In this paper, a two-period frame transient switching control scheme for buck converters using coupled-inductor auxiliary circuit to improve the load dynamic response is proposed. The proposed auxiliary circuit only works during a transient event, while in steady state only the main converter side of the coupled inductor is used with a constant-frequency capacitor-current hysteresis control (CF-CCHC) scheme. The transient event consists of two periods, in the first of which, the coupled inductor is inversely connected in parallel using a logic switching control scheme, so as to reduce the equivalent inductance sharply and achieve a small voltage-deviation transient regulation. In the second transient period, the auxiliary-circuit side inductor works in a freewheeling state and the converter behaves as in an approximate steady state using the CCHC scheme. With three switching actions of the auxiliary circuit, the proposed scheme is implemented using a digital processor and an analog control circuit. With the application of the proposed control scheme to a 24-12 V synchronous buck converter, its feasibility is validated in simulation and experiment. Experimental results demonstrate that the settling time of the proposed scheme, with its coupled coefficient of 0.57, has improved by more than 83% compared to the time-optimal control scheme, and the voltage deviation has improved by more than 77% as well.