When buck-boost transformers are connected to buck the input voltage, how are the primary and secondary voltages affected?

When buck-boost transformers are used to buck (reduce) the input voltage, the relationship between the primary and secondary voltages is characterized by subtraction. In this configuration, the primary voltage is connected in a manner that allows the secondary voltage to effectively reduce the total voltage seen by the load.

Specifically, in a bucking scenario, the secondary winding opposes the primary voltage, resulting in a lower output voltage than what is supplied to the primary. This is achieved by the specific wiring of the transformer, where the voltage drop across the secondary counters the voltage from the primary. Therefore, the output voltage can be calculated by subtracting the voltage induced in the secondary from the primary voltage.

This contrasts with the other options: adding would imply a higher voltage output, remaining the same does not reflect the function of a buck-boost transformer, and fluctuating indicates an inconsistency or instability, which is not a typical outcome of proper usage of buck-boost transformers. Thus, the adjustment to the secondary voltage when bucking is a direct result of the subtraction process, leading to the correct understanding of why the secondary voltage is less than the primary voltage in this context.