Functions of filter, decoupling and bypass capacitors in circuit board design

Functions of filter, decoupling and bypass capacitors in circuit board design

The circuit board assembly and processing manufacturers introduce the functions of filter, decoupling and bypass capacitors in circuit board design

The filter capacitor is used in the power supply rectifier circuit to filter out the AC components. Make the output DC smoother.

The decoupling capacitor is used in the amplifier circuit where AC is not needed to eliminate self excitation and make the amplifier work stably.

The bypass capacitor is used when there is a resistance connection. It is connected at both ends of the resistance to make the AC signal pass smoothly.

1. Understanding of energy storage function of decoupling capacitor

1) The decoupling capacitor is mainly used to remove the interference of high-frequency signals such as RF signals, and the interference enters through electromagnetic radiation. In fact, the capacitor near the chip also has the function of energy storage, which is the second.

You can think of the general power supply as Miyun Reservoir. Every household in our building needs water. At this time, the water does not come directly from the reservoir. It is too far away. When the water comes, we are thirsty. The actual water comes from the water tower on the top of the building. The water tower is actually a buffer.

From a microscopic perspective, when high-frequency devices are working, their current is discontinuous and their frequency is very high. However, there is a distance between the device VCC and the general power supply. Even if the distance is not long, when the frequency is very high, the impedance Z=i * wL+R, the inductance of the line will also have a great impact, which will lead to the devices being unable to be supplied in time when they need current. The decoupling capacitor can make up for this deficiency. This is one of the reasons why many circuit boards place small capacitors at the VCC pins of high-frequency devices

(A decoupling capacitor is usually connected in parallel on the vcc pin, so that the AC component is grounded from this capacitor.)

2) High frequency switching noise generated by active devices during switching will propagate along the power line. The main function of the decoupling capacitor is to provide a local DC power supply to the active devices to reduce the transmission of switching noise on the board and guide the noise to the ground

2. Difference between bypass capacitance and decoupling capacitance

Decoupling: Remove RF energy from high-frequency devices into the distribution network during device switching. The decoupling capacitor can also provide a localized DC voltage source for the device, which is particularly useful in reducing the cross board surge current.

Bypass: Transfer unwanted common mode RF energy from components or cables. This is mainly to eliminate the unintentional energy entering the sensitive part by generating AC bypass, and also to provide baseband filtering function (bandwidth limited).

We can often see that a decoupling capacitor is connected between the power supply and the ground, which has three functions: first, it serves as the energy storage capacitor of the integrated circuit; The second is to filter out the high-frequency noise generated by the device and cut off its transmission path through the power supply circuit; The third is to prevent the noise carried by the power supply from interfering with the circuit.

In electronic circuit, decoupling capacitor and bypass capacitor both play the role of anti-interference. Different capacitors are called different due to their different positions. For the same circuit, the bypass capacitor takes the high-frequency noise in the input signal as the filtering object and the high-frequency clutter carried by the front stage as the filtering object. The decoupling capacitor, also known as decoupling capacitor, takes the interference of the output signal as the filtering object.

Selection of filter capacitor

After passing through the rectifier bridge, there is pulsating DC with a large fluctuation range. Later, two capacitors are generally used. The large capacitor is used to stabilize the output. As we all know, the voltage at both ends of the capacitor cannot jump, so the output can be smooth. The small capacitor is used to filter out high-frequency interference, so that the output voltage is pure. The smaller the capacitor, the higher the resonant frequency, and the higher the interference frequency that can be filtered out. Circuit board assembly, circuit board design and circuit board processing manufacturers introduce the functions of filter, decoupling and bypass capacitors in circuit board design