Suppression for the output ripple and noise / Selection of output filter capacitance

In general applications, the output end does not need the external capacitance, and the DC-DC Converters can work well, but in some applications that are more sensitive to ripple, the additional filter capacitance is needed.

The frequency of the ripple (f) depends on the switching frequency of the DC-DC Converters. Reducing the ripple can be solved by increasing the value of filter capacitance. According to the switch frequency, the capacitance of different materials can be used. If the frequency is less than 100KHz, the high frequency aluminum capacitance or tantalum capacitance can be used; the frequency range of 100kHz-500kHz can be used by tantalum capacitance or polymer capacitance ; the frequency range of 200kHz-5MHz can use large-capacity multi-layer stone (X7R or X5R) to filter noise peaks.

The output should not be increased with a large electrolytic capacitance. Due to the high switching frequency (200-400KHz) of most DC / DC Power modules , there are higher requirements for the ESR and ESL of the output capacitance, and increasing the capacity of electrolytic capacitance can not only not effectively reduce the ripple, but also bring some negative effects. Due to the output external capacitor charging, the startup current will increase, and the front-level DC-DC Converter is required to provide high instantaneous power and require sufficient surplus of input insurance. A large capacity load, for DC-DC Power supply with cut-off protection, means a transient over-current, the DC-DC Power modules may be under continuously restarted protection, and the output voltage will not be established.