SMPS Duty Cycle Calculator

A switched-mode power supply (SMPS) converts DC voltage by rapidly switching a transistor on and off and using an inductor to store and transfer energy. The duty cycle D — the fraction of time the switch is on — sets the voltage conversion ratio. Unlike linear regulators that burn excess voltage as heat, SMPS converters achieve 85–95% efficiency, making them essential for battery-powered devices and high-current applications.

A Buck converter steps voltage down (Vout < Vin). During the on-time, current flows through the inductor and load; during the off-time, the inductor’s stored energy keeps current flowing through the freewheeling diode. A Boost converter steps voltage up (Vout > Vin): the inductor charges during on-time and adds its voltage to Vin during off-time. A Buck-Boost can produce an output voltage higher or lower than the input (with inverted polarity in the basic topology).

Choosing component values is critical. The inductor must be large enough to keep the converter in continuous conduction mode (CCM) at the minimum load. The output capacitor must be large enough to keep the output voltage ripple within spec. Higher switching frequencies allow smaller components but increase switching losses.

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