Low-Power Solid-State Transformers to Replace Line-Frequency Class 2 Transformers

Class 2 transformers are small line-frequency transformers widely used for control systems requiring 24 VAC signaling, including residential and commercial HVAC systems, industrial controls, and much more. These transformers have large standby losses, low efficiencies, large weights, and high costs. In this work, we propose a power electronic alternative in the form of a low-power solid-state transformer. We design and test two 40 VA 120 VAC to 24 VAC solid state transformers, including a two-stage and a single-stage topology. Both converters provide higher efficiencies across all load ranges compared to the Class 2 line-frequency transformers, especially at light load (5 VA) with improvements of 19.6% and 29.9%, respectively. Standby losses for the two are 417 mW and 196 mW, compared to an average of 2.8 W standby loss for 40 VA Class 2 line-frequency transformers.