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What Is a Buck Converter?

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  • Written By: Geisha A. Legazpi
  • Edited By: Lauren Fritsky
  • Last Modified Date: 18 April 2014
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A buck converter is a converter that decreases the voltage that reaches a direct current (DC) load. For instance, the output of a 24-volt (V) direct current (VDC) power source may be stepped down to energize 12-volt equipment. A well-designed buck converter is able to provide a steady output voltage under varying load conditions and maintain very low-power dissipation, leading to high efficiency.

Linear regulators may be a simple solution for low-power requirements. They make use of a series pass power transistor that controls the current across the load to produce the steady DC output voltage. If a 50-VDC supply used a linear regulator to drive a 25-V load at 1 ampere (A), the power across the load as well as the linear regulator will be 25 watts (W). This means 50 W is being drained from the 50-V power supply. The conversion efficiency is 50%, but if a buck converter is used and the power dissipation in the buck converter is 2.5 W while the power at the load is still 25 W, the efficiency is about 90%.

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The buck converter is a DC to DC converter that operates much like a switched-mode power supply (SMPS), which controls the duty cycle of a fixed frequency square wave. When the load requires less current, the “on” time of the square wave is low, but when the load requires a current very close to the limit of the SMPS, the “on” time goes beyond 85%. The switched DC uses a series inductor with fast-switching diode that uses the inductive backflow to sustain energy transfer when the main driving element is off for brief periods.

For an application that is opposite that of the buck converter, the step-up boost converter (SUBC) is used. The SUBC generates an output voltage that is higher than its input. In this application, the inductor in series with the load converts a collapsing magnetic field into a DC voltage that is higher than the circuit’s input voltage.

Another electrical power conversion device is the buck-boost converter that can work either as a buck converter or as a boost converter. In solar energy systems, it is possible to obtain a wide DC voltage range depending on the availability of sunlight. A battery bank charged by a photovoltaic array may have a voltage ranging from 40 to 56 VDC. If a sensitive load requires 47 to 49 VDC, then a buck-boost converter will work as a boost converter when the battery bank voltage is less than 47 V. The converter will work as a buck converter when the voltage is more than 49 V.

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