There are two power inputs to the power-supply board, PWR-N and PWR-R (nominal and redundant, respectively). The power input is nominally 5 V ±5% and up to 12 A. Each power input goes to the TPS7H2201-SP device, a power switch that is used to switch power from either the nominal or the redundant power input to an array of point-of-load (POL) converters. The power switches are controlled by a fault detection circuit using the OFF(H) signal which, when asserted, switches off the power switches. During normal operation only one power switch is provided with power.

When power is applied to the nominal power input, the nominal power switch is turned on and the redundant one turned off. When power is applied to only the redundant power input, the nominal power switch is turned off and the redundant one turned on. When power is applied to both nominal and redundant power inputs, the nominal power switch is turned on and the redundant one turned off, so that power is drawn from the nominal power input only.

The power switches protect the SpaceFibre routing switch from reverse polarity and overvoltage up to 7 V. The switched 5-V rail provides the main supply voltage (Vmain) to several POL DC-DC converters and to a linear regulator. Each of these devices can operate with an input voltage of up to 7 V. The power switches provide overvoltage protection which switches off the Vmain power if the input voltage exceeds the maximum power input voltage.

The power switches have overcurrent and overtemperature protection and also a current sense output. The overcurrent limit is set to 17.6 A maximum, which considers the normal maximum current, current inrush during power up, and 20% tolerance on the overcurrent limit setting. Power is switched off when the overcurrent condition has been present for around 1 ms. There are two current sense outputs, IMON, one on each power switch device.