AT16192U1 - Switched AC direct driver for LEDs - Google Patents

Abstract

A driver is provided for driving at least one string of LEDs with a rectified AC voltage, with input terminals (C1, C2) for supplying a rectified AC voltage, a control circuit (CC) to which a signal indicating the amplitude or the phase of the AC voltage is supplied , and a switching network (SN) with switches, each switch being arranged such that it selectively switches or activates a sub-string (S6, S7) of one or more LEDs of the LED string (L1) in parallel, wherein the control circuit (CC) is designed such that it controls each of the switches independently such that the combined forward voltage of the activated sub-strings (S6, S7) of LEDs follows the amplitude of the rectified AC voltage stepwise, characterized in that the control circuit is designed to have at least two active ones Alternate sub-strings (S6, S7) of LEDs, the same combined Durchl Assspannung during the period of a step.

Description

description

SWITCHED AC DIRECT DRIVER FOR LIGHT-EMITTING DIODES The invention relates to a driver for driving at least one string of light-emitting diodes and in particular for driving a LED string with a rectified AC voltage directly. “Direct control” means that, for example, there is no associated AC-DC converter (AC / DC converter) for converting an AC voltage into a DC voltage for controlling the LED string. The LED string comprises input terminals at which the supplied AC voltage, in particular a rectified AC voltage, is provided.

Fig. 1a shows a diagram of an AC half-wave. 1b schematically shows an approach from the prior art, in which a controller controls switches arranged in parallel with light-emitting diodes in order to selectively activate the light-emitting diodes. However, the LED string sections all have the same length and thus the same forward voltage V _F1 , ..., V _F4 drops on each LED of the light-emitting diodes connected in series with a resistor R.

In Fig. 1a, the hatched areas show current losses that result from the difference in the actual voltage development and the forward voltage of the gradually activated LEDs of the LED strand shown in Fig. 1b. This leads to the step-shaped curve as shown in FIG. 1a, each step being the result of the activation of an LED.

The invention thus provides a solution for driving an LED strand directly from an AC supply voltage, while losses are reduced when the LED strand is actuated.

Thus, the invention provides a driver for driving an LED string and a method for driving the string of light emitting diodes according to the independent claims. Further aspects of the invention are defined in the dependent claims.

In a first aspect, a driver for driving at least one string of light-emitting diodes with a rectified AC voltage is provided, which input terminals, to which a rectified AC voltage is supplied, a control circuit to which a signal indicating the amplitude or phase of the AC voltage is supplied , as well as a switching network with two or more switches, each switch being arranged for optional parallel connection or activation of a partial strand made of one or more light-emitting diodes of the LED strand.

The control circuit is designed so that it controls each of the switches independently in such a way that the combined forward voltage of the activated sub-strands of LEDs follows the amplitude of the rectified AC voltage step by step.

Preferably, the control circuit is designed such that it alternates at least two active sub-strands of light-emitting diodes, which have the same combined forward voltage during the period of a step.

The control circuit can control the switches according to an electrical parameter, e.g. control a measured alternating voltage amplitude value, the number of activated partial strands depending on the current value of the electrical parameter.

A (preferably linear) current regulator can be provided in series with the light emitting diodes. The control circuit can be supplied with a signal which represents the voltage drop across the current regulator. The voltage drop across the current regulator can be used as at least one electrical parameter, on the basis of which the control circuit activates / deactivates the LED sub-strands.

An additional LED sub-string (or another LED sub-string with a higher forward voltage) can be activated when the voltage drop across the current regulator / 17

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An LED sub-string can be deactivated (or replaced by a further activated LED sub-string with a lower forward voltage) if the voltage drop across the current regulator falls below a predetermined threshold value.

The control circuit can control the setpoint (nominal current value) of the current regulator in order to control the energy supplied to the LED string.

The nominal current value can be changed in order to achieve a dimming of the light-emitting diodes.

The control circuit can directly or indirectly detect the peak amplitude of the AC voltage and set the current value of the current regulator depending on the same, e.g. to ensure that the energy supplied to the LED string is constant, even if there is a fluctuating AC supply voltage.

The control circuit can activate a certain partial strand depending on the electrical parameter, e.g. at least a certain sub-strand for a certain value or a range of values.

