Power: Step-Down Regulators—Part 1: The Fundamentals

Sept. 20, 2004

What's a buck converter? A buck converter, or step-down voltage regulator, provides either isolated or non-isolated, switch-mode dc-dc conversion with the advantages of simplicity and low cost. The figure shows a simplified non-isolated buck converter that accepts a dc input and uses pulse-width modulation (PWM) of switching frequency to control the output of an internal power MOSFET (Q1). An external Schottky rectifier diode, together with external inductor and output capacitors, produces the regulated dc output. The regulator IC compares a portion of the rectified dc output with a voltage reference (VREF) and varies the PWM duty cycle to maintain a constant dc output voltage. If the output voltage wants to increase, the PWM lowers its duty cycle to reduce the regulated output, keeping it at its proper voltage level. Conversely, if the output voltage tends to go down, the feedback causes the PWM duty cycle to increase and maintain the proper output.What's the difference between non-isolated and isolated regulators? An isolated converter employs a transformer to provide dc isolation between the input and output voltage. The non-isolated converter usually employs an inductor, and there is no dc voltage isolation between the input and the output. The vast majority of applications do not require dc isolation between input and output voltages.What's the effect of switching frequency? Switching frequency determines the physical size and value of external filter inductors and capacitors. The higher the switching frequency, the smaller the physical size and component value. However, there is an upper frequency limit where either magnetic losses in the inductor or switching losses in the regulator IC and power MOSFET reduce efficiency to an impractical level.

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About the Author

Sam Davis 2

Sam Davis was the editor-in-chief of Power Electronics Technology magazine and website that is now part of Electronic Design. He has 18 years experience in electronic engineering design and management, six years in public relations and 25 years as a trade press editor. He holds a BSEE from Case-Western Reserve University, and did graduate work at the same school and UCLA. Sam was the editor for PCIM, the predecessor to Power Electronics Technology, from 1984 to 2004. His engineering experience includes circuit and system design for Litton Systems, Bunker-Ramo, Rocketdyne, and Clevite Corporation. Design tasks included analog circuits, display systems, power supplies, underwater ordnance systems, and test systems. He also served as a program manager for a Litton Systems Navy program.

Sam is the author of Computer Data Displays, a book published by Prentice-Hall in the U.S. and Japan in 1969. He also authored the book Managing Electric Vehicle Power. He is also a recipient of the Jesse Neal Award for trade press editorial excellence, and has one patent for naval ship construction that simplifies electronic system integration.

You can also check out additional articles on his other author page.