WO2016049671A1

WO2016049671A1 - Switched direct ac driver for leds

Info

Publication number: WO2016049671A1
Application number: PCT/AT2015/050240
Authority: WO
Inventors: Istvan Bakk
Original assignee: Tridonic Jennersdorf Gmbh
Priority date: 2014-09-30
Filing date: 2015-09-28
Publication date: 2016-04-07
Also published as: GB201417345D0; AT16192U1; DE102015202814A1; GB2530766A

Abstract

A driver for driving at least one string of LEDs with a rectified AC voltage is provided, comprising input terminals for supplying a rectified AC voltage, a control circuitry being 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 selectively shunting or activating a substring of one or more LEDs of the LED string, wherein the control circuitry is designed to control each of the switches independently such that the combined forward voltage of the activated substrings of LEDs follows the amplitude of the rectified AC voltage in incremental steps, characterized in that the control circuitry is designed to alternate at least 2 active substrings of LEDs having the same combined forward voltage during the time period of one step.

Description

Switched direct AC driver for LEDs

The invention relates to a driver for driving at least one string of LEDs and in particular to directly drive an LED string with a rectified AC voltage. "Direct drive" means that there is e.g. no dedicated AC/DC-converter for transforming an AC voltage to a DC voltage for driving the LED string. The LED string comprises input terminals at which the supplied AC voltage and more particularly a rectified AC voltage is provided.

Fig. 1 a shows a diagram of an AC voltage half-wave. Fig. 1 b displays schematically a state of the art approach in which a regulator controls switches arranged in parallel to LEDs in order to selectively activate the LEDs. However, the LED string portions are all of the same length and hence the same forward voltage V_Fi , V_F4 drops at each LED of the LEDs connected in series to a resistor R. In Fig. 1 a, the hatched areas show power losses resulting from the difference of the actual voltage development and the forward voltage of the incrementally activated LEDs of the LED string shown in Fig. 1 b. This results in the stepped curve shown in Fig. 1 a, where every step is the result of the activation of one LED. The invention hence provides a solution for driving a LED string directly from an AC supply voltage while reducing losses when the LED string is operated.

Therefore, the invention provides a driver for driving a LED string as well as a method for driving the string of LEDs according to the independent claims. Further aspects of the invention are defined by the dependent claims.

In a first aspect, a driver for driving at least one string of LEDs with a rectified AC voltage is provided, comprising input terminals for being supplied with a rectified AC voltage, a control circuitry being supplied with a signal indicating the amplitude or the phase of the AC voltage, and a switching network comprising two or more switches, each switch being arranged for selectively shunting or activating a substring of one or more LEDs of the LED string.

The control circuitry is designed to control each of the switches independently such that the combined forward voltage of the activated substrings of LEDs follows the amplitude of the rectified AC voltage in incremental steps. Preferably the control circuitry is designed to alternate at least two active substrings of LEDs having the same combined forward voltage during the time period of one step.

The control circuitry can control the switches in accordance with an electrical parameter, e.g. a measured AC voltage amplitude value, and wherein the number of activated substrings is function of the current value of the electrical parameter.

A (pref. linear) current regulator may be provided in series with the LEDs. The control circuitry may be provided with a signal representing the voltage drop across the current regulator. The voltage drop across the current regulator may be used as at least one electrical parameter on the basis of which the control circuitry activates/deactivates the LED substrings.

An additional LED substring (or another LED substring having a higher forward voltage) may be activated if the voltage drop across the current regulator exceeds a given threshold value.

An LED substring may be deactivated (or replaced by another activated LED substring having a lower forward voltage) if the voltage drop across the current regulator falls below a given threshold value.

The control circuitry may control the setpoint (nominal current value) of the current regulator in order to control the power supplied to the LED string. The nominal current value may be changed on order to achieve dimming of the LEDs.

The control circuitry may detect directly or indirectly the peak amplitude of the AC voltage and set the nominal current value of the current regulator as a function thereof, e.g. in order to ensure that the power supplied to the LED string is constant even when having a fluctuating AC supply voltage.

The control circuitry may activate a specific substring depending on the electrical parameter, e.g. at least one specific substring for a specific value or a value range. The control circuitry may cyclically activate all substrings corresponding to the electrical parameter value or value range by switching the respective switches, in particular the control circuitry is configured to periodically switch on and off the respective switches.

In parallel to each substring a capacitor can be connected.

The capacitor can drive the associated substring, when the associated switch is set to shunt/bypass the substring or parts thereof, e.g. at least one LED of the substring.

At least two substrings can be of different length, e.g. comprising a different number of LEDs.

The control circuitry may incrementally activate and deactivate specific substrings depending on an increase or decrease of the electrical parameter value, e.g. following an AC voltage sine wave.

The control circuitry can control a current regulator of the LED string. The control circuitry can receive a feedback signal, e.g. a voltage measurement signal, from a shunt at the voltage regulator indicating in particular a current flow through the LED string.

The control circuitry can be an ASIC or microcontroller.

Each substring can comprise at least one LED.

In another aspect a method for driving at least one string of LEDs with a rectified AC voltage is provided, comprising input terminals for supplying a rectified AC voltage, a control circuitry being 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 selectively shunting or activating a substring of one or more LEDs of the LED string. The control circuitry controls each of the switches independently such that the combined forward voltage of the activated substrings of LEDs follows the amplitude of the rectified AC voltage in incremental steps. Preferably the control circuitry alternates at least 2 active substrings of LEDs having the same combined forward voltage during the time period of one step.

Further, a driver for driving at least one string of LEDs with a rectified AC voltage is provided, comprising input terminals for supplying a rectified AC voltage, a control circuitry being 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 selectively shunting or activating a substring of one or more LEDs of the LED string, wherein the control circuitry is designed to control each of the switches independently such that the combined forward voltage of the activated substrings of LEDs follows the amplitude of the rectified AC voltage in incremental steps, characterized in that the substrings contain LEDs according to a Frobenius number set.

