EP2624663B1

EP2624663B1 - A LED circuit

Info

Publication number: EP2624663B1
Application number: EP13153604.7A
Authority: EP
Inventors: Serban Bruma; Gert Frederik Jozef Huysmans
Original assignee: Etap NV
Current assignee: Etap NV
Priority date: 2012-02-03
Filing date: 2013-02-01
Publication date: 2015-05-27
Anticipated expiration: 2033-02-01
Also published as: NL2008231C2; EP2624663A1

Description

The invention relates to a LED circuit, comprising a LED module and a driver circuit for feeding the LED module, the driver circuit having a pair of input terminals forming a line input terminal and a neutral input terminal, respectively, and a pair of output terminals forming input terminals of the LED module including a substrate, a heat sink and a multiple number of LED elements connected in series.
LED circuits are commonly known for application in light armatures, e.g. for lighting offices and private houses. Known LED circuits include a LED module with LED elements generating a light beam, and a driver circuit providing the power supply for the LED module. A multiple number of LED elements can be integrated on a common substrate, for cost purposes. In order to cool the LED elements, a heat sink can be provided.
When integrating the LED elements on a common substrate, the LED elements can be provided with a corresponding heat pad that is capacitively coupled to the heat sink. Due to the physical structure of the substrate and the LED elements, the heat pads are also capacitively coupled to the anode and cathode of the corresponding LED elements. In principle, the heat pads can be formed as a single, common heat pad.
The phenomenon of mains voltage surge is mainly due to thunderstorm and/or lightening activity in the atmosphere, and may cause damage to electric equipment. Surge magnitudes and durations are categorized based upon geographical location of the facility, exposure to thunderstorm activity and electrical system location. A surge magnitude of 4 kV is not uncommon. Further, the duration of a surge is typically in a range of tens to hundreds of microseconds.
Patent publications US2010/127625 and EP2290777 describe how mains-carried voltage surges or transients can be absorbed at or redirected from a LED driver input, such as to protect the LED load. Both documents use a combination of varistors and filters.
The protection of electric equipment against voltage surges is highly specialized. Usually, the manufacturer of the supply device feeding the electric equipment takes care of measures against damage to the electric equipment. As an example, a driver circuit may include a surge protection device interconnected between a line input terminal and a neutral input terminal.
It is an object of the present invention to provide a LED circuit according to the preamble that is better protected to the occurrence of mains voltage surges. Thereto, the LED circuit according to the invention comprises a shunt circuit by-passing the capacitive coupling between the anode and cathode of the LED elements on the one hand, and the corresponding heat pad on the other hand, wherein the shunt circuit has a low impedance for transient signals.
The invention is partly based on the insight that protection of the driver circuit may not be enough for protection of the LED circuit as a whole, even if a surge pulse enters the LED circuit via a driver circuit that is protected by a surge protection device. It has been realized that surge caused damage may even occur if the driver circuit does not explicitly include a so-called protected earth terminal (class II type power supply).
The invention is also partly based on the insight that the substrate material forming a capacitor between the heat pad on the one hand, and the anode and cathode of the corresponding LED elements on the other hand, also called substrate capacitor, is typically made from ceramic material having a less controlled thickness, and vulnerable for voltage breakdown when a surge occurs having a voltage magnitude that is substantially larger than usual operational voltage levels.
The invention is further partly based on the insight that the capacitance value of the capacitor between the heat pad on the one hand, and the anode and cathode of the LED elements on the other hand, is typically, although not precisely controlled during manufacturing of the LED module, significantly smaller than the capacitance value of a capacitor between the head pad and the heat sink, and significantly smaller than the capacitance value of a driver capacitor, rendering the substrate capacitor even more vulnerable to damage in the LED circuit when a surge pulse occurs.
By applying a shunt circuit by-passing the substrate capacitor, the substrate is advantageously protected since the shunt circuit forms a parallel path section for the surge pulse travelling in the circuit.
In this context it is noted that the expression "low impedance" means that the absolute value of the optionally complex impedance is smaller than other series impedances that the transient signal encompasses when traveling through the LED circuit, preferably one order of magnitude smaller. Further, the expression "transient signal" means in this context any signal that varies over time in the micro seconds range, e.g. over a time period ranging from circa 10 micro seconds to circa 100 micro seconds, such as a mains voltage surge signal.
Patent publications US6140585 and US4621199 describe how high voltage transients can be diverted from sensitive communication and computer circuits. Both documents use capacitors to create low-impedance diversion or bypass paths for voltage transients.
In an advantageous embodiment according to the invention, the shunt circuit includes a shunt capacitor. Alternatively, the shunt circuit includes an electrically conducting path.
Preferably, the shunt capacitor is dimensioned such that its capacitance is large relative to the capacitance of the substrate capacitor, so that the substrate capacitor is exposed to only a very minor portion of the surge pulse.
In order to improve the protection against a-symmetric surge even further, the LED circuit can be provided with a surge protection device connected between one of the input terminals of the driver circuit on the one hand, and the protective earth terminal on the other hand, thus limiting the surge amplitude on the primary side of the driver circuit.
Additional advantageous embodiments according to the invention are described in the following claims.
By way of example only, embodiments of the present invention will now be described with reference to the accompanying figures in which

