说明：  分析仿真Buck 、Boost 和Buck-Boost三种直- 直电路，建立输入24V输出、负载2A电流的3种基本电路仿真模型，要求： buck电路5V输出，Boost48V输出，Buck-boost分别输出5/48V的仿真模型
(This paper analyzes and simulates buck, boost and buck boost direct-to-dc circuits, and establishes three basic circuit simulation models of input 24 V output and load 2A current. The requirements are: Buck circuit 5V output, boost 48V output, buck boost output 5 / 48V respectively)