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Auxiliary Power Solution For EV Chargers

Dec 21, 2022      View: 299

Traditional fuel vehicles have shifted to cleaner, more efficient electric vehicles (EVs). While much work remains to be done on the vehicles themselves, providing a sufficient number of charging stations worldwide is a challenge for industry and governments. Mileage anxiety - the fear of being stranded - and the time it takes to charge an EV are often cited as reasons for lower-than-expected EV sales. Addressing this challenge will significantly benefit automakers, the driving public, and the environment.

Some of the first cars built were electric. However, the situation changed when the electric starter was invented. This pushed the internal combustion engine (ICE) car to the forefront, while the electric car became history. During the oil crisis of the early 1970s and California's zero-emissions mandate in the 1990s, there was renewed interest in electric vehicles, but they did not become mainstream.

In recent years, new battery technologies have emerged. Combined with widespread environmental concerns and a customer base seeking cleaner alternatives to fossil fuels, electric vehicles are finally becoming a viable technology.

The market is maturing rapidly but is still in its early stages, and there are various vehicle types to choose from. Hybrid vehicles contain an internal combustion engine and an electric motor, and they can be designed with either or both for propulsion. Pure battery types are all-electric and rely on the charge stored in the battery to drive. The market can be divided into two segments - hybrids that use ICE to charge their batteries (HEVs, MHEVs) and those that can be plugged in and charged (BEVs, PHEVs).

As battery technology improves and electric vehicles become more efficient, their range is increasing and beginning to approach that of internal combustion engine vehicles. Many of the most common journeys, such as the daily commute, can be completed without recharging. Nonetheless, the range anxiety associated with long EV trips remains a barrier preventing many customers from adopting the new technology. When ICE vehicles run low on fuel, it is simple and quick to refuel at a traditional gas station. On the other hand, charging an EV's battery can take hours, depending on various factors such as the state of charge, battery capacity, charger type, and more. Charging for hours on the go is impractical, so EVs are charged more frequently at their "destination."

EV Charger Overview and Market

The EV charging market is driven by sales of EVs themselves and, more specifically, by sales of pluggable charging types (i.e., BEVs and PHEVs). According to a recent report by McKinsey & Company, the approximately 8 million BEVs and PHEVs on our roads will grow significantly to approximately 120 million by 2030.

By 2030, there will be 18 million BEVs / PHEVs and about 13 million chargers in the US, with one charger for every 1.38 vehicles. In the EU, with 15 million chargers for 29 million vehicles - one charger for every 1.87 vehicles - the quality of service for EV drivers will be slightly worse - although more compact geography may compensate for this to some extent.

Most charging will be done at home in the early stages of electric vehicle development, especially in the U.S. and Europe. While the future will depend on many factors, over time, there will be a gradual shift to charging EVs at work, alongside highways, and in other public places.

The type of charger deployed will depend on how the technology evolves, where it is charged, and the power available at that location. While there is much discussion of DC-driven "fast chargers" that can charge vehicles in minutes, these are only feasible in some locations and will remain a minority - particularly in the U.S. - due to high installation and maintenance costs and high power and voltage requirements.

AC vs. DC Charging

Given the different charging locations and power supply methods, it is not surprising that different types of chargers are deployed. The primary description will be determined by the type of power provided to the vehicle and the power that can be delivered.

Several standards have emerged within the industry that defines different levels of charging stations. A common standard is SAE J1772, published by SAE International, which defines four charger levels.

  • AC Level 1 designates the ability to deliver up to 16 A or 1.9 kW at 120 V AC
  • AC Class 2 designates 208 to 240 V AC voltage stations up to 80 A or 19.2 kW
  • DC level 1 designates DC voltages up to 1000 V and 80 A or 80 kW
  • DC Class 2 designates DC voltages up to 1000 V and 400 A or 400 kW

DC Levels 1 and 2 are collectively called Level 3 or DC Fast Chargers. Other standards exist that define similar but different voltage and power levels.

Batteries can only be charged by DC, while the grid is always AC, so at some point in the power train, a conversion (rectification) is required. This conversion is done within the charging port for higher-power DC chargers, which means that DC power is supplied to the vehicle. This is typically fed directly into the battery management system (BMS), which then manages and monitors the charging of the battery.

There is no rectification inside the charging port when AC is charging, so AC power is supplied to the vehicle. The onboard charger (OBC) then rectifies that voltage and changes the voltage level to fit the BMS/battery string if necessary.

