Key to Mild Hybrid Electric Vehicles: Belt/Integrated Starter Generator

For more than a century, vehicles have been powered by internal combustion engines (ICE). But you’ve no doubt noticed that things are changing rapidly with the introduction of electric vehicles (EVs). The entire automotive industry and mainstream news media are discussing this topic.

When we think of EVs, many of us think of pure battery electric vehicles (BEVs). However, the EV market segment includes mild hybrid electric vehicles (MHEVs), hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and BEV variants. MHEV architectures offer a low-impact path for automakers to modify existing vehicle platforms and reduce CO2 emissions, while still enabling MHEVs to travel long distances regardless of whether charging stations are available.

The belt starter generator (BSG) or integrated starter generator (ISG) is part of this rapid electric vehicle development, dedicated to MHEVs. This device effectively combines the functions of the starter motor and alternator in ICE cars and supports the creation of MHEVs.

MHEVs typically have two batteries: a “conventional” 12 V lead-acid battery and a 48 V lithium-ion (Li-Ion) battery. The 12 V battery powers many of the “conventional” systems, while the 48 V battery powers higher loads such as starting the vehicle or providing electrical boost/drive for the BSG/ISG.

Implementing a BSG/ISG on an ICE vehicle enables a host of additional features, including start-stop, energy recuperation during coasting/braking, ICE power generation, and even electric drive (or boost), depending on the vehicle situation. Some of these features may be so subtle that drivers won’t notice their MHEV is different from a conventional ICE vehicle unless the internal combustion engine is switched off during use.

Functionality and performance depend on the location of the BSG/ISG in the powertrain, as shown in Figure 1. This location also determines whether the device is a BSG (P0 or possibly P2) or an ISG (P1, possibly P2, P3 & P4).

While an MHEV is not a zero-emission vehicle (ZEV) like a BEV, it does reduce CO2 emissions compared to an ICE vehicle. This is particularly noticeable in slow-moving traffic in large cities, where air pollution is often the worst and has the greatest impact on human health.

If installed in the P0 or P1 position, functionality is limited to start-stop and energy recovery. While the P0 and P1 positions make it easier to integrate the unit, the benefit to emissions is minimal, as there is no energy recovery if the ICE is not spinning. In this configuration, the ICE shut-off algorithm is less aggressive, which reduces CO2 savings.

When positioned further back in the driveline (P2-P4), energy recovery is possible during coasting or braking if the ISG operates as a generator. The movement of the vehicle will turn the rear axle or propshaft even if the ICE is switched off. As the ISG is now independent of the ICE, more aggressive switch-off algorithms are possible, resulting in even greater CO2 savings.

Furthermore, in these positions, electric drive is possible, which means that the vehicle can be moved by the ISG while operating as a motor. This is useful in two situations: starting in stop-and-go traffic, or starting the journey from a standstill and then starting the ICE to gain speed.

BSG/ISG power outputs typically range from 5 kW to 25 kW or more, but are usually targeted at the lower end of the market due to the potential for slippage in belt drive schemes when combined with high levels of torque.

Currently, MHEVs account for one-third of all electric vehicle sales. This proportion is expected to remain stable until at least 2026, while MHEV sales will continue to grow at a compound annual growth rate (CAGR) of nearly 20%.

High-performance BSG/ISG are essential for MHEVs to be able to achieve their CO2 reduction targets. The constant electrical load is high and there are frequent energy peaks to cope with. BSG/ISG are often installed where they must withstand high temperatures as well as dust and moisture in the environment. Given all this, it is a challenge to design a unit that is small, light, yet powerful, efficient and reliable over the long term.

ON Semiconductor has a variety of advanced semiconductor technologies suitable for BSG/ISG design. Its high-power 80 V and 100 V MOSFETs can be used as discrete components or integrated into automotive power modules (APM17M) to provide enhanced performance and simplified design. All its devices for automotive applications meet the AECQ-101 standard, AQG-324 standard (module) and production part approval process (PPAP) standard.