The tutor transferred me 800 yuan and asked me to simulate a circuit (power supply design)

Write it at the front

The pleasant and decadent Sunday is coming , I am planning what to write on my blog this week , Suddenly I received a message from my tutor ,What？ Give me a subsidy ？

In this way, you can't go to the canteen for a meal at noon , Soon! , I said ： thank you .

But the follow-up proved that I was still too young , There is no free lunch in the sky .

But for this kind of professional correlation , And can learn a lot from the task , I'm still very happy to do it , I'm afraid it's repetitive （ For me, ） Worthless manual labor .

Go to school , Of course, to contact new things , Learn new knowledge , There's nothing wrong with helping the teacher do something , But the premise is to let students feel that they are growing , If it is a win-win situation （ Full of desire to survive ）.

Oh, I digress , Specific circuit diagrams may be project related , We're not going to let it go , But it does not prevent me from analyzing it 、 Deconstruction and reorganization .

Junior high school students design a power circuit , What would he do ？

Why do you say that you are a junior high school student , Because I found that the circuit knowledge I learned in junior high school , But the electrical knowledge learned in high school is much more useful for circuit analysis , For example, series parallel connection , For example, Ohm's law . Even if it is learned later KVL、KCL It is also based on series parallel connection .

High school physics is mainly discussed in depth in junior high school , Gradually go to the micro , Charge, for example , Electronics, etc. use mechanical knowledge to solve the problems encountered in electricity .

So now let a junior high school student design a step-down circuit , What would he do ？

Ohm's law

No one who believes in electronic people will not remember this law . With the help of this Law , Junior high school students can also design a step-down circuit , As shown in the figure ：

Let's have a good analysis , According to the requirements of power supply design , To evaluate this circuit .

What are the requirements for a good power supply ？

First of all, the power supply must be brought Power supply Of , And it's offering The voltage should be stable , And when the load changes, there should be good Load response capability .

And a little bit more , For an embedded system , Voltage supply demand is not unique .

Okay , There are these requirements , Let's analyze this circuit ：

• 1、 Can only depress the pressure

• 2、 The driving ability is too weak
  When you want a high current drive , It's hard to achieve , Because it is impossible to flow more than R1 Current .

• 3、 The output is not stabilized
  When a load is applied , It's like giving R2 A resistor is connected in parallel , The equivalent circuit R2 The resistance must be higher than 200K Small , The output is not 8V.

• 4、 Poor load response
  When the load changes , Insufficient current , Only the voltage can be changed , Cause great voltage fluctuation .

Take a closer look. , It's all wrong , But don't forget that each of us came from that state , Start with the most basic principles , A step forward .

Remember this circuit diagram , Temporarily press no table （ There will be ）, Let's talk about power design first .

Power Design

For an embedded hardware system , Power module design is the top priority , The quality of power supply directly affects the reliability of embedded hardware system .

The factors that should be considered in the design of power supply include the output voltage 、 electric current , Input voltage 、 electric current , Safety factors , Electromagnetic compatibility and electromagnetic interference , Volume limit 、 Power consumption limitation and cost limitation .

Power tree

First of all, I want to give you a concept —— Power tree .

According to the estimated current , And current distribution of each voltage level , Draw a more visual tree view .

To put it bluntly, it is a hardware system , Different circuit components require different operating voltages , But the input voltage of the whole system is only one , So it's the only input （ It's usually 12V perhaps 24V）, Into a voltage that can be used by the processor and various peripherals （ Common are 5V、3.3V、1.8V）.

Simple classification

People like to classify things around them , According to different classification standards , The results are also different , The power circuit is also , But we can't change without our ancestors , No matter how the standard is changed , Nothing more than communication （AC） And DC （DC） Love and hate between .

AC To DC

AC to DC , The most common is Various household appliances , As we all know, what is connected to the power grid is 220V Alternating current (ac) , Most household appliances are DC .

AC To AC

Communication to communication , such as Isolation transformer , prevent 220V Directly connected to the ground .

DC To AC

DC to AC , This category belongs to inverter

image Solar panels , Convert the direct current converted from solar energy into alternating current for use ;

perhaps Variable frequency refrigerator and air conditioner , In general, it is to put AC go to DC Then go to the AC, Because in DC , Changing the frequency is simple .

DC To DC

All kinds of common 12V turn 5V、3.3V、1.8V Circuit , For driving CPU And various peripherals . such as Low differential pressure LDO And switching power supply .

Classification chart

For Xiaobai, who just got started , The most common , The most common is DC-DC, DC to DC , And the most common one is LDO Linear voltage stabilizing circuit .

So I will focus on this later .

LDO Linear regulator

Internal principle of linear regulator

The world's first integrated circuit linear regulator 1970 Year of introduction
The famous LM317,1976 (National Semiconductor)Robert Dobkin and Bob Widlar designed

Have you ever thought about how the linear regulator can stabilize the voltage ？ Why is the voltage at the output of the linear regulator so stable ？

In fact, let's think about various examples in life or work ： A stable system , What's the most important ？

For example, your family , A happy family , What is the most important thing among the members inside ？ It's communication .

you 're right , It's communication , It has another name in the system —— feedback .

The circuit designed by the junior middle school students mentioned above can be taken out again .

