EP2342946A1

EP2342946A1 - Circuit arrangement and method for operating an oled

Info

Publication number: EP2342946A1
Application number: EP08875272A
Authority: EP
Inventors: Thomas Siegmund
Original assignee: Osram GmbH
Current assignee: Osram GmbH
Priority date: 2008-11-04
Filing date: 2008-11-04
Publication date: 2011-07-13
Also published as: WO2010051832A1; CN102204406A; US20110204792A1

Abstract

The present invention relates to a circuit arrangement for operating an OLED, comprising a power supply, which is coupled to the OLED in order to supply the latter with power, wherein the circuit arrangement furthermore comprises an OLED voltage measuring device, which is coupled to the OLED and is designed to provide at its output a signal, which correlates with the voltage declining over the OLED, an evaluation device (10), which is coupled to the output of the OLED voltage measuring device and is designed to provide at its output a first signal, if the voltage, declining over the OLED, lies above a specifiable threshold value, and a second signal, if the voltage, declining over the OLED, lies below the specifiable threshold value, an electronic switch (T4) with a reference electrode, a working electrode and a control electrode, wherein the control electrode is coupled to the output of the evaluation device (10), and at least one ohmic resistance (R₇; R₈; R₉; R₁₀), wherein the series connection of the at least one ohmic resistance (R₇; R₈; R₉; R₁₀) and the working electrode - reference electrode segments of the electronic switch (T₄) is connected parallel to the OLED. The invention, moreover, relates to a corresponding method for operating an OLED.

Description

Confession

Circuit arrangement and method for operating an OLED

Technical area

The present invention relates to a circuit arrangement for operating an OLED, comprising a Stromquel- Ie, which is coupled to supply the OLED with the OLED. It also relates to a method for operating an OLED.

State of the art

Commercially available OLEDs consist of millimeter-thin glass substrates, on the back of which the light-emitting material layers and the electrodes for contacting are applied. External mechanical action can break the glass of the OLED. As a result, the current distribution in the carrier and in the applied OLED material layers is inhomogeneous. Depending on the design, a large crack may lead to a reduction in the area through which the current flows, as far as a complete separation of the electrodes or a complete interruption of the current flow.

In such an error case, it comes - viewed from the outside - to an impedance increase of the OLED. In a series connection of several OLEDs, the current is readjusted by the driver to the constant value as a reaction to the disturbance and thus more power is delivered to the defective OLED. Only when a maximum output voltage of the OLED connected in series is exceeded, the driver switches off. An early and rapid detection of such a single OLED defect in a series connection is thus very difficult, since initially only a very small part of the current is obstructed by the crack, which can not be separated from the total current in terms of measurement and additionally during operation Fluctuations due to the OLED parameter dispersion of different batches is subject.

Constant current operation results in the following estimation of the OLED voltages: For a series connection of 50 OLEDs and a total operating voltage of 200 V, a tolerance of approx. 10% overall is to be expected in normal operation. If the maximum output voltage is limited to + 15%, the maximum output voltage is 230 V. The defective OLED is then supplied with 30 V instead of a nominal 4 V. The power consumed at the defective OLED is exaggerated by a factor of 7.5 before the shutdown. The inhomogeneous current flow can lead to a further increase in the power density and to a local overheating of the current-carrying regions of the defective OLED. As a result, specified clearances and creepage distances, insulation regulations and mechanical integrity can no longer be guaranteed. For a complete break with interruption at the crack point even about 200 V on. The insulating light-emitting film can be damaged at these points and the insulation of the OLED can be impaired. In summary, therefore, there is a considerable security risk.

From the prior art, such as the US 20040201985 Al or the data sheet of the component LM3553 from National Semiconductor, are only linear regulated or switched current drivers are known, which are offered for the operation of inorganic LED with input side or output side monitoring a maximum voltage. These are not suitable for eliminating the security risk.

