Electronic Design

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

Sponsored by: NATIONAL SEMICONDUCTOR

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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