3 Design Considerations for Next Generation EV Charging Stations – Charged EVs

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How to make the charging stations of tomorrow more efficient, highly reliable and safer

The global adoption of hybrid and electric vehicles depends on the network of fast charging stations available. Users don’t want to run out of juice where they can’t charge their vehicles.

Widespread adoption also depends on reducing charging time, with the goal of getting EV charging times as close to the time it takes for a full tank of conventional fossil fuel. This requirement stipulates that high-power charging stations have more than 50 kW of charging power, and the challenge is that these types of fast charging stations are relatively more expensive.

Given the growing importance of fast-charging stations, the demand for higher power levels, and the significant investment to provide both, it is crucial that designers of electric vehicle charging systems plan for efficiency, increased reliability and safety.

  • Efficiency. DC fast charging systems must convert energy efficiently. It is crucial to minimize power conversion losses to ensure maximum power to charge vehicle batteries while reducing heat build-up.
  • Reliability. After installation, EV charging equipment must operate reliably for at least ten years, even in very harsh outdoor conditions, to ensure an acceptable return on investment.
  • Security. Above all, user safety is paramount. Until recently, the public did not have access to more power than is available from wall outlets in their homes. The advent of DC charging stations has changed this by making 400 to 1000 V DC available. EV chargers must protect consumers from the threat of electric shock and other hazards.
DC charging station and functional parts requiring circuit protection.

Efficiency

Power semiconductors convert alternating current to the direct current needed to charge a vehicle’s batteries. The power semiconductor device controls charging by switching to match the charge level to what the vehicle battery requires, which naturally results in power losses in the form of heat. This heat can create engineering challenges in an EV charging application.

This is why the use of advanced devices based on SiC and GaN technologies in power conversion makes sense; compared to silicon devices, they offer ultra-fast switching that results in lower power losses.

SiC MOSFETs that combine high operating voltages and fast switching speeds are now available, a combination not found with traditional power transistors. Automotive charging applications must operate at high junction temperatures and exhibit low gate resistance, low output capacitance, low gate charging, and ultra-low on-resistance. Charging station designers prefer devices that offer high power density and reduce the size and weight of filter components, thereby reducing costs and space requirements.

Reliability

DC charging stations are expensive compared to mobile devices designed for a lifespan of three to five years, so end users need them to last more than ten years to get an acceptable return on investment. Costs for semiconductor content range from $350 in AC chargers to over $3,500 in a 350 kW charging system. This is why proper circuit protection is so important: it allows the investment to operate reliably for a longer period of time.

Semiconductors are usually made from silicon or silicon carbide and have low thermal resistance capability, which makes them very susceptible to electrical threats. They must be protected from overcurrents by fuses.

While conventional fuses can protect most devices, specialized high-speed DC fuses are needed to protect MOSFETs, IGBTs, thyristors, and diodes used in power converters (rectifiers, inverters, etc.). These fuses are designed with a specific time-current characteristic and operate very quickly compared to traditional AC input fuses.

Power surges are another threat to sensitive semiconductors. For example, an EV charger near an industrial facility that uses large motors may experience surges in the power supply caused by the switching on and off of these motors. Additionally, lightning strikes near the charging station result in electromagnetic energy that can induce a surge on neighborhood power lines that could propagate into the charger via AC power input lines. To absorb this energy, the charging station must use surge protection to prevent damage to the sensitive electronic components that operate the charger.

Security

The two biggest security threats to an EV charging station are electric shock and overcurrent. An electric shock is usually the result of a ground fault.

Electrical shock
A ground fault is an unintentional contact between a live conductor and earth or the equipment frame. Insulation breakdown is the typical culprit. In addition, dust and humidity can cause unplanned trips for electricity. Wet and dusty environments, such as those found around outdoor equipment, require careful design.

TVS diodes (such as Littelfuse SMF Series Surface Mounted TVs) are specifically designed to protect sensitive electronic equipment from lightning-induced voltage transients and other transient voltage events.

AC ground fault protection is required on the input side of the design to protect components from damage and consumers from electric shock if the equipment chassis or enclosure is energized. A ground fault protection device uses a current transformer on the phase conductors to ensure that all current from the source flows back to those same conductors, or it reads the current in the connection between the transformer neutral and Earth. A ground fault anywhere in the system will return current through this path.

Also, ground fault protection is required on the output side, so that when a consumer picks up a nozzle capable of 1000 V, the handle or frame is not powered. A DC ground fault monitor is installed on the output side to detect any ground leakage and immediately shut off the power. Because the output side is ungrounded, the ground fault monitor depends on a ground reference module between the two buses to establish a neutral point, which is used as a reference to detect faults at low level ground.

Overcurrent

By their nature, vehicle charging stations are connected to a power source with a high available fault current. Electrical faults, including those that trigger ground faults, can draw high current which can be very destructive, damaging components, bending bus bars, starting fires and even causing an arc flash incident – a kind explosion that could injure or kill anyone standing nearby.

Varistors are designed for applications requiring high peak currents and high energy absorption capability (see Littelfuse UltraMOV Metal Oxide Varistors Series).

Select fuses based on their breaking capacity, their rating based on normal operating current, and their time-current curve characteristics. “Current-limiting fuses” operate quickly in the event of a high-value overcurrent, which limits the peak through current.

Unless interrupted quickly, even moderate overcurrents can overheat system components, damage insulation and damage conductors. However, the worst damage will be to the many electronic components, which are sensitive to low value overcurrents.

Make circuit protection a pre-think, not an after-thought, in DC charging station design

Circuit protection devices today encompass many different technologies, each with specific application solutions in mind. Although many devices may work, it is best to select a component with the ideal technology for the application. In a DC charging system, high power transient voltage suppression diodes (TVS) or metal oxide varistors (MOV) generally provide the best type of suppression. Other types of protectors, such as gas discharge tubes (GDTs), protective thyristors and multilayer varistors (MLVs) or combinations of suppression devices, are often specified.

SiC-based MOSFETs from Littelfuse are optimized for high power, low resistance, and low power conversion losses not available with traditional silicon devices.

When protecting sensitive circuits, it is essential to know the time it takes for a transient suppressor to begin operating. For example, if the suppressor is slow acting and a fast rising transient appears in the system, the voltage across the protected load may rise and cause severe damage before the suppression can initiate.

Download the Guide to Littelfuse Supercharged Solutions for Electric Vehicle Charging Stations from TTI for more information.

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