EV Charger Auxiliary Power Supply & Converter Considerations

Jun 3, 2022
Electric vehicle charging connectors
EV chargers are becoming ubiquitous, to the point that legislation now requires new domestic and industrial buildings to include charging points. While the main power conversion technology is an area of intense innovation, there is also a need for low-power auxiliary AC/DC and DC/DC converters. This article examines the specifications these low-power auxiliary converters must meet.

Table of Contents

EV Charger Auxiliary Power Supply & Converter Considerations

The uptake of electric vehicles (EVs) in all forms has accelerated rapidly. As many as 6.75 million units were sold globally in 2021, representing a 108% increase over 2020 according to the ‘EV-Volumes’ database (Figure 1).

Global EV Sales Growth 2012-2021

Fig 1: Global growth in EV sales, with permission, https://www.ev-volumes.com/


The factors driving this increase are clear – enhanced environmental awareness, rising fuel prices, and CO2-reduction targets set by governments. In the UK, laws coming into force in 2022 will require every newly built home with associated parking to include a charging point. Meanwhile, the proposed ban on the sale of new petrol-only and diesel-only vehicles has been advanced from 2040 to 2035 in several European countries. Public chargers are also being rolled out rapidly with increasing functionality. For example, in Germany from June 2023, all new charging points must incorporate a debit or credit card reader for easier accessibility.

Chargers can vary in complexity from a simple, slow, domestic single-phase AC source for on-board charging to ultra-fast DC charging at 800V or higher, fed from utility three-phase AC, with multiple processors and interfaces for controlling power delivery, safety functionality, and connectivity via the cloud for secure reporting and billing.

EV Chargers Require Auxiliary Power

Every DC charger requires a range of auxiliary power rails. Although the main multi-kW power converter may generate low-power DC rails as a side function, perhaps from a winding off the PFC inductor, this is rarely practical for several reasons. For instance, when unloaded, the DC charger’s main converter operates inefficiently and causes substantial losses. Therefore, if ‘housekeeping’ power is needed on standby, it is best derived from low-power AC/DC converters, with the main converter disabled. Low-power AC/DC converters are designed for high efficiency at typical operating levels.

Furthermore, having an independent supply makes the start-up and shut-down of the main converter more secure and predictable. Separate low-power AC/DC converters also allow isolated DC return lines for different parts of the system, helping to avoid ground loops, EMC issues, and safety concerns with accessible interfaces. Auxiliary AC/DC converters can also be followed by isolated or non-isolated DC/DC converters to generate any required point-of-load voltages while meeting specific regulation and noise-level requirements.

Overvoltage Category, Surge Protection and Environmental Requirements for EV Chargers

Auxiliary AC/DC and DC/DC converters in EV DC chargers must withstand specific environmental challenges and are expected to have long life and high reliability. At a minimum, industrial-grade parts are required, but specific standards must also be met, for example EN 61851-23 ‘Electric vehicle conductive charging system, DC charging stations’.

This standard covers many areas and references other documents, specifying, for example, that the supply to the EV charger must conform to overvoltage category (OVC) III or IV. In practice, this means that most industrial-grade AC/DC converters are unsuitable, typically rated at category II for mains installations, after some voltage transient limiting device. The OVCs refer to transients that may occur due to lightning strikes, for example, summarized in Figure 2 with associated impulse voltages according to IEC 60664-1.
IEC surge protection device standards diagram
Fig 2: OVCs according to different standards
Surge protection devices (SPDs) are shown in Figure 2 (Class B, C, and D), allowing the OVC to be reduced in severity from the input to a building distribution system toward end equipment. Class A SPDs, not shown, are part of the overground LV distribution system. A Class B SPD, characterized by a 10/350µs current waveform, is typically a gas discharge tube or spark gap, while a Class C SPD is characterized by an 8/20µs current waveform and Class D by a combination of 1.2/50µs voltage waveform and 8/20µs current waveform.

Classes C and D are typically metal oxide varistors (MOVs), with a Class D type always preceded by a Class C. MOVs have a limited lifetime, with clamp voltage decreasing with each surge until normal operating voltage is approached and leakage current rises to the point of overheating and failure. External MOVs rated as SPDs typically include a visual health indication and are often in a DIN rail format. Some offer remote health monitoring as an option.

