Posted on 10 June 2014

eScooter is Gaining Speed

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The life of Swiss postmen in some rural areas may just become a bit easier: In a pilot project they will be equipped with eScooters. Complex power electronics is required to drive the electrically powered two-wheelers and intelligent thermal management ensures reliable operation. This is where HSMtec enters the picture.

Author: Johann Hackl, PM HSMtec, Häusermann, Gars am Kamp, Austria

The main requirement of electrically powered bicycles for postmen is to transport them versatile, speedy and effortless to their destination in the countryside – environmentally friendly and resource conserving. The eScooter’s basis is the motor control unit from Rising-edge GmbH (Image 1), and a three-phase permanent magnet synchronous motor with up to 15 kW gives a comfortable driving experience as well as maneuverability. The motor controller provides an output current of up to 270 A with 160 A continuously. The computing power necessary for calculating the field-oriented vector control (FOC) is provided by a FPGA. A three-phase full bridge module with six MOSFETs acts as output stage. Since this is a THT module, the motor currents have to flow through the PCB.

HSMtec makes it possible to integrate the full motor control unit on a single board

Routing high currents of 100 A or more through the PCB requires efficient thermal management, a technological challenge for board layout engineers. Power semiconductors for high current and high voltage applications become increasingly complex and create high thermal losses. The excess heat needs to be conveyed away quickly. However, high currents need broad copper paths which take much space. And space is at a premium on miniaturized boards.

This ongoing miniaturization of control circuits also makes it necessary to integrate sensitive SMT components on the same board as the high current components. This poses various challenges for design engineers. On the one hand, the wire cross sections have to be as large as possible in order to avoid overheating by high currents. At the same time, minimum distances to the sensitive wires of the control circuit are required.

The most common solution places control and power electronics on separate boards using a connector. Placing both functional units on one board would save money and space but this doesn’t work with conventional PCB technologies. Using thick copper technology deals with the high currents but not with the fine structures of the control circuits. So we need a PCB technology that allows realizing broad wires for heat transport as well as very fine structures.

Intelligent thermal management with massive copper

HSMtec is a suitable solution for this problem. The PCB technology conforms to DIN EN 60068-2-14 and JEDEC A 101-A and has been audited for aviation and automotive. It works selectively: massive copper is integrated into the board only where high currents actually have to fl ow through it, as profile or wire. At this point in time, profiles with 500 μm height and widths between 2,0 mm and 12 mm in variable length are available, as well as wires with a diameter of 500 μm (Image 2).

The visible copper wires and profiles provide efficient thermal management

Those copper elements are bonded substance-to-substance to the etched conductive patterns using ultrasonic welding. This works in any layer of a FR4-based multilayer. HSMtec allows reducing the heat resulting from high currents quickly to acceptable partial and system temperature. The integrated copper elements can cope with currents up to 400 A.

A look at the specific thermal conductivity shows the importance of a continuous metallic path from heat source to heat sink and demonstrates the potential of HSMtec. The thermal conductivity of copper is a factor 1000 higher than FR4. A thermally optimized layer structure allows for rapid heat spreading and supports the full thermal concept. A real world example: An area of 10 mm x 10 mm can be perforated with more than 400 drill holes with a diameter of 0,25 mm each. The area then consists of 10% copper and its effective thermal conductivity rises to 30 W/m•K. This means that this assembly conducts heat a hundred times better than FR4 and still ten times better than the best heat conducting substrates (Figure 3).

Specific thermal conductivity of different materials

HSMtec for reliable eScooters HSMtec makes it possible to combine large diameters for heat transfer with very fine structures (Figure 4).

Bending position: Constructing self-supporting multidimensional PCBs with the integrated wires or profiles. Heat and high voltages can be routed through dedicated paths

This solves the problem and provides optimum heat transfer from the power component to the heat sink. So, HSMtec is very suitable for use in the control unit of a DC motor with an IGBT or for electromobility. In this application area, weight and size are of importance, but also the ability to cope with high conductivity in order to dissipate heat rapidly (Figure 5).

Significant space savings by coexistence of control units and LED on one board

Traditionally, the eScooters motor control required special assemblies in order to route large currents on the PCB. This has changed with HSMtec. This technology makes it possible to integrate the full motor control module on 228 mm x 75 mm on a single board (Figure 6).

The unpopulated motor control: The first step is the assembly of the output stage, an integrated three-phase full-bridge module with six MOSFETs

The massive copper elements in the four-layer board conduct current while dissipating heat rapidly. Since there are neither screwed connections nor cables necessary between module and PCB, this leads to higher reliability. The motor controller doesn’t just regulate the engine but also the recuperation of braking energy. This requires trading-off the mechanical brake power with the rate of recuperation in order to gain as much energy as possible while still executing the right amount of braking force.

At the moment, HSMtec is used in prototypes of the eScooter’s power-train electronics. Its Small size allows space-saving installation directly on the motor wing. The enormous amount of computing power available through the FPGA may turn the motor controller into a central vehicle computer in the future. It could then provide additional functions like diagnosis and data logging, fleet management using GPS and GSM or display and instrument control. The HSMtec board plays a vital part in the development of power and cost efficient DC motor control in restricted space. That means it contributes significantly to future mobility and environment protection.


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