The invention is also now described with reference to the figures. In particular, the figures show:

Fig. 1 a a half-wave of an AC voltage;

Fig. 1 b a prior art adaptive LED circuit;

Fig. 2 an LED string according to the invention;

Fig. 3 measurement points on a half-wave of an AC voltage;

Fig. 4 a circuit arrangement according to the invention;

Fig. 5 another LED string according to the invention;

Fig. 6 yet another LED string according to the invention;

Fig. 7 yet another LED string according to the invention;

Fig. 8a to 8e yet another LED string according to the invention. An AC voltage and in particular a rectified AC voltage directly supplies at least one LED string, which is divided into at least two, typically a plurality of, substrings, which are selectively switches to bypass LEDs of the LED string/substrings. A switching network is provided in which at least one switch is associated with a substring, comprising at least one LED so that at least one substring can be completely bypassed/shunt or activated by switching the associated switch.

The switches of the switching network can be controlled by a control circuitry, and may also be arranged to shunt or activate portions of the substrings. The control circuitry can be a microcontroller, IC or ASIC, which controls the switches, for example transistors, FETs, MOSFETs, to selectively short circuit or bypass substrings of the LED string, so that the substrings do not take up energy. More than one LED strings can be arranged in parallel or in series.

The control circuitry tracks or monitors the AC voltage or current development. Especially an electric parameter of the AC voltage is monitored, e.g. the amplitude of the AC voltage or AC current. In the following, for reasons of clarity, only the term AC voltage is used.

Depending on the detected AC voltage or electric parameter select substrings of the LED string are activated or deactivated. In particular, the activation of substrings corresponds to their forward voltage and the detected electrical parameter and in particular the control circuit decides to activate substrings of the LEDs having a combined forward voltage corresponding to the detected electrical parameter or amplitude value of the AC voltage.

In the LED string there might be more than one substring or a combination of substrings which have the same combined forward voltage. One aim of the invention is to alternately operate at least two substrings which have the same combined forward voltage. Hence, in a time period in which, in a correspondence with the detected electrical parameter, a specific LED substring or a combination of substrings is normally activated, the invention allows switching between substrings of LEDs having the same (combined) forward voltage. That means that instead of operating only one substring or combination of substrings, in the same time period at least two substrings are activated and the control circuit is designed to alternate these at least two active substrings. In this way, substring operation can be "multiplexed".

For example, during a time period of the AC sine wave, a certain substring of the LED string should be switched on. The corresponding switch of the switching network is on or conductive and therefore the substring is deactivated or bypassed. It may be the case that only one substring is to be activated but no further substring should be activated. However, there might be two substrings with the same forward voltage. If this is the case, the activated substring is not always the same in this time period but rather the substrings with the same forward voltage can be cyclically or periodically switched on and off. This has the benefit of providing a better use of all LEDs and, due to the high switching frequency, the multiplex use will lead no flickering effects, which otherwise might be detectable by the human eye. The switching frequency of the alternating activation of the substrings may therefore preferably be are above 90 Hz. In parallel to each LED substring, a capacitor might be connected. In view of the fact that there is a multiplex use of the substrings, as previously described, during a switch-on period of each substring, the capacitor will be charged. Once the associated switch of the switching network is switched to its conductive state, which will lead to a short circuiting of the LEDs of the respective substring, the charged capacitor will continue to drive the LEDs of the substring for a time depending on the dimension of the capacitor. After the switch is switched to its conductive state, the capacitor discharges supplying the LEDs of the substring and hence leads to an improved use ratio of each LED, but also prevents flickering when it is switched to another substring. Further, the subdivision scheme of the LED string is that the LED string does not need to be divided into substrings of equal lengths, but essentially into substrings of different length. However, there might be more than one substring of the same length, but typically there is at least one substring which is shorter or longer. If the same LEDs are used throughout the LED string, the length of a substring can be regarded as the number of LEDs in the substring. If LEDs of different kind are used, the substrings can be classified according to their forward voltage. In this case, the substrings can be of different classes, but two or more substrings may be of the same class. Activating substrings of different lengths, in result, allows operating the LEDs of the LED string and controlling its forward voltage in a manner that more closely follows the AC voltage or the electrical parameter over the voltage/current cycles or phase.

The control circuitry activates the substrings depending on the required forward voltage and in particular calculates a selection of the substrings which best matches the detected electrical parameter of the voltage curve followed by the rectified AC voltage.

For example, the LED string may comprise a substring i, which has 2'^~1 LEDs for i={1 ,..., n}. Then, the control circuitry may activate the substring containing one LED in case of a low amplitude of the AC voltage or a specific value of the electrical parameter surpasses or exceeds a specific threshold level. When the AC voltage increases, the substring with two LEDs may be operated. When the AC voltage increases more, the substring with two LEDs and the substring with one LED may be operated. Afterwards the substring with four LEDs might be operated and so on. This might resemble a somewhat binary counting activation scheme. However, in case there are two substrings with e.g. four LEDs, these substrings might be operated alternately.

Therefore, increments can be made with a fine grain resolution, which is comparable to the resolution of an AC/DC converter. Losses will be reduced during operation of the LED string as the activated LED strings allow to more closely follow the AC voltage applied to the LED string. The losses in particular are a function of the difference of the supply AC voltage/current amplitude or of the AC sine wave and the forward voltage of the currently activated strings. The losses essentially occur at a current regulator connected in series to the LED string, which has to dissipate or use up excess voltage.

The control circuitry can be a microprocessor, which controls the switches of the switching network provided for each substring. The control circuitry can also control a current regulator, e.g. a single current regulator for the LED string, and may receive, as a feedback signal, e.g. a voltage measurement signal from a shunt in series to the voltage regulator indicating the current flow through the LED string. The feedback signal may be compared to a given nominal value. The control circuitry can then control the current regulator to match the nominal value.

Fig. 2 shows an LED string L1 according to the invention with 90 LEDs, 10 substrings S1 to S10 and, consequently, a switching network SN of 10 switches to selectively short cut the respective substrings S1 to S10. Input terminal C1 and C2 are also shown which are supplied with a rectified AV voltage. The substrings S1 to S10 may contain different numbers of LEDs, for example there might be, as shown in Fig. 2, five substrings S1 to S5 with 14 LEDs (not all LEDs are shown), two substrings S6, S7 of 7 LEDs, one substring S8 of three LEDs, one substring S9 of two LEDs and one substring S10 of one LED. Hence, the shown LED string L1 has 90 LEDs with 10 substrings S1 to S10 and 10 switches to selectively shunt or activate the substrings S1 to S10. It is also possible to have 10 current regulators instead of the switches of the switching network SN.

