2 Selecting C_Bulk capacitance

The server front end PSU shown in Figure 1-1 generally consists of two stages: an AC/DC rectifier stage and an isolated DC/DC converter stage, with peak efficiency targets of >98.5% and >97.5%, respectively. In order for the isolated DC/DC stage to achieve an efficiency target >97.5%, the operational input voltage range of the isolated DC/DC converter (V_Bulk) generally has to be limited inside 320 V to 410 V (V_Bulk,max). Assuming a nominal bulk capacitor voltage (V_Bulk,nom) of 390 V, Equation 1 calculates the capacitance required for holding up 3 kW for 10 mS as:

Considering V_Bulk voltage ripple and capacitance tolerance, the system shown in Figure 1-1 would require a capacitor with over 1.3 mF of capacitance. It is notable that the capacitor energy used to hold up the output voltage after AC dropout is only 32.6% of the total energy stored in the bulk capacitor during normal operation.

Inserting a baby boost converter stage in between the AC/DC rectifier stage and the isolated DC/DC converter stage (as shown in Figure 2-1) makes it possible to turn off the bypass field-effect transistor (FET) and enable the baby boost converter to allow the charging of C_BB to above 320 V from C_BULK after AC dropout. V_Bulk can then go much lower than 320 V, thus requiring less capacitance on the bulk capacitor to hold the output voltage for the same amount of time.

Assuming that V_Bulk can go down to 240 V (V_Bulk,min) with the baby boost converter during the AC dropout period, using Equation 1 equates to a required C_BULK of 635 μF, using 62% of the total capacitor energy.