LED module and method for operating at least one LED

An LED module (10) for operating at least one LED (18) comprising an input (E1, E2) adapted for coupling to a DC voltage source (UE); an output (A1, A2) adapted for coupling to the at least one LED (18); a filter device (14); and a DC-DC converter (16); wherein the filter device (14) and the DC-DC converter (16) are coupled in series between the input (E1, E2) and the output (A1, A2), and wherein the output (DCA1, DCA2) of the DC-DC converter (16) is coupled to the output (A1, A2) of the LED module (10); wherein the LED module further comprises an electronic switch (Q1), which is configured to couple the DC-DC converter (16) to the input (E1, E2) of the LED module (10) and to decouple it therefrom.

RELATED APPLICATIONS

This is a U.S. national stage of application No. PCT/EP2007/051367, filed on Feb. 13, 2008.

FIELD OF THE INVENTION

The present invention relates to an LED module for operating at least one LED comprising an input for coupling to a DC voltage source, an output for coupling to the at least one LED, a filter device; and a DC-DC converter, wherein the filter device and the DC-DC converter are coupled in series between the input and the output, and wherein the output of the DC-DC converter is coupled to the output of the LED module. The invention moreover relates to a circuit arrangement comprising an electronic ballast, and to a method for operating at least one LED using an LED module.

BACKGROUND OF THE INVENTION

The problem area on which the present invention is based can best be discussed with reference to the accompanyingFIGS. 1 to 3. Thus, in modern LED lighting systems, use is increasingly being made of a multiplicity of parallel-connected LED modules10a,10b, etc., which are operated from a domestic power supply system, which provides the voltage UN, via a central electronic ballast12. UNis 230 V AC, for example, in Germany. The electronic ballast12provides a voltage UEat its output, which voltage can be 24 V DC, for example. In this case, the electronic ballast12is generally short circuit-proof and equipped with a current monitoring system that triggers in the event of overcurrent. After the removal of a possible short circuit on the secondary circuit the electronic ballast12starts up again independently. In order to avoid unintended malfunctions of the system, for example switching off, flicker or periodic flashing during operation, the switch-on current of the parallel-connected LED modules10a,10bmust not reach the overcurrent switch-off threshold of the electronic ballast12.

FIG. 2shows the temporal profile of the input voltage UEof the light modules10a,10bknown from the prior art, and also the temporal profile of the input current IEfor a selected one of said light modules10a,10b. It is established here that the maximum switch-on current IEis 300 mA approximately 2.20 ms after switch-on. In continuous operation, the current TE is only 76 mA. On account of this high switch-on current, the maximum permissible number of light modules10a,10boperated in parallel from an electronic ballast12has to be severely limited.

FIG. 3shows by way of example in a schematic illustration the construction of one of these LED modules10known from the prior art. This LED module comprises an input having a first E1and a second E2input terminal. This is followed by a filter device14comprising an inductance L1and a capacitor C1. The filter device14has the terminals FE1and FE2on the input side and the terminals FA1and FA2on the output side. The filter device14is followed by a DC-DC converter16having input terminals DCE1and DCE2and output terminals DCA1and DCA2. The filter device14serves for ensuring high-frequency decoupling between the electronic ballast12and the DC-DC converter16in order to avoid EMC interference. The output terminals DCA1and DCA2simultaneously form the output terminals A1and A2of the LED module10. By way of example, an LED18is connected thereto. In this case, the LED18can be arranged on the same circuit board as the rest of the circuit, but provision can also be made for leading out the terminals A1, A2from the circuit board on which the LED module10is realized, in order to couple a separately arranged LED18to the LED module10. In this case, the switch-on current IEis essentially limited by the inductance L1and the nonreactive resistance of said inductance L1. This furthermore results in the disadvantage that the operating current brings about a voltage drop at the nonreactive resistance of the inductance L1and, consequently, permanent losses and additional heating occur in the LED module10.

