Buck Converter Calculator

Size a buck power stage and check real inductors, capacitor banks and IC limits across the input-voltage range, with conduction-mode checks and current waveforms.

Method, formulas and references

Model and operating limits

This is a preliminary continuous-conduction buck design based on TI SLVA477B. Efficiency is a user estimate: D = Vout/(ηVin), so the converter also covers its own losses. This efficiency-adjusted duty is a sizing approximation, not a device-specific switching simulation. In a buck the inductor's average current equals the load current at every input voltage.

The initial inductance follows TI: ΔIestimate = ripple ratio × Iout; L = Vout(Vin,nom − Vout)/(ΔIestimate fs,min Vin,nom). Enter the actual part value before finalising a design. Current screening uses Lmin = L(1 − tolerance) and fs,min. Inductance reduction under DC current, temperature and saturation must be included in the effective minimum value you choose.

ΔIL = (Vin − Vout) D / (fs Lmin); Ipeak = Iout + ΔIL/2; Ivalley = Iout − ΔIL/2; IL,rms = √(Iout² + ΔIL²/12).

ΔIL grows with input voltage, so ripple, peak current and the CCM boundary are worst at Vin,max. CCM requires Ivalley > 0, i.e. a load above ΔIL/2. Boundary/DCM operation and forced-PWM reverse current are not solved. If any point in the checked range leaves CCM, range-wide current and capacitor results are withheld.

Capacitors and ripple

Ceff = Cnom × (1 − tolerance) × retained fraction. Retention is taken from the selected capacitor's voltage and temperature data; it is not inferred from an X5R/X7R label. Enter complete-bank capacitance, ESR and RMS ratings. A value of zero for the bank means sizing only.

The output capacitor sees only the triangular inductor ripple: Cout,min = ΔIL/(8 fs ΔVout), Icout,rms = ΔIL/√12, and ESR ripple = ESR × ΔIL. The sum of capacitive and ESR ripple is a conservative upper bound, since their extrema need not coincide; ESL spikes are excluded.

The input capacitor carries the pulsed switch current minus its average, which makes it the harder-working capacitor in a buck:

Icin,rms = √(D(Iout² + ΔIL²/12) − (D Iout)²) ≈ Iout √(D(1 − D)); Cin,min = Iout D(1 − D)/(fs ΔVin).

The RMS current peaks at D = 0.5, so that input voltage (Vin = 2Vout/η) is always evaluated when it lies inside the range. Input ESR ripple is bounded by ESR × Ipeak, the current step at turn-on. The displayed minimum is the larger of the charge-balance estimate and the IC datasheet minimum. Source impedance, cable inductance, start-up and load transients can demand substantially more capacitance. Output capacitance must also stay within the IC's compensation/stability recommendations.

Rating checks and range screening

The input range is sampled at 201 evenly spaced voltages plus the nominal input and the 50 % duty point. Checks apply at the entered load and constant efficiency, not a full load/temperature sweep.

Current-limit ceiling = Ilim,min − ΔIL/2 at the worst-case ripple, for peak-current-limited ICs and only where the limit exceeds ΔIL. This excludes thermal limits, start-up and protection behaviour. A saturation-current check at operating peak does not certify survival at the maximum IC current limit.

Shortest on-time = Dmin/fs,max at Vin,max; shortest off-time = (1 − Dmax)/fs,max at Vin,min. The minimum on-time is the usual limit for high step-down ratios at high frequency. The switch is screened against Vin,max + Vf (the switch node swings from Vin to −Vf) and the diode against Vin,max. Voltage-rating passes cover steady state only; additional transient and ringing headroom is required. Unspecified ratings remain unchecked.

Total estimated loss = Pout(1/η − 1). Winding I²R, capacitor ESR and diode losses (Vf × Iout(1 − D), largest at Vin,max) are a partial breakdown of that budget, never added to it. MOSFET conduction/switching loss, inductor core loss and thermal behaviour are not separately modelled.

References

TI SLVA477B — Basic Calculation of a Buck Converter's Power Stage, 2015. Equation 1 of the main text is used; the appendix prints η in the numerator.
R. W. Erickson, D. Maksimović, Fundamentals of Power Electronics, Springer — input-capacitor RMS current including the inductor ripple.

Related guides

Buck Converter Formulas: Duty Cycle, Inductor and Capacitors, Worked Through

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