www.osram-oled.com User guideline for operation of OLEDs with electronic driver systems The operation of OLEDs with electronic drivers is similar to anorganic LEDs for the most part. In a majority of applications, standard LED drivers may be used also for OLEDs. Nevertheless, there are some important basic rules and OLED specific characteristics, that have to be considered. Optimal driving mode: Constant current operation and series connection OLEDs have to be operated with constant current drivers. For correct operation of more than one OLED at one single constant current (CC) driver, the OLEDs have to be connected in series connection. Alternatively, multi channel drivers with more than one individually current controlled output channel may be used. Due to degradation processes, the luminous output of the OLED decreases with increasing operation time. Additionally, the static resistance of the OLED increases. In constant voltage operation, the rise of the static resistance leads to a decreasing operation current with increasing operation time. As this happens additionally to the normal luminous degradation, the L70 point of luminous output is reached earlier (figure 1). Hence, constant voltage mode results in reduced lifetime and is not recommended. Figure 1: Decay of current and luminance with constant voltage operation Knowing this, constant current operation is suggested as an optimal solution: The OLED current will be kept constant over the full operation time, while the forward voltage of the OLED increases due to the rise of the static resistance (so-called voltage ageing). The L70 point of luminous output will be reached by far later, the operation time of the OLED is maximized (figure 2). 1 Leitfaden für den Betrieb von OLEDs an elektronischen Treibern Figure 2: Reduced decay of luminance with constant current operation The luminous flux of the OLED is proportional to the operation current, but not proportional to the operating voltage. The Voltage-Luminance characteristic is very steep at the nominal operating point. This means, that very small changes of the operation voltage or small changes of the characteristic (by temperature, production tolerances) will result in significantly high changes in the luminous output. The CurrentLuminance characteristic is much more flat at the nominal operating point. Figure 3: Voltage-Luminance and Current-Luminance characteristics of an OLED 2 Leitfaden für den Betrieb von OLEDs an elektronischen Treibern This means that small changes in operation current or small changes of the characteristic (by temperature, production tolerances) will only lead to small changes of the luminous output (figure 3). Comparing both characteristics leads to the result, that current controlled mode is the best solution for driving OLEDs. Production tolerances result in much more luminance deviations in constant voltage mode than in constant current operation. Hence, constant voltage drivers would need to be adjusted extremely exact to the correct OLED voltage, which is not possible with most of the available devices. Also constant voltage operation combined with a series resistor allows only very imprecise adjustment of the operation point and leads – especially due to the voltage ageing – to an early decrease of the luminance. Constant current operation ensures maximum OLED lifetime and keeps the luminance stable over a wider variation of environmental parameters like: Temperature Ageing Production tolerances Tolerances of the driver device Adjustment of the optimal operation current and ensuring an appropriate signal shape The nominal current of the OLED must be set by adjusting the DC amplitude (not by the duty cycle of a pulse width modulation, PWM) and the superimposed ripple current must only be small. For ensuring maximum OLED lifetime, the nominal OLED current has to be checked in the datasheet and accordingly adjusted via the DC amplitude of the driver. Exceeding the nominal DC current level will lead to a disproportionate decrease of the OLED lifetime, which cannot be compensated by proportional adjustment in the time domain (PWM). Therefore, adjustment of the driver output current with a higher DC signal as the allowed datasheet current and then reducing the mean value by a pulse width modulation (PW M) is not recommended as it leads to reduced lifetime (figures 4 and 5). Figure 4: Correct shape of the operation current 3 Leitfaden für den Betrieb von OLEDs an elektronischen Treibern Figure 5: Reduction of lifetime by exceeding the nominal DC current (although the mean value is correct) Example: Theoretically, a 200mA OLED could be operated by a 350mA standard LED driver which is dimmed to a mean current of 200mA by PWM with 57% duty cycle. For the above named reasons, this would decrease the OLED lifetime significantly and is therefore not recommended. Remark: The description above is about the setting of the nominal operation current for 100% light output. For dimming in the application (decreasing operation current lower than 100%), PWM is still allowed. Please refer to the chapter “dimming” below. Adjustment of the DC level of the output current is possible with more and more LED drivers by now: DIP-Switch (e.g. OTi DALI 2x300 CS, see below): Adjustment of different fixed output current values by choosing the correct switch combination at the driver hardware. LEDset interface: Free DC current adjustment in a certain range by connecting a setup resistor to the LEDset