An auxiliary circuit for reducing an output voltage ripple of a DC-DC converter includes: a capacitor configured to perform charging and discharging, a main buck converter including a first switch and a second switch connected to a voltage source, and a first inductor connected between the capacitor and a contact point of the first switch and the second switch, the first inductor having a first inductor current that flows therethrough, and an auxiliary buck converter including a third switch and a fourth switch respectively connected to the first switch and the second switch in parallel, and a second inductor connected between the capacitor and a contact point of the third switch and the fourth switch, the second inductor having a second inductor current that flows therethrough, and the auxiliary buck converter configured to control the second inductor current by generating two envelopes with different heights to compensate for a difference between the first inductor current and an output current.