LED lighting system and control method

Abstract

For an efficient dimming LED lighting system, a system is created that includes a voltage source, an electronic switch controlled by a pulse-width modulation (PWM) drive signal, and an LED lighting module connected in series. The LED includes a group of LEDs and a constant current driver circuit that drives a constant current through the group of LEDs. A bypass branch connected in parallel with the voltage source and the electronic switch is adapted so that when the electronic switch is open, the power supplied to the constant current driver circuit is sufficient to maintain operation of the constant current driver circuit, ensuring deep dimming performance.

Description

[Technical Field]

[0001] The present invention relates to the field of lighting systems, and more particularly to the field of LED lighting systems. The present invention relates particularly, but not exclusively, to LED lighting systems that include a dimming function based on interruption of the supply of current to a lighting load. [Background technology]

[0002] There are many different drive schemes for driving lighting fixtures that use LEDs. For example, complex circuits such as high-frequency switch-mode power conversion circuits are used in mains-powered lighting systems; they enable high power factor and low total harmonic distortion, but are a high-cost solution. Low-cost circuits, for example, use linear drivers. The most basic drive circuit may simply supply a DC voltage to the LED through a so-called ballast resistor, which acts as a current limiter.

[0003] Complex drivers incorporate dimming functionality, for example, with a dimming input providing control of the settings of a switch-mode power converter. To implement dimming in simpler circuits, the current supply to the LED device may be cut off, for example, using a series switch controlled using a pulse-width modulated (PWM) signal. The duty cycle of the PWM signal then determines the light output.

[0004] For an LED to perform optimally, it is driven with a current as specified by the manufacturer to achieve the desired light output at its most efficient operating point. This can be most easily achieved by connecting the LED to a voltage source in series with a resistor as described above. This is a low-cost solution that is generally used for low- and medium-power LEDs. The current through the LED is therefore (V VS -V LED ) / R, where V VS is the voltage supplied by the voltage source, and V LEDis the forward voltage of the LED and R is the resistance of the series resistor. However, this resistor introduces losses and therefore reduces the efficiency of the lighting system.

[0005] To improve this efficiency, the voltage drop across the resistor needs to be kept as low as possible. One way to do this is to place multiple LEDs in series. Keeping the light output of multiple LEDs the same compared to a single LED requires a lower driver current, which reduces resistor losses. However, it also means that if the forward voltage of the LED differs from the expected value, for example due to manufacturing errors or tolerances, the light level will be significantly different. To eliminate this variation problem, V VS and V LED A large difference between the two is required, but this results in large losses.

[0006] A constant current circuit can be used to regulate the current through the LED to eliminate the need for a current limiting resistor and improve efficiency, at the cost of slightly more complex circuitry and therefore increased system cost. Summary of the Invention [Problem to be solved by the invention]

[0007] One problem with using constant current circuits is that they tend to have an associated start-up time before delivering constant current. This means that for standard PWM dimming schemes such as those described above, which generally operate at about 1 kHz or above, there is a lower limit to the duty cycle and therefore the dimmed LED lighting system.

[0008] There remains a need for a driver architecture that allows for low-cost dimming based on interruption of the current supply to an LED device, yet can achieve a wide range of dimming levels. [Means for solving the problem]

[0009] The invention is defined by the claims.

[0010] According to an example according to an aspect of the present invention, a voltage source; an electronic switch in series with the voltage source, controlled by a pulse width modulated (PWM) drive signal; a bypass branch in parallel with the electronic switch; an LED lighting system comprising an LED lighting module in series with the voltage source and the electronic switch, the LED lighting module comprising: a constant current drive circuit; and a group of LEDs, the constant current drive circuit is adapted to drive a constant current through the LEDs; An LED lighting system is provided in which the bypass branch and / or voltage source are adapted such that when the electronic switch is open, the power supplied to the constant current drive circuit is sufficient to maintain operation of the constant current drive circuit.