The control circuit can cyclically activate all the sub-strands corresponding to the electrical parameter value or value range by switching the respective switches, in particular the control circuit is designed such that it switches the respective switches on and off periodically.

A capacitor can be connected in parallel with each branch.

The capacitor can drive the assigned sub-string if the assigned switch is set so that it connects / bridges the sub-string or parts thereof in parallel, e.g. at least one LED of the branch.

At least two sub-strands can have a different length, for example with a different number of light-emitting diodes.

The control circuit can gradually activate and deactivate certain partial strands depending on an increase or decrease in the electrical parameter value, for example following a sine curve of the AC voltage.

The control circuit can control a current regulator of the LED strand.

The control circuit may receive a feedback signal, e.g. a voltage measurement signal, received from a shunt on the voltage regulator, which in particular indicates a current flow through the LED string.

The control circuit can consist of an ASIC or microcontroller.

[0025] Each sub-string can comprise at least one LED.

In a further aspect, a method for driving at least one string of light-emitting diodes with a rectified AC voltage is provided, with input terminals for supplying a rectified AC voltage, a control circuit to which a signal indicating the amplitude or phase of the AC voltage is supplied, and a switching network with switches, each switch being arranged for optional parallel connection or activation of a partial strand of one or more LEDs of the LED strand.

The control circuit controls each of the switches independently in such a way that the combined forward voltage of the activated partial strands of light-emitting diodes follows the amplitude of the rectified AC voltage step by step.

Preferably, the control circuit allows at least two active sub-strands of light-emitting diodes to alternate with the same combined forward voltage during the period of a step.

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Patentamt Furthermore, a driver for driving at least one strand of LEDs with a rectified AC voltage is provided, the input terminals for supplying a rectified AC voltage, a control circuit which is supplied with a signal indicating the amplitude or the phase of the AC voltage, and a Switching network comprising switches, each switch being arranged for optional parallel connection or activation of a sub-strand of one or more light-emitting diodes of the LED strand, the control circuit being designed in such a way that it independently controls each of the switches in such a way that the combined forward voltage of the activated sub-strands of light-emitting diodes follows the amplitude of the rectified AC voltage step by step, characterized in that the sub-strands contain LEDs in accordance with a set Frobenius number set.

The invention will now be described with reference to the figures. In particular, the figures show in:

1a shows a half-wave of an AC voltage;

1b shows an adaptive LED circuit known from the prior art;

Figure 2 Fig. 3 Fig. 4 Fig. 5 Figure 6 an LED strand according to the invention; Measuring points on a half-wave of an AC voltage; a circuit arrangement according to the invention; another LED strand according to the invention; again a further LED strand according to the invention.

An AC voltage and in particular a rectified AC voltage directly supplies at least one LED strand, which is divided into at least two, usually several sub-strands, which optionally represent switches to bridge light-emitting diodes of the LED strand / sub-strands.

A switching network is provided in which at least one switch is assigned to a sub-string that comprises at least one LED, so that at least one sub-string can be completely bridged / connected in parallel or actuated by switching the assigned switch.

The switches of the switching network can be controlled by a control circuit and can also be arranged such that they connect or activate sections of the sub-strands in parallel. The control circuit can be a microcontroller, IC or ASIC, which switches the switches, for example transistors, FET, MOSFET, in order to optionally short-circuit or bridge partial strands of the LED strand, so that the partial strands do not absorb any energy. More than one LED string can be arranged in parallel or in series.

The control circuit tracks or monitors the AC or current development. In particular, an electrical parameter of the AC voltage is monitored, e.g. the amplitude of the alternating voltage or alternating current. For the sake of clarity, only the term AC voltage is used below.

Depending on the detected AC voltage or the electrical parameter, selected sub-strands of the LED strand are activated or deactivated. In particular, the activation of partial strings corresponds to their forward voltage, and the detected electrical parameter and in particular the control circuit decides to activate partial strands of the LEDs with a combined forward voltage in accordance with the detected electrical parameter or amplitude value of the alternating voltage.

[0043] There may possibly be more than one partial strand or a combination of partial strands which have the same combined forward voltage in the LED strand. An object of the invention is to alternately actuate at least two partial strands that have the same combined forward voltage. The invention thus enables a period of time in which a specific one corresponds to the detected electrical parameter

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LED sub-string or a combination of sub-strands is normally activated, switching between sub-strings of LEDs with the same (combined) forward voltage. This means that instead of actuating only one partial strand or a combination of partial strands, at least two partial strands are activated in the same time period and the control circuit is designed such that it can alternate these at least two active partial strands. This enables a “multiplexing” of the actuation of the partial strand.

