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Buck Converter Design Calculator

Design a synchronous buck (step-down) converter: calculate duty cycle, inductor value, output capacitor, and input capacitor.

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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%.

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