“Capacitor is the most common device in circuit design and one of the passive components. An active device is simply a device that requires energy (power) power, called an active device, and a device that does not require power (power) is a passive device. .
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Capacitor is the most common device in circuit design and one of the passive components. An active device is simply a device that requires energy (power) power, called an active device, and a device that does not require power (power) is a passive device. .
Capacitors generally have a variety of functions and uses, such as: in bypass, decoupling, filtering, energy storage; in completing oscillation, synchronization and time constant…
Let’s analyze it in detail:
Blocking DC: The function is to prevent DC from passing through and allow AC to pass.
Bypass (Decoupling): Provides a low impedance path for certain parallel components in an AC circuit.
Bypass capacitor: Bypass capacitor, also known as decoupling capacitor, is an energy storage device that provides energy for a device. It utilizes the frequency impedance characteristic of the capacitor. The frequency characteristic of an ideal capacitor decreases as the frequency increases, just like a pond, it can make the output voltage output uniform and reduce the load Voltage fluctuation. The bypass capacitor should be as close as possible to the power supply pin and ground pin of the load device, which is an impedance requirement.
When drawing the PCB, special attention should be paid to suppress the ground potential rise and noise caused by excessive voltage or other input signals only when it is close to a certain component. To put it bluntly, the AC component in the DC power supply is coupled to the power supply ground through a capacitor, which plays a role in purifying the DC power supply. As shown in Figure C1, it is a bypass capacitor, and it should be as close to IC1 as possible when drawing.
Figure C1
Decoupling capacitors: Decoupling capacitors take the interference of the output signal as the filtering object. The decoupling capacitor is equivalent to a battery, and it is charged and discharged so that the amplified signal will not be disturbed by the sudden change of the current. Its capacity depends ON the frequency of the signal and the degree of ripple suppression. The decoupling capacitor acts as a “battery” to meet the change of the current of the drive circuit and avoid mutual coupling interference.
Bypass capacitors are actually decoupled, but bypass capacitors generally refer to high-frequency bypasses, that is, to improve a low-impedance discharge path for high-frequency switching noise. The high-frequency bypass capacitor is generally relatively small, and generally takes 0.1F, 0.01F, etc. according to the resonant frequency. The capacity of the decoupling capacitor is generally larger, which may be 10F or larger, which is determined according to the distribution parameters in the circuit and the change of the driving current.Figure C3 is the decoupling capacitor
Figure C2
The difference between them: Bypass is to take the interference in the input signal as the filtering object, while decoupling is to take the interference of the output signal as the filtering object to prevent the interference signal from returning to the power supply.
Coupling: As a connection between two circuits, allowing an AC signal to pass and travel to the next stage circuit.
The capacitor is used as the coupling element to transmit the signal of the previous stage to the next stage, and to cut off the influence of the DC of the previous stage on the latter stage, so that the circuit debugging is simple and the performance is stable.
If no capacitor is added, the AC signal amplification will not change, but the working points at all levels need to be redesigned. Due to the influence of the front and rear stages, it is very difficult to debug the working points, and it is almost impossible to achieve in multi-stage.
Filtering: This is very important for the circuit, and the capacitors behind the CPU are basically used for this purpose.
That is, the larger the frequency f, the smaller the impedance Z of the capacitor. At low frequencies, the capacitor C has a relatively large impedance Z, and useful signals can pass through smoothly; at high frequencies, the capacitor C has a very small impedance Z, which is equivalent to short-circuiting high-frequency noise to GND.
Filtering effect: ideal capacitor, the larger the capacitor, the smaller the impedance, and the higher the passing frequency.
Electrolytic capacitors are generally more than 1uF, and the inductance component is large, so the impedance will be large after the frequency is high. We often see that sometimes there is an electrolytic capacitor with a large capacitance connected in parallel with a small capacitor. In fact, the large capacitor can pass low frequencies, and the small capacitor can pass high frequencies, so that high and low frequencies can be fully filtered out.
The higher the frequency of the capacitor, the greater the attenuation. The capacitor is like a pond, and a few drops of water are not enough to cause a big change. That is to say, the voltage fluctuation is not very large when you can buffer the voltage, as shown in Figure C2:
Figure C3
Temperature compensation: Compensate for the influence of other components’ insufficient adaptability to temperature to improve the stability of the circuit.
Analysis: Since the capacity of the timing capacitor determines the oscillation frequency of the line oscillator, the capacity of the timing capacitor is required to be very stable and does not change with the change of ambient humidity, so that the oscillation frequency of the line oscillator can be stabilized.
Therefore, capacitors with positive and negative temperature coefficients are used in parallel for temperature complementation.
When the working temperature increases, the capacity of Cl is increasing, while the capacity of C2 is decreasing. The total capacity of the two capacitors in parallel is the sum of the capacities of the two capacitors. Since one capacity is increasing and the other is decreasing , so the total capacity remains basically unchanged.
Similarly, when the temperature decreases, the capacity of one capacitor decreases while the other increases, and the total capacity is basically unchanged, which stabilizes the oscillation frequency and achieves the purpose of temperature compensation. Timing: Capacitors are used in conjunction with resistors to determine the time constant of a circuit.
When the input signal transitions from low to high, it is input to the RC circuit after buffering 1.
The characteristics of capacitor charging make the signal at point B not jump immediately following the input signal, but have a gradually increasing process.
When it becomes large enough, buffer 2 flips, and a delayed low-to-high transition is obtained at the output.
Time constant: Take the common RC series connected integrator circuit as an example, when the input signal voltage is applied to the input terminal, the voltage on the capacitor gradually rises.
The charging current decreases as the voltage rises. The Resistor R and the capacitor C are connected to the input signal VI in series, and the capacitor C outputs the signal V0. When the value of RC (τ) and the input square wave width tW satisfy: τ 》》tW, such a circuit is called an integrating circuit.
Tuning: System tuning of frequency-dependent circuits, such as cell phones, radios, and televisions.
Varactor Tuned Circuit Because the resonant frequency of an LC-tuned oscillator circuit is a function of lc, we find that the ratio of the maximum to minimum resonant frequencies of the oscillator circuit varies with the square root of the capacitance ratio.
The capacitance ratio here refers to the ratio of the capacitance at the minimum reverse bias voltage to the capacitance at the maximum reverse bias voltage. Thus, the tuning characteristic of the circuit (bias-resonant frequency) is essentially a parabola.
Rectification: Turning on or off a semiconductor switching element at a predetermined time.
Energy Storage: Stores electrical energy for release when necessary. Such as camera flashes, heating equipment, etc.
Generally, electrolytic capacitors have the function of energy storage. For capacitors with special energy storage function, the mechanism of capacitor energy storage is electric double layer capacitor and Faraday capacitor.
Its main form is supercapacitor energy storage, in which supercapacitors are capacitors that utilize the principle of electric double layers.
When an applied voltage is applied to the two plates of a supercapacitor, the positive electrode of the plate stores positive charge, and the negative plate stores negative charge, just like ordinary capacitors.
Under the action of the electric field generated by the charges on the two electrode plates of the supercapacitor, opposite charges are formed on the interface between the electrolyte and the electrodes to balance the internal electric field of the electrolyte.
The positive and negative charges are arranged in opposite positions on the contact surface between two different phases with a very short gap between the positive and negative charges. This charge distribution layer is called an electric double layer, so the capacitance is very large.