Protection Functions

Protection functions for a power supply unit are important not only in terms of safety, but also for the reliability of the circuit. There are several major types of protection functions: input (over-voltage, input voltage drop with non-functional), output (overload, short circuit, reverse voltage, and over-voltage), and temperature (excessive self-heating, and increased ambient temperature). Although not all of these functions are necessary in all power supply circuits, most of these functions are either essential or desirable. As noted in the previous section, most of the required protection functions are integrated into the control IC. The job of a design engineer entails the proactive use of such functions.

Figure 38

In particular, in the case of AC-DC conversion, since the AC is input from the outlet, in some cases the input power is subject to massive surges due to lightning. Also, the input voltage is high, and even in homes a current as high as 30A to 50A can prevail. Therefore, it does not take a flash of genius to realize that if an unexpected event occurs the ensuing damage can be severe and extensive.

In the schematic diagrams of AC-DC conversion circuits presented in this article, An AC line fuse is omitted. In most power circuits, a fuse is provided at the source. Because this is something to be taken for granted, remember that a fuse is omitted in most schematic diagrams.