Designing a Light Electric Vehicle

The LEV market is highly fragmented and growing rapidly

The term LEV describes a range of two, three and four-wheeled vehicles which carry either passengers or cargo. This definition covers a range of vehicles, including:

• Electric bicycle (e-bike) and electric kick-scooter
• Electric powered 2/3 wheelers - e-moped, e-scooter, e-rickshaw, e-motorcycle
• MicroEV – electric microcars, low-speed electric vehicles (LSEV), neighbourhood EV (NEV), electric quadricycle
• Other electric transporters (e.g., e-forklift, e-golf cart, e-ATV) with less than 200V batteries

LEVs tend to be driven over short distances at low speeds, most often in densely populated urban environments, and their simple configuration makes them extremely user-friendly. Their primary user benefits include low operating costs, ease of use, simple charging and, most importantly, zero emission levels. As LEVs continue to evolve, they are integrating more automated vehicle technology, including sensors and ADAS functionality.

Figure 1 – A graph showing the distribution of LEVs against their energy demands. Source: onsemi

Global e-bike Market Data Source: Mordor Intelligence

There are multiple ways in which LEVs can be classified, including by power levels and battery voltage (Figure 1).

As can be expected with such a new and emerging technology, the market is currently highly fragmented with as yet, no established major players. The earliest market to emerge and currently the fastest growing is in the Asia-Pacific region, with Europe close behind at 20% of the global market share.

According to a recent study by Markets and Markets, a global revenue impact and advisory organisation, the global LEV market is projected to grow from US$78.5 Bn in 2022 to US$122.7 Bn by 2027, equating to a CAGR of 9.4%^[1]. Within this overall LEV market, Mordor Intelligence, a leading market intelligence and advisory firm, forecast that global e-bike sales will grow from US$28.87 billion in 2023 to USD 52.59 billion by 2028, a CAGR of 12.74%^[2].

These figures correlate with projected sales of Brushless Direct Current Motors, (BLDC), the primary means of powering LEVs, which, according to Grand View Research, a leading market research and consulting company, are projected to grow at a CAGR of 6.5% between 2023 and 2030^[3]. Furthermore, this study estimates that sub-3kW BLDCs – used by e-bikes and e-scooters – account for the predominant share of this market, fig 2.

Again, as to be expected in such a new and fast-growing market, regulations are emerging on a regional basis with a global regulatory framework yet to emerge. With e-bikes, for example, in the USA, a straightforward approach defines 3 classes, based on propulsion control (pedal or throttle) and maximum speed – 20 mph or 28 mph. In Europe, LEVs are classified as ‘L-Category vehicles’, and subdivided into seven groups, each defined by power output, number of wheels, seating layout and weight.

Figure 3 – The BLDC market share is dominated by sub 3kW applications Data Source: Grand View Research

Figure 4 – The key electronic subsystems within the e-bike Source: onsemi

LEV can be classified in multiple ways

As has already been seen, LEVs can be classified according to power levels, battery voltage, speed, propulsion control method, and many more. LEV designers must thus grapple with a wide spectrum of options when selecting components for the key electronic subsystems within an LEV. A later section will look at this challenge in more detail, but first it is helpful to review what these key subsystems are, with reference to the e-bike, figure 4.

The power train or motor drive system of an e-bike, and any LEV is based upon a BLDC, controlled by Variable Speed Drive (VSD), system. The power of the bike and its range are dependent upon the voltage and capacity of the battery pack, table 1, and most e-bikes today use either 36V or 48V batteries. A typical 10-cell, 36V battery can deliver 14 Ah at 504Wh, enough to power a pedal-controlled e-bike, (Pedelec), for anywhere between 60km and 150km.

The Human-Machine Interface is a key subsystem of any LEV, and these systems are becoming more powerful and complex as automated vehicle technology trickles down. As a minimum, the HMI allows the rider to control the speed of the e-bike while incorporating additional functionality such as battery monitoring and management, temperature sensing, USB ports, and increasingly, a wireless communications interface. Wireless communications are becoming indispensable within LEVs for a range of use cases, including mobile phone connectivity, tracking and payment for rental e-bikes and scooters.

