EP2897444B1 - Circuit for LED current supply - Google Patents

Description

The invention relates to a circuit for an LED power supply, in particular for the electrical supply of an arrangement of one or more LEDs connected, for example, in series (light-emitting diodes, which also include organic light-emitting diodes).

LEDs are being used to an ever greater extent for lighting equipment such as indoor and outdoor lights. Circuit arrangements, so-called LED drivers, are known for the electrical supply of the LEDs. In order to be able to operate different LED arrangements, different types of LEDs or LEDs with different brightness, a circuit arrangement for power supply is often provided in the prior art, which has several connection terminals, each connection terminal being intended to deliver a predetermined current intensity (see e.g. Figure 1a ).

DE 10 2013 206 309 A1 discloses the operation of lighting means, in particular LEDs, with a circuit which provides a constant average controlled first current at a pair of connecting terminals for a first LED channel and a second circuit control which provides a regulated output current for a second LED channel.

EP 0 660 648 A1 discloses a dimming circuit for an LED array. A regulated output current for an arrangement of LEDs connected in parallel and in series is provided at a pair of connections.

" Optotronic LED Power Supply Product Data Sheet OTe 25 / 220-240 / 420 CS Constant Current LED Power Supply 290mA-350mA-420MA ", July 23, 2013 , discloses a circuit for LED power supply according to the preamble of claim 1.

A disadvantage of these LED drivers, however, is that the supply current for the LEDs through the driver arrangement can only be selected from a limited number of permanently specified current intensities.

Other approaches from the prior art provide for the LED driver to be provided with a software interface so that the current source in the LED driver can be set electronically. However, this solution requires a lot of effort both on the hardware side, in order to provide a freely programmable power supply, and on the software side, in order to be able to program the driver as required.

Further approaches in the state of the art provide for an LED driver to be equipped with dip switches or with wire bridges, which make it possible to select different currents. The disadvantage of this arrangement, however, is that only a limited number of discrete current intensities can be selected.

The object of the present invention is to provide a circuit for LED power supply which can implement setting of the supply current intensity for an arrangement of at least one LED in the simplest possible way.

To achieve the object, the invention provides a circuit for LED power supply which comprises: an electrical ballast for connecting an arrangement of at least one LED, the electronic ballast having a first connection terminal to which the arrangement of at least one LED is connected an electrode, preferably the anode, is connected, and has a plurality of second connection terminals, to which the arrangement of at least one LED with the second electrode, preferably with the cathode, can optionally be connected to the at least one LED with one of the selected second connection terminal corresponding To supply current strength, wherein a trimming resistor is also provided, which is connected in parallel to two of the second connection terminals in order to set the selected supply current strength.

According to the invention, a trimming resistor is connected in parallel between two second connection terminals, at least one of which is not occupied by the second electrode of the LED arrangement. The current intensity is influenced by the trimming resistor, which is made available by the electronic ballast at the selected connection terminal. In this way, the current intensity for supplying the at least one LED can be set very easily. The restriction to the discrete current strengths specified by the electronic ballast is thereby overcome without having to influence the electronic ballast itself.

According to a preferred embodiment, the trimming resistor is connected between the selected second connection terminal, to which the second electrode of the arrangement of at least one LED is applied, and a further second connection terminal, which is at a lower or higher potential than the selected connection terminal. In one embodiment, for example, the anode of the LED arrangement lies against the first connection terminal and the cathode of the arrangement comprising at least one LED lies against the selected second connection terminal. In this embodiment, the trimming resistor is connected between the selected second connection terminal and a connection terminal with a lower potential. Depending on the size of the trimming resistor, the supply current strength in this embodiment is set to a value which is between the current intensity is which corresponds to the current intensity of the selected second connection terminal and the one further second connection terminal to which the trimming resistor is clamped. If, for example, the current intensity of the selected second connection terminal corresponds to 200 mA and the further second connection terminal to which the trimming resistor is connected corresponds to a current intensity of 700 mA, the supply current intensity can be between 200 mA (trimming resistor very high) and 700 mA, depending on the size of the trimming resistor mA (trim resistance very low) can be set.

