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This application is the U.S. National Phase application under 35 U.S.C. § 371 of International Application No. PCT/EP2016/055569, filed on Mar. 15, 2016 which claims the benefit of Patent Application No. PCT/CN2015/075136, filed on Mar. 26, 2015 and European Patent Application No. 15165202.1, filed Apr. 27, 2015. These applications are hereby incorporated by reference herein.

This invention relates to LED driver circuits and driving methods. It is of particular interest for LED tubular lighting but also has more general applicability.

Luminaires with fluorescent tubular lamps are widely used in offices. In Europe, luminaires with multiple lamps in series are also used. For example, luminaires with three 60-cm tubular lamps are popular, such as troffers. In general, this type of luminaire has two lamps in series, driven by a high power ballast, and the third lamp is driven by a low power ballast.

The structure of this type of luminaire is shown in FIG. 1.

A first lamp 10 is driven by a low power ballast 12 and a dummy starter 14. Second and third lamps 16 and 18 are in series, and are driven as a single entity by a high power ballast 20. Each has its own dummy starter 17, 19.

There is an increasing demand for LED replacements for the fluorescent tubular lamps, and in particular which do not need the luminaire to be changed. There is accordingly a very large retrofit market for tubular LED lamps, for replacing traditional fluorescent tubular lamps. However it is challenging to make an LED driver design which enables series connection of the LED lamps with their associated driver. Etc usb devices driver download for windows 10 laptop.

It is easy to connect traditional fluorescent lamps in series, since the equivalent impedance of traditional fluorescent lamp is negative. This means the input voltage for each lamp can self-regulate when they are in series.

For an LED lamp, it is more challenging to connect the LEDs in series. In particular, the impedance of a closed loop driver is positive, which gives rise to positive feedback and the possibility of an unbalance of input voltage on each lamp.

For example, if two closed loop drivers are connected in series it becomes difficult to provide the required balance between the circuits, as a result of circuit tolerances. More specifically, for a closed loop circuit, the driver will try to keep the driver output stable, but the input impedance of the closed loop driver will change following the input voltage. For example R=Uin²/P, where P is the power, Uin is the voltage and R is the resistance. If P is fixed then R will change to follow Uin.

In a series connection, the increased resistance will in turn result in an increased voltage allocated to that driver. With two closed loop drivers connected in series, there is a possibility that one driver develops a larger and larger voltage than the other driver, and the imbalance becomes worse and worse until there are abnormal driving conditions. Even if the system reaches a stable steady state, there may still be an imbalance between the two drivers. This means that different lamps may have different outputs, and instability may also give rise to flicker.

A possible solution is to use open loop drivers. An open loop driver has an input impedance which behaves as a fixed resistor. Since an open loop driver has a relatively stable input impedance, it is easier to provide balancing between them. However, open loop control means the light output is unregulated during input or output disturbances, and the output is influenced significantly by tolerances of the main parameters of the LED driver.

EP2257124A1 discloses a circuit for connecting a low current lighting circuit to a dimmer. It has a switch that closes when the output of the bridge rectifier falls below 200 volts. When the switch closes, it provides a resistive load that provides holding current for the triac of the dimmer.

It would be advantageous to provide an LED driver circuit which enables closed loop current control or closed loop power control and that can operate normally in the context of series connection with another LED driver, avoiding the technical problem of unbalanced voltage developed on this and the another LED driver due to their negative impedance.

A basic idea of the embodiments of the invention is the two LED drivers work complementarily in each half cycle: in one given half cycle there is one and only one LED driver is powered by the AC input power; and in next half cycle the other LED driver is powered by the AC input power. This complementary operation is implemented by the complementarily open/closing of two shorting element across each LED driver. Thus a technical problem of unbalance voltage developed on the two LED drivers, in case they work simultaneously, is overcome.

To better address this concern, the invention is defined by the claims.

According to examples in accordance with an aspect of the invention, there is provided a first LED driver circuit comprising:

a first input interface for receiving an AC power input and providing a first output, wherein said first output is adapted to be connected to a second LED driver circuit;

a first LED driver powered by the first output;

a first shorting element for shorting the first output;

a first control arrangement for controlling the first shorting element wherein the first control arrangement is adapted to close the first shorting element during only one subset of half cycles of the AC power input to short circuit the first LED driver and to prevent the first LED driver from being powered by the AC power input, when the second LED driver circuit is powered by the AC power input complementarily with respect to the unpowered first LED driver and, open the first shorting element during the other subset of half cycles to allow the AC power input to power the first LED driver, when the second LED driver circuit is not powered by the AC power input complementarily with respect to the first LED driver; wherein the half cycle is the positive amplitude duration or the negative amplitude duration of the AC power input.

This driver circuit is able to be controlled to draw power only during the other subset of half cycles (i.e. only the positive phases or the negative phases) of an AC power input, while in the one subset of half cycles the first output is short-circuited thereby disabling the first LED driver and allowing the power delivered to a second LED driver circuit. The first LED driver may be connected in series with the LED driver circuit which acts alternatively/complementarily with respect to the first LED driver, namely drawing power from the AC power input during the subset of half cycles and being disabled during the other subset. The problem of a shared voltage across a series string of LEDs is thus avoided. In this way, two LED drivers may operate in a time division manner, and stability and balancing issues are overcome. The LED driver (of each driver circuit) may implement closed loop feedback control, by which is meant that there is negative feedback from a sensor (such as a current sensor) for controlling a current or power delivered to the load.

In a further embodiment, the first LED driver circuit may further comprise:

a first buffering capacitor between the first shorting element and the first LED driver; and

a first blocking diode in a forward direction from the first shorting element to the first buffering capacitor.

Many LED drivers need a buffering capacitor before the driver to smooth the input power. If the input of the LED driver is short-circuited, there is a risk that this buffering capacitor is discharged by the short-circuiting. To address this concern, the blockin

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