In the case of an UPS, system efficiency is critical to maximize battery charge utilization. In a dual-stage UPS, where the DC to AC conversion is typically based on an H-bridge, there is less variation in efficiency over the entire load range. However, for the DC-DC step-up, there can be several topologies. For higher efficiency, an LLC topology is preferred. The LLC can be designed to work at resonance at a nominal battery voltage. However, when the battery is fully charged, the LLC moves away from resonance to a higher frequency of operation, and when the battery is fully discharged, the LLC tries to operate below resonance. In these regions, the LLC 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, while the DC-AC conversion is based on an H-bridge inverter. The LLC resonant topology is made to work at fixed resonant frequency without regulating DC-DC output voltage i.e., open-loop operation, while the AC output voltage is regulated by varying the modulation index of the H-bridge stage.
