How to make a 10kW Bidirectional AC/DC Converter
2023. 5. 25
This whitepaper starts with who needs a high power bidirectional AC/DC converter and follow that ups with what recent developments have occurred that makes such designs commercially more viable, before going into any further details.
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Introduction
Handling 10kW of power in both directions (from AC to DC and back) is technically challenging, but understanding the available topologies and techniques simplifies the task significantly. At the start of any project, it's crucial to first ask, "What is the demand?" before considering "How can I construct it?" Many projects begin the wrong way, based on the misguided principle that if something is built, someone will eventually buy it. While this idea worked in the 1989 film Field of Dreams, it rarely succeeds in practice. So, let’s begin by examining who needs a high-power bidirectional AC/DC converter, followed by a look at recent developments that have made such designs more commercially viable, before diving into further details.
The Case for Bidirectional Power
If you examine the literature, prototype designs, and evaluation boards for bidirectional power supplies, you’ll find that they are becoming increasingly common. Why the sudden interest in bi-directionality? One key reason is electric vehicles, or more specifically, their battery packs, which serve as storage for renewable energy.
Renewable energy has become a significant topic in many countries. In the United States, it is the fastest-growing energy source, with a 100% growth rate from 2000 to 2018. The UK, for the first time last year, generated more power from zero-carbon energy sources than from fossil fuel-based power stations, despite relying on fossil fuels for more than 75% of its electricity less than a decade ago. Additionally, Austria, where RECOM Power’s energy-neutral headquarters are located, leads the European green energy movement, with around 72% of its electricity needs being met by zero-carbon sources.
However, not all countries benefit from a long coastline to house nuclear power stations or convenient snow-capped mountains and lakes. Many countries must rely on wind, solar, or small-scale hydroelectric power, which can be unreliable. Low river water levels in summer can reduce run-of-river power production, and peak electrical demand often occurs on windless days or at night. One potential solution to ensure continuity of supply is to use the electrical power stored in electric vehicle (EV) batteries to help balance supply and demand in a Vehicle-to-Grid (V2G) system. Over the next decade, Germany is expected to have around seven million electric vehicles, each with an onboard battery capacity of 20-100kWh. Even if only 20% of this capacity is available at any given time, it still represents 140GW — more than the combined capacity of 100 nuclear power stations.
The key to a successful V2G system is the combination of bidirectional energy flow and artificial intelligence (AI). Since most vehicles are parked more than 95% of the time, an electric vehicle (EV) plugged into a charging station while the owner is at work can decide whether to continue charging its battery or release part of its stored charge back to the grid during peak demand. This decision depends on known or predicted usage patterns, allowing the EV to adapt its state of charge accordingly. Given that most daytime journeys are under 37km, it’s not always necessary for the vehicle to remain fully charged between daily trips. To enable this, a bidirectional charger/mains inverter is required to facilitate the flow of energy in both directions. Importantly, the bidirectional charging station itself does not need to be intelligent, as the necessary processing power is already embedded in the AI system of the electric vehicle.
Having established the potential need for millions of bidirectional AC/DC power supplies to meet the projected increase in electric vehicles (EVs) by 2030, the next step is to evaluate whether it is commercially viable to produce them. Two recent developments have significantly simplified and reduced the cost of bidirectional designs. The first is the introduction of new topologies that are particularly suited for bidirectional current flow. 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, while offering substantially better efficiency.
Renewable energy has become a significant topic in many countries. In the United States, it is the fastest-growing energy source, with a 100% growth rate from 2000 to 2018. The UK, for the first time last year, generated more power from zero-carbon energy sources than from fossil fuel-based power stations, despite relying on fossil fuels for more than 75% of its electricity less than a decade ago. Additionally, Austria, where RECOM Power’s energy-neutral headquarters are located, leads the European green energy movement, with around 72% of its electricity needs being met by zero-carbon sources.
However, not all countries benefit from a long coastline to house nuclear power stations or convenient snow-capped mountains and lakes. Many countries must rely on wind, solar, or small-scale hydroelectric power, which can be unreliable. Low river water levels in summer can reduce run-of-river power production, and peak electrical demand often occurs on windless days or at night. One potential solution to ensure continuity of supply is to use the electrical power stored in electric vehicle (EV) batteries to help balance supply and demand in a Vehicle-to-Grid (V2G) system. Over the next decade, Germany is expected to have around seven million electric vehicles, each with an onboard battery capacity of 20-100kWh. Even if only 20% of this capacity is available at any given time, it still represents 140GW — more than the combined capacity of 100 nuclear power stations.
The key to a successful V2G system is the combination of bidirectional energy flow and artificial intelligence (AI). Since most vehicles are parked more than 95% of the time, an electric vehicle (EV) plugged into a charging station while the owner is at work can decide whether to continue charging its battery or release part of its stored charge back to the grid during peak demand. This decision depends on known or predicted usage patterns, allowing the EV to adapt its state of charge accordingly. Given that most daytime journeys are under 37km, it’s not always necessary for the vehicle to remain fully charged between daily trips. To enable this, a bidirectional charger/mains inverter is required to facilitate the flow of energy in both directions. Importantly, the bidirectional charging station itself does not need to be intelligent, as the necessary processing power is already embedded in the AI system of the electric vehicle.
