12.7 Summary

In this chapter you have examined the basic operation of voltage regulators. Their purpose is simple: to provide constant, non-varying output voltages despite changes in either the AC source, or in the load current demand. Voltage regulation circuits are an integral part of just about every piece of modern electronic equipment. Due to their wide use, a number of specialized voltage regulator ICs are available from a variety of manufacturers.

Voltage regulation may be achieved through two main methods. These methods are linear regulation and switching regulation. In both cases, a portion of the output voltage is compared to a stable internal reference. The result of this comparison is used to drive a control element, usually a power transistor. If the output voltage is too low, the control element allows more current to flow to the load from the rectified AC source. Conversely, if the output is too high, the control element constricts the current flow. In the case of the linear regulator, the control element is always in the active, or linear, state. Because of this, the linear regulator tends to dissipate quite a bit of power and, as a result, is rather inefficient. On the plus side, the linear regulator is able to quickly react to load variations, and thus exhibits good transient response.

In contrast to the linear regulator, the control device in the switching regulator is either fully on or fully off. As a result, its power dissipation tends to be reduced. For best performance, fast control devices are needed. The control device is driven by a pulse-width modulator. The output of this modulator is a rectangular pulse whose duty cycle is proportional to the load-current demand. As the control device produces current pulses instead of a constant current, some means of smoothing the pulses is necessary. This function is performed by an 𝐿𝐶 filter. The main advantage of the switching regulator is its high efficiency. On the down side, switching regulators are somewhat more difficult to design, do not respond as fast to transient load conditions, and tend to radiate high frequency interference.

No matter what type of regulator is used, power dissipation can be rather large in the control device, so heat sinking is generally advisable. Heat sinks allow for a more efficient transferal of heat energy to the surrounding atmosphere than the control device exhibits on its own.