Inverting Buck-Boost Converter Design

Inverting converter operation principle animation

On this page are presented basic design equations of the inverting buck-boost converter. Below the equations there
is an interactive online simulation circuit along with component selection steps. In addition to basic components the
circuit also contains eventual parasitic elements of the main converter components.

Key design equations:

Continuous conduction mode (CCM):

Current ripple ratio : $r=\frac{I_{L_{ripple}}}{I_{L1_{AVG}}}=\frac{I_{L_{ripple}}*(1-D)}{I_{o}}$

Voltage conversion ratio : $V_{o}=\frac{V_{i}*D-V_{DS_{Q1}}*D-V_{D1}*(1-D)}{1-D}$

RMS current in input capacitor: $I_{C_{in}}=\frac{I_o}{1-D}*\sqrt{D*(1-D+\frac{r^2}{12})}$

RMS current in inductor: $I_{L1_{RMS}}=\frac{I_o}{1-D}*\sqrt{(1+\frac{r^2}{12})}$

RMS switch current: $I_{Q1_{RMS}}=\frac{I_o}{1-D}*\sqrt{D*(1+\frac{r^2}{12})}$

Average current in Diode: $I_{D1_{AVG}}=I_o$

Average current in inductor: $I_{L1_{AVG}}=\frac{I_o}{1-D}$

Inductor curent ripple: $I_{L_{ripple}}=\frac{V_o+V_{D1}}{L*f_s}*(1-D)$

Output voltage ripple: $\frac{I_o*D}{f_s*C_o}+(\frac{I_o}{1-D}+\frac{I_{L_{ripple}}}{2})*ESR_{C_o}$

Suggested reading:
1. Basic Calculation of Inverting Buck-Boost Converter's Power Stage
2. Understanding Buck-Boost Power Stages in Switchmode Power Supplies
3. Switching Power Supply Design and Optimization

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Open project to simulate circuit, see "Notes" for component selection info - the circuit is simulatable online