In public EV charger installations, especially, if the environment is OVC IV, an SPD will likely be in place to reduce it to OVC III and provide an AC rail for the main power converter. Another AC power rail with a further SPD for Class II is not guaranteed, so any auxiliary AC/DC power supply must typically withstand Class III transients, precluding most commercial types.

Safety clearances in equipment for an overvoltage class also relate to altitude. No correction is needed up to 2000m, but clearances must increase progressively at higher elevations, for example x1.48 at 5000m. This is sometimes overlooked, but eight capital cities exceed 2000m elevation and require EV charging points.

Safety Standards for EV Charger Auxiliary Power Supplies

The current EN 61851-23:2014 still references EN 60950-1 as a safety standard, although this became obsolete at the end of 2020 and was replaced by EN 62368-1, which is normally acceptable in EV applications. Users should verify the exact specification required. For example, EN 61851-1 requires safety isolating transformers to meet IEC 61558-1. EN 62368-1 refers to this standard as an option with some additions. AC/DC converters holding IEC 61558-1 certification are therefore a safer choice. Power supplies certified to IEC/EN 60335-1 may also be relevant for chargers with a maximum output of 120VDC, such as for plug-in hybrids or e-scooters with 48V or 72V batteries.

The AC supply voltage for EV DC chargers depends on the location, ranging from domestic single-phase 115/230VAC to three-phase 400VAC or 480VAC. Low-power auxiliary AC/DC converters in three-phase systems are typically connected phase-to-neutral, requiring operation from 277VAC nominal in 480VAC systems, although some low-power AC/DCs can operate at the maximum phase-to-phase voltage in 480VAC delta systems, up to 528VAC.

Environmental and Operating Conditions for EV Chargers

EN 61851-23 specifies the environment for a DC EV charger in terms of minimum pollution: degree 3 for outdoor use and degree 2 for indoors, except in industrial areas where it must also be degree 3. Pollution degree 3 is defined as ‘Conductive pollution or dry non-conductive pollution that becomes conductive due to condensation.’ In practice, electronics must be coated, encapsulated, or have increased creepage distance to avoid malfunction or voltage breakdown in damp, dirty, or dusty conditions – typical in garages or open-sided parking spaces.

Thermal ratings of EV charger electronics must also match potentially severe conditions, from sub-zero temperatures to +60°C or higher in outdoor installations in full sun. Industrial-grade AC/DC supplies rated for -40°C to +85°C ambient are generally adequate.

Off-the-shelf parts are available

RECOM RAC05-K/480 with dimensions
Fig 3: The RAC05-K/480 rated for 528VAC input with OVC III rating
Despite the complexity of low-power EV DC charger requirements, a range of off-the-shelf products from RECOM fits many applications. These include 3W to 40W miniature encapsulated RAC series modules suitable for high pollution environments. In addition to the standard input range of 85–264VAC, some variants are rated to 305VAC for 277VAC nominal, and the RAC05-K/480 is rated up to 528VAC input (Figure 3).

All modules are rated from -40°C to at least +80°C ambient and are available with OVC III rating as standard or option. Safety certification is comprehensive, with IEC/EN 62368-1 minimum, and some modules also include IEC/EN 61558 or EN 60335-1 for household use and sometimes IEC/EN 60601-1 for medical applications.

RECOM also supplies a comprehensive range of DC/DC converters suitable for gate drive power for the main inverter, isolated communication interfaces, isolated auxiliary rails, and non-isolated point-of-load converters. All converters are rugged and reliable for demanding EV charger environments.

Conclusion

EV DC charging is an emerging market with specific technical requirements. Cost and size are drivers, but RECOM offers parts that meet needs for modular auxiliary AC/DC power supplies as well as general-purpose DC/DC converters.
Applications
  Series
1 RECOM | RAC05-K/480 Series | AC/DC, THT, 5W, Single Output
Focus
  • Ultra-wide input range 85-528VAC
  • OVC III input rating without additional fuses
  • Operating temperature range: -40°C to +80°C
  • Overvoltage and overcurrent protected