Driving of OLEDs
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).
Figure 2: Reduced decay of luminance with constant current operation
Figure 3a: Voltage-Luminance characteristics of an OLED
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 Current-Luminance characteristic is much more flat at the nominal operating point.
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.
Figure 3b: Current-Luminance characteristics of an OLED
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:
- 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 (PWM) is not recommended as it leads to reduced lifetime (figures 4 and 5).
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 tab OTi DALI 2x300 CS): 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.
Figure 6: Schematic presentation of ripple current
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.
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 disproportionally (figure 7a and 7b), 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.
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 previous 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.
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).
Figure 8: Logarithmic dimming characteristic with 8 Bit resolution
Output Voltage and Isolation
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
- additionally in case of reduced DC current (e.g. with amplitude dimming)
- additionally in case of their highest temperature (ambient and self heating)
- in case of their minimal production tolerance (datasheet: forward voltage Vf,min)
- OLEDs have their maximum voltage
- in case of their maximum production tolerance (datasheet: Vf,max)
- in case of increased current (e.g. operation with higher brightness than nominal)
- in case of their lowest temperature
- 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.Basic knowledge
OTi DALI 2x300 CS
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.