Inverting Buck-Boost Converter Calculator

An inverting buck-boost converter delivers an output voltage that is negative relative to the input ground while stepping the voltage magnitude up or down. The core calculations rely on the steady-state equations of switch-mode power supplies. First, the duty cycle (D) is determined by the absolute input voltage (V_in) and output voltage (V_out) using the formula: D = V_out / (V_in + V_out). Second, the required inductance (L) to maintain continuous conduction mode is calculated using the input voltage, duty cycle, switching frequency (f_s), and inductor ripple current (ΔI_L) via the equation: L = (V_in * D) / (f_s * ΔI_L).

Imagine you are designing a power supply that takes a positive input voltage of 12V and converts it to a regulated -15V output. Your switching frequency is set to 250 kHz (250,000 Hz), and you want to limit the inductor ripple current to 0.8A. First, calculate the duty cycle: D = 15 / (12 + 15) = 15 / 27 ≈ 0.5556 (or 55.56%). Next, substitute these values into the inductance formula: L = (12 * 0.5556) / (250,000 * 0.8) = 6.6667 / 200,000 = 0.00003333 henries, which equals 33.33 microhenries (μH). Therefore, an inductor of at least 33.33 μH is needed to handle these operational parameters.

Always account for component tolerances when selecting your final inductor value; choosing an inductor that is 20% higher than the calculated minimum ensures you remain in continuous conduction mode under light loads. Pay close attention to the peak current rating of your semiconductor switches, as inverting topologies often experience higher peak currents than standard step-down converters. Finally, ensure your output capacitors have low Equivalent Series Resistance (ESR) to minimize output voltage ripple in high-frequency designs.