Next Generation PCB-mounted relays in EV charging systems
2021 is shaping up to be the year in which adoption of the Electric Vehicle takes off. First half sales in the key markets of China, the USA and Europe, accounted for 26% of all new vehicle sales, globally and, in the UK alone, recent figures indicate that Battery Electric Vehicles, (BEVs) accounted for 15.2% of the market in September. Initial consumer reticence over price and range have been partially addressed by advances in battery technology and increased choice as the world’s auto-manufacturers bring more models to market. Other factors such as the global pandemic, increasing concerns over climate change and, more recently, fuel shortages in some markets, have undoubtedly also contributed to the EV’s growth.
Figure 1: Global EV sales have experienced strong growth in 2021
Source: Rho Motion
As Electric Vehicle, (EV) popularity continues to grow, the barrier to their continued adoption has shifted from range to charging. Although charging either at home or the office covers around 80% of all EV journeys, significant concerns remain for longer journeys, over both availability of charging stations and charging times. The ongoing viability of EVs demands that charging station numbers match that of the traditional filling station and there is therefore a strong industry focus on EV charging.
The EV charging station market
Reflecting this focus, the global market for charging stations - or electric vehicle supply equipment, (EVSE) – is projected to grow from its 2020 value of 2,115 million units to over 30,7 million by 2027 – a CAGR of 46.6%
As with any emerging market, a number of alternative charging technologies have emerged as various manufacturers have developed their own proprietary systems. Japanese companies, including Nissan and Mitsubishi have adopted the CHAdeMO system, which allows bidirectional charging, enabling EV owners to sell power back to utility grids. In Europe, BMW, Daimler-Benz, Ford, and the Volkswagen group are establishing a multi-vendor, multi-technology standard by backing the Combined Charging System, (CCS). Meanwhile, Tesla, has recently announced that its proprietary supercharging network, with 25,000 charging points globally, (as of May 2021), will be opened to EVs from other manufacturers.
The EV charging station
Current EV charging stations use either Alternating Current, (AC), or Direct Current, (DC) to deliver charge to the vehicle. Since batteries require to be charged by DC current, the difference between the two methods of charging is based upon the location of the rectification. Most EVs will accept AC power and rectify it onboard, whereas some can accept a direct DC feed, with rectification handled by the charging station, figure 2.
IEC 61851, a key standard covering electric vehicle charging systems, defines four different charging modes, as illustrated in Table 1.
Table 1: IEC 61851 defines four different EV charging modes
Level 2 charging are currently the most commonly deployed systems, in homes, workplaces and public locations, typically delivering 40A of power. Level 3, DC charging stations, using 380V three phase supplies, can deliver more power and therefore faster charging, since they can use larger, more efficient, and better-cooled rectification circuitry than would be possible in an onboard charger. Level 3 chargers are therefore the focus of much industry development as manufacturers strive to bring charging times down to traditional “fill-up” times.
These trends towards higher power and voltage levels present several challenges to the designer and place demands on the components within the charging system, including the relays which are essential to the overall functional safety.
Relays play a key role in EV Charging
The main function of a relay within a charging station, figure 3, is to switch power to the EV. In practice, a number of relays are used within the EVSE, to connect vehicles to, or isolate vehicles from, the charger, and to ensure user safety by managing potential hazard conditions such as ground faults or creepage currents. A control module within the EVSE controls the main relay, ensuring that it is not activated until a number of safety conditions are established. Once these conditions are met, the main relay switches power to the high voltage terminals of the charging cable, beginning the charging cycle. Charging cable connectors often have a mechanical or electronic latch which, when disengaged, causes the control module to de-activate the main relay. This prevents arcing at the terminals when the cable is plugged or unplugged at the EV charging port.
Figure 3: A typical EVSE contains a number of relays
to ensure safety during EV charging
Care must be taken when choosing the optimum relay for an EVSE. Relays must be able to withstand high inrush and short circuit currents and also high voltage spikes of 10kV or more. High currents must be handled for sustained periods and the devices must be capable of repeatedly switching high voltages. For basic, level 1 or 2 chargers the relays need to handle 16A at 250V AC whereas, in advanced, mode 3 EVSEs they must deal with 32A at 380V AC in a three-phase system. Safety dictates that the main relay should be a normally open, (NO), design, so that current to the EV is cut off if the charger fails. Low contact resistances are essential to minimise losses and hold currents for normally open relays must be low, ensuring overall EVSE efficiency.
With pressure on efficiencies, size and costs, PCB mounted relays are highly desirable since they avoid the additional bill-of-material and assembly related costs of their standalone counterparts. PCB mounted relays also improve reliability of any design by eliminating interconnecting leads between board sockets and relays.
PCB mounted relays, on the other hand, often struggle to meet the demands of higher voltage charging systems due to the clearances between their contacts and creepage distances between the relay’s board connections. The Panasonic HE-R series, the world’s first PCB based relay that can be used as the main switching element in 3 phase systems, is a notable exception.
The Panasonic HE-R relay
Figure 4: The Panasonic HE-R relay
The compact size of the HE-R relay enables the EVSE designer to integrate the switching function on the PCB, delivering considerable space & cost savings. The device has 4 NO contacts, each capable of switching 40A at ambient temperatures up to +85°C. An optional normally closed, (NC) contact can be used as a feedback contact, handling low level loads down to 10mA/5VDC.
Designers can benefit from the advanced technology inside the IEC62955 compliant HE-R relay to control 3 phase systems directly on the PCB, offering significant opportunities for both AC and DC charging stations.
The availability of EV charging stations is key to the ongoing adoption of EVs and developers face numerous challenges, designing for higher voltages and power levels, whilst meeting increasingly stringent efficiency targets.
Relays play an important role in the safety of EV charging systems and the evolution of PCB-mounted relays brings benefits in terms of cost, reliability and space. Until now PCB relays have been limited to lower-voltage level applications but the Panasonic HE-R relay, capable of handling 40A, represents a real breakthrough for EVSE design.
Find out more about Panasonic’s range of high power relays, including the HE-R series. Alternatively if you’re ready to take the next step with your designs, get in touch with our team of field application engineers.
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About Author
Adam Losonczy, Avnet Abacus
As Supplier Development Manager, Adam is responsible for supporting and managing key supplier relati...
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Next Generation PCB-mounted Relays In EV Charging Systems | Avnet Abacus