Of course, the invention can also provide an electronic ballast in which the connection terminal for the anode of the arrangement can be selected at least one LED. In this case, in the embodiment cited above, the trimming resistor is applied between the selected second connection terminal and a further second connection terminal which has a higher potential than the selected second connection terminal.

According to a preferred embodiment the trim resistor is adjustable, e.g. in the form of a rotary or slide potentiometer. In this embodiment, the supply current strength can be set in a particularly simple manner. If, for example, the trimming resistor is connected to the selected second connection terminal and a further second connection terminal with a higher or lower potential, and the resistance can be adjusted from almost zero to almost insulating, any supply current intensity between the current intensity of the selected one can be set by adjusting the adjustable trimming resistor corresponds to the second connection terminal and the current intensity which corresponds to the further second connection terminal can be set.

According to a preferred embodiment, the electrical ballast has at least three, four, five, six, seven or eight second connection terminals. Although the invention can already be implemented with two second connection terminals, a larger number of second connection terminals is advantageous because this allows the upper and lower limit values for the adjustable supply current to be selected in a larger range or with a finer gradation.

According to a preferred embodiment, the electronic ballast is set up in such a way that the second connection terminals each supply a constant current intensity through the at least one LED. For this purpose, the electronic ballast acts as a constant current source, the constant current source making available different but constant current intensities at the respective second selectable connection terminals via a voltage divider. The one or more LEDs are preferably supplied with a constant current source (in contrast to a constant voltage source) in order to be able to operate different numbers of LEDs or LEDs of different construction types. Furthermore, LEDs can also age over time, which changes their internal resistance. By supplying a constant current (as opposed to a constant voltage), the aging effect then has little or no effect on the brightness of the LED.

For example, depending on the selected second connection terminal, the constant current intensity can be a current between 10 mA and 5000 mA, preferably between 30 mA and 1500 mA, e.g. between 200 mA and 700 mA. The currents mentioned can e.g. can be tapped with five different intermediate values from the electronic ballast. The trimming resistor enables a further setting option between the respective constant currents mentioned.

According to a preferred embodiment, the arrangement of at least one LED comprises a row of several LEDs, the first and the second electrode of the arrangement being defined by the outermost electrode of the first or the last LED within the row. It is also possible for one or more LEDs or rows of LEDs to be connected in parallel. In each case, the common anode or the common cathode of the first or last LED in the multiple LED rows connected in parallel forms the first or second electrode of the arrangement.

The arrangement of several LEDs in a row has the advantage that the supply voltage for the LEDs is higher, so that the power loss due to the ohmic resistance in the supply lines is lower. Even if an LED should fail within a row of LEDs, this does not necessarily lead to the failure of the entire row of LEDs, because in the case of a defective LED, a short circuit is usually formed across the defective LED. For safety, however, several rows of LEDs can be provided in parallel, so that at least the LEDs in the remaining parallel rows continue to operate if one LED in a row causes an interruption due to a defect. The arrangement of one or more LED rows can be operated on the power supply according to the invention, regardless of the number of LEDs within the rows, because the power supply provides a constant supply current in accordance with the selected connection terminals and the trimming resistor.

According to a preferred embodiment, the plurality of second connection terminals in the electronic ballast are each arranged at a node between a row of resistors, a voltage dropping across the row of resistors. The supply voltage at the first connection terminal for supplying the connected arrangements of at least one LED can be regulated such that a constant current intensity always flows between the first connection terminal and the selected second connection terminal. For this purpose, the voltage for the LED supply at the first connection terminal is readjusted according to the measured voltage drop across the series of resistors, so that the current through the arrangement of at least one LED remains constant. If different second connecting terminals are selected for connecting the second electrode of the LED arrangement, while the voltage drop is always measured across the whole series of resistors, a correspondingly different constant current is established through the LED arrangement. This corresponds to a constant current source with several selectable discrete currents. In connection with the further development according to the invention, however, the advantage is achieved that the constant current intensity can be freely adjusted over a wide range with the aid of the trimming resistor.

According to a preferred embodiment of the invention, the trimming resistor can also be formed by a low-resistance resistor, in particular by a wire bridge. This embodiment can be particularly simple and inexpensive, e.g. manufacture in a robot assembly system.

