TECHNICAL FIELD

The invention relates to an LED circuit, and more specifically to an LED circuit protected against surge currents. BACKGROUND ART

Penetration of LED lighting to both indoor and outdoor markets took place in the past years. However, high failure rate of LED panels and, therefore, field returns within warranty period are common. Surges are identified as a primary root cause of LED failures.

It is common for LED circuits to be produced on metal-core printed circuit boards (MCPCB) that can handle the occuring heat loads and transfer them efficiently to a heatsink. The so-called heat spreader metal polygon that surrounds an LED is attached to a thermal pin and to one of the electrodes of an LED, either an anode or a cathode. This heat spreader is isolated from the metal substrate of the MCPCB by a thin layer of isolation.

Therefore, the heat spreader area forms a parasitic capacitor one electrode of which is a metal part of the heat spreader and another electrode is a part of the metal substrate of the MCPCB.

When the common mode surge is applied, the parasitic capacitors are getting charged and high peak current flows through the LEDs. This high peak current may damage the LEDs. In particular, common-mode surges may pass through a Y-capacitor providing a path for this current from the primary side to the secondary side of an LED driver and are applied to the LED circuit. Further, the surge current is conducted via the parasitic capacitance to the ground connection of the circuit board. The LEDs are in the path of the surge current, because the heat spreaders are electrically connected to the LEDs.

WO 201 1/158196 Al describes a light- emitting apparatus including a substrate, an LED string mounted on the substrate, in which a plurality of light emitting diodes are connected in series, a power supply line connected in series to the LED string, and a plurality of protection capacitors, which are arranged between the power supply line and a connection between the LEDs, wherein one half of the protection capacitors is connected to the positive power supply line and the other half is connected to the negative power supply line.

SUMMARY OF THE INVENTION

It may be considered an object to provide an LED circuit with reduced production costs of the LED circuit.

It would be advantageous to have a surge protection integrated into the LED circuit in a way to easily allow stacking of several LED circuits while maintaining the surge protection.

It may also be considered an object to provide an LED circuit, wherein the

LEDs are efficiently protected against damage caused by current surges.

Finally, it may be considered an object to provide an LED circuit that is particularly compact and to reduce the complexity of the circuit layout.

To better address one or more of these concerns, in a first aspect of the invention an LED circuit according to claim 1 and an LED panel according to claim 12 are presented. Dependent claims refer to preferred embodiments of the invention.

The invention is based on a new understanding of failures of LEDs in a LED circuit due to surge phenomena. The inventors have realized that LEDs can withstand forward surge currents which are at least one order of magnitude higher than reverse surge currents which are strong enough to damage the LEDs. The LEDs are mainly getting damaged by reverse current when reverse voltage that exceeds the specification of the LEDs is applied. The reverse current is an avalanche breakdown current that appears when the reverse voltage is applied. Also, an internal TVS diode, which may be part of a high-power LED, is not strong enough to clamp the reverse surge current down to a safe reverse voltage level. On the other hand, medium-power LEDs do not have an internal TVS diode and therefore tolerate no reverse current at all.

The present inventors have recognized that surge protection components connected to an electrical discharge path may be used to avoid damage, and that the number of surge protection components does not have to be equal to the number of LEDs. As will become apparent, the connection of a surge protection component to every node that contains a parasitic capacitance is not required to protect each of the LEDs from current surges.

According to the invention, an LED circuit is provided with a multiple number of LED arrangements connected in series at interconnection terminals on a circuit board, wherein each LED arrangement comprises at least two LEDs connected in series.

Additionally, an electrical discharge path is arranged in parallel to the series connection of LED arrangements. To protect the LEDs in the LED circuit from current surges, a surge protection component is arranged between each interconnection terminal and the discharge path, whereas the connection between at least two LEDs of each LED arrangement is isolated from the discharge path.