A control circuitry CC activating or deactivating the switches may monitor and evaluate the electrical parameter, for example representing the value of the AC input voltage and in particular the amplitude of the AC voltage, and based thereon, activate an increasing number of LEDs. For example, if there is a plurality of measurement points at which the electrical parameter is detected, in particular the value of the amplitude of the AC voltage, at each measurement point a different number of LEDs may be activated.

When the AC voltage increases, as shown in Fig. 3, and there are 15 measurement points (as illustrated in Fig. 3 by vertical lines at each measurement point 1 -15 in the phase where the AC voltage increases), at each measurement point a substring may be activated. For example, at measurement point 1 , the substring S10 with one LED may be activated, at measurement point 2, substring S9 with two LEDs may be activated. Three LEDs at measurement point 3 may be activated by combining the activation of substrings S10 and S9, but also substring S8 may be activated additionally or alternatively. Hence, the substrings can be combined to increment the number of LEDs activated when the AC voltage increases.

At the point where 7 LEDs are to be activated, the substrings S6 and S7 may be activated (i.e. the respective switches of the switching network SN are switched off) in an alternating fashion by cyclically switching on and off substrings S6 and S7. Substrings S6 and S7 may also be alternately controlled when 8 LEDs are activated at measurement point 8, with the addition of Substring 10.

Hence, in case a number of LEDs should be activated and there is more than one substring containing a number of LEDs or having the same forward voltage, these substrings can be alternately activated. For example, if 14 LEDs need to be activated, at least two strings of the LED substrings S1 to S5 containing 14 LEDs can be activated in an alternating fashion in the same time period in which typically one LED string is activated. Hence, changes of the electrical parameters detected can be followed with a finer grained resolution compared to LED strings with substrings containing only the same number of LEDs. If at least two strings with the same number of LEDs are alternately activated, the flickering effect can be reduced.

As can also be seen in Fig. 2, for each substring a capacitor CP1 to CP8 can be provided. The respective capacitor then can, when the switch of the respective substring S1 to S8 is activated (the switch is put into its conductive state), maintain the supply of the LEDs of the substring and hence allows to bridge a time period in which another substring is activated. However, not all substrings need to be provided with a capacitor (cf. e.g. substrings S9 and S10 of Fig. 2).

Fig. 4 shows an example for the LED string L1 of Fig. 2 together with a control circuitry CC and in particular a microcontroller which is adapted to control the switches of the switching network SN. The switching network SN consists of all switches of the substrings. As can be seen, the control circuitry 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. To monitor the AC voltage, the control circuitry CC can facilitate an A/D-converter (analog to digital-converter).

The control circuitry also may contain a current sink (current regulator set to a given nominal current value), at which the output current of the LED string can be monitored, e.g. also by using an A/D-Converter. The control circuitry may also control a current regulator CR to regulate the current through the LED string. The numbers above the substrings of the LED string indicate an (approximate) number of LEDs per substring (which may need to be rounded to a natural number), where n typically is a natural number. The current regulator CR, on the other hand, may be controlled via a digital to analog-converter in the control circuitry CC. The (pref. linear) current regulator is provided in series with the LEDs. The control circuitry may be provided with a signal representing the voltage drop across the current regulator. The voltage drop across the current regulator may be used as at least one electrical parameter on the basis of which the control circuitry activates/deactivates the LED substrings.

The control circuitry may control the setpoint (nominal current value) of the current regulator in order to control the power supplied to the LED string.

The nominal current value may be changed on order to achieve dimming of the LEDs.

The AC voltage sensing may be used for a rough setting of the activated/deactivated substrings of LEDs) and for the current setting of the current regulator (e.g. to compensate/control for lower AC voltage, or for protection of higher voltage) The measured voltage drop across the current regulator may be used by the control circuitry e.g. as follows:

If the voltage drop across the current regulator is larger than the voltage drop of the regulator (Vsw+R^*l) plus the forward voltage Vf of the shortest LED substring, then the smallest LED substring is activated, or a larger LED string, and the smallest LED substring off ("Small" and "large" refer to the combined forward voltage Vf of a LED substring). This allows monitoring of the LED string (as tolerances can be higher than the lowest segment voltage), and compensate for tolerances, temperature, and detect failure. Fig. 5 shows an alternative LED string L1 ' with so called floating switches and input terminals C1 ', C2'. The floating switches can be MOSFET switches. The floating switches can be used in conjunction with the current regulator CR. A floating switch can integrate a MOSFET switch to shunt LED current as a line transitions. As the line transitions through a cycle, the floating switch monitors for a zero crossing at which time the internal switch is either opened or shorted to steer the current away from the LEDs of the LED substring. The floating switches do not need to directly control the output power of the LED current, but may direct current to the LEDs or bypass the LEDs. One example for a floating switch is the Texas Instruments floating switch TPS9241 1 x, a floating switch for offline AC linear direct drive of LEDs.

In particular, depending on the voltage supplied to the LED string, the floating switches can autonomously determine whether a required voltage is reached and may then activate the corresponding substring. A control circuitry is then not required. Again, current regulator CR at the output terminal of the LED string is provided.

Fig. 6 shows an LED string L1 " as a combination of the inventive approach, in which substrings of different length are selectively activated by a control circuitry CC, and a "classical" tapped series circuitry CT with current regulators CR1 to CR5. For each substring of the classical tapped series circuitry CT, the control circuitry CC can control the current regulator CR1 to CR5 in order to control a current of the LED string and/or respective substrings. The terminals of the LED string L1 " are shown as terminals CT, C2".

Hence, depending on the input voltage level a specific amount of LEDs can be turned on. The current is selected to match the input voltage level and is controlled via the D/A-Converter, an integrated PWM signal, a digitally selective current, a resistor ladder and/or external D/A-converter. There is a feedback voltage, preferably between LED strings and a current regulator, supplied to the control circuitry CC, CC to check whether the adaptation is accurate. This leads to little access voltage and variations resulting from thermal differences or tolerances can be compensated by respective regulation performed by the control circuitry CC, CC. At low voltages, the current can be increased, while the capacitors will integrate. The solution can especially be used with phase-cut dimming.

