EP3100589A1

EP3100589A1 - Circuit arrangement minimizing reverse voltage drop across a light emitting diode

Info

Publication number: EP3100589A1
Application number: EP14703800.4A
Authority: EP
Inventors: Onder Sunetci; Hakan Ozkahraman
Original assignee: Arcelik AS
Current assignee: Arcelik AS
Priority date: 2014-01-31
Filing date: 2014-01-31
Publication date: 2016-12-07
Also published as: WO2015113616A1

Abstract

The present invention relates to a LED driving circuit (1) comprising at least a first switching element (4) and at least a second switching element (5), each one of the first and second switching elements (4, 5) being in series electrical connection with at least a first and at least a second LED (2, 3) such that said at least first and at least second LEDs (2, 3) in association with each one of the first and second switching elements (4, 5) are independently drivable.

Description

CIRCUIT ARRANGEMENT MINIMIZING REVERSE VOLTAGE DROP ACROSS A LIGHT EMITTING DIODE

The present invention relates to a reverse-voltage protection circuit arrangement in order for protecting a light emitting diode (LED) against reverse voltage drop and therefore preventing failure thereof.
It is well-known that semiconductors such as light emitting diodes (LEDs) are prone to damage from reverse voltages to the extent that the LED will be damaged at the moment power is applied to the same. As such, LEDs require reverse voltage protection necessitating special circuit-level designs.
It is also known that in a matrix format, LEDs might typically be arranged in rows and columns, each LED then being addressable by specifying its location in terms of rows and columns. As microcontrollers have respective pins with a maximum output current, in order for driving multiple LEDs on at once, transistors can typically be used as switching elements to perform control of the power source. As LEDs can be subject to reverse voltage when they are driven in multiplex, a circuit arrangement is needed in this regard to prevent failure of the semiconductor LED elements.
The present invention proposes a circuit design distinguishable in that it provides reverse voltage protection for a given group of LEDs in association with a given switching element in the form of a transistor, whereby the nominal value of the reverse voltage a LED in said group of LEDs can be subjected to is negligible.
Among others, a prior art publication in the technical field of the invention may be referred to as US2011017972, which discloses a light emitting structure including a substrate having a first face, a second face, and a p-n junction formed within the substrate between a p-type layer and an n-type layer, wherein the p-type layer and the n-type layer are adapted as a reverse voltage protection diode. A buffer layer is provided on the substrate, and a light emitting diode (LED) is fabricated on the buffer layer. The LED is then electrically coupled to the reverse voltage protection diode in an anti-parallel diode pair configuration.
It is therefore known that reverse voltage protection for a given LED can typically be ensured by an anti-parallel diode to prevent the reverse breakdown of the LED. Likewise, a second switching element can be used such that the LED is disposed between the two switching elements in series. It is, however, to be noted that these alternative solutions are not preferable as they incur extra manufacturing costs, namely for the additional diodes or the additional switching elements, an additional diode being needed for every LED element and an additional switching element being needed for every existing transistor. Extra costs can be a concern especially considering the required number of additional elements in case of a large LED matrix.
The present invention provides an electrical circuit with a plurality of LEDs driven on/off by a plurality of switching elements, the latter in turn being controlled by a microcontroller, whereby application of reverse voltage to a LED associated with a switching element during its OFF state is prevented, as is provided by the characterizing features defined in Claim 1.
Primary object of the present invention is therefore to provide an electrical circuit with a plurality of LEDs driven on/off by a plurality of switching elements, whereby application of reverse voltage to a LED associated with a given switching element during its OFF state is prevented.
The present invention proposes a LED driving circuit comprising a plurality of switching elements, each one powering a plurality of LEDs. Said switching elements may preferably be bipolar junction transistors (BJTs) triggered on and off by a control signal controlled by a microcontroller.
A reverse voltage limiting resistance is provided between the emitter and the collector of the switching elements, the latter preferably being PNP BJT transistors such that said switching elements are in parallel with the reverse voltage limiting resistance. The value of the reverse voltage limiting resistance is determined such that the LEDs are not drivable by the current flowing through said reverse voltage limiting resistance, that is, the current flowing through the same is not high enough to drive the LEDs associated with said switching element.
Accompanying drawings are given solely for the purpose of exemplifying an electrical circuit having LED reverse voltage protection function, whose advantages over prior art were outlined above and will be explained in brief hereinafter.
The drawings are not meant to delimit the scope of protection as identified in the claims nor should they be referred to alone in an effort to interpret the scope identified in said claims without recourse to the technical disclosure in the description of the present invention.
Fig. 1 demonstrates a simplified circuit diagram of a LED driving circuit in which the current flowing through a turned-on LED complete its path through a not turned-on LED.
Fig. 2 demonstrates a simplified circuit diagram in which the LED driving circuit provides that a voltage value greater than zero is reached at the common anode node according to the present invention.
The following numerals are used in the detailed description:

