating voltage supply V_AC+ is less than a defined threshold value (for example equal to 10% of a reference voltage value V_REF), the LED string 5 is disconnected from the rectified alternating voltage V_RTF; when the value of the alternating voltage supply V_AC+ is instead greater than or equal to the defined threshold value, the LED string 5 is connected to the rectified voltage V_RTF.
Advantageously, the disconnecting stage is implemented using a reference voltage V_REF generated by the current regulator 4 (for example, equal to 17 Volts), using the pulse-width modulator already used to regulate the luminous intensity of the LED string 5 and using a solid-state relay that interrupts the electrical connection between the rectified alternating voltage V_RTF and the LED string of 5.
In particular, the disconnecting stage comprises:

a pulse-width modulator (for example internal to the current regulator 4) configured to generate a square-wave periodic signal S_PWM having a period width varying as a function of a configuration signal;
an input terminal adapted to receive a reference voltage V_REF (for example, generated by the current regulator 4);
a comparator comprising a first input terminal adapted to receive a divided reference voltage (i.e. obtained by means of a voltage divider dividing the reference voltage V_REF), a second input terminal adapted to receive the square-wave periodic signal S_PWM and an output terminal adapted to generate a comparison signal having a high or low logical value, as a function of the comparison between the values of the divided reference voltage and the periodic square-wave signal S_PWM;
a solid-state relay configured to connect or disconnect the LED string 5 to/from the rectified alternating voltage V_RTF, as a function of the value of the comparison signal.

It will be described hereinafter the operation of the electronic driving circuit 1 according to the first embodiment, also referring to Figures 1 and 4A-4C.
For the purposes of explaining the operation thereof, the following assumptions are to be considered:

the rectifier 3 is a full-wave diode bridge;
the LED string 5 is composed of the series connection of four equal LEDs 5-1, 5-2, 5-3, 5-4 having the same threshold voltage Vt (for example, equal to 3.5 Volts);
the electronic switch 6 is implemented with a transistor 6-1 of the n-channel IGBT type;
the current regulator 4 is the integrated circuit ACS1404 by Altoran Chip & Systems, comprising four input terminals It₁, It₂, It₃, It₄ connected to the anode a₂ of the second LED 5-2, to the anode a₃ of the third LED 5-3, to the anode a₄ of the fourth LED 5-4 and to the cathode c₄ of the fourth LED 5-4, respectively;
the bias stage 8 is implemented with a voltage divider composed of the resistors 8-1, 8-2, wherein the voltage V_g of the gate terminal G of the IGBT 6-1 is equal to the divided voltage of the terminal common between the resistors 8-1 and 8-2;
a discharge circuit 9 is present and it is implemented with a resistor 9-1.

In the instants comprised between t0 and t3 (t3 excluded) the rectified alternating voltage V_RTF has an increasing sinusoidal trend from the null value to the value of the threshold voltage V_th: the IGBT transistor 6-1 is in the cut-off state, the capacitor 7 is discharged in part through the LED string 5 and in part through the resistor 9 and this allows to maintain the LEDs 5-1, 5-2, 5-3, 5-4 in the conduction state.
At the instant t3 the rectified alternating voltage V_RTF reaches a value equal to three times the threshold voltage Vt, i.e. V_RTF= 3*Vt.
Part of the current I₃ flowing in the third LED 5-3 enters the current regulator 4, whilst another part flows into the resistor 8-1 and then into the resistor 8-2: consequently, the voltage of the gate terminal V_g has a transition from a low voltage value to a high voltage value (for example, equal to 5 Volts) and thus the IGBT transistor 6-1 goes into the conduction state: consequently, the capacitor 7 starts charging to the value of the rectified alternating voltage V_RTF.
Therefore, in the first embodiment the value of the threshold voltage Vth is equal to three times the threshold voltage Vt of each LED of the string 5, i.e. Vth= 3*Vt.
At the instants following t3, the rectified alternating voltage V_RTF continues to have an increasing sinusoidal trend and thus the operation is analogous to that at the instant t3, i.e. the IGBT transistor 6-1 conducts and the capacitor 7 is charged to the value of the rectified alternating voltage V_RTF.
At a certain instant, the rectified alternating voltage V_RTF reaches a value equal to four times the threshold voltage Vt: the voltage of the gate terminal V_g continues to have a high value (in the example considered, equal to 5 Volts) and thus the IGBT transistor 6-1 remains in the conduction state and the capacitor 7 continues to charge.
Subsequently, the rectified alternating voltage V_RTF continues to have an increasing sinusoidal trend up to the maximum value and then has a decreasing sinusoidal trend until returning to a value equal to three times the threshold voltage Vt at the instant t7, i.e. V_RTF= 3*Vt: during this interval of time, the IGBT transistor 6-1 remains in the conduction state and thus the capacitor 7 remains charged.
After the instant t7, the rectified alternating voltage V_RTF falls below the value 3*Vt, the value of the voltage V_d3 of the cathode of the third LED 5-3 decreases and thus the voltage of the gate terminal V_g has a transition from the high voltage value to the low voltage value: consequently, the IGBT transistor 6-1 enters the cut-off state and the capacitor 7 starts discharging in part through the resistor 9-1 and in part through the LED string 5, maintaining not only the first and second LEDs 5-1, 5-2, but also the third and fourth LEDs 5-3, 5-4 in the conduction state.
At the instants comprised between t7 and t10, the rectified alternating voltage V_RTF continues to have a decreasing sinusoidal trend until reaching the null value: the capacitor 7 continues to discharge in part through the resistor 9-1 and in part through the LED string of 5, maintaining the first and second LEDs 5-1, LED 5-2 and also the third and fourth LEDs 5-3, 5-4 in the conduction state.
At the instants comprised between t10 and t13, the operation of the electronic driving circuit 1 is analogous to that previously described for the instants comprised between t0 and t3; therefore, the IGBT transistor 6-1 is in the cut-off state, the capacitor 7 is discharged in part through the LED string 5 and in part through the resistor 9 and this discharging maintains the third and fourth LEDs 5-3, 5-4, in addition to the first and second LEDs 5-1, 5-2, in the conduction state.

