Abstract
In power electronics, there is the need to develop solutions to increase the power density of converters. Interleaved multicellular transformers allow interleaving many switching cells and, as a result, a possible increase in the power density. This converter is often composed of a magnetic core having the function of an intercell transformer (ICT) and, depending on the complexity of the designed architecture, its shape could be extremely complex. The switching frequencies (1-10 MHz) for the new wide band gap semiconductors (SiC, GaN) allow to interleave switching cell at higher frequencies than silicon-based semiconductors (<1 MHz). Intercell transformers must follow this increase in frequency times-fold the number of switching cells. Current applications for ICT transformers use Mn-Zn based materials, but their limit in frequency drive raises the need of higher frequency magnetic materials, such Ni-Zn ferrites. These materials can operate in medium and high power converters up to 10 MHz. We propose to use Ni0,30Zn0,57Cu0,15Fe2O4 ferrite and to compress it by cold isostatic pressing (CIP) into a a green ceramic block and to machine it to obtain the desired ICT of complex shape prior sintering. We compare the magnetic permeability spectra and hysteresis loops the CIP and uniaxially pressed ferrites. The effect of temperature and sintering time as well as high-pressure on properties will be presented in detail. The magnetic properties of the sintered cores are strongly dependent on the microstructure obtained.
Highlights
Power electronics is a branch of electrical engineering that concerns static energy converters
One strategy arises from the developments in materials science, as properties evolve with the emergence of new materials, another strategy comes from the materials processing progress to shape and tailor the real object needed in the applications
The cold isostatic pressing method allows increasing the mechanical cohesion of the green ceramic enough to permit a machining step required to realize complex and large cores required for the interleaved converters
Summary
Intercell transformer (ICT) design is based on sharing the magnetic flux between the different phases of a converter (Fig. 1A). The shape of the magnetic cores needed is very specific and complex in order to link the different phases within the ICT (Fig. 1B). A classical method in ceramic processing technology to produce complex parts consists in machining a sintered raw core to the desired shape. We propose to develop another technique consisting in cold isostatic pressing to shape a green ceramic, followed by machining to the complex shaped core, and sintering. The cold isostatic pressing method allows increasing the mechanical cohesion of the green ceramic enough to permit a machining step required to realize complex and large cores required for the interleaved converters. The comparison is made trough the measurement of permeability spectra and hysteresis loops
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