This setup advantageously allows to easily redirect a surge current away from the LEDs of the LEDs circuit and in particular to prevent any of the LEDs in the circuit to be exposed to a reverse current which could damage the LED. The use of one relatively small and cost-efficient surge protection component per interconnection terminal may allow a reduced number of electronic components, resulting in a more compact circuit layout and allowing a more cost-efficient production of the LED circuit. The connection between at least two of the LEDs of each LED arrangement is isolated from the discharge path, i. e. not provided with a surge protection component. Thus, a reduced number of surge components is required, such as e. g. only half the number of LEDs or less. Further, the invention may allow series connection of several LEDs and could provide surge protection per several LEDs while keeping a good level of protection.

According to the invention, a multiple number of LED arrangements are connected in series at interconnection terminals. Under a multiple number of LED

arrangements, at least two LED arrangements are understood, but preferably more LED arrangements may be connected in series, for example three, four or five, but also a much larger number, such as e. g. more than 10, more than 20 or even 100 or more. It should be recognized that logically grouping the LEDs into LED arrangements still allows free placement, and in particular equidistant placement for all LEDs.

Each LED arrangement according to the invention comprises at least two

LEDs connected in series. Each LED arrangement may comprise more than two LEDs, e. g. three, four or more.. The number of LEDs (n) in each LED arrangement is generally only limited by the ratio between LED forward voltage and allowable reverse voltage. The allowable reverse voltage of each LED in the LED arrangement shall be larger than (n-1) times the LED forward voltage. According to a preferred embodiment of the invention, the number of LEDs in an LED arrangement is less or equal to 10, in particular preferably less or equal to 5, and in an especially preferred embodiment of the invention precisely 2. Generally, an LED arrangement may further comprise other electronic components than LEDs. According to a preferred embodiment of the invention, an LED arrangement comprises only LEDs which in a particularly preferred embodiment of the invention are all connected in series. In a especially preferred embodiment of the invention, all LED arrangements comprise only LEDs connected in series.

According to one aspect of the invention, at least one connection between two LEDs of each LED arrangement, preferably all connections between LEDs of each LED arrangement are electrically isolated from the discharge path.

In principle, the multiple LED arrangements of the LED circuit can be freely chosen and may differ from one another, in particular with regard to their circuit structure and/or number of LEDs. According to a preferred embodiment of the invention, all LED arrangements of the LED circuit are of identical structure. Such a layout is convenient for mass production and would easily allow to add or remove a certain number of LED arrangements from the LED circuit to adjust the LED circuit for a specific application.

Generally, any kind of LED component, comprising OLEDs, light emitting diodes and any other solid state lighting element, may be chosen for the LED circuit, and each LED arrangement may contain any number of different LEDs. In particular, the LEDs may differ in their size, color or light intensity. Each LED may also have further electronic components integrated or inseparably joint, such as a suppressor diode, in particular an internal suppressor diode. According to a preferred embodiment of the invention, all LEDs of one LED arrangement are identical. Also preferably, the LEDs of one LED arrangement and in particular preferably, all LEDs in the LED circuit are low-, medium- or high-power LEDs.

Each of the interconnection terminals is connected to at least two LED arrangements. Most preferably, the interconnection terminals are each connected to two adjacent LED arrangements.

In the LED circuit, an electrical discharge path is arranged in parallel to the LED arrangements. In general, the electrical discharge path may be formed by any conductive element or even by several, possibly different conductive elements. Preferably, the electrical discharge path is arranged on the circuit board and in particular preferably, the electrical discharge path is comprised of a conductor of the circuit board.

According to another aspect of the invention, each interconnection terminal is connected to the discharge path at least by a surge protection component, effectively isolating the LED arrangements from the neighbouring LED arrangements. Preferably, each LED arrangement is enclosed by surge protection components.

According to one preferred embodiment of the invention, a first terminal of the surge protection component is electrically connected between two adjacent LED

arrangements, in particular to the interconnection terminal of both LED arrangements, and a second terminal of the surge protection component is connected to the discharge path. In a particularly preferred embodiment of the invention, each of the interconnection terminals between two LED arrangements is directly connected to the first terminal of one surge protection component, whereas the second terminal of each of the surge protection components is directly connected to the electrical discharge path.

According to a preferred embodiment of the invention, the discharge path is at one end directly connected to a first end of the LED arrangements connected in series and at the other end connected via a capacitor to a second end of the LED arrangements. In particular preferably, the discharge path is directly connected to the first end in such a way that between each of the surge protection components connected to the discharge path and the end of the discharge path connected to the first end of the LED arrangements no other electric component is arranged, i. e. the discharge path is comprised of a conductor providing direct electrical connection of each of the surge protection components.

