Power Electronics
The buck-boost is a hybrid of the buck and the boost: it can produce an output both lower and higher than its input. It is built with a single inductor, one switch, and one diode — but there is a price: the output is inverted relative to the input (negative). So from +12 V input you can produce −5 V or −20 V.
Operation has two phases: while the switch conducts, the inductor connects directly to the input and stores energy (no power reaches the output at that moment); when the switch turns off, the inductor dumps its stored energy into the load through the diode, but the current direction makes the output negative. The average output is Vout = −Vin·D/(1−D). At 50% duty |Vout| = Vin; below it steps down, above it steps up.
Where is it used? When you need to generate a negative supply from a positive source (e.g. the −V rail for op-amps), or when the input voltage wanders both below and above the output. If the inverted polarity is a problem, you move to non-inverting derivatives such as the SEPIC or Ćuk.
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