A modular design approach for cost-optimised low-voltage inverters

Micro-mobility applications, like electric bikes, scooters, skateboards, and three-wheel auto rickshaws, generally contain low-voltage inverters. Typically, they require a modest power level — between 200 W and 2 kW.

Other LV inverter systems applications include cordless power tools, hand-held garden tools, lawnmowers and other Applied solutions A modular design approach for cost-optimised low-voltage inverters By G. Valente, and A. Johnston domestic appliances, and automotive auxiliaries. An inverter’s requirements clearly depend on the specific application; however, cost reduction is often the priority for low-voltage, low-power inverters.

This influences other requirements and the overall design solution. To meet demanding cost constraints, inverters based on low voltage systems often adopt Brushless DC (BLDC) motors controlled with a six-step commutation method (sometimes referred to as trapezoidal control) — which we’ll explore herein.

Among the different low-voltage inverter applications explored, the micro-mobility sector was selected as a cardinal case study for a modular inverter design, as the application demands increased reliability, safety, and performance compared to other typical applications like domestic appliances. Furthermore, by providing a convenient and affordable alternative to driving, electric micro-mobility can help decrease the number of cars on the road, reducing both traffic and pollution.

Another advantage of electric micro-mobility is its general accessibility; these vehicles are small and lightweight, inherently more affordable, and easy to use and store. They can easily be used with public transportation and stored in a small apartment or office, providing a compelling commute option that is highly efficient and more sustainable than alternative options. Micro-mobility is an attractive option for people who do not have access to a car or who live in areas with limited parking. Unlike hybrid or battery-electric cars, which require costly charging infrastructure costs, electric micro-mobility vehicles can be charged using a standard electrical outlet on the existing infrastructure. They cost far less overall simply owing to their physical size and material content. The end user can use them for a fraction of the cost of driving any car.

Inverter design for micro-mobility

We adopted a model-based systems-engineering approach to improve overall performance and reduce the need for custom, application-specific solutions. This approach embodied conventional Systems Engineering methods and processes and advanced model-based design and analysis to better control and optimise the design solution. The objective was to reach an object-oriented design solution that could theoretically be reused in many different applications, as previously described.

You can see a summary of the Systems Engineering methods adopted to complement the standard electronics design workflow in Figure 1.

Figure 1. Summary of Systems Engineering methods adopted to complement the electronics design workflow.

The scalability required was formalised through the requirements capture exercise, which inherently provided strong design cues at both the architecture level and the lower-level technology and component choice. Validation of requirements was supported by several iterations, considering the functionality required and the technology available to deliver the functionality at the desired performance, as well as exploring conceptual system solutions to define the system’s boundary better.

Design modularity was achieved by dividing the inverter into different functional blocks, directly influenced by the Functional Modelling and N2 analysis results, as depicted in Figure 2, where the system boundary interfaces and the main functional block interconnections are shown.

The cardinal inverter system requirements are presented in Table 1. In addition, the inverter was designed assuming a 3-phase motor winding and a passively cooled power stage, which is common in micro-mobility applications, where the heat to be dissipated is relatively low, and the system cost and complexity need to be reduced as much as possible.

Figure 2. Inverter System Boundary diagram (simplified) with functional interfaces and main functional blocks defined.

The requirements for the individual functional blocks were then derived and decomposed from the system-level requirements. The high-level functional block diagram, as presented in Figure 2, may be employed for any low-voltage and low-power inverter, provided that the functions and requirements of the individual blocks are carefully derived, taking into account the application.

Code development and design optimisation through detailed analysis

 Different modelling and analysis simulations were carried out on the inverter design to help validate requirements, thoroughly investigate the intended solution’s performance and key components’ performance, and verify that the proposed solution fully satisfies the requirements before any manufacturing commences.

Several simulation runs have been carried out using the extensive software suite provided by Hexagon MI, the details of which will be described in a series of additional publications. Table 2 provides a synopsis of the performed analysis and the main motivations for conducting them.