Parallel or high current rated LEDs can be used for the substrings, which are higher utilized and/or where no capacitors are used for integrating the current (for example the strings with one and two but also maybe three LEDs). In particular, the control circuitry in a first step may detect a line voltage and (e.g. in a second step) detect a line voltage and a forward voltage difference at a set current. Then, the substrings to be turned on are determined and the other strings are bypassed. Then, the current on the activated strings is controlled. The invention is especially advantages as low cost components can be used, for example a MO Cortex microprocessor, a linear current regulator and bypass switches such as FETs, BJTs (bipolar transistors) or

optocouplers can be used, which allow reduction of the costs for the respective circuit. The LED substrings may have a different number of LEDs and different combined forward voltages.

One example would be to divide the LED string in a "binary encoded" style, i.e. the first LED substring has one LED, the next one 2 LEDs, the following 4 LEDs etc.

According to a specific embodiment, the LEDs may especially be switched in a Frobenius number set, which allows that any voltage can be achieved by a

combination of the substrings with a desired resolution. The minimum resolution is a forward voltage of a single LED. In other words, the invention also aims at solving a knapsack problem with a series combination of LED substrings to best adapt to the input AC voltage. One example is, using a series of substrings that can be bypassed in the order of 14+14+14+14+14+7+7+3+2+1 LEDs. The number of LEDs used in the LED string all together can be adapted from 1 to 90 LEDs. Thus, the string voltage can be vary for example between 3,2 Volts to 288 Volts, with a resolution of 3,2 Volts (forward voltage of a single LED). The higher utilized substrings can have more LEDs in parallel as the average power on these strings will be higher. Now two examples of a switch method will be given: a. Is it possible to detail "the frobenius number set" it can be

1 +2+3+7+7+14+14+14+14+14 (for 230V), or 1 +2+3+7+14+14+14+14+14 (a bit less efficient over 230V) or

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

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

2+4+8+16+16+32 or

5+5+10+20+20+20 or

1 +2+3+7+7+20+20+20

Or b. Switching sequence such that all strings are best utilized so

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 ,14a,1 4b+ 1 , 14c+2, 14d+3, 14e+3+ 1 ,7a+7b+3+2

The invention hence also provides a solution to overcome line voltage fluctuations or tolerances as well as problems resulting from phase-cut dimming.

The LED string is hence adapted to be driven by an AC driving voltage. The current is regulated by series current resistor, a current sink or a linear control. The LED string can be partitioned into a number of LED substrings and especially adhering to the Frobenius number set. The substrings can be included in the current flow by activating or deactivating the switches to adapt best to the input voltage and minimize the voltage drop on the linear current regulator circuit. A knapsack problem is solved with the series combination of LED substrings to best adapt to the input AC voltage. There can also be some substrings that contain a few numbers of LEDs which, where the average current or power per junction is higher, within a full AC cycle can contain parallel LEDs allowing a higher utilization of the substring and the entire string without exceeding the maximum allowed current.

Fig. 7 illustrates a schematic diagram of a driver module L1 for driving LEDs according to another embodiment of the invention. The circuit of Fig. 7 is similar to the circuit of Fig. 4. In the example of Fig. 7 the current regulator (current source 11 ) with the transistor M1 is shown as a controllable current source 11 (acting as a current regulator). The current source 11 (current regulator) may be controlled by control circuitry CC. There may be placed a capacitor C1 , C2, C3, C4, C5 arranged in parallel to each of LED strings and form a part of a substring. The capacitors C1 , C2, C3, C4, C5 act as energy storage elements and may filter out high frequency parts of the LED current and may smooth the LED current. The invention is not limited to four substrings or LED strings D1 , D2, D3, D4, D5, D, D7, D8. E.g. the driver module can comprise a fifth substring or LED string (not shown) comprised in the load path 4 in series with the four substrings or LED strings D1 , D2, D3, D4, D5, D, D7, D8. Also, a fifth bypass module (not shown) can be provided in parallel to the fifth LED string.

For a low value of the AC voltage only one substring or LED string will be switched on for a certain time of a switching period within the defined voltage range, the other LED strings being bypassed. For a higher value, two LED strings will be on, while at least another LED string will be bypassed. For an even higher value of the AC voltage, all substrings or LED strings will be switched on, such that the current will flow through the all substrings or LED strings and the overall light output can be increased.

Advantageously, there is no switched current source in the LED driver of the invention. A switch may however be used in a bypass module coupled to a subgroup of the LEDs.

The first input terminal C1 which forms the first input with the higher electric potential may be connected to a first terminal of a resistor R1 (not shown). A diode F1 , e.g. a transient-voltage-suppression (TVS) diode, may be provided between the second terminal of the resistor R1 and the second input terminal 3. This optional diode TVS may be used for protecting the driver module L1 e.g. from voltage spikes.

The preferably rectified input voltage VA is applied to a resistor R2 and a capacitor CX (not shown) that are connected in series between node A and ground. The optional elements R1 , R2 and CX may present in order to enable a dimming operation using a phase cut dimmer as they form a passive bleeding circuit. Furthermore, the mentioned elements R1 , R2 and C1 represent damping elements avoiding ringing effects - in view of capacities provided on the dimmer - caused when operated with usual dimmers. The invention enables a driver module L1 for driving LEDs with less flicker and higher usage of the LED and thus higher efficiency of the overall LED unit compared to known solutions.

The substrings or LED strings may be arranged on the same board, e.g. the same PCB, as the control circuitry CC and the bypass modules. In an alternative

embodiment the substrings or LED strings can be arranged on one or more PCBs separated from the board with the control circuitry CC and the bypass modules which enables a modular approach for building up the driver module L1 . The driver module L1 for driving LEDs D1 , D2, D3, D4 shown in Fig. 7 is supplied with an input voltage Vin in the form of an alternating voltage such as a mains voltage. The alternating voltage is applied between a first input terminal C1 and a second input terminal C2. The input voltage Vin (or V1 ) is applied to a rectifier for converting the alternating voltage (AC) to a rectified voltage (DC).

