ARRANGEMENT AND METHOD FOR CONTROLLING LIGHT-EMITTING DIODES IN ACCORDANCE WITH AN INPUT VOLTAGE LEVEL, BY MEANS OF A CAPACITOR AND SWITCH

An arrangement and a method for controlling an array of light-emitting diodes includes a capacitor arranged in series with an electronic switch between one end of a first segment of the array and a constant-current source. A first terminal of the capacitor is connected to the one end and a second terminal of the capacitor is connected to the switch. The switch is connected to a control unit for control purposes. The second terminal of the capacitor is connected to the constant-current source via an additional switch and to one end of a subsequent segment of the array, and, by the connection of the additional switch, to a control unit for control purposes

The invention relates to an arrangement for actuating light-emitting diodes, comprising an input, to which an AC input voltage can be applied, and an array of LEDs connected in series, which array is connected to the outputs of the arrangement for actuating light-emitting diodes and is divided into at least two segments, and wherein each segment of the array is connected at one end at least indirectly to a constant current source.

The invention also relates to a method for actuating light-emitting diodes, in which an array of light-emitting diodes connected in series is provided, which array is divided into segments, wherein each segment can contain a plurality of light-emitting diodes and has a first connection and a second connection, and wherein the array is operated on a rectified AC input voltage (VDC) in such a way that the segments are switched on and off successively depending on the amplitude of the AC input voltage (VDC).

LEDs (light-emitting diodes) are increasingly used for lighting purposes since they have a number of advantages over conventional light-emitting means such as incandescent lamps or fluorescent lamps, in particular a low energy requirement and a longer life. Owing to their semiconductor-typical current-voltage characteristic, it is expedient to operate LEDs using a constant current.

During operation of light-emitting means comprising LEDs from a lighting mains, therefore, circuitry measures need to be taken in order to produce the required constant direct current with the low voltage of typically 3 . . . 4 V per LED from a high AC voltage supply, which may have voltage values of 230 VAC, for example. These values can typically apply to so-called white LEDs and may be different for other LEDs.

In addition to the widespread use of so-called AC-to-DC converters, which usually consist of a rectifier and a switched mode power supply, a method is known in which an array of LEDs connected in series is actuated directly from the rectified AC voltage via one or more linear current sources.

This arrangement is also referred to as direct AC LED. For this purpose, advantageously the LED array can be divided into segments, which are energized individually or connected in series corresponding to the instantaneous AC voltage. The number of LEDs connected in series and therefore the forward voltage of the entire LED array is thus configured such that it corresponds to a notable proportion of the amplitude of the mains voltage, which may be in the region of 80 to 90% of the amplitude of the mains voltage, for example.

The voltage drop across the linear current source is therefore kept low, which results in a comparatively high degree of efficiency. At a relatively low instantaneous voltage, only part of the LED array, corresponding to the arrangement-side segmentation of the LEDs, is likewise actuated with a relatively low voltage drop across the associated current source. As a result, the angle of current flow is increased within a half-period, which results in more uniform light emission. Optionally, the current from the linear current source or current sources can be modulated corresponding to the instantaneous mains voltage in order to increase the power factor, i.e. to keep the harmonics content of the supply current low.

Advantages of this known method over the use of AC-to-DC converters are the smaller structural form and lower costs of the drive electronics and improved EMC (electromagnetic compatibility) of the arrangement since no quick switching edges occur.

A principle disadvantage consists in the high degree of ripple of the light emission at twice the mains frequency, which sensitive people find bothersome. Even when there is constant energization of the LEDs, the light emission is reduced when fewer segments than are arranged in the LED array are active.

If the instantaneous voltage at which the LED arrays are actuated falls below the forward voltage of the first segment of the arrays, the current becomes zero, i.e. there are two gaps in each period in which there is no energization of the LEDs. In contrast to the filament of an incandescent lamp, which has considerable thermal inertia and therefore damps the ripple of the power supplied, the light emission of an LED follows the current practically without any delay. In particular these energization gaps can result in an impression of flicker of the lighting which is found to be unpleasant.

