说明：  本程序主要是单片机控制的DC—DC开关电源系统的软件设计。 闭环反馈控制功能：在开环控制的基础上，设置一个反馈控制触发按键，该按键按下时，启动闭环控制程序，将电源输出电压反馈到单片机中，并由闭环算法调整输出矩形波的占空比，使得重新输出的电压与数码管显示电压误差减小。
(A DC-DC system controlled by single chip, including the realization of the hardware as well as the software of the system. The chip AT89S52, chip TL494, and the A/D chip ADC0804 matching with the outside circuit realize the hardware part of the system. The software controls the PWM to generate a DC power supply ranging from 5V to 10V in condition of an input voltage ranging from 20V to 30V. The integration of two parts implements the effect of Open-loop Control and Close-loop Control.)