The embodiment of Fig. 7 preferably comprises a bridge rectifier D1 comprising four diodes in bridge configuration. The output of the bridge rectifier D1 1 , D12, D13, D14 is a full-wave rectified voltage V provided between a positive terminal + and a negative terminal - of the rectifier. The negative terminal - corresponds to ground, while the positive terminal + at node A represents voltage VA.

An advantage of the LED driver according to the present invention is that it is easily dimmable when using usual dimmers as for example phase cut dimmers, wherein the voltage generated by such a phase cut dimmer may be applied to the input terminal C1 , C2 of the driver module.

A load path 4 comprising substrings or LED strings D1 , D2, D3, D4, D5, D, D7, D8 is connected to node A, i.e. to the voltage VA. A first LED string D1 , D2 is thereby connected in series with a second LED string D3, D4 within the load path 4. Each LED string D1 , D2, D3, D4, D5, D, D7, D8 comprises at least one LED, preferably a plurality of LEDs connected in series and/or optionally in parallel. In the particular embodiment of Fig. 8a, the two LEDs D1 , D2 of the first LED string schematically represent a plurality of LEDs coupled in series. Also, the two LEDs D3, D4 schematically represent a series of coupled LEDs for the second LED string. The anode of the LEDs is connected towards node A. For the particular LED sets of Fig. 8a, this means that the anode of the first LED D1 of the first LED string is coupled to node A and voltage VA.

The load path 4 comprises a series connection of two or more substrings or LED strings each comprising at least one LED D1 , D2, D3, D4, D5, D6, D7, D8.

Each LED string D1 , D2 comprises a bypass module W1 which is connected in parallel to one of the substrings or LED strings D1 , D2 and which is adapted to selectively bypass the LED string D1 , D2. A control circuitry CC (might be also referenced as control unit CU) is adapted to activate one or more of the bypass modules W1 , W2, W3, W4 in case the voltage across the load path 4 is between a first and a second voltage level, whereby the control circuitry CC is adapted to selectively change the activated bypass module, preferably that all bypass modules W1 , W2, W3, W4 are activated at least once with the period where the voltage across the load path 4 is between a first and a second voltage level. For example the first voltage level may be equal to the forward voltage of the full LED string. The second voltage level may be equal to the forward voltage of one LED string.

All bypass modules W1 , W2, W3, W4 may be de-activated in case the voltage across the load path 4 is as above the first voltage level. All bypass modules W1 , W2, W3, W4 are activated in case the voltage across the load path 4 is as below the second voltage level.

At least two of the bypass modules W1 , W2, W3, W4 may be activated at the same time when the voltage across the load path 4 is between the first and a third voltage level.

As an example the load path comprises a series connection of a number of N substrings or LED strings each comprising at least one LED D1 , D2, D3, D4, D5, D6, D7, D8. Preferably for this example all substrings or LED strings comprise the same number of LED. Again a bypass module W1 , W2, W3, W4 is connected in parallel to each of the N substrings or LED strings D1 , D2 and which are adapted to bypass the LED string D1 , D2. The control circuitry CC may detect a value M which is the next higher multiple of the forward voltage of one LED string D1 , D2 in relation to the actual voltage across the load path 4. A number of (M-1 ) bypass modules W1 , W2, W3, W4 may be deactivated at the same time when the voltage across the load path 4 is below the product of M multiplied by the forward voltage (Vf group) of one LED string D1 , D2.

A number of (M-1 ) bypass modules W1 , W2, W3, W4 may be deactivated at the same time when the voltage across the load path 4 is above the product of M-1 multiplied by the forward voltage of one LED string D1 , D2. This means, if

(M-1 ) x Vf group < Vin < M x Vf group than

(M-1 ] groups are bypassed.

All bypass modules W1 , W2, W3, W4 may be deactivated at the same time when the voltage across the load path 4 is above the forward voltage of all Substrings or LED strings D1 , D2, D3, D4, D5, D6, D7, D8.

As an example in a minimal configuration the load path 4 comprises at least a first LED string D1 , D2 with one or a plurality of LEDs and at least a second LED string D3, D4 with one or a plurality of LEDs. The second LED string D3, D4 is being connected in series with the first LED string D1 , D2. A first bypass module W1 is connected in parallel to the first LED string D1 , D2 and is adapted to bypass the first LED string D1 , D2. A second bypass module W2 is connected in parallel to the second LED string D3, D4 is adapted to bypass the second LED string D3, D4. The first bypass module W1 and the second bypass module W2 are controlled by a control circuitry CC. The control circuitry CC is adapted to deactivate the bypass the first bypass module W1 and the second bypass module W2 in case the voltage across the load path 4 is above a certain first voltage level.

The control circuitry CC is adapted to alternatingly bypass either the first LED string D1 , D2 or the second LED string D3, D4 in case the voltage across the load path 4 is below a certain first voltage level. The alternating bypassing of either the first LED string D1 , D2 or the second LED string D3, D4 in such case (where the voltage across the load path 4 is below a certain voltage level) may follow a switching pattern where the first LED string D1 , D2 and the second LED string D3, D4 are turned on and off alternatingly following a pattern or by random selection.

As an extension of this example the load path 4 may comprise at least a third LED string D5, D6 with one or a plurality of LEDs.

The third LED string D5, D6 is being connected in series with the first LED string D1 , D2 and the second LED string D3, D4. A third bypass module W3 is connected in parallel to the third LED string D5, D6 and is adapted to bypass the third LED string D5, D6. The control circuitry CC may be adapted to alternatingly bypass two substrings or LED strings selected out of the first LED string D1 , D2, the second LED string D3, D4 and the third LED string D5, D6 in case the voltage across the load path 4 is below a certain first voltage level.

The control circuitry CC may be adapted to alternatingly bypass either the first LED string D1 , D2, the second LED string D3, D4 or the third LED string D5, D6 in case the voltage across the load path 4 is below a certain second voltage level.

The control circuitry CC may adapted to selectively activate the bypass modules with a frequency at least ten times higher than the frequency of the AC mains voltage, preferably above 500 Hz.

