Skip to content
RFrftools.io
Power

Buck Converter Design Calculator

Design a synchronous buck converter: calculate duty cycle, inductor, output and input capacitors, peak current, and ripple. Free, instant results.

Loading calculator...

Formula

D=VoutVin,Lmin=(Vin−Vout)⋅Dfsw⋅ΔILD = \frac{V_{out}}{V_{in}},\quad L_{min} = \frac{(V_{in}-V_{out})\cdot D}{f_{sw}\cdot \Delta I_L}

Reference: Erickson & Maksimovic, "Fundamentals of Power Electronics" 3rd ed.

D— Duty cycle
Vᵢₙ— Input voltage (V)
Vₒᵤₜ— Output voltage (V)
fₛw— Switching frequency (Hz)
ΔIL— Inductor current ripple (A)

How It Works

The buck converter calculator computes duty cycle, inductor value, and capacitor requirements for step-down DC-DC conversion — essential for point-of-load regulators, battery chargers, and embedded power supplies. Power electronics engineers, hardware designers, and SMPS developers use this tool to achieve 90-98% efficiency in voltage reduction. According to Erickson & Maksimovic's 'Fundamentals of Power Electronics' (3rd ed.), synchronous buck converters reach 97% efficiency at 500 kHz switching frequency with properly selected GaN FETs exhibiting 5 mΩ Rds(on). The duty cycle D = Vout/Vin determines the voltage conversion ratio in continuous conduction mode (CCM). Inductor selection follows L = Vout(1-D)/(fsw × ΔIL), where 20-40% peak-to-peak ripple current is standard per TI application note SLVA477. Output capacitor requirements depend on ripple voltage target: Cout = ΔIL/(8 × fsw × ΔVout) for ceramic capacitors, with ESR-dominated ripple requiring ESR < ΔVout/ΔIL. Modern integrated converters (TI TPS62840, 60 nA quiescent) enable 95% efficiency even at 1 µA load current.

Worked Example

Design a 12 V to 3.3 V buck converter for a Raspberry Pi supply at 3 A maximum load. Target specifications: <30 mV output ripple, >92% efficiency, 500 kHz switching frequency. Step 1: Calculate duty cycle — D = 3.3/12 = 0.275 (27.5%). Step 2: Select inductor for 30% ripple — ΔIL = 0.3 × 3 A = 0.9 A. L = 3.3 × (1-0.275)/(500k × 0.9) = 5.3 µH. Use standard 4.7 µH (Würth 744373680047) with 8.5 A saturation current. Step 3: Calculate output capacitance — Cout = 0.9/(8 × 500k × 0.03) = 7.5 µF minimum. Use 3 × 22 µF/10V X5R ceramics (effective 45 µF after DC bias derating). Step 4: Select controller — TI TPS54360 (60 V input, 3.5 A output) with integrated compensation. Step 5: Verify efficiency — Estimated: conduction loss = 3² × 0.07Ω = 0.63 W, switching loss ≈ 0.3 W. Total loss ≈ 0.93 W. Efficiency = 9.9 W/(9.9 + 0.93) = 91.4%.

Practical Tips

  • ✓Per TI's 'Power Supply Design Seminar', use ceramic capacitors with X5R or X7R dielectric — Y5V capacitors lose 80% capacitance at DC bias and exhibit ±22% tolerance
  • ✓Implement spread-spectrum frequency modulation (SSFM) to reduce EMI peaks by 10-15 dB — TI TPS65281 varies switching frequency ±6% to spread harmonics
  • ✓Place input and output capacitors within 5 mm of the IC pins to minimize parasitic inductance — 10 mm trace adds 10 nH, causing 500 mV voltage spikes at 50 A/µs di/dt

Common Mistakes

  • ✗Neglecting inductor saturation current — a 10 µH inductor rated for 2 A saturates at 3 A peak (DC + ripple), losing 80% of inductance and causing output voltage collapse
  • ✗Using electrolytic capacitors at high frequency — aluminum electrolytics have 100-500 mΩ ESR at 500 kHz, causing 90-450 mV ripple versus <10 mV with MLCC ceramics
  • ✗Ignoring input capacitor requirements — input current is pulsed at D × Iload; inadequate input capacitance causes 30-50% higher input ripple, failing EMI requirements