Presentation of the invention

Therefore, the object of the present invention is to further develop a circuit arrangement mentioned at the beginning or a method mentioned at the outset such that the existing security risk when operating OLED can thereby be reduced.

This object is achieved by a circuit arrangement having the features of patent claim 1 and by a method having the features of patent claim 10.

The present invention is based initially on the finding that OLED tolerances and parameter fluctuations add up adversely in a series connection. An improvement is therefore only possible by monitoring a single OLED.

Since the OLED current flowing through the series connection of several OLEDs is always regulated to a constant value, the OLED voltage of a single OLED prepared via a network is therefore analyzed in terms of time by means of a voltage threshold or voltage window. In this case, the setting can be made much more sensitive than monitoring the output voltage of a driver operating the entire series connection of several OLEDs. This makes it possible to detect an OLED break at an early stage, in particular already in the development phase. Local overheating at the OLED fault can be reliably prevented. The circuit can be implemented by means of SMD components and integrated directly (chip on glass) or by means of a flex board on the back of the O1ED or in the mounting frame of the OLED. There is thus no security risk; the relevant safety regulations can be complied with.

Moreover, the electronic switch and the ohmic resistor deactivate the damaged OLED upon detection of an OLED break. Since the current then flows through the series connection of the ohmic resistance and the transistor, the other OLEDs of the series connection of several OLEDs can continue to be operated without interruption.

In a preferred embodiment, the evaluation device of the series circuit comprises a threshold device, an amplifier device and a holding member. In this way, it can be ensured in a particularly simple manner that upon detection of a faulty OLED, the associated electronic switch is permanently driven by a signal which enables bridging of the defective OLED by the series connection of the at least one ohmic resistor and the electronic switch ,

The threshold device preferably comprises a Zener diode. As a result, a voltage threshold can be defined very precisely in a simple manner, which is used to detect an OLED break. The holding member is preferably designed to provide a voltage of a predeterminable amplitude over a predefinable period of time. This allows the control of the electronic switch and thus the bridging of a defective OLED permanently. The remaining OLEDs of the series connection can therefore continue to be operated permanently.

Preferably, therefore, the amplifier device and the holding member are designed to provide at its output a signal, in particular for switching on the electronic switch.

In a preferred embodiment, the evaluation device comprises a voltage divider which is designed to provide at its tap a voltage which is correlated with the voltage drop across the OLED. As a result, it is possible in particular to generate a voltage which can be further processed by a microcontroller. In a preferred embodiment, therefore, the threshold value device, the amplifier device and the holding member is realized by a microcontroller.

For filtering in coupled-in interference and of RF components, it is furthermore preferred if a filter device is coupled between the OLED voltage measuring device and the evaluation device.

In a particularly preferred embodiment, the at least one resistor is dimensioned such that the voltage drop across the series connection of the at least one ohmic resistance and the distance between the working electrode reference electrode of the electronic switch and the supply voltage for the evaluation device, in particular re the holding member, and / or the electronic switch is usable. As a result, no additional power supply for the mentioned components needs to be provided, so that a particularly cost-effective implementation is made possible.

Further advantageous embodiments will become apparent from the dependent claims.

The preferred embodiments presented with reference to the circuit arrangement according to the invention and their advantages apply, as far as applicable, correspondingly to the method according to the invention.

Brief description of the drawings

In the following, embodiments of a circuit arrangement according to the invention will now be described in more detail with reference to the accompanying drawings. Show it:

1 shows a first embodiment of a circuit arrangement according to the invention, realized with discrete components.

2 shows a second exemplary embodiment of a circuit arrangement according to the invention, wherein, however, parts of the evaluation device are realized by a microcontroller;

FIG. 3 an embodiment according to FIG. 1 in a more detailed representation; FIG. 4 shows the time profile for various signals of a circuit arrangement according to the invention, measured on an intact OLED;

5 shows the time profile for various signals of a circuit arrangement according to the invention, measured on a defective OLED; and

Fig. 6 shows the time course for various signals to a circuit arrangement according to the invention measured during load shedding.

