Abstract: A single-fault tolerant DC-to-DC converter having an output voltage clamp is capable of sourcing current to and sinking current from a load while maintaining a well regulated secondary bus voltage V.sub.bus. The clamp is disabled when the DC-to-DC converter is sourcing current so that all of the current is delivered to the load and is enabled when V.sub.bus exceeds a clamp set point voltage V.sub.clamp to sink the excess current and clamp V.sub.bus at V.sub.clamp. To provide single fault tolerance, the clamp includes a fault detection clamp element that is controlled by the DC-to-DC converter so that they operate mutually exclusively and a regulator clamp element whose resistance is set in a negative feedback control loop so that, when the fault protection clamp element is activated, both clamp elements sink the excess current and control the secondary bus voltage.