SUMMARY OF THE INVENTION

One object of the present invention is to provide an LED module of generic type arranged in such a way that a larger number of LED modules can be operated from an electronic ballast than is possible in the prior art. Another object is to provide a corresponding method for operating at least one LED.

These objects are achieved by means of an LED module comprising the features of patent claim1and also by means of a method comprising the features of patent claim13. In accordance with a further aspect of the invention, moreover, a circuit arrangement comprising the features of patent claim12is provided.

It is noted that as the input current reaches amplitude values that are all the higher, the lower the input voltage UEat the instant when the DC-DC converter is switched on. Accordingly, if the input voltage UEhas already increased to higher values before the DC-DC converter is switched on, then a switch-on process distinguished by a significantly lower input current can thereby be realized. As a result, it is then possible for significantly more LED modules to be operated from one and the same electronic ballast. Moreover, the inductance L1can thus be dimensioned with no consideration for limiting the input current and thus with lower impedance (lower DC resistance), that is to say with lower losses, than in the prior art.

The above-described insight can be realized the most simply in that an LED module according to the invention furthermore comprises an electronic switch, which is designed to couple the DC-DC converter to the input of the LED module and to decouple it therefrom. A delayed switch-on of the DC-DC converter can thus be realized in a simple manner.

A particularly preferred embodiment is distinguished by the fact that it furthermore comprises a timing element, which is designed to bring about switching of the electronic switch that is temporally delayed depending on the amplitude of the signal at the input of the LED module. What can be realized as a result of this is that the electronic switch couples the DC-DC converter to the input of the LED module automatically only when a sufficiently high input voltage is available. The presence of a sufficiently high input voltage subsequently has the consequence that the switch-on current turns out to be comparatively low. Through suitable dimensioning of the timing element, it is thus possible to define when the DC-DC converter is turned on.

Particularly preferably, the time constant of the timing elements of different light modules which are coupled to one and the same electronic ballast is varied. As a result, the variation of the switch-on instant increases, that is to say that fewer LED modules switch on simultaneously and the peak value of the cumulative switch-on current again turns out to be lower.

Preferably, the electronic switch has a control electrode, an operating electrode and a reference electrode wherein the LED module furthermore comprise a voltage divider having a first and a second nonreactive resistor, wherein the series circuit formed by the first and the second nonreactive resistor is coupled in parallel with the input of the LED module. Preferably, in this case the control electrode of the electronic switch is coupled to the junction point between the first and the second nonreactive resistor. The voltage obtained at the voltage divider is thereby utilized for switching the electronic switch on and off.

Preferably, the timing element is realized by at least one capacitor being coupled in parallel with the second nonreactive resistor. As a result, the outlay on additional components becomes minimal.

Two variants of the invention are presented below, these variants differing in where the electronic switch is arranged.

In accordance with a first variant, the electronic switch is coupled between the input of the LED module and the filter device. In this case, it is particularly preferred if the input of the LED module has a first and a second input terminal, wherein the second input terminal is connected to a reference potential, and the filter device has a first and a second filter device input terminal, wherein the first filter device input terminal is coupled to the first input terminal of the LED module, wherein the reference electrode of the electronic switch is coupled to the second input terminal of the LED module, wherein the operating electrode of the electronic switch is coupled to the second filter device input terminal.

Preferably, a first diode is furthermore provided, which is coupled in parallel with the second nonreactive resistor, and is oriented in such a way that it clamps the control electrode of the electronic switch to the potential of the second input terminal. Polarity reversal protection is thereby realized, which prevents the electronic switch from being destroyed in the event of polarity reversal of the voltage UEon the input side. In normal operation, the diode is operated in the reverse direction and is therefore as it were “invisible”.

In the second variant, the electronic switch is coupled between the filter device and the DC-DC converter. Preferably, in this case the DC-DC converter has a first and a second input terminal, the filter device has a first and a second filter device output terminal, wherein the second filter device output terminal is coupled to the reference electrode of the electronic switch, wherein the operating electrode of the electronic switch is coupled to the second input terminal of the DC-DC converter.

