In school, the ideal model of electronic devices is usually used for learning, but when you want to use it, you need to know something else. . .
PCB layout capacitance can't be messy
There are many ways for capacitors. For most novices, they just need to master the following. Don't talk nonsense, just go to the dry goods.
Types of capacitors
1. According to the structure, it can be divided into: fixed capacitor, variable capacitor, and fine-tuning capacitor.
2. According to the dielectric material, it can be divided into: gas dielectric capacitor, liquid dielectric capacitor, inorganic solid dielectric capacitor, organic solid dielectric capacitor and electrolytic capacitor.
3. According to polarity, it is divided into: polar capacitor and non-polar capacitor. The most common thing we see is an electrolytic capacitor.
The role of capacitance
Classified according to the function of the capacitor in the circuit:
Coupling capacitor ( acting to block DC and AC)
Filtering capacitor (removing the signal in a fixed frequency band from the total signal)
Decoupling capacitor (eliminating harmful low frequency between each amplifier Cross-connect)
High-frequency damping capacitor (eliminates the high-frequency noise that may occur in the amplifier)
Resonant capacitor (used in the LC resonant circuit)
Bypass capacitor (removes a certain frequency band signal from the signal)
Neutralizing capacitor (eliminates Self-excited in high-frequency amplifier)
Timing capacitor (plays the role of controlling the size of the time constant)
Capacitor PCB layout
1. The placement of the capacitors in the PCB: The
capacitor input in the switching power supply: input via -> large capacitor -> small capacitor -> the input pin of the power chip
output L -> large capacitor -> small capacitor ——>Output via
2. The filter capacitor is used in the power rectifier circuit to filter out AC components. Make the output DC smoother. .
Decoupling capacitor used in place of the AC amplifying circuit is not required, to eliminate the self-excited, so that the amplifier stable.
The bypass capacitor is used when a resistor is connected, and it is connected to both ends of the resistor to allow the AC signal to pass smoothly.
3. The decoupling capacitor is mainly to remove the interference of high frequency such as RF signal, and the way of interference is through electromagnetic radiation. (Electromagnetic radiation is the transmission of momentum and energy in the form of waves by electric and magnetic fields which oscillate in the same direction and are perpendicular to each other. Emission or propagation in the air forms electromagnetic radiation.)
In fact, the capacitor near the chip also has the function of storing energy, which is second.
Don't underestimate the capacitor. In the high-frequency circuit, the incorrect placement of the capacitor may make your board fail to achieve the expected function, but you don't know what went wrong.
Capacitor selection
1. The accuracy is 5%, 10%.
2. If the temperature coefficient is lower or higher than the temperature range, the capacitance will decrease.
SMD capacitors are often MLCC capacitors, such as NP0.
It is better to use NP0 for important occasions.
3. The higher the withstand voltage, the smaller the capacitance (choose a higher value, preferably around 80%).
Tantalum capacitors: leave 50%, such as the maximum voltage of 8V, withstand voltage To 16V. (Otherwise, it may explode and catch fire)
4. Temperature range:
Electrolytic capacitors below 105 degrees, tantalum capacitors and NP0 capacitors for high temperature of 150 degrees.
5. ESR equivalent series resistance, the output end of the DCDC power supply should use capacitors with very low ESR, MLCC capacitors or tantalum capacitors, ceramic capacitors.
(The short leg of the in-line electrolytic capacitor is negative)
The closer the capacitor is to the chip, the better. There will be electromagnetic waves or something. . . . .
Inductor selection
The coil is an inductor, (air-core inductor). If an iron core is inserted in the middle, the inductance will increase hundreds of times.
1. The accuracy is not very high, generally 30%.
2. The DC resistance of the inductor, DCR, is determined by the diameter of the wire and the number of turns.
3. Current capacity irms: the maximum allowable current capacity
4. Saturation current isat: the current is too large, and the inductance decreases.
The number of chip inductance is generally three digits, the last digit is the secret of 10, the number before the last digit is the multiplier, and R is the decimal point.
Generally, the saturation current is smaller than the current capacity, to 80% margin.
Structure: 1. Shielded inductance and unshielded inductance (interference to the surrounding circuit)
Type: color ring inductance,
color code inductance
, I -shaped inductor (NXP smart car detects magnetic field using this type of inductance)
plastic sealed I- shaped inductor,
patch winding inductance
hollow core Inductance,
constant voltage adjustable inductance,
magnetic loop inductance, and so on.
Diode selection
1. Maximum rectified current
2. Maximum reverse working voltage (Rectification has higher requirements for this, generally using Schottky diodes, fast recovery diodes SS36, FR107)
3. Maximum operating frequency
Schottky diode selection diagram:
Function:
1. Prevent the reverse connection of the circuit 2. Freewheeling
circuit, give the inductor Xu Liu (BUCK circuit)
3. Limit the amplitude, and ground the diode (the amplitude cannot be greater than the conduction voltage)
4. Rectification (https://editor.csdn.net /md/?articleId=111570545)
Transistor
Function: 1. Switch function: this is the most commonly used
MOS tube
1. Switch function
2. Isolation function