Inverting Buck-Boost Converters

Geoffrey Hunter

mbedded.ninja Author

Inverting Buck-Boost Converters

An inverting buck-boost is a type of switch-mode power supply (SMPS) that converts an input voltage into a higher or lower output voltage. It is given the name inverting because it generates a negative output voltage.

The basic schematic of a inverting buck-boost converter. SW1 is typically a MOSFET switched by control logic (not shown).

Output Voltage

Again, the output voltage for an ideal SEPIC is purely determined by the input voltage and the duty cycle $D$, as given in the following equation:

$$\begin{align} V_{OUT} = -\frac{D}{1 - D} V_{IN} \end{align}$$

To give you an idea of how varying the duty cycle can produce either a higher or lower output voltage, see the below image which shows how the output voltage can vary from a small fraction of $V_{IN}$ to many times larger than $V_{IN}$, and they are equal when the duty cycle is set at 50%.

Relationship between duty cycle and the voltage ratio for a inverting buck-boost converter, ignoring the sign (so applicable for a SEPIC also). Dotted line drawn where $V_{OUT} = V_{IN}$, at $D=0.5$.

As the duty cycle approaches 100%, the ideal output voltage approaches infinity! In practise, non-idealities and component absolute maximums limit the the output voltage to something in the range of 10x the input voltage.