Electronic Design - Capacitors for pcb circuit board design

1. The structure and characteristics of the capacitor designed by PCB
When a potential is applied to the conductor, the conductor is charged. But for the same potential, the amount of charge contained in a conductor varies due to its own structure. The ability of a conductor to contain charge is called the capacitance of PCB design. Generally, the charge Q (Coulomb) contained in a conductor is proportional to its potential V (Volt, related to the earth), that
Is the capacitance of the conductor's PCB design. The unit of capacitance designed by PCB is farad (F).
An insulating medium is inserted between two parallel metal plates, and the lead electrode becomes a capacitor of PCB design. Its circuit symbols are capacitors designed with polarized PCB and capacitors designed with non-polarized PCB.
If the capacitor designed by PCB is charged, charge will accumulate on the two electrode plates of the capacitor designed by PCB. The capacitor designed for PCB whose capacitance is C is charged with constant current intensity I as a graph. It is assumed that the capacitor designed by PCB is initially uncharged, that is, the initial voltage across it is equal to zero. Let us recall the definition of electric current: the activity of electric charge in a conductor constitutes an electric current. The amount of electric charge flowing through the cross-section of the conductor per unit time is called the current intensity.
That is, the capacitor designed by PCB whose capacitance is C is under the action of constant current intensity I, the voltage V at both ends rises linearly with timet.
The higher the voltage across the capacitor of the PCB design, the more charge it contains, that is, the greater the energy storage. However, the dielectric strength of the insulating medium between the two plates of the capacitor designed by PCB is limited. If the electric field strength between the two plates is too high, the insulating medium may be broken down and the capacitor designed by PCB will be short-circuited. Therefore, it is necessary to coordinate the withstand voltage of the capacitor designed by PCB in the application.
Conclusion: The capacitor designed by PCB has the function of containing charge in the circuit, that is, the function of storing energy. It takes time to store energy in a capacitor designed by PCB, so the voltage at both ends of a capacitor designed by PCB cannot change suddenly. And the larger the capacity of the PCB design, the more energy can be stored. The two most important parameters of a capacitor designed by PCB are the capacitance and withstand voltage of its PCB design.
2. RC charging and discharging circuit
The circuit is represented by an RC charging and discharging circuit. Assuming that the initial voltage at both ends of the capacitor designed by PCB is zero, the power supply charges the capacitor designed by PCB through the resistor R at the moment when switch K and terminal 1 are turned on. At this time, the charging current of the capacitor designed by PCB is the maximum E/R. When charging with this current, the rising curve of VC is a linear straight line.
However, because the charging current is during the entire charging process, as VC rises, the charging current intensity IC gradually decreases, and the amplitude of the rise of VC gradually decreases until it rises to the power supply voltage E, and the charging current is zero at the same time. This makes the actual VC upward curve. VC rises exponentially, and the expression for its change with time t is:
Among them, is the time constant.
It can be seen that the larger the series resistance R, the smaller the charging current, and the longer the charging time; the larger the capacitance C of the PCB design, the more charge required (that is, the more energy is stored), and the charging time is also The longer.
When the capacitor designed by PCB is overcharged, VC is equal to E. At this time, switch K and terminal 2 are connected, and the capacitor designed by PCB is discharged through R, and the discharge current is, and VC is gradually reduced. At the moment when terminal 2 is turned on, the discharge current is the maximum, but with the decrease of VC, the discharge current also gradually decreases, until VC is 0V, and the discharge current is also 0. In this way, the drop curve of VC when the capacitor designed by PCB is discharged.
3. Capacitive reactance of capacitor designed by PCB
In the circuit, the capacitance of PCB design has a very important function, which is to pass AC and block DC. If a DC voltage is applied to one end of the capacitor designed by PCB, after the capacitor designed by PCB is stable (that is, after the charging and discharging process is completed), the voltage cannot be sensed at the other end of the capacitor designed by PCB, that is, the DC is separated. This can also be seen from the RC charging and discharging circuit; if the input Vi is an AC signal, Vo will output an AC signal of the same frequency, and the higher the input AC signal frequency, the greater the output Vo amplitude, that is, the AC signal After this PCB design capacitor.
In fact, we can understand that the amplitude and direction of the AC signal change with time, and the capacitance of the PCB design has an inert response to the voltage, that is, the voltage across it cannot change suddenly. When the potential of one plate of the capacitor designed by PCB changes quickly with the input signal, the voltage at both ends of the capacitor designed by PCB changes slowly, and the potential of the other plate that caused it also changes in the same way. In this way, although there is some loss (the voltage across the capacitor of the PCB design has changed a little after all), it is also equivalent to the AC signal passing through the capacitor of the PCB design. Moreover, the faster the input signal changes (that is, the higher the frequency), the larger the capacitance of the capacitor designed by PCB (that is, the slower the voltage across it changes), the easier it will pass.

4. The filtering function of the capacitor designed by PCB
We can manufacture filters by applying the characteristics of capacitors designed by PCB. The circuit is a high-pass filter, that is, the higher the frequency of the input signal, the easier it is to pass, and the lower the frequency, the more difficult it is to pass. DC is not allowed to pass, so that low-frequency components in the signal can be filtered out. The opposite circuit is a low-pass filter, which can filter out high-frequency components in the signal.
(a) High-pass filter (b) Low-pass filter
5. Classification of capacitors commonly used in PCB design
The selection of capacitors for PCB design should be cautious. Generally, you can choose more well-known capacitor brands for PCB design, such as capacitors designed by TDKpcb, capacitors designed by Yageo PCB, etc., as a guarantee of quality.
(1) Capacitors designed by aluminum electrolytic PCB
The capacitor designed by aluminum electrolytic PCB is a capacitor designed with polarity PCB. In the circuit, its "+" pole must be connected to the end with a higher potential.
Advantages: large capacity, able to withstand large pulsating currents.
Defects: large capacity error and large leakage current; ordinary electrolytic PCB design capacitors are not suitable for high frequency and low temperature applications, and should not be used at frequencies above 25kHz.
Uses: low frequency bypass, signal coupling, power supply filtering.
(2) Capacitors designed by tantalum electrolytic PCB
Capacitors designed for tantalum electrolytic PCB are also capacitors designed for polarized PCB.
Advantages: temperature characteristics, frequency characteristics and reliability are better than those of ordinary electrolytic PCB design capacitors, especially the leakage current is very small, the life is long, the capacity error is small, and the volume is small, and the largest PCB circuit board design can be obtained under the unit volume. Capacitor voltage product.
Defects: Poor ability to withstand pulsating current. If damaged, it is easy to short-circuit, and the price is higher.
Usage: In many places, it can replace the capacitors designed by aluminum electrolytic PCB and used in ultra-small and high-reliability equipment.
(3) Capacitors designed by monolithic ceramic PCB
It is a capacitor designed with a large amount of PCB at present.
Advantages: temperature and frequency stability are very good, low loss, long life.
Defects: Can not be made into capacitors designed for large-capacity PCB.
Uses: high frequency filtering, oscillation and coupling, etc.