A further disadvantage in terms of circuitry in respect of the actuation consists in that the switchover thresholds of the individual segments need to be matched to the number of LEDs per segment and the actual forward voltage.

Thus, the object of the invention consists in specifying an arrangement and method for actuating light emitting diodes whereby improved actuation of the LEDs is achieved without the efficiency and/or the harmonic content being impaired.

In an arrangement for actuating light-emitting diodes, it is therefore proposed that a capacitance is arranged in a series circuit with an electronic switch between one end of a first segment (for example LED-S3) and the constant current source, wherein a first connection of the capacitance is connected to the end and a second connection of the capacitance is connected to the switch, in that the switch is connected, for actuation, to a control unit, that the second connection of the capacitance is connected, via a further switch, to the constant current source and to one end of a segment (for example LED-S4) following the first segment, and that the further switch is connected, for actuation, to a further control unit.

With reference to an example of a circuit shown inFIG. 7, charging of the capacitance CER takes place, when an electronic switch TCC is switched on, via the path of the input of the AC input voltage VDC, the LED segment LED-S1, the capacitance CER itself, the closed electronic switch TCC and the constant current source ILED, which is connected to the second input GND of the actuation arrangement1and the ground potential thereof. This charging operation begins with the case where the AC input voltage VDC has exceeded the forward voltage of the segment LED-S1. During charging of the capacitor CER, the potential at the node VCER between the capacitor CER and the electronic switch TCC also increases, which results in the switch TC1arranged between the end of the first segment LED-S1and the constant current source ILED being switched off.

The charging of the capacitor CER is continued until the forward voltage of the segments LED-S1and LED-S2is exceeded. For this case, the switch TCC is opened and the charge remains on the capacitance CER.

In order to improve the flicker index, the opening of the switch TCC is advantageous, but is not absolutely essential for implementing the invention per se.

The charge collected on the capacitance CER is used for closing the current gaps occurring at twice the line frequency. These occur when the AC input voltage VDC falls below the forward voltage of a single segment. The arrangement is dimensioned in such a way that the voltage VCERon the capacitor CER is greater than the forward voltage of an LED segment. In the current gaps, the capacitor is now connected to an LED segment (LED-S4) by means of a suitable switch (TER) or else only to one or more LEDs within the LED segment, wherein the capacitor CER is discharged via the LED segment or the LEDs and said LEDs illuminate. By virtue of this light emission from the LEDs during the time of a current gap in which there is no light output in accordance with the prior art, the ripple of the light emission is improved.

Further configurations provide for charging a plurality of capacitors and therefore supplying a voltage to a plurality of LED segments or LEDs in the current gaps. Alternatively, the capacitors can be connected in series so as to increase the resultant voltage in the current gaps and therefore, for example, an LED segment or individual LEDs can be caused to illuminate at a higher forward voltage.

A further possibility consists in the use of a current limitation or current regulation arrangement in the discharge circuit. In this way, by appropriate dimensioning of the current regulation, the energy stored in the capacitor or the capacitors is output uniformly and as long as the forward voltage of the LEDs permits this, completely during the current gap. In addition, such regulation of the brightness and the illumination duration of the LEDs is possible.

In a configuration of the invention, it is provided that a second capacitance is arranged in a second series circuit with an electronic switch between one end of a subsequent segment and the constant current source.

In addition to a first capacitance, a second capacitance is introduced into the arrangement, and this capacitance is likewise charged for the case where the amplitude of the AC input voltage VDC is sufficiently high. This second capacitance is connected in series with the first capacitance in the current gaps by means of a suitable arrangement of switching elements, wherein the voltages across the capacitances add up VCER1+VCER2=VGES. By virtue of this measure, an LED segment can be operated in a current gap even for the case where a voltage VCER1or VCER2on a Single capacitance does not reach the magnitude of the forward voltage of the LED segment.

Alternatively, it is also possible for only one LED or a plurality of LEDs within an LED segment to be operated in this embodiment.

