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While in numerous power electronics applications, transformers and inductors with nonlayered windings are used, the absence up until now of any theoretical or even empirical model for their HF effective resistance calculation leads magnetic component designers to make high-error approximations. The typical approach until now has been to consider them as layered and apply some of the existent relevant models. The present paper establishes a new semiempirical model for the accurate determination of HF copper losses in windings with the random conductor distribution, a case that cannot be treated by any analytical method. This model is based on the statistical treatment of numerical results coming from a large number of simulations carried out with finite-element analysis software, and incorporates only three easily determinable parameters. The selected range for each of these parameters ensures that the model is suitable for the majority of practical applications. The theoretical analysis is verified by experimental measurements on different forms of winding geometries. A detailed investigation of the new formula reveals its inherent advantages on copper loss calculation when designing nonlayered coils. Index Terms--Coils, HF transformers, loss measurement, skin effect, transformer windings.

Active clamp topologies of low power dissipation have become a very attractive solution in order to limit overvoltages in flyback converters. Although many suitable topologies have been introduced for the case of discontinuous conduction mode (DCM), where the duty cycle value depends on the load level, in continuous conduction mode (CCM) it is more difficult to appropriately design such topologies so as to 'sense' load changes--due to the small duty cycle divergence under wide load variation. Taking for granted that in order to achieve high power-factor correction in these converters, CCM is a more attractive mode of operation, a drastic solution for this case that will manage to eliminate voltage stresses under wide load changes has become very essential. For this purpose, this paper presents an active clamp topology with small power dissipation, suitable for flyback converters operating in CCM mode. Its main idea is the use of a load-dependent current source, consisting of an auxiliary converter operating in DCM mode. Experimental results highlight the effectiveness of the proposed topology under wide load changes, establishing it as an appropriate solution in order to develop flyback converters, even at the power range of 500 W. Index Terms--Active voltage clamp, flyback converter, snubbers.

Field-oriented-controlled induction motor drives have been widely used over the last several years. Conventional direct stator-flux-oriented control schemes have the disadvantage of poor performance in the low-speed operating area when the stator flux is calculated using the voltage model, due to the stator resistance uncertainties and variations. In this paper, a new closed-loop stator-flux estimation method for a stator-flux-oriented vector-controlled induction motor drive is presented in which the stator resistance value is updated during operation. This method is based on a simple algorithm capable of running in a low-cost microcontroller, which is derived from the dynamic model of the induction machine. The effects of stator resistance detuning, especially in the low-speed operating region, are investigated and simulation results are shown. The motor drive system as well as the control logic and the resistance estimator are simulated and characteristic simulation results are derived. In addition, the proposed control scheme is experimentally implemented and some characteristic experimental results are shown. The simulation as well as the experimental results reveal that the proposed method is able to obtain precise flux and torque control, even for very low operating frequencies. Index Terms--Field-oriented control, induction machine, stator-flux orientation, stator resistance identification.

The output characteristics of Buck zero-voltage switching quasi-resonant converters (ZVS-QRCs), presented in previously published works, are calculated considering equal input and output energies. Nevertheless, measurements that were taken with breadboarded Buck ZVS-QRCs demonstrate an important deviation between the results of the theoretical analysis and the experimental ones. This deviation becomes greater when the load current increases. This paper deals with a novel simulation method to approximate the real behavioral characteristics of Buck ZVS-QRCs. Analytical expressions are derived to calculate the power losses in each converter component as well as the dc-Voltage conversion ratio. Computer simulation results are confronted, to experimental ones proving the effectiveness of the proposed method. Index Terms--DC-DC converters, power losses, soft switching, zero-voltage switching quasi-resonant converters.