Vicor’s DC high-density power conversion technology enables Dynamis’ transition to electric racing vehicles

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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Dynamis in action

 

Dynamis PRC is the Formula Student team of Politecnico di Milano. It comprises more than 100 students who design and manufacture a racing car prototype every year, for racing in the international Formula SAE championship. Formula SAE is the third prototype championship after Formula One and the FIA World Endurance Championship. It now supports three competition categories – combustion, electric and driverless.

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After 15 years in the combustion category, Dynamis PRC has reached fourth position, and the first in Italy, in the Formula Student Combu­stion world ranking. 2019 was their most successful year with excellent results in the Netherlands, Italy, Germany, and Hungary. In 2020, they moved to the electric category – but full competition, and the feedback it would have generated, was denied because of Covid. Nevertheless, Dynamis managed to gain third place in the static events and simulations that did take place during the pandemic.

Three events are planned for 2021 - Formula Student Netherlands, Formula ATA in Italy, and Formula Student East in Hungary.

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The move from combustion to electric vehicles

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While changing from combustion to electric created many new challenges, it also allowed many improvements over combustion. Firstly, weight was reduced, as the original engine, which weighed over 20Kg, was replaced by four 3.5Kg electric motors, one mounted on each wheel. Each motor is independently controlled to optimise both traction and braking performance, through a torque vectoring algorithm developed by the Politecnico. The power train delivers up to 80 kW, an improvement on previous results.

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Introducing the new electric drive also provided an opportunity to redesign many components, and add new ones. One outstanding example was the accumulator, which comprised 660 Sony VTC6 cells, placed in a 132s5p configuration to deliver up to around 550V, with a capacity of 7.3kWh. Weighing 45Kg, it provided 30 minutes’ driving autonomy.

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The Dynamis team designed a set of seven electronics printed circuit boards (PCBs) to monitor and manage the accumulator; these boards additionally controlled the vehicle’s high voltage and low voltage circuits, as well as a low voltage LiFePO4 battery also included in the electrical system. Originally, this battery supplied all the vehicle’s 12V low voltage power, but, even though it could be recharged, it was found to have very low autonomy. Accordingly, Dynamis decided that a better approach would be to augment the battery’s power by stepping down the voltage from the high voltage accumulator and extracting 12V, using Vicor converters, for the more demanding part of the low-voltage load.

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Therefore, the low voltage system now comprises two 12V circuits, each with their own dedicated power supply, for maximum reliability. One circuit, powered by the low voltage battery, supplies the on-board PCBs, sensors, and telemetry equipment. The other uses a DC-DC converter system fed by the high voltage accumulator. This was designed by Dynamis and based on two Vicor modules. It had to source 600W – 50A at 12V – to supply the cooling fans and cooling water pump. For simplicity, the two circuits were kept entirely independent, with no ability to switch between the power sources.

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Above all, the DC-DC converter system had to be reliable, as a fault would cause the cooling system to stop, leading to an uncontrolled rise in motor and inverter temperatures. However, weight and size were also key concerns, as the DC-DC converter could only be justified if it saved weight and space compared with simply increasing the low voltage battery capacity. The overriding concern was to keep the car’s total weight below 200kg, to maximise competitiveness.

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Additionally, the DC-DC system had to accommodate an input supply from the accumulator that ranged from 550V down to 330V as the batteries discharged during a race.

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Dynamis could not find any commercially available single converter which could satisfy these requirements while meeting the weight and size constraints. The input voltage swing created the greatest challenge of all.

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To overcome this difficulty, they divided the battery pack into two voltage sources in series; each source supported half of the voltage swing. Implementing a serial-input, parallel-output DC-DC converter stage this way considerably simplified the task. This solution also reduced the current load on each converter.

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This solution depended on the availability of a DC-DC converter module that could operate in array mode. The system also had to be galvanically isolated, and able to withstand at least 2.5 kV, as required by the Formula’s Student Rulebook.

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A module - based DC-DC conversion solution

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After an extended search for suitable DC-DC products, Dynamis settled on Vicor’s DCM4623 module, as it satisfied all their system requirements. While providing over 4kV input to output isolation, each module could deliver up to 600W power, for a total maximum of 1.2kW. The DCMs’ voltage input range of 160V to 420V after startup is the widest in their category – and their trimmer-controlled,  regulated output voltage, from 11.5V to 15.5V, allowed Dynamis to easily compensate output drift arising from temperature and load variations.

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The modules are optimized for array operation, and their Fault and Enable signals facilitate their integration into Dynamis’ battery management system (BMS).

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The DCMs, measuring 47.91 x 22.8 x 7.21mm and weighing 29.2g, critically  also help Dynamis to comply with their size and weight restraints, keeping the vehicle’s weight below 200kg to improve competitiveness.

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The DCM modules were well-documented and easy to use; nevertheless, Dynamis needed deep understanding of the products’ full capabilities to achieve the best possible design. For this reason, Vicor’s quality of support was as valuable to Dynamis as the DCM modules’ specifications:

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“As important as the product specification, we found in Vicor a partner that was willing to support our project by supplying us with their modules, and also assisting us in the design process” commented Marta Bragotto, Electronics and Powertrain - Logistics Manager, “They made their engineers available to us, along with excellent whitepapers and technical documentation.”

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However, it was very much a two-way conversation. It was valuable to Vicor to learn how their modules are enabling innovation, and contributing to the move from combustion to electric vehicle technology.

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Future plans

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Dynamis also believes that Vicor’s products will allow them to easily fulfil the next stage in their vehicle’s evolution. As a legacy from their combustion cars, the current vehicle uses 12V for all its low voltage components. However, when cables supplying high power cooling equipment are restricted to 12V, they become large and heavy due to the high current they must carry – and this is a significant drawback in electric vehicles.

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Accordingly, Dynamis is going to focus next year on developing a dual low voltage system, with 48V for the cooling equipment, and 12V for all the onboard electronics. They intend to do this by using Vicor modules to convert the accumulator’s high voltage down to 48V – and then further modules to convert from 48V to 12V. This will allow them to achieve a lighter system with reduced cable losses.

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In fact, this evolution may be even more far-reaching. Dynamis is highly satisfied with the development and test results obtained to date. This is encouraging them to consider the idea of completely replacing the low voltage battery with an array of 48V and 12V Vicor DC-DC converters, powered by the accumulator, and capable of completely meeting the vehicle’s 48V/12V power requirement.