According to a first aspect, an uninterruptable power supply is provided. In one example, the uninterruptable power supply comprises a controller configured to provide a first current reference signal, the first current reference signal having a periodic waveform including a first half period and a second half period, each half period having a period end, the waveform comprising a substantially rectified sine wave modified such that a value of the rectified sine wave is equal to zero for a predetermined period of time prior to the period end of the second half period, a positive current loop control circuit configured to receive the first current reference signal and provide an output signal to the positive boost circuit, and a negative current loop control circuit configured to provide an output signal to a negative boost circuit.