How to make a 10kW bidirectional AC/DC Converter

Handling 10kW of power in both directions (from AC to DC and back) is technically challenging, but understanding the topologies and techniques available makes the job a lot easier. At the beginning of any project, you should always ask yourself “What is the demand?” before you go on to ask yourself “How can I construct it?” Too many projects start the wrong way around, following the principle that if it is built, someone will eventually come to buy it. Although this concept worked in the 1989 film “Field of Dreams”, it rarely works in practice. So, let us begin with who needs a high power bidirectional AC/DC converter and follow that up with what recent developments have occurred that makes such designs commercially more viable, before going into any further details.

If you check the literature, prototype designs and evaluation boards for bidirectional power supplies are appearing everywhere. Why the sudden interest in bi-directionality? One of the main reasons is electric vehicles, or more exactly, their battery packs, as a storage medium for renewable energy.

Renewable energy is now a hot topic in many countries: it is the fastest growing energy source in the United States, with a 100% growth rate from 2000 to 2018. The UK produced more power from zero-carbon energy than from fuel-based power stations for the first time last year, despite having more than 75% of the electrical power derived from fossil fuels less than a decade ago. Moreover, Austria, where RECOM Power has its energy-neutral headquarters, is at the forefront of the European green energy project, with around 72% of its electricity needs coming from zero-carbon sources. However, not every country has the advantages of a long coastline where nuclear power stations can be tucked away out of sight or convenient local snow-capped mountains and lakes. Most countries have to rely on wind, solar or small-scale river hydroelectric power, which are not always very reliable sources since low river water levels in summer limit run-of-river power production and peak electrical demand often comes during windless days or at night.

One of the solutions to ensuring supply continuity is, of course, to use the combined electrical power stored in the batteries of electric vehicles (EV) to help balance out supply and demand in a so-called Vehicle-to-Grid (V2G) system. Within the next ten years, there is likely to be around seven million electric vehicles in Germany alone, each with 20-100kWh of on-board battery capacity. Even if only 20% of that capacity is available at any one time, that still represents 140GW, or more capacity than 100 nuclear power stations.

The key to a successful V2G system is the combination of bidirectional energy flow and artificial intelligence (AI). Most vehicles spend more than 95% of their time parked. If an EV is plugged into a charging station while the owner is at work, the EV can determine whether to continue recharging its battery or to release part of its stored charge back to the grid at peak times, thus adapting its state of charge depending on known or predicted usage patterns. As the majority of daytime journeys are less than 37km per day, it is not always necessary that the vehicle is fully charged or even remains so in between the daily journey times. But to do this requires a bidirectional charger/mains inverter to transfer the electrical power in both directions. Note that the bidirectional charging station does not need to be intelligent; the necessary processing power is already incorporated into the AI system built in to the electric vehicles.

Having established the potential need for millions of bidirectional AC/DC power supplies to meet the estimated increase in EVs by 2030, the next stage is to ask if it is commercially viable to build them. There are two relatively recent developments that have made bidirectional designs substantially simpler and cheaper to realize the first is the introduction of new topologies that lend themselves particularly well to bidirectional current flow and the second is the maturation of new technologies, such as Silicon Carbide (SiC) high power switching transistors, which are now price-competitive with the long-established Insulated Gate Bipolar Transistor (IGBT) technology, but with significantly better efficiency savings.

Unidirectional AC/DC battery chargers have been on the market for many years. They generally use the following arrangement, with some proprietary variations:


Mains powered battery chargers are essentially AC/DC converters (the PFC stage), followed by a DC/AC converter (the transformer driver stage), followed by an AC/DC converter (the rectifier and output filter stage), followed by a battery charging interface. Depending on the battery voltage and power levels, the transformer driver stage can use single-ended, push-pull, phase shift full bridge or LLC topologies, but in almost every battery charger application, power factor correction on the input and some sort of battery interface to handle reverse polarity protection and match the charging voltage and current profile to the cell chemistry is required.

To make such a design bi-directional could be realized by adding an inverter stage in parallel with the existing schematic:


However, this way of adding bi-directionality is inefficient in the use of the existing components and adds significant cost because two transformers are needed. If the market is for millions of such bi-directional power supplies, then the cost of each unit becomes a very significant factor. A better solution would be to use a topology that is inherently bi-directional with only one isolating transformer.


To go through the design considerations for such a product, we can analyse each stage in turn, comparing the conventional unidirectional topology with alternate bidirectional alternatives. As the desired result is a power supply that works in both directions, we can start at the end, as it is also the beginning.