Efficient regenerative power is becoming a real differentiator

The electric vehicle momentum has reached a tipping point, and it is difficult to imagine a future in which electric vehicles cease to be a significant presence on the road. This will revolutionize not only our purchasing preferences and driving habits, but also the way we think about mobility.

Imagine a world before Henry Ford. With few places to refuel, early users carried fuel cans strapped to the outside of their cars. Concern about cruising range is nothing new. However, little consideration has been given to how long it takes to refuel a vehicle powered by an internal combustion engine. Faster than feeding and watering horses. That's the big appeal of owning a car, and the lack of care. Mechanics took the place of the stables and the true cost of ownership should have been revealed eventually, but the wheels were already turning.

"Turning the wheel" is not a metaphor, it is ultimately "turning the wheel." With the shift to EVs, the wheels are turned by an electric motor instead of a reciprocating engine, but the purpose is the same. The big difference, however, is in the way energy is exchanged. In an internal combustion engine, motion is obtained by converting chemical energy (fuel) into kinetic energy (motion), which in turn is converted into heat, the entropic state of all energy.

In electric vehicles, there is another step in this process, recovering unused kinetic energy. This is called regenerative braking. It turns the motor into a generator and feeds the generated electricity back into the battery. This increases the cruising range of the EV, but how much it depends largely on the efficiency of the regenerative stage.

The motor/generator is already optimized for high efficiency in both motor and generator modes. Another important stage is the inverter. A circuit that converts the high voltage from the battery into alternating current to drive the motor. The amplitude and frequency of this AC waveform determine the rotational speed. Traction motors are generally three-phase, so the inverter must generate three AC cycles from the DC voltage of the battery. Therefore, it is necessary to convert the voltage of 800V DC to about 180kW AC, and the efficiency at this stage greatly affects the overall performance and cruising range provided by automakers.

Naturally, this is where the design focus is. To increase inverter efficiency, use components with lower losses. Until recently, IGBTs had the advantage in terms of conduction losses, but they also incurred significantly increased switching losses at turn-off. This was a good trade-off, especially given the low cost of IGBTs, since typical motor drives have relatively low switching frequencies. SiC cascode JFETs have been steadily replacing IGBTs in this field, due to their lower switching and conduction losses. There are two reasons for this. First, as mentioned above, IGBTs have a slower turn-off speed because charge from the bipolar current is trapped. On the other hand, SiC cascode JFETs have a faster turn-on/turn-off switching speed and lower switching losses because only electrons flow. More importantly, when an IGBT conducts in both the forward and reverse directions, a PN junction, either from the IGBT itself or from a parallel diode, is always present in the current path. Because SiC material has low resistance and no voltage drop across a PN junction, SiC cascode JFETs have low conduction losses at all current levels, but have a significant advantage at the low power levels most commonly found in electric vehicles. Also, because SiC cascode JFETs do not require a parallel diode, they do not experience a "knee" voltage (after the switching dead time) at either forward or reverse current.

The mode of operation is correlated with the power factor (PF). If PF is positive, the circuit is in inverter mode, drawing energy from the battery to drive the motor. If PF is negative, the circuit acts as a rectifier, returning energy to the battery. Ideally, PF should be as close to +1 or -1 as possible to maximize efficiency.

By changing the PF, the losses of the FETs used become clear. The important factors here are the forward and reverse conduction losses and the turn-on and turn-off switching losses. These are added together to calculate the losses for each FET. In inverter and rectifier modes, most of the conduction losses are due to forward and reverse currents. Note that the forward current flows from the drain to the source (from the collector to the emitter in the case of IGBTs). IGBTs for motor drives only allow current to flow in the forward direction, so a parallel diode is required to allow current to flow in the reverse direction. This results in different conduction losses depending on the current direction, and the heat generated by the IGBT and diode also differs. On the other hand, SiC cascode JFETs pass forward and reverse currents through the same chip with the same conduction losses (after the dead time), which allows for higher chip utilization and higher power density.

When designing for high efficiency in both inverter and rectifier modes, one of the metrics to check is the reverse recovery charge and switching loss at turn-on of each FET. For example, if the bottom FET of a half bridge turns on after the top FET has reversed current, the top FET will reverse recover. This has the effect of causing residual current to flow through the lower FETs of the half-bridge, increasing switching losses at turn-on. Thus, reverse recovery charge is an important parameter for FETs. In fact, SiC devices have a much lower reverse recovery charge, so using SiC devices instead of IGBTs typically yields efficiency gains of a few percent. This is a big advantage in terms of cruising range and vehicle costs.

ON Semiconductor has conducted several experiments comparing its SiC cascode JFETs to IGBTs in these applications and can share design tools that enable engineers to quickly simulate the performance of the parts under different operating conditions such as PF, battery voltage, number of phases, and motor output power.

One thing is certain: regenerative braking is becoming more important to the end customer, and efficiency levels need to be carefully considered during the design phase.

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