Supercharged protection solutions for EV charging stations
Electric and hybrid electric vehicle (EV/HEV) adoption by the public depends on having a vast network of available charging stations. Today’s EV consumers don’t want to run out of juice in “the middle of nowhere” and be stuck in a place where they cannot easily recharge their vehicles — and charge them quickly.
Bottom line: Most important to EV/HEV adoption is increasing the charging speed. The goal is to get EV charging time on par with a conventional fossil fuel fill-up. This in turn depends on high-power charging stations having more than 50 kW of charging power, which is more expensive.
Considering the importance of charging stations, the high-power level and the investment required, it is essential that EV charging system designers plan for safety, efficiency and reliability. (See an example EV charging station in Figure 1.)
Safety, efficiency and reliability
Until the recent advent of DC charging stations, the general public has not had access to power higher than the 120 V they see at home from their wall sockets. EV chargers must deliver from 400 to 1,000 V DC or better while minimizing the threat of electrical shock and other hazards.
It is crucial that DC fast charging systems have the most efficient power conversion possible. By minimizing power conversion losses, designers ensure the maximum amount of power delivered for charging the vehicle’s battery while reducing heat buildup.
To ensure an acceptable return on investment, EV charging equipment must operate dependably for at least 10 years, even in very harsh outdoor conditions.
The biggest threats to an EV charging station’s safety are electrical shock and overcurrent.
Electrical shock most frequently is the result of a ground fault. In turn, ground faults result from an unintended contact between an energized conductor and the ground or the equipment’s frame. The typical culprit is insulation breakdown. Additionally, dust and moisture can create unintended pathways for electricity. Electronics engineers must be diligent when designing high-powered chargers for the wet and dusty environments found outdoors.
To protect components from damaging faults, AC ground-fault protection is needed on the input side of the design. This will also protect consumers from electric shock in the event that 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.
Proper ground-fault protection is also required at the output side, so that when a consumer picks up a nozzle capable of 1,000 V, the handle or the frame is not energized. 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.
EV charging stations, by their nature, are connected to a power supply that has a high available fault current. Electrical faults, including those that start ground faults, can draw very destructive high current, subsequently damaging components, twisting bus bars, starting fires and even causing arc-flash incidents — a kind of explosion that could seriously injure anyone standing nearby.
Proper overcurrent protection results from selecting fuses based on three things: 1) their interrupting capacity; 2) their rating based on normal operating current; and 3) their time-current curve characteristics. Consider using “current-limiting fuses” because they operate quickly in the event of a high-value overcurrent, which limits peak let-through current.
Even moderate overcurrent conditions can overheat system components, resulting in damage to insulation and conductors, unless the overcurrent is interrupted quickly. However, many electronic components may experience extensive damage as they are susceptible to even low-value overcurrents.
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. This process naturally incurs power losses in the form of heat. The heat can create unique engineering challenges in an EV charging application.
That’s why advanced technology devices based on silicon carbide (SiC) and gallium nitride (GaN) technologies are utilized in power conversion; compared to regular silicon (Si) devices they provide ultra-fast switching along with lower power losses.
SiC metal oxide–semiconductor field-effect transistor (MOSFET) devices (like the one shown in Figure 2) are available that blend both fast switching speeds and high operating voltages, a combination that is typically not available with traditional power transistors.
To be useful in automotive charging applications, SiC MOSFETs must include:
- Operation at high junction temperatures
- Low gate resistance
- Low gate charge
- Low output capacitance
- Ultra-low on-resistance
Designers prefer devices that offer high power density and reduce the size and weight of filter components, thus reducing the cost and space requirements.
While consumer electronics like smartphones and laptops are engineered for a lifetime of three to five years, EV DC charging stations are expensive, so buyers need them to last for at least 10 years in order to get an adequate return on their investment. To put the investment into perspective: The semiconductor content alone ranges in value from $350 in an AC charger to more than $3,500 in a 350 kW charging system. To keep that investment working reliably for a longer period of time, it makes sense for electronics designers to also invest in proper circuit protection.
Like most electronics components, semiconductor devices are also sensitive to electrical threats and must be protected from overcurrent by fuses. These devices are typically fabricated from either silicon or silicon carbide and have low thermal withstand capacity.
While conventional fuses may be sufficient to protect most of these devices, specialized high-speed DC fuses are required to protect power semiconductor devices such as MOSFETs, insulated-gate bipolar transistors (IGBTs), diodes and thyristors used in power converters (inverters, rectifiers) incorporated in charging stations. Unlike traditional AC input fuses, these DC fuses are developed with a specific time-current characteristic so that they operate very quickly.
Overvoltage is another threat to sensitive semiconductor devices. 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 the sensitive electronics that make the charger work.
Different technologies are used to create a variety of circuit protection devices. While many types of devices may work, it is better to select a device with 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 — such as protection thyristors, gas discharge tubes (GDTs) and multi-layered varistors (MLVs), or combinations of suppression devices — are often specified.
When used to protect sensitive and highly valuable electronic circuits, the length of time a TVS diode or TVS diode array requires to begin its suppression function becomes extremely important. If the suppressor is slow-acting and a fast-rising transient appears on the system, then the voltage across the protected load can rise to a damaging level before suppression kicks in.
Reliability, efficiency and safety are all achievable in EV charging station designs so long as proper circuit protection is a forethought in the design process. Littelfuse offers a white paper on this topic that includes block diagrams and specific device recommendations.
Download Littelfuse’s applications guide, Supercharged Solutions for EV Charging Stations, from their website.