Buck Converter
Design a switching buck converter in seconds. Enter your input voltage, output voltage, load current, and switching frequency, and this calculator instantly solves the duty cycle, minimum inductor value, and output capacitor size — the three numbers every power supply designer needs before picking real components.
What is a Buck Converter?
A buck converter (step-down converter) is a DC-to-DC power converter that reduces voltage efficiently through switching rather than heat dissipation. Unlike a linear regulator, which can waste 50% or more of the input power as heat, a well-designed buck converter routinely reaches efficiencies above 95%.
It works by rapidly switching a transistor (usually a MOSFET) on and off, chopping the input voltage into a high-frequency square wave. An inductor and capacitor then filter that square wave back into a smooth, stable, lower DC voltage — with far less energy lost as heat than a linear regulator would waste.
Why Use a Buck Converter Calculator?
Manually working through buck converter design math is easy to get wrong — a single misplaced unit in the inductor ripple current formula can leave you with a part that saturates under load or a converter that never hits your target output voltage. This buck converter designer automates the standard equations used in datasheets from Texas Instruments, Analog Devices, and every major switching regulator IC vendor, so you get consistent, checkable numbers before you touch a breadboard or lay out a PCB.
How to Use This Buck Converter Designer
- Input Voltage (V) — the DC voltage feeding the converter (e.g., 12V from a car battery or wall adapter).
- Output Voltage (V) — your target regulated output (e.g., 5V or 3.3V).
- Output Current (A) — the maximum load current your circuit will draw.
- Switching Frequency (kHz) — the MOSFET switching speed, typically 100kHz–2MHz depending on the controller IC.
- Inductor Ripple Current (%) — how much ripple you'll allow relative to the load current, usually 20–40%.
The calculator returns your duty cycle, minimum inductor value, and minimum output capacitor value instantly.
Buck Converter Design Formulas Explained
Designing a buck converter comes down to solving for two key components: the inductor (L) and the output capacitor (Cout).
1. Duty Cycle
D = (Vout / Vin) × Efficiency
The ideal (lossless) duty cycle is simply Vout/Vin. Real converters run slightly higher than this to compensate for switching and conduction losses.
2. Inductor Selection (L)
The inductor is sized to limit peak-to-peak ripple current (ΔIL) to a target fraction of the load — typically 20–40%.
L = (Vout × (Vin − Vout)) / (ΔIL × fsw × Vin)
Where fsw is the switching frequency. A larger inductor reduces ripple but is physically bigger and slower to respond to load transients; a smaller inductor is compact but increases ripple current and core losses.
3. Output Capacitor Selection (C)
The output capacitor smooths the remaining voltage ripple (ΔVout). The ideal minimum capacitance assumes negligible ESR:
Cmin = ΔIL / (8 × fsw × ΔVout)
In real designs, ESR (Equivalent Series Resistance) often dominates the ripple budget more than raw capacitance does — low-ESR ceramic or polymer capacitors are typically required to hit tight ripple targets at high switching frequencies.
Practical Applications of Buck Converters
Buck converters are the backbone of power delivery in nearly every modern electronic device:
- USB Chargers & Power Banks — stepping 12V car battery power down to 5V for phones and USB devices.
- Computer Motherboards — multiphase buck converters (VRMs) step 12V from the PSU down to sub-1V rails for the CPU core.
- LED Drivers — constant-current buck topologies drive high-power LEDs efficiently without wasting power as heat.
- Battery-Powered Electronics — converting a higher battery pack voltage down to the regulated rail needed by microcontrollers and sensors.
- Solar & Renewable Energy Systems — stepping down panel or battery-bus voltage to match load requirements.
Frequently Asked Questions
Buck Converter vs. Linear Regulator (LDO): Which is better?
Use a linear regulator (like an LM7805) for low-power or noise-sensitive analog circuits where efficiency isn't critical. Use a buck converter when current is high (>0.5A) or the voltage drop is large (e.g., 24V to 5V) — a linear regulator would dissipate that difference as heat and can overheat almost instantly.
What is switching frequency, and how do I choose it?
Switching frequency is how fast the MOSFET turns on and off. Higher frequencies (e.g., 1MHz) allow smaller physical inductors and capacitors but increase switching losses and MOSFET heating. Lower frequencies (e.g., 50kHz) are more efficient but need larger, bulkier components. Most general-purpose designs land between 300kHz and 1MHz.
What happens if the inductor saturates?
If current exceeds the inductor's saturation rating, its inductance collapses toward zero and it behaves like a short circuit. This causes a sudden current spike that can destroy the MOSFET or the switching controller — always choose an inductor with a saturation current rating comfortably above your calculated peak current.
How do I choose the right output capacitor type?
For high switching frequencies, prioritize low-ESR ceramic (X5R/X7R) or polymer capacitors — ESR often contributes more to output ripple than the raw capacitance value at frequencies above a few hundred kHz.
Is a higher duty cycle better or worse for efficiency?
Neither inherently — duty cycle is set by the Vout/Vin ratio, not chosen for efficiency. However, very high duty cycles (>90%) or very low ones (<10%) can push some controller ICs outside their recommended operating range, so check your controller's datasheet limits.