For example, in a period of the alternating current sinusoid, a certain sub-string of the LED string should be switched on. The corresponding switch of the switching network is switched on or conductive, and the sub-branch is therefore deactivated or bridged. It may be the case that only a partial strand should be activated, but no further partial strand should be activated. However, there could be two sub-strands with the same forward voltage. In this case, the activated partial strand is not always the same during this period, but rather the partial strands can be switched on and off cyclically or periodically with the same forward voltage.

This has the advantage of better use of all light-emitting diodes and, owing to the high switching frequency, the multiplex use does not lead to flickering effects which could otherwise be perceived by the human eye. The switching frequency of the alternating activation of the partial strands can therefore preferably be above 90 Hz.

A capacitor could be connected in parallel with each LED sub-string. In view of the fact that there is multiplexing of the sub-strings, as described above, during the on-time of each sub-string, the capacitor is charged. As soon as the assigned switch of the switching network is switched to its conductive state, which leads to a short-circuiting of the light-emitting diodes of the respective sub-string, the charged capacitor continues to control the LEDs of the sub-string over a period of time depending on the size of the capacitor. After the switch is switched to its conductive state, the capacitor relieves the supply of the light-emitting diodes of the sub-string and thus leads to an improved usage ratio of each light-emitting diode, but also prevents flickering when switching to another sub-string.

Furthermore, the subdivision scheme of the LED strand is that the LED strand does not have to be divided into partial strands of the same length, but essentially into partial strands of different lengths. There could be more than one strand of the same length, but there is usually at least one strand that is shorter or longer. If the same light emitting diodes are used over the entire LED string, the length of a partial string can be regarded as the number of light emitting diodes in the partial string. If different types of light-emitting diodes are used, the sub-strands can be classified according to their forward voltage. In this case, the partial strands can come from different classes, but two or more partial strands can also come from the same class.

The activation of sub-strands of different lengths enables the LEDs of the LED strand to be actuated and its forward voltage to be controlled in a way that follows the AC voltage or the electrical parameter more precisely via the voltage / current cycles or phase.

The control circuit activates the partial strands depending on the forward voltage required and in particular calculates a selection of the partial strands that best matches the detected electrical parameter of the voltage curve, which is followed by the rectified AC voltage.

[0050] For example, the LED strand can comprise a sub-strand i, which has 2 ^I_1 light-emitting diodes with i = {1, ..., n}. The control circuit can then activate the sub-string containing an LED if a low amplitude of the AC voltage or a specific value of the electrical parameter exceeds or exceeds a specific threshold value. If the AC voltage rises, the sub-string can be operated with two LEDs.

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If the AC voltage continues to rise, the partial string can be actuated with two light-emitting diodes and the partial string with an LED. Then the sub-string could be actuated with four LEDs and so on. This could correspond to an approximately binary counting scheme. However, if there are two sub-strands with, for example, four LEDs, these sub-strands could be operated alternately.

Extensions can thus be carried out with a fine grain resolution which is comparable to the resolution of an AC / DC converter. Losses are reduced during the actuation of the LED string, since the activated LED strings enable the AC voltage applied to the LED string to be followed more precisely. The losses represent in particular a function of the difference between the supply alternating voltage / current amplitude or the alternating current sine curve and the forward voltage of the currently activated strings. The losses essentially occur on a current regulator connected in series with the LED strand, which converts or consumes excess voltage got to.

The control circuit can consist of a microprocessor, which controls the switches of the switching network, which are provided for each branch. The control circuit can also control a current regulator, e.g. a single current controller for the LED string, and can be used as a feedback signal e.g. receive a voltage measurement signal from a shunt in series with the voltage regulator that indicates the current flow through the LED string. The feedback signal can be compared to a predetermined nominal value. The control circuit can then control the current regulator so that it matches the nominal value.

