Calculation of the size of the power supply filter capacitor

The inductance impedance is proportional to the frequency, and the capacitance impedance is inversely proportional to the frequency. Therefore,Inductance can block high-frequency pass through, and capacitance can block low-frequency pass through.The two can be combined to filter various frequency signals.For example, in a rectification circuit, the capacitor can be connected to the load or connected to the load to filter out the AC ripple ..

Capacitor filtering is a voltage filter that directly stores the pulsating voltage to smooth the output voltage.High Output voltage, close to the peak AC voltage; suitable for small current,The smaller the current, the better the filtering effect..

Inductor filtering is current filtering, which is used to generate electromagnetic induction through current to smoothly output current.Low output voltage, lower than the valid value of AC voltage;Suitable for large current, the larger the current, the better the filtering effect.

Many of the characteristics of capacitance and inductance are the opposite.

In general, electrolytic capacitors are used to filter out low-frequency signals in the current. However, even low-frequency signals have frequencies of several orders of magnitude. Therefore, in order to be suitable for use at different frequencies, electrolytic capacitors are also divided into high-frequency capacitors and low-frequency capacitors (the High Frequency here is relative).

The low-frequency filter capacitor is mainly used for filtering mains filtering or transformer rectification. Its working frequency is the same as that of mains 50Hz. The high-frequency filter capacitor is mainly used for filtering after switching power supply rectification, the operating frequency is several thousand Hz to tens of thousands Hz. When we use low-frequency filter capacitors for high-frequency circuits, the high-frequency characteristics of low-frequency filter capacitors are poor. In high-frequency charging and discharging, the internal resistance is large and the equivalent inductance is high. Therefore, large heat is generated due to frequent electrolyte polarization during use. The high temperature will make the electrolyte inside the capacitor gasification, the pressure inside the capacitor increases, and eventually lead to the drum package and burst of the capacitor.

The size of the power supply filter capacitor, which is usually designed. 4.7u is used for filtering low frequency and 0.1u is used for filtering high frequency. The capacitance of 4.7uf is used to reduce the output pulsation and low frequency interference, the 0.1uf capacitor is used to reduce high-frequency interference caused by transient changes of load current.Generally, the larger the front, the better. The difference between the two capacitors is about 100 times. Power supply filtering, switching power supply, depending on how large your ESR (capacitance equivalent series resistance), and the choice of High Frequency Capacitor is best in its self-resonance frequency. A large capacitor is used to prevent surge, and the mechanism is like a large reservoir with stronger flood control capabilities; a small Capacitor Filters high-frequency interference, and any device can be equivalent to a series parallel circuit with resistance, inductance, and capacitance, there is self-resonance, only in this self-resonance frequency, the equivalent resistance is the smallest, so the best filter!

The equivalent model of a capacitor is an inductance L, a resistor R and a capacitor C in series,

The inductance L is obtained from the capacitor lead, and the resistance R represents the active power loss of the capacitor. Therefore, capacitor C can be equivalent to the series LC loop for its resonance frequency. The conditions for series resonance are WL = 1/WC, W = 2 * pI * F to obtain this formula F = 1/(2PI * LC )., the minimum impedance at the center frequency of the LC loop is pure resistance, so the center frequency is filtered. the size of the lead inductor varies depending on the thickness. The inductance of the grounding capacitor is generally about 1MM and 10 NH, depending on the grounding frequency.

Parameters for capacitor filtering design should be considered: ESR, ESL, withstand voltage value, and resonance frequency.