The Engineer’s Guide to Bi-Directional Power System Design

May 2, 2018

Power components that allow energy to flow in either direction are a relatively new concept, with one of the first articles discussing bi-directional power design appearing in Electronic Design magazine in 2014. Initially many of the applications focused on battery charging and discharge, particularly back-up power supplies, but since then power engineers have found a range of applications for bi-directional design techniques.

The ability to step voltages up or down and to provide isolation is analogous to AC transformers, leading to this class of components being called “DC transformers” by many engineers.

Although engineers have only recently adopted this approach to designing systems, the bi-directional capability of the Sine Amplitude Converter (SAC) technology isn’t new. As Harry Vig explains, Vicor began to highlight a bi-directional power conversion capability only when it became clear that power developers would benefit from this capability.


Products that Support Bi-Directional Energy Flow

The BCM (Bus Converter Module) is the most popular bi-directional power component, offering voltage transformation and isolation. BCMs are available to support a wide range of voltages, from a few volts on the low side, to high-side voltages that extend to 700V.

The isolation provided by the BCM, however, means that additional circuitry is needed if the component is to be started by applying voltage to the high side. Non-Isolated Bus Converters (NBMs) eliminate this issue while offering a more cost-effective solution when isolation is not required. There are several additional benefits of choosing a non-Isolated Bus Converter, particularly an improvement in efficiency and power density over an isolated BCM.


Bi-Directional Power Design Resources

bi-directional converter application topologies

The Four Bi-Directional Converter Application Topologies

The best introduction to designing with bi-directional power components is our webinar Enabling Bi-Directional Energy Flow Using DC Power Transformers, which explains how to design bi-directional architectures and use them to improve system performance. The blog includes some Q&A responses from Harry after the webinar, with his discussion about the scalability of bi-directional power design being a very popular topic.

Engineers considering using bi-directional architectures can get detailed technical information from Vicor white papers. One describes the reverse mode operation of SACs, while another explains how to start a BCM in reverse mode in “DC Transformer” applications without losing the isolation between the two sides of the converter.

Architecting and analyzing bi-directional systems is enabled by the PowerBench Whiteboard, The capability to analyze NBMs in bi-directional configurations was introduced in 2016 to our online power design tools.


Bi-Directional Applications

Improving the performance of battery systems in energy storage applications is still a popular use of bi-directional power design. Vicor customers have used the approach in a range of applications that make use of batteries, including home energy storage.  And one customer even used a bi-directional architecture to develop a Li-Ion battery tester.

Another popular application for “DC transformers” is reducing the losses associated with long power cabling. The design of a tethered UAV is one example where stepping the voltage up to reduce the I2R losses in the cable not only increases electrical efficiency, but also allows the use of smaller, lighter cables, improving a key metric for UAVs: power-to-weight ratio.

An extension of this application is enabling easy system upgrades in applications such as telecommunications. Upgrading the performance of a system with remote units such as a cellular system that upgrades microcell base stations to 3G usually means power consumption increases. With cabling already laid to the remote units, often the cables aren’t able to carry the increased power without increasing the voltage used to decrease the current flowing. Bi-directional components provide a “matched pair” that will step up the voltage and then step it down at the remote unit without taking up a large amount of space or impacting on the system efficiency.


Meeting the Demands of Future Power Systems

After his webinar, Harry Vig explained that bi-directional power design was so interesting to engineers because the pressure on engineers to maximize efficiency means they need to look at every aspect of power flow in their systems. Bi-directional approaches using “DC transformers” allow engineers to optimize performance without compromising on real estate. Check out the other questions and answers from his webinar to make sure you are ready to take advantage of bi-directional power design today.


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