[0011] This LED lighting system allows for dimming of a single LED or a group of LEDs. A PWM signal with a duty cycle selected for the desired dimming amount is applied to an electronic switch. When the switch is open, the LED is intended to be turned off. A bypass branch is in parallel across the switch. The parallel combination of the switch and the bypass branch is connected in series with a voltage source and an LED lighting module. The LED lighting module consists of an LED group and a constant current driver. The constant current driver ensures a constant current through the LED group, which ensures proper light output as specified by the LED group manufacturer. This contrasts with the more basic system of series static resistors, which results in lower efficiency and less consistent light output characteristics. Furthermore, several LED lighting modules can be connected together in parallel.

[0012] The bypass branch allows the constant current drive circuit to remain powered even when the LED is turned off, and the power provided can be used, for example, as a supply voltage that is sufficient to keep the drive circuit operational.

[0013] When certain constant current drive circuits are used, there is a switch-on delay of several microseconds. PWM-controlled LED systems usually operate at PWM frequencies higher than 1 kHz, so the delay can be significant, especially for low duty cycles of the PWM signal. For example, with a 4 kHz PWM signal, a dimming level of 1% results in a 2.5 μs pulse, which corresponds to a switch-on delay. Therefore, the lowest possible dimming level is limited.

[0014] Furthermore, when the constant current drive circuit is switched on, large switch-on currents can occur, which can require additional delays to ensure these currents are controlled, which can result in an additional delay of several microseconds. These delays increase the minimum possible dimming level. By adding a bypass branch, the constant current drive circuit can be continuously powered even when the switch is open, eliminating this problem.

[0015] The bypass branch and / or the voltage source may be adapted, for example, so that when the electronic switch is open, the voltage across each of the LEDs in the LED group remains below its respective threshold voltage. The addition of the bypass branch may also cause the LEDs to emit light at a very low, but still visible, level. This is particularly problematic when several LED lighting modules are in parallel, as a higher current is required to continue powering all of the constant current drive circuits, resulting in a higher current through the LED group. By keeping the voltage across the LEDs below their threshold voltage, no current flows through the LEDs. Thus, when the electronic switch is open, the constant current drive circuit remains switched on, but the LEDs are switched off.

[0016] The LED group may, for example, have one or more LEDs connected in series. This means that the overall threshold voltage of the LED group can be selected so that it does not reach that threshold voltage when the electronic switch is open. Each additional LED placed in series increases the threshold voltage. The desired illumination level can also be selected based on the number of LEDs in the group.

[0017] The bypass branch may, for example, comprise a resistor that reduces the voltage across the LED group so as to prevent any current from flowing through the LED group when the electronic switch is open, but allows a voltage high enough to ensure operation of the constant current driver circuit.

[0018] The bypass branch comprises, for example, a Zener diode that provides a certain voltage drop and thus reduces the voltage across the LED group when the electronic switch is open so as to prevent any current from flowing through the LED group, but allows a voltage high enough to ensure operation of the constant current driver circuit.

[0019] The bypass branch may comprise, for example, a resistor and a Zener diode in series. If one of the Zener diode or the resistor does not allow a voltage low enough to switch off the LED and high enough for the operation of the constant current driver, an additional resistor in series with the Zener diode can be used.

[0020] The constant current drive circuit is, for example, a buck converter, which can be used in a known manner to maintain a constant current across the LEDs by fast switching, providing an electrically efficient design but a relatively high circuit cost.

[0021] The constant current drive circuit may alternatively be a linear constant current converter, which keeps the current constant across the LEDs by operating the drive transistor in its linear region, which is more efficient than a simple resistor circuit and has lower system cost than using a buck controller.

[0022] The electronic switch is, for example, a transistor.

[0023] The system may further comprise a plurality of LED lighting modules, each including a respective constant current drive circuit and a respective group of LEDs, each LED lighting module being in a respective parallel branch, and the bypass branch being adapted such that when the electronic switch is open, the voltage across each constant current drive circuit remains sufficient to operate the constant current drive circuit. Thus, the lighting system may comprise a plurality of LED branches.

