Chapter 6 - Transformer-isolated Circuits

Abstract

This chapter discusses detailed designs of transformer isolated converters. Their primary application is in off-line power supplies, but these designs are also useful in applications where safety isolation is required or where the input voltage can vary above and below the output voltage. The output can be connected directly to the control IC in applications where isolation is not required. Using an optocoupler is the “easiest” way to transfer the output voltage information across the isolation barrier to a control IC on the primary side. Optocoupler phototransistors are built with a large base region to improve the conversion of light to current. The large base region creates a larger input capacitance and reverse transfer capacitance than in regular transistors. Although it is only a few picofarads, the Miller effect can amplify the capacitance to a much larger value. The Miller capacitance creates a pole at a fairly low frequency.