Buck converter is actually first switching mode power supply topology (first DC-DC converter) ever made. Being generally very efficient and consisting of only two passive elements that can store energy (capacitor and inductor), and only one active switch and one clamping switch (diode), it was very natural to think about re-arrangement of this elements in order to obtain another topologies. If the components are arranged like in figure:
Basic buck-boost converter topology is obtained. Buck-boost DC-DC converter is different from other smps topologies, since it is smps with negative output voltage. During “ON” time (active switch is “ON”), the inductor is charging with the flux, like in figure bellow.
When main switch is “OFF” energy stored in the inductor tries to release and according to Lenz’s law, since positive voltage caused the inductor current, after disconnecting the inductor from positive voltage source, negative voltage is induced, and diode is clamped. Then circuit topology is like in figure bellow.
Inductor current keeps it’s direction, but path is now closed through resistive load and output capacitor. Current is drawn from power ground potential, so output must be negative. Another buck-boost converter specific is that absolute value of the output voltage can be either higher or lower then the input voltage, depending on duty cycle D value. If buck-boost DC-DC converter running in continuous conduction mode (CCM), output voltage is:
So if D<0.5, output voltage absolute value is less then the input voltage, and vice versa. Modified version of transformer isolated buck-boost converter will result as very useful flyback configuration, that will be discussed later.