As a first end of the LED arrangements, an electrical terminal or connector is understood, which is the first, outmost of the plurality of LED arrangements connected in series. Preferably, the first end is part of, or at least connected to, a positive rail of the LED circuit, wherein the positive rail may be electrically connected to a power source or a driver circuit. Accordingly, the second end of the LED arrangements is the other outmost electrical terminal or connector of the plurality of LED arrangements connected in series, arranged furthest from the first end. Preferably, the second end is part of, or at least connected to, a negative rail of the LED circuit, wherein the negative rail may be electrically connected to ground.

In a preferred embodiment of the invention, the structure of the circuit is such that two interconnection terminals at each LED arrangement are connected to each other via two surge protection components.

According to a preferred embodiment of the invention, the surge protection component is a capacitor. Preferably, the surge protection component is a bypass capacitor electrically connected between one interconnection terminal and the electrical discharge path. According to another preferred embodiment of the invention, all bypass capacitors provided as surge protection components have the same capacitance to ensure that all interconnection terminals stay at almost the same electrical potential. The application of identical capacitors as surge protection components advantageously allows to stack an arbitrary number of LED arrangements without any necessity of adjustments of the surge protection component.

In a preferred embodiment of the invention, a capacitor connecting the discharge path with the second end of the LED arrangements has the same capacitance as the bypass capacitors utilized as surge protection component. In particular, the capacitor connecting the discharge path with the second end of the LED arrangements may be identical to the bypass capacitors.

In another possible embodiment of the invention, the surge protection component is a diode, which is preferably connected with its anode to an interconnection terminal and with its cathode to the electrical discharge path.

In a preferred embodiment of the invention, the circuit board of the LED circuit is a metal-core printed circuit board (MCPCB), which is beneficial due to its high thermal conductivity, in particular in combination with medium- or high-power LEDs.

According to further a preferred embodiment of the invention, an electrically conductive element is electrically connected to each of the LEDs and eletrically isolated from the circuit board. Such a conductive element may e. g. serve as a heat spreader, made for instance out of metal material. Particularly preferred, the electrically conductive element is arranged with a surface in parallel to the surface of the circuit board, in particular at a small distance, in particular less than 1 mm, preferably less than 500 μηι, to the circuit board.

In a particularly preferred embodiment of the invention, the electrically conductive element is separated from a surface of the circuit board by a thin layer of an insulator, for example a thin layer (e.g. 1mm or less) of white plastic isolation or another thin polymer layer.

According to a particularly preferred embodiment of the invention, the electrically conductive element is a heat spreader connected to either an anode or a cathode of an LED and is capacitively coupled to the circuit board. Additionally, the heat spreader may be attached to a thermal pin of the LED to provide a better thermal conductivity between the LED, in particular a high-power LED, and the heat spreader. In general, the heat spreader can be made out of any electrically conductive material and can be built in any conceivable shape. Preferably, the heat spreader is made from material of high thermal conductivity, such as aluminum, copper, silver, gold or mixtures thereof. The heat spreader may for example be a heat spreader metal polygon. Also preferably, the heat spreader has a large surface adapted to the shape of the surface of the circuit board. Preferably, the heat spreader surrounds the LED at least at one side of the LED for an optimal thermal coupling.

According to a preferred embodiment of the invention, each LED is connected to one electrically conductive element and all electrically conductive elements are isolated from one another.

In a possible embodiment of the invention, a conductive part of the circuit board, in particular a large conductive area of a metal-core printed circuit board, and the electrically conductive element, in particular a large area of the electrically conductive element close to the conductive part of the circuit board, arranged at a short distance and electrically isolated by the thin layer of the insulator, form a parasitic capacitor. This parasitic capacitor on one hand provides an additional electrical path between an electrical component connected to the electrically conductive element, in particular the anode or the cathode of a LED connected to the heat spreader of the LED, and the circuit board, in particular an area of the circuit board electrically connected to the ground of the LED circuit. On the other hand, whenever the power state of the LED circuit changes, the parasitic capacitors charge or discharge, contributing to the current in the circuit and possibly creating undesired current surges.

