说明：  参考对buck 电路符号模型的推导，推导buck-boost 电路的符号模型，并与采用详细器件模型的电路进行仿真比较，要求输出两种模型的输出电压波形。
(Reference model for the derivation of the circuit symbols buck, buck-boost circuits derived symbolic model, and with more devices using the circuit model simulation comparison of the two models required output voltage waveform.)