In a method for actuating light-emitting diodes, it is proposed that in the event that a preset switch-on threshold of the AC input voltage (VDC) is exceeded, a charging operation of at least one capacitance, fed by the AC input voltage (VDC), is started, and that, in the event that a second switching threshold of the AC input voltage (VDC) is undershot, discharge of the capacitance takes place via a segment.

The method provides for charging of a charging capacitor above a threshold value. For this purpose, a comparison between the AC input voltage and the switch-on threshold which was previously assigned a voltage value takes place. However, the method does not absolutely require this comparison. Alternatively, a connection of a charging capacitor to one end of an LED segment results in not only a current flow through the segment (LED-S1) itself and the further switching elements thereof, but also a charging current for the capacitance CER being generated once the forward voltage for the element in question has been reached. For this purpose, the end of the segment LED-S1is connected to a constant current source ILED via a closed switch TC1. The capacitance CER is also connected to this constant current source ILED via a closed switch TCC in parallel therewith. By means of suitable actuation of the switched TC1and TCC, it is ensured that the following LED segments (LED-S2, LED-S3, . . . ) can also be operated with an increasing AC input voltage VDC, and that the charge can be kept at the capacitor CER.

For the case where the AC input voltage VDC falls below a second switching threshold, which is preset prior to the method sequence, the charged capacitor CER is connected to an LED segment in such a way that the charge of the capacitor CER is discharged via the LEDs in the segment and, by virtue of this discharge current, a light emission of the LEDs takes place.

In one embodiment of the invention, it is provided that the switch-on threshold is at a higher voltage value for the AC input voltage (VDC) than the second switching threshold.

The method can detect the case of the forward voltage of the first segment LED-S1being undershot and can therefore start the capacitor discharge via this or another segment. In order to charge the capacitor CER, it is necessary that the AC input voltage (VDC) has exceeded at least the value of the forward voltage of the first segment LED-S1.

In another embodiment of the invention, it is provided that the discharge of the capacitance takes place only via some of the LEDs arranged in the segment.

In addition to the possibility of switching all of the LEDs in a segment into the discharge circuit of the capacitance, in accordance with the method also only some of the LEDs in the segment can be included. This can be advantageous, for example, for the case where the voltage VCERon the capacitance CER does not reach the magnitude of the forward voltage of the segment containing a plurality of LEDs.

FIGS. 1 and 2show two possible embodiments of an arrangement1for actuating light-emitting diodes5in accordance with the prior art. So-called direct AC LED drivers each having four LED segments6, which are denoted by LED-S1to LED-S4, are illustrated. The array4is fed from the rectified mains voltage VDC2, wherein a ground-side current source8ILED generates a constant current.

In the illustration shown inFIG. 1, the segments6are short-circuited by the switching elements SW1to SW3, which can be embodied as MOSFETs, for example, corresponding to the instantaneous voltage present across the array4.

In the configuration shown inFIG. 2, the segment taps7are connected to the common current source8ILED corresponding to the instantaneous voltage across the array4by means of the switching elements SW1to SW3. A control unit CRL serves the purpose of distributing the current among the number of segments6appropriately for the instantaneous voltage. The current source8ILED can optionally be modulated corresponding to the instantaneous mains voltage VDC.

The automatic matching of the switching thresholds to the forward voltage of the segments in accordance with the invention will be described below.

FIG. 3shows the principle using the example of three segments6LED-S1to LED-S3of an LED array4comprising any desired number of LEDs5in the respective segment6. The number of segments6can be increased as desired, which is illustrated by a dash-dotted line at the connection7of the segment6-LED-S3in the figure. Likewise, the number of LEDs5per segment6is freely selectable.

The anode of the “upper” LED5of the segment LED-S16is connected to the supply voltage VDC2, i.e. the rectified mains voltage. Each segment6of the array4has a first and a second connection7. InFIG. 3, the first connection of the first segment6is connected to the voltage VDC. The second connection7of the first segment6is connected to the first connection of the following segment6of the array4. In addition, this second connection7is connected to a switching means9,10, . . . .

