说明：  一种新颖的拓扑结构，四开关BUCK-BOOST变换器，能够实现双向的BUCK功能，BOOST功能，而且能视情况自动切换BUCK和BOOST
(A novel topology, four switch buck-boost converter, can achieve two-way BUCK function, BOOST function, and can automatically switch BUCK and BOOST as the case)