Regenerative power efficiency is becoming a real differentiator

The momentum behind electric vehicles has reached an inflexion point, it is hard to picture a future in which EVs are not a significant presence on our roads. This changes a lot, not just our buying preferences and driving habits, but the way we think about mobility.

Imagine the world before Henry Ford. Places to refuel would have been sparse, it was common for early adopters to carry fuel cans strapped to the outside of the vehicle. Range anxiety is nothing new. There was little consideration, however, about how long it would take to refuel a vehicle powered by an internal combustion engine. It was always going to be quicker than feeding and watering a horse, after all. In fact, that was probably one of the main attractions of owning an automobile; the fact that it required much less consideration. Mechanics replaced stable hands and the real cost of ownership would have become apparent eventually, but the wheel had already turned.

Turning wheels is more than a metaphor in this case, because ultimately that is what it is all about. The move to EVs means those wheels are now turned through an electric motor, rather than a reciprocating engine, but the goal is the same. One thing that is significantly different, however, is the way energy is exchanged. With an internal combustion engine, chemical energy (fuel) is transformed into kinetic energy (motion), which is subsequently transformed into the entropic state of all energy, heat, in exchange for movement.

With EVs, there is another stage in this process that captures the unused kinetic energy. This has become known as regenerative braking, but what it really means is using the motive power of the vehicle to turn the electric motor, rather than the other way around. This turns the motor into a generator and the electricity produced is fed back into the battery. This extends the range of the EV, but by how much is very dependent on the efficiency of the regenerative stages.

The motor/generator is already optimized to be highly efficient in both motor and generator modes. The other key stage is the inverter. This is the circuit that turns the high voltage from the battery into alternating current (AC) to drive the motor. The amplitude and frequency of the AC waveform will determine the speed of rotation. Typically, traction motors are three-phase, so the inverter needs to generate three AC cycles from the DC battery voltage. This can be as much as 800 V DC converted to around 180 kW AC, so the efficiency in this stage is crucial to the overall performance and range offered by the vehicle manufacturer.

Not surprisingly, this is where much of the design effort has been focused. Making the inverter as efficient as possible means using components that have minimum losses. Until recently, IGBTs had a conduction loss advantage at the expense of significantly higher turn-off switching loss. Because the typical motor drive switching frequency is relatively low, this was a good tradeoff, especially considering the low cost of IGBTs. Silicon Carbide (SiC) FETs have been steadily displacing IGBTs in this application space because they offer both lower switching and conduction losses. There are two reasons for this. First, as mentioned, IGBTs have a slow turn-off speed due to trapped charges from bipolar current. SiC FETs on the other hand have fast turn-on and turn-off switching speeds because only electrons flow, resulting in low switching loss. More importantly, an IGBT always has a PN junction in its current path, either from the IGBT itself or from its anti-parallel diode, during forward or reverse conduction respectively. Due to the lower resistance of SiC material and the elimination of the PN junction voltage drop, SiC FETs have lower conduction loss at all current levels, but a significant advantage at low power where an EV operates most often. A SiC FET requires no anti-parallel diode, so no “knee” voltage with either forward or reverse current (after switching deadtime).

The operating mode correlates with the power factor (PF). If the PF is positive, the circuit is in inverter mode, taking energy from the battery to drive the motor. If the PF is negative, the circuit operates as a rectifier, putting energy back into the battery. Ideally, the PF is as close to either +1 or -1 as possible to maximize efficiency.

Changing the PF highlights the losses of the FETs used. The key metrics here are the forward and reverse conduction losses, and the turn-on and turn-off switching losses. These are added together to give a total loss per FET. In inverter or rectifier mode, most of the conduction loss is from forward or reverse current, respectively.  Note that forward current flows drain-to-source (or collector-to-emitter for an IGBT). An IGBT intended for motor drives conducts only in the forward direction, and so it requires an anti-parallel diode for reverse current flow. This means that the conduction loss and hence IGBT and diode heating is different based on current direction. SiC FETs on the other hand conduct forward and reverse current with the same conduction loss (after deadtime) and through the same chips, so chip utilization is better and power density can be higher.

When designing for high efficiency in both inverter and rectifier modes, one of the metrics to check is the reverse recovery charge, as well as the turn-on switching loss of each FET. For example, if the bottom FET in a half-bridge turns on after the top half has had current flowing in the reverse direction, the top part goes through reverse recovery. This causes some residual current to flow into the bottom FET of the half-bridge, which has the effect of increasing its turn-on switching loss.  Thus, the reverse recovery charge is a critical FET parameter. In fact, it is because the reverse recovery charge of SiC devices is so low that SiC devices in place of IGBTs in general delivers efficiency gains of several percentage points. This translates into real benefits in terms of range or vehicle cost.

UnitedSiC has conducted several experiments to compare its SiC FETs against IGBTs in these applications. It can also share design tools that let engineers quickly simulate the performance of its parts for different operating conditions, such as PF, battery voltage, number of phases, and motor output power.

One thing seems certain, regenerative breaking is becoming more relevant to the end customer and the efficiency levels deserve to be considered closely at the design stage.