According to the invention, an LED panel is provided, comprising an LED circuit as well as a driver circuit for the LED circuit.

In general, the driver circuit may be any circuit suitable for supplying the LED circuit with electrical operating power. The driver circuit may be separated from the LED circuit or may be arranged on the same circuit board. Preferably, the driver circuit is a separate circuit electrically connected to the LED board. Also preferably, the driver circuit is disposed to provide power to circuit boards with a varying number of LED arrangements. One of the functions of the driver circuit may be to provide a constant current to the LED circuit. Another function of the driver circuit may be to transform the input power from mains power or any other AC or DC power source to a suitable level for the LED circuit. According to a preferred embodiment of the invention, the LED panel comprises a y-capacitor arranged in the driver circuit, forming a path for the current between a primary and a secondary side of the driver circuit to the LED circuit. In such an isolated driver, the function of the y-capacitor may be to reduce or suppress electrical noise, in particular common-mode electrical noise, and/or to provide protection against electrical shocks. In particular, the driver may further comprise an transformer, isolating the primary and the secondary side.

According to another preferred embodiment of the invention, the LED panel comprises a non-isolated driver which is built without a transformer and/or a y-capacitor.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In the figures,

figure 1 shows a schematic circuit diagram in accordance with a first embodiment of an LED circuit;

figure 2 shows two equivalent electrical circuit diagrams for each of both possible directions of current surges in the LED circuit according to figure 1 ;

figure 3 shows a schematic circuit diagram of an LED panel comprising an LED circuit according to the first embodiment as presented in figure 1 and

figure 4 shows a schematic circuit diagram in accordance with a second embodiment of the LED circuit.

DESCRIPTION OF EMBODIMENTS

Figure 1 shows in a schematic representation an electric circuit of a first embodiment of an LED circuit 1 comprising a number of LED arrangements 2 connected in series on a circuit board 4, wherein each of the LED arrangements 2 is comprised of two LEDs 5 connected in series. For connecting two adjacent LED arrangements 2,

interconnection terminals 3 are provided between the LED arrangements 2, connecting the LED arrangements 2 directly in series.

A break in the circuit diagram indicates that the overall LED circuit 1 may contain an arbitrary number of such LED arrangements 2 connected in series. It should be recognized that the strucure of the LED circuit 1 as shown in Fig. 1 is such that all individual LEDs 5 are connected in series. The logical grouping of each two series connected LEDs 5 into an LED arrangment 2 serves, as will become apparent, for illustrating the structure of the circuit 1 regarding surge protection, but does not mean that the placement of the LED components 5 on a circuit board would be such that the LEDs within each LED arrangement are arranged close together, in fact it is preferred that all LEDs 5 are provided equally spaced.

Additionally, an electrical discharge path 6 is provided, connecting a first end 10 of all LED arrangements 2 connected in series with a second end 1 1 of all LED

arrangements 2 connected in series via a capacitor 12, wherein the first end 10 of the LED arrangements 2 is provided to be connected to a positive terminal of a power supply and the second end 1 1 is provided to be connected to a negative terminal of a power supply.

The discharge path 6 is comprised of a single conductor arranged in parallel to the series connection of LED arrangements 2. The capacitor 12 between the second end 1 1 of the LED arrangements 2 and the electrical discharge path 6 is provided to keep all positions of the electrical discharge path 6 at the same electrical potential and to avoid to short-circuit the LED arrangements 2 connected in series.

Each of the LEDs 5 is mounted on a heat spreader metal polygon 13, which is connected to a thermal pin as well as to one of the anode or the cathode of the corresponding LED 5. In particular, in figure 1 the heat spreader metal polygon 13 is connected to the cathode of the LED 5. Further, the heat spreader metal polygon 13 is arranged with a flat surface parallel to a surface area of the circuit board 4.

The circuit board 4 is a metal-core printed circuit board which has a metal surface 14 at positions where a heat spreader metal polygon 13 of an LED 5 can be placed to provide an optimal heat dissipation of the heat emitted from the LED in operation. In particular for an LED circuit 1 comprising high-power LEDs 5, large heat spreader metal polygons 13 and correspondingly large metal surface areas 14 of the circuit board 4 are necessary.

