【Introduction】With the popularity of electric vehicles, electric vehicle chargers have become so ubiquitous that there is now legislation requiring new residential and industrial buildings to have charging points. While main power conversion technology is an area of constant innovation, there is also a need for low power auxiliary AC/DC and DC/DC converters. This article discusses the specifications that these low power auxiliary converters need to meet.
Various forms of electric vehicles (EVs) have become increasingly popular. According to “EV-Volumes” data (Figure 1), global sales will reach 6.75 million vehicles in 2021, an increase of 108% over 2020 sales.
Figure 1: Global electric vehicle sales growth, data source www.EV-volumes.com
The “driving factors” behind this growth are obvious: increased environmental awareness, rising fuel prices and government-set CO2 reduction targets. In the UK, certain laws will come into force in 2022 that will require every new home with a parking space to have a charging point. Meanwhile, a proposed ban on the sale of new gasoline-only and diesel-only vehicles in several European countries has been brought forward from 2040 to 2035. Public charging stations are also being rolled out rapidly and with increasing functionality. From June 2023, for example, all new charging stations in Germany must be equipped with debit or credit card readers for ease of use.
Therefore, the complexity of the charger can range from simple on-board slow charging (household single-phase AC power supply) to 800V or higher ultra-high-speed DC fast charging (powered by industrial three-phase AC power), with multiple processors and interfaces to control Power delivery and security features, as well as security reporting and billing services via the “cloud”.
Electric Vehicle Chargers Need Auxiliary Power
Every DC charger requires auxiliary power. While a main converter of a few kW can generate low power auxiliary power, possibly from the windings of the PFC Inductor, doing so is usually not advisable for various reasons. For example, when there is no load, the low efficiency of the main power converter of the DC charger will cause a lot of losses. Therefore, if auxiliary power is required for standby, it is best to get it from a low power AC/DC converter and turn off the main converter at the same time to reduce losses. Low power AC/DC converters can exhibit high conversion efficiency when used as an auxiliary power supply. In addition, having an independent auxiliary power supply makes the startup and shutdown of the main converter safer and more reliable. Using a separate low-power AC/DC converter also provides isolated power to other functional circuits in the system, which helps avoid ground loops, EMC issues, and safety issues at the interface. Of course, an isolated or non-isolated DC/DC converter can be connected in series behind the auxiliary AC/DC converter, and adjusted to generate the working voltage required by each functional circuit and meet the noise level requirements.
Harsh environment where the charger is located
Auxiliary AC/DC and DC/DC converters for EV DC chargers must overcome specific environmental challenges, and they are generally expected to have a long lifetime and some reliability. Devices are required to be at least “industrial grade” and also need to meet specific standards, such as EN 61851-23 “Conductive charging systems for electric vehicles: DC electric vehicle charging stations”.
The standard covers many areas and references other documents, but specifically states that EV chargers must be powered by overvoltage category (OVC) III or IV. Most industrial-grade AC/DC converters on the market today are not compliant because early designs are usually designed with reference to Class II standards. OVC refers to transient overvoltages such as those caused by lightning strikes; the relevant impulse voltages according to IEC 60664-1 are summarized in Figure 2.
Figure 2: Overvoltage categories (OVC) for different standards
From the input of the building to the terminal equipment of the power distribution system, Surge Protective Devices (SPDs) are shown in Figure 2 (Class B, C and D), and they can dissipate/absorb the impulse energy of the surge. Unmarked Class A SPDs are part of a low voltage power distribution system, Class B are characterized by 10/350µs current waveforms and typically use gas discharge tubes. Class C SPDs are characterized by an 8/20µs current waveform, while Class D are characterized by a combination of a 1.2/50µs voltage waveform and an 8/20µs current waveform. Class C and D usually use metal oxide varistors (MOV), and there is always a Class C in front of Class D. MOVs have a limited lifetime, the clamping voltage decreases with each surge strike until it approaches normal operating voltage, and the leakage current continues to increase until it overheats and fails. For this reason, the MOV used by SPDs is usually in DIN rail format, so that its health status can be visualized.
