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The electric vehicle (EV) revolution isn’t coming—it’s here. With the seismic transition from a traditional, fossil fuel-powered mentality to going all-electric with a more high-tech approach, EVs are on the road and they’re here to stay.

Accompanying this new methodology and technology are a host of accompanying issues, one of the major ones being the vast network of charging stations needed to accommodate this growing fleet of vehicles on the road. Some of the fundamental issues include the number of stations available (no one wants to run low on power with no fueling station in sight), the charging times (attempting to get closer to the time it takes for a conventional fill-up) and the power level of charging stations (providing more than 50kW of power).

Faced with these challenges, design engineers are focusing on three major areas of concern as they anticipate the EV powering needs of today and the future: safety, efficiency and reliability.

Safety

User safety is of great concern in EV innovation and technology. Until DC charging stations came along, the average driver was not accustomed to accessing power higher than the 120V they experience at home in the wall socket. With today’s EV chargers delivering 400V to 1,000V of DC power, there are potentially significant safety threats from two critical areas: electrical shock (usually as a result of a ground fault) and overcurrent.

Electrical Shock

A ground fault is an inadvertent contact between an energized conductor and ground or the equipment frame. The typical causes of a ground fault are insulation breakdown and dust and moisture interruption along the electrical pathway.

On the input side of the design, AC ground-fault protection is needed to protect components from damaging faults and to protect users from electric shock should the equipment frame or housing become energized. A ground-fault protection device uses a current transformer on the phase conductors to ensure that all current coming from the source returns on those same conductors, or it reads the current in the connection between the transformer neutral and ground. A ground fault anywhere in the system will return current through this path.

Ground-fault protection is also required at the output side to avoid having drivers picking up a nozzle capable of high power output (400V to 1,000V) whose handle or frame is energized. To avoid this, a DC ground-fault monitor is installed on the output side to detect any earth leakage and shut off power immediately. As the output side is not grounded, 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 low-level ground faults.

Overcurrent

Here’s the dilemma: EV charging stations need high amounts of voltage to service the current demands and numbers of vehicles using them, but if that power source exceeds normal levels and beyond, all sorts of incidental and fundamental consequences can occur.

In fact, unless the elevated current amount is interrupted quickly, even at moderate levels, the results could be numerous and possibly catastrophic: overheating systems, damaging insulation, bending and twisting bus bars, permanently damaging key electrical components, starting fires and even causing arc-lash incidents that could severely injure or kill a user.

The solution to avoiding these serious threats is to select fuses based on their interrupting 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 peak let-through current.

Efficiency

One of the primary reasons for making the extensive and fundamental change from traditional fossil fuel energy sources to electric is for the improved efficiency, productivity and overall higher performance that the all-electric solution provides. Faster, leaner, cleaner, better. But fundamental changes can often bring about fundamental challenges, especially for the designers responsible for creation, implementation and execution.

In creating DC fast charging systems, power conversion is a crucial element. Minimizing losses in power conversion ensures the maximum amount of power is delivered for charging the vehicle’s batteries and, in the process, reducing heat buildup within all the systems.

Power semiconductor devices convert AC power into the DC power needed to recharge vehicle batteries. To match the level of charge to what the vehicle battery needs, the power semiconductor device controls the charge through switching, a process that naturally incurs power losses in the form of heat.

That’s why advanced devices based on silicon carbide (SiC) and gallium nitride (GaN) technology are utilized in power conversion. Compared to silicon devices, these provide ultra-fast switching for lower power losses. SiC MOSFET devices are now available that blend high operating voltages and fast switching speeds, a combination typically not available with traditional power transistors. For automotive charging applications, they must operate at high junction temperatures and feature low gate resistance, low gate charge, low output capacitance and ultra-low on-resistance.

Reliability

Effectively solving potential safety and efficiency issues is one thing, but if the equipment isn’t dependable at a consistent and long-term rate, the EV solution is not a revolution people will support for the long-term future. The solution to the reliability issue rests on the protective measures and components used to withstand the electric shocks and overcurrents that threaten them.

Unlike many consumer devices like laptops, which are engineered for a lifetime of only three to five years, DC charging stations are expensive. Buyers need them to last for 10 years or more even in the most severe outdoor conditions to get a return on their investment. The value of semiconductor content alone ranges from $350 for an AC charger to more than $3,500 for a 350kW charging system. The higher the quality of the circuit protection, the longer it lasts and the better investment it makes.

Because semiconductor devices are sensitive to electrical threats and must be protected from overcurrent, fuses are necessary. These devices are typically fabricated from silicon or silicon carbide and have low thermal withstand capacity. Conventional fuses are sufficient to protect most of these, but specialized high-speed DC fuses are needed to protect power semiconductor devices such as MOSFETs, IGBTs, diodes and thyristors used in power converters. Such fuses are engineered with a specific time-current characteristic so that they operate very quickly compared to traditional AC input fuses.

Another threat is overvoltage. If an EV charger is located near an industrial facility with large motors, the switching on and off of those motors can produce voltage surges in the power supply. Also, if there is a lightning strike near the charging station, the electromagnetic energy may induce a voltage surge on the power lines in the neighborhood that could propagate into the charger via the AC power input lines. Overvoltage protection devices must be used to absorb that energy, preventing it from damaging sensitive electronics.

Circuit protection devices are made with different technologies. While many types of devices may work, it is better to select a device having the ideal technology for that application. In a DC charging system, a high-power transient voltage suppressor (TVS) diode or metal oxide varistor (MOV) is usually the best type of suppression device. Other types of protectors are often specified, such as protection thyristors, gas discharge tubes and multi-layered varistors (MLV) or combinations of suppression devices.

The Drive of The Future

Worldwide EV sales doubled from 2020 to 2021 and some sources forecast that EVs will account for as high as 30 percent of new car sales in the U.S. and Europe by 2025. The commitment to EVs has been made. Now, high quality equipment is needed to support the investment.

In the exciting and industry-changing world of EV charging station development, many challenges await, ranging from the number of stations needed, to acceptable charging times, to ensuring appropriate power levels. Fortunately, today’s design engineers, with a careful eye toward user safety and equipment efficiency and reliability, are well on the road to making EVs the drive of the future.