To realize the buck/Strengthen aspect, all that is needed is two independent switches so which the AC input is often connected to one of several two Principal taps, even though the load is connected to another, So using the main transformer's Major windings as an autotransformer. For big power units, https://lukascfhec.anchor-blog.com/14092201/the-definitive-guide-to-uninterruptible-power-supply-calculator