EV Charger Auxiliary Supply Considerations

EV charger installations are increasing exponentially

The uptake of electric vehicles (EVs) in all of their forms has really taken off. As many as 6.75 million units were sold globally in 2021, signifying a 108% increase over the sale of EVs in 2020 according to the ‘EV-Volumes’ database (Figure 1) [1].

The “drivers” accounting for this increase are rather obvious – enhanced environmental awareness, spiraling fuel prices, and CO2-reduction targets set by governments. In the UK, certain laws will come into force in 2022, which will require every newly-built home with associated parking to include a charging point.

At the same time, the proposed ban on the sale of new petrol-only and diesel-only vehicles has been advanced to 2035 from 2040 in several European countries. Public chargers are also being rolled out rapidly with more and more functionalities. For example, in Germany from June 2023, all of the new charging points must have a debit or credit card reader incorporated in them for easier accessibility.

Chargers can, therefore, range 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, having multiple processors and interfaces for control of power delivery and safety functionality as well as connectivity via ‘the cloud’ for secure reporting and billing.

EV chargers need auxiliary power

Every DC charger needs a range of auxiliary power rails. Although the main multi-kW power converter may be able to generate low-power DC rails as a side function, perhaps from a winding off of the PFC inductor, this is rarely sensible for various reasons. For instance, when unloaded, the DC charger’s main converter operates with poor efficiency and causes substantial losses. So, 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 their typical operating levels.

Further, having an independent supply makes the start-up and shut-down of the main converter much more secure and predictable. Having separate low-power AC/DC converters also allows isolated DC return lines to be generated for different parts of the system, which can help avoid ground loops, EMC issues, and safety concerns with accessible interfaces. Of course, auxiliary AC/DC converters can also be followed up by isolated or non-isolated DC/DC converters to generate any needed ‘Point of Load’ voltages with their specific regulation and noise-level requirements are fulfilled.

The tough charger environment

Auxiliary AC/DC and DC/DC converters in EV DC chargers must meet specific environmental challenges, and there is an expectation anyway that they will have a long life and be duly reliable. At a minimum, ‘industrial-grade’ parts are required, but there are specific standards to meet, for example, as in the case of EN 61851-23 ‘Electric vehicle conductive charging system, DC charging stations’.

The said standard covers many areas and references many other documents, but specifically it states, for example, that the supply to the EV charger must conform to the over-voltage category (OVC) III or IV. This in practice means that even most of the industrial-grade AC/DC converters would be unsuitable, being typically rated at category II for mains installations, after some voltage transient limiting device. The OVCs refer to the transients that may be present because of lightning strikes, for example, and are summarized in Figure 2 with the associated impulse voltages according to IEC 60664-1.

Surge protection devices (SPDs) are shown in Figure 2 (Class B, C, and D), and they allow the OVC to be reduced in severity, away from the input to a building distribution system towards the 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, effectively a spark gap, while a Class C SPD is characterized by an 8/20µs current wave and Class D characterized by a combination 1.2/50µs voltage waveform and 8/20µs current waveform.

Both Class C and D are typically metal oxide varistors (MOVs), and a Class D type will always have a Class C preceding it. MOVs have a limited lifetime, with their clamp voltage decreasing with each surge event until the normal operating voltage is approached and leakage current increases to the point of overheating and failure. For this reason, external MOVs rated as SPDs will normally have a visual indication of health and typically be in a DIN-Rail format. Some can also indicate health remotely as an option.

In an EV charger installation, especially a public one, if the environment is OVC IV, it can be expected that an SPD will be in place anyway to reduce it to OVC III and provide an AC rail for the main power converter. It is not guaranteed that another AC power rail is available with a further SPD for Class II, so any auxiliary AC/DC power supply will typically need to withstand Class III transients, precluding most commercial types that are available. There is also a relationship between the safety clearances required in any equipment for an over-voltage class, on the one hand, and altitude on the other. There is no correction up to 2000m but clearances must be increased by a progressively larger amount at altitude, for example, x1.48 at 5000m. This is sometimes neglected as a consideration, but there are eight capital cities in the world over 2000m elevation, which are certain to require EV charging points.

Safety standards for EV auxiliary supplies

The current version of EN 61851-23:2014 still references EN 60950-1 as a safety standard, although this standard became obsolete at the end of 2020 and was replaced by EN 62368-1, which would normally be acceptable in EV applications. However, the user would need to verify the exact specification required. For example, EN 61851-1 requires that safety isolating transformers should meet IEC 61558-1. EN 62368-1 refers to this standard as an option with some additions and despite its limitations. AC/DCs that already hold the IEC 61558-1 certification, therefore, are a safer bet. Power supplies with certification to IEC/EN 60335-1 can also be relevant for chargers with a maximum output of 120VDC, such as might be used for plug-in hybrids or e-scooters with 48V or 72V batteries.

The AC supply voltage for EV DC chargers will depend on the location, and it might be a domestic single-phase 115/230VAC, three-phase 400VAC, or 480VAC. Low-power auxiliary AC/DC converters in three-phase systems will be typically connected from phase to neutral, which will require operation from 277VAC nominal in 480VAC systems, although some low-power AC/DCs are available that can operate on the maximum phase-to-phase voltage in 480VAC ‘delta’ systems, up to 528VAC.

The physical environment for EV chargers

EN 61851-23 states that the environment for a DC EV charger is specified for minimum pollution, i.e., degree 3 for outdoor use and degree 2 for indoors, except in industrial areas where it must be degree 3 as well. Pollution degree 3 is defined as follows: ‘Conductive pollution or dry non-conductive pollution, which becomes conductive due to condensation, which is to be expected.’. In practice, this means that electronics must be coated or encapsulated or have greatly increased creepage distance to avoid malfunction and voltage breakdown if the surroundings are damp, dirty, or dusty – a typical environment in a garage or open-sided parking space.

Thermal ratings of EV charger electronics must also match the potentially severe environment with sub-zero temperatures and up to +60 degrees and more in outdoor installations in full sun. However, ‘industrial-grade’ ratings of AC/DC supplies of typically -40°C to +85°C ambient temperature are likely to be adequate.

Off-the-shelf parts are available

Despite the complexity of the requirements for low-power EV DC chargers, a range of off-the-shelf products from RECOM that fit many applications are available. These include 3W to 40W miniature encapsulated modules in the RAC series 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 for up to 528VAC input (Figure 3).

All of the modules are rated from -40°C to at least +80°C ambient operation and are available with OVC III rating either as the standard or as an option. Safety certification of all of these modules is comprehensive with IEC/EN 62368-1 being the minimum, with some of the parts even including IEC/EN 61558 or EN 60335-1 for household use and sometimes even medical IEC/EN 60601-1.

RECOM can also supply a comprehensive range of DC/DC converters suitable for gate drive power for the main inverter or isolated communication interfaces, isolated auxiliary rails, and non-isolated point-of-load converters. All of these converters have the ruggedness and quality that is suited to the demanding environment in which they are to be used.

Conclusion

EV DC charging is an emerging market with its particular technical requirements. Cost and size are also drivers, but RECOM has parts that match not only modular auxiliary AC/DC power supplies but also general-purpose DC/DC converters.

References

[1] https://www.ev-volumes.com/