[0024] According to an example embodiment of the present invention, there is provided a method of controlling an LED lighting system, the method comprising: controlling an electronic switch using PWM control to thereby implement dimming, the electronic switch being in series with a voltage source and an LED lighting module; using the voltage source to power a constant current drive circuit of the LED lighting module when the electronic switch is closed, thereby driving a constant current through LEDs of the LED lighting module; and using the voltage source to supply power to the constant current drive circuit through a bypass branch in parallel with the electronic switch when the electronic switch is open, wherein the power supplied is sufficient to maintain operation of the constant current drive circuit.

[0025] In an example, when the electronic switch is open, the voltage across each of the LEDs in the group remains below its respective threshold voltage.

[0026] The bypass branch comprises, for example, a resistor, a Zener diode, or a resistor and a Zener diode in series.

[0027] Powering the constant current drive circuit may include, for example, operating a buck converter or a linear constant current converter.

[0028] In an example, when the electronic switch is closed, the method includes powering constant current drive circuits of a plurality of LED lighting modules, each including a respective constant current drive circuit and a respective group of LEDs, each LED lighting module being in a respective parallel branch, and when the electronic switch is open, the voltage across each constant current drive circuit remains sufficient to operate the constant current drive circuit.

[0029] These and other aspects of the invention will be elucidated and elucidated with reference to the following embodiments. [Brief explanation of the drawings]

[0030] For a better understanding of the present invention, and to show more clearly how the same may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings, in which: [Figure 1A] A basic LED lighting system is shown, consisting of one LED in series with a voltage source and a resistor. [Figure 1B] A basic LED lighting system is shown, consisting of a group of LEDs in series with a resistor. [Figure 2A] 1 shows an LED lighting system with a buck controller that provides a constant current through a group of LEDs. [Figure 2B] 1 shows an LED lighting system with a linear constant current converter that provides a constant current through the LEDs. [Figure 3] 2B shows the LED lighting system of FIG. 2B with a PWM-driven electronic switch in series with the voltage source and LED lighting module for dimming. [Figure 4A] 4 shows the LED lighting system of FIG. 3 with a first bypass branch including a resistor in parallel with the electronic circuit. [Figure 4B] 4 shows the LED lighting system of FIG. 3 with a second bypass branch including a Zener diode in parallel with the electronic circuit. [Figure 4C] 4 shows the LED lighting system of FIG. 3 with a first bypass branch in parallel with the electronic circuit, the first bypass branch including a resistor and a Zener diode connected in series. [Figure 5] 1 shows an LED dimmable lighting system comprising multiple LED lighting modules connected in parallel. DETAILED DESCRIPTION OF THE INVENTION

[0031] The present invention will be described with reference to the drawings.

[0032] It should be understood that the detailed description and specific examples, while indicating exemplary embodiments of the devices, systems, and methods, are for purposes of illustration only and are not intended to limit the scope of the invention. These and other features, aspects, and advantages of the devices, systems, and methods of the present invention will become better understood from the following description, appended claims, and accompanying drawings. It should be understood that the figures are merely schematic and are not drawn to scale. It should also be understood that the same reference numerals are used throughout the figures to indicate the same or similar parts.

[0033] The present invention provides an LED lighting system comprising a voltage source, an electronic switch controlled by a pulse-width modulated (PWM) drive signal, and an LED lighting module, all connected in series. The LED lighting module comprises a group of LEDs and a constant current driver circuit adapted to drive a constant current through the group of LEDs. In addition, the LED lighting system further comprises a bypass branch in parallel with the electronic switch. The bypass branch and / or the voltage source are adapted such that when the electronic switch is open, the power supplied to the constant current driver circuit is sufficient to maintain operation of the constant current driver circuit.

[0034] Figures 1A and 1B show examples of LED lighting circuits 100, 101. The circuit of Figure 1A includes a voltage source 110, a resistor 120, and an LED 130 in series. Figure 1B shows the LED lighting circuit 100 of Figure 1A with an LED group 131 instead of the single LED 130. The LED group 131 includes multiple LEDs 132 connected in series.

[0035] The current level required for the LED to output the desired light level is specified by the manufacturer, but must also be limited to prevent damage to the LED. This drive current is provided in both examples by connecting a resistor in series with a voltage source.