The entire LED array6is fed from a common ground-side current source8ILED via these switching means9,10which can be switched on and off. Above the current source8, there are so-called cascode elements TC1and TC29,10, formed by MOSFETs, bipolar transistors or IGBTs, for example, as switching means for each current path n. A series circuit of two transistors, wherein the “lower” transistor (in the case of an n-channel or NPN transistor) performs the function of control, while the “upper” transistor is used for increasing the dielectric strength and/or the output impedance, is referred to as a cascode.

n stages within the arrangement, which each comprise an n-th LED segment6and at least one n-th switching means9or10, are formed in such a way. The first stage comprises the first segment6of the array4and the first switching means9. In addition, another element actuating the first switching means9can also be included. In the example shown inFIG. 3, this is a first comparator or amplifier11AMP1.

The cascode elements9,10limit the voltage VQ across the current source8and take up some of the difference between the instantaneous VDC and the forward voltage of the active segments6of the LED array4. The gate or base voltage VGC applied to the cascode elements9,10determines the maximum voltage VC. It is advantageous for automatic threshold adaptation to keep this voltage low.

If the voltage VDC2increases starting from a value less than the forward voltage of the segment LED-S16, first the segment LED-S16will begin to conduct current when the forward voltage is reached. If the current limited by the current source8has been reached and VQ has reached the value limited by the cascode element9,10, on a further increase in the VDC2, the segment voltage VS1increases, while VQ remains approximately constant.

First there is no current flowing through the segment LED-S26, and the segment voltage VS2approximately corresponds to the voltage VQ.

If VDC reaches the sum of the forward voltages of LED-S16and LED-S26, LED-S26also begins to conduct, and the current is divided between TC19and TC210. The summation current is furthermore determined by the common current source ILED. On a further increase in VDC2, the voltage VS2now increases in comparison with VQ. This increase indicates that LED-S26is conducting, and the current path via TC19can be disconnected. The disconnection can take place, for example, via an amplifier or comparator11AMP, whose comparison value is a settable magnitude above the voltage VQ. In order to avoid oscillations around the switching point, it is advantageous to provide a comparator11with a hysteresis. This applies in particular to the case where MOSFETs with a relatively high resistance are used as cascode elements9,10. When using bipolar transistors, the base current of said bipolar transistors needs to be limited.

Gradual disconnection, for example by means of an amplifier or a simple inverter with a gradual amplification in place of the comparator, is advantageous for avoiding possible noise emission owing to the switchover operations.

Takeover of the current by TC210without switching of TC19is likewise possible by virtue of a control voltage VG2>VG1being applied, as illustrated inFIG. 4. When the segment LED-S26becomes conducting, TC210increases the voltage VQ and TC19is automatically turned on. The voltage difference between VG214and VG113needs to be sufficiently high for TC19to turn off safely, however, which is particularly important when integrating and using MOSFETs with a relatively high resistance.

In the case of a relatively large number “n” of LED segments, this can result in a considerable “scatter” of the controlling gate voltages VG1to VGn. Therefore, the combination of graded actuation voltages with the disconnection of proceeding current paths is advantageous.

If the LED array4consists of more than two segments6, the described procedure is repeated with a further increase in VDC2for the subsequent stages or current paths n+1, n+2 . . . etc. For the “last” segment6of the array4, a cascode element9,10is not absolutely necessary, but is advantageous in terms of circuitry for limiting the voltage VQ. This last cascode element9,10does not need to be switched.FIG. 3illustrates, by way of example, two cascode elements9and10.

Once VDC2has exceeded its amplitude and there is a decrease in the voltage again, the cascode elements9,10are activated again in the reverse order corresponding to the instantaneous voltage with the same circuitry.

FIG. 5shows the voltage profiles during a half-period using the example of an LED array4consisting of four segments6with the same number of LEDs5. In the illustration, no LED5is operated in the region around the zero crossing of the grid-side AC voltage2and there is no LED current flowing. Over the further course of time of a positive half-cycle, the voltage VDC2increases until the forward voltages of the LEDs5in the segment VLED-S16is reached, current is flowing through the segment VLED-S16and this segment6therefore illuminates. Over the further course of the positive half-cycle, the voltage VDC2continues to increase until the forward voltages of the LEDs5in the segments VLED S16and VLED S26are reached. After this time, current also flows through the segment VLED-S26, which now likewise illuminates.

