axm-2017-05-broadcom-v3

Optocouplers Help Promote Safe, Efficient EV Charging Stations

Hong Lei Chen – Product Manager, Isolation Products Division – Broadcom

This is an abridged version. The entire article can be found here.

The worldwide electrification of transportation has grown rapidly through recent years. The global electric-vehicle (EV) stock was about 180,000 by the end of 2012. This number grew by 3.7 times, reaching more than 665,000 through the end of 2014 per International Energy Agency (IEA) Global EV Outlook reports. The report forecasts that by 2020, approximately 20 million EVs will be on the road.

Rapid growth of the EV fleet is driving strong demand for a charging infrastructure to extend the vehicles’ travel range. An EV charging station, also called Electric Vehicle Supply Equipment (EVSE), supplies electric energy to the EVs while providing a network connection. EVs in this context refer to plug-in electric vehicles, including all-electric cars or battery electric vehicles (BEVs), electric buses, and plug-in hybrids (PHEVs). Figure 1 shows an EV at a charging station.

1. Here’s a typical scene of an electric vehicle being charged at a charging station.

IHS Automotive forecasts the installation base for global EV charging stations to skyrocket from 1 million units in 2014 to 13.6 million by 2020. The market-research firm estimates that there will be 4.3 million units installed in the Americas; 4.1 million units in Europe, the Middle East, and Africa (EMEA); and 5.3 million in Asia (including Japan).

Governments such as those in Germany, China, and the United States are steadily making more funds available to develop charging infrastructure. China, for example, plans to deploy 4.5 million EV charging stations by 2020. This effort will support the plan of cumulative production and sales of 5 million units of BEVs and PHEs by 2020, reports www.gov.cn, a website run by the central Chinese government. Compared to 31,000 charging stations built through the end of 2014, the target of 4.5 million units implies a whopping compound annual growth rate (CAGR) of 129%.

AC or DC Charging?

Putting aside the complication of standards, there are primarily two ways to transfer electricity from outside the vehicle to the battery inside: AC or DC. The grid transmits power in AC form, and energy stored in the on-board battery is in DC. Therefore, a charger is required to do the conversion job.

Depending on whether the charger is installed inside of the vehicle or not, chargers can be categorized into on-board charger (OBC) and off-board charging station. An OBC accepts an AC power source from the main supply available at home and at the consumer’s workplace and converts it to DC to charge the battery. Typically, AC charging is slow due to the charger’s limited power rating—a constraint arising from the limitations of allowable weight, space, and cost.

The DC charging method is often used in off-board charging stations. It supplies regulated DC power directly to the batteries inside the vehicle. Because the DC charging equipment is installed at fixed locations with little constraint of size, its power rating can be as high as hundreds of kilowatts. 

An EV charging station, also called Electric Vehicle Supply Equipment (EVSE), supplies electric energy to the EVs while providing a network connection.

2. The DC fast-charging method shortens the charging time from hours to minutes.

Charging-Station Topology and Safety Isolation

The need for safety isolation is present in all functions of an EV’s on-board electronic systems as well as in EV charging stations. On-board systems include the high-voltage battery management system, DC-DC converter, electric motor drive inverter, and on-board charger. For on-board systems, optocouplers must provide reinforced reliability and safety insulation capability, which suits applications such as gate driving, current/voltage sensing, and digital communication. Discussions in this article will focus on the isolation solution for off-board charger designs, which often find industrial-grade devices to be sufficient.

An EV charging station typically includes functional blocks, such as the AC-DC rectifier, power-factor-correction (PFC) stage, and DC-DC conversion to regulate the voltage to a level that’s suitable for charging the vehicle battery. Figure 3 shows a simplified block diagram of a DC-charging-station design. In high-frequency isolation topology, galvanic isolation is provided in the DC-DC converter stage by a high-frequency transformer. In addition, multiple isolation devices provide various signal isolation functions while maintaining a safety isolation barrier between the high-voltage power section and low-voltage controller section. Within all of these stages, power devices like MOSFETs and IGBTs are used to perform the switching functions.

3. The charging control carries out calculations and control instructions to fulfill the designed function.

The EV charging infrastructure is a key factor in widespread EV adoption.

Located in the center of the system is the microcontroller unit (MCU), which controls the PFC and DC-DC converter with pulse-width-modulation (PWM) signals. The charging control is based on voltage, current information, and other data, such as temperature, user inputs, etc., to carry out calculations and control instructions to fulfill the designed function. Digital communication ports are used to communicate between EVSE and the EV for charging control and between EVSE and the charging station control center and thereafter to the cloud for charging data reporting, remote monitoring, and diagnostics.

Optocouplers Deliver Galvanic Isolation, Efficient Charging

As seen in Figure 3, a safety isolation barrier is built up along the line formed by optical coupling points of the various optocouplers. This is important to ensure that the design safety aspects comply with regulatory standards. Besides galvanic isolation, the other key factor that often requires close attention in power converters, including the one in the EV charging station, is power-conversion efficiency. This article introduces how to use several optocouplers from the catalog to implement efficient charging station designs for safety isolation.

Conclusion

Ultimately, EVs help reduce dependence on petroleum within the world of transportation while tapping into an often relatively inexpensive source of electricity. They also help reduce the emissions of greenhouse gases and other pollutants, which can be further improved as electricity generation portfolios add more renewable sources. 

The EV charging infrastructure is a key factor in widespread EV adoption. In an EV charging station, especially DC fast charging, complex power-supply systems are employed to deliver huge amounts of energy to the battery in the vehicle within a short period of time. Safety isolation is imperative, since the low-voltage control system, high-voltage power system, and user-accessible user interface coexist in a single charging station.

Efficiency in energy conversion is another critical design consideration in EV chargers. Optocouplers, such as the gate drivers, voltage sensors, current sensors and digital communication optocouplers, deliver both safety isolation and respective electrical function in a single package, helping lead the way toward highly efficient systems.


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