[0036] The drive current through the LED is (V VS -V LED) / R, where V VS is the voltage supplied by the voltage source, and V LED is the forward voltage of the LED, and R is the resistance of the series resistor. The voltage drop across the LED, the forward voltage, is relatively constant over a range of drive currents.

[0037] Therefore, the value of the resistor can be determined together with the voltage source at the forward voltage and drive current specified by the LED manufacturer. However, this resistor introduces losses and therefore reduces the efficiency of the system.

[0038] For example, if LED 130 in Figure 1A has a forward voltage of 3V at 200mA and voltage source 110 supplies 10V, resistor 120 would need to be a 35Ω resistor to supply 200mA, which would result in a resistor loss of 1.4W.

[0039] If LED group 131 in Figure 1B has three LEDs and voltage source 110 is the same as in the example of Figure 1A, a current of 66.6 mA through LED group 131 and a forward voltage of 9 V are required to achieve the same light level as in the example above. In this case, the value of resistor 121 needs to be 15 Ω. This results in a resistor loss of 0.066 W, approximately 21 times less than the resistor loss in the example of Figure 1A.

[0040] This efficiency improvement is due to the reduced voltage drop across the resistor. Unfortunately, the forward voltage of an LED is a production factor, meaning that this value can vary by a few percent.

[0041] 1A, if the LED forward voltage due to tolerance is 10% higher, the forward voltage required by LED 130 is 3.3 V. For a resistance value of 35 ohms, this means that the current through LED 130 will decrease by 5% to 191 mA, resulting in 5% lower light output.

[0042] 1B, if the LED forward voltage due to tolerance is 10% higher, the forward voltage required by LED group 131 is 9.9 V. For a resistance value of 15 ohms, this means that the current through LED group 131 will be reduced by 90% to 6.66 mA, resulting in 90% less light output.

[0043] In summary, given a lower voltage drop across resistor 120, lower resistor losses are achieved, increasing the efficiency of the lighting system. However, this makes the light output of the system more susceptible to changes due to variations in the forward voltage of LED group 131. This is because of the smaller headroom between the voltage supplied by voltage source 110 and the forward voltage of LED group 131, which means that small variations in forward voltage result in larger variations in drive current.

[0044] 2A shows an example of an LED lighting circuit 200 comprising a voltage source 210 and an LED lighting module 220. The LED lighting module 220 comprises a constant current drive circuit 240 and an LED group 230. The constant current drive circuit 240 is adapted to control a constant current through the LED group 230.

[0045] The constant current driver circuit 240 is a current buck controller circuit. It is powered by supply rails having a supply voltage VCC and a ground voltage GND. These supply rails are derived from the output of the voltage source 210. The constant current driver circuit, for example, has a minimum supply voltage (relative to ground) at which the circuit remains operational.

[0046] A constant current through the LED string 230 is achieved at high switching speeds by a buck controller, in a known manner. The current is sensed by a current sense resistor 270. The current buck controller 240 controls the series main converter switch 260 in response to the monitored current across the sense resistor 270. Capacitor C1 and inductor L1 form the tank circuit of the buck converter, and diode D1 is the buck converter's freewheeling diode. This provides an efficient design and consistent light output from the LED string 230. However, this solution requires the use of an inductor and a high-speed buck controller, increasing system costs.

[0047] 2B shows an alternative example of an LED lighting circuit 201, again comprising a voltage source 210 and an LED lighting module 221. The LED lighting module 221 comprises a constant current driver circuit 241 and an LED group 230. The constant current driver circuit 241 in this example is a linear constant current converter.

[0048] The constant current converter has a main transistor 261 (T1) operated in its linear region. Again, a current-sensing resistor 270 senses the current through the LEDs 230. The resulting voltage is compared in comparator U1 to a reference represented by voltage source 251. The comparator's output determines the conduction state, and therefore the output impedance, of transistor T2. Transistor T2 forms a voltage divider with resistor R1 such that the voltage at the junction between transistor T2 and resistor R1 varies depending on the current through the LEDs. This controls the base voltage of main transistor T1. In this manner, a feedback path is created that adjusts the conduction state of T1 to maintain the desired current set by reference 251. If the current decreases, T2 is forced into a low-impedance state to provide current regulation (in this case, an increase), thus increasing the voltage at the base of T1.