LED driving circuit
First LED
Second LED
First switching element
Second switching element
Anode node
Reverse voltage limiting resistance

The present invention provides an electrical circuit in the form of a LED driving circuit (1) having a plurality of LEDs (2, 3) driven on/off by a plurality of switching elements (4, 5), the latter in turn being controlled by a microcontroller, whereby application of reverse voltage to a LED (2, 3) associated with a switching element (4, 5) during the OFF state of said given switching element (4, 5) is prevented.
The electrical circuit of the invention conventionally comprises a power stage converting AC voltage to DC voltage and supplying a microcontroller (not shown) and other electrical circuit elements. The LED driving circuit (1) can typically be connected to a digital output of the microcontroller. A LED matrix can be arranged in rows and columns such that a certain number of LEDs are addressable by specifying a certain location in terms of rows and columns. For the sake of simplification, the accompanying figures, solely provided for the purpose of exemplifying the embodiments of the present invention, demonstrate LEDs that are driven on and off in a one-dimensional manner. It is readily apparent to the skilled person that a given LED can be driven on the basis of specific rows and columns.
The LED driving circuit (1) of the present invention comprises at least a first switching element (4) and at least a second switching element (5), each one independently supplying power to at least a first LED (2) and at least a second LED (3). Said switching elements (4, 5) can optionally be bipolar junction transistors (BJTs) triggered on and off by a respective control signal of the microcontroller.
According to the present invention, the transistors can be realized in the form of PNP BJTs, whose emitters are connected to the DC terminal and bases are drivable by a digital output. When a certain number of LEDs (2, 3) are powered by the first switching element (4), the current flowing through a turned-on LED can complete its path through a not turned-on LED in direct electrical connection, i.e. in series with the second switching element (5), in which case a reverse voltage drop is likely to cause damage to a LED (2, 3) that is connected to said second switching element (5) currently not turned on. The current flowing through a turned-on LED completing its path through a not turned-on LED is illustrated by the dashed lines in Fig. 1.
In order for preventing damage to the LEDs (2, 3), a reverse voltage limiting resistance (7) is accordingly provided between the emitter and the collector of the switching elements (4, 5), i.e. the PNP transistors. The fact that a switching element (4, 5) is in parallel with an ohmic reverse voltage limiting resistance (7) provides that in the event said switching element (4, 5) is not in conduction, a voltage value greater than around zero can be reached at the common anode node (6) or the collector node of the switching element (4, 5). This will generate a smaller reverse voltage across a LED (2, 3) in series with the switching element (4, 5). The circuit arrangement of the invention therefore ensures that reverse voltage protection is imparted to a given LED (2, 3) associated with the currently non-conducting switching element (4, 5).
To this end, when a first switching element (4) is turned on, the emitter-collector current flows through the transistor to supply the LEDs (2, 3) being electrically connected to said first switching element (4). Since the currently non-conducting second switching element (4, 5) is also in parallel with a reverse voltage limiting resistance (7), the amount of current flowing through a turned-on LED (2, 3) supplied by the first switching element (4) and completing its path through a not turned-on LED (2, 3) associated with the second switching element (5) will be determined by the electrical potential at the anode node (6), the value of which is in turn determined by the reverse voltage limiting resistance (7) according to the invention.
If the second switching element (5) is not conducting at this time, the voltage drop across the reverse voltage limiting resistance (7) will determine the voltage value at the common anode node (6). It is to be noted that the value of the reverse voltage limiting resistance (7) is determined such that the LEDs (2, 3) are not driven on. In other words, the amount of resistance is selected such that the current flowing through the same is not sufficient to drive the LEDs (2, 3) associated with said second switching element (5).
In a nutshell, the present invention proposes a LED driving circuit (1) comprising at least a first switching element (4) and at least a second switching element (5), each one of the first and second switching elements (4, 5) being in series electrical connection with at least a first and at least a second LED (2, 3) such that said at least first and at least second LEDs (2, 3) in association with each one of the first and second switching elements (4, 5) are independently drivable.
The present invention further proposes that each one of the first and second switching elements (4, 5) are in parallel with a reverse voltage limiting resistance (7) disposed between a DC terminal and a common anode node (6) of said at least first and at least second LEDs (2, 3).
In a further embodiment of the invention, the switching elements (4, 5) are bipolar junction transistors (BJTs) triggered on and off by a respective control signal.
In a still further embodiment, the bipolar junction transistors’ (BJTs) emitters are connected to a DC terminal and bases are drivable by a control signal.
In a yet still further embodiment, said reverse voltage limiting resistance (7) is provided between the emitter and the collector of the switching elements (4, 5), the latter being PNP BJT transistors.
In a yet still further embodiment according to the present invention, the value of the reverse voltage limiting resistance (7) is determined such that the LEDs (2 ,3) are not drivable by the current flowing through the reverse voltage limiting resistance (7). The amount of resistance in the reverse voltage limiting resistance (7) is selected such that the current flowing through the same is not sufficient to drive the LEDs (2, 3) associated with the switching element (4, 5).
The present therefore affords a LED driving circuit (1) having LED reverse voltage protection function while at the same time minimizing extra costs without compromising the functional effectiveness compared to alternative solutions.

