David Buso

OLED Electrical Equivalent Device for Driver Topology Design

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 equivalent circuit model with temperature coefficient and intrinsic capacitor

This paper presents an organic light-emitting diode (OLED) equivalent circuit model with a temperature coefficient and an intrinsic capacitor. In order to precisely describe the OLED nonlinear V-I characteristic at different ambient temperatures, four parameters, i.e., the maximum operating point, the rated operating point, the knee point, and the temperature coefficient, are obtained from a manufacturers datasheet to establish an improved Taylor-series-based OLED equivalent circuit model. Furthermore, the proposed model can be used to generate a V-I lookup table to build a voltage-controlled piecewise linear (VPWL) behavior model that is conveniently used for the time-domain and frequency-domain circuit simulations of OLED drivers. In addition, the intrinsic capacitor of the OLED is voltage dependent and can be modeled with a piecewise-linear model associated with the built VPWL behavior model for OLED circuit simulations. Finally, the proposed model is compared with the experimental results to confirm the mutual agreement.

Modeling of Acoustic Resonances in HID Lamps - Coupling Between Standing Waves and Arc Bending

We have developed a 3D model, based on the resolution of the propagation of acoustic waves, in the real geometry of the lamp burner. This model provides the frequency spectrum of the different modes which can take place in the lamp. It helps in the understanding of the coupling between the acoustic modes and the arc geometry. Especially, it has shown that the acoustic modes have an influence on the convective fluxes which causes arc bending, especially in the electrodes neighborhood.

Influence of Auxiliary Heating on the Degradation of Fluorescent Lamp Electrodes Under Dimming Operation

Saving energy using dimmable electronic ballasts for fluorescent lamps is becoming a popular solution. Under dimming operation, lamp lifetime can be drastically reduced, if electrode auxiliary heating is not correctly set. In this paper, electrode degradation of a dimmed fluorescent lamp is investigated. Electrode degradation is evaluated by a simple electrical measurement, giving indirectly a measure of barium losses from the electrode surface. This paper shows how the experimental setup has been implemented. Then it shows experimental results obtained on a test run, and finally discusses setup limits. It is shown that strong electrode degradation caused by excessive heating conditions is identifiable within 500 h of operation using this technique.

Model investigation on moving striations in low pressure Ar–Hg discharge: electron response to striation-like fields

Simulation of the electron response to striation-like fields in fluorescent lamps is performed based on the numerical solution of the Boltzmann equation. Calculation results are presented and analysed. It is indicated that in a fluorescent lamp under the given condition, Ar is dominant in the energy loss by elastic collision and Hg is leading in the energy loss by inelastic collision. Furthermore, according to the analyses on relaxation length, the high energy part of the electron distribution function is non-local, which may cause ionization instabilities. By comparison with other works, it is here confirmed that metastable Ar atoms are crucial for the appearance of moving striations.

Influence of Driving Current on Photometric Performances of a White Light OLED

In this article, we demonstrate how driving current affects photometric performances of a white light OLED (WOLED) for general lighting purpose. Experiments reveal that ripple current with 40% amplitude, 50Hz-100Hz frequency, and sinusoidal and triangle waveform has little influence on the test OLED. In PWM mode, both duty cycle and frequency of pulse current affect the OLEDs performance such as efficacy and CCT. Such influences become worse at high frequency and low duty cycle. The reason for this influence is considerable difference between electron and hole mobility, and the movement of recombination zone therefrom.

Influence of driving current on photometric performances of a white light OLED

In this paper, we demonstrate how driving current affects photometric performances of a white light organic light emitting diode (OLED) for general lighting purpose. Experiments reveal that a ripple current with 40% amplitude, 50-Hz-100-kHz frequency, and sinusoidal and triangle waveform has little influence on the test OLED. In the pulse-width modulation mode, both duty cycle and frequency of pulse current affect OLEDs performance, and more particularly, their efficacy and correlated color temperature. Such influences become worse at high frequency and low duty cycle. The reason for this influence is a considerable difference between electron and hole mobility, and a time-dependent distribution of charge carrier within the structure leading to a movement of recombination zone therefrom.

Study of ferroelectric material to optimize supply voltage in n-type organic transistors

In this study, we tried to evaluate the impact of two dielectric materials (PMMA and P(VDF-TrFE)) on the performance of n-type organic field-effect transistors (OFETs). PMMA is the reference as a dielectric layer in OFETs and P(VDF-TrFE) is a ferroelectric material which has a high permittivity (e R =10). For this study, we elaborated four different structures and we reduced supply voltage by 2 with a stack of P(VDF-TrFE)/PMMA in comparison to an OFET with a PMMA layer.

OLED Equivalent Circuit Model With Temperature Coefficient and Intrinsic Capacitor

This paper presents an Organic-Light-Emitting-Diode (OLED) equivalent circuit model with temperature coefficient and intrinsic capacitor. In order to precisely describe the OLED non-linear V-I characteristics at different junction temperatures, four parameters: maximum operating point, rated operating point, knee point, and temperature coefficient, are obtained from the OLED V-I curves to establish the improved Taylor series based OLED model. Furthermore, in order to perform the transient and AC simulations of the OLED drivers, the proposed model can be used to generate the V-I look-up table for the voltage-controlled-piecewise-linear (VPWL) behavior model for the time-domain and frequency-domain circuit simulations. Besides, the intrinsic capacitor of the OLED is voltage dependent and can be modeled with the piecewise-linear (PWL) behavior model to associate with the built VPWL behavior model for the OLED circuit simulations. Finally, the measured results are provided to compare with the modeled V-I curves, transient simulations, and AC simulations of the OLED.

Bidirectional OLED buck driver

This paper presents a bidirectional OLED buck driver with the proposed variable-frequency control mechanism. The bulky parasitic capacitor of the OLED causes the significant delay on the dimming response of the OLED. Therefore, the bidirectional OLED buck driver with the variable-frequency control mechanism is proposed to increase the dimming response. Finally, the prototype circuit of a 6.4W bidirectional OLED buck driver with the variable-frequency control mechanism is built to verify the response performance.