Fig. 2 shows an LED strand L1 according to the invention with 90 LEDs, 10 sub-strands S1 to S10 and consequently a switching network SN of 10 switches in order to shorten the respective sub-strands S1 to S10. The input terminals C1 and C2, which are supplied with a rectified AC voltage, are also shown. The sub-strands S1 to S10 can contain a different number of LEDs, for example, as shown in FIG. 2, five sub-strands S1 to S5 with 14 LEDs (not all LEDs are shown), two sub-strands S6, S7 with 7 LEDs Sub-string S8 with three LEDs, a sub-string S9 with two LEDs and a sub-string S10 with 1 LED are present. Thus, the LED string L1 shown has 90 LEDs with 10 partial strings S1 to S10 and 10 switches in order to optionally connect or activate the partial strings S1 to S10 in parallel. It is also possible to have 10 current regulators instead of the switches of the switching network SN.

A control circuit CC, which activates or deactivates the switches, can monitor and evaluate the electrical parameter, which, for example, represents the value of the input AC voltage and in particular the amplitude of the AC voltage, and on this basis activate a larger number of light-emitting diodes. If, for example, there are a large number of measuring points at which the electrical parameter is recorded, in particular the value of the amplitude of the alternating voltage, a different number of light-emitting diodes can be activated at each measuring point.

If the AC voltage increases, as shown in FIG. 3, and there are 15 measuring points (as shown in FIG. 3 by vertical lines at each measuring point 1-15 in the phase in which the AC voltage increases), then at each measuring point a partial strand can be activated. For example, the partial strand S10 can be activated with one LED at the measuring point 1, and the partial strand S9 can be activated with two LEDs at the measuring point 2. Three LEDs can be activated at measuring point 3 by combining the activation of the sub-branches S10 and S9, but the sub-branch S8 can also be activated additionally or alternatively. The partial strings can thus be combined in order to expand the number of activated LEDs when the AC voltage increases.

At the point at which 7 LEDs are to be activated, the sub-branches S6 and S7 can be activated alternately (i.e. the respective switches of the switching network SN are switched off) by cyclically switching the sub-branches S6 and S7 on and off. The sub-strands S6 and S7 can also be controlled alternately if 8 LEDs on the

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Measuring point 8 can be activated, with sub-string S10 being added.

If a number of LEDs should be activated and there is more than one partial strand which contain a number of LEDs or have the same forward voltage, these partial strands can thus be activated alternately. If, for example, 14 LEDs have to be activated, at least two strands of the LED sub-strands S1 to S5 with 14 LEDs can be activated alternately in the same period in which an LED strand is normally activated. Changes in the recorded electrical parameters can thus be tracked with a more fine-grained resolution compared to LED strings with partial strands which only contain the same number of LEDs. If at least two strings with the same number of LEDs are activated alternately, the flickering effect can be reduced.

As can also be seen from FIG. 2, a capacitor CP1 to CP8 can be provided for each branch. When the switch of the respective sub-string S1 to S8 is activated (the switch is switched to its conductive state), the respective capacitor can maintain the supply of the light-emitting diodes of the sub-string and thus enables a period of time in which another sub-string is activated , However, not all sub-strands need to be provided with a capacitor (see e.g. sub-strands S9 and S10 from FIG. 2).

FIG. 4 shows an example for the LED strand L1 from FIG. 2 together with a control circuit CC and in particular a microcontroller which can control the switches of the switching network SN. The switching network SN consists of all switches of the sub-strands. As can be seen, the control circuit CC monitors the AC input voltage, which in particular is rectified, and preferably uses digital control outputs to activate or deactivate the switches of the switching network SN. In order to monitor the AC voltage, the control circuit CC can provide an A / D converter (analog-digital converter).

The control circuit may also include a current sink (current regulator set to a predetermined current rating) at which the output current of the LED string can be monitored, e.g. also by using an A / D converter. The control circuit may also control a current regulator CR to regulate the current through the LED string. The numbers above the sub-strands of the LED string indicate an (approximate) number of LEDs per sub-string (which may have to be rounded to a natural number), where N is normally a natural number. The current regulator CR, on the other hand, can be controlled via a digital / analog converter in the control circuit CC.

The (preferably linear) current regulator is provided in series with the LEDs. A signal can be supplied to the control circuit which represents the voltage drop across the current regulator. The voltage drop across the current regulator can be used as at least one electrical parameter, on the basis of which the control circuit activates / deactivates the LED sub-strands.

An additional LED sub-string (or a further LED sub-string with a higher forward voltage) can be activated when the voltage drop across the current regulator exceeds a predetermined threshold value.

