Buck-Converter Design Demystified

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

Adding the output-voltage ripple due to capacitance value (the first term in Eq. 4) and the output-capacitor ESR (the second term in Eq. 4) yields the total output-voltage ripple for the stepdown converter:

A decent stepdown converter usually achieves an output-voltage ripple of less than 2% (40 mV in our case). For a 560-µF output capacitance, Eq. 5 yields 18.8 mΩ for the maximum calculated ESR. Therefore, choose a capacitor with ESR that's lower than 18.8 mΩ and a capacitance that's equal to or greater than 560 µF. To achieve an equivalent ESR value less than 18.8 mΩ, you can connect multiple low-ESR capacitors in parallel.

Fig. 3 presents output-ripple voltage versus output capacitance and ESR. Because our example uses tantalum capacitors, capacitor ESR dominates the output-voltage ripple.

Input Capacitor Selection

The input capacitor's ripple-current rating dictates its value and physical size, and the following equation calculates the amount of ripple current the input capacitor must be able to handle:

Fig. 4 plots ripple current for the capacitor (shown as a multiple of the output current) against the input voltage of the buck converter (shown as a ratio of output voltage to input voltage). The worst case occurs when V_IN = 2V_OUT (V_OUT/V_IN = 0.5), yielding I_OUTMAX / 2 for the worst-case ripple-current rating.

The input capacitance required for a stepdown converter depends on the impedance of the input power source. For common laboratory power supplies, 10 µF to 22 µF of capacitance per ampere of output current is usually sufficient. Given the design parameters of Fig. 1, you can calculate the input-ripple current as 3.16 A. You then can start with 40 µF in total input capacitance and can adjust that value according to subsequent test results.

Tantalum capacitors are a poor choice for input capacitors. They usually fail “short,” meaning the failed capacitor creates a short circuit across its terminals and thereby raises the possibility of a fire hazard. Ceramic or aluminum-electrolytic capacitors are preferred because they don't have this failure mode.

Ceramic capacitors are the better choice when pc-board area or component height is limited, but ceramics may cause your circuit to produce an audible buzz. This high-pitched noise is caused by physical vibration of the ceramic capacitor against the pc board as a result of the capacitor's ferroelectric properties and piezo phenomena reacting to the voltage ripple. Polymer capacitors can alleviate this problem. Polymer capacitors also fail short, but they are much more robust than tantalums, and therefore are suitable as input capacitors.

Diode Selection

Power dissipation is the limiting factor when choosing a diode. The worst-case average power can be calculated as follows:

where V_D is the voltage drop across the diode at the given output current I_OUTMAX. (Typical values are 0.7 V for a silicon diode and 0.3 V for a Schottky diode.) Ensure that the selected diode will be able to dissipate that much power. For reliable operation over the input-voltage range, you must also ensure that the reverse-repetitive maximum voltage is greater than the maximum input voltage (V_RRM ≥ V_INMAX). The diode's forward-current specification must meet or exceed the maximum output current (i.e., I_FAV ≥ I_OUTMAX).

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