[0049] The constant current driver circuit 241 is operated by supply rails having a supply voltage VCC and a ground voltage GND, which are derived from the output of a voltage source 210. The constant current driver circuit may be an integrated circuit (with peripheral circuit components), which again has a minimum supply voltage (relative to ground) at which the circuit remains operational.

[0050] This provides an efficient design and consistent light output from the LED array 230. A linear constant current converter has lower system cost and a simpler design than a buck controller.

[0051] In Figures 2A and 2B, unlike the lighting circuit shown in Figures 1A and 1B, the current through the LED group 230 remains constant even if there is a difference between the actual LED group forward voltage and the expected value provided by the manufacturer.

[0052] FIG. 3 shows an example of an LED lighting circuit 201 having a pulse width modulation (PWM) controlled electronic switch 350 connected in series with a voltage source 210 and an LED lighting module 320, adapted for the circuit design of FIG. 2B.

[0053] The electronic switch 350 may be, for example, a transistor or another switch known to those skilled in the art that can be switched on and off by an electronic signal.

[0054] The PWM-controlled electronic switch 350 allows for dimming of the light output of the LED group 230. This is accomplished by regularly and repeatedly opening and closing the electronic switch 350, connecting and disconnecting the voltage source 210 from the LED lighting module 221. By decreasing the duty cycle, the average light output from the LED group 230 is reduced, an effect perceived by the human eye as dimming. By increasing the duty cycle, the average light output from the LED group 230 is increased up to a maximum level at which the LED group 230 is always on. Thus, the duty cycle can be selected for a desired level of dimming.

[0055] This dimming method becomes problematic when the constant current drive circuit is a current buck converter, a linear constant current converter, or any other constant current system that requires time to start up. In the case of the linear constant current converter 241 shown in Figure 3, the start-up time, i.e., the time it takes for the electronic device to begin functioning, is typically a few microseconds. During this start-up time, large switch-on currents can lead to damage to the LED cluster 230, so additional measures are taken to limit the current, introducing additional delays.

[0056] PWM-controlled LED systems typically operate at PWM control frequencies above 1 kHz. Therefore, the period is less than 1 ms. Considering a PWM control frequency of 1 kHz, at 0.1% dimming, the LED lighting module 230 is connected for only 1 microsecond during an on / off cycle. Due to issues with high switch-on currents during startup of electronic devices, the connection of the constant current driver circuit may need to be delayed. If this delay time is only 2 microseconds, the minimum dimming level of the LED lighting system is at least 0.2%. If the PWM control frequency is increased to 5 kHz, this minimum dimming level increases to 1%. Therefore, if there is a startup time, there is a lower limit to the dimming level.

[0057] Therefore, to overcome this minimum dimming level, the occurrence of the start-up time needs to be prevented. To achieve this, the present invention is based on an embodiment in which the constant current driver circuit is continuously powered. However, this is not easy to achieve in a system with a series disconnect switch 350 (i.e., a single-wire system) in that the switch 350 removes power from the LED lighting module and therefore from the constant current driver circuit.

[0058] 4A, 4B and 4C show an example of a lighting system according to the invention, based on a modification of the circuit of FIG.

[0059] FIG. 4A shows a lighting system 400 having an electronic switch 350 driven by a PWM signal, a voltage source 210, and an LED lighting module 221 in series, as described above.

[0060] The LED lighting system 400 further comprises a bypass branch 460 connected in parallel with the electronic switch 350 .

[0061] The bypass branch 460 comprises a resistor 461 in this first example.

[0062] Adding resistor 461 in parallel with electronic switch 350 provides a path through which current can flow when electronic switch 350 is open. This makes it possible to maintain the power supply to linear constant current converter 241, and in particular the power supply VCC to comparator U1 and other circuit components. Therefore, linear constant current converter 241 can operate continuously even when electronic switch 350 is open. This means that there is no longer a lower limit on the dimming level, as linear constant current converter 241 does not need to be activated.

