Transformers and coupled inductors with optimum interleaving of

Details Transformers and coupled inductors with optimum interleaving of Transformers and coupled inductors with optimum interleaving of

1993-05-07

1996-08-06

Thomas, Laura

Inductor devices

Winding formed of plural coils

Two windings

336229, 336200, H01F 2728, H01F 500

Patent

active

055437737

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

This invention relates to the concept, design, construction and deployment of electrical power transformers and coupled inductors primarily for application in switch-mode power supplies, and in particular to the concept and methods of making such components whereby high frequency eddy current loss mechanisms are minimised. These devices can be used to overcome the problems of low efficiency and excessive power dissipation in high frequency switching applications.


BACKGROUND OF THE INVENTION

Conventional power transformer windings used in switch mode power supplies are usually either of the cylindrical or sandwich type (FIG. 1). The main flux produced by either winding arrangement is carried by the high permeability (usually E-shaped ferrite) core. Assuming that the current-turns product in each winding is equal (i.e. balanced mmfs, no magnetising current) the leakage flux due to the winding current is as shown (FIG. 1). This condition is approximated to in most conventional designs but applications of the present invention are not restricted to these cases. Referring to the mmf (NI) diagram it is evident that the leakage flux density: flux is to induce eddy currents in the windings which become more severe as the flux density increases. Referring to the flux-linkage (N.phi.) diagram, the distribution of the leakage inductance (flux linkage per amp, N.phi./I) is shown. The leakage inductance can be visualised as the solid of revolution of the N.phi. diagram about the central axis.


Eddy Current Winding Loss

The two main mechanisms contributing to a.c. winding loss are well known. The skin effect is caused by the current flowing within a conductor setting up a magnetic field which then induces eddy currents in the conductor. The direction of eddy current flow is such as to cancel out the main current in the centre of the conductor but reinforce it as the edges (FIG. 2). Thus the current tends to flow in a skin, around the outside of the conductor, and the effect is characterised by a skin depth, .delta., defined by: ##EQU1## where f=frequency, .mu.o=permeability of free space and .sigma.=conductivity of conducting material.
The proximity effect (1) is caused by the leakage flux due to all the windings (as in FIG. 1) inducing eddy currents in each winding in such a direction as to cancel the main current at the outer edge and to reinforce it at the inner edge (FIG. 3).
Hence the action of these mechanisms is to force the current to flow non-uniformly in the conductors, increasing the effective resistance. The resistance increase due to skin effect depends entirely on conductor dimensions, material and operating frequency whereas proximity effect depends on overall transformer construction and geometry also. Proximity effect is hence less easy to quantify but one method has been presented by Dowell (1).
It is generally the aim of power transformer design to minimize the leakage flux density, thus minimizing proximity effect losses, and hence minimizing a.c. resistance, and leakage inductance. (In High Frequency Power Transformers this is generally to reduce winding conduction losses and uncoupled magnetic energy storage rather than to reduce voltage regulation with load, as in the case of mains frequency transformers). Leakage flux density, B.sub.1, can clearly be reduced by: cylindrical arrangement or a low wide sandwich transformer. giving a modified mmf diagram such as that of FIG. 4.
Additionally eddy current losses can be mimimized by using conductors less than two skin depths thick, or multiple stranded conductors such as Litz wire.
For very low Leakage transformers, particularly in radio equipment, a bifilar winding arrangement is used in which the primary and secondary windings are twisted together prior to winding. This ensures that, for an N turn winding, the peak mmf is never greater than one Nth of its value in a single layer conventional arrangement; i.e. each primary turn is interleaved with a secondary turn. Such transformers are usually wound on a toroidal core to g

REFERENCES:
patent: 3263191 (1966-07-01), Arvonio
patent: 4103267 (1978-07-01), Olschewski
patent: 4258467 (1981-03-01), Witte
patent: 4536733 (1985-08-01), Shelly
patent: 4577175 (1986-03-01), Burgher et al.
patent: 4689594 (1987-08-01), Kawabata et al.
patent: 5039964 (1991-08-01), Ikeda
patent: 5140706 (1992-08-01), Maeda et al.
patent: 5257000 (1993-10-01), Billings et al.
patent: 5331536 (1994-07-01), Lane

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