Electric vehicle infrastructure-based chargers made easy
The number of electric vehicles (EVs) and plug-in hybrid vehicles (PHEVs) on the road is growing. In fact, the governor of California has set a goal for 1.5 million EVs on the road by 2025 — and that’s just one state in the U.S. Globally, sales could be much higher with projections in Europe reaching 3 million EV/PHEVs by 2020; authorities in China are setting an even loftier goal of having 5 million pluggable vehicles by 2020. With this growth, it only makes sense that there will also be a boom in the demand for vehicle charging stations.
Range anxiety is seen as one of the largest barriers to achieving consumer confidence in EVs. Plentiful and readily available charging stations will help relieve this anxiety and further increase EV popularity. A handful of free charging stations can already be seen at office complexes, parking structures, restaurants and shopping centers, but the need for “pay-to-charge” stations will increase, as will the need for more technology and communications in these systems. The technical demands for these systems will undoubtedly grow while system developers will be tasked with the challenge of increasing functionality while keeping the units small and uncomplicated.
Wireless charging and communications
Many of the pay-to-charge stations found in urban areas today look and operate similar to parking meters, except with the addition of a charging cable so users can plug in vehicles. There are three common types (or levels) of charging stations:
- Level 1 and Level 2 charging stations are “metered” AC power sources that utilize on-board charging functions of the EV.
- Level 3 includes DC “fast chargers.” These bypass vehicle power factor correction (PFC) and feed 400 VDCs to the batter charging stage.
While all power levels and stages are different, the need to meter the electricity usage and offer the capability to charge the customer a fee is the same. In “pay-to-charge” stations, it is also necessary to communicate with the back-end network for credit card charging, charging a mobile subscribers cellular phone plan, or even processing cash-based transactions. This functionality calls for the system to have a flexible architecture.
What does this mean for the technology used? Near-field communications (NFC), necessary for mobile payments, is a very short-range communications standard that acts in principle very closely to radio frequency (RF) identification. Each smartphone, or NFC-enabled device, has its own unique identifying code associated with a payment account. Ethernet, power-line communications (PLC) and Wi-Fi are necessary for payment processing, as well as for advanced metering and other control functions. Communications with the vehicle being charged are also required. Most EVs require communications with the charging station via CAN, RS232, Ethernet, PLC or with Pulse Width Modulation (PWM) signaling. So, how do the designers of these pay-to-charge stations keep designs relatively simple and cost effective while accomplishing all of the requirements necessary in these systems?
An easy solution to this challenge is to use an embedded controller or processor that offers NFC, PLC, Wi-Fi, CAN and 10/100 Ethernet communications along with the ability to handle the metering, housekeeping and power-stage control all in a single device. This way, developers can keep printed circuit board space and bill of materials costs to a minimum, while also integrating all vital communications and advanced protection functions into the system. An example of such an integrated embedded controller would be the C2000™ C28x + ARM® Cortex®-M3 based dual-core microcontrollers (MCUs) from Texas Instruments (TI). These MCUs can handle the power-stage control in additional to the necessary measurement, communications and interface requirements.
Serving as the foundation in metering systems are the analog interface and processing capabilities of the embedded controller. Using a device that contains this analog integration, designers can easily implement the required voltage and current monitoring required for single and three-phase AC measurements, as well as monitor output levels in the higher output DC-based systems.
Breaking down the design needs
We will divide the system into two sub-sections to simplify the diagrams presented:
1. The power supply that is being monitored
2. The low-voltage communications side of the system
Since we are dealing with both low- and high-voltage systems, we must also consider the requirements for isolation between the high- and low-voltage systems. As previously mentioned, EV chargers are currently classified into three categories: Levels 1 and 2 (AC charging) and Level 3 (DC fast charging). In the Level 1 and 2 systems, the charging station architecture looks very similar to a standard metering application found in most smart grid applications, as shown in Figure 1. The meter is simply connected across a single- or three-phase AC source (common grid), and there are no power control stages within the system. It operates much the same as a residential meter, monitoring the flow of power through the system, with the added functions of communications both to the vehicle under charge and to the payment gateway. The system may also include safety monitoring and disconnects.
Both Level 1 and Level 2 chargers utilize the vehicles on-board charging system, which includes the power factor correction boost stage and the high-voltage DC charging circuit. Level-1 chargers are based on the standard 120/240VAC level, offering up to 16 amps of charge. Level 2 charging can utilize either 240VAC or 480V 3-phase AC, but both are limited to 32A. Again, in either Level 1 or Level 2 cases, the charger is simply acting as a metered interface between the utility grid and the vehicle being charged, with no energy-conversion stages.
Figure 1 – Simplified signal chain of a ‘smart’ infrastructure charging station
DC fast-charging systems operate very differently, converting the AC mains voltage levels to a boosted DC level, capable of delivering up to 400 amps. While a Level 1 or 2 charger can charge the common EV in four to eight hours, the DC boost charger can offer the same level of charge in as little as 20 to 30 minutes. Although the power stages are quite different between Levels 1 and 2 when compared to Level 3, the metering application is common to all three as the metered input is always AC mains and ahead of any PFC stages.