In the different embodiments, the same reference numerals are used for the same and similar components. They are therefore introduced only once.

Preferred embodiment of the invention

1 shows a schematic representation of a first embodiment of a circuit arrangement according to the invention. In this case, an OLED is connected in series between a power supply. The current flowing through the _OLED is denoted by I _OLED , and the voltage drop across the _OLED is denoted by U _OLED . A filter Fi, realized in the present case by a capacitor Ci, serves to filter in coupled-in disturbances and HF components. An evaluation device 10 comprises a threshold device 12 on the one hand and a step Vi with an amplifier device and a holding member on the other. The threshold device 12 includes the series connection of an ohmic resistor R ₄ and a Zener diode ZDi. The output of the stage Vi is coupled to the control electrode of an electronic switch T ₄ , which is connected in parallel with the parallel connection of ohmic resistors R7 to RIO of the OLED is. The threshold device 12 and the stage Vi are designed so that when the OLED voltage U _{OLED is} greater than the blocking voltage of the Zener diode ZDi, this leads to the full-level control of the stage Vi. The fully controlled state is permanently retained at the output by means of a holding element. This state corresponds to the status of a faulty OLED.

The then positive output signal of the stage Vi is fed to the base of the transistor T ₄ , which then turns on and bridges the defective OLED in a low-resistance manner via the load resistors R ₇ to Rio and thus takes over the current.

It should be particularly pointed out that the dimensioning of the resistors R ₇ to Ri ₀ connected in parallel takes place in such a way that the total voltage _drop of U _R7 + U _0E (T ₄ ) is sufficient to supply the circuit _arrangement , in particular the holding member and the electronic switch T4.

The connecting lines between the OLED and the evaluation device 10 serve in the present case as an OLED voltage measuring device.

In Fig. 2, the threshold device 12 comprises a voltage divider with the resistors R ₇ and Rs, wherein the resistor R _7, a capacitor C ₄ is connected in parallel. The stage Vi is realized by a microprocessor μC whose input is coupled to the tap of the voltage divider R ₈ / R ₇ , C ₄ and whose output is coupled to the base of the transistor T ₄ .

Fig. 3 shows in greater detail an embodiment of the schematic diagram of FIG. 1. This is the Filter Fl realized by the capacitor Ci. The evaluation device 10 comprises the ohmic resistor R ₄ and the Zener diode ZDi. The stage Vi comprises the components C ₂ , Ri, Ti, R ₂ , R ₃ , C ₃ , D ₂ , T ₂ , D ₃ . The electronic switch and the load comprise the components R ₅ , T ₃ , R ₆ , T ₄ , R ₇ , R ₈ , R ₉ and Ri ₀ .

In a preferred embodiment according to FIG. 3, the zener diode ZDi is dimensioned such that the OLED is switched off when the voltage U _OLED exceeds twice the nominal value. In other embodiments, the shutdown threshold may be between 10% and 300% when the nominal value is exceeded.

As can be seen from Fig. 3, the circuit arrangement consists of a few SMD components, works independently and wins its own power supply from the OLED current I ₀ LED- In case of failure of the OLED in operation, the defective OLED is typically on the order of less switched off as 2.3 μs. The current flow is permanently reduced by the electronic switch T ₄ , so that only the damaged OLED remains dark and no overheating or flashovers occur.

4 shows the time profile of the current I _OLED through the OLED, the voltage U OLED dropping across the _OLED and the sum of the current I _OLED through the OLED and the current I _L through the circuit structure connected in parallel with the OLED. As can be clearly seen, the voltage U _{OLED increases} very quickly to a constant value after switching on, whereas the current I _OLED through the OLED only gradually increases to a constant value. The same applies to the sum of the current I _OLED and the Current I _L. The current I _L is therefore approximately zero. This is an intact OLED.

