Many types of switching regulators exist, which are classified in various ways, depending upon one’s point of view. In this section, we classify switching regulators by input power, circuit types, and function and operation.
<Types of Switching Regulators by Circuit Configuration>
First of all, switching regulators can be divided into DC/DC and AC/DC converters, depending upon whether the input power is DC (direct current) or AC (alternating current). Each type, in turn, has isolated and non-isolated subcategories.
In the isolated type, the input (primary) and the output (secondary) are isolated. For isolation, the type of component frequently employed is a transformer. In industrial and medical applications where a high degree of safety is mandatory in the event of a malfunction, the isolated type is used as a standard device. The non-isolated type is characterized by the presence of electrical conduction between the input and output. Most voltage converters used on circuit boards for which isolation is not deemed essential are of the non-isolated type.
Construction of an isolated or non-isolated converter involves the use of a suitable circuit type. The circuit types include rectifying and flyback type, each comprising different components, circuit size, and operating principles.
Focusing now on classification by function and operating method, we will discuss switching regulators in terms of DC/DC converters. Since the AC/DC converter, after performing rectification and smoothing of the AC input in the initial stage, basically operates as a DC/DC converter, we may assume that both the AC/DC and DC/DC converters behave alike.
In a DC/DC conversion, a switching regulator can step up or step down the input voltage. As an application of this capability, conversions involving stepping up/down and inversion are also possible. Depending on the specific function that is required, different circuit configurations and ICs must be selected.
There are Pulse Width Modulation (PWM) and Pulse Frequency Modulation (PFM) modes to control the output voltage. PWM is a mode that delivers regulation by adjusting the on/off time ratio at a constant switching cycle (frequency). PFM uses a fixed on/off time with a variable frequency; more on this later.
The current mode, voltage mode, and hysteresis represent feedback control methods designed to regulate the output. This will also be discussed in detail later.
Switching regulators comprise combinations of these options. The optimal combination is selected based on the intended application, input/output conditions, the required specifications and performance goals, and by consideration of limiting factors such as cost and size. To make the best possible choice, we need to know the pros and cons of each option.