Having established the potential need for millions of bidirectional AC/DC power supplies to meet the projected increase in electric vehicles (EVs) by 2030, the next step is to evaluate whether it is commercially viable to produce them. Two recent developments have significantly simplified and reduced the cost of bidirectional designs. The first is the introduction of new topologies that are particularly suited for bidirectional current flow. 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, while offering substantially better efficiency.
Unidirectional vs Bidirectional AC/DC
Unidirectional AC/DC battery chargers have been on the market for many years. They generally use the following arrangement, with some proprietary variations:
Fig. 1: General arrangement of an AC/DC battery charger
Mains-powered battery chargers are essentially composed of an AC/DC converter (the PFC stage), followed by a DC/AC converter (the transformer driver stage), an AC/DC converter (the rectifier and output filter stage), and a battery charging interface. Depending on the battery voltage and power levels, the transformer driver stage can employ single-ended, push-pull, phase-shift full-bridge, or LLC topologies. In nearly every battery charger application, power factor correction (PFC) on the input is required, along with a battery interface to handle reverse polarity protection and adjust the charging voltage and current to match the specific chemistry of the battery cells.
To make this design bidirectional, it could be achieved by adding an inverter stage in parallel with the existing schematic:
Fig. 2: General arrangement of an AC/DC battery charger with parallel inverter for bidirectional energy flow
However, this approach to adding bi-directionality is inefficient in terms of utilizing the existing components and introduces significant cost due to the need for two transformers. If the market demands millions of such bidirectional power supplies, the cost of each unit becomes a crucial factor. A more efficient solution would be to use a topology that is inherently bidirectional, requiring only a single isolating transformer.
Fig. 3: General arrangement of an AC/DC bidirectional battery charger/discharger
To explore the design considerations for such a product, we can analyze each stage individually, comparing the conventional unidirectional topology with alternative bidirectional options. Since the goal is to create a power supply that operates in both directions, we can start with the end result, as it also marks the beginning of the process.
Fig. 1: General arrangement of an AC/DC battery charger
Mains-powered battery chargers are essentially composed of an AC/DC converter (the PFC stage), followed by a DC/AC converter (the transformer driver stage), an AC/DC converter (the rectifier and output filter stage), and a battery charging interface. Depending on the battery voltage and power levels, the transformer driver stage can employ single-ended, push-pull, phase-shift full-bridge, or LLC topologies. In nearly every battery charger application, power factor correction (PFC) on the input is required, along with a battery interface to handle reverse polarity protection and adjust the charging voltage and current to match the specific chemistry of the battery cells.
To make this design bidirectional, it could be achieved by adding an inverter stage in parallel with the existing schematic:
Fig. 2: General arrangement of an AC/DC battery charger with parallel inverter for bidirectional energy flow
However, this approach to adding bi-directionality is inefficient in terms of utilizing the existing components and introduces significant cost due to the need for two transformers. If the market demands millions of such bidirectional power supplies, the cost of each unit becomes a crucial factor. A more efficient solution would be to use a topology that is inherently bidirectional, requiring only a single isolating transformer.
Fig. 3: General arrangement of an AC/DC bidirectional battery charger/discharger
To explore the design considerations for such a product, we can analyze each stage individually, comparing the conventional unidirectional topology with alternative bidirectional options. Since the goal is to create a power supply that operates in both directions, we can start with the end result, as it also marks the beginning of the process.
Step 1. The Battery Interface
Each battery type has a unique cell chemistry that requires a specific charging profile. For instance, a 48V Lithium-Ion battery pack should be charged initially with a constant current, followed by constant voltage charging until it reaches full saturation.
After that, the charging process should be interrupted, as Lithium-Ion packs cannot tolerate overcharging (a trickle charge could damage the battery by causing metallic lithium to plate onto the anode). However, the charging process should not be stopped prematurely, as the full charge capacity lags significantly behind the constant current cut-off point (Figure 4).
In an EV charging scenario, the user and safety interface must also be considered. Most charging cables are equipped with a data bus to facilitate the necessary handshaking and negotiation with the EV before power is supplied. Furthermore, charging stations typically feature an LCD display that shows essential information such as the state of charge, charging voltage and current, expected duration, and cost.
Since this microprocessor-based interface is already in place, ...
After that, the charging process should be interrupted, as Lithium-Ion packs cannot tolerate overcharging (a trickle charge could damage the battery by causing metallic lithium to plate onto the anode). However, the charging process should not be stopped prematurely, as the full charge capacity lags significantly behind the constant current cut-off point (Figure 4).
In an EV charging scenario, the user and safety interface must also be considered. Most charging cables are equipped with a data bus to facilitate the necessary handshaking and negotiation with the EV before power is supplied. Furthermore, charging stations typically feature an LCD display that shows essential information such as the state of charge, charging voltage and current, expected duration, and cost.
Since this microprocessor-based interface is already in place, ...
Fig. 4: Typical Li-Ion charging profile (Source: Batteryuniversity.com)
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