Another type of regulator, the switching regulator, uses a transistor switch with an inductor or transformer to change a dc-input voltage. Figure 2a shows a buck-switching regulator that steps down a voltage and that operates analogously to a water wheel (Figure 2b). The device's rate of rotation is analogous to the current following through the inductor.
Just like an inductor, the water wheel cannot instantaneously stop or start. The figure may give some insight about why engineers often refer to the diode as "freewheeling." When the valve turns off, the inertia of the water wheel creates a powerful suction. The wheel needs water to keep rotating, and the check valve provides this function.

A boost converter employs the same water-wheel analogy (Figure 3). Many engineers have trouble with magnetic circuits because their high reactance means that the current does not track the voltage as it does in a resistor. An intuitive understanding of the buck and boost converters allows you to understand the more complicated architectures, such as Cuk, boost-buck, and SEPIC.
Converters can also use transformers to create an isolated output (Figure 4). Flyback converters, which differ from forward converters only in the polarity of the output diode, use a transformer as a choke. They store energy in the magnetic field when the switch is closed and as current increases in the primary. When the switch opens, the energy in the magnetic field discharges through the secondary. Designers favor flyback converters for their low cost and their ability to make multiple outputs that all track each other reasonably well.

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