This procedure is illustrated further until all segments6VLED-S1to VLED-S4have current flowing through them and illuminate. Once the maximum of the voltage VDC2has been reached, this voltage decreases sinusoidally, which results in the forward voltage of the segment VLED-S46no longer being reached. This results in an interruption of the current flow in the segment VLED-S46and therefore in disconnection thereof. Then, the segments VLED-S36, VLED-S26and VLED-S16are disconnected successively, as a result of which there is no longer a current flowing through the array4.

The embodiment with identical segments6can be advantageous for the provision of an application, but is not a precondition for the functionality of the method. The voltage drop VQ across the current source8has not been included in the illustration for reasons of better understanding.

FIGS. 3,4and6show the constant current source8with a control input, via which the constant current can be controlled. Thus, the current profile of the constant current source can optionally be matched to the for example sinusoidal current profile of the rectified pulsating input voltage VDC by means of the input voltage VDC2. This matching results in an improvement of the so-called power factor owing to the reduction of disruptive harmonics.

For operation of an LED luminaire using a dimmer, which operates by means of a phase-gating method (triac) or phase-chopping method (MOSFET or IGBT), a current path needs to be provided for charging a capacitor, which determines the current flow angle within a half-cycle of the mains voltage.

The previously described circuit1only conducts current when the forward voltage of the first LED segment6has been reached and only then can the time-determining capacitor be charged. Without further measures, therefore, the maximum current flow angle that can be achieved with a dimmer is reduced. In order to avoid this shortening, it is advantageous to design an additional current path which is already active when the mains voltage VDC is still lower than the forward voltage of the first segment6, for example LED-S1.

This current is referred to as “bleeder current” since it is not used for actuating the LEDs5themselves. InFIG. 6, the circuit shown inFIG. 4has been extended by a cascode or switching element TCBL16and a comparator or amplifier15AMPBL in accordance with the same principle. The bleeder current flows until VDC has exceeded the forward voltage of the segment LED-S16. In this case, the voltage VS1increases and the comparator15AMPBL deactivates the bleeder path. While TCBL16is active, the current source ILED8provides the bleeder current.

The polarity of the described topology can be reversed, i.e. the current source8is then connected to the positive supply voltage (VDC)2and the cathode of the “lowermost” LED5is connected to the negative supply (GND). It is likewise easily possible for a high-side current source to be controlled by a ground-side or floating-potential current sensor.

The filling of the so-called current gaps in accordance with the invention during actuation of the LEDs is described below with reference to the circuit arrangement shown inFIG. 7.

As soon as a cascode element9conducts the current of the current source ILED8in the event of an increase in the voltage VDC2, as described previously, the voltage drop thereof increases corresponding to the difference between the voltage VDC and the summation voltage of the active segment(s) (LED-S1, . . . ) until the next cascode element10takes over the current. This current flow in the linear range of the element9can be used to charge a capacitor17. The charging voltage can be up to the forward voltage of the “next” segment (for example LED-S2) without the summation current and the current flow in the LED segments6being impaired. This charging operation can be performed for a single cascode element or for a plurality of cascode elements9,10with a corresponding plurality of capacitors17, which are not illustrated inFIG. 7.

If the capacitor17has not been charged up to the forward voltage of the next segment6(for example LED S2) during the rising edge of the voltage VDC, it can be charged further during the falling edge of the voltage VDC as long as the voltage difference between the instantaneous voltage VDC and the voltage across the capacitor17is still greater than the voltage across the capacitor17itself.

The distribution of the current between the “regular” path for operating the LEDs5of the segments6and the path for charging the capacitor17or a further capacitor, advantageously takes place in accordance with the same method as has been previously described for automatic matching to the forward voltages of the LED segments6.

