US Patent 7362206 Variable transformer

A variable transformer has an effective turns ratio that can be varied by electronic switching. By varying the effective turns ratio, the output voltage can be controlled precisely for a varying input voltage and load regulation. The variable transformer is a modular transformer in which one or more of the modules can be effectively removed from the variable transformer by turning on an ac switch so as to short-circuit the secondary winding of the module. When the ac switch is on, the rectifiers of the module must be turned off, if they are synchronous rectifiers. If they are diodes, they will be off by being back-biased when the ac switch is on. Improved transformer modules are shown having reduced leakage inductance.