A practical and unique control technique to enhance efficiency of dual-stage DC-AC power inverter

In the case of an uninterruptible power supply (UPS) or DC-AC power inverter, system efficiency is critical to maximize battery back-up time. In a dual-stage UPS or DC-AC power inverter, where the DC to AC conversion is typically based on an H-bridge configuration, there is less variation in efficiency over the entire load range as it is fed from a fixed DC bus. However, for the DC-DC step-up, there can be several topologies. For higher efficiency, an LLC resonant topology is preferred. The LLC converter can be designed to work at resonance at a nominal battery voltage. However, when the battery is fully charged, the LLC converter moves away from resonance to a higher frequency of operation, and when the battery is fully discharged, the LLC converter tries to operate below resonance. In these regions, the LLC converter is not operating at peak efficiency, which reduces the critical battery backup time for a UPS.In this paper, a dual-stage digital control DC-AC power inverter is proposed where the DC-DC step-up conversion is based on a full bridge LLC resonant topology, while the DC-AC conversion is based on an H-bridge inverter. The LLC resonant topology is made to work at a fixed resonant frequency without regulating DC-DC converter output voltage, i.e., open-loop operation, while the AC output voltage is regulated by varying the modulation index (MI) of the H-bridge stage. The fixed operating frequency of the LLC converter ensures maximum efficiency irrespective of battery voltage. The DC-DC stage open loop operation means lower computational load on the microcontroller. Based on the proposed scheme, a digitally controlled 1500VA inverter has been designed and developed, generating a sinusoidal 230V AC output from any battery providing a nominal voltage range of 42-60V DC.