[0063] Preferably, the LED string 230 is turned off by ensuring that the voltage drop across resistor 461 results in a voltage across the LED string that is less than the total threshold voltage of the LEDs. However, the voltage supply VCC to the linear constant current converter 241 remains higher than its minimum operating voltage (described above), allowing the linear constant current converter to operate. Thus, there is a range of voltages between the minimum operating voltage of the constant current drive circuit and the forward voltage of the LED string, and by providing a drive voltage within this range, the LED string can be turned off while the constant current drive circuit remains active.

[0064] The minimum supply voltage VCC is e.g. 2.5 V, therefore the operating voltage of the LED string needs to be higher than this. The resistor has a value such that the current through the LEDs is as low as possible while the linear constant current converter is operating.

[0065] During this period, no current flows through the current sense resistor 270, causing transistor T2 to be turned off (or in its lowest conducting state, corresponding to the lowest output voltage from comparator U1), thus causing a pull-up of the base voltage of T1. Thus, while switch 350 is open and the circuit is powered by the bypass path, and the LED is off, the sense resistor current is null, and therefore the current transistor T1, controlled by the switch, is maximally open.

[0066] When switch 350 is closed, transistor T1 again conducts current, which is regulated by the constant current drive circuit.

[0067] Thus, even when there is a very low duty cycle, the circuit is ready to supply current to the LEDs, i.e., the circuit is already powered and the transistor is in a suitable conducting state to supply current as soon as the forward voltage of the LEDs is reached.

[0068] There is a response time for the circuit to return the current transistor to the correct value, depending on the sensing resistor, with the risk of a "flash" current. This problem depends on the time, and therefore on the speed of the circuit's control loop. It should not be acceptable for such a flash to be visible, and even more importantly, the LED must be able to handle any current spikes that may occur without damage.

[0069] The gain of transistor T1 can be limited so that the maximum current does not become too high.

[0070] 4B shows a second example of a lighting system 401 that is identical to FIG. 4A except for a bypass branch, which in this example includes a Zener diode 462. This provides a step voltage drop (which is current independent). Again, the Zener diode 462 provides a path through which current can flow when the electronic switch 350 is open. The Zener diode 462 is adapted to reduce the voltage across the LED group 230 to a voltage lower than the forward voltage of the LED group 230, preventing current flow across the LED group 230 and thereby preventing light emission.

[0071] As explained above, the voltage supply to the linear constant current converter 241 remains above its minimum operating voltage, allowing the linear constant current converter to operate.

[0072] Figure 4C shows a third example of a lighting system 403 that is identical to Figure 4A except for a bypass branch that includes a Zener diode 462 in series with a resistor 464. Resistor 464 may also prevent undesirably high current through the LED lighting module 221 when the switch is open.

[0073] 4A-4C, the number of LEDs connected in series in the LED group 230 can be selected to achieve a desired total forward voltage of the LED group 230. The forward voltage of the LED group is the sum of all of the forward voltages of the LEDs in the LED group. This allows the minimum operating voltage of the constant current driver circuit to be set at an appropriate level below the total threshold voltage of the LEDs.

[0074] 5 shows an example of an LED lighting circuit 500 having in series a PWM controlled electronic switch 350, a voltage source 510, and an LED lighting module 520. In parallel with the PWM controlled electronic switch is a bypass branch 560. Additionally, there are multiple optional LED lighting modules 521 in parallel with each other.

[0075] The multiple LED lighting modules 520, 521 allow multiple LED groups with multiple threshold voltage and drive current options to be added to the LED lighting system. The bypass branch 560 is adapted accordingly to the multiple LED lighting modules to ensure that the voltage across the multiple constant current drive circuits remains high enough for continued operation when the electronic switch 550 is open. The bypass branch 560 is also adapted to ensure that the voltage across the multiple LED groups 530 remains lower than the forward voltage of the LED group 530 when the electronic switch 550 is open, thereby preventing light emission.

