Power Supplies for EV Charging Stations

­EVs can now achieve similar mileage as gas-powered vehicles, but it still takes more time to charge an EV than to refuel a gas vehicle. Charge time is approximately equal to the amount of energy delivered to the battery divided by the charging power; therefore, it is desirable to use as much power as possible to charge the battery. However, within the vehicle, increasing the power of the on-board charger (OBC) adds weight and cost. Increasing the battery capacity also increases weight and cost. The balancing of these trade-offs results in a variety of OBC power levels and battery sizes.

Outside of the EV, the power source must have the capacity to support the desired charging power, but that is not always the case. To protect the EV and the power source from overload and other issues, electric vehicle supply equipment (EVSE), commonly referred to as charging stations, are placed between the power source and the EV. This equipment provides power to and communicates with the EV to determine the source’s power rating and charge status information so that the OBC does not overload it and charging proceeds safely. The EVSE also needs its own dedicated power supply to operate.

EVSE CHARGE LEVELS

The appropriate EVSE power level for any location depends on ac power restrictions, cost, size, and charge time, which impact power supply selection.

EVSEs have been divided into multiple levels by standards organizations based on standard voltages and electrical connections available. While standards exist with a varying number of levels (SAE 1772 contains four) EVSEs have generally been split into three levels. Levels 1 and 2 are for slow and quick ac charging, while Level 3 is for dc fast charging.

EVSE Level 1

Level 1 EVSEs (Table 1) are rated up to 1.92 kW (120 V ac * 16 A) and is based on the ratings of a U.S. household 120 V wall outlet. For a plug-in hybrid vehicle (PHEV), with a battery capacity around 20 kWh, this means a charge time of over 10 hours. For longer-range EVs, with batteries around 100 kWh, this means 52 hours to fully recharge a battery.

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Table 1: EVSE charging levels

Another way to look at it is how many miles of range an hour of charging adds to the battery (equation 1).

If 100 kWh gets you 373 miles (600km), then 1 kWh provides about 3.7 miles of range and an hour charge at 1.92 kW adds 7.1 miles.

EVSE Level 2

The EVSE Level 2 is rated up to 19.2 kW (240 V Ac * 80A). This level can operate at reduced power off a household 240 V outlet or with a dedicated installation and equipment can achieve the full power range. This level tends to be larger and more expensive than level 1 and is often installed on the wall in a garage or as a charging station in a parking lot. The maximum charge rate of Level 2 is approximately 71 miles added per hour.

EVSE Level 3

EVSE Level 3 differs significantly from Level 1 and 2 EVSE because the charger inside the EV is bypassed and an off-board charger inside the EVSE performs the ac/dc conversion and sends dc power to the BMS directly (Figure 2). This raises the size and expense of the equipment but enables very high-powered charging.

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Figure 2: Dc charging diagram

Being located off-board means that the size and weight of the charger are notlimiting factors, and because the OBC is bypassed, charging is not limited by the size of the OBC. The size and cost (>$100k) prohibit private use and Level 3 EVSE will be the least common of the three, found at strategic locations where fast charging is required, such as along highways.

Level 3 has a maximum charge power of 400 kW, which can increase the charge rate of our example to 1,480 miles added per hour.

Powering the EVSE

There are four main blocks that need dc power:

●      MCU

●      Contactor

●      Control Pilot

●      Display/Indicators

An ac-dc power supply converts the ac source to dc to power the internal devices. For electro-magnetic compliance, a filter will be needed between the power supply and the source. This may be internal or external to the power supply and include surge protection. 12 V is a common choice for the output voltage and main power rail. This rail can power the contactor, the display, and one end of the control pilot (CP). The CP is a communication signal that uses PWM to send and receive current and status information from the BMS.

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Figure 3: Typical level 1 Ac charging block diagram

Dc to dc conversion is needed to create the 5 V and -12 V rails for the MCU and CP. The 5 V rail could be generated by an LDO or, for a more efficient option, a switching regulator, such as the VXO7805-500. A switching converter is required for the -12V rail as a linear regulator cannot convert from positive to negative voltages.

Charging for EVSE level 3 is different from levels 1 and 2 because the charger is located inside the EVSE. Figure 4 shows a typical level 3 block diagram.

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Figure 4: Typical level 3 EVSE block diagram

The level 3 EVSE in Figure 4 uses 24 V instead of 12 V for the main power rail. This rail powers an LCD display, security monitoring, and the contactor control. There is no control pilot in this case and the BMS communicates directly with the EVSE over CAN bus. This removes the need for a -12 V rail and the associated regulator. There is still a regulator used to power the MCU. A second power supply is used to power the BMS with 12 V. Both power supplies operate from a single phase of the three-phase input to the EVSE.

Power Supply Requirements

Power supplies intended for residential locations should meet class B electromagnetic emissions limits to prevent interference with other devices. Both level 1 and 2 EVSE may be found in homes and will need to meet class B limits. Non-residential locations typically need to meet the less stringent Class A limits. In these cases, significant space can be saved by using smaller filters.   

To meet the full power range of Level 2, a dedicated hardwired installation will be needed. These installations place the connection much closer to the utility source, in so-called over-voltage category III (OVC III) locations, and lead to higher transient voltage potential than category II locations, such as a typical residential outlet. Level 3 EVSE will also have dedicated grid connections. Power supplies in these applications should have increased surge protection and be designed for use in OVC III locations. Standard power supplies are typically designed for OVC II locations.

Level 2 and 3 EVSEs that are to be located outdoors need a wide temperature range. External temperatures can easily exceed 40°C or fall below -20°C. Power supplies with extended temperature ranges of -30°C to 70°C, such as the VGS-100D, help ensure safe operation in all seasons. Over-temperature protection provides an extra level of protection for today’s unpredictable climate. Outdoor applications can also be subject to more dust and debris. Conformal coating the PCB or fully encapsulating the power supply provides protection from this.                             

To achieve the highest output power, Level 3 EVSE are often powered from three phase voltages up to 480 V. The auxiliary power supplies for these applications must be rated up to a 305 V ac input to handle the 277 V ac single phase line voltages.              

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Table 2: Power supply features important to EVSE levels

CUI EVSE Power Solutions

CUI’s PBO-C series provides a compact, wide input (85~305 V ac), ac-dc conversion.EMI filters are large and bulky, and rather than containing a general-purpose filter that may be oversized or poorly placed, the circuit can be optimized to suit emission and form factor goals.

To protect against foreign objects and debris in outdoor locations, encapsulated power supplies, such as the PSK-D series, are a good choice.This series is fully encapsulated, has a wide temperature range, and is rated for OVC III use. For higher power needs the metal cased VGS-D series has solutions up to 320 W with conformal coating and DIN rail mounting options available.

CUI, Inc.