Buck-Boost Transformer

A special purpose 2- or 4-coil transformer used to produce modest increases or decreases in the utilization voltage at a load site. The low-voltage coil(s), which typically have rated voltages of 5% to 15% of the high-voltage coils, and in use, the high- and low-voltage, coils, are connected in series to produce an  autotransformer arrangement. If primary voltage is applied to the high voltage coil and load voltage is taken from the series coil combination, the low-voltage coil adds to, or boosts, the load utilization voltage. Conversely, reductions in load utilization voltage occur when these primary and secondary connections are reversed causing the low-voltage coil to buck the supply voltage. A typical 4-coil buckboost transformer would have two 120 V primary coils and two 12 V secondary coils, which could be used to produce voltage ratios of (120/132), (120/144), (240/252), and (240/264). In a basic buck-boost converter, the  inductor accumulates energy from the input voltage source when the transistor is on and releases energy to the output when the transistor is off. It can be viewed as a buck converter followed by a boost converter with topologic simplification. In a buck-boost converter, the output voltage vo is related to the input voltage vi by vo = vid/(1 − d) and it can be controlled by varying the duty ratio d. Note
that the output voltage is opposite polarity to the input. Also called a buck-boost converter, up-down transformer or up-down converter