In a regulated power supply, a diode connected across the input and output terminals of a regulator is used to:

In regulated power supplies, especially those utilizing linear voltage regulator ICs, a protection diode is frequently placed in reverse bias across the input and output terminals of the regulator. This diode is crucial for preventing the output voltage from exceeding the input voltage. If the input power is suddenly turned off or shorted, the large capacitors on the output side could hold a higher voltage than the input side. This would reverse-bias the internal components of the regulator, potentially destroying it. The diode provides a safe discharge path for the output capacitors to empty back into the input side. Distractors involving RF bypass, additional capacity, or primary transformer fluctuations describe entirely different circuit mechanisms.