Calculating ripple voltage and current
To choose the right capacitor for the input filter of a switching regulator, for example, the capacitance needed to achieve a desired voltage ripple can be calculated, if the operating conditions of the regulator are known. When the capacitance is calculated, a candidate component can be identified, and the ripple current determined from the known ESR. This ripple current must be within the capacitor’s ripple current handling capability, if the device is to be suitable for use. This is where selection can become difficult, because both ESR and capacitance are known to vary with temperature, operating frequency, and the applied DC bias.
The capacitance can be calculated using the equation (from TI Application Report SLTA055)
Where CMIN = minimum capacitance required
IOUT = output current
dc = duty cycle (usually calculated as dc=Vout/(Vin*Eff))
fSW = switching frequency
VP(max) = peak-to-peak ripple voltage
Assuming, for example, a regulator with 12v input; 5v output; 2amp output; 85% efficiency; 400kHz switching, and an allowable input ripple voltage of 65mV:
Note that the chosen device must provide this value of capacitance at the regulator operating frequency of 400kHz.
The rms value of the peak-to-peak ripple voltage can be calculated from the equation:
The ripple current in the capacitor can then be calculated by applying Ohm’s law, if the capacitor’s ESR is known.