In this case, the capacitor17behaves in the same way as a segment with variable voltage.FIG. 7shows a corresponding circuit detail for an energy reserve capacitor CER and two LED segments LED-S1and LED-S2. Once the voltage VDC has exceeded the forward voltage of the first segment LED-S16, the voltage VS1increases and the capacitor CER17is charged via the cascode element TCC20. As long as VDC increases more quickly than the voltage across the capacitor CER17, the potential at the node VCER19is also increased, and the first control unit AMP111switches off the first switch TC19, and the total current of the current source ILED8is used for charging the capacitor CER17.

If the increase in voltage of VDC is insufficiently steep for the capacitor CER17to be able to take up the total current, the voltage at the node VCER19is reduced, and the switch TC19becomes active. Linear actuation of the switches9,10and20embodied as cascode elements is particularly advantageous here in comparison with switching using comparators in order to avoid switching to and fro of the current between the electronic switches TCC20and TC19, for example.

If the voltage VDC reaches the sum of the forward voltages of the segments LED-S16and LED-S26, the voltage VS2increases and the charging operation of the capacitor CER17is terminated. The switch TC19has either already switched off or is switched off by the increase in voltage at the node VCER19. If necessary, the voltage VS2can also additionally be used in order to deactivate the switch TC19.

If all of the LED segments6are active, a capacitor can be charged from the difference between the AC input voltages VDC and the sum of the forward voltages of the segments VLED(VLED=VLED-S1+VLED-S2+VLED-S3+ . . . VLED-Sx). Since the profile of the mains voltage in the region of the amplitude is quite flat and therefore the time available for charging a capacitor (for example CER17) is relatively long, a comparatively large amount of charge can be accumulated on the capacitance here.

It is not absolutely necessary to stop the charging operation of a capacitor (for example17) when the next LED segment6(for example LED-S2) becomes active, but rather a capacitor17can also be charged in parallel with two or more segments6. This simplifies the circuitry complexity involved, but also increases the flicker index, i.e. the relative ripple of the luminous flux, based on the waveform of the total current ILED.

In one embodiment, which is illustrated inFIG. 8, in addition to this first capacitor17a second capacitor18is arranged in the circuit and is charged in the manner described above.

Some of the energy stored in the capacitor17or in the capacitors17and18can be used to reduce the ripple of the luminous flux occurring at twice the line frequency, specifically to close the energization gap which arises when the voltage VDC falls below the forward voltage of an individual segment6(LED-S1). For this purpose, it is necessary for the capacitor voltage to be higher than the forward voltage of at least one LED segment6. Depending on the dimensions of the circuit, it may be necessary to connect the capacitors17,18in series with one another during the discharge operation.

A possible arrangement with four segments (LED-S1to LED-S4) and two capacitors17and18, which are charged sequentially and discharged in series for filling the current gap, is shown inFIG. 8. In order to simplify the illustration, the bleeder current has not been taken into consideration inFIG. 8. It goes without saying that this circuit part known fromFIG. 6can also be used in the arrangement shown inFIG. 8.

As the voltage VDC increases, first the cascode elements TC19, TC210and TC321become conductive successively, and the current of the constant current source ILED8flows through the segments6LED-S1, LED-S1+LED-S2and LED-S1+LED-S2+LED-S3, in the same order. If the voltage VS3reaches the voltage still remaining at the capacitor CER117plus the diode forward voltage of the diode D122, a charging current is fed into the capacitor CER117and, in the case of a further increase in the voltage VDC, the voltage VCER1across the capacitance CER1also increases. The control unit AMP323turns on the switch TC321, and the total current of the current source ILED8is available for charging the capacitor CER117.

A precondition for this is that the change in voltage dVDC/dt is greater than the change dVCER1/dt in the case of the current ILED8. The capacitor CER117therefore needs to be selected to be sufficiently large. If this condition is not met, the current of the current source ILED8is divided between the cascode elements TC321and TC120, and only so much current is used for charging the capacitance CER117that dVCER1/dt and dVDC/dt are identical. The energization of the LEDs5of the segment6is not influenced by this, however.

