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Typically, the hold-up time requirement is met by using enough capacitance on the 48V line, such that if power is lost even at the minimum permissible line voltage, the charge in the cap will be sufficient to keep the power supply operating for the duration of the hold-up time.
All you need really is a voltage comparator to tell your system that the voltage has dropped below a certain level and so power will be lost.
Then you calculate the value of your hold-up capacitor, knowing that the energy the system needs is the energy the cap delivers as it discharges between the two voltage levels:
Vth= threshold voltage of your comparator circuit (say 48V-15%=40.8V, call it 40V, to allow for tolerances in the sensing circuit)
Vmin= the minimum voltage at which the system power supply can still maintain regulation (typically 35~36V, call it 35V)
where P is the total power needed by the system (at the power supply input)
t=hold-up time (4ms)
As an example, if the required power is 100W, the capacitor value is:
A 2200µF capacitor would then be required. But you have to adjust the capacitance, to account for the cap's tolerance (typically -20%), since the calculated value represents the absolute minimum required to just meet the hold-up requirement, without any margin.