Frequently Asked Questions

Per Mohan's 'Power Electronics' (3rd ed.), losses include: conduction (Irms² × Rds(on)), typically 1-3% at full load; switching (½ × Vin × Iout × (tr+tf) × fsw), 1-5% at 500 kHz; gate drive (Qg × Vgs × fsw), 0.1-0.5%; inductor DCR (Iout² × DCR), 0.5-2%. GaN FETs achieve 99% efficiency by reducing switching losses 10× versus silicon MOSFETs.
Higher frequency enables smaller LC components but increases switching losses. Per Analog Devices AN-1471: 100-300 kHz for automotive/high power (>10 W), 300 kHz-1 MHz for consumer (1-10 W), 1-3 MHz for mobile/compact designs (<1 W). EMI requirements may mandate frequencies outside broadcast bands (AM: 530 kHz-1.7 MHz).
Yes — multiphase buck converters power server CPUs at 200+ A. Intel VR14 specification requires 12 V to 1.0 V at 300 A with <20 mV load transient response. This uses 6-8 phases with 50 A per phase, achieving 92% efficiency. Per Infineon application notes, current sharing accuracy of ±3% between phases is critical.
Instability typically results from insufficient phase margin (<45°) in the feedback loop. Ceramic output capacitors have low ESR, eliminating the ESR zero that stabilizes Type II compensation. Solutions: use Type III compensation (adds 2 zeros), select controllers designed for ceramic capacitors (TI TPS62913), or add small ESR with 10-50 mΩ resistor in series with output capacitor.
Modern buck controllers integrate cycle-by-cycle current limiting at 120-150% of rated current. Per TI SLVA504, hiccup mode protection reduces average power during shorts to <5% of normal operation, preventing thermal damage. For critical applications, add output fuse (fast-blow, 125% of max load) and input reverse-polarity protection (P-FET or ideal diode controller).
L = Vout × (1-D) / (fsw × ΔIL), where D = Vout/Vin. For 12 V → 5 V at 2 A, 300 kHz, targeting 30% ripple: D = 0.417, ΔIL = 0.6 A. L = 5 × 0.583 / (300k × 0.6) = 16.2 µH. Use 22 µH standard value. Per Würth Elektronik guidelines, select inductor with Isat ≥ Iout + ΔIL/2 = 2.3 A and Irms rating ≥ Iout = 2 A.
Output ripple sources per TI SLVA630: (1) Inductor ripple current charging capacitor — reduce L or increase Cout. (2) Capacitor ESR — MLCC ceramics have 2-10 mΩ ESR versus 50-500 mΩ for electrolytics; 0.6 A ripple × 100 mΩ ESR = 60 mV ripple. (3) PCB layout — keep high-current loop (FET-inductor-capacitor-FET) under 2 cm². (4) Insufficient output capacitance — X5R ceramics lose 50% capacitance at rated DC voltage.
At light load, fixed losses dominate: gate drive power = Qg × Vgs × fsw (e.g., 20 nC × 5 V × 500 kHz = 50 mW), controller quiescent current (1-5 mA × Vin), and synchronous FET body diode conduction. Per TI TPS62840 datasheet, pulse-frequency modulation (PFM) maintains >90% efficiency down to 1 µA load. Without PFM, efficiency at 1% load drops to 50-60%.

Related Articles

Power Electronics

Buck Converter: Inductor, Capacitor & Efficiency

How to design a synchronous buck converter from scratch. Calculates duty cycle, inductor value, output capacitor, and estimates efficiency with worked examples.

Power Electronics

Magnetics Optimizer: Pareto-Optimal Transformers

A 48 V to 12 V, 36 W flyback through the Magnetics Optimizer: the loss-versus-size Pareto front, core loss from datasheets, and what interleaving is worth.

Power Electronics

SMPS Buck Converter Stability: Monte Carlo Tuning

A 12 V to 5 V buck through the SMPS Control Loop Stability Analyzer: phase margin, Monte Carlo yield, and why a ceramic output capacitor breaks a stable loop.

Advanced Simulation Tools

SMPS Control Loop Stability Analyzer

Model switch-mode power supply control loops using small-signal transfer functions. Get Bode plots of T(s), extract phase margin, gain margin, and crossover frequency, then run Monte Carlo trials over component tolerances (L ±20%, C ±20%, ESR ±50%, load ±30%) to compute manufacturing yield.

Magnetics & Transformer Design Optimizer

NSGA-II optimizer finds Pareto-optimal core and winding designs across 40 cores (E, ETD, PQ, RM and toroids) and 104 core/material combinations, with TDK and Ferroxcube ferrites and Micrometals iron powder. Minimizes total loss and volume simultaneously — shows the full efficiency-vs-size trade-off frontier.

Shop Components

As an Amazon Associate we earn from qualifying purchases.

DC-DC Buck Converter Modules

Adjustable step-down converter modules for bench and prototype use

Amazon →

LDO Voltage Regulator Kit

Assorted low-dropout linear regulators for prototyping

Amazon →

Electrolytic Capacitor Kit

Aluminum electrolytic capacitor kit for power supply filtering

Amazon →

Power Inductor Kit

Assorted shielded power inductors for switching supply designs

Amazon →

Related Calculators

Power

LDO Thermal

Calculate LDO regulator power dissipation, junction temperature, thermal margin, and minimum dropout voltage for thermal design validation.

Power

Voltage Divider

Calculate voltage divider output voltage, current, Thévenin impedance, and power dissipation from Vin, R1, and R2. Ideal for bias networks and level shifting.

Power

RC Snubber

Size an RC damping snubber from the ringing you measure: the ring frequency alone and with a known capacitor added give the parasitic capacitance and inductance, the snubber resistor and capacitor, the power the resistor dissipates and the time constant, checked against the minimum on-time.

Power

LED Resistor

Calculate the correct current limiting resistor for an LED. Shows exact value, nearest E24 standard, actual current, and power dissipation.