A second switch is needed to disconnect the input from the output. Because the input is connected directly to the output through the inductor and rectifier, turning off the PWM controller does not disconnect the input from the load. High pulsed input currents can result in electromagnetic interference (EMI) issues and lower conversion efficiencies. For example, a design that triples the input voltage also draws triple the input current. The input current increases proportionally with the boost gain. To maintain stable operation, practical boost ratios are usually 3X or less. Increasing battery cell voltages is a common application for boost converters.
In a boost converter, the output voltage can be equal to or higher than the input voltage. This becomes a synchronous buck converter when D1 is replaced with a FET for synchronous rectification. Figure 1: Asynchronous buck converter basic schematic. Replacing the rectifier diode with a FET for synchronous rectification results in a synchronous buck with significantly improved conversion efficiency. To simplify output filtering, the inductor is usually specified so the output ripple is less than 30% of the load current. The current delivered to the load is the average value of the inductor current. The inductor and rectifier are key elements in a buck converter. Among its many applications, the buck converter is widely used in distributed power systems to convert the bulk distribution voltage (usually 12 to 48 Vdc) to lower voltages such as 3.3V or 1.8V to power integrated circuits such as microprocessors, field programmable gate arrays, and so on. It steps down an input voltage to a lower output voltage. The buck converter is the most common power conversion topology (Figure 1). This article also does not consider various switched capacitor topologies, a review of those approaches can be found in “ Switched capacitor power conversion for electronic systems.” The next FAQ will review two- and four-switch power conversion topologies. Specialty topologies such as the Joule Thief are not included. This FAQ reviews some of the most common one-switch topologies. DC/DC conversion is the basic building block for many power converter designs including AC/DC power supplies, battery chargers, and uninterruptible power systems.
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