In a paid charger application of any of the charging levels, we have the need (or potential need, depending on billing and communications options) for:
- Metering of the actual power usage of the vehicle under charge (usually in kWh)
- Fault management and system protection
- Payment processing (credit card, smart card, bill collector or cellular billing through NFC with a cellular telephone)
- Merchant processing communications (Wi-Fi, Ethernet or PLC)
- Charge-management communications to the vehicle (CAN, RS232, Ethernet, Power Line Communication or PWM signaling)
The metering system can be easily partitioned to include all of the functions into a single embedded processor using a dual-core processor with a subsystem. Many silicon vendors also offer multiple solutions for radio communications and system-level isolation. The system can be broken into smaller sub-segments based on the functions listed above, starting with the requirement for metering and determining the kilowatt hours (kWh) to be billed to the customer.
Figure 2 – Multi-phase metering connections to analog sub-system
As demonstrated in Figure 2, the metering stage takes advantage of the analog system of the dual-core device, utilizing the internal ADCs and processing power of the CPU (in this case, the C28x DSP core) combined with a current transformer. For increased tamper resistance, a shunt resistor circuit may also be added. When combined with a real-time clock, the processing for measuring kWh becomes a standard voltage and current measurement that can easily be handled with a combination of up to seven analog to digital converter inputs of the C2000 MCU, depending if both the current transformer and shunt resistor are used in parallel as well as the total number of phases.
By utilizing a digitally controlled closed loop over-current protection scheme, shown in Figure 3, system-level safety is increased with the addition of a physical relay on the main charging bus. With the on-chip analog comparators of the dual-core device, their outputs tied to a standard GPIO, and using the relay driver (DRV110), a closed loop “smart” circuit protection scheme can be implemented that offers both user or CPU-controlled reset while maintaining low-power architecture and reducing the number of required external components (See Figure 3).
Figure 3 – Implementing line disconnect with the DRV110 and relay
There are also several elements deployed in dual-core microcontroller architectures that can be utilized to enhance the overall safety of the system. With two independent processors in the controller, one can be used to check for proper operation of the other one on a scheduled basis. In addition, critical computations can be run on both processors in parallel and checked for correctness before the result is used by the system.
A similar checking method can also be deployed on the digital and analog I/O modules in the microcontroller. Critical system signals can be connected to multiple I/O modules in the microcontroller, and the result from each module can be checked for correctness. Other methods that can be used to increase the safety of the system include enabling integrated hardware memory checking mechanisms like error correcting code (ECC). Many implementations of ECC hardware can automatically detect and correct single bit memory errors. In addition, they can also detect and report double bit errors. This type of ECC scheme can be used to effectively enhance both the reliability and safety of the system. Clock signals are also very critical to proper operation of microcontrollers; therefore, taking advantage of integrated clock failure detection logic is an important measure to enhance the safety of the system. Power supply fluctuations can also cause malfunctions and indeterminate behavior of the system, so utilizing power monitoring circuitry and employing brown out reset and recovery methods are important in the safety of the system.
Integrating the payment processing functions
Moving on through the signal chain, the next thing to take into account is payment processing. If a device has an industry-standard ARM Cortex-M3 core, there is the ability to run merchant processing services in the main controller. Primary forms of processing are either through direct credit card swiping, a bill or coin collector, or NFC with a smartphone.
Processing credit cards directly requires more processing power, but the ARM Cortex-M3 core, as well as many other solutions, can handle this. For example, another ADC input in an MCU or other embedded processor can read a credit card directly from a magnetic tape head. Solutions for decoding the magnetic tape area are readily available, or a solution could be developed in house. Technically, bill or coin collection systems can be implemented with the same ARM Cortex-M3 core, but to keep things simple, this will be treated as a separate system using a digital interface to the C2000 dual-core host MCU.
- NFC enables users to tap smartphones against an NFC-enabled payment gateway. These uses are asked for a PIN number similar to using a debit card. A secure transaction to the bank or payment account is made, authenticated, and then the user is charged accordingly. By combining the processing power of a dual-core device, with an NFC chipset, such as the TI TRF7970, developers are able to implement this function directly into the host processor, further reducing the need for additional components.
- Communication layers can be supported by many embedded processors. For example, the C2000 dual-core MCU supports the IPv6 10/100 TCP/IP stack in software with support for the internal Ethernet MAC for wired Ethernet.
- Wireless connections are also supported by many dual-core devices – wired Ethernet, wireless Wi-Fi communications (i.e. TI’s SimpleLink CC3000 solution) through a self-contained wireless solution offering an easy solution for wireless connectivity,
- PLC is a flexible option for areas with no Wi-Fi or Ethernet infrastructure. Designers can leverage the computational power of the CPU in a dual-core device. Low-frequency narrowband standards as PRIME, G3, CENELEC and FlexOFDM, all configurable on the same device, in addition to the host communications and measurement functions expressed earlier.
As the EV and PHEV markets continue to grow to include payment and metering options, it’s possible to easily and cost effectively meet these needs by using a single dual-core embedded processor or microcontroller.
For additional information, visit www.ti.com/c28x_arm_cortex-m3.