How to choose a filter capacitor

Selecting appropriate filter capacitors can improve product reliability and stability.

The main parameters of the capacitor are: withstand voltage, operating temperature, capacitance value, equivalent series resistance (ESR), etc. When designing the filter circuit, we need to choose the appropriate filter capacitor according to the actual needs.

The capacitors used for filtering mainly include: electrolytic capacitors, tantalum capacitors, and ceramic capacitors.

1. How to choose the withstand voltage of the filter capacitor?

Of course, the higher the withstand voltage value, the better, but the capacitor with the same capacitance value, the higher the withstand voltage value, the larger its volume and the higher the price. In fact, as long as the withstand voltage value is greater than 2 times of the working voltage, it can work very reliably. If it is not 2 times the withstand voltage, at least 1.5 times the withstand voltage value. For example, for a circuit with a working voltage of 10V, we can choose a capacitor with a withstand voltage of 25V.

2. How to choose the working temperature of the filter capacitor?

In order to make the capacitor work stably and reliably for a long time, a certain margin should be reserved for the working temperature. When the working environment temperature exceeds 60°C, it is recommended to choose a capacitor of 105°C.

3. How to choose the capacitance value of the filter capacitor?

In the capacitor power supply circuit, the AC component can be filtered out to make the power supply more stable and smooth. In low-frequency circuits, capacitors with larger capacitance values can be selected. But in high-frequency switching power supply circuits, the larger the filter capacitor, the better. Capacitor impedance and frequency characteristics need to be considered in high frequency circuits. Capacitors need a lower equivalent impedance at the operating frequency to have good filtering effect.

The size of the capacitance value can refer to the calculation formula of the predecessors: C》0.289/{f×(U/I)×ACv} ACv is the ripple factor, the unit is %.

4. How to choose the equivalent series resistance (ESR) of the filter capacitor?

In the low frequency filter circuit, it is enough to pay attention to the capacitance value, and it is not necessary to pay too much attention to the ESR. When the operating frequency of the power supply increases, the capacitance value will drop rapidly, so in the high-frequency power supply circuit, we need to choose a filter capacitor with small ESR.

5. What kind of capacitor should be selected as filter capacitor?

The ESR of electrolytic capacitors is the largest, the ESR of tantalum capacitors is smaller than that of electrolytic capacitors, and the ESR of ceramic capacitors is the smallest.

The capacitance value of electrolytic capacitors is relatively large, and the withstand voltage is also high; the withstand voltage value of tantalum capacitors is generally low, and the tantalum capacitors are small in size and stable in performance; ceramic capacitors are generally small in size, but the capacitance value is not too large.

It is necessary to select qualified capacitors according to capacitance value, size, withstand voltage, temperature resistance, ESR, cost, etc.