Overview

Transformer-isolated switching power supply topologies offer a great many advantages over the non-isolated topologies. Three of the major factors are: it is very easy to create multiple output voltages from the same transformer, transformers create a natural dielectric boundary from the input to output circuits, and the input voltage levels are independent from the output voltages. Unfortunately, multiwinding magnetics are more expensive to manufacture than simple inductors, but this disadvantage may be attractive in the context of the overall system cost.

Transformer-isolated topologies can be used to create multiple output non-isolated converters or to dielectrically isolate a section of the system such as a floating sensor circuit. They are very flexible.

Transformer-isolated topologies are also used as the basis of all of the 48 VDC (telecom input bus voltages) and the AC/DC converters. Here, the advantage provided by isolation due to the construction of the transformer plays a very important role in the many "hazardous" applications that can present several thousands of volts between the input and output(s). This topic is covered in Chapter 11.

When testing these types of supplies, it is good to isolate the ground connection of the oscilloscope and the other instruments with an earth ground isolation plug. Otherwise you will be connecting the various isolated circuits together, which may result in pyrotechnic light shows on your workbench.

Ray Mack delves into the circuits needed for these topologies in an understandable way.

Marty Brown

In this chapter, we will look at detailed designs...