Soft switched high frequency ac-link converter

Variable frequency drives typically have employed dc voltage or current links for power distribution between the input and output converters and as a means to temporarily store energy. The dc link based power conversion systems have several inherent limitations. One of the important limitations is the high switching loss and high device stress which occur during switching intervals. This severely reduces the practical switching frequencies. Additionally, while the cost, size, and weight of the basic voltage sourced PWM drive is attractive, difficulties with input harmonics, output dV/dt and over-voltage, EMI/RFI, tripping with voltage sags, and other problems significantly diminish the economic competiveness of these drives. Add-ons are available to mitigate these problems, but may result in doubling or tripling the total costs and losses, with accompanying large increases in volume and weight. This research investigates the design, control, operation and efficiency calculation of a new power converter topology for medium and high power ac-ac, ac-dc and dc-ac applications. An ac-link formed by an inductor-capacitor pair replaces the conventional dc-link. Each leg of the converter is formed by two bidirectional switches. Power transfer from input to output is accomplished via a link inductor which is first charged from the input phases, then discharged to the output phases with a precisely controllable current PWM technique. Capacitance in parallel with the link inductor produces low turn-off losses. Turn-on is always at zero voltage as each switch swings from reverse to forward bias. Reverse recovery is with low dI/dt and also is buffered due to the link capacitance.