SMPS Duty Cycle Calculator
Calculate duty cycle, inductor, and capacitor for Buck, Boost, and Buck-Boost switched-mode power supplies.
Component Values
Results
Inductor voltage waveform (V_L)
Buck vs Boost vs Buck-Boost: Which Topology Do You Need?
Use this calculator when designing a DC-DC power stage from scratch or verifying component values for an existing design. The three most common topologies: Buck (step-down) for converting 12V→5V or 5V→3.3V in embedded systems; Boost (step-up) for powering 5V circuits from a 3.7V Li-Po cell or running LEDs from a low-voltage source; Buck-Boost for battery-powered devices where the battery voltage swings across the output voltage (e.g., a 4.2V→3V Li-Ion driving a 3.3V rail). Example: 12V input, 5V/1A output, 100kHz, 30% ripple — the calculator gives D=41.7%, L≥58µH, C≥265µF.
The duty cycle is the starting point for everything. For a Buck: D=Vout/Vin (higher Vin means shorter on-time). For a Boost: D=1−Vin/Vout (larger step-up means longer on-time). The minimum inductor value keeps the converter in Continuous Conduction Mode (CCM), where inductor current never reaches zero — CCM gives lower peak currents and more predictable control. Go below this inductance and you enter Discontinuous Conduction Mode (DCM), which is fine but requires different control compensation.
Higher switching frequency means smaller passives but more switching losses in the MOSFET and diode. Most designs land between 100kHz and 500kHz — above that, PCB layout becomes critical. For a first prototype, use a switching regulator IC (LM2596, TPS5430, XL4016) which integrates the MOSFET and control — these calculators help you size the external inductor and capacitor shown in the datasheet application circuit.
Buck
D = Vout / VinBoost
D = 1 - Vin / VoutBuck-Boost
D = Vout / (Vin + Vout)Key Points
- Buck: D = Vout/Vin — output is always lower than input
- Boost: D = 1 − Vin/Vout — output is always higher than input
- Buck-Boost: D = Vout/(Vin+Vout) — can go higher or lower
- Higher switching frequency → smaller inductor and capacitor
- Ripple current is typically 20–40% of output current
- SMPS efficiency: 85–95% vs 30–60% for linear regulators
Applications
- Phone chargers and USB power adapters
- Laptop and computer power supplies
- Battery-powered IoT and embedded devices
- Solar panel MPPT charge controllers
- LED driver circuits (constant current)
- Automotive 12V-to-5V and 12V-to-3.3V converters
Did you know? Switch-mode power supplies can achieve 90–98% efficiency — vs. 30–60% for linear regulators. The key: instead of dissipating excess energy as heat, SMPS switches the transistor at high frequency (100 kHz–10 MHz), storing energy in an inductor and releasing it controlled.