A power conversion system and method includes a DC-DC converter and an auxiliary circuit configured to ensure that a minimum input voltage is provided to the DC-DC converter during power interruption and for at least a predetermined hold-up time period. The auxiliary circuit includes an energy storage device, an auxiliary energy source for charging the energy storage device, and a clamping circuit to limit the energy stored by the energy storage device to a threshold voltage. A discharge time of the energy storage device from the threshold voltage to the minimum voltage thereby exceeds the predetermined hold-up time, but is only incrementally greater such that the size of the energy storage device is substantially reduced. The auxiliary energy source may typically be a current source, with the clamping circuit being control logic effective to disable the current source as a voltage across the storage device approaches the threshold.