Buck Converter Design Calculator
Design step-down DC-DC buck converters. Calculate duty cycle, inductor, capacitor, and ripple currents for efficient power conversion.
Buck Converter Formulas
Duty Cycle: D = Vout / Vin
Inductor: L = Vout × (Vin - Vout) / (Vin × f × ΔIL)
Output Voltage Ripple: ΔVout = ΔIL / (8 × f × C)
Input Current: Iin = Iout × Vout / Vin (ideal)
Power: Pin = Vin × Iin; Pout = Vout × Iout
Common Buck Converter Applications
| Application | Vin → Vout | Current | fsw (typical) |
|---|---|---|---|
| USB Power Delivery | 20V → 5V | 3–5 A | 500 kHz |
| 12V to 3.3V (SBC/FPGA) | 12V → 3.3V | 1–3 A | 300–500 kHz |
| Battery Charger | 48V → 12V | 5–10 A | 100–200 kHz |
| LED Driver | 24V → 12V | 2–4 A | 200–500 kHz |
Frequently Asked Questions
What is a buck converter?
A buck converter (step-down DC-DC converter) reduces a higher DC input voltage to a lower DC output voltage using a switching transistor, inductor, diode, and capacitor. Buck converters are highly efficient and widely used in power supplies, battery chargers, and embedded systems.
How does a buck converter work?
The MOSFET switches on/off at high frequency. When ON, current ramps up through the inductor and charges the output capacitor. When OFF, the inductor discharges through the freewheeling diode, maintaining continuous output current. The inductor smooths current ripple; the capacitor smooths voltage ripple.
What is duty cycle?
Duty cycle (D) is the fraction of time the MOSFET is ON per switching period. Formula: D = Vout / Vin (ideal). For example, converting 12V to 5V requires D ≈ 0.42 (42% ON time). Duty cycle directly determines output voltage.
How do I choose inductor value?
Inductor selection depends on desired output current ripple and switching frequency. Typical ripple: 20–40% of output current. Formula: L = Vout × (Vin - Vout) / (Vin × f × ΔIL). Higher L = lower ripple but larger, more expensive inductor.
Why is inductor ripple current important?
Inductor ripple current affects output voltage ripple, EMI, and MOSFET stress. High ripple current increases core losses and winding losses. Typical design targets: 20–40% ripple for general applications, <15% for precision analog.
How do I choose output capacitor?
Output capacitor reduces voltage ripple and provides load transient response. Capacitance requirement depends on ripple voltage and inductor ripple current. Typical ripple: <1% of output. Larger C = lower ripple but increased cost and board space.
What switching frequency should I use?
Common frequencies: 100 kHz (simple, lower cost), 500 kHz (compact), 1–2 MHz (very compact, high efficiency). Higher frequency = smaller inductors/capacitors but higher switching losses and EMI. Choose based on application and inductor availability.
What is output voltage ripple?
Output voltage ripple is caused by inductor current ripple charging/discharging the output capacitor. Formula: ΔVout = ΔIL / (8 × f × C). Minimize by increasing C or f. Typical targets: <1–2% of output voltage.
What is input voltage ripple?
Input-side ripple comes from the inductor ripple current drawn from the input source. An input capacitor reduces this ripple. Ripple frequency = 2 × switching frequency (due to diode conduction). Larger input cap = lower ripple.
Why does duty cycle have a maximum?
Duty cycle is limited by practical constraints: MOSFET on-resistance losses increase with D, inductor saturation risk, and heat dissipation. Most designs stay below D = 0.8–0.85 to maintain efficiency and stability.
What is continuous conduction mode (CCM)?
In CCM, the inductor current never reaches zero—it continuously flows during both ON and OFF phases. This reduces output ripple and input ripple. CCM requires sufficient output load current.
What is discontinuous conduction mode (DCM)?
In DCM, inductor current reaches zero before the next switching cycle begins. DCM occurs at light loads when inductor current is low. Output ripple increases in DCM. Most designs avoid DCM for stable operation.