Buck Converter Calculator
Generated infographic and interface snapshot for Buck Converter Calculator
Size the inductor and capacitor before you burn the board.
Free Buck Converter Calculator Online — Big Das
The buck (step-down) converter is the workhorse of DC-DC design. The Big Das Buck Converter Calculator computes duty cycle, required inductor, output voltage ripple, and whether your design stays in continuous conduction mode — with switch/diode drop corrections and presets like 24 V → 5 V.
What Is a Buck Converter?
A buck converter chops the input voltage with a high-side switch, then smooths the result through an inductor and capacitor to produce a lower, regulated output. It's more efficient than a linear regulator because the switch is either fully on or fully off — almost never dissipating power in between.
The two big design questions:
- How big must the inductor be to keep current ripple at your target (typically 20–40 % of load current)?
- *Is the converter in CCM or DCM?
- Continuous conduction mode is predictable; discontinuous mode changes the transfer function and raises ripple.
How to Use the Calculator
- Pick a preset (24 V → 5 V or 12 V → 3.3 V) or type your own Vin and Vout.
- *Enter the load current
- Iout in amps.
- Set the switching frequency — modern buck ICs run from 100 kHz to several MHz.
- Set the ripple target as a percentage of Iout; 30 % is a common starting point.
- Enter your output capacitance to see the resulting voltage ripple.
- Choose the rectifier — async (diode drop) or synchronous (MOSFET drop) — to refine the duty cycle.
The Formulas Used
Duty (async diode): D = (Vout + Vd) / (Vin + Vd)
Duty (synchronous): D = Vout / (Vin − Vsw)
Inductor: L = (Vin − Vsw − Vout) × D / (fsw × ΔI)
Ripple target: ΔI = ripple% × Iout
Output ripple: ΔV = ΔI / (8 × fsw × Cout)
CCM/DCM boundary: Lcrit = (1 − D) × Vout / (2 × fsw × Iout)
If your chosen L is greater than Lcrit, the converter runs in continuous conduction mode at full load.
Worked Example
24 V to 5 V at 2 A, 200 kHz, 30 % ripple, 47 µF output cap, async diode with 0.5 V drop.
Duty: (5 + 0.5)/(24 + 0.5) = 5.5/24.5 ≈ 22.45 %
ΔI target: 0.30 × 2 = 0.6 A
Inductor: (24 − 5) × 0.2245 / (200,000 × 0.6) ≈ 35.5 µH → pick 33 µH or 39 µH
Output ripple: 0.6 / (8 × 200,000 × 47e-6) ≈ 8 mV (ideal capacitor)
Lcrit: (1 − 0.2245) × 5 / (2 × 200,000 × 2) ≈ 4.8 µH — well below our 35 µH, so the design is firmly CCM.
Common Use Cases
Point-of-load rails for microcontrollers, FPGAs, and sensors.
USB-C PD or barrel-jack input stepped down to system logic voltage.
Choosing between a 100 kHz chunky-inductor design and a 2 MHz compact one.
Sanity-checking a reference design's inductor before you copy it onto your PCB.
Frequently Asked Questions
Why does 30 % ripple current come up so often?
It's a sweet spot: higher ripple shrinks the inductor but increases conduction loss, output ripple, and EMI; lower ripple needs a bigger, slower inductor. 20–40 % of full load is the classic compromise.
What changes between async and synchronous?
An async buck's catch diode drops ~0.3–0.7 V, so the duty cycle must stretch slightly to compensate, and the diode burns power at high currents. A synchronous (MOSFET) rectifier drops much less, improving efficiency — especially at low output voltages.
My calculated ripple is small but my scope shows huge spikes — why?
The formula assumes an ideal capacitor. Real electrolytics and even ceramics have ESR (and ESL) that add an ESR×ΔI ripple component, plus switching spikes from layout parasitics. Use low-ESR ceramics close to the IC.
What happens in DCM?
Below the critical inductance, inductor current hits zero each cycle. The duty-cycle equation no longer holds, ripple grows, and the control loop's behavior changes. Many buck ICs handle it gracefully, but your ripple and efficiency estimates must be re-done for DCM.
Can I use this for a boost or buck-boost?
No — the transfer functions differ. Use the Big Das Boost Converter Calculator for step-up stages.