An LED sub-string can be deactivated (or replaced by a further activated LED sub-string with a lower forward voltage) if the voltage drop across the current regulator falls below a predetermined threshold value.

The control circuit can control the setpoint (nominal current value) of the current regulator in order to control the energy supplied to the LED string.

The current rating can be changed on instruction to achieve dimming of the LEDs.

The control circuit can directly or indirectly detect the peak amplitude of the AC voltage and set the current rating of the current regulator as a function thereof, e.g. to ensure that the energy supplied to the LED string is also present when a

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The detection of the AC voltage can be used for a rough setting of the activated / deactivated sub-strands of LEDs and for the current setting of the current regulator (e.g. to compensate / control a lower AC voltage or to protect a higher voltage).

The measured voltage drop across the current regulator can be used by the control circuit, for example, as follows:

If the voltage drop across the current controller is greater than the voltage drop of the controller (Vsw + R * l) plus the forward voltage Vf of the shortest LED sub-string, then the smallest LED sub-string is activated, or a larger LED string, and the smallest LED - Sub-string is switched off ("small" and "large" refer to the combined forward voltage Vf of an LED sub-string).

This enables the monitoring of the LED string (since tolerances can be higher than the lowest segment voltage) and the compensation of tolerances, temperature and detection errors.

Figure 5 shows an alternative LED string L1 'with so-called float switches and input terminals C1', C2 '. The float switches can be MOSFET switches. The float switches can be used in conjunction with the CR current regulator. A float switch can incorporate a MOSFET switch to parallel LED current when a line transition occurs. If line transitions occur in a cycle, the float switch monitors at a zero crossing the time at which the internal switch is either opened or short-circuited in order to conduct the current away from the LEDs of the LED sub-string. The float switches do not have to directly control the output power of the LED current, but they can conduct current to the LEDs or bridge the LEDs. An example of a float switch is the Texas Instruments float switch TPS92411x, a float switch for offline AC linear direct drive of LEDs.

In particular, depending on the voltage supplied to the LED strand, the float switches can independently determine whether a required voltage has been reached and can then activate the corresponding sub-strand. A control circuit is then not necessary. Again, a current regulator Cr is provided on the output terminal of the LED string.

6 shows an LED string L1 "as a combination of the inventive approach, in which partial strings of different lengths are optionally activated by a control circuit CC", and a "classic" tap series circuit CT with current regulators CR1 to CR5. For each partial line of the classic tap series circuit CT, the control circuit CC 'can control the current controller CR1 to CR5 in order to control a current of the LED line and / or corresponding partial lines. The terminals of the LED line L1 “are shown as terminals C1“, C2 “.

[0073] A certain amount of LEDs can thus be switched on depending on the input voltage level. The current is selected so that it corresponds to the input voltage level and is controlled via the digital-to-analog converter, an integrated PWM signal, a digitally selectable current, a resistance conductor and / or an external digital-to-analog converter. There is preferably a feedback voltage between LED strings and a current regulator, which is fed to the control circuit CC, CC 'in order to check whether the adaptation is correct. This leads to a low access voltage and fluctuations or tolerances resulting from heat differences can be compensated for by a respective regulation which is carried out by the control circuit CC, CC '. At low voltages, the current can be increased while the capacitors integrate. The solution can be used in particular with phase gating dimming.

Parallel LEDs or LEDs with a high rated current can be used for the partial strands which are subjected to higher loads and / or if no capacitors for integration

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Patent office of the current can be used (for example, the strands with one and two, but possibly also three LEDs). In particular, the control circuit can detect a line voltage in a first step and (e.g. in a second step) detect a line voltage and a forward voltage difference at a desired current. Then the partial strands to be switched on are determined and the other strands are bridged. The current on the actuated lines is then controlled. The invention is particularly advantageous since inexpensive components can be used, for example an MO Codex microprocessor, a linear current regulator and bypass switches such as FET, BJT (bipolar transistors) or optocouplers, which enable a cost reduction for the respective circuit.

The LED sub-strands can have a different number of LEDs and different combined forward voltages.

An example would be to subdivide the LED string in a "binary coded" manner, i.e. the first LED sub-string has 1 LED, the next 2 LEDs, the following 4 LEDs etc.

