The reason and analysis of DC power supply needing filtering
Ideal stable voltage power supply without filtering at the outlet. It is true that filter capacitors reduce the AC impedance of the power supply. The reason is that there is always internal resistance. At the output end of the power supply, a capacitor is added, so that the current increases instantaneously and maintains the capacitor in a short time, and the current decreases instantaneously and maintains a capacitor that can be reversed in a short time. A significant portion of these transient currents are switched directly across the capacitors rather than through the internal resistance of the power supply, thereby reducing the AC impedance of the power supply.
There is a similar application circuit board IC power supply decoupling capacitor, which also plays this role, because the internal resistance of the power supply, the impedance of the transmission line, and the decoupling capacitor can be partially changed immediately and directly in the capacitor current exchange, reducing the current change of the line. On the one hand, there are It is beneficial to the power supply of integrated circuits, and on the other hand, it can also help reduce the impact on other integrated circuits.
From this point of view, using an ideal power supply, the decoupling capacitor cannot be omitted due to the imperfection of the wire.
There is no need to consider the situation of DC. In fact, all power supplies have driving capabilities. Under the premise of satisfying the driving capabilities, the output power can remain relatively stable. The requirement of driving ability is also the requirement of internal resistance to a certain extent, but it is different, the internal resistance must form a pressure drop, and stable power supply, due to the use of closed-loop control, within the range of driving ability, it can theoretically be guaranteed to be constant Output.
The so-called capacitive reduction of AC impedance is mainly aimed at high-frequency AC impedance, because even if the power supply is closed-loop, there are requirements for response speed. Due to momentary changes in load current, its response cannot keep up, resulting in slight fluctuations in voltage. By adding capacitance, the instantaneous change in the power supply current can be reduced, thereby changing the characteristics of the power supply.
For power filters, I just talked about the effect of reducing AC impedance. In fact, reducing output ripple is more important. For dry cells, the output contains no ripple and requires no filtering.
However, an increase in capacitance, especially near the IC power pins, is necessary for complex circuits from the viewpoint of AC impedance reduction mentioned earlier.