While AC charging posts are simpler and cheaper, the downside is that they require OBCs, which add weight to the vehicle and thus shorten its range. Finding the right space for an OBC can be a challenge due to the very tight space in modern vehicles, and in all cases, the power density must be as high as possible. This can lead to operation at elevated temperatures, which stresses the components. Because the OBC (by definition) goes anywhere with the vehicle, the unit is subject to shock and vibration on every trip.

In theory, DC charging stations do eliminate the need for OBCs. However, at least for now, most vehicles are equipped with OBCs and have bypass capability, so they do not unnecessarily rectify DC. Including an OBC is useful if the only available charging station in the range is an AC stack.

Considering the amount of energy stored in vehicles that may be parked at home or work most of the time, energy companies are realizing the opportunity to use this reserve power as a way to load balance the grid and help reduce additional demand generation capacity. During peak demand periods/high electricity costs, vehicles can drain their batteries to power homes or contribute to the grid. When electricity prices fall, usually overnight, vehicles recharge their batteries and are ready for the morning commute.

In the early stages, using the energy stored in the vehicle's battery requires a bi-directional charger/inverter, which also allows the power to be returned to the grid.

Inside the charging pile

While the high power path between the grid and the vehicle is the primary power conversion required within AC and DC charging posts, these complex devices include many control and protection circuits that require dedicated power solutions.

In essence, AC and DC charging systems are very similar, with the main difference being the location of the AC to DC conversion. AC chargers have a simple straight-through contactor, and the conversion takes place in the vehicle. In the case of DC charging, the charging post has a large AC-DC converter built into it.

The simplest charging post is the Level 1 ac charging unit, which consists mainly of a large relay to switch the charging power to the OBC on the vehicle. In the case of the power rail, the primary rail is 12 V dc, usually from a miniature board-mounted AC/DC with external EMC circuitry for a more compact design.

The control pilot that communicates the state of charge and monitors the protected ground connection requires +/-12 V DC power. The positive rail comes from the AC/DC unit, and the negative rail can be generated with a small PCB-mounted DC-DC converter.

To power the microcontroller, the 12 V supply rail must be stepped down to a lower voltage - shown here as 5 V dc, but it can be lower depending on the device selected. Voltage for the MCU can be generated in several ways. Since isolation is not required, a small low dropout regulator (shown here) can be used, or again a simple "step-down" converter (non-isolated point-of-load - NiPOL) can be used as a more efficient option.

Level 2 AC charging units tend to be more complex than Level 1 units, with additional features and functions requiring more auxiliary power. However, there is typically more room to work in a Level 2 charger, so the design can be simplified by using ac/dc with internal EMI filters.

Downstream of the ac-dc, the solution is much the same as before. A dc-dc generates a negative rail for controlling the conduction, but this time we show a dc-dc converter for the 5 V rail instead of an LDO, although both can be used.

When moving to a dc-dc charger, the architecture changes. Additional functions such as safety monitoring, control electronics, LCDs and measurements, and the need to power BMS units in the vehicle require more power and rails.

Three-phase AC power, typically 480 V AC, is supplied to the charger. The 150 W ac/dc supplying the auxiliary rails must have an extra wide input so that they can use 277 V ac single phase. Power factor correction (PFC) is required at this power level and is included in every AC-DC power supply. In the case of Figure 8, the main auxiliary rail is 24 V and supplies power to the control, monitoring, and display circuits. A second AC/DC power supply provides a 12 V power rail that feeds directly to the BMS unit via a connector.

Similarly, the MCU requires a 5 V power supply generated directly from the 24 V power rail via a 5 V DC-DC converter.

In addition to providing the correct voltage, there are several other factors that affect the power solution used. High ambient is to be expected in these densely packaged high-power systems, especially when operating in hot countries/regions. However, in the winter months in colder countries, the power solution may be required to start up correctly from sub-zero temperatures if the charging station is dormant.

EMI performance is important, is a known quantity, and is repeatable when using off-the-shelf modules. Especially for Class 3 DC chargers, surge performance is an important consideration, which may require using a power supply with higher surge immunity or adding protection circuitry to the power system.

These examples show a typical arrangement of charging posts in each of the three tiers. In actual applications, there will be differences and may have more or fewer characteristics. However, the principles for designing and selecting modular power supplies and converters for any charging post are the same.

Despite competing standards, EV charging posts are growing rapidly, and power delivery is increasing so that EV batteries can eventually be charged in large quantities in just a few minutes. There is a wide range of power requirements in these power systems, all of which can be addressed using off-the-shelf modules.

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