The method of double averaging: an approach for modeling power-factor-correction switching converters

This paper describes the modeling of power-factor-correction converters under average-current-mode control, which are widely used in switch-mode power supply applications. The objective is to identify stability boundaries in terms of major circuit parameters for facilitating design of such converters. The approach employs a double averaging procedure, which first applies the usual averaging over the switching period and subsequently applies generalized averaging over the mains period. The resulting model, after two averaging steps and application of a harmonic balance procedure, is nonlinear and capable of describing the low-frequency nonlinear dynamics of the system. The parameter ranges within which stable operation is guaranteed can be accurately and easily found using this model. Experimental measurements are provided for verification of the analytical results.     

A novel method for elimination of line-current harmonics insingle-stage PFC switching regulators

This paper studies a particular single-stage power-factor-correction (PFC) switching regulator employing a discontinuous-conduction-mode (DCM) boost-input cell and a continuous-current-mode (CCM) forward output cell. Although this single-stage PFC regulator can provide a reasonably high power factor when its PFC stage is operating in discontinuous mode, substantial reduction in line-current harmonics is possible by applying a suitable frequency-modulation scheme. This paper derives a frequency-modulation scheme and proposes a practical implementation using a simple translinear analog circuit. A quantitative analysis on the total harmonic distortion (THD) of the line current when the circuit is subject to a limited range of frequency variations is presented along with some considerations for practical design. Experimental data obtained from a prototype confirms the effectiveness of the proposed frequency-modulation scheme. The proposed analog translinear circuit allows custom integrated circuit implementation, making it a viable low-cost solution to the elimination of line-current harmonics in switching regulators     

Increased expression of peripheral benzodiazepine receptors in the facial nucleus following motor neuron axotomy

Peripheral benzodiazepine receptors (PBRs) are expressed in a variety of tissues but are normally found at low levels in the brain. Following various types of nerve injury, a reactive gliosis results that exhibits a high expression of this receptor. To further characterize the expression of PBRs following neuronal injury, we evaluated PBR expression in the facial nucleus following facial nerve axotomy (FNA). Injury to a peripheral nerve results in a complex series of metabolic and morphological changes around the injured neuron. Transections of the facial nerve results in a rapid activation of both astrocytes and microglia around axotomized motor neurons. FNA resulted in an increase in the staining for both astrocytes (glial fibrillary acidic protein) and activated microglia (OX42). There was also a reduction in synaptic contacts with the motor nucleus as evidenced by reduced staining for the synaptic marker, synaptophysin. In sections labeled with [3H]-PK11195, the subsequent autoradiograms displayed marked increases in the labeling for PBRs. This increase was observed at 5, 7 and 10 days after nerve transection. The increase was primarily in the level of expression (Bmax), with no change in the affinity of the ligand (Kd). The increase in PBR expression after FNA supports the hypothesis that PBRs can be used as a sensitive marker for CNS injury.     

Aesthetic outcome of breast implant removal in 85 consecutive patients

As we began to see increasing numbers of women concerned about their gel-filled breast implants, we became aware that we could not advise them with any degree of confidence what they might expect in terms of aesthetic result after implant removal. We decided to review the records and outcomes over a 2-year period of a number of patients who underwent implant removal. Eighty-five consecutive patients were reviewed, 69 of whom had undergone cosmetic augmentation and 16 of whom had breast reconstruction with silicone gel implant(s). Thirty-nine of the 69 cosmetic augmentation patients had removal of implants alone, and 27 had removal accompanied by mastopexy. Three had reaugmentation with saline-filled implants; one had replacement with saline-filled implants. Fifteen of the 16 reconstruction patients underwent autogenous tissue transfer. Preoperative and postoperative photographs of all patients were mixed randomly and rated by two independent raters in four aesthetic categories on a five-point scoring system. Repeatability was measured several weeks later, when each rater scored randomly selected photographs from this patient pool. The patients also performed their own outcome evaluations by means of questionnaire. We discovered that cosmetic augmentation patients who undergo implant removal only often suffer adverse aesthetic results. The postremoval appearance of many cosmetic augmentation patients actually will be improved over their preoperative appearance when mastopexy is performed in conjunction with implant removal. The study demonstrated that patients with certain body types could expect a particular outcome; i.e., women with asthenic builds and older patients with lax, striated breast skin generally had unsatisfactory aesthetic outcomes with implant removal only. Patients selected for autogenous breast reconstruction had favorable results, with extended latissimus dorsi and TRAM flaps yielding equally good outcomes. The study allows us to offer patients an optimistic view of postoperative results following breast implant removal. We have begun to advise selected patients that implant removal accompanied by mastopexy provides a more pleasing aesthetic outcome than implant removal alone.     

