7.1 Introduction

In Part 1, characteristics of most of the frequently used topologies were discussed in sufficient depth to permit the choice of a topology most suited to the power supply specifications.

Most frequently, the topology will be selected to minimize the power transistor's off-voltage stress at high line and peak current stress at maximum output power. Other considerations are to minimize parts count, cost, and required volume of the complete supply. Minimizing potential RFI problems is also a frequent factor in choice of topology.

After a topology is selected, the next major decisions are to select an operating frequency and transformer core size which yields the specified maximum output power for the smallest-size transformer.

To make the frequency and transformer core selection, it is necessary to know the numerical relations between available output power and transformer parameters such as iron area, core window or bobbin winding area, peak flux density, operating frequency, and coil current density. In the following sections, equations giving these relations will be derived for most of the frequently used topologies.

These relations can be used in equation form to select a transformer core and operating frequency. But in equation form, they require a tentative guess as to the required core and frequency,...