Fig. 1 shows a known LED circuit;
Fig. 2 shows a LED circuit according to the invention, and
Fig. 3 shows another LED circuit according to the invention.

Fig. 2

Fig. 3

Figure 1 shows a known LED circuit 1. The LED circuit 1 includes a driver circuit 2 and a LED module 3. The driver circuit 2 has a pair of input terminals 4 forming a line input L terminal 4a and a neutral input N terminal 4b, respectively, and a pair of output terminals 5a,b forming input terminals for the LED module 3. As shown in Fig. 1, the driver circuit 2 typically includes a transformer 6 with a primary and secondary, mutually inductively coupled coil 7a,b for transforming the input voltage to a desired voltage or current feeding the LED module 3.
The driver circuit 2 also includes a surge protection device (SPD) 8 interconnecting the line input L terminal 4a and the neutral input N terminal 4b. The surge protection device 8 limits the voltage between the input terminals 4, e.g. in case a surge occurs between said terminals 4. The voltage protection device 8 can be implemented as a varistor, e.g. a metal oxide varistor (MOV), and is also known as a transient voltage surge suppressor (TVSS). The driver circuit 2 further includes in the electrical circuit representation a driver capacitor C_driver interconnecting an input terminal 4b with an output terminal 5b of the driver circuit 2. The driver capacitor C_driver includes a physical capacitor, implemented for counteracting electromagnetic interference (EMI) effects, and a parasitic capacitor. The capacitance value of the driver capacitor C_driver is typically several nF, e.g. 3 nF.
In the known LED circuit 1, the LED module 3 includes a substrate, a heat sink 10 and a multiple number of LED elements 11a,b connected in series, e.g. 10 LED elements. Fig. 1 shows an electrical circuit representation of the module 3. The cathode 14 of the individual LED elements 11 is connected to the anode 13 of the subsequent LED element 11, forming a chain or cable of LED elements 11. The anode 13a of a first LED element 11 is electrically connected to a first output terminal 5a of the driver circuit 2, while the cathode 14b of a last LED element is electrically connected to a second output terminal 5b of the driver circuit 2. Further, the LED elements 11 are provided with a common heat pad 12. The heat pad 12 is coupled to the heat sink 10 via a heat sink capacitor C₁. Further, the heat pad 12 is coupled to the anode 13a,b and the cathode 14a,b of the LED elements 11 via parallel capacitors represented by a single substrate capacitor C₂.
The known LED circuit 1 also includes a physical earth (PE) connection 15 that is connected to the heat sink 10, either explicit via an electric wire or via other components in the physical construction of the LED circuit 1 (e.g. if no explicit PE terminal is arranged such as the so-called class II type).
When a so-called asymmetric surge voltage 20 occurs between one of the driver input terminals 4 on the one hand, and the physical earth connection PE on the other hand, the surge voltage will pass over by the LED power supply to the LED module 3. Figure 1 shows a possible path 21 that the surge voltage may follow. The path 21 comprises a path section 21a including the driver capacitor C_driver, a path section 21b including a driver output terminal 5a, a path section 21c including the substrate capacitor C₂ and the heat sink capacitor C₁, and a path section 21d including an electrical connection to the physical earth connection PE. When the surge voltage passes the substrate capacitor C2, a breakdown of the LED substrate may occur.
Figure 2 shows a LED circuit 1 according to the invention. Here, the LED circuit 1 comprises in addition a shunt circuit including two shunt capacitors C_Y,1; _Y,2 by-passing the capacitive coupling between the common heat pad 12 on the one hand and the anode and cathode 13, 14 of the LED elements 11 on the other hand. In the shown embodiment, each of the shunt capacitors C_{Y,1; Y,2} is, via one of their terminals, connected to the heat sink 10. The other terminal of the capacitors is connected to an anode 13a or a cathode 14b of the LED elements 11. More specifically, a first shunt capacitor C_Y,1 is connected between the heat sink 10 and a first input terminal 5a of the LED module 3, while a second shunt capacitor C_Y,2 is connected between the heat sink 10 and a second input terminal 5b of the LED module 3. In the shown embodiment, not only the substrate capacitor C₂ is protected against breakdown, but also the heat sink capacitor C₁.
It is noted that, as an alternative, a shunt capacitor C_Y can be interconnected between the heat pad 12 on the one hand and the anode or the cathode of a LED elements 11 on the other hand, thus protecting the substrate capacitor C₂. Further, the number of shunt capacitors C_Y may be chosen otherwise. As an example, the number of shunt capacitors C_Y is one, three or even more, e.g. five.
The capacitive value of the heat sink capacitor C₁ is typically in the order of several nF, e.g. circa 3 nF. Further, the capacitive value of the substrate capacitor C₂ is typically in the order of hundreds pF, e.g. circa 300 pF. The mentioned capacitive values are exemplary. Apparently, the values of the heat sink capacitor C₁ and the substrate capacitor C₂ may deviate, e.g. depending on components, technology and/or design.
Preferably, the capacitance of the shunt capacitor C_Y is at least an order higher than the capacitance of the substrate capacitor C₂, so that, for transient signals, the impedance of the shunt capacitor C_Y is significantly smaller than the impedance of the substrate capacitor C₂. Similarly, the capacitance of the shunt capacitor C_Y is at least an order higher than the capacitance of the driver capacitor C_driver. Then, the shunt circuit has a low impedance for transient signals.