Experimental and theoretical comparison of NIR spectroscopy measurements of cerebral hemoglobin changes

Two near-infrared spectroscopy (NIRS) methods are available for measuring changes (Delta) in total cerebral hemoglobin concentration (CHC): 1) a continuous measurement of the changes in total hemoglobin concentration (Delta[Hb]tot) and 2) the difference between two absolute measurements of CHC, each derived from a small, controlled change in inspired O2 fraction. This paper investigates the internal consistency of these two methods by using an experimental and theoretical comparison. NIRS was used to measure [Hb]tot in five newborn piglets before and after a change in arterial PCO2. Delta[Hb]tot demonstrated a low coefficient of variation of 2.8 +/- 2.8 (SD) % which allowed changes in CO2-cerebral blood volume reactivity to be clearly discriminated. However, a high coefficient of variation of 22.8 +/- 3.5% on the DeltaCHC measurements obscured any CO2 reactivity changes. A theoretical analysis demonstrates the effects of optical pathlength, background absorption, scatter, and blood vessel diameter on both methods. For more accurate monitoring of CHC, individual measurements of optical pathlength and more accurate pulse oximetry are required.     

The stress-whitened damage zone of PVC blends

The stress-whitened damage zone that formed ahead of a semicircular notch during slow tensile loading has been measured from optical micrographs of translucent blends of poly(vinyl chloride) (PVC) with experimental chlorinated polyethylene (CPE) resins. When the zone was small, the plane strain condition applied and from the elastic stress distribution a constant mean stress condition was found at the boundary of the crescent-shaped zone. The critical mean stress did not depend on the chlorine content or the chlorine distribution of the experimental CPE resin used in the blend. While the critical mean stress decreased as the amount of CPE in the blend was increased, the critical volume strain, calculated from the bulk modulus, was independent of composition and was thought to be the controlling parameter for stress-whitening. When the zone was larger, the shape was qualitatively described by concepts of stress redistribution in the presence of a plastic zone ahead of the notch. Macroscopic flow and necking were only detected near the maximum in the stress-displacement curve.     

DNA recognition by the EcoK methyltransferase. The influence of DNA methylation and the cofactor S-adenosyl-L-methionine

The methyltransferase of the EcoK type I restriction/modification system is trimeric, M2S1, where the S subunit determines the sequence specificity of the enzyme. The methyltransferase has a strong preference for hemimethylated substrate DNA and, therefore, we have investigated the effect of the methylation state of DNA on binding by the enzyme, together with the effects on binding of the cofactor S-adenosyl-L-methionine. Our results indicate that the methyltransferase has two non-interacting S-adenosyl-L-methionine binding sites, each with a dissociation constant of 3.60 (+/- 0.42) microM determined by equilibrium dialysis, or 2.21 (+/- 0.29) microM determined by the displacement of a fluorescent probe. Ultraviolet light-induced crosslinking showed that S-adenosyl-L-methionine binds strongly only to the modification (M) subunits. Changes in the sedimentation velocity of the methyltransferase imply a protein conformational change due to S-adenosyl-L-methionine binding. Gel retardation results show that the binding of S-adenosyl-L-methionine to the methyltransferase enhances binding to both specific and non-specific DNAs, but the enhancement is greater for the specific DNA. Differences in binding affinities contribute to the recognition of the specific nucleotide sequence AAC(N)6GTGC by the methyltransferase in preference to a non-specific sequence. In contrast, although the complexes of unmodified and hemimethylated DNAs with the methyltransferase have different mobilities in non-denaturing gels, there appears to be no contribution of binding affinity to the distinction between these two substrates. Therefore, the preference for a hemimethylated substrate must be due to a difference in catalysis.