The control circuitry CC may be adapted to selectively activate each bypass module at least once during the time period where the voltage over the load path 4 is between the product of (M-1 ) multiplied by the forward voltage of one LED string D1 , D2 and the product of M multiplied by the forward voltage of one LED string D1 , D2. An example for the invention shall be further explained by the figures 8b to 8e. The load path 4 is formed by four substrings or LED strings with four bypass modules in parallel. The first LED string D1 , D2, the second LED string D3, D4, the third LED string D5, D6 and the forth LED string D7, D8 are arranged in a series connection. A first bypass module W1 is connected in parallel to the firs LED string D1 , D2 and is adapted to bypass the third LED string D1 , D2. A second bypass module W2 is connected in parallel to the firs LED string D3, D4 and is adapted to bypass the third LED string D3, D4. A third bypass module W3 is connected in parallel to the third LED string D5, D6 and is adapted to bypass the third LED string D5, D6. A forth bypass module W4 is connected in parallel to the third LED string D7, D8 and is adapted to bypass the third LED string D7, D8.

The control circuitry CC may be adapted to detect the input voltage which forms the voltage over the load path 4. Depending on the detected voltage over the load path 4 the control circuitry CC can determine the number of substrings or LED strings which have to be bypassed and thus de-activated and can determine the number of substrings or LED strings which have to be activated and thus shall not be bypassed. According to the invention the control circuitry CC may keep the number of activated substrings or LED strings constant for a certain voltage over the load path but may alter the selection which substrings or LED strings out of the load path 4 are activated at the same time. The remaining substrings or LED strings may be bypassed and thus de-activated at the same time.

Figure 8b shows an example with a voltage over the load path 4 which is above the forward voltage of a single LED string but below the double of the forward voltage of a single LED string. This means that at such voltage level an activation and operation of a single LED string is efficient. Therefore the control unit activates as a first step the bypass modules W1 , W2 and W3 and whereby on the forth LED string D7, D8 is activated. After a certain time, e.g. 1 ms, the control circuitry CC activates the bypass module W4 in combination with the activation of the bypass modules W2 and W3 and de-activates the bypass module W1 . This means that only LED string D1 , D2 is activated now. As a next step the control circuitry CC activates the bypass module W1 in combination with the activation of the bypass modules W3 and W4 and de-activates the bypass module W2.The LED string D3, D4 is now activated. As a next step the control circuitry CC activates the bypass module W2 in combination with the activation of the bypass modules W1 and W4 and de-activates the bypass module W3. The LED string D5, D6 is now activated. In a next step the cycle starts again and the control unit activates the bypass modules W1 , W2 and W3. The forth LED string D7, D8 is activated. This alternating activation of substrings or LED strings will continue as long as the voltage over the load path 4 is in the range between the forward voltage of a single LED string and the double of the forward voltage of a single LED string. The switching period for activation of one LED string may be for instance 1 ms which is equal to a switching frequency of 1 kHz. Compared to the 50Hz or 60 Hz frequency of the AC mains voltage such frequency is high and not visible for a human eye.

For the example of figure 8b the number M would equal two. At every time there is only one LED string active as (M-1 ) equals one. Figure 8c shows an example with a voltage over the load path 4 which is above the double of the forward voltage of a single LED string but below the triple of the forward voltage of a single LED string. This means that at such voltage level an activation and operation of two Substrings or LED strings in series is efficient. Therefore the control unit activates as a first step the bypass modules W1 and W2 and whereby on the third LED string D5, D6 and the forth LED string D7, D8 are activated. After a certain time, e.g. 1 ms, the control circuitry CC activates the bypass module W3 in combination with the activation of the bypass modules W2 and de-activates the bypass module W1 . This means that first LED string D1 , D2 and forth LED string D7, D8 are activated now. As a next step the control circuitry CC activates the bypass module W4 in combination with the activation of the bypass modules W3 and de-activates the bypass module W2 and W1 .The first LED string D1 , D2 and second LED string D3, D4 are now activated. As a next step the control circuitry CC activates the bypass module W2 in combination with the activation of the bypass modules W1 and deactivates the bypass module W3 and W4.The LED second LED string D3, D4 and the third LED string D5, D6 are now activated. In a next step the cycle starts again and the control unit activates the bypass modules W2 and W3. The forth LED string D7, D8 and the first LED string D1 , D2 are activated.

This alternating activation of Substrings or LED strings will continue as long as the voltage over the load path 4 is in the range between the double of forward voltage of a single LED string and the triple of the forward voltage of a single LED string. Again the switching period for activation of one LED string may be for instance 1 ms which is equal to a switching frequency of 1 kHz.

For the example of figure 8c the number M would equal three. At every time there are two Substrings or LED strings active as (M-1 ) equals two.

Figure 8d shows an example with a voltage over the load path 4 which is above the triple of the forward voltage of a single LED string but below the quadruple of the forward voltage of a single LED string. This means that at such voltage level an activation and operation of three Substrings or LED strings in series is efficient.

Therefore the control circuitry CC activates as a first step the bypass modules W1 and the second LED string D3, D4, the third LED string D5, D6 and the forth LED string D7, D8 are activated. After a certain time, e.g. 1 ms, the control circuitry CC activates the bypass module W2 and de-activates the bypass modules W1 , W3 and W4. This means that first LED string D1 , D2, third LED string D5, D6 and forth LED string D7, D8 are activated now. As a next step the control circuitry CC activates the bypass module W3 and de-activates the bypass module W2, W4 and W1 .The first LED string D1 , D2, second LED string D3, D4 and the forth LED string D7, D8 are now activated. As a next step the control circuitry CC activates the bypass module W4 and de- activates the bypass module W1 , W3 and W2.The first LED string D1 , D2, second LED string D3, D4 and the third LED string D5, D6 are now activated. In a next step the cycle starts again and the control unit activates the bypass modules W1 . The forth LED string D7, D8, third LED string D5, D6 and the second LED string D3, D4 are activated.

For the example of figure 8d the number M would equal four. At every time there are three Substrings or LED strings active as (M-1 ) equals three.

Figure 8e shows an example with a voltage over the load path 4 which is above the quadruple of the forward voltage of a single LED string. This means that at such voltage level an activation and operation of all four Substrings or LED strings in series is efficient. Therefore the control circuitry CC de-activates all of the bypass modules and the first LED string D1 , D2, the second LED string D3, D4, the third LED string D5, D6 and the forth LED string D7, D8 are activated. For the example of figure 8e the number M would equal five. At every time there are all four Substrings or LED strings active as (M-1 ) equals four.

