John T. Hall

An ultra-compact transformer for a 100 W to 120 kW inductive coupler for electric vehicle battery charging

A new generation of electric vehicles is being developed. A key problem to be solved is that of charging the batteries. One means of charging uses inductive coupling. The inductive coupling approach is essentially a transformer with a removable primary winding connected to a charging unit via a cable. The secondary and the core of the transformer are on the vehicle. This paper presents an inductive coupler which has been demonstrated delivering from 100 W to 120 kW continuously at a frequency of 75 to 120 kHz. The transformer is very compact (<100 in/sup 3/). The primary purpose of this paper is a discussion of the power transformer. In addition the paper briefly addresses how the design of a magnetic device, which is usually a strictly technical exercise between engineers, is impacted when it is directly accessible to consumers in a mass market.

Optimizing AC and DC winding losses in ultra-compact, high-frequency, high-power transformers

Removing heat from the windings of a high power (>100 kW) transformer is a significant thermal challenge, especially in the case of the ultra-compact electric vehicle inductive charging transformer. In this paper, nine different transformer design techniques are outlined, analyzed and compared for optimization of the primary and secondary winding resistances to minimize copper losses in a high-power, high-frequency transformer. For the inductive charging transformer example illustrated, these techniques reduce the total winding losses by as much as 5 to 1 and the AC winding losses by 14 to 1. The techniques used are a mixture of well-known approaches, such as winding interleaving, and other lesser-known approaches, such as flux shaping. The winding losses are calculated using a two-dimensional finite element analysis (FEA) program, and using one-dimensional equations for comparative purposes. These techniques result in a high-power transformer with a power density greater than 1300 W/in/sup 3/.