Finally, battery charging is straightforward for an e-bike or scooter, as they use electric mains, but higher-level LEVs may incorporate onboard charging subsystems. Having outlined the key subsystems within an LEV, we will now consider the choices and decisions facing the LEV designer.

Table 1 – Battery voltage determines e-bike performance Source: onsemi

Figure 5: A Typical VSD Source: onsemi

Powertrain solutions

The design of the motor drive system heavily influences the overall performance and characteristics of the LEV, impacting speed, torque, and weight. Figure 5 shows an example of a variable speed drive, (VSD), for a 3-phase BLDC, based on an inverter with three half-bridge units which control the torque and speed of the motor.

The voltage and power requirements of the application largely dictate the choice of MOSFET or IGBT as switching device within the VSD and, since we are focusing on the sub-3 kW LEV classification, we will assume the use of MOSFETs for both the switching devices and the gate drivers. The challenge then is to optimise the MOSFET selection, recognising that there is no single “best” choice since any selection involves numerous trade-offs – with cost an obvious one!

Beyond the top-level parameters of current-handling and peak-voltage ratings, which must match the motor’s load requirements, the designer must consider factors such as RDS(on) and gate capacitance. Figure 6 illustrates the trade-offs involved when choosing an RDS(on) value with lower-on resistances; a reduction in conduction losses and an increase in drive efficiency, but the potential for increased costs, and the associated higher gate charges reduce switching times.

Total gate charge, QG, is another key parameter to be considered when choosing a MOSFET. QG determines how much gate current is required to turn the MOSFET on and off, and this value, in conjunction with the required switching rise and fall times, determines how much gate current is required.

Once the minimum required gate drive current has been determined, a suitable gate driver, capable of selecting the amount of current needed to achieve the desired rise and fall times, must be selected. There is a wide variety of available devices with different gate drive current capabilities, wide supply voltage ranges, and additional integrated features, including current-sense amplifiers and protection circuits. onsemi’s NCD83591, for example, is an easy-to-use, multipurpose 3−phase gate driver optimised for low BOM cost. The wide operating voltage range makes this device ideal for industrial and commercial applications from 5V (min) up to 60V (max) and the device features include Gate Sensing for Cross Conduction Protection and Optimised Dead Time and individual Six Gate Control Mode.

Figure 6 – Optimising RDS(on) involves multiple tradeoffs Source: onsemi

The designer has the additional choice of using discrete components or modules, with discretes generally more suited to low and medium voltages and modules for higher voltages. onsemi’s APM17 48V drive, figure 7, is a reference design based on the APM17M-NX module which showcases the design possibilities at 48 V.

HMI and wireless communications solutions

Figure 8 shows a high-level schematic of a typical HMI and control system for an e-bike, showing how the various functions described above can be implemented using devices from the onsemi portfolio.

As discussed above, wireless communications are of increasing importance within the LEV and onsemi devices are available to support BLE, Sub-GHz and NFC/RFID protocols. The RSL15 and RSL10 are both Arm® Cortex®−M33 processor-based BLE 5.2 wireless MCUs designed for connected smart devices and offer rich feature sets. Ultra-low power narrowband sub-GHz communications are supported by the AXMOF343 system-on-chip while NFC/RFID communications can be enabled using the N24RF64 RFID/NFC dual interface tag.

Figure 7: onsemi’s APM17 drive Source: onsemi

Figure 8: E-Bike HMI subsystem Source: onsemi

Throttle control is obviously a key function of the HMI and effective throttle control interfaces can be implemented using the NCS2003 operational amplifier and NCD98010 analogue to digital converter devices.

The devices within the HMI subsystem are powered by the vehicle’s battery and voltage regulation and protection are important functions which are provided by devices such as the NCP330 soft load switch, the FAN48632 regulator and the FAN53600 step-down switching voltage regulator.

Finally, no HMI system would be complete without a battery status monitor – or fuel gauge – and onsemi’s LC709204F module provides a highly accurate, ultra-low power solution.

Battery management

LEV battery technologies vary widely across market regions, with Lead Acid popular for e-bikes in China and Asia, due to its low cost and ease of recycling. In Europe however, Li-Ion and Li-Po (polymer) are the most common technologies, offering higher efficiencies and longer life cycles, while requiring more sophisticated battery management systems (BMS).