The world is entering a period of rapid buildout of EV charging infrastructure. Average total cost of EV ownership is falling below that of comparable internal combustion vehicles while EV adoption rates continue to trend upwards. Expect to see more and more electric vehicle chargers in parking lots and gas stations.
Porticos has broad experience in the engineering design of electric vehicle chargers. In this series of posts we’ll explore the intricacies of electric vehicle chargers from the perspective of new product development.
Read Previous Post: EV Charger Engineering Part II: AC Chargers
Welcome back to Electric Vehicle Chargers, A Development Perspective.
In Part 2 of this series introduced Level 1 and Level 2 AC chargers, including some of their specifications and design challenges. Today, we introduce DC charging.
Introduction to DC Charging
DC charging (sometimes called Level 3 charging or Supercharging) is a huge step up from AC Level 1 and Level 2. DC chargers can deliver a staggering 80kW of power to an electric vehicle. To put this in perspective, one can fully charge an EV in 15 minutes. What makes this immense improvement in charge capacity possible? The answer lies in the AC/DC conversion.
Any EV charging system is going to have some AC/DC conversion. After all, grid power is AC, and EV batteries are DC devices. In an AC charging system, the EV uses an onboard AC/DC converter to generate DC power for the battery, and the EVSE is simply switching on and off AC power. This architecture is simple and cost-effective for both the EV and EVSE… but it is not fast.
In a DC charging system, the AC/DC conversion happens in the Electric Vehicle Supply Equipment (EVSE), not the vehicle. This is the key to achieving much higher power because it bypasses the “bottleneck” that is the onboard AC/DC converter.
To support the higher power, DC charging systems need to be powered by at least a three-phase 440V AC service. While technically “low voltage,” it is not typically available in residential buildings or office space.
Compared to AC chargers, DC charges can be expensive. These power converters often require custom magnetics, advanced semiconductor technologies, sophisticated controls, and elaborate thermal management strategies.
Power Conversion
Designing an 80kW AC/DC converter is fraught with engineering challenges. They require extensive knowledge of power electronics. The engineer must give careful consideration to many design parameters including topology, efficiency, thermal management, and safety. The block diagram below shows the basic elements of this type of design.
The rectification stage is responsible for turning the AC voltage to a rough DC voltage and also handling power factor correction (PFC). When selecting a topology, efficiency is one of the most important considerations. A mere 1% inefficiency represents 800 watts of waste heat that must be dissipated. For this stage, either a “totem pole” converter or a Vienna converter are two of the best choices. Both offer very high theoretical efficiency.
The second stage is the DC/DC conversion stage. This takes the rough DC voltage generated by the rectifier and steps it up to the output voltage required. Resonant LLC converters are good candidates, again due to high theoretical efficiency and also good utilization of the magnetic core of the switching transformer. Adding parallel Interleaved phases can also increase efficiency and power output capability.
Power Controller
Critical to overall performance is a power controller. The controller carefully monitors current, voltage, phase angle, and temperature, and operates the power electronics accordingly. Controlling this type of supply requires more than just a simple integrated circuit. Multiple feedback loops, high loop bandwidth, and safety-critical features require a dedicated processor with custom firmware.
It is also worth noting that just because an EV can be charged fast doesn’t mean that it should be. Fast charging is handy in a pinch but repeated fast charging will eventually degrade the life of the battery much faster than the slower charging represented by Level 1 and Level 2 (AC) chargers.
Implementation
Most DC charger setups are found in public spaces and dedicated charging stations. DC chargers are not typically installed in homes. They are expensive, and most homes do not have electric services capable of supplying them.
It’s important to understand that the charger is not the only item that determines practical charging speed. Several other factors are involved:
- Physical limitations of the EV
This includes the connector materials and internal wiring. 80A is a lot of current and requires heavy-duty connections. - Battery Design
Factors like chemistry and thermal management play a big role in defining a safe charging rate.
Summing up: just because a charger can supply a high level of electrical power doesn’t mean that an EV can or will accept it.
It is also worth noting that just because an EV can be charged fast doesn’t mean that it should be. Fast charging is handy in a pinch but repeated fast charging will eventually degrade the life of the battery much faster than the slower charging represented by Level 1 and Level 2 (AC) chargers.
More to Learn
Contact Porticos for engineering assistance with any aspect of EV charger design, and stay tuned for the next article in this series!