According to further preferred embodiments, more than one trimming resistor can be provided between two of the second connection terminals. In this way, there are even more options for adjusting the current through the LED arrangement for a given electrical ballast. For example, the multiple trimming resistors can also be formed by multiple wire bridges.

Figuren 1a und b

zeigen im Vergleich einen LED-Treiber, welche eine LED ohne und mit Trimmwiderstand betreiben, wobei Figur 1b eine erste Ausführungsform der Erfindung zeigt.

Figur 2

zeigt eine Schaltung zum Betreiben einer LED-Anordnung mit Trimmwiderstand.

Figur 3

zeigt eine Schaltung zum Betreiben einer LED-Anordnung in verschiedenen Konfigurationen mit und ohne Trimmwiderstand.

Figur 4

zeigt eine weitere Schaltung zum Betreiben einer LED-Anordnung.

Figur 5

zeigt eine weitere Schaltung zum Betreiben einer LED-Anordnung, wobei der Trimmwiderstand durch eine Drahtbrücke gebildet ist.

Figur 6

zeigt eine weitere Schaltung zum Betreiben einer LED-Anordnung mit zwei Trimmwiderständen, die als Drahtbrücken ausgeführt sind.

Further features and advantages of the invention will become clear from the following description of a preferred embodiment, which is described in connection with the accompanying figures. The figures show the following:

Figures 1a and b

show in comparison an LED driver that operates an LED with and without a trimming resistor, with Figure 1b Figure 3 shows a first embodiment of the invention.

Figure 2

shows a circuit for operating an LED arrangement with a trimming resistor.

Figure 3

shows a circuit for operating an LED arrangement in various configurations with and without a trimming resistor.

Figure 4

shows a further circuit for operating an LED arrangement.

Figure 5

shows a further circuit for operating an LED arrangement, the trimming resistor being formed by a wire bridge.

Figure 6

shows a further circuit for operating an LED arrangement with two trimming resistors, which are designed as wire bridges.

Referring to the Figures 1a and 1b an LED driver 4 is shown, to which an LED 2 is connected. It should be understood that the LED 2 only symbolically represents an arrangement of LEDs, which can also include several LEDs in a series or parallel connection.

The LED driver 4 has a first connection terminal (LED +) to which the anode of the LED 2 is connected. Furthermore, the LED driver 4 has several, in the example shown, four second connection terminals LED, to which the cathode of the LED 2 can optionally be connected. Every second connection terminal LED- is designed to provide a constant current to supply the LED, with the connections providing discrete currents of 200 mA, 350 mA, 500 mA and 700 mA. In the figure 1a the connection terminal with 500 mA is selected to connect the cathode of LED 2. Accordingly, the LED is supplied with a current of 500 mA.

In the Figure 1b the LED is connected between the connection terminal LED + and the connection terminal LED- with 200 mA. Furthermore, a _trim resistor R _{Trim is} connected in parallel between the connection terminal LED- with 200 mA and the connection terminal LED- with 700 mA. Without the trimming resistor, a current of 200 mA would flow through the LED 2. However, the current strength is increased by the trimming resistor R _Trim . The current intensity through the LED is between 200 mA and 700 mA, depending on the height of the trimming resistor. By choosing the _trim resistor R _Trim , the supply current through the LED can therefore be adjusted. According to one embodiment, the trimming resistor R _{Trim can be} an adjustable potentiometer which can be adjusted between a very low-resistance and a very high-resistance value. This allows the current intensity through the LED to be freely set from 200 mA to 700 mA.

Referring to the Figure 2 is shown a circuit diagram according to the invention. The electronic ballast 4 (in the Figures 1a and 1b referred to as LED driver 4) includes as in Figures 1a and 1b a first connection terminal LED + for connecting the cathode of an LED arrangement 2 and four second connection terminals LED-, which provide discrete currents of 200 mA, 350 mA, 500 mA and 700 mA. The LED arrangement 2 is in the Figure 2 shown as a row of several LEDs. The anode of the first LED in the LED row is connected to the connection terminal LED +, while the cathode of the last LED in the row is connected to the connection terminal for 200 mA. In addition, a _trim resistor R _{Trim is} connected in parallel between the connection terminal for 200 mA and the connection terminal for 700 mA. As in connection with the Figure 1b explained, the current intensity for supplying the LED arrangement can be set between 200 mA and 700 mA by selecting the trimming resistor.

