Explain capacitance characteristics and settings in PCB design

1. Structure and characteristics of capacitors designed for PCB

When a potential is applied to a conductor, it carries an electric charge. However, for the same potential, the amount of charge a conductor can hold varies with its own structure. The ability of a conductor to hold electric charges is called capacitance. Generally, the charge Q (coulomb) contained by a conductor is proportional to its potential V (volt, relative to the earth), that is, there is

C is the designed electrical capacity of the PCB of the conductor. The unit of capacitance in PCB design is Farad (F).

The insulating medium is inserted between two parallel metal plates, and the lead out electrode becomes the capacitor of PCB design. Its circuit symbols are capacitors designed with polar PCB and capacitors designed with non-polar PCB.

If the capacitor designed with PCB is charged, the charge will accumulate on the bipolar plates of the capacitor designed with PCB. Schematic diagram for charging a PCB designed capacitor with constant current intensity I with a capacitance of C. Assume that the capacitor in PCB design is initially free of charge, that is, the initial voltage at its two ends is equal to zero. We recall the definition of current: the charge flows in the conductor to form a current. The amount of charge flowing through the conductor cross section in a unit time is called the current intensity. That is, it exists in the capacitor designed for PCB, so it is.

That is, under the action of constant current intensity I, the voltage V at both ends of a PCB designed capacitor with a capacitance of C increases linearly with time t.

pcb board

The higher the voltage at both ends of the capacitor in PCB design, the more charge it will contain, that is, the greater the energy storage. However, the dielectric strength between the two electrode plates of the capacitor designed with PCB is limited. If the electric field strength between the two electrode plates is too high, the dielectric may be broken down, thus making the capacitor designed with PCB short circuited. Therefore, the withstand voltage of capacitors designed with PCB should be considered in application.

Conclusion: The capacitor designed with PCB has the function of holding charge in the circuit, that is, storing energy. The capacitor designed with PCB needs time to store energy, so the voltage at both ends of the capacitor designed with PCB cannot change suddenly. The larger the capacitance of PCB design, the more energy can be stored. The two most important parameters of the capacitor in PCB design are the capacitance and withstand voltage of its PCB design.

2. RC charging and discharging circuit

The circuit is a schematic diagram of RC charging and discharging circuit. Assume that the initial voltage at both ends of the capacitor designed for PCB is zero, and the power supply charges the capacitor designed for PCB through resistance R at the moment when switch K and terminal 1 are connected. At this time, the charging current of the capacitor designed for PCB is the maximum E/R. If the current is continuously used for charging, the rising curve of VC is a linear straight line.

However, since the charging current is during the whole charging process, with the increase of VC, the charging current strength IC gradually decreases, so the increase of VC gradually decreases until it reaches the power supply voltage E, and the charging current is 0. This makes the actual VC rise curve. VC rises according to the exponential law, and the expression of its change with time t is:

Where, is the time constant.

It can be seen that the larger the series resistance R is, the smaller the charging current is, and the longer the charging time is; The larger the capacitance C of PCB design is, the more charge is required (that is, the more energy is stored), and the longer the charging time is.

When the capacitance of PCB design is fully charged, VC is equal to E. At this time, the switch K is connected to the 2 terminals, and the capacitor designed by PCB discharges through R, the discharge current is, and VC gradually decreases. At the moment of connecting the 2 terminals, 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 decline curve of VC during capacitor discharge in PCB design.

3. Capacitance reactance of PCB design capacitor

The capacitance of PCB design plays an important role in the circuit, which is to connect AC and isolate DC. If a DC voltage is applied to one end of the PCB designed capacitor, after the PCB designed capacitor is stabilized (that is, after the charging and discharging process is completed), the voltage cannot be felt at the other end of the PCB designed capacitor, that is, the DC is separated, as can 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 amplitude of the output Vo, that is, the AC signal passes through the capacitance designed by this circuit board.

In fact, we can understand that the amplitude and direction of the AC signal change with time, and the response of the capacitor in PCB design to the voltage is sui generis, that is, the voltage at its two ends cannot change suddenly. When the potential of one electrode plate of the capacitor designed with PCB changes rapidly with the input signal, the voltage at both ends of the capacitor designed with PCB changes slowly, causing the potential of the other electrode plate to change in the same way. In this way, although there are some losses (after all, the voltage at both ends of the PCB designed capacitor has changed a little), it is also equivalent to the AC signal passing through the PCB designed capacitor. Moreover, the faster the input signal changes (i.e., the higher the frequency), the larger the capacitor capacity of PCB design (i.e., the slower the voltage changes at both ends), and the easier it is to pass.

4. Filtering function of capacitor in PCB design

Using the characteristics of capacitors designed by PCB, we can make filters. The circuit is a high pass filter, that is, the higher the frequency of the input signal, the easier it is to pass through, and the lower the frequency, the harder it is to pass through, and the DC is not allowed to pass through, so that the low-frequency component in the signal can be filtered out. In contrast, the circuit is a low-pass filter, which can filter out the high-frequency components of the signal.

(a) High pass filter (b) Low pass filter

5. Classification of capacitors for common PCB design

The capacitance of PCB design should be carefully selected. Generally, we can choose a more well-known capacitor brand of PCB design, such as the capacitance of TDKPCB design, the capacitance of Guoju PCB design, etc., as a guarantee of quality.

(1) Capacitance of Aluminum Electrolytic PCB Design

The capacitance designed for aluminum electrolytic PCB is the capacitance designed for polar PCB. In the circuit, its "+" pole must be connected to the end with higher potential.

Advantages: large capacity, can withstand large pulsating current.

Disadvantages: large capacity error and large leakage current; Capacitors designed with common electrolytic PCB are not suitable for high frequency and low temperature applications, and should not be used at frequencies above 25kHz.

Purpose: Low frequency bypass, signal coupling, power filter.

(2) Capacitance of tantalum electrolytic PCB design

The capacitance of tantalum electrolytic PCB design is also the capacitance of polar PCB design.

Advantages: The temperature characteristics, frequency characteristics and reliability are superior to those of capacitors designed with ordinary electrolytic PCB. In particular, capacitors with minimal leakage current, long life, small capacity error and small volume can obtain the largest capacitor voltage product of PCB design per unit volume.

Disadvantages: The ability to withstand pulsating current is poor. If it is damaged, it is easy to be short circuited, and the price is high.

Purpose: It can replace the capacitor designed by aluminum electrolytic PCB in many PCB applications, and is used in ultra small and high reliability equipment.

(3) Capacitance of monolithic design

It is a capacitor for PCB design with large demand in PCB industry at present.

Advantages: good temperature and frequency stability, low loss and long service life.

Disadvantages: It cannot be made into a capacitor with large capacity PCB design.

Purpose: High frequency filtering, oscillation and coupling.