Regenerative power efficiency is becoming a real differentiator
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Electric vehicles will eventually become the preferred mode of transportation. From the outside, electric vehicles look the same as the cars on the road today. What is different is the motor inside and how energy is exchanged, which involves many trade-offs. This blog explores the design considerations for electric vehicles while maintaining efficiency, overall performance, and driving range.
This blog post was originally published by United Silicon Carbide (UnitedSiC), which joined the Qorvo family in November 2021. The addition of UnitedSiC, a leading manufacturer of silicon carbide (SiC) power semiconductors, enables Qorvo to expand into fast-growing markets such as electric vehicles (EVs), industrial power supplies, circuit protection, renewable energy, and data center power.
The momentum behind electric vehicles has reached an inflection point where it’s hard to imagine a future without many electric cars 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 a world before Henry Ford. There were few places to refuel, so early car owners would often strap their gas cans to the outside of their cars. Range anxiety was also common. Yet few people thought about how long it took to refuel their internal combustion engine cars. After all, it was faster than feeding and watering a horse. In fact, this may have been a major attraction of car ownership; we didn’t have to think about it too much. Mechanical parts replaced the horsemen, and the actual cost of ownership would eventually become apparent, but the wheels were already turning.
At this point, turning the wheels is more than just a metaphor; it’s what cars are all about. Electric cars mean that the wheels are driven by an electric motor rather than a piston engine, but the goal is the same. However, there is a significant difference in the way the energy is exchanged. In an internal combustion engine, chemical energy (fuel) is converted into kinetic energy (motion), which is then converted into the entropy state of all energy, namely heat, to move the vehicle.
For electric vehicles, there is another phase in the process, which is to capture unused kinetic energy. This process is called regenerative braking, but what it really means is that the vehicle's power is used to turn the motor, rather than the motor powering the vehicle. The motor then becomes a generator, and the electricity produced is fed back into the battery. This can increase the range of an electric vehicle, depending largely on the efficiency of the regeneration phase.
The optimized motor/generator is very efficient in both motor and engine mode. Inversion is another critical stage. The inverter circuit is responsible for converting the high voltage output of the battery into alternating current (AC) to drive the motor. The amplitude and frequency of the AC waveform determine the speed. Typically, the traction motor is a three-phase motor, so the inverter needs to convert the DC battery voltage into three AC cycles. For example, 800 V DC is converted to about 180 kW AC, so the efficiency of this stage is crucial to the overall performance and driving range that automakers can provide.
Unsurprisingly, this is where the design effort is focused. Maximizing inverter efficiency requires using components with the lowest losses. So far, IGBTs have held the advantage in terms of conduction losses, but their turn-off switching losses are more significant. This is a good compromise because typical motor drive switching frequencies are relatively low, and IGBTs are also low cost. Silicon carbide (SiC) FETs have steadily replaced IGBTs in this application area due to their lower switching and conduction losses. There are two main reasons for this. First, as mentioned earlier, IGBTs are slow to turn off because they collect charge from the bipolar current. On the other hand, since only electrons flow, SiC FETs switch on and off quickly, so their switching losses are also low. More importantly, there is usually a PN junction in the current path of the IGBT, which can come from the IGBT itself or its anti-parallel diode, formed during forward or reverse conduction, respectively. Because SiC material has lower resistance and eliminates the 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 EVs most often operate. SiC FETs do not require an anti-parallel diode, so there is no “knee” voltage (after the switching dead time) for forward or reverse current.
The operating mode is related to the power factor (PF). If the PF is positive, the circuit is in inverter mode and draws energy from the battery to drive the motor. If the PF is negative, the circuit is in rectifier mode and feeds energy back to the battery. Ideally, the PF should be as close to +1 or -1 as possible to maximize efficiency.
Changing the PF highlights the losses of the FETs used. The key indicators of loss are forward and reverse conduction losses, as well as turn-on and turn-off switching losses. These four terms are added together to give the total losses of each FET. In inverter mode or rectification mode, most of the conduction losses occur in the forward or reverse current, respectively. Note that the forward current is the current flowing from the drain to the source (from the collector to the emitter for IGBTs). IGBTs used for motor drives only conduct in the forward direction, so an anti-parallel diode is required to conduct the reverse current. Therefore, the conduction losses and the heating of the IGBT and the diode will vary depending on the direction of the current. On the other hand, SiC FETs conduct forward and reverse currents through the same chip with the same conduction losses (after dead time), so the chip utilization is higher and the power density is higher.
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