When installing EV chargers (especially public ones), if the environment is OVC IV, we can expect to have SPD anyway to drop the rating to OVC III and provide AC power to the main power converter, but this is not guaranteed only OVC The AC/DC power supply of II can work safely and reliably, so any auxiliary AC/DC power supply usually needs to have the ability to withstand the transient voltage of OVC III, based on which most of the current commercial power supplies are not suitable. There is also a relationship between the safety clearance required for the overvoltage class of any device and the altitude above sea level. When the altitude is below 2000m, there is no need to consider the correction factor, but at higher altitudes, the electrical clearance must be gradually increased. For example, at 5000m, a multiplication factor of x1.48 is required. Sometimes this important consideration is overlooked, but there are 8 countries in the world with capitals above 2000 m above sea level, and there will definitely be EV charging stations in the future.
Safety Standards for Auxiliary Power Supply of Electric Vehicles
The current version of EN 61851- 23:14 still cites EN 60950-1 as the safety standard, although that standard became obsolete at the end of 2020 and was replaced by EN 62368-1, which is generally acceptable in EV applications of. However, users need to verify the exact specifications required. For example, EN 61851-1 requires safety isolation transformers to comply with IEC 61558-1. EN 62368-1 includes the standard as an additional option, although it has limitations. Therefore, an AC/DC that already has IEC 61558-1 certification is a safer choice. The IEC/EN 60335-1 certified power supply is also suitable for chargers with a maximum output of 120VDC, such as can be used in plug-in hybrid vehicles or electric scooters with 48V or 72V batteries.
The AC power supply voltage of the EV DC charger will be determined by the location of the application. It may be single-phase 115/230VAC for household use, three-phase 400VAC or 480VAC for industrial/commercial use. The low-power auxiliary AC/DC converter for three-phase power supply is usually connected to the phase line and the neutral line, and needs to work at a nominal voltage of 277VAC in a 480VAC system, while some low-power AC/DC can directly operate at a maximum voltage of 480VAC delta system Phase-to-phase voltage operation up to 528VAC.
The Physical Environment of an EV Charger
EN 61851-23 stipulates that the environmental requirements of EV DC chargers require a minimum pollution level, that is, PD3 for outdoor use and PD2 for indoor use, but it must also be PD3 if it is in an industrial area. PD3 pollution is defined as follows: “Conductive pollution, or dry non-conductive pollution that becomes conductive due to condensation.” In fact, this means that Electronic equipment must be coated or encapsulated, or significantly increase creepage distance to avoid malfunctions and voltage breakdowns in wet, dirty or dusty environments, which are the most common environments in garages or open parking spaces.
The thermal rating of the EV charger device must also match the potentially harsh environment, such as sub-zero temperatures and up to 60+ degrees outdoors in sunlight. Typically an “industrial grade” AC/DC power supply with an ambient temperature of -40°C to +85°C is sufficient.
Provide ready-made modules
Although the requirements for low power EV DC chargers are complex, RECOM offers a range of off-the-shelf products suitable for many applications. There are RAC series AC/DC modules in 3W to 40W miniature package modules for highly polluted environments. In addition to the “standard” input range of 85–264VAC, some models are rated at 305VAC at a nominal 277VAC, and the RAC05-K/480 is rated at up to 528VAC (Figure 3). All AC/DC modules have an ambient operating temperature range of -40°C to at least +80°C and are OVC III rated or optional. All AC/DC modules provide full safety certification, IEC/EN 62368-1 is the minimum requirement, some modules are certified to IEC/EN 61558 or EN 60335-1 for household use, and some are even certified to IEC/EN 60601-1 for medical treatment.
Figure 3: The RECOM RAC05-K/480 is rated at 528VAC input and complies with OVC Class III
RECOM also offers a complete range of DC/DC converters for main inverter gate drive power, isolated communication interfaces, isolated auxiliary power supplies and non-isolated converters. These converters are rugged and high quality to work in harsh environments.
EV DC charging is an emerging market with special technical requirements. Cost and size are also driving factors, but RECOM’s products can be used not only with modular AC/DC auxiliary power supplies, but also with general purpose DC/DC converters.
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