| Abstract |
High frequency switching power converters are widely considerd interesting low-cost alternatives to their linear or SCR controlled counterparts because of their higher efficiency, smaller volume and weight. To a large extent, the cost of such switching power converters is determined by the size of the filter components and the isolation wide band power transformer(s). Converter cost can be reduced by using high switching frequencies, which lower the overall dimensions of the filter and the transformers. The advent of low-cost power MOS transistors allows switching frequencies up to 200 kHz. However, increasing the switching frequency introduces high constraints on the design of the power wide band isolation transformers. For example, the leakage inductance can cause severe voltage spikes during the turn-off transition of the transistors. At high switching speeds, leakage inductances behave as series impedances, and thus limit peak power transfer (Bonte, Baart 1983b). The combination of the transformer leakage inductances and winding capacitances results in high frequency (3-30 MHz) resonating modes. The energy accumulated in such modes increases with the switching frequency and rise time, and cannot be recuperated efficiently in most cases. Most of the published guidelines for high frequency power transformer design advocate the use of ferrite pot cores (Snelling, 1969). Pot core solutions are suggested because of their low achievable stray vs. magnatization inductances and low resistive losses. In this note, however, we show that better results can be obtained using toroidal ferrite core pulse transformers, both as to volume per unit power as well as to high frequency behavior. Furthermore, we present a simple, yet robust high frequency model for the pulse transformer which is far easier to use than the more complicated models in (O'Meara, 1961; Müller and Buckow, 1984). We first analyse published guidelines for the design of pulse transformers. Next we propose our own design, which outperforms the published data on several counts. Finally, we present a simple and robust high frequency network model for our transformer design. |
