Fixed-ratio converters unlock innovation in the battery lifecycle

Battery production is a core technology driver of the continued spread of electrification across many industries, which not only improves productivity but also brings environmental advantages. Driven by electric vehicles (EVs) and renewable energy storage systems, the battery industry has become one of the fastest growing industries in the world.

According to the International Energy Agency (IEA), global battery demand has surged tenfold from 43.8GWh per year in 2016 to 550.5GWh per year in 2022. As demand continues to grow, every stage of the battery life cycle is worth exploring. The entire battery life cycle is divided into four main stages: battery formation, battery testing, application and recycling. Shortcomings at any stage may hinder the development of the battery industry and the popularization of electrification. Existing power conversion technology is not conducive to the development of the battery life cycle and has become an obstacle to the advancement of the market. Vicor's high power density fixed ratio converter technology brings an innovative approach to improve sustainability and cost-effectiveness at all stages of the battery life cycle.

In high voltage battery systems, DC-DC power conversion is a fundamental aspect of the power delivery architecture.

DC-DC conversion is usually achieved through switch-mode power converters (such as buck or boost topologies) or low dropout regulators (LDOs). These power converters may be effective, but their output rigidity and low conversion efficiency limit the flexibility and performance of the power delivery network (PDN). The high voltage of current battery systems highlights this shortcoming.

To overcome these shortcomings, Vicor developed fixed ratio converters that provide highly efficient isolated conversion in a small package for loads converting from high voltage to low voltage (often referred to as safety extra low voltage). Similar to a transformer in an AC-AC solution, a fixed ratio converter is used to perform DC-DC conversion, with an output voltage that is a fixed fraction of the input voltage (see Figure 1).

Figure 1: The bidirectional fixed-ratio converter can operate as either a buck converter (K ​​= 1/12) or a boost converter (K ​​= 12/1). This bidirectional conversion capability in a single module creates unprecedented application scenarios for the battery industry.

Just as a transformer's ability to step down or step up voltage is defined by the turns ratio of its windings, a fixed-ratio converter's ability to step down voltage is determined by its K factor, which is the ratio of its ability to step down voltage (see Figure 2).

Unlike traditional DC-DC converters, which regulate the output voltage, fixed-ratio converters do not provide output regulation. These devices can also operate autonomously, requiring no loop feedback or external control mechanisms.

Fixed-ratio converters have several distinct advantages over traditional converters.

Figure 2: Vicor's BCM fixed-ratio converters support a variety of different K-factor and output power configurations to meet the needs of most applications.

Bidirectional conversion

Fixed-ratio converters operate independently of an external host or controller, making them inherently bidirectional. That is, depending on the direction of current flow, only one converter block is needed to both step up and step down the voltage. As a result, fixed-ratio converters can bring unprecedented flexibility and simplicity to power delivery networks (PDNs) that rely on bidirectional current flow.

Flexibility and scalability

Fixed ratio converters are easily paralleled to meet higher power requirements. Designers can easily add multiple fixed ratio converter modules and connect them in parallel to scale the system to meet the required output power requirements. Similarly, designers can connect multiple fixed ratio converters in series to achieve unique voltage conversion ratios based on their cascade K factors. In these cases, the converters need to be power matched to ensure safe and reliable operation.

Finally, fixed ratio converters achieve excellent power conversion efficiency in a small package. Conventional buck or boost converters have a maximum conversion efficiency of only about 90%, while the Vicor BCM® family of fixed ratio converters has a conversion efficiency of nearly 98%. This not only improves the sustainability of the application, but also significantly reduces thermal management overhead.

Battery Formation

The first stage of the battery life cycle is battery formation. During this stage, newly produced batteries must go through the formation cycle process, which is the first time the battery cells are charged and discharged. During this process, the battery is cycled many times and the solid electrolyte interface (SEI) layer is gradually formed. The speed of this process depends on the chemical properties of the battery cell, so the time taken for battery formation is mostly fixed.

Battery formation cycling requires an underlying power delivery network (PDN) that can support repeated charge and discharge cycles.

