M255 SERIES HOLD-UP POWER SUPPLY

Date: June 9, 2021

Rev (g)

Page 4 of 5

Description of Operation

The holdup unit is meant to be installed in front of a power supply, and not a standalone. It is used for input

voltage holding when a power failure occurs, such that if the input power returns to nominal limits in a

defined period of time, the load can't tell that a power failure had happened.

This unit is designed to be used with most of Milpower Source's DC/DC converters and DC/AC inverters

(consult factory for applicability to specific models).

During normal operation, an internal converter charges the external capacitor to 51 V.

In case a power failure occurs where the input voltage drops below 16 V, the input is disconnected from the

output via a MOSFET switch, and the external cap is connected to the output pins through another MOSFET

switch. The external cap is then discharged into the cascaded power supply, dropping towards 16V, where

the cascaded power supply ceases to operate.

When input power returns (it can happen during cap discharge or after complete loss of power), the

MOSFET switches disconnect the output from the external cap, reconnect the input to the output, and the

internal converter recharges the external cap back to 51 V, preparing it for the next power failure.