As an additional protection measure, the LED circuit 2 shown in Fig. 2 comprises further surge protection devices (SPD) 30, 31 connected between the line input terminal 4a and the neutral input terminal 4b on the one hand and the protective earth terminal 15 on the other hand.
When the LED circuit 1 of the invention is exposed to a surge voltage 20 occurring between one of the driver input terminals 4 on the one hand, and the physical earth connection PE on the other hand, the surge voltage will pass over by the LED power supply to the LED module 3 via another path 22. The modified path 22 that the surge voltage may follow comprises a path section 22a including the driver capacitor C_driver, a path section 22b including a driver output terminal 5a, a path section 22c including the first shunt capacitor C_Y,1, a path section 22d including the heat sink 10, and a path section 22e,f including an electrical connection to the physical earth connection PE. Advantageously, the modified path 22 does not include the substrate capacitor C₂, thereby reducing the chance to damage of the substrate capacitor considerably. The LED module 3 is now adequately protected. It is noted that, as an alternative route, the modified voltage path may include the path section 22a, a path section including the other driver output terminal 5b, a path section including the second shunt capacitor C_Y,2, a path section including the heat sink 10, and a path section 22e,f including the electrical connection to the physical earth connection PE.
On the primary side, at the input of the driver circuit 2, the SPD's 30, 31 limit the amplitude of the surge voltage. Further, on the secondary side, at the output of the driver circuit 2, the shunt capacitors C_Y shunt the substrate capacitor C₂ of all LED elements 11, so that any electrical stress on the substrate of the LED's is lowered considerably.
Figure 3 shows another LED circuit according to the invention. The common heat pad 12 shown in Figs. 1 and 2 is now replaced by a first heat pad 12' and a second heat pad 12", each being capacitively coupled to the heat sink 10 via a corresponding capacitor C'₁, C"₁. Further, the heat pads 12', 12" are capacitively coupled, via substrate capacitors C'₂, C"₂, to the anode and cathode of corresponding groups of LED elements. Here, the circuit also includes two shunt circuits each including an electrically conducting path 40, 41 by-passing the substrate capacitor C₂. A first electrically conducting path 40 is connected between the first input terminal 5a of the LED module and a first heat pad 12', while a second electrically conducting path 41 is connected between the second input terminal 5b of the LED module and a second heat pad 12". By by-passing the substrate capacitors C'₂, C"₂, via electrically conducting paths 40, 41, a surge voltage surge may follow one or both shunt circuits, so that the substrate capacitors are not exposed to the surge voltages. In order to guarantee proper functioning of the LED module during normal operation, the first heat pad 12'and the second heat pad 12" are mutually electrically isolated, forming a so-called split heat pad design.
More generally, the two shunt circuits are each formed as an electrically conducting path, respectively, wherein the first shunt circuit is connected to a first set of mutually connected heat pads and wherein the second shunt circuit is connected to a second set of mutually connected heat pads. The heat pads may be grouped in at least two common heat pad sets that are mutually electrically isolated. In a first example, a first set of heat pads is formed by a first group of mutually electrically connected heat pads 12', while a second set of heat pads is formed by a second group of mutually electrically connected heat pads 12", as shown in Fig. 3. Then, the first set is electrically isolated from the second set. However, the heat pads may also all be isolated from each other. Each heat pad is then capacitively coupled to the heat sink, and the to the anode and cathode of the corresponding LED element.
The invention is not restricted to the embodiments described herein. It will be understood that many variants are possible.
It is noted that, in principle, a shunt circuit can be omitted in the embodiments shown in Figs. 2 and 3, e.g. to save components. However, then, only one end of the LED elements chain is protected.
It is also noted that one of the shunt circuits shown in Fig. 3 can be replaced by a shunt capacitor described in view of the Fig. 2. Then, the heat pads can be integrated in a single, common heat pad. The shunt capacitor can then be connected to either the heat pad or to the heat sink.
It is further noted that the LED driver circuit may include further components, such as an AC/DC converter for converting the input AC voltage to a DC voltage for feeding the LED module. Further, additional circuitry may be included, e.g. shunt circuits by-passing an individual LED element in case of a defect.
It is also noted that the LED circuit according to the invention can, in principle, also be applied using a driver circuit that is not provided with a surge protection device between the input terminals of the driver circuit.
Further, in accordance with an aspect of the invention, the LED circuit does include a single surge protection device connected between one of the input terminals on the one hand and the protective earth terminal on the other hand. Alternatively, the LED circuit does not include a surge protection device between the input terminals of the driver circuit on the one hand and the protective earth terminal on the other hand, e.g. when the capacitance of the shunt capacitor is so large that a change of damage to the substrate capacitor due to a surge voltage is only minor.
These and other embodiments will be apparent for the person skilled in the art and are considered to lie within the scope of the invention as defined in the following claims.

