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Class 2 transformers are small line-frequency transformers that are widely used for control systems that require 24 VAC signaling, including residential and commercial HVAC systems, industrial control systems, doorbells, and much more. In this work, we sampled and tested seven Class 2 transformers, each across different operating conditions, in order to characterize their efficiencies and note their shortcomings. We also provide possible improvements and solutions. We see on average a peak efficiency of 84.43% with 5.37 W of power loss when operated at 75% (30 VA output power) of their rated power, a 1.84% efficiency drop from the temperature rise that occurs at steady state when operated with full load, 2.8 W of no-load loss at 120 VAC input, and a no-load loss contribution of over 50% when operating at less than 75% load power. With these values, there is a clear goal to strive for in order to improve or create an alternative to these Class 2 transformers. 

Seven different miniaturized low-power isolation transformer topologies are analyzed using FEA simulation. Performance optimizations of fixed-area transformers with different operation frequencies and heights, with and without magnetic cores, are performed. The comparison of optimization results reveals performance potential and trade-offs of the different topologies under different restrictions. More than 1 W of power transfer is possible at more than 85% efficiency in 1 mm2 of footprint area for several different topologies. 

Efficient high-conversion-ratio power delivery is needed for many portable computing applications which require sub-volt supply rails but operate from batteries or USB power sources. In such applications, the power management unit should have a small volume, area, and height while providing fast transient response. Past work has shown favorable performance of hybrid switched-capacitor (SC) converters to reduce the size of needed inductor(s), which can soft-charge high-density SC networks while supporting efficient voltage regulation [1-5]. However, the hybrid approach has its own challenges including balancing the voltage of the flying capacitor and achieving safe but fast startup. Rapid supply transients, including startup, can cause voltage stress on power switches if flying capacitors are not quickly regulated. Past approaches such as precharge networks [3] or fast balancing control [5] have startup times that are on the order of milliseconds. This paper presents a two-stage cascaded hybrid SC converter that features a fast transient response with automatic flying capacitor balancing for low-voltage applications (i.e., 5V:0.4 to 1.2V from a USB interface). The converter is nearly standalone and all gate drive supplies are generated internally. Measured results show a peak efficiency of 96.9%, <; 36mV under/overshoot for 1A/μs load transients, and self-startup time on the order of 10μs (over 100× faster than previous works).