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Learning From the Past: EV High Power Density Trends

May 20, 2021 by Steve Arar

With EVs being a hot topic recently, what trending designs were used in the past could be learned from? Oak Ridge National Laboratory aims to do just that through its benchmarking project.

Car manufacturers are making massive investments to set new benchmarks in the performance of future EVs. A significant improvement is impossible without the incorporation of highly advanced technologies.

Recently, Oak Ridge National Laboratory (ORNL) has been conducting a benchmarking project that examines the performance of the main electronic components employed in today’s on-the-road EVs, HEVs, and PHEVs. The results of this project are published as an educational resource for industry, academia, and the general public.

Inspired by this project, we’ll take a look at some of the main design trends that can play a significant role in widespread EV adoption. Let’s first take a look at a typical HEV power control unit.     

 

Breaking Down a 2010 Prius: Enter the PCU

The first interesting design trend to dig into is the power control unit (PCU) of the 2010 Prius. The PCU circuit diagram (Figure 1) shows a 201.6 V battery is connected to the low voltage side of the DC-DC boost converter. This converter provides a boosted voltage ranging from 202 to 650 Vdc on the DC link.

A few other features of this PCU are:

  • The PCU includes motor and generator inverters.
  • The "motor inverter” converts the power provided by the DC link into AC to power the motor.
  • The DC link is also connected to the generator inverter.

 

The 2010 Prius PCU.

Figure 1. The 2010 Prius PCU DC-DC converter diagram. Image used courtesy of Burress et al and Oak Ridge National Laboratory

 

The DC bus voltage is adjusted to ensure efficient operation under light load conditions and depends on driving conditions such as acceleration and regenerative braking conditions. A 470 V, 315 μF capacitor is placed at the DC-DC converter input, and a smoothing capacitor is placed at the boosted level is a 750 V, 888 μF capacitor. 

Figure 2 shows how these circuit components and other required blocks, such as the cooling infrastructure, are arranged in the Prius PCU compartments.

 

The 2010 Prius compartments for the inverter and converter assembly.

Figure 2. The 2010 Prius compartments for the inverter and converter assembly. Image used courtesy of Burress et al and Oak Ridge National Laboratory

 

Power is a typical trend when it comes to EVs; another primary power trend, aside from the DC-DC converter, is achieving high power density, specifically through bus voltage and motor speed.

 

Achieving High Power Density with Bus Voltage and Motor Speed

The second trend is achieving high power density. To achieve this, it is necessary to reduce the size and weight of different system components to increase the running distance per battery charge. One example, in the case of the Prius's PCU, there was a reduction of the total mass from 21.2 kg in the 2004 Prius to only 13.0 kg in the 2010 Prius. 

According to the ORNL study, the three key factors that have enabled this dramatic mass reduction are increased DC bus voltage, motor speed, and a more efficient cooling structure. 

This trend can be observable in different generations of Prius. While the first generation of Prius's bus voltage and motor speed was 375 V and 6000 rpm, the 2017 Prius employed a DC link of 600 V and a motor speed of 17000 rpm. This addition reduced the size of the motor and inverter and thus helping to create higher power density.  

Another trend on high power density is by creating a more efficient thermal design.

 

A Power-dense Solution: Using an Efficient Thermal Design

Though thermal efficiency is always a challenge in all aspects of design, it can be an even larger issue when considering the PCU. In an EV, the most expensive component tends to be the PCU because their blocks require significant semiconductor components, such as IGBTs and diodes, to output the large current for EV traction. 

The current handling capability determines the size and cost of these devices. An efficient cooling strategy must prevent the power devices from overheating with the large current required for EV traction. In other words, an efficient cooling mechanism allows us to supply a given output current with relatively smaller devices leading to a more power-dense solution. 

As shown in Figure 3, some manufacturers have attempted to shrink the heat conduction path from the power devices to the coolant channel to reduce the overall thermal resistance. It is essential to avoid mechanical stress and fractures with this compact solution by understanding the mismatches between the coefficients of thermal expansion of different layers.

 

Thermal conduction paths of a 2004 Prius (left) and a 2010 Prius (right).

Figure 3. Thermal conduction paths of a 2004 Prius (left) and a 2010 Prius (right). Image used courtesy of Burress et al and Oak Ridge National Laboratory

 

Another commonly used technique to enhance the heat radiation and lower the thermal resistance is the double-sided cooling methodology. In this case, special semiconductor packaging accommodates heat dissipation from both sides of the die.

 

Power module cooling structures (a) single-sided cooling and (b) double-sided cooling.

Figure 4. Power module cooling structures (a) single-sided cooling and (b) double-sided cooling. Image courtesy of Hirao et al and IPEC

 

Thermal efficiency is just one of many ways to provide power density, a final solution to look at in the scope of EV trends is using inverters and motors.

 

Benefits of Integrating Inverters and Motors

Physical integration of the inverters and motors into a single casing can be an effective strategy to increase the power density. 

 

An integrated electric drive that puts different components such as the electric motor (1), current sensors (2),  water cooling plates (3), power PCB (4) and control PCB (5), into a common housing.

Figure 5. An integrated electric drive that puts different components such as the electric motor (1), current sensors (2),  water cooling plates (3), power PCB (4), and control PCB (5), into a common housing. Image used courtesy of Nikouie et al and SPEC

 

This integrated solution can lead to a more compact design with less cabling. Moreover, using a typical cooling system for both the motor and drive increases the system power density with an integrated electric drive. A bonus to this solution is that it can reduce electromagnetic interference (EMI).

 

The Effects of Power Devices and Modules

With today's silicon-based power devices, the efficiency of power modules can be above 90%. Migrating to wide-bandgap (WBG) devices, such as SiC MOSFET and GaN HEMT, can help achieve efficiency numbers above 98%. 

Hence, a further increase in efficiency might seem insignificant. However, this is not the case because even a slight increase in efficiency will reduce the produced heat. This reduction in produced heat will consequently reduce the size of the heatsink and cooling system leading to a more power-dense solution. 

When designing power modules, it is crucial to minimize parasitic inductance to maximize the benefits of the available high-performance devices. The packaging materials and heat management solutions, such as the double-sided cooling technique discussed above, can play a significant role in the system's performance.

 

Despite Challenges, the Future is Hopeful

In addition to the issues discussed above, there are many other challenges that should be overcome to enable an efficient, power-dense solution. Design of the motors, generators, passive magnetic components, the employed inverter topology as well as the DC link capacitor technology is some of these design challenges.

Though these challenges exist, there is hope for progress with how booming the EV industry is and now focused engineers are on creating and re-innovating solutions.