Claims

A LED driving circuit (1) comprising at least a first switching element (4) and at least a second switching element (5), each one of said first and second switching elements (4, 5) being in series electrical connection with and powering at least a first and at least a second LED (2, 3) such that said at least first and at least second LEDs (2, 3) in association with each one of the first and second switching elements (4, 5) are independently drivable characterized in that,

the switching elements (4, 5) are in parallel with a reverse voltage limiting resistance (7) disposed between a DC terminal and a common anode node (6) of said at least first and at least second LEDs (2, 3).
A LED driving circuit (1) as in Claim 1 characterized in that the switching elements (4, 5) are bipolar junction transistors (BJTs) triggered on and off by a control signal.
A LED driving circuit (1) as in Claim 2 characterized in that said bipolar junction transistors’ (BJTs) emitters are connected to the DC terminal and bases are drivable by a control signal.
A LED driving circuit (1) as in Claim 3 characterized in that said reverse voltage limiting resistance (7) is provided between the emitter and the collector of the switching elements (4, 5), the latter being PNP BJT transistors.
A LED driving circuit (1) as in Claim 1 characterized in that the value of the reverse voltage limiting resistance (7) is determined such that the LEDs (2 ,3) are not drivable by the current flowing through the reverse voltage limiting resistance (7).
A LED driving circuit (1) as in Claim 5 characterized in that the amount of resistance in the reverse voltage limiting resistance (7) is selected such that the current flowing through the same is not sufficient to drive the LED (2, 3) associated with the switching element (4, 5).