The invention discloses a switch assembly. The switch assembly comprises a unit chain which is connected in series with an inductor. The unit chain comprises a plurality of half-bridge active clamping units which are successively connected in series. Each half-bridge active clamping unit has a conducting state and a clamping state. When all half-bridge active clamping units are in the conducting state, the switch assembly is closed. When all half-bridge active clamping units are in the clamping state, the switch assembly is open. By clamping or turning on the half-bridge active clamping units one by one, the voltage across both ends of the switch assembly rise or fall step-by-step, which reduces the device requirement of the switch assembly. The invention discloses a modular multi-level DC/DC converter. The switch assembly replaces a single-power device in a conventional DC/DC converter to acquire the modular multi-level DC/DC converter. The device requirement in the modular multi-level DC/DC converter is reduced. The invention discloses a modulation method and a clamping capacitor voltage balancing method.