For off-line rectified-AC applications, the total input bulk capacitor (C_BULK) should be sized to provide energy from the peak of the minimum input AC-line rms voltage (V_IN(MIN)) to the minimum allowable voltage (V_BULK(MIN)) for the power conversion stage. Due to the transition-mode operation, too low of V_BULK(MIN) selection results in higher rms current at V_IN(MIN) and affects the full-load efficiency, while too high of V_BULK(MIN) enlarges the volume of the bulk capacitor. This equation does not account for the hold-up time requirement over AC-line dips and drop-outs.

Equation 19.

For DC-input applications, the value of C_BULK depends on the nature of the input:

• If the DC source is subject to brief interruptions (similar to dips and drop-outs of an AC-line), then the value of C_BULK is calculated in a similar manner as for C_BULK(MIN) above. Δt_{DC_drop} represents the maximum expected interruption time of the DC input.
  
  Equation 20. ${\mathrm{C}}_{\mathrm{B}\mathrm{U}\mathrm{L}\mathrm{K}\left(\mathrm{M}\mathrm{I}\mathrm{N}\right)}=\frac{\frac{{\mathrm{P}}_{\mathrm{O}}}{\mathrm{\eta }}\times {\mathrm{\Delta }\mathrm{t}}_{\mathrm{D}\mathrm{C}\_\mathrm{d}\mathrm{r}\mathrm{o}\mathrm{p}}}{\left({\mathrm{V}}_{\mathrm{D}\mathrm{C}\_\mathrm{I}\mathrm{N}\left(\mathrm{M}\mathrm{I}\mathrm{N}\right)}^2-{\mathrm{V}}_{\mathrm{B}\mathrm{U}\mathrm{L}\mathrm{K}\left(\mathrm{M}\mathrm{I}\mathrm{N}\right)}^2\right)}$
• If the DC source is consistently steady such as from a high-voltage battery, then the value of C_BULK should be sufficient to effectively by-pass the high-frequency primary switching current to avoid conducting significant EMI noise back to the source.C_BULK may be made up of more than one capacitor. Select standard values with sufficient margin to the calculated C_BULK(MIN) to allow for tolerance and aging.