Fill in the blank<br/> <img src="https://static.zxtiku.com/wangke/imgs/topic/3be9ee3774cb2ec8ffd0762f43af7847.png"/><br/> For the Buck converter, suppose Us=10V, L=1H, and R=10Ω. If the duty cycle D=20%, the average output voltage U=[填空1]V.