Power Electronics

Turning Power Conversion Technology "Green"

DR. SLOBODAN CUK has devoted many hours to seeking improvements to dc-dc and ac-dc power conversion. Starting with this issue of Power Electronics Technology, he will describe his “Green Dream Technology” that solves power conversion problems thought to be impossible to solve.

Component technology for power conversion has progressed dramatically in the last two decades with the development of switching devices such as MOSFET's with low gate drive and switching losses at moderate 100 kHz switching frequencies. Likewise, multi-layer chip capacitors have been improved through a dramatic increase of their ripple currents and a simultaneous reduction in size. Unfortunately, there has not been as much progress in magnetic components, transformers and inductors. Only small incremental improvements have been made in core loss reduction and the increase of saturation flux densities.

Despite these advanced component technologies, power electronics technology as a whole has progressed slower because of the limitations presented by the Square-wave PWM switching method and corresponding conventional converter topologies. Present PWM switching methods impose inherently large fluxes on magnetic components no matter what converter topology is used. In addition, conventional dc-dc converters store dc energy in inductors, which seriously impacts their size, efficiency and transient response. Many resonant methods related to PWM switching also failed to resolve these problems.

Dr. Cuk's solution for dc-to-dc converters is a new storage less-switching method implemented with new converter topologies so that ac flux and size of inductors and transformers is reduced tenfold with no increase in switching frequency. In addition, storage less-switching method eliminates dc energy storage and provides a naturally fast transient response. The new technology results in efficiencies over 99% while operating between 50 kHz and 100 kHz.

This storageless switching method can be modified for low output voltage, high current POL converter applications. For example, 12V to 1V conversion results in total efficiencies over 97%. Even the “impossible” conversion range from 48V to 1V is achievable using new POL switching methods.

Present ac-dc converters suffer from limitation of conventional PWM switching methods to process input power through two or more cascaded power stages. They have a mandatory front-end bridge rectifier followed by a boost PFC converter, and another cascaded dc-dc converter providing isolation and output voltage regulation.

Dr. Cuk has developed a single-stage ac-dc conversion method, the hybrid-switching method, which eliminates the need for a full-bridge rectifier and results in a True Bridgeless PFC converter and an Isolated Bridgeless PFC converter. This new hybrid-switching method is a unique combination of PWM and resonant switching and employs only three switches. As described in the article, published in this issue, the hybrid-switching method provides a solution in a single-stage power processing that results in 98% efficiency. Dr. Cuk has another ac-dc converter solution in the works: a three-phase Bridgeless Bi-directional PFC converter for a single-stage power exchange between current three-phase utility transmission line and future DC transmission lines powered by solar and wind power.

These new switching techniques have moved the field of power electronics much closer to a green power technology. The variety of converter topologies based on the new switching methods already provide unprecedented performance benefits for both ac-dc and dc-dc conversion, including POL applications. Further efficiency improvements are forthcoming with the development of the new switching devices, which would take full advantage of new switching methods.

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