Claims

A LED circuit, comprising a LED module and a driver circuit for feeding the LED module, the driver circuit having a pair of input terminals forming a line input terminal and a neutral input terminal, respectively, and a pair of output terminals forming input terminals of the LED module including a substrate, a heat sink and a multiple number of LED elements connected in series, characterised in that
the LED elements are provided with a corresponding heat pad that is capacitively coupled to the heat sink and to the anode and cathode of the LED elements, wherein the LED circuit comprises a shunt circuit by-passing the capacitive coupling between the anode and cathode of the LED elements on the one hand, and the corresponding heat pad on the other hand, and wherein the shunt circuit has a low impedance for transient signals.
A LED circuit according to claim 1, wherein the shunt circuit includes a shunt capacitor.
A LED circuit according to claim 1 or 2, including two shunt circuits by-passing the capacitive coupling between the anode and the cathode of the LED elements on the one hand, and the corresponding heat pad on the other hand, wherein a first shunt circuit is connected to a first input terminal of the LED module and wherein a second shunt circuit is connected to a second input terminal of the LED module.
A LED circuit according to claim 2 or 3, wherein the shunt capacitor is connected to the heat sink.
A LED circuit according to any of the preceding claims, wherein the heat pads are electrically mutually interconnected forming a common heat pad.
A LED circuit according to any of the preceding claims, wherein the capacitance of the shunt capacitor is at least an order higher than the capacitance of the assembled capacitance between the heat sink on the one hand and the anode and cathode of the LED elements on the other hand.
A LED circuit according to any of the preceding claims, wherein the driver circuit includes a driver capacitor having a capacitance that is at least an order lower than the capacitance of the shunt capacitor.
A LED circuit according to any of the preceding claims, wherein the shunt circuit includes an electrically conducting path.
A LED circuit according to claims 3 and 8, wherein the two shunt circuits are each formed as an electrically conducting path, respectively, wherein the first shunt circuit is connected to a first set of mutually connected heat pads and wherein the second shunt circuit is connected to a second set of mutually connected heat pads.
A LED circuit according to claim 9, wherein the first set of heat pads forms a first common heat pad capacitively coupled to the heat sink, and wherein the second set of heat pads forms a second common heat pad capacitively coupled to the heat sink.
A LED circuit according to any of the preceding claims, further comprising a protective earth input terminal for connection to the heat sink.
A LED circuit according to claim 11, further comprising a surge protection device connected between the line input terminal or the neutral input terminal on the one hand and the protective earth terminal on the other hand.