Industry Article

EV Charging: Auxiliary Power Solutions

April 26, 2022 by Ron Stull, CUI Inc

In the rapidly expanding EV market, engineers must have highly reliable, rugged auxiliary power supplies that meet the needs of essential service equipment. In this article, learn more about auxiliary power solutions.

According to data compiled by Statista, the global market for electric vehicles in 2026 is expected to be four times larger than it was in 2020. As a result, engineers are striving to achieve better charging speeds, longer run times, and more environmentally friendly solutions for the batteries that power EVs. 

However, charging the batteries is not the only challenge these engineers are facing. There are auxiliary circuits responsible for supporting critical service equipment that must also be powered. These auxiliary circuits, which may include elements such as displays and payment systems, face their own set of constraints that include space, voltage range, operating temperature, and circuit protection. Reliable and rugged auxiliary power solutions are necessary to meet the demands of this rapidly growing EV market. 


Service equipment and auxiliary circuitry also need power when charging an electric vehicle. Image courtesy of Shutterstock


The EV Battery Charging Process

AC/DC Stage

It is essential to review the overall EV battery charging process to understand the need for auxiliary power solutions. The first EV battery charging process is the AC/DC stage, where power from the AC source (single or polyphase) is converted to high voltage DC during this stage. 

The AC/DC stage requires PFC (Power Factor Correction), and both PFC and rectification functions are typically combined into a single high-frequency switching converter. In addition, the AC/DC converter may require EMI filtering and surge protection. Finally, the Battery Management Unit is also crucial and controls the charging process to protect, monitor, and balance the battery cells.

DC/DC Stage

Because the battery requires specific voltage and current charge profiles, a DC/DC stage is necessary. The DC/DC converter provides constant current or constant voltage to the battery management system.

There are also two power systems within the general EV charging application: 

  • Primary system - converts power to charge the battery
  • Auxiliary system - powers service equipment such as displays, relays, and payment systems. And the design of the auxiliary system is just as important as that of the primary system.

However, before diving into the specifics of chargers for auxiliary systems, a short discussion of how the chargers for the primary systems are classified is in order.


Onboard and Offboard Chargers for EVs

Primary system chargers can be categorized as either onboard or offboard. With an onboard charger (or OBC), all phases of battery charging take place onboard the vehicle. Therefore, these chargers must be compact, light, and cost-effective while supporting the necessary charging speeds and vehicle range. Note that OBCs can vary in charging speed and power level. In addition, some are bi-directional to support reverse power transfer from vehicle-to-grid (V2G) or vehicle-to-vehicle (V2V).

Offboard chargers have all the elements of an on-board charger (high power AC/DC + DC/DC) plus those of the EV service equipment (low power AC/DC, auxiliary equipment), all of which are located outside of the vehicle. For off-board chargers, an AC/DC or DC/DC power supply is used to convert the mains or high voltage DC bus on the converter primary to the low voltage needed by the control circuitry. 

Off-board chargers are not subject to the size and weight constraints of OBCs, but this also makes them capable of much higher power levels. However, the high cost to produce, install and maintain off-board DC chargers, combined with the fact that they are not for home use, limits their proliferation.

To further understand the relationship between primary system power needs and how they relate to auxiliary power systems, note that there are three commonly used charging levels associated with EVs:

  • Level 1 AC OBC - AC (120 V) Up to 1.9 kW, which can be charged from a standard 120 V outlet in a home and provides 3 to 5 miles of range per hour of charging
  • Level 2 AC OBC - AC (208 - 240 V) Up to 19.2 kW, which provides 12 to 80 miles of range per hour of charging
  • Level 3 DC Off-board - DC (600-1000 V) Up to 400 kW, which provides 180 to 1200 miles of range per hour (3 to 20 miles per minute) and is also known as DC fast charging

Each of these charging levels and the OBC or offboard charger associated with them requires auxiliary circuitry to power service equipment that may be specific to the charging level provided.


Service Equipment and Auxiliary Power Needs

Each charging level just discussed has key elements of electronics and circuitry to enable their practical use. As an example, consider level 1 AC, the simplest charging system. As illustrated in Figure 2, additional power is also needed for an MCU (Microcontroller Unit), control pilot, and indicator LED.


Figure 1. Illustration of the auxiliary power needs for a level 1 AC charging unit.  Image courtesy of CUI


Level 2, shown in Figure 3, is more complex and includes an LCD display and electronic lock that will also require auxiliary power. A Level 3 DC charger also includes service equipment such as security monitoring, LCD, control electronics, and measurement.


Figure 2. Illustration of the auxiliary power needs for a level 2 AC charging unit. Image courtesy of CUI


CUI Auxiliary Power Solutions for EVs

Finding a reliable, robust solution for powering auxiliary circuits for EV chargers can be challenging. For example, higher charging levels usually require three phase 480 V which necessitates a wide input range of 85 to 305 VAC, requires circuit protections, and options that can operate over a significant temperature range.

DC/DC Conversions

In off board dc charging applications, the CUI AE40-UW Series meets these needs. These DC/DC converters can be used to convert the high voltage DC created by the high power AC/DC used for charging into the low voltage needed for auxiliary circuits. These DC/DC converters support high voltage inputs up to 1500 VDC and outputs from 12 to 24 VDC.

Economical AC/DC conversion

Consider the CUI PBO-5C product line of low-power (3.3 to 5.1 W) AC/DC chargers. This product line consists of small, open frame board mount AC/DC power supplies with the wide input voltage range needed for EV applications, ranging from 85 to 305 VAC, and both overcurrent and short circuit protection.

Compact Solutions

For situations that are highly space-constrained and may be subject to significant environmental temperature changes, a good option would be the PSK-30D series of compact board mount encapsulated AC/DC power supplies. They support the same wide input voltage range as the PBO-5C but offer 30 W of power and include overcurrent, overvoltage, and short circuit protection. They have a wide operating temperature range of -40 to 85°C. 

Feature Rich Solutions

When an auxiliary charging solution requires a higher power output in the range of 150 to 153 W and would benefit from active PFC and output trim control, consider the CUI VGS-150D product series. These rugged AC/DC power supplies support input voltages from 85 to 305 VAC and outputs from 12 to 48 VDC. For protection, the CUI VGS-150D chassis mount power supplies provide additional protections, including input overvoltage category III design and input surge resilience (300 VAC, 5 seconds).


Open-frame design of the CUI VGS-150D supports convection cooling. Image courtesy of CUI



The power requirements for EVs go beyond what is needed to charge the batteries—auxiliary circuits for service equipment also need power and have voltage requirements different from those of the battery. And to remain competitive in the rapidly expanding EV market, engineers must have highly reliable, rugged auxiliary power supplies that meet the needs of essential service equipment. Whether the need is for an AC/DC converter that can handle temperature extremes or a DC/DC converter that is compatible with off-board chargers, CUI offers viable solutions and has team members with the knowledge and experience to help you find the right product to meet your needs.


Feature image used courtesy of Ernest Ojeh/Unsplash