Buck Converter Calculator: Complete Design Guide
A buck converter - also called a step-down converter - is a switching DC-DC power supply that efficiently converts a higher input voltage into a lower regulated output voltage. Unlike a linear regulator that wastes the voltage difference as heat, a buck converter rapidly switches a transistor and uses an inductor and capacitor to store and smooth energy, reaching efficiencies of 85-98%. This calculator computes every key design parameter and shows the result on a live schematic and four dynamic waveforms, making it an ideal tool for engineers, students, and hobbyists designing switching regulators.
How a Buck Converter Works
During the switch-on phase, current flows from the input through the high-side switch and inductor to the load, storing energy in the inductor while the capacitor charges. During the switch-off phase, the inductor maintains current flow through the freewheeling diode (or synchronous low-side switch), delivering its stored energy to the load. The fraction of each cycle that the switch is on is the duty cycle, and it directly sets the output voltage.
Duty Cycle
The duty cycle is the heart of buck converter operation. For an ideal converter it equals the simple voltage ratio; including efficiency raises it slightly because the converter must draw a little extra to cover its losses.
• D = Duty cycle (0-1)
• VOUT = Output voltage (V)
• VIN = Input voltage (V)
• η = Efficiency (fraction)
Inductor Selection
The inductor sets the ripple current - the triangular ripple riding on top of the DC output current. A common rule of thumb targets 20-40% of the output current. Too little ripple needs a large, expensive inductor; too much ripple stresses the capacitor and switch.
L = (VIN − VOUT) × D / (ΔIL × fSW)
Peak current: Ipeak = IOUT + ΔIL / 2
Always choose an inductor with a saturation current rating above the calculated peak current, with margin for transients.
Output Capacitor Selection
The output capacitor absorbs the inductor's ripple current and holds the output voltage steady between switching events. Its value is chosen to keep the output voltage ripple within a target, often 0.5-2% of the output voltage.
COUT = ΔIL / (8 × fSW × ΔVOUT)
In real designs the capacitor's equivalent series resistance (ESR) often contributes more ripple than its capacitance, so low-ESR ceramic or polymer capacitors are preferred.
Input Current, Power, and Efficiency
A buck converter steps voltage down and current up: the input current is lower than the output current. Because the converter conserves power (minus losses), the input power is the output power divided by efficiency, and the difference is dissipated as heat.
PIN = POUT / η
IIN = PIN / VIN
PLOSS = PIN − POUT
Quick Reference: All Buck Converter Formulas
| Parameter | Formula | Typical Range |
|---|---|---|
| Duty Cycle | D = VOUT / (VIN × η) | 0.1 – 0.9 |
| Ripple Current | ΔIL = ripple% × IOUT | 20 – 40% of IOUT |
| Inductor | L = (VIN − VOUT) × D / (ΔIL × fSW) | µH – mH range |
| Peak Current | Ipeak = IOUT + ΔIL / 2 | Always > IOUT |
| Output Capacitor | COUT = ΔIL / (8 × fSW × ΔVOUT) | µF range |
| Output Power | POUT = VOUT × IOUT | — |
| Input Power | PIN = POUT / η | — |
| Input Current | IIN = PIN / VIN | Always < IOUT |
| Power Loss | PLOSS = PIN − POUT | 2 – 15% of POUT |
| CCM Boundary | ΔIL < 2 × IOUT | Trough current > 0 A |
Worked Example: 24 V → 12 V at 3 A
Reading the Waveforms
- Inductor Current: The triangular ripple rising during switch-on and falling during switch-off, centered on the DC output current. Its slope and amplitude change with inductor value and duty cycle.
- Switch-Node Voltage: The square wave at the junction of the switch, diode, and inductor - high (~V_IN) when the switch is on, near zero when off. Its high-time fraction equals the duty cycle.
- Duty Cycle vs. Output Voltage: A straight line showing how the duty cycle scales with the desired output, with a marker at your operating point.
- Power Loss vs. Efficiency: A curve showing how losses fall as efficiency rises, with a marker at your chosen efficiency.
Continuous vs. Discontinuous Conduction Mode
These formulas assume continuous conduction mode (CCM), where the inductor current never reaches zero. At light loads or with small inductors the converter may enter discontinuous conduction mode (DCM), changing the voltage relationship. Keeping the ripple below twice the output current (so the trough stays above zero) ensures CCM.
