Systems, methods, and devices which estimate the inductor current in a power factor correction (PFC) converter for use with AC/DC converters. A control system for use with the PFC takes as input the input voltage and the output voltage of the PFC. Control signals for power semiconductor subcircuits in the PFC are then output from the control system. The control system uses an adaptive observer sub-circuit that estimates the inductor current and the bus voltage. The adaptive observer uses an adaptive updater which uses both the estimates and the estimate error to update the adaptive observer's estimates.