In this paper, a hardware equivalent of an organic light-emitting diode (OLED) was designed and investigated. This substitution OLED device is based on a circuit-equivalent OLED model and can be used to design and test OLED dedicated drivers. Indeed, OLEDs are available on the market, but they are still very expensive and hard to obtain. Compared to a real OLED, the substitution device is cheap and robust and can be easily duplicated. Moreover, a photodetector is not required to measure the light output waveform. This can be simply done by measuring a voltage across a resistance. This model can be used, for instance, to simulate a large OLED panel made of several associated single OLEDs for various series/parallel connection strategies. It can also be used to simulate aging phenomena by changing the values of some of its components. This might be useful for the definition of strategies to compensate aging effects like luminous flux depreciation. Another advantage of such a device is its use for power supply tests as it could serve as a substitution load, at maximum deviation from standard OLED electrical characteristics. We discuss the theoretical model that was used as a basis for developing the device. The accuracy of the model was then evaluated, particularly in pulsewidth-modulation dimming conditions. Then, the hardware equivalent device was compared to a real OLED. Finally, an example of the potential use of this substitution device is given: It was successfully used to investigate the “overdrive” technique in order to increase OLED light output rise time. This technique improves the light output rise time by a factor of over 4.

OLED Electrical Equivalent Device for Driver Topology Design

Ladder network circuits for controlling operation of a string of light emitting diodes (LEDS). The circuits include a number of sections connected in series. Each section includes one or more LED junctions, a variable resistive element coupled to the LED section, and a switch coupled to the variable resistive element and the LED section for controlling activation of the LED. The sections can include a transistor coupled between the switch and variable resistive element. The series of sections are connected to an AC power source, rectifier, and dimmer circuit. When receiving power from the power source, the sections activate the LEDs in sequence throughout the series of the sections. The dimmer circuit controls activation of a selection of one or more of the sections in order to activate in sequence the LEDs in only the selected sections, providing for both dimming and color control of the LEDs.

Transistor ladder network for driving a light emitting diode series string

The equivalent circuit model for two commercial high power LEDs for illuminance is proposed and extensive measurements have been carried out to verify the accuracy of the model. In addition, the frequency response of two LEDs in the optical domain can be estimated using the transfer function of electrical equivalent circuit. It is shown that, the electrical parasitics in the chip and packaging introduce significant attenuation beyond a resonance frequcency. Care should be taken when designing high-speed driver and/or equalization circuits for VLC systems.

/pdf/equivalent-circuit-model-of-high-power-leds-for-vlc-systems-127o2vumz6.pdf

Equivalent Circuit Model of High Power LEDs for VLC Systems

Networked light-emitting diode (LED) lighting systems form an important part of smart buildings and the Internet of Things. Plug-and-play extra-low-voltage LED lighting systems are attractive because they can be installed and reconfigured or expanded by the end user. Adding network/control/programming abilities to these LED lighting systems increases the appeal to the end user—to be in charge of electricity usage as well as the aesthetics involved. However, these LED lamps as well as billions of other proliferating dc-based electronic appliances all require some form of dc power supply which hampers power-line networking and is a threat to the future success of such systems in smart buildings. A novel power-line interfacing technique, coupling diversity, is tested and modeled to investigate its potential to mitigate this threat which is posed by the worldwide increasing number of dc power supplies. Simulations and live results have yielded gains of up to 2 dB for 12-V LED systems. However, the most important result is a more consistent performance displayed by the novel technique when parameters such as cable topology, cable length, voltage, and impedance levels are varied.

Modeling of Coupling Diversity for Extra-Low-Voltage Power-Line Communication Networked LED Lighting in Smart Buildings

The design and advantages of a novel light emitting diode (LED) bandwidth extension technique, based on the adoption of negative impedance converters (NICs), are discussed in the context of visible light communication systems. The design principles of the proposed negative impedance converter are introduced, with analytical derivations of the impedance frequency behaviour and the circuit performance frequency limitations. A two-transistor circuit is designed to generate a negative capacitance over the range −3 to −5 nF and is experimentally demonstrated, using discrete transistors, passive elements and commercially available LED constructed on a PCB, with frequencies up to 50 MHz. NIC design considerations necessary to obtain optimum LED bandwidth extension are discussed and outlined. Measurements demonstrate advantageously the optically lossless nature of the proposed solution and that, unlike traditional passive equalization or pre-distortion based bandwidth extension techniques, substantial improvement in the LED bandwidth, of up to 400 % can be obtained without compromising the output optical power.

/pdf/use-of-negative-impedance-converters-for-bandwidth-extension-2dfszjr2za.pdf

Use of Negative Impedance Converters for Bandwidth Extension of Optical Transmitters

We performed measurements on a number of pre-production DH LEDs biased from 1 to 10 mA and, for each case, we obtained a broad-band input impedance plot which gives both magnitude and phase as a function of frequency. We deduced from these plots the carrier lifetime, differential diode resistance, diffusion capacitance, parasitic resistance, and the ideality factor of each diode. The parameters were then used in the equivalent circuit to calculate the modelled input impedance of the device which was then compared with the measured result. There is good agreement over a wide frequency range, at least up to the frequency equal to 1/(carrier lifetime). The existence of a maximum phase lag previously noticed in single-port laser measurements is fully accounted for in our model.

G.N. French

Papers

Determination of LED equivalent circuits using network analyser measurements