The linear regulator, also called the 3-terminal regulator, is a familiar device capable of stepping down the DC in a simple manner. Basically this device consists of three terminals, input, output, and GND, with an output voltage that is preset to an industry standard voltage. In addition, there is a type that permits adjustable output through the use of external resistors and a type that has a built-in on/off (shutdown) capability. Depending upon the specific function supported, the number of terminals provided in a linear regulator can vary.
In terms of operating mechanism, the linear regulator operates in complete linear feedback loop control, where an error amplifier monitors the voltage that is fed back from the output and regulates the output so that it remains constant despite input variation or output load fluctuations. Devoid of any switching operations, the linear regulator is free of any noise or ripples originating from switching.
While the method of using it is simple, the operation of the linear regulator requires caution in terms of loss = heat. As illustrated in Figure 15, in the linear regulator the product of the voltage difference between input and output and the current that flows into the input terminal amounts to power loss, which manifests itself in the form of heat. Without a heat radiator plate, linear regulators can handle at most 2W. Obviously, a large loss means poor efficiency.
For application in AC/DC converter, because the input to a linear regulator IC cannot accommodate the voltage as large as 140V that is produced by direct rectification of 100VAC, it would not be feasible to replace the switching DC/DC part of a switching system-based AC/DC conversion with a linear regulator IC. It is possible to make a linear regulator in a discrete configuration through the use of high voltage-tolerance transistors. However, considering the problem of heat if 140V DC voltage is stepped down to 12V, for example, the use of a linear regulator would not be a realistic option. The problem becomes compounded when we consider the requisite circuit design and the footprint requirement including the provision of a heat sink.
For these reasons, the transformer system is commonly used to provide DC/DC conversion (stepping down regulation) based on a linear regulator. A linear regulator can be used at an efficiency that is not extremely poor and with a tolerable level of heat dissipation, provided that the transformation (stepping down) using a transformer is optimized, under a condition where the input/output difference in the linear regulator does not grow very large.
In addition, because the linear regulator has a ripple rejection capability, in certain situations it can be used to reject any residual ripple from the DC after it has been smoothed out. In noise-sensitive applications the advantages of such a combination can be beneficial.
・Although linear regulators are commonly used in the transformer system, the maximum allowable loss is about 2W, and efficiency may be a concern.
・Because the linear regulator is free of any switching noise, it can be beneficial in noise-sensitive applications.