interface terminals at the driver. DALI intelligent: Current adjustment by programming the correct value via DALI commands by using a USB-DALI configuration interface (e.g. OSRAM DALI Magic). Using the official dimming interface of a driver (1-10V, Poti, DALI, DMX) for adjusting the nominal current is not recommended, as this is often working with PWM (lifetime reduction) and also often not sufficiently accurate. For lifetime optimization and accurate luminance adjustment, the adjustment - and current-control-accuracy of the driver should be ±5% or better. The operation current must only have low ripple current. Typical LED drivers are designed for lowest hardware cost and often do not provide a clean DC signal at the output. There is a ripple current (a low or high frequency AC signal) superimposed to the nominal DC level, often with a sinusoidal or triangular waveform shape (figure 6) High ripple current reduces the OLED lifetime and should be less than ±15% of the mean DC current, as shown in the following figure. 4 Leitfaden für den Betrieb von OLEDs an elektronischen Treibern Figure 6: Schematic presentation of ripple current Voltage- and current spikes may damage the OLED. A big difference between OLED and LED can be seen in the parasitic capacitance of the component. An OLED has a relatively high value of its capacitance, which many of LED drivers are not designed for. This may cause high current- or voltage spikes at the moment of driver turn-on/off or also at the edges of the PWM signal. Only drivers are allowed, that produce voltage spikes of less than 5% of the nominal voltage and where current spikes are less than 15% of the nominal current value. Dimming of OLEDs The adjustment of the nominal current must be realized by the DC amplitude of the driver (see above). Nevertheless, dimming of OLEDs is allowed also by pulse width modulation (PWM). Reduction of the DC amplitude for dimming will increase the OLED lifetime dis proportionally (figure 7), but may lead to color shifts at white OLEDs. Reduction of the mean current by pulse width modulation (PWM) will increase the OLED lifetime not that much (only proportionally, figure 7) but keeps the color point of white OLEDs stable. 5 Leitfaden für den Betrieb von OLEDs an elektronischen Treibern Figure 7: Dimming by amplitude (left) and PWM (right) If using PWM-Dimming, it has to be ensured, that no significant voltage or current spikes may occur at the PWM edges. Please refer to the information about a clean current signal in the prev ious chapter. A logarithmic dimming characteristic is recommended. For LED lighting, logarithmic dimming is already common to avoid visible dimming steps in the region of low luminance levels. Drivers for effect-lighting, architainment, sound and stage applications usually do not use logarithmic but linear dimming, often only with low dimming step resolution (e.g. DMX with 8 Bit = 255 steps). As OLEDs cause less glaring than LEDs, the negative effect of linear dimming may be even more visible as in LED applications. Figure 8: Logarithmic dimming characteristic with 8 Bit resolution 6 Leitfaden für den Betrieb von OLEDs an elektronischen Treibern Therefore, a logarithmic dimming characteristic (figure 8) is generally recommended for OLED applications. If this is ensured, also the standard dimming resolution of 8 Bit (255 steps) will be enough for most applications. If drivers with linear dimming are used anyway, at least a sufficiently high resolution should be ensured (e.g. 10 or 12 Bit). Choosing the correct driver output voltage range and ensuring sufficient safety isolation The output voltage range of the driver must be chosen appropriately according to the applications boundary conditions. LED drivers have a limited range for varying their output voltage in order to keep the output current constant. The lower and upper limit of that range must be chosen properly. OLEDs have their minimum voltage o in case of their minimal production tolerance (datasheet: forward voltage V f,min) o additionally in case of reduced DC current (e.g. with amplitude dimming) o additionally in case of their highest temperature (ambient and self heating) OLEDs have their maximum voltage o in case of their maximum production tolerance (datasheet: V f,max) o in case of increased current (e.g. operation with higher brightness than nominal) o in case of their lowest temperature o at the end of their lifetime (ensure 1V extra per OLED for voltage ageing effect) As OLEDs have to be connected in series connection when used with constant current drivers, the determined lower and upper voltage limits of a single OLED, like described above, have to be multiplied by the number of OLEDs in series connection. It has to be considered that typical drivers may shut down or even do not startup if the minimum output voltage is not reached or the maximum limit is exceeded. Therefore, appropriate safety margins to the limits are recommended. Enough safety isolation between OLEDs and touchable or earthed parts of the luminaire must be ensured according to the maximum driver output voltage and the relevant safety standards. The luminaire manufacturer or integrator of OLEDs is responsible for the application to be compliant with all relevant standards (e.g. luminaire standard EN60598). Most important is a sufficient isolation of electrically live parts (OLED edges, contacts, wires) against touchable or earthed parts, e.g. by isolating materials or sufficient clearance and creeping distances according to the standard. Using drivers with SELV equivalent output voltage helps to reduce standard requirements for the isolation and may be helpful for a simplified luminaire design. Especially for ensuring low self heating of the driver and keeping electromagnetic radiation below the maximum allowed limit, standard LED drivers are recommended which are completely integrated in a housing, and, amongst others, use the following label markings: CE, VDE, SELV, MM Drivers from the OSRAM product portfolio offer lowest integration efforts for luminaire manufacturers, while ensuring maximum safety and user comfort. 