[0076] The present invention has been described with reference to a linear constant current driver. This provides a low-cost solution. However, the same approach can be applied to a buck converter. Any other constant current converter can be used. Essentially, a current driver is a current source circuit, and any suitable current source circuit can be used.

[0077] The invention is for example interesting for LED systems, such as LED strips, which are driven using a DC voltage, which may for example be obtained by rectification from the mains.

[0078] The system operates at a voltage ranging from, for example, 2 V to 50 V. The maximum voltage depends on the IC process and is, for example, in the range of 18 V to 50 V. As an example, the operating range of the IC may be from 2.5 V to 18 V.

[0079] Those skilled in the art can understand and effect variations to the disclosed embodiments in practicing the claimed invention, from a study of the drawings, the specification and the appended claims. In the claims, the word "comprises" does not exclude other elements or steps, and the singular does not exclude a plurality.

[0080] The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

[0081] It should be noted that when the term "adapted to" is used in the claims or specification, the term "adapted to" is intended to be equivalent to the term "configured to."

[0082] Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. a voltage source; an electronic switch in series with said voltage source, controlled by a PWM drive signal; a bypass branch in parallel with the electronic switch; an LED lighting system comprising an LED lighting module in series with the voltage source and the electronic switch, the LED lighting module comprising: a constant current drive circuit adapted to receive power via a power supply having a return coupled between the electronic switch and the LED lighting module; an LED group including a plurality of LEDs connected in series; the constant current drive circuit is adapted to drive a constant current through the LEDs; the bypass branch is adapted so that when the electronic switch is open, the power supplied to the constant current drive circuit is sufficient to maintain power supply to the constant current drive circuit; An LED lighting system, wherein the bypass branch is adapted to provide a voltage across the group of LEDs such that the voltage drop across the bypass branch is less than a total threshold voltage of the LEDs.

2. The LED lighting system of claim 1 , wherein the bypass branch comprises a resistor.

3. 3. An LED lighting system according to any one of claims 1 to 2, wherein the bypass branch comprises a Zener diode.

4. 3. An LED lighting system according to any one of claims 1 to 2, wherein the bypass branch comprises a resistor and a Zener diode in series.

5. 5. The LED lighting system according to claim 1, wherein the constant current drive circuit is a buck converter.

6. 5. The LED lighting system according to claim 1, wherein the constant current drive circuit is a linear constant current converter.

7. 7. The LED lighting system according to claim 1, wherein the electronic switch is a transistor.

8. 8. An LED lighting system according to any one of claims 1 to 7, comprising a plurality of LED lighting modules, each comprising a respective constant current drive circuit and a respective group of LEDs, each LED lighting module being in a respective parallel branch, the bypass branches being adapted such that when the electronic switch is open, the voltage across each constant current drive circuit remains sufficient to operate the constant current drive circuit.

9. 1. A method for controlling an LED lighting system, comprising: controlling an electronic switch using PWM control to thereby implement dimming, the electronic switch being in series with a voltage source and an LED lighting module; using the voltage source to power a constant current drive circuit of the LED lighting module when the electronic switch is closed, thereby driving a constant current through an LED group of the LED lighting module, the LED group including a plurality of LEDs connected in series; and using the voltage source to supply power to the constant current drive circuit through a bypass branch in parallel with the electronic switch when the electronic switch is open, wherein the supplied power is sufficient to maintain power supply to the constant current drive circuit, and the bypass branch is adapted to result in a voltage drop across the LEDs that is less than a total threshold voltage of the LEDs.

10. 10. The method of claim 9, wherein the bypass branch comprises a resistor, a Zener diode, or a resistor and a Zener diode in series.

11. 11. The method of any one of claims 9 to 10, wherein powering the constant current drive circuit comprises operating a buck converter or a linear constant current converter.

12. 12. The method of any one of claims 9 to 11, comprising the step of powering constant current drive circuits of a plurality of LED lighting modules, each including a respective constant current drive circuit and a respective group of LEDs, when the electronic switch is closed, each LED lighting module being in a respective parallel branch, and when the electronic switch is open, the voltage across each constant current drive circuit remains sufficient to operate the constant current drive circuit.

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