Controlled transformer

A controllable high-frequency transformer for switching power supplies is described, which combines the properties of a transformer and magnetic amplifier by using two pairs of low-cost ferrite U-cores. A primary winding provides excitation to two flux paths, one path couples to a secondary winding and one to a control winding. The control power can be quite low relative to the output power since the control current is only magnetizing current for the control flux path, while the output power current can be many times the magnetizing current as in a normal transformer. The controlled output voltage can be any value, set by the number of turns in the secondary winding, since the control is achieved by steering flux. Thus, when zero output is required, nearly all the volt seconds (flux change) occurs in the control flux path, and very little in the power flux path