According to a special embodiment, the LEDs can be switched in particular in a Frobenius number set, which enables each voltage to be achieved by a combination of the partial strands with a desired resolution. The minimum resolution is a forward voltage of a single LED. In other words, the invention also aims to solve a backpack problem with a row combination of LED partial strings in order to best adapt the AC input voltage. One example is the use of a series of sub-strands that can be bridged in the order of 14 + 14 + 14 + 14 + 14 + 7 + 7 + 3 + 2 + 1 LEDs. The total number of LEDs used in the LED string can be adjusted from 1 to 90 LEDs. Thus, the string voltage can vary, for example, between 3.2 volts and 288 volts with a resolution of 3.2 volts (forward voltage of a single LED). The higher-utilized partial strands can have more LEDs connected in parallel, since the average energy on these strands is higher.

[0078] Two examples of a switching method are given below:

[0079] a. If it is possible to exactly determine “the Frobenius number set”, then it can be 1 + 2 + 3 + 7 + 7 + 14 + 14 + 14 + 14 + 14 (at 230V) or 1 + 2 + 3 + 7 + 14 + 14 + 14 + 14 + 14 (slightly less effective over 230V) or 2 + 4 + 8 + 16 + 16 + 16 + 16 or

2 + 2 + 4 + 4 + 8 + 16 + 16 + 16 + 16 or

Be 2 + 4 + 8 + 16 + 16 + 32 or 5 + 5 + 10 + 20 + 20 + 20 or 1 + 2 + 3 + 7 + 7 + 20 + 20 + 20 [0080] or [0081] b. the switching sequence is such that all strands are best utilized

1,2,3,1 + 3,2 + 3,1 + 2 + 3,7a, 7b + 1,7a + 2,7b + 3,7a + 3 + 1,7b + 3 + 2,7a + 3 + 2 + 1.14 a, 14b + 1.14c + 2.14d + 3.14e + 3 + 1.7a + 7b + 3 + 2 ........

The invention thus also provides a solution for overcoming line voltage fluctuations or tolerances, as well as problems resulting from phase-angle dimming.

The LED strand can thus be driven by an AC drive voltage. The current is regulated by a series current resistor, a current sink or a linear control. The LED string can be divided into a series of LED partial strings, which in particular follow the Frobenius number set. The sub-strands can be included in the current flow by activating or deactivating the switches to best adapt to the input voltage and the voltage drop across the linear current

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To minimize patent office regulator circuit. A backpack problem is solved with the row combination of LED partial strings for optimal adaptation to the AC input voltage. There may also be some sub-strings that contain some numbers of LEDs, which, if the average current or the energy per transition is higher, may contain parallel LEDs within a complete alternating current cycle, which enable a higher utilization of the sub-strand and the entire strand, without the to exceed the maximum permissible current.

It goes without saying that while the switching network is used for switching LEDs, it is also possible to apply the switching network and the drive method to other loads supplied with an AC voltage, in which the load is dynamically an electrical parameter, in particular should follow an amplitude of an AC voltage. When using the inventive method, parts of the load can be activated dynamically in order to follow the character development of the electrical parameter.

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Claims (17)