Claims

Electronic driving circuit (1) for driving a string (5) of light-emitting diodes, the circuit comprising:
- a current regulator (4) comprising a first input terminal (It₀) configured to receive a rectified alternating voltage (V_RTF) and a plurality of second input terminals (It₁, It₂, It₃, It₄) connectable to respective different voltages present in the string (5) of light-emitting diodes, the current regulator (4) being configured to regulate the value of the current flowing through the string of light-emitting diodes;

- an electronic switch (6) configured to switch between a closed and an open position, as a function of a value of a control signal (V_g);

- a capacitor (7) adapted to be interposed between the electronic switch (6) and the string (5) of light-emitting diodes;
characterised in that the electronic driving circuit further comprises:
- a bias stage (8) consisting of a voltage divider (8-1, 8-2) connected to the ground reference, wherein said bias stage comprises an input terminal configured to receive one of the voltages internal to the string (5) of light-emitting diodes and an output terminal configured to generate, as a function of the voltage received at its input terminal, said control signal (V_g) controlling the electronic switch (6),
wherein the control signal (Vg) generated by the bias stage (8) is the divided voltage of the voltage divider (8-1, 8-2).
Electronic driving circuit according to claim 1, wherein:
- when the rectified alternating voltage (V_RTF) has values lower than a threshold voltage (V_th), the bias stage (8) is configured to generate the control signal (V_g) having a first value which opens the electronic switch (6), so that the capacitor (7) is discharged at least in part through the string of light-emitting diodes;

- when the rectified alternating voltage (V_RTF) has values greater than the threshold voltage (V_th), the electronic switch (6) is configured in the closed position, so that the capacitor (7) is charged.
Electronic driving circuit according to claims 1 or 2, wherein the electronic switch (6) is an n-channel insulated-gate bipolar transistor (6-1) having a collector terminal (C), an emitter terminal (E) and a gate terminal (G), wherein:
- the collector terminal (C) is connected to a first terminal of the capacitor (7);

- the emitter terminal (E) is connected to a ground reference voltage;

- the gate terminal (G) is connected to the output terminal of the bias stage (8) and is configured to receive the control signal (V_g) having a voltage value controlling the switching of the n-channel insulated-gate bipolar transistor (6-1) between a conduction state wherein the capacitor (7) is charged and an interdiction state wherein the capacitor (7) is discharged at least in part through the string of light-emitting diodes;

- the second terminal of the capacitor (7) being connectable to the rectified alternating voltage and to a first terminal (a₁) of the string (5) of light-emitting diodes;

- the current regulator (4) being connectable to a second terminal (c₄) of the string (5) of light-emitting diodes.
Electronic driving circuit according to any one of the previous claims, further comprising a discharge circuit (9) connected to the capacitor (7), the discharge circuit being in particular a resistor (9-1) connected between the capacitor and the ground reference voltage.
Electronic driving circuit according to claim 3 or according to claim 4 when depending on claim 3, wherein the divided voltage of the voltage divider (8-1, 8-2) is adapted to control the gate terminal (G) of the insulated-gate bipolar transistor (6-1).
Electronic driving circuit according to any one of the previous claims, wherein the voltage received at the input terminal of the bias stage (8) is the voltage of the anode terminal (a₄) of the last light-emitting diode (5-4) of the string of light-emitting diodes.
Electronic driving circuit according to any of claims from 1 to 5, wherein the voltage received at the input terminal of the bias stage (8) is the voltage of the anode terminal (a₃) of the last but one light-emitting diode (5-3) of the string of light-emitting diodes.
Electronic driving circuit according to any of the previous claims, further comprising a diode bridge rectifier (3) configured to receive an alternating voltage (V_AC+, V_AC-) and to generate therefrom the rectified alternating voltage (V_RTF).
Lamp for illuminating environments comprising a string (5) of light-emitting diodes and the electronic driving circuit according to any of the previous claims, wherein the first terminal (a₁) of said string of light-emitting diodes is connectable to the rectified alternating voltage and wherein the second terminal (c₄) of said string of light-emitting diodes is connected to the current regulator (4) of said electronic circuit (1).
Lamp according claim 9, wherein the string (5) of light-emitting diodes comprises a plurality of segments connected to the plurality of second input terminals (It₁, It₂, It₃, It₄) of the current regulator (4) of the electronic driving circuit respectively, each segment comprising the series connection of a plurality of light-emitting diodes.
Integrated circuit comprising at least one electronic driving circuit according to any of the claims from 1 to 8.