As soon as a power supply designer needs to comply with military standards, he might need to take into account the transparency or holdup requirement. Transparency requirement is the minimum amount of time during which input power can go away but the equipment is expected to remain operational.
This power interruption duration can range from 50ms all the way to 1000ms for DO160 or Mil-Std 704 standards for example. The simplest way to achieve such a hold up function, is to connect to input bus a huge tank capacitor that will store energy during normal operation, and restore it during power interruption.
The necessary capacitance value is given by the following formula :
Total Capacitance= 2 x P x Δt / (η x (V12 – V22))
P = Power at the load
Δt = Hold Up time required
η = Converter efficiency
V1 = Charged capacitor voltage before power drop out
V2 = Final input voltage before power supply shut down .
As an example, achieving 200ms transparency on a 28V input bus, a 25W DC/DC converter with 9-36V input voltage range requires a capacitor value as high as 25 000µF / 50V. Not only will this capacitor be very big, but it will also result in very high inrush current to charge it, unless some additional circuit comes into play to limit it. Here below is an example of typical Holdup circuitry:
The table hereafter shows the drop out voltage V1 – V2 considering different conditions of capacitor charge voltage (such as in normal operation, emergency operation or low transient on the input bus), before power drop out, down to 9V, the shut-down voltage value of the DC/DC converter.
To help reduce size, inrush current and monitor the state of charge, an integrated high voltage module increases the voltage the