If after a further increase in the voltage VDC, the forward voltage of the segment6LED-S4is also exceeded, the charging operation of the capacitance CER117is ended by a switching operation of the control unit AMPC124and switch TCC120. Until the voltage. VCER2remaining on the capacitance CER218plus the diode forward voltage of the diode D225has been reached, the switch TC426conducts the current of the current source ILED8. Then, the operation described above for the switches TC321and TCC120in the cascode elements TC426and TCC227is repeated, and the capacitance CER218is charged. This charging operation is ended when the voltage VDC has fallen so far back again, once its amplitude has been exceeded, that the diode D225turns off. Then, the switch TC426again takes over the current of the current source ILED8. The cascode element TCC227does not need to be actuated, but can be continuously active. This can be achieved, for example, by virtue of the fact that the gate of this MOSFET switch27is connected to the voltage VDC. The diodes D122and D225prevent the discharge of the capacitors CER117and CER218in the case of a falling edge of voltage VDC.

The charges accumulated in the capacitances CER117and CER218are used, by way of example, in order to energize the segment6LED S4as soon as the voltage VDC falls into the range or below the forward voltage of the segment6LED-S1. The control signal required for this purpose is obtained in the same way as has already been described above for the actuation of the bleeder current and has been illustrated in the associatedFIG. 6.

In one embodiment, possibly the units AMPER28for controlling the energy reserve and AMPBL15can be combined to form one unit. A stage for level matching LS29controls, using a control signal CRLER, a switching element TER30, which connects the capacitors CER117and CER218in series with one another. The segment6LED-S4is now fed from the summation voltage of the two capacitors17and18. The current is defined via a current source IER31. The current source IER31can be arranged in the discharge path at any desired point. The discharge of the capacitances17and18connected in series by means of the switch TER30takes place beginning from the first connection of the capacitance17via the LEDs5of the segment LED-S46and the fifth switch TC426, the sixth switch TCC227to the second connection of the second capacitor CER218and from the first connection of this capacitor18further via the current source IER31, the switching element TER30to the second connection of the first capacitor17.

Combining the current source IER31with the switching element TER30in terms of circuitry can be advantageous in particular for an integrated solution.

FIG. 9illustrates, by way of example, the voltage profiles at the capacitances CER117, CER218and the summation voltage (VCER1+VCER2) when the capacitances17and18are connected in series with one another.

FIG. 9illustrates, in the background, the segment voltages (VDC, VS1, VS2, VS3and VS4) as are already known fromFIG. 5. For better understanding, only the lower region of the voltage-time profile shown inFIG. 9is illustrated in enlarged form in detail inFIG. 10.

For this example, it is assumed that the capacitance CER218is greater than the capacitance CER117. This assumption is not necessary for the function of the circuit, however.

The dimensioning of the constant current source for the discharge current IER31should take place in such a way that, in the case of a minimal supply voltage VDC, the summation voltage at the end of the discharge operation is even higher than the forward voltage of the segment6LED-S4. This ensures that the current remains constant at a maximum level during the entire gap and the efficiency of the circuit in relation to the selected topology of the LEDs5is at a maximum.

Since more charge can be stored in the capacitor CER218in the case of a higher supply voltage VDC, control of the current of the source IER31depending on the level of the supply VDC or the voltage difference between VDC and the forward voltage of the LED array4is also advantageous.

By way of example, the discharge operation ends when the voltage VDC is again high enough for the segment6LED-S1to be energized. A discharge operation which is extended on both sides can be expedient if the current of the source ILED8is controlled in order to improve the power factor, i.e. in order to reduce the harmonic content in the line current depending on the instantaneous voltage VDC, and the current in the segment6LED-S1is initially lower than the current of the source IER31. Since the available charge in the capacitances CER117and CER218is limited, the current of the source IER31needs to be reduced if the discharge time is extended.

The described operation is repeated in each half-period.

Alternatively embodiments are as follows:a further segmentation of at least one LED segment6, with the result that the voltage of a single capacitor17is sufficient for energizing a subsegment of this type,different forward voltages of the segments6of an LED array4, with the result that the voltage of an individual capacitor17is sufficient for energizing a segment6with a relatively low forward voltage,a separate LED5or LED array4which is energized from the energy reserve,the use of the energy reserve for energizing a different LED segment than the last LED segment6.

LIST OF REFERENCE SYMBOLS