This paper discusses a Feedback Injection Technique (FIT) to simplify the line/load transient and AC response testing of dc-dc power supplies. The FIT exploits the power summing amplifier feature of op-amps embedded in the feedback control loop of dc-dc regulators, thus realizing a dynamic source and a dynamic load to observe the transient and AC response of another power supply connected to its input or output by means of an oscilloscope. Examples of line transient and load transient tests and PSRR measurements are presented in the paper, realized with the Texas Instruments PMLK Series BUCK and LDO boards, allowing an easy implementation of dynamic testing of power supplies based on the FIT concept.