The size and value of the input capacitor is determined by the converter transient performance capability. The minimum amount of required input capacitance is 470 µF, with an RMS ripple current rating of 500 mA. 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.

For high-performance/transient applications, or wherever the input source performance is degraded, 1000 µF of input capacitance is recommended. The additional input capacitance above the minimum level insures an optimized performance.

Ripple current (rms) rating, less than 100 mΩ of equivalent series resistance (ESR), and temperature are the main considerations when selecting input capacitors. The ripple current reflected from the input of the PTH08T210W module is moderate to low. Therefore any good quality, computer-grade electrolytic capacitor will have an adequate ripple current rating.

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. Adding one or two ceramic capacitors to the input attenuates high-frequency reflected ripple current.