Buck-Converter Design Demystified

Jun 1, 2006 12:00 PM
By Donald Schelle and Jorge Castorena, Technical Staff, Maxim Integrated Products, Sunnyvale, Calif.

Stepdown (buck) switching converters are integral to modern electronics. They can convert a voltage source (typically 8 V to 25 V) into a lower regulated voltage (typically 0.5 V to 5 V). Stepdown converters transfer small packets of energy using a switch, a diode, an inductor and several capacitors. Though substantially larger and noisier than their linear-regulator counterparts, buck converters offer higher efficiency in most cases.

Despite their widespread use, buck-converter designs can pose challenges to both novice and intermediate power-supply designers because almost all of the rules of thumb and some of the calculations governing their design are hard to find. And though some of the calculations are readily available in IC data sheets, even these calculations are occasionally reprinted with errors. In this article, all of the design information required to design a buck converter is conveniently collected in one place.

Buck-converter manufacturers often specify a typical application circuit to help engineers quickly design a working prototype, which in turn often specifies component values and part numbers. What they rarely provide is a detailed description of how the components are selected. Suppose a customer uses the exact circuit provided. When a critical component becomes obsolete or a cheaper substitute is needed, the customer is usually without a method for selecting an equivalent component.

This article covers only one stepdown regulator topology — one with a fixed switching frequency, pulse width modulation (PWM) and operation in the continuous-current mode (CCM). The principles discussed can be applied to other topologies, but the equations do not apply directly to other topologies. To highlight the intricacies of stepdown converter design, we present an example that includes a detailed analysis for calculating the various component values. Four design parameters are required: input-voltage range, regulated output voltage, maximum output current and the converter's switching frequency. Fig. 1 lists these parameters, along with the circuit illustration and basic components required for a buck converter.

Inductor Selection

Calculating the inductor value is most critical in designing a stepdown switching converter. First, assume the converter is in CCM, which is usually the case. CCM implies that the inductor does not fully discharge during the switch-off time. The following equations assume an ideal switch (zero on-resistance, infinite off-resistance and zero switching time) and an ideal diode:

where f_SW is the buck-converter switching frequency and LIR is the inductor-current ratio expressed as a percentage of I_OUT (e.g., for a 300-mA_p-p ripple current with a 1-A output, LIR = 0.3 A/1 A = 0.3 LIR).

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