Development of High Power, High Frequency Magnetics for the Future Power Electronics Applications. Kapila Warnakulasuriya, Carroll & Meynell Transformers Ltd, UK,kapila@carroll-meynell.com Farhad Nabhani, Teesside University, United Kingdom, F.Nabhani@tees.ac.uk Vahid Askari, Teesside University, United Kingdom, v.askari@tees.ac.uk
Abstract
In this paper the optimum design of high power high frequency magnetics is discussed based on a 50kW application operating at 50 kHz. An in depth theoretical explanation on minimizing the losses at high frequency is discussed. A novel approach to the windings that makes the high frequency conductor losses due to proximity effect almost frequency independent is presented. Theoretical explanation of this phenomenon is given. Further how this approach will make it possible to develop a new generation of magnetics that enable power electronics designers to go for significantly high operating frequencies is discussed. The challenge of handling high temperature rise due to high power density of such high power high frequency magnetics is addressed in this paper with a practical solution for that. The theoretical findings are verified based on a set of prototypes developed for a 50kW DC DC converter operating at 50kHz. 1. Introduction Energy efficiency, low noise, reduced mass and dimensions are becoming more and more vital factors in designing magnetics for all kinds of modern power electronics applications such as rolling stock, renewable energy and similar applications. In order to achieve high degree of miniaturization, designers move into significantly high operating (switching) frequencies in the development of modern high power converters and inverters. There is also a clear trend that the operating frequencies used in these applications will continue to increase. Applications like traction, ship and basically any mobile platform with a converter on board require light weight and compact converters to exploit the space available on board more effectively. They often require galvanic isolation for safety or other reasons. Therefore high power high frequency transformers which offer galvanic isolation and a small volume are of increased importance [1]. In addition to the galvanic isolation magnetic components including transformers and inductors perform functions of harmonics filtering, energy storage, level shifting, current sensing and parameter matching for power stages as well as control circuitries in a power converter. They often determine the converter size [2], [3]. It has been a long held view that with a continuous increase in operating and /or switching frequency a continuous decrease in physical size of magnetics would follow. However the heat removal surface of the magnetic components decreases as a result of the higher density design; on the other hand, core and winding loss densities increase correspondingly. Therefore attention needs to be paid to magnetic material selection and associated core loss calculations, especially for high frequency high density magnetics and power converter design [2]. The selection of suitable core material out of several Ferrite grades and Nanocrystalline materials is discussed in this paper with modern approaches of estimating core losses in each type under non sinusoidal excitations of high frequencies such as 50kHz. The advantages of Nanocrystalline that the author has discussed in [11] for 20kHz applications reduce to some extent at this 50kHz application. The effects that high frequency currents such as 50kHz square and triangular waveforms have on conductor losses are explained. A novel winding technique that brings down the significance of proximity effect at high frequencies in transformer windings is presented in this paper. Several core material options such as Ferrite and Air cored inductor are discussed for the series inductor of converters operating in the range of 50kHz. Further a method of minimizing high frequency conductor losses of inductor windings carrying 50kHz currents of significant amplitudes is presented. A novel cooling technique for these compact magnetics with high power densities is discussed. A set of prototypes were developed based on the theoretical findings and tested on a DC/DC converter. Verification of theoretical findings based on practical observations is carried out.
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