FIG. 5 shows the corresponding time profiles for a defective OLED: After switching on, the voltage U _OLED across the OLED briefly rises sharply. This is registered by the protective device according to the invention and leads to Leitendschalten the switch T _4th The current I ₀ LED through the OLED therefore remains at zero, the switch T ₄ takes over the current that has flowed through the OLED in the example of FIG. As can be clearly seen, the detection takes place shortly after switching on during startup of the arrangement, in particular at currents I _OLED <600 μA.

Fig. 6 shows the course of the corresponding variables in a load shedding. It can be seen that here too the protective circuit responds after a brief rise in the voltage U _OLED and as a result the current I _{OLED returns} to zero. As is apparent from the curve of the total current I _O + IL ^¬ LED's, all of the current now flows through the switch T _4, the remaining OLED of a series connection of several OLED are therefore supplied with power. Only the defective OLED was switched off.

Claims

claims

A circuit arrangement for operating an OLED comprising: a power source coupled to supply the OLED to the OLED;

characterized,

that the circuit arrangement further comprises:

an OLED voltage measuring device coupled to the OLED and configured to provide at its output a signal correlated with the voltage drop across the OLED;

an evaluation device (10), which is coupled to the output of the OLED voltage measuring device and is designed to provide at its output a first signal when the above

OLED dropping voltage is above a predetermined threshold, and to provide a second signal when the voltage drop across the OLED voltage is below the predetermined threshold;

an electronic switch (T ₄ ) with a reference electrode, a working electrode and a control electrode, wherein the control electrode is coupled to the output of the evaluation device (10); and

at least one ohmic resistor (R7; Rs; R9; Rio); wherein the series circuit of the at least one ohmic resistor (R ₇ ; Rs; R9; Rio) as well as the distance working electrode - reference electrode of the electronic switch (T ₄ ) is connected in parallel to the OLED.

2. Apparatus according to claim 1,

characterized,

in that the evaluation device (10) comprises the series connection of a threshold value device (12), an amplifier device and a holding element (V ₁ ).

3. Apparatus according to claim 2,

characterized,

in that the threshold device (12) comprises a Zener diode (ZD ₁ ).

4. Device according to one of claims 2 or 3,

characterized,

the holding member (V ₁ ) is designed to provide a voltage of a predeterminable amplitude over a predeterminable time period.

5. Device according to one of claims 2 to 4, characterized,

the amplifier device and the holding member are designed to provide at their output a signal for switching on the electronic switch (T ₄ ).

6. Device according to one of the preceding claims,

characterized,

the evaluation device (10) comprises a voltage divider (R ₇ , R ₈ , C ₄ ) which is designed to provide at its tap a voltage which is correlated with the voltage drop across the OLED.

7. Device according to one of claims 2 to 6,

characterized,

the threshold value device (12), the amplifier device and the holding member are realized by a microcontroller (μC).

8. Device according to one of the preceding claims,

characterized,

in that a filter device (Fi) is coupled between the OLED voltage measuring device and the evaluation device (10).

9. Device according to one of the preceding claims,

characterized,

in that the at least one resistor (R ₇ ; Rs; R9; Rio) is dimensioned in such a way that the above the series connection of the at least one ohmic resistor (R ₇ ; Rs; R9; Rio) and the distance between the working electrode reference electrode of the electronic switch ( T ₄ ) falling voltage as a supply voltage for the evaluation device (10), in particular the holding member and / or the electronic switch (T ₄ ), is usable.

10. A method of operating an OLED comprising the steps of:

a) supplying the OLED from a power source;

b) measuring the voltage dropped across the OLED;

c) evaluating the measured voltage and providing a first signal when the voltage drop across the OLED exceeds a predetermined one

Threshold is, and providing a second signal when the voltage drop across the OLED voltage is below the predetermined threshold; and

d) coupling the first or the second signal to the control electrode of an electronic switch (T ₄ ), wherein the distance working electrode - Reference electrode of the electronic switch (T ₄ ) in series with at least one ohmic resistance (R ₇ ; Rs; R9; Rio) is connected, and wherein this series connection of the OLED is connected in parallel.