For the example of the figures 8b to 8e the first voltage level is equal to the forward voltage of the full LED string. The second voltage level is equal to the forward voltage of one LED string.

All bypass modules W1 , W2, W3, W4 are de-activated in case the voltage across the load path 4 is as above the first voltage level (example of figure 8e). All bypass modules W1 , W2, W3, W4 may be activated in case the voltage across the load path 4 is as below the second voltage level (example not shown).

At least one bypass module is activated in case the voltage across the load path 4 is as above the second voltage level. Three of the four bypass modules are activated in case the voltage across the load path 4 is as above the second voltage level but below the double of the second voltage level. The double of the second voltage level equals the third voltage level in this example. At least two of the bypass modules W1 , W2, W3, W4 would be activated at the same time when the voltage across the load path 4 is between the first and a third voltage level.

The examples shown in the figures 8a to 8e describe a typical sequence of operation of a driving module L1 according to the invention for a rising edge of the AC mains voltage which might be applied at the input terminals of the driving module L1 . Those examples show the sequence during the first 90° of one cycle of the AC mains voltage. When the AC mains voltage decreases again the same sequence will be initiated in reverse direction.

The number of LEDs per LED string would be selected according the nominal peak voltage of the AC mains voltage. For instance in case of a 230V AC mains voltage the nominal peak voltage would be 325V. This means in case of the four Substrings or LED strings of this example that the overall forward voltage of the load path and thus of the overall series connection of LED would be selected to be around 280V in order to enable stable and efficient operation of all four Substrings or LED strings. As the example of figures 8b to 8e is using four Substrings or LED strings the forward voltage of a single LED string would be a quarter of 280 V meaning it equals 70V. If the forward voltage of a single LED is around 3.5 V (three point five Volts) it would need 20 LEDs per LED string.

The number of Substrings or LED strings may vary but the number of parallel bypass modules has to be selected according to the number of Substrings or LED strings. The same applies for the number of control outputs of the control circuitry CC in order to control the bypass modules.

The control circuitry CC may be adapted to detect the waveform and shape of the input voltage, whereby the control circuitry CC may control the bypass modules according to the detected input voltage, e.g. may detect a dimming information or an emergency lighting situation, and preferably may perform a dimming operation. For instance the control circuitry CC may detect a phase cut signal, whereby the dimming level is depending on e.g. the phase cut angle or a digitally coded phase cut signal. The control circuitry CC may detect an emergency lighting situation. For instance there might be supply system where the mains supply is switched over from a AC mains to a DC voltage or a rectified AC voltage. The control circuitry CC may detect such change of the input voltage and may change control of the bypass units accordingly. For instance in case of a switch over to a DC supply the control unit may activate a certain number of bypass modules with a given or random switching pattern. Preferably there is only a limited number of Substrings or LED strings active at one time. The number of Substrings or LED strings which shall be active at one time during emergency may be programmed or defined by a user. The driver module L1 may also comprise a wired or wireless interface for dimming control, e.g. a DALI interface, an 1 -10V interface, a Bluetooth interface, a Zigbee interface or an infrared interface.

Dimming of the driver module L1 may be performed by limitation of the number of Substrings or LED strings which are active at the same time or by introduction of pauses where all Substrings or LED strings are bypassed (by activating all bypass modules for a short time). Such pauses may be repeated at high frequency which is not visible for the human eye. According to another example of the invention the load path may comprise a series connection of a number of several Substrings or LED strings each comprising at least one LED D1 , D2, D3, D4, D5, D6, D7, D8. Preferably for this example at least of the Substrings or LED strings comprises less LED than the other Substrings or LED strings. The control circuitry CC may activate this LED string in order to compensate variations in the input voltage. The activation of this LED string with less LED combined with other Substrings or LED strings can enable that the voltage of the activated part of the load path (of all activated

Substrings or LED strings) may better fit to the actual input voltage. The activation of this LED string with less LED may be independent from the switching (activation) scheme of the other Substrings or LED strings as described for this invention.

In addition to this example there might be another LED string with more LED than the LED string with less LED but with less LEDs than the other Substrings or LED strings. For instance there might be formed a driver module L1 with three Substrings or LED strings of four LED, one LED string with two LED and one LED string with only one LED. Of course there might be again two Substrings or LED strings with only one LED which might also be activated alternatingly (in case one LED string with one LED should be activated).

The driver module L1 may comprise a current regulator (current source 11 ) for controlling the current flowing through the LEDs D1 , D2, D3, D4, D5, D, D7, D8 and thus through the Substrings or LED strings. The current regulator (current source 11 ) is advantageously operated so that the current through the LEDs follows the shape of the sine-wave of the rectified voltage VA which is the voltage over the load path 4. The current regulator (current source 11 ) is set up for driving a non-constant current through the load path 4 and thus through the Substrings or LED strings. With the optional control of the LED current by the current regulator (current source 11 ) the variation of the input voltage may be compensated. For the example of the figures 8b to 8e where only four Substrings or LED strings are existing the forward voltage of one LED string is relatively high and thus the voltage range for one of the switching schemes of figures 8b to 8e are wide. This means that the operation voltage of a LED string will have a difference of 70V within such range. The current regulator (current source 11 ) may comprise a switch in the form of a transistor M1 for controlling the current through the LEDs. Said transistor M1 is connected in series with the LED sets 5, 6, and particularly with the second LED set 6. Said transistor M1 is implemented as a power transistor. The transistor M1 may be a field-effect transistor (FET), and preferably an N-channel metal-oxide-semiconductor field effect transistor (MOSFET).

To control the current through the LEDs, the transistor M1 is advantageously operated in the linear mode i.e. in the ohmic mode. In this linear mode, the transistor M1 is turned on and the gate-source voltage of the transistor M1 is above the threshold voltage Vth. The characteristic of drain current versus drain-to-source voltage is nearly linear for e.g. small values of the drain-source voltage.

The current regulator (current source 11 ) and the transistor M1 may be controlled by the control circuitry CC.

According to the present invention, a bypass module W1 is connected in parallel to the first string of LEDs D1 , D2 so that the LED string D1 , D2 can be bypassed depending on the voltage VA applied to the load path 4.

