4S6TMP.com — Buck, Boost, and Buck-Boost DC-DC design
Buck (step-down): Output voltage is always less than input. High efficiency, simple control. Duty cycle D = Vout/Vin. Common for converting 12V to 5V or 3.3V.
Boost (step-up): Output voltage is always greater than input. Duty cycle D = 1 - Vin/Vout. Common for LED drivers, battery-powered systems.
Buck-boost: Output can be above or below input (inverted). Useful when input voltage varies above and below the desired output, like battery systems.