问题 问答题 论述题

分析USB单端反激式充电器控制电路原理。

答案

参考答案:

控制电路原理:当接通电源后,220V交流通过D2整流,C1滤波,通过电阻R2给Q1提供基极启动电流,使Q1开始微导通,其集电极电流Ic线性增长,在L2中感应出使Q1基极为正、发射极为负的正反馈电压,通过C2和R3,送到Q1基极,使Q1迅速饱和;

同时,感应电动势给C3充电,随着C3电压升高,Q1基极电位逐渐下降,Ic开始减小,L2感应出使Q1基极为负,发射极为正的电压,使Q1迅速截止,完成一个振荡周期。C3逐渐放电,Q1基极电压逐渐升高,从而开始第二个周期,不断循环;

当负载变轻或电源电压升高等原因导致输出电压升高时,Z1击穿,IC1发光二级管电流增大,IC1中光敏三极管电流增大,L2反馈绕组中的感应电压经IC1中的光敏三极管到Q2基极,Q2基极电流增大,集电极电流增大,Q1基极电流减小,集电极电流减小,负载能力变小,从而导致输出电压降低。当输出电压降低后,Q2取样后又会截止,Q1的负载能力变强,输出电压升高,这样起到自动稳压作用;

过流过载保护功能:当负载过载或者短路时,Q1的集电极电流增大,Q1的发射极电阻R2上产生较高的压降,这个过载或者短路产生的高电压经过R4让Q2饱和导通,从而让Q1截止,停止输出,防止过载损坏。

判断题
阅读理解

Answer the questions(根据短文内容回答下列问题)

Your Nose is Busy and Important

Your nose has many important jobs, and is always busy.

Smelling

Smelling helps you gather information about the world around you. A good scent (气体),like flowers, brings you pleasure. A bad scent, like smoke or sour milk, warns you of danger. Most things around you release scent particles (微粒) into the air. These invisible chemicals enter the nose when you breathe. Nerves in the nose send information to the brain to identify a smell. You are able to recognize thousands of smells.

Breathing

Most of the air you breathe enters through your nose. The nose uses hair and mucus (粘液) to clean dust and germs (细菌) out of the air. The air that enters your nose contains oxygen. Your body uses oxygen to unlock the energy in the food you eat.  Even when you sleep, your body needs energy to keep your heart beating and your brain working.

Tasting

Your nose helps you enjoy food. Flavor is a mixture of taste and smell. The smell of the food helps you identify what you are eating. The sense of smell contributes more to (贡献) food flavor than the sense of taste .When you are sick and have a stuffy nose, your food doesn’t taste as good because without smell, food loses most of its flavor.

Smell memory

Smells can bring back memories. Your brain uses a process to create a picture in your mind from the odors(气味)you smell. Smells can affect your feelings because the part of the brain that helps you identify smells is close to the area that affects your emotions, mood and memory. More smell memories are collected as you get older.

Your nose has many important jobs, and is more than just a smeller!

小题1:How many important jobs does a nose do according to the passage? What are they?

小题2:Smelling helps you recognize thousands of smells, doesn’t it?

小题3:What does the nose use to clean dust and germs out of air?

小题4:What happens to your food if you are sick and have a stuffy nose?

小题5:How do smells bring back memories?

小题6:What do you think of your nose? Why do you think so?