Design Tips and Best Practices
| Goal | Action |
|---|---|
| Lower ripple current | Increase inductor value or switching frequency |
| Smaller components | Raise switching frequency (watch switching losses) |
| Lower output ripple | Larger / lower-ESR output capacitor |
| Higher efficiency | Use a synchronous (low-side MOSFET) topology |
| Reliable inductor | Rate saturation current above I_peak with margin |
| Stay in CCM | Keep ΔI_L below ~40% of I_OUT |
Common Applications
- Point-of-load regulators powering CPUs, FPGAs, and microcontrollers
- Automotive electronics stepping 12V/24V rails down to 5V or 3.3V
- LED drivers providing constant current at lower voltages
- Battery-powered devices maximizing efficiency and run time
- USB-PD and adapter step-down stages
Frequently Asked Questions
Can a buck converter output a voltage higher than its input?
No. A buck converter can only step voltage down. For step-up conversion use a boost converter, or a buck-boost for both.
What happens if I increase the switching frequency?
Higher frequency lets you use a smaller inductor and capacitor for the same ripple, but increases switching losses in the transistor and diode, which can lower efficiency and raise temperature.
Why is my real duty cycle higher than Vout/Vin?
Because the converter must supply its own losses. Dividing by efficiency accounts for this, giving D = Vout / (Vin × η).
CCM vs DCM — Key Differences
| CCM (Continuous Conduction Mode) | DCM (Discontinuous Conduction Mode) | |
|---|---|---|
| Inductor current trough | > 0 A — never reaches zero | = 0 A — reaches zero each cycle |
| Condition | ΔIL < 2 × IOUT | ΔIL ≥ 2 × IOUT |
| Duty cycle formula | D = VOUT / (VIN × η) | More complex — depends on load |
| Output ripple | Lower (standard formulas apply) | Higher — needs larger capacitor |
| When it occurs | Normal/heavy load conditions | Light load or large inductor |
| This calculator | ✅ All formulas valid | ⚠️ Results approximate only |
Buck Converter vs Boost vs Linear Regulator
| Feature | Buck Converter | Boost Converter | Linear Regulator (LDO) |
|---|---|---|---|
| Output vs Input | VOUT < VIN | VOUT > VIN | VOUT < VIN |
| Efficiency | 85 – 98% | 80 – 95% | (VOUT/VIN) × 100% |
| Noise / Ripple | Low (switching artefacts) | Higher (large output cap needed) | Very low (ideal for analog) |
| Component count | L, C, switch, diode | L, C, switch, diode | IC only (no inductor) |
| Heat dissipation | Low — losses as switching | Low — losses as switching | High — (VIN − VOUT) × IOUT |
| Best for | High-current step-down | Step-up from battery/low rail | Low-noise / low-dropout analog |
Frequently Asked Questions
Can a buck converter output a voltage higher than its input?
No. A buck converter can only step voltage down (VOUT < VIN). For step-up use a boost converter, or a buck-boost for both directions.
What happens if I increase the switching frequency?
Higher frequency allows a smaller inductor and capacitor for the same ripple, but increases switching losses in the transistor and diode, which can lower efficiency and raise temperature. Typical modern ICs use 200 kHz – 2 MHz.
Why is my real duty cycle higher than VOUT/VIN?
Because the converter must supply its own losses. D = VOUT / (VIN × η) accounts for this. A 90% efficient converter needs about 11% more duty cycle than the ideal formula predicts.
How do I reduce output voltage ripple?
Increase the output capacitor value, use a lower-ESR type (ceramic or polymer), increase the switching frequency, or increase the inductor value to reduce ΔIL. All four reduce output voltage ripple.
What is the minimum input voltage for a buck converter?
The output voltage sets the minimum: VIN(min) = VOUT / Dmax. Most integrated buck controllers have a maximum duty cycle of 85–100%, setting the minimum input accordingly.
Related Calculators
- Boost Converter Calculator — step-up DC-DC design
- DC Power Calculator — P = V × I
- Inductors in Series Calculator
- Resistor Wattage Calculator — power dissipation
- Ohm's Law Calculator