Expectations 1. Driver for controlling at least one strand of LEDs with a rectified AC voltage, with: Input terminals (C1, C2), to which a rectified AC voltage is supplied, - A control circuit (CC), which is supplied with a signal indicating the amplitude or phase of the AC voltage, and - A switching network (SN) with switches, each switch for the optional parallel connection or activation of a partial line (S6, S7) of one or more LEDs of the LED line (L1) is arranged, the control circuit (CC) being designed such that it controls each of the switches independently in such a way that the combined forward voltage of the activated partial strands (S6, S7) of light-emitting diodes follows the amplitude of the rectified AC voltage step by step, - a current regulator in series with the string of LEDs, - The control circuit is provided with a signal that represents the voltage drop across the current regulator. 2. Driver for controlling at least one strand of LEDs with a rectified AC voltage, with: Input terminals (C1, C2), to which a rectified AC voltage is supplied, - A control circuit (CC), which is supplied with a signal indicating the amplitude or phase of the AC voltage, and - A switching network (SN) with switches, each switch for optional parallel connection or activation of a sub-string (S6, S7) of one or more LEDs (L1) is arranged, the control circuit (CC) being designed so that it switches each of the switches independently controls in such a way that the combined forward voltage of the activated partial strands (S6, S7) of light-emitting diodes gradually follows the amplitude of the rectified AC voltage, characterized in that the control circuit (CC) is designed such that it has at least two active partial strands (S6, S7) alternated by LEDs that have the same combined forward voltage during the period of a step. Driver according to one of the preceding claims, wherein the control circuit (CC) is designed to switch the switches according to the electrical parameter, e.g. of an AC voltage amplitude value, and the number of activated partial strands (S6, S7) corresponds to a value of the electrical parameter. 4. Driver according to one of the preceding claims, wherein the control circuit (CC) is designed such that it activates only a specific sub-branch (S6, S7) depending on the electrical parameter, for example at least one specific sub-branch (S6, S7) for a specific one Value or range of values. 5. Driver according to one of the preceding claims, wherein the control circuit (CC) is designed such that it cyclically activates all sub-strands (S6, S7) according to the electrical parameter value or value range by switching the respective switches, in particular the control circuit (CC ) designed so that it switches the respective switches on and off periodically. 6. Driver according to one of the preceding claims, wherein a capacitor (CP6, CP7) is connected in parallel to each sub-strand. 7. Driver according to claim 6, wherein the capacitor (CP6, CP7) is arranged such that it continues to drive the associated sub-strand (S6, S7) when the associated switch 10/17 AT 16 192 U1 2019-03-15 Austrian Patent office is set for parallel connection of the partial string (S6, S7) or parts thereof, e.g. at least one LED of the branch (S6, S7). 8. Driver according to one of the preceding claims, wherein at least two sub-strands (S7, S8) have different lengths, e.g. with a different number of LEDs. 9. Driver according to one of the preceding claims, wherein the control circuit (CC) is designed such that it controls a current regulator (CR) of the LED strand (L1). 10. Driver according to one of the preceding claims, wherein the control circuit (CC) is designed to receive a feedback signal, e.g. receives a voltage measurement signal from a shunt on the voltage regulator, which in particular indicates a current flow through the LED strand (L1). 11. Driver according to one of the preceding claims, wherein the control circuit (CC) is an ASIC or a microcontroller. 12. Driver according to one of the preceding claims, wherein each sub-strand (S6, S7) comprises at least one LED. 13. Method for driving at least one strand of LEDs with a rectified AC voltage, with: Input terminals (C1, C2) for supplying a rectified AC voltage, - A control circuit (CC), which is supplied with a signal indicating the amplitude or phase of the AC voltage, and - A switching network (SN) with switches, each switch for optional parallel connection or activation of a sub-string (S6, S7) of one or more LEDs (L1) is arranged, the control circuit (CC) independently controlling each of the switches in such a way that the Combined forward voltage of the activated partial strings (S6, S7) of light-emitting diodes gradually follows the amplitude of the rectified AC voltage, characterized in that the control circuit (CC) alternates at least two active partial strands (S6, S7) of light-emitting diodes that have the same combined forward voltage during the Show period of a step. 14. Driver for controlling at least one strand of LEDs with a rectified AC voltage, with: Input terminals (C1, C2) for supplying a rectified AC voltage, - A control circuit (CC), which is supplied with a signal indicating the amplitude or phase of the AC voltage, and - A switching network (SN) with switches, each switch for the optional parallel connection or activation of a partial line (S6, S7) of one or more LEDs (L1) is arranged, the control circuit being designed such that it controls each of the switches independently in this way that the combined forward voltage of the activated partial strings (S6, S7) of light-emitting diodes gradually follows the amplitude of the rectified AC voltage, characterized in that at least two LED partial strands (S6, S7) have different combined forward voltages, preferably the LED partial strands contain LEDs according to a Frobenius set of numbers. 6 sheets of drawings 11/17 AT 16 192 U1 2019-03-15 Austrian Patent Office 1.6 Fig. 1a 12/17 AT 16 192 U1 2019-03-15 Austrian Patent Office 13/17 AT 16 192 U1 2019-03-15 Austrian Patent Office 3.6 Fig. 3 14/17 AT 16 192 U1 2019-03-15 Austrian Patent Office 4.6 15/17 AT 16 192 U1 2019-03-15 Austrian Patent Office 16/17 AT 16 192 U1 2019-03-15 Austrian Patent Office 6.6 (° * - , CM σ> * - oo Γ ** ·. 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