7 Leitfaden für den Betrieb von OLEDs an elektronischen Treibern The flat OLED allrounder from OSRAM: OTi DALI 2x300 CS For ensuring the above described OLED requirements in combination with maximum application flexibility, OSRAM has developed a dedicated OLED driver: the OPTOTRONIC OTi DALI 2x300 CS (figure 9). Figure 9: OPTOTRONIC OTi DALI 2x300 CS This driver offers a maximum of features and flexibility for the operation of OLEDs: DC Input with 24V or 48V SELV (e.g. supplied by constant voltage drivers from OSRAM) 2 constant current output channels for 2-6 OLEDs per channel OLED conform operation DALI dimming interface TouchDIM interface (dimming by a simple push button) Direct and simple choice of 4 different output currents by DIP-Switch Future proof by option for reprogramming the output current in a range of 100 -300mA Mechanical height of only 20mm allows maximum integration freedom, including comfortable screw terminals and integrated strain-relief. 8 Leitfaden für den Betrieb von OLEDs an elektronischen Treibern Operation of multi-segment OLEDs Multisegment OLEDs (more than one independently controllable light segment in one sing le OLED component) are typically constructed with a common cathode. The different segments have an own anode contact each, but only one common cathode contact for all segments. The cathodes of the single segments are connected OLED internally (figure 10). Figure 10: Circuit diagram of a multi-segment OLED with common cathode Most of the available multichannel LED drivers are not common cathode compatible. Also, an external connection of the cathode output of multiple single channel drivers is not allowed for most drivers (max. allowed touch current according to safety standard). For choosing the correct driver for multi segment OLEDs, it has to be checked in the driver datasheet, whether a connection of the cathode output of several drivers is explicitly allowed. In case of doubt, the driver manufacturer may be asked for further information. In case of the development of own special multi channel driver circuits, also common -cathode capability has to be ensured: Most of typical LED driver controller ICs are not common-cathode compatible, as the LED load is not ground referenced typically (low-side current sense, shunt resistor between cathode and circuit ground). A driver topology with ground referenced load output (e.g. high-side shunt) has to be used. Simple and cost efficient designs may also be realized by using a two-terminal current source (linear regulator for low- OR high-side connection) or also with a ground referenced high-side linear regulator. Short circuit protection and usage of special driver topologies Despite of high quality standards, OLEDs may be damaged during operation, e.g. by wrong handling (mechanical stress, reverse connection). A typical failure mode of OLEDs (in contrast to LEDs) is short circuit like behaviour. 9 Typically, there is no short circuit in the common meaning (zero-ohm-short), but a local low-ohmic bypass of the organic diode. Almost the full operating current flows through this bypass and causes abnormal local heat dissipation. Driven in constant current operation, this low-ohmic bypass resistor only causes a reduction of the voltage at the OLED contacts, but not a voltage decrease to zero. Therefore, a driver topology must be used, which is able to control the current at nominal level also in this failure mode. Ideally, the driver is able to detect that failure mode of the OLED and shuts the output down. Boost converter (e.g. in battery driven devices) have a DC path from input to output and are intrinsically not short circuit proof. Especially, if the output voltage falls to a value below the input voltage of the boost converter (due to the low-ohmic-behaviour), there may be uncontrolled excessive current flow. Additional short circuit detections in boost converters typically have high threshold current levels and may not detect such a low-ohmic-bypass failure mode safely, resulting in high local heat dissipation at the OLED surface. Therefore, such topologies are not suitable for OLED operation. There are preferred topologies like Buck-, Buck-Boost or SEPIC-Converters, that are able to stop uncontrolled DC current from input to output or may control the output current also in case of low output voltage. An additional undervoltage detection may help to detect such a failure mode and shut down the driver for safety reasons. OSRAM OLED GmbH OSRAM OLED GmbH Head Office: Head Office: Wernerwerkstrasse 2 Wernerwerkstrasse 2 93049 Regensburg, Germany 93049 Regensburg, Germany Phone +49 941 850-0 Phone +49 941 850-0 www.osram-oled.com Fax +49 941 850-XXXX www.osram-oled.com 10 07/14 Subject to change without notice. Errors and omission excepted. Leitfaden für den Betrieb von OLEDs an elektronischen Treibern