If we use an adjusting tube Q1（ You can change your voltage output according to the information , Just use it as a tap ） Instead of R1;R2 It is equivalent to our load circuit .

In this case , We can detect the voltage at the output , Feedback to our comparator , Then the comparator gives us Q1,Q1 Through the feedback information, you can control your output voltage , So as to achieve a stable output .

The transistor Q1 What are the characteristics of ：

• It's like putting a... Between input and output Resistance to withstand the voltage drop between the two nodes
• The input and output currents are the same , therefore , The product of voltage difference and current , Turned into LDO Heat dissipation power
• The work efficiency = Output power / The input power and thus the conversion efficiency are inversely proportional to the input / output pressure difference . The heat dissipation power is proportional to the input and output pressure difference .

Analyze some simple LDO circuit

Classical circuit 7850

A very typical three terminal regulator , My teacher said that when he went to school , His teacher often gives this example , An input capacitor , An output capacitor , On three feet .

Its internal schematic diagram is like this , It uses a low-power MOS Tube to drive a high-power MOS tube , That is, adjust the pipe part , Here is the feedback circuit .

Classical circuit LM1117

This regulator is no stranger to new engineers , It can be seen in almost all power circuits .

Vout Adjustable , By default, you can directly ADJ Grounding ,3.3V perhaps 5V; You can adjust the voltage by changing the resistance R2 The resistance of .

His internal principle is the same , Adjusting tube part and detecting feedback circuit part .

In fact, most engineers are designing LDO When the circuit , Refer to the typical circuit in the data book of the linear regulator , What do people say , Let's just do it .

Use LDO Key performance parameters

Minimum pressure drop

The minimum voltage drop determines whether your circuit can work properly .

for instance , Do you think this circuit can work normally ？

First , We need to check AMS1117 The workbook will reveal

When the output current is 1A When , The typical value of pressure drop is 1.3V, The voltage range of lithium battery in normal working state is 4.5V—3.3V, Do you want 3.3 Output ,3.3+1.3=4.6V.

And our common lithium battery , It simply can not provide such a high voltage value .

So this circuit is difficult to work properly .

What does the pressure drop have to do with ？ That must be the adjustment tube .

There are several characteristics ：

• Different chip design processes and structures , Pressure drop varies
• Voltage drop is closely related to working current （ The adjusting tube has an internal impedance ）
• The greater the current , Higher temperature , The greater the pressure drop
• The lower the pressure drop, the better the efficiency , Of course, the more expensive

Input voltage

Hot plug effect , In fact, each wire is similar to an inductance , With inductance, there is no shortage of oscillation . Probably 5V It's stable , When it is just inserted, it can reach 10V.

Therefore, the selection of input voltage shall comply with the following points as far as possible ：

• The highest transient value of input voltage shall be considered for device withstand voltage
• Surge voltage shall be considered for input voltage cables with long conductors
• The higher the withstand voltage , Usually the higher the price
• Usually press 2 It is safer to take the withstand voltage value more than times

Input current

This is easy to understand , You can't pull a cart with a small horse , If you can't output the required current, you can't make the circuit work properly .

Pay attention to the following points when choosing ：

• The output current capability of the chip , It represents an ideal situation
• The actual output capacity is related to the input / output voltage difference and current
• The greater the output current required , The more serious the heat dissipation problem is
• Usually high current applications , Replace with switching power supply LDO

Thermal resistance parameters

Often look at three parameters ：

TJMAX： The maximum temperature at which the chip core works normally
0JC： Every... From the chip core to the chip shell W temperature rise
0JA： Every from the chip core to the external environment W temperature rise

What we should pay attention to most is 0JA This parameter .

5V turn 3.3V electric current 0.5A ambient temperature 25°C： Chip temperature =25+(5-3.3)x0.5x40=59°c

Ripple / noise

Ripple is a phenomenon caused by the voltage fluctuation of DC stable power supply , Because the DC stable power supply is generally formed by the AC power supply through rectification and voltage stabilization , It is inevitable that there are some AC components in the DC stability , The AC component superimposed on the DC stability is called ripple .

Remember an important parameter PSRR, The output is greater than the input , then 20log once .

And the following features ：

• LDO The ripple noise is usually less than 0.1mV
• The ripple noise of switching power supply is usually greater than 10mV
• PSRR Is a very important noise Inhibition indicators
• The lower the frequency of the noise , The harder it is to suppress

Transient response

Transient response , Can be suppressed by increasing the capacitance , But it can't be eliminated , And the capacitance should not be too large , Otherwise, it will prolong the reaction time of re stabilization .

When choosing , Pay attention to a few points ：

• Important parameters of power supply stability
• Accidental system reset , It may be that the transient response of the power supply is poor
• Transient response is related to sudden change amplitude of load current
• Transient response is related to sudden change slope of load current

At the end

There are so many ways for a small power supply design , The above contents may not even be one thousandth of the power supply design .

That's it today , Good morning , Good afternoon and good night ！

（ I went to watch the ball , Warriors are really annoying , Green, take it. Hurry up ）

reference ：

http://t.csdn.cn/UQmPx

https://www.cnblogs.com/iron2222/p/15852617.html

https://www.bilibili.com/video/BV1ya411L7Jj?vd_source=3602ab285cd1b8d5fa78a0f57368918c