Useful Tips | The role of capacitance between VCC (power supply) and GND (ground)

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Useful Tips | The role of capacitance between VCC (power supply) and GND (ground) [Copy link]

Useful Tips | The role of capacitance between VCC (power supply) and GND (ground)

Function: Power input/output filter capacitor, mainly used to stabilize the output, which is beneficial to voltage regulation.

The main function of capacitor

Voltage stabilization
The reason for connecting a capacitor between the power supply and the ground is that it has two functions, energy storage and bypass. Energy storage: The power consumption of the circuit is sometimes large and sometimes small. When the power consumption suddenly increases, if there is no capacitor, the power supply voltage will be pulled down, generating noise and ringing. In severe cases, it will cause the CPU to restart. At this time, a large-capacity capacitor can temporarily release the stored energy to stabilize the power supply voltage, just like the relationship between a river and a reservoir. Bypass: The circuit current often pulsates. For example, the synchronous frequency of a digital circuit will cause the power supply voltage to pulsate. This is an AC noise. A small-capacity non-polar capacitor can bypass this noise to the ground (capacitors can pass AC and block DC. The passband of small-capacity capacitors is much higher than that of large capacitors). This is also to improve stability.

Power supply filtering
Capacitance of capacitor = Dielectric constant Area/Distance = εS/d. Usually ε and d are not easy to change, and only S can be changed to change the capacitance. When the capacitor is very large, S must be large. In order to reduce the volume, it has to be wound, but winding will inevitably increase the inductance (although symmetrical double winding). The capacitor is actually a combination of R, L, and C. In this way, the relative inductance L of the large capacitor is also large. For example: when using a 2200uF capacitor wave, it is very good for low frequency 50Hz, but it is useless for high frequency (K, MHz) because L is too large. Therefore, experts pay great attention to power supply filtering and will use large, medium, and small capacitors to filter low, medium, and high frequencies respectively.

Practical Application

01

The capacitor connected in parallel between the DC power supply (Vcc) and the ground can be called a filter capacitor. The filter capacitor filters out the clutter and AC components of the power supply, compresses the smooth pulsating DC, and stores electrical energy. The value is generally 100-4700uF. The value is related to the load current and the purity of the power supply. The larger the capacity, the better. Sometimes there will be a capacitor with a smaller capacity next to the large capacitor, called a high-frequency decoupling capacitor, which is also a form of filtering. It is used to filter out high-frequency clutter in the power supply to prevent the circuit state from self-excitation and stabilize the circuit working state. The value is generally 0.1-10uF, and the value is related to the frequency of the filtered clutter.
The role of this connection is generally called "decoupling", also called "de-interconnection" and "bypass" capacitors, which are often arranged near the power supply, IC and functional module circuits. Non-inductive ceramic chips and monolithic capacitors are preferred.
The function is to provide a path for high-frequency signals, reduce the internal resistance of the power supply, remove the influence of the power supply and ground wires "taking a long line" on the copper-clad board, and prevent "harmful interconnection" between the various circuits of the public power supply, etc.
10nF is commonly used.

On the development board, there are usually many 0.1uF non-electrolytic capacitors and 10uF electrolytic capacitors between the DC power supply and the ground.
The purpose of these capacitors is to make the impedance between the power line and the ground line low, and the power supply is close to the ideal voltage source. You can say that it is a filtering effect, but you need to figure out what wave is filtered. It is not to filter the ripple of the power supply, but the ripple caused by the change of the current of a certain chip on the power line, so that it does not affect other chips.
The use of 0.1uF non-polar capacitors and 10uF electrolytic capacitors in parallel is because the parasitic inductance of electrolytic capacitors is relatively large and the ability to eliminate high-frequency ripples is poor. The parasitic inductance of non-polar capacitors is small and the ability to filter high-frequency ripples is better. However, if the capacity is selected according to the requirements of low frequency, the non-polar capacitor is too large and the cost is also high, while the electrolytic capacitor is small and the price is cheaper for the same capacity. Therefore, two capacitors are used in parallel. If
you design the circuit yourself, you should also use it in this way, and the position and routing of each capacitor are very particular. There are only two principles:
01
The connection lines from both ends of each small-capacity non-polar capacitor to the power pin and ground pin of the chip should be as short as possible, the shorter the better.
02
The power supply is usually introduced from other circuit boards, and electrolytic capacitors are usually only one or two on each circuit board. If there is only one electrolytic capacitor, put it where the power enters the circuit board. At this time, the electrolytic capacitor is of course far away from each chip, but because the electrolytic capacitor mainly works at a lower frequency, it doesn't matter if it is a little farther away. If two electrolytic capacitors are used on the circuit board, the other one should be placed near the chip that consumes the most power.

These are related to the layout of the circuit board components and the arrangement of the ground line (multilayer boards usually have a ground layer). For
noise below 10MHz, a 0.1μF capacitor works well. According to C=1/F, 0.1μF is used for 10MHz.
Simply put, the interference is grounded through the capacitor.

injoinic11

Thanks Thanks