The heat spreader metal polygons 13 and metal surface areas 14 are electrically isolated from one another by a thin layer of a white plastic insulator 15 arranged between each pair of one heat spreader metal polygon 13 and one metal surface area 14 of the circuit board 4.

The two surfaces of each heat spreader metal polygon 13 and the corresponding metal surface area 14 of the circuit board 4 at close distance to one another form a parasitic capacitor 16 with the thin layer of white plastic insulator 15 as the dielectric, wherein those parasitic capacitors 16 provide an electrical path for current surges, in particular reverse surge currents, which would be likely to damage the LEDs 5, between the ends 10, 1 1 of the LED arrangements 2 connected in series and a ground connection of the circuit board 4.

To prevent such current surges from damaging the LEDs 5, at each

interconnection terminal 3 a bypass capacitor 7 is connected as a surge protection component with a first terminal 8 connected to the interconnection terminal 3 and with a second terminal 9 connected directly to the electrical discharge path 6, keeping all interconnection terminals 3 on the same electrical potential and providing an additional path for current surges, wherein no LED 5 or at least no LED 5 in the reverse direction is part of the additional path.

Figure 2 shows two electrical equivalent circuit diagrams for each of both possible directions of current surges in an LED circuit 1 according to the first embodiment as presented in figure 1. The direction of the corresponding current surge is indicated by plus- and minus-signs, figure 2 A corresponding to a reverse current surge and figure 2 B corresponding to a forward current surge.

As illustrated in figure 2 A, one path for a reverse current surge is provided through a parasitic capacitor 16 and a bypass capacitor 7 and a second path is provided through another parasitic capacitor 16, one LED 5b of the two LEDs 5a,b of an LED arrangement 2 and another bypass capacitor 7. The current surge passes the active LED 5b in the forward direction, where the LED 5 can withstand the current surge, whereas the other LED 5a of the LED arrangement 2, where the current surge would pass in the backboard direction, possibly damaging the LED 5, is not in the path of the current surge and therefore protected.

In figure 2 B the path of a forward current surge is illustrated, wherein also one path is provided for the current surge directly through an parasitic capacitor 16 as well as a bypass capacitor 7. A second path is provided through one LED 5 a of the LEDs 5a,b of the LED arrangement 2, which is the other one of both LEDs 5a,b compared to the opposite current surge direction. Also for this direction of the current surge, the current only passes through the active LED 5 a in the forward direction and is therefore unlikely to damage the LED 5.

Figure 3 shows a schematic circuit of an LED panel comprising an LED circuit according to the first embodiment as presented in figure 1 , illustrating the path of a current surge from the primary side 23 of a driver circuit 21 through a y-capacitor 22 to the secondary side 24 of the driver circuit 21 into the LED circuit 1 comprising - in this example only - three LED arrangements 2 with two LEDs 5 each connected in series. The LED arrangements 2 are protected at interconnection terminals 3 by bypass capacitors 7, which are connected to an electrical discharge path 6. The path of the current surges from the LED circuit 1 on the secondary side 24 of the driver circuit 21 back to the primary side 23 is provided by the secondary side ground 25 and the primary side ground 26.

In figure 4, a schematic circuit of a second embodiment of the LED circuit 1 is shown. The only difference between the first embodiment shown in figure 1 and the second embodiment of the LED circuit 1 is that instead of the bypass capacitors 7 protection diodes 7 are provided connecting each interconnection terminal 3 with the electrical discharge path 6.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments.

For example, while in the above described embodiments each LED arrangement is comprised of two LEDs connected in series, such that one surge protection component is provided for two LEDs and the total number of surge protection components corresponds to half the number of LEDs, a different structure could be provided to achieve a different ratio. For example, three LEDs could be connected in series for each LED arrangement, with the connections between the LEDs isolated from the discharge path 7, such that only one surge protection component would be provided for three LEDs, leading to a 1 :3 ratio.

Other variations of the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a"or "an" does not exclude a plurality. A single unit may fulfill the function of several items recited in the claims.

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