In this case, a typical PDN takes three-phase AC input from the grid, rectifies it to high-voltage DC, and then uses multiple stages of DC-DC conversion to reach the nominal voltage required to charge the battery cells (e.g., 4.2V) (see Figure 3). The final voltage required to charge the battery varies from battery to battery, depending on the battery cell chemistry, but it is standard practice across the industry to use several intermediate voltage drops when converting AC to a lower DC bus voltage (e.g., 12V).

Discrete component solutions are difficult to design, require extensive in-house power expertise, and have large bill of materials requirements. This increases costs, creates supply chain challenges, and slows time to market. Discrete solutions limit flexibility because they are limited to predefined output voltages. When different voltages are required due to different battery chemistries, it is more cost-effective if designers can create a flexible solution that can be modified based on the specific chemistry of the battery. When using discrete solutions, the battery formation system lacks flexibility and cannot be dynamically modified to accommodate multiple battery types.

During the constant current conversion stage, designers can use fixed ratio converters to easily step down higher DC voltages to safer, lower voltages without the need for discrete or single-module solutions. By connecting one or more fixed ratio converters in parallel, they can create modular power delivery networks that are easily scalable .

This allows designers to design systems that cycle multiple batteries simultaneously, achieving higher throughput, power density, and efficiency. In addition, this architecture allows designers to easily change the PDN to complete the required DC-DC conversion based on the unique nominal voltage of the battery. Without discrete components, the solution is more flexible, which can speed up time to market and reduce failure rates.

In terms of energy conservation, the inherent bidirectional conversion capability of fixed ratio converters is particularly effective during the battery formation process. Using fixed ratio converters, battery manufacturers can switch between charge and discharge cycles with ease, knowing that the fixed ratio converter will automatically step up to a predefined higher voltage when discharging and step down when charging. This unique feature improves the energy efficiency of the formation process and reuses energy during the formation cycle.

In addition, the fixed ratio converter has an efficiency of up to 97.9%, and the power loss during the bidirectional conversion process is negligible. Without a fixed ratio converter, multiple components (one for buck and one for boost) would be required to achieve this bidirectional conversion function. This would consume more power and increase the number of components due to inefficiency.

Figure 3: Battery manufacturers can use fixed-ratio converters to integrate bidirectional conversion capabilities into the battery-to-battery power network while improving efficiency.

Battery Test

The second phase of the battery life cycle is battery testing. During this phase, manufacturers combine battery cells into larger battery packs. While the charging and discharging of battery cells is subject to certain time requirements (depending on the chemistry), the production of battery packs is not subject to these constraints, but still faces the same throughput challenges.

For example, each battery cell must be properly tested and precisely measured to ensure that multiple cells can be combined into a larger battery pack. Then, the larger battery pack also needs to undergo rigorous testing. This is not a value-added step, so the faster manufacturers can complete this step, the lower the overall cost of the battery pack.

To accommodate a variety of battery voltages and power levels, the power supply network must be flexible and scalable. At the same time, in order to test more batteries in a shorter time in the same physical space, the power supply network needs to provide high throughput. Therefore, battery pack test equipment requires a modular and scalable power supply network to meet specific test requirements and test volumes. As in the battery formation stage, the standard power supply network of battery test equipment also needs to convert three-phase AC power to the nominal voltage of the battery cell (see Figure 4).

By using a fixed ratio converter in the constant current conversion stage of the power delivery network, battery test designers no longer need to laboriously design intermediate conversion stages. Instead, they can confidently utilize a fixed ratio converter to manage the constant current conversion. This allows them to focus on the final stage of the conversion process, which is matching the voltage to the nominal voltage of the battery cell for testing. This simplified architecture enables designers to create flexible modular systems and easily modify them for different test requirements.

Another important advantage of fixed ratio converters is power density. With extremely high power efficiency and small form factor, fixed ratio converters can support several kilowatts of power and hundreds of volts in an industry-leading small design. This allows the construction of higher throughput testers, installing more test equipment in the same space, creating the opportunity to test more cells simultaneously.

Figure 4: Fixed-ratio converters bring high power density to battery test power networks, increasing test throughput by fitting more test equipment in the same space.

Challenges in Real-World Battery-Powered Networks

When the batteries finally leave the factory and are put into practical use, the challenges for the power supply network are not over.