The electronic ballast acts as a constant current source. For this purpose, the electronic ballast measures the voltage drop across a series of resistors R ₁ , R ₂ , R ₃ and R ₄ , ie between the first connection terminal at U _{I_sense} and GND. Depending on the measured voltage drop across the resistor chain, the voltage at the connection terminal LED + is readjusted so that a constant current of 200 mA flows through the LED arrangement 2. The electronic ballast also sees further connection terminals at the nodes in the resistor chain R ₁ to R ₄ . If the LED arrangement with the cathode is connected to one of these further connection terminals, a correspondingly lower voltage drop is measured over the entire chain of resistors R ₁ to R ₄ , so that the current control leads to a higher supply current flowing through the LED arrangement 2, leads.

The following generally applies to the current through the LED arrangement 2: $${\mathrm{I.}}_{\mathit{LED}}=\frac{U_{\mathrm{I.}\_\mathit{sense}}}{\mathrm{R.}}$$

The resistor R is shown in the circuit arrangement Figure 2 formed by the parallel connection of (R ₁ + R ₂ + R ₃ ) with R _Trim and the resistor R ₄ in series. The following therefore applies to the current through the LEDs $${\mathrm{I.}}_{\mathit{LED}}=\frac{U_{\mathrm{I.}\_\mathit{sense}}}{{\mathrm{R.}}_{\mathrm{4th}}+\frac{\left({\mathrm{R.}}_1+{\mathrm{R.}}_2+{\mathrm{R.}}_3\right)\ast {\mathrm{R.}}_{\mathit{Trim}}}{{\mathrm{R.}}_1+{\mathrm{R.}}_2+{\mathrm{R.}}_3+{\mathrm{R.}}_{\mathit{Trim}}}}$$

It is easy to see from the formula that for a very low-ohm _trim resistor, ie R _Trim << (R ₁ + R ₂ + R ₃ ), there is a supply _current through the LED I _LED = U _{I_sense} / R ₄ . With a very high-resistance trimming resistor, ie R _Trim >> (R ₁ + R ₂ + R ₃ ), the result is a current through the LED of I _LED = U _{I_sense} / (R ₁ + R ₂ + R ₃ + R ₄ ). The current can therefore be set between 200 mA and 700 mA according to the upper and lower connection terminal for the cathode of the LED arrangement.

Figure 3 shows the circuit arrangement Figure 2 , the circuit with a trim resistor is shown in the last column, while a circuit without a trim resistor is shown in columns I ₁ to I ₄ .

Without a trimming resistor, a current of 200 mA, 350 mA, 500 mA or 700 mA can be selected depending on the selection of the connection terminal for the cathode of the LED arrangement. In the circuit according to the invention with a trimming resistor, the current can be dependent on the trimming resistor, as in connection with the Figure 2 explained between 200 mA and 700 mA can be freely set depending on the value of the trimming resistor in relation to the resistor R ₁ + R ₂ + R ₃ .

In the embodiments of the invention shown in the figures, the cathode of the LED arrangement is connected to the same connection terminal as one end of the trimming resistor. This is not absolutely necessary. The trimming resistor can also be connected between two other connection terminals of the electronic ballast, which are provided for the cathode of the LED arrangement. The current strength available for supplying the LED can be calculated accordingly from the series or parallel connection of the resistors R ₁ to R ₄ and R _Trim , as shown above.

The LED arrangement 2 in the Figures 1a and 1b consists of only one LED while in the Figures 2 and 3 an arrangement 2 of several LEDs in series is shown. It is to be understood, however, that the LED arrangement 2 can generally be formed from a series and / or parallel connection of several LEDs. The arrangement in a row has the advantage that a higher voltage can be used to supply the LEDs, so that the power loss due to the ohmic resistance of the supply cables is reduced. The parallel connection has the advantage that in the event of a defect in an LED that causes the circuit to be interrupted, the other LEDs connected in parallel remain in operation. Since the electronic ballast regulates to a constant current, it is irrelevant in the context of the maximum supply voltage made available how many LEDs are connected in series to the electronic ballast. If LEDs are connected in parallel, the supply current is increased accordingly. This can be done by selecting the second connector and the strength of the trim resistor.

