Abstract-This paper presents an LED driver circuit consisting of multiple linear current regulators and a voltage preregulator with adaptive output voltage. In the proposed driver, the output voltage of the preregulator is always self-adjusted so that the voltage across the linear current regulator of the LED string with the highest voltage drop is kept at the minimum value that is required to maintain the desired string current. Because the linear current regulators in this driver operate with the minimum voltages, the driver efficiency is maximized. The performance of the proposed driver was experimentally verified on a four-string LED setup with eight white LEDs in each string. The measured efficiency improvement of the linear current regulators was approximately 15% compared to the corresponding implementation with a constant preregulator voltage.Index Terms-Adaptive drive voltage, LED driver, linear current regulator, sequential pulsewidth modulation (PWM) dimming.
Abstract.We investigate the coupling between neutral hydrogen atoms and protons in the corona for a range of proton temperatures reaching a maximum of 6 x 10 • K, as recently inferred from observations of the Ly a spectral line profiles by Kohl et al. [1996]. We adopt the approach used by Olsen et al.
Abstract. We investigate the preferential acceleration and heating of solar wind alpha particles by the resonant cyclotron interaction with parallel-propagating left-hand-polarized ion cyclotron waves. The Alfvdn wave spectrum equation is generalized to multi-ion plasmas and a Kolrnogorov type of cascade effect is introduced to transfer energy from the low-frequency Alfvdn waves to the highfrequency ion cyclotron waves, which are assumed to be entirely dissipated by the wave-particle interaction. In order to distribute the dissipated wave energy among the alphas and protons, the quasi-linear theory of the wave-particle interaction is used along with the cold plasma dispersion relation, and a power law spectrum of the ion cyclotron waves is assumed, with the spectral index as a free parameter of the model. The set of three-fluid solar wind equations and the Alfvdn wave spectrum equation are then solved in order to find fast solar wind solutions. It is found that the effect of the alpha particles on the dispersion relation, omitted in most previous wave-driven solar wind models, has a significant influence on the preferential acceleration and heating of the alphas, especially in the region close to the Sun. With this effect included, the alpha particles can be accelerated to a bulk flow speed faster than the protons by a few hundred kilometers per second and heated by the resonant cyclotron interaction to more than mass-proportional temperature values at several solar radii. However, this mechanism does not yield a differential speed of the order of an Alfvdn speed and a mass-proportional temperature for the alphas beyond 0.3 AU, as observed, which confirms the same conclusion reached previously by Isenberg and Hollweg [1983] for nondispersive ion cyclotron waves.
Abstract. We present for the first time a one-dimensional, four-fluid turbulencedriven solar wind model in order to investigate the preferential acceleration and heating of heavy ions by the resonant cyclotron interaction with parallel-propagating left-hand-polarized ion cyclotron waves. The model contains four species: electrons, protons, alpha particles, and one species of minor ions. A Kolmogorov type of cascade effect is introduced to transfer energy from the low-frequency Alfv•n waves to the high-frequency ion cyclotron waves, which are assumed to be entirely dissipated by the wave-particle interaction. The quasi-linear theory of the waveparticle interaction is invoked to distribute the dissipated wave energy among the three ion species based on a given power law spectrum of the ion cyclotron waves and the cold plasma dispersion relation. It is found that in terms of the cold plasma dispersion relation, the dispersion generated by all ion species has an appreciable influence on both the behavior of the major species and the preferential acceleration and heating of the minor ions. The larger the number of species included in the dispersion relation is, the stronger preferential acceleration and heating produced by the waves for the heavy ions close to the Sun will be. A detailed comparison is carried out between two cases, one with and the other without the dispersive effect of the minor ions. Although the solutions for the two cases are somewhat different, they predict a more or less similar behavior of the minor ions, which essentially agrees with recent observations from SOHO. This indicates that the resonant cyclotron interaction may be responsible for the preferential acceleration and heating of minor ions in the fast solar wind. Furthermore, the influence of minor ions on the proton-alpha solar wind is found to be dominated by the dispersive effect of the minor ions. Even though such an influence is exaggerated by the cold plasma dispersion relation, it is still small and remains within the present observational uncertainties. Therefore minor ions may be treated approximately as test particles in the solar wind.
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