9.1.1.2.3 Input Capacitor Information

The size and value of the input capacitor is determined by the converter’s transient performance capability. This minimum value assumes that the converter is supplied with a responsive, low inductance input source. This source should have ample capacitive decoupling, and be distributed to the converter via PCB power and ground planes.

Ceramic capacitors should be located as close as possible to the module's input pins, within 0.5 inch (1,3 cm). Adding ceramic capacitance is necessary to reduce the high-frequency ripple voltage at the module's input. This reduces the magnitude of the ripple current through the electroytic capacitor, as well as the amount of ripple current reflected back to the input source. Additional ceramic capacitors can be added to further reduce the RMS ripple current requirement for the electrolytic capacitor.

The main considerations when selecting input capacitors are the RMS ripple current rating, temperature stability, and less than 100 mΩ of equivalent series resistance (ESR).

Regular tantalum capacitors are not recommended for the input bus. These capacitors require a recommended minimum voltage rating of 2 × (maximum dc voltage + ac ripple). This is standard practice to ensure reliability. No tantalum capacitors were found with a sufficient voltage rating to meet this requirement.

When the operating temperature is below 0°C, the ESR of aluminum electrolytic capacitors increases. For these applications, OS-CON, poly-aluminum, and polymer-tantalum types should be considered.