Symmetrical power converter

Abstract

A switched-capacitor circuit has two capacitors and two MOSFETs that cross-couple the capacitors, connecting the anode of one to the cathode of the other, and vice-versa. When either MOSFET is on, the capacitors are in series; the order alternates as the MOSFETs alternate. A reversing cyclical voltage suitable as a primary drive for a transformer is generated. If the MOSFETs alternate with no dead-time, a square-wave excitation is generated. With off-time, a pwm excitation is generate. Charge is maintained on the switched-capacitors using a symmetrical common-mode inductor. A bifilar winding is center-tap as its input, and the ends of the bifilar winding are connected to the capacitors. The capacitors are effectively in parallel. Because the charging current flows and returns through each leg of the inductor equally, it cannot magnetize the inductor core or cause any flux change. Because any voltages induced in the windings are common-mode, flux change in the core does not affect the charging current. The ac voltage generated when the capacitors switch is across the full inductor. Not only does the inductance attenuate any noise, the center-tap is between equal and opposite negative and positive voltages, which cancel. There is very little noise at the input. The circuit is reciprocal, so it can be used to rectify a transformer output. Two can be used as a bi-directional transformer isolated power converter. Several modules using 1 to 1 transformers can be stacked for a power converter having a higher ratio of input to output voltage.