Particularly preferably, a first diode is in turn provided, which is coupled in parallel with the second nonreactive resistor, and is oriented in such a way that it clamps the control electrode of the electronic switch to the potential of the second filter device output terminal. This in turn serves as polarity reversal protection, as already discussed above in connection with the first variant.

The preferred embodiments discussed with regard to an LED module according to the invention and their advantages also hold true, if applicable, for the circuit arrangement according to the invention and the method according to the invention for operating at least one LED.

In a particularly preferred embodiment of a circuit arrangement according to the invention, the components of the timing elements of the LED modules have a tolerance of at least 5%, wherein nonreactive resistors of the timing elements have, in particular, a tolerance of 5% and capacitors of the timing elements have, in particular, a tolerance of 10%.

This dimensioning specification makes it possible to ensure, in a particularly simple manner, that the delay until the respective DC-DC converters of different LED modules are switched on differs from one another. The respective maximum switch-on currents therefore occur at different points in time, whereby the maximum switch-on current to be provided by the electronic ballast is reduced further.

DETAILED DESCRIPTION OF THE DRAWINGS

The reference symbols introduced with regard toFIG. 3will continue to be used for the embodiments of an LED module according to the invention as shown inFIGS. 4aand4bin so far as they relate to identical or similar elements. Therefore, they will not be introduced again.

The embodiment in accordance withFIG. 4ahas a block20comprising an electronic switch Q1. The control electrode S is connected to the junction point between two nonreactive resistors R1, R2, which form a voltage divider. The reference electrode B of the electronic switch Q1is coupled to the second input terminal E2of the LED module10and the operating electrode A of the electronic switch Q1is coupled to the second filter device input terminal FE2. In order to realize a timing element, a capacitor C2is connected in parallel with the nonreactive resistor R2. A diode D1clamps the control electrode S of the electronic switch Q1to the potential of the second input terminal E2. When the supply voltage UEis switched on, the electronic switch Q1is initially at high impedance, and the DC-DC converter16is therefore not in operation. C2is charged via R1slowly with the time constant τ=R1*C2. As soon as the threshold voltage UGSbetween gate and source of the electronic switch Q1, realized as a MOSFET, for the switch-on of the electronic switch Q1has been reached, said switch starts to conduct and thus puts the DC-DC converter16into operation. In one preferred exemplary realization, R1is 51 kΩ, R2is 4.7 kΩ, C2is 2.2 μF, and an Si2316DS was used as the electronic switch Q1.

In the embodiment in accordance withFIG. 4b, the block20is coupled between the filter device14and the DC-DC converter16. In this case, the second filter device output terminal FA2is coupled to the reference electrode B of the electronic switch Q1, and the operating electrode A of the electronic switch Q1is coupled to the second input terminal DCE2of the DC-DC converter16.

Reference will be made below once again toFIG. 2and for the first time toFIG. 5: in the temporal profile of the input voltage UE, shortly after the rise a first bend can be ascertained. This is the instant that identifies the switch-on of the DC-DC converter16. The time period t1, identifying the time duration from the switch-on of the LED module10until the switch-on of the DC-DC converter16, is 2.2 ms inFIG. 2, and 31.2 ms inFIG. 5owing to the delay according to the invention. As a consequence, the amplitude of the maximum switch-on current IEdecreases from 300 mA (seeFIG. 2) to 188 mA (seeFIG. 5).

A particularly simple variation of the switch-on instant of a plurality of LED modules10a,10boperated in parallel from an electronic ballast12can be achieved by choosing the components C2and R1with a higher tolerance.

After the switch-on, the operating current flows via the low-impedance drain-source path of the turned-on electronic switch Q1. Through a suitable selection of the transistor, for example the resistance RDS onof the transistor Si2316DS is approximately 50 mΩ, permanent losses are minimized and, consequently, disadvantageous additional heating is avoided in the LED module.