According to the invention, electronic ballasts can be provided with more or fewer connection terminals for the cathode of the LED arrangement. In the simplest embodiment, only two cathode connection terminals are required. However, electronic ballasts with more than four connection terminals, as shown in the figures, can be used.

Furthermore, the invention is not limited to electronic ballasts which only provide different connection terminals for the cathode. It is also possible to use ballasts to use, which instead or in addition provide several connection terminals for the anode of the LED arrangement.

The trimming resistor can be switched between connection terminals with a lower or higher potential than the connection terminal selected for the LED arrangement 2 in order to increase or decrease the LED supply current.

In Figure 4 shows a further embodiment of a circuit arrangement with a trimming resistor, the trimming resistor between two second connecting terminals, which are in the Figure 4 designated as 23 and 27 is connected. The LED arrangement 2 is connected between the first connection terminal LED + and a selected second connection terminal, which in this example forms the terminal 22. In contrast to the embodiment according to Figure 1b In this embodiment, the trimming resistor is not connected to the selected second connection terminal to which the one electrode of the LED arrangement is connected.

Figure 5 shows a further circuit arrangement with a trimming resistor, which in this embodiment is formed by a wire bridge. This embodiment represents the special case that the trimming resistor is low-resistance.

In the embodiment according to Figure 6 two trimming resistors are connected between two second connection terminals. The trimming resistors in this embodiment are also low-ohmic, ie formed by a wire bridge.

List of reference symbols:

2

Arrangement of at least one LED

4th

electronic ballast

LED +

first connection terminal

LED

second connection terminal

R _Trim

Trim resistance

R ₁ to R ₄

Resistors in the electronic ballast

U _{I_sense}

Voltage drop across R ₁ to R ₄

I _LED

Current through the arrangement of at least one LED

Claims (8)

1. An LED power supply circuit comprising:

   an electric ballast (4) for connection of an array (2) of at least one LED, the electronic ballast having a first connection terminal (LED+) to which the array (2) of at least one LED is connected with an electrode, preferably the anode, and a plurality of second connection terminals (LED-), to which the array (2) of at least one LED can be selectively connected with the second electrode, preferably to the cathode, in order to supply the array of at least one LED with a discrete current intensity corresponding to the selected second terminal (LED-) from the plurality of second terminals, the supply current area being a constant current intensity;

   characterized in that the plurality of second terminals (LED-) in the electronic ballast (4) are each located at a respective node between a series of resistors (R1 to R4) forming part of the ballast (4), the circuit further comprising a trimming resistor (R_Trim) connected between two of the plurality of second terminals (LED-), wherein the circuit is arranged to further adjust the selected discrete supply current intensity by means of the trimming resistor, the electronic ballast being arranged to measure a voltage (U_{I_sense}) dropping across the series of resistors (R1 to R4) for controlling the adjusted discrete supply current intensity for the arrangement (2) of at least one LED.
2. The circuit for LED power supply according to claim 1, wherein the trimming resistor (R_Trim) is connected between the selected second terminal (LED-) to which the second electrode of the array (2) of the at least one LED is applied and a further second terminal (LED-).
3. The circuit for supplying LED power according to one of the preceding claims, wherein the trimming resistor (R_Trim) is adjustable.
4. The circuit for LED power supply according to one of the preceding claims, the electrical ballast (4) having at least three, four, five, six, seven or eight second terminals (LED-).
5. The circuit for LED power supply according to claim 4, wherein the constant current intensity provides a current between 10 mA and 5000 mA, preferably between 30 mA and 1500 mA, depending on the selected second terminal (LED-).
6. The circuit for supplying LED power according to any of the foregoing claims, comprising a plurality of trimming resistors (R_Trim) each connected between two of the second terminals (LED-) to further adjust the supply current intensity.
7. The circuit for LED power supply according to one of the preceding claims, wherein the one or, when referring back to claim 6, several of the trimming resistors (R_Trim) are low-impedance, in particular formed by a wire bridge.
8. A system comprising a circuit for supplying power to LEDs according to one of the preceding claims and the arrangement of the at least one LED, wherein the arrangement of the at least one LED comprises a row of several LEDs, wherein the first and second electrodes are defined by the outermost electrodes of the first and the last LED in the one row.

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