Simulation analysis tool and description	Simulation tool snapshot
Elements: Modelica-based system-level 1D BLDC motor control modelling where the six-step commutation and trapezoidal motor control algorithm are implemented and model-in-loop (MIL) verification tested.
PICLS: Rapid 2D thermal analysis where different component layouts can be quickly assessed to achieve an optimum heat distribution on the PCB.
Cradle:   High-fidelity 3D CFD analysis where the temperature distribution on the designed PCB can be accurately predicted for the adopted cooling method (i.e., passive cooling in this case study).
Cradle/scSTREAM and Nastran: Combined thermo-structural analysis where the thermal simulation undertaken with scSTREAM drives the structural simulation in Nastran to evaluate the solder joint durability when subjected to power cycles.
CAE Fatigue: Long-term fatigue failure and durability analysis where frequency and time-based loading have been applied to evaluate the expected reliability and lifetime of the inverter product and to predict (and validate) the potential root cause failure mode.
Marc: Marc: Non-linear and multi-physics simulations where the coupling between Joule-losses caused by electrical current, thermal heat power dissipation, and mechanical stresses induced by mismatched coefficients of thermal expansion between different materials can be evaluated and used to better understand and optimise the inverter reliability and improve lifetime.

Table 2. Summary of Model-Based Design and Analysis Conducted.

Electronic Design Automation (EDA)

EDA software carried out the schematic capture and the final PCB layout. Figure 3 shows a high-level summary of the schematic capture of the proposed inverter design, aligned to the aforementioned logical architecture, whereby the schematics for the functional blocks have been organised into specific subsystems managed by the EDA software.

Figure 3. Summary of EDA showing Schematic Capture of Inverter Architectural elements.

Figure 4 shows the final PCB layout, whereby the copper-filled areas are hidden to better show the components and key routing, supported by the 3D view of the final and populated PCB assembly, which was used for the aforementioned multiphysics simulations.

Figure 4. Final Inverter design: (a) PCB layout and (b) 3D view of the populated PCB assembly.

Inverter rapid prototyping and testing

Hardware-in-Loop (HIL) testing was conducted on a rapid prototype to verify the proposed design against its requirements and to validate the model developed — following which an inherent Digital Twin of the inverter solution was formulated.

The prototypic PCB assembly was carried out within Hexagon’s Applied Solutions Group facilities. Figure 5 shows a snapshot of the local PCB assembly, which employed a mix of hand-worked, pick-and-place, and automated oven soldering and inspection techniques.

The motor control algorithm was developed via model-based design and analysis, using the Modelica-based 1D simulation tool Elements (see Table 2), and then converted into source code supported by Code Composer Studio (CCS), the Texas Instruments IDE and flashed onto the target micro-controller. The inverter was then tested locally in the Applied Solutions Group facilities using an off-the-shelf BLDC Motor as illustrated in Figure 5 (b) supported by a simple spin and load rig.

 

Figure 5. (a) Rapid Prototype PCB assembly process and (b) initial commissioning and calibration testing setup.

Summary and conclusions

The proposed design approach, underpinned by systems engineering and optimised through modelbased design, enabled a scalable, cost-optimised, low-voltage inverter suitable for the reliability and integrity requirements of the electric micro-mobility application. Thanks to the modular approach, the design can be easily adapted to meet the requirements of different applications by modifying the individual logical blocks that constitute the inverter but without the need to change the architecture or their interfaces, e.g. higher or lower power applications, or alternative applications such as an automotive shift actuator, commercial power tool or domestic appliance.

The manufactured inverter prototype met all the initial design requirements, and this was possible thanks to the extensive multi-physics simulation analysis that had been carried out to achieve a “right first-time” solution, where thermal performance together with shock, vibration, and durability analysis could be performed to ensure the inverter could achieve the reliability and lifetime targets. The detailed description of the simulation workflows implemented using the Hexagon MI software suite and the results will be presented in a series of additional publications in due course.