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.
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 fSW is the buck-converter switching frequency and LIR is the inductor-current ratio expressed as a percentage of IOUT (e.g., for a 300-mAp-p ripple current with a 1-A output, LIR = 0.3 A/1 A = 0.3 LIR).
An LIR of 0.3 represents a good tradeoff between efficiency and load-transient response. Increasing the LIR constant — allowing more inductor ripple current — quickens the load-transient response, and decreasing the LIR constant — thereby reducing the inductor ripple current — slows the load-transient response. Fig. 2 depicts transient response and inductor current for a given load current, for LIR constants ranging from 0.2 to 0.5.
Peak current through the inductor determines the inductor's required saturation-current rating, which in turn dictates the approximate size of the inductor. Saturating the inductor core decreases the converter efficiency, while increasing the temperatures of the inductor, the MOSFET and the diode. You can calculate the inductor's peak operating current as follows:
For the values listed in Fig. 1, these equations yield a calculated inductance of 2.91 µH (LIR = 0.3). Select an available value that is close to the calculated value, such as a 2.8 µH, and make sure that its saturation-current rating is higher than the calculated peak current (IPEAK = 8.09 A).
Choose a saturation-current rating that's large enough (10 A in this case) to compensate for circuit tolerances and the difference between actual and calculated component values. An acceptable margin for this purpose, while limiting the inductor's physical size, is 20% above the calculated rating.
Inductors of this size and current rating typically have a maximum dc resistance range (DCR) of 5 mΩ to 8 mΩ. To minimize power loss, choose an inductor with the lowest possible DCR. Although data sheet specifications vary among vendors, always use the maximum DCR specification for design purposes rather than the typical value, because the maximum is a guaranteed worst-case component specification.