Regenerative power efficiency is becoming a real differentiator

The momentum for electric vehicles has reached an inflection point, and it’s hard to imagine a future without many electric vehicles on the road. This has dramatically changed not only our buying preferences and driving habits, but also the way we think about cars.

Let's imagine the world before Henry Ford. There were very few places to refuel, so early owners often strapped gas cans to the outside of their cars. Range anxiety is also common. However, few people consider how long it takes to refuel an internal combustion engine car. After all, it’s faster than feeding and watering the horse. In fact, this is probably one of the main attractions of owning a car; because we don't have to think about it too much. Mechanical components replace the groom, and the actual cost of ownership will eventually become apparent, but the wheels are already turning.

At this point, turning the wheel isn't just a metaphor, it's what the car is about. Electric cars mean the wheels are driven by electric motors rather than piston engines, but the goal is the same. However, there are significant differences in the way energy is exchanged. In an internal combustion engine, chemical energy (fuel) is converted into kinetic energy (motion), which is then converted into the entropic state of all energy, heat, to enable movement of the vehicle.

For electric cars, there's another stage in the process, which is capturing unused kinetic energy. This process is called regenerative braking, but what it really means is using the vehicle's power to turn the electric motor, rather than having the electric motor power the vehicle. This turns the motor into a generator, and the electricity generated is fed back into the battery. This increases the driving range of electric vehicles, with the exact increase largely depending on the efficiency of the regeneration phase.

The optimized motor/generator is highly efficient in both motor and engine modes. Inversion is another critical stage. The inverter circuit is responsible for converting the high voltage output from the battery into alternating current (AC) to drive the motor. The amplitude and frequency of the AC waveform determine the rotational speed. Typically, traction motors are three-phase motors, so an inverter is required to convert the DC battery voltage into three AC cycles. For example, 800 V DC is converted into approximately 180 kW AC, so efficiency at this stage is critical to the overall performance and driving range provided by the car manufacturer.

Unsurprisingly, this is where the design efforts focus. Maximizing the efficiency of your inverter requires using components with the lowest losses. So far, IGBTs have the advantage in terms of conduction losses, but their turn-off switching losses are more significant. Because typical motor drive switching frequencies are relatively low, this is a good compromise and the cost of the IGBTs is also lower. Silicon carbide (SiC ) FETs have steadily replaced IGBTs in this application due to their lower switching losses and conduction losses . There are two main reasons for this. First, as mentioned earlier, the IGBT turns off slowly due to the charge collection from the bipolar current. SiC FETs, on the other hand, switch on and off faster because only electrons flow, so their switching losses are lower. More importantly, there is usually a PN junction in the current path of the IGBT, which may come from the IGBT itself or its anti-parallel diode, formed during forward or reverse conduction respectively. Because the SiC material has lower resistance and eliminates PN junction voltage drop, SiC FETs not only have lower conduction losses at all current levels, but also have significant advantages at the low powers where electric vehicles most often operate. SiC FETs do not require anti-parallel diodes, so there is no "knee point" voltage (after switching dead time) for forward or reverse current flow.

The operating mode is related to the power factor (PF). If PF is positive, the circuit is in inverter mode and needs energy from the battery to drive the motor. If PF is negative, the circuit is in rectifier mode, feeding energy back to the battery. Ideally, the PF should be as close to +1 or -1 as possible to maximize efficiency.

Changing PF will highlight the losses in the FET used. The key indicators of losses are forward and reverse conduction losses, and turn-on and turn-off switching losses. Adding these four terms gives the total loss in each FET. In inverter mode or rectifier mode, most conduction losses arise from forward or reverse current flow respectively. Note that forward current is the current that flows from drain to source (in the case of IGBT, from collector to emitter). IGBTs used for motor drives only conduct forward current, so an anti-parallel diode is required to conduct reverse current. Therefore, the conduction losses and the heating conditions of IGBTs and diodes will vary depending on the direction of the current. SiC FETs, on the other hand, conduct forward and reverse current through the same chip with the same conduction losses (after dead time), so the chip utilization is higher and the power density is higher.