The output capacitor value is determined by two factors in a power supply design: steady state ripple voltage and the output transient voltage response. The capacitor values for the transient response are typically specified as a load step from no load to full load of the power supply design.

Where

• I_step is the largest step in load current
• t_response is the approximate response time
• ΔV_out is the allowable output voltage change

Where

• f_c is the approximate crossover frequency, typically set to one-tenth the switching frequency
• T_sw is the switching period expected at the initial load condition before the load step

The ripple voltage in steady state has two major contributors: the change in the output voltage due to the charge and discharge of the output capacitors in every switching cycle, and the step in the output voltage due to the equivalent series resistor of the capacitors. An additional margin is placed on the ESR calculation to account for variance and aging of the

Where

• V_ripple is the allowable voltage ripple for a design
• N_PS is the primary to secondary turns ratio
• I_out,max is the maximum output load current
• i_{pk, max} is the primary side maximum peak current of the transformer

The final output capacitor value is the larger of the C_out and C_ripple values. The estimated crossover frequency largely determines the value of capacitance to use. For example, a crossover frequency of 2.5kHz suggests to use 900μF, while a 6.5kHz crossover is much lower capacitance, about 350μF. Use multiple capacitors to lower the equivalent ESR and get the actual capacitance close to the nameplate capacitance. Multiple capacitors increase robustness by accounting for DC derating and temperature rating fluctuations in the capacitance value.