In many emerging battery-powered applications, such as tethered robots or underwater vehicles (ROVs), renewable energy storage systems such as solar and wind power, and electric vehicles, there is a growing demand for very high voltage power supply (see Figure 5). For example, the power supply architecture of electric vehicles is transitioning from 400V to 800V to increase power and improve efficiency.

At the same power, the higher the voltage, the lower the supply current. Therefore, one advantage of high voltage supply is higher efficiency, because the lower current can reduce I2R ^losses . This makes the application more efficient and also reduces thermal management overhead.

Additionally, high-voltage power delivery allows for a reduction in the wire gauge in the vehicle’s wiring harness. With less current being delivered, designers can use smaller diameter cables, which reduces system weight, material requirements, and cost.

Obviously, for this high voltage system to operate successfully, these high voltages for power delivery need to be converted to low voltages for use at the point of load. At this stage in the battery life cycle, fixed ratio converters add value by providing a simple and efficient means of DC power conversion.

以系留机器人为例，利用 K 因数为 1/16 的固定比率转换器，设计人员能以 97.9% 的效率，将电压从用于供电的高压（例如 800V _DC ）降至较低的电压，例如 48V _DC 。在 48V _DC 的基础上，设计人员可以使用效率为 90% 的常规降压转换器，获得微控制器单元（MCU）最终所需的 3.3V 电压。如果没有固定比率转换器，从 800V 到 3.3V 的整个转换过程将以 90% 的效率进行，产生的损耗将显著高于固定比率转换器架构。

图 5：系留机器人等应用可以使用固定比率转换器实现高压供电，而不会在转换为低压的过程中产生显著的功率损耗。

Challenges in battery recycling

The final stage in the battery life cycle is recycling.

Battery recycling is a high-power electrochemical process that chemically separates the raw materials and elements in the battery for future recovery and reuse. As with other industrial stages of the battery life cycle, the power supply network needs to convert the three-phase AC input voltage to high-power DC and finally step it down to a lower voltage to power the recycling equipment (see Figure 6).

From the perspective of the power supply network, the battery recycling process generates a lot of heat. Therefore, the components of the power supply network must be able to operate reliably at high temperatures. Similarly, power density is becoming more and more important in power supply network design, requiring small form factor design and efficient power conversion.

Fixed ratio converters provide an extremely power dense DC-DC conversion solution capable of supporting thousands of watts and hundreds of volts in a very small form factor.

Figure 6: BCM fixed ratio bus converter Enables reliable high power voltage conversion in the high temperature environment of a battery recycling plant.

For example, the Vicor BCM6123 fixed-ratio bus converter has a power density of up to 2352 W/in³ (see Figure 7). With this power density, designers can easily meet the temperature and performance requirements of the battery recycling plant. As power demands continue to grow, the modular power architecture using fixed-ratio converters allows the system to scale with minimal overhead.

Enhancing the Battery Ecosystem with Fixed-Ratio Converters

At every stage of the battery life cycle, the demand for efficient, high power density and scalable high voltage power delivery networks is growing. The success of the entire battery life cycle depends on the success of each stage. Whether it is battery formation, testing, actual application or recycling, the entire battery life cycle can benefit from fixed ratio converters. Compared with traditional power conversion solutions, fixed ratio voltage converters have unprecedented efficiency and compact size, while also having unique features such as bidirectional conversion.

Figure 7: The Vicor BCM6123 fixed-ratio bus converter module provides 24V output voltage and 62.5A output current in a 61.0 x 25.14 x 7.26mm ChiP™ package.

Vicor BCM® products Using a Sine Amplitude Converter (SAC™) topology, they can operate at higher frequencies than PWM-based solutions. The BCM family of fixed-ratio converters also offers a variety of form factors and power levels to meet the needs of a wide range of high-voltage applications. In addition to the BCM family, Vicor offers a wide range of fixed-ratio converters to meet the needs of many other applications.

BCM fixed ratio converters will play an important role in the current fast-growing battery manufacturing market. It supports higher throughput, higher efficiency, and can be scaled to meet the needs of any application. Regardless of the application or life cycle stage, fixed ratio power converters are the ideal solution for the booming contemporary battery industry, where traditional power conversion methods are currently unable to meet the needs.

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