The required bulk capacitance value depends on the target minimum bulk capacitor ripple voltage at minimum AC input line, minimum line frequency and on the power level of interest. As a way of estimating, use 1.5-μF to 2-μF per Watt of rated, continuous power to achieve approximately 70 V to 80 V minimum at 88 VRMS input. This case indicates a required bulk capacitance of between approximately 100 μF and 130 μF. Alternatively, the required capacitance may be explicitly calculated for a specific set of requirements using Equation 21.
Using the parameters in Table 7, this calculates a required CBULK of 130 μF.
To help reduce differential mode (DM) emissions for conducted EMC compliance, the bulk capacitance has been split into two separate capacitors C5 and C7 in Figure 44, with a small DM choke L2 inserted between the capacitors. The total resulting capacitance of 127 μF is close to the required minimum requirement per Equation 21, and the design results in a small decrease in the actual bulk capacitor minimum ripple voltage.
Next, verify that the choice of bulk capacitance satisfies the X-capacitor discharge constraints for rate of discharge by the load when X-capacitor sampling is inactive, per Equation 5. The bulk capacitance should be less than the value calculated by Equation 22.