The bypass module W1 may comprise a MOSFET, a bipolar transistor or two transistors Q2, Q3 arranged according to a Darlington circuit 23. The transistors Q2, Q3 are e.g. in the form of bipolar junction transistors, and preferably of the PNP-type. Alternatively, the transistors Q2, Q3 can also be of the NPN-type, or they can be of opposite type, one NPN and one PNP, and arranged according to a Sziklai configuration.

Both transistors Q2, Q3 may have a common collector in that their respective collectors are connected together. The transistors are further on coupled such that the emitter current of the transistor Q3 becomes the base current of the transistor Q2. The transistor Q2 is connected as an emitter follower and the transistor Q3 as a common emitter amplifier.

According to the present invention, the bypass module W1 is adapted to bypass the LED string D1 , D2 when the rectified AC voltage VA is below a given threshold. On the contrary, the bypass module W1 is switched off if said rectified AC voltage VA is above said given threshold. Above this threshold, the shunting transistor 23 (Darlington circuit 23) is switched off so that current will flow through the LED set 5. The switching of the bypass module may be controlled the control circuitry CC.

The reason for switching operative the LED string D1 , D2 above said given threshold is the efficiency of the driver module. Above said threshold the voltage VA applied to the load path 4 is indeed sufficient for lighting said LED string D1 , D2. On the other hand, if the applied voltage VA is too low, i.e. below said threshold, the voltage across both substrings or LED strings D1 , D2, D3, D4 will not be sufficient to switch on both Substrings or LED strings D1 , D2, D3, D4.

The control circuitry CC may be formed by a microcontroller or other kind of an integrated circuit for instance an application specific integrated circuit or a FPGA. The energy supply for the control unit may be coupled to the current regulator and may be powered out of the current regulator.

It has to be understood that while the switching network is used for switching LEDs, it is also possible to apply the switching network and the driving method to other loads supplied with an AC voltage, in which the load should dynamically follow an electrical parameter, especially an amplitude of an AC voltage. By using the inventive method, portions of the load can be dynamically activated to follow the sign development of the electrical parameter.

Claims

A driver for driving at least one string of

LEDs with a rectified AC voltage, comprising:

- input terminals (C1 , C2) being supplied with a rectified AC voltage,

- a control circuitry (CC) being supplied with a signal indicating the

amplitude or the phase of the AC voltage, and

- a switching network (SN) comprising switches, each switch being arranged for selectively shunting or activating a substring (S6, S7) of one or more LEDs of the LED string (L1 ),

wherein the control circuitry (CC) is designed to control each of the switches independently such that the combined forward voltage of the activated substrings (S6, S7) of LEDs follows the amplitude of the rectified AC voltage in incremental steps,

- a current regulator in series with the string of LEDs,

- the control circuitry being provided with a singal representing the voltage drop across the current regulator.

A driver for driving at least one string of

LEDs with a rectified AC voltage, comprising:

- input terminals (C1 , C2) being supplied with a rectified AC voltage,

- a control circuitry (CC) being supplied with a signal indicating the

amplitude or the phase of the AC voltage, and

characterized in that

the control circuitry (CC) is designed to alternate at least two active substrings (S6, S7) of LEDs having the same combined forward voltage during the time period of one step.

3. Driver according to any one of the preceding claims, wherein the control circuitry (CC) is configured to control the switches in accordance with the electrical parameter, e.g. an AC voltage amplitude value, and wherein the number of activated substrings (S6, S7) corresponds to a value of the electrical parameter.

4. Driver according to any one of the preceding claims, wherein the control

circuitry (CC) is configured to only activate a specific substring (S6, S7) depending on the electrical parameter, e.g. at least one specific substring (S6, S7) for a specific value or a value range.

5. Driver according to any one of the preceding claims, wherein the control

circuitry (CC) is configured to cyclically activate all substrings (S6, S7) corresponding to the electrical parameter value or value range by switching the respective switches, in particular the control circuitry (CC) is configured to periodically switch on and off the respective switches.

6. Driver according to any one of the preceding claims, wherein in parallel to each substring a capacitor (CP6, CP7) is connected.

7. Driver according to claim 8, wherein the capacitor (CP6, CP7) is arranged to continue to drive the associated substring (S6, S7), when the associate switch is set to shunt the substring (S6, S7) or parts thereof, e.g. at least one LED of the substring (S6, S7).

8. Driver according to any one of the preceding claims, wherein at least two

substrings (S7, S8) are of different length, e.g. comprising a different number of LEDs.

9. Driver according to any one of the preceding claims, wherein the control

circuitry (CC) is configured to control a current regulator (CR) of the LED string (L1 ).

10. Driver according to any one of the preceding claims, wherein the control

circuitry (CC) is configured to receive a feedback signal, e.g. a voltage measurement signal, from a shunt at the voltage regulator indicating in particular a current flow through the LED string (L1 ).

1 1 . Driver according to any one of the preceding claims, wherein the control circuitry (CC) is an ASIC or microcontroller.

12. Driver according to any one of the preceding claims, wherein each substring (S6, S7) comprises at least one LED.

13. Method for driving at least one string of

LEDs with a rectified AC voltage, comprising:

- input terminals (C1 , C2) for supplying a rectified AC voltage,

- a control circuitry (CC) being supplied with a signal indicating the

amplitude or the phase of the AC voltage, and

wherein the control circuitry (CC) controls each of the switches

independently such that the combined forward voltage of the activated substrings (S6, S7) of LEDs follows the amplitude of the rectified AC voltage in incremental steps,

characterized in that

the control circuitry (CC) alternates at least two active substrings (S6, S7) of LEDs having the same combined forward voltage during the time period of one step.

14. A driver for driving at least one string of

LEDs with a rectified AC voltage, comprising:

- input terminals (C1 , C2) for supplying a rectified AC voltage,

- a control circuitry (CC) being supplied with a signal indicating the

amplitude or the phase of the AC voltage, and

wherein the control circuitry is designed to control each of the switches independently such that the combined forward voltage of the activated substrings (S6, S7) of LEDs follows the amplitude of the rectified AC voltage in incremental steps,

characterized in that

at least two LED substrings (S6, S7) have different combined forward voltages, preferably the LED substrings contain LEDs according to a Frobenius number set.