Supercharged solutions that provide fast charging reliability, efficiency and safety

The introduction of electric vehicles will reduce pollution and help slow down the effects of climate change. A significant barrier to the wider use of electric cars is both a public charging infrastructure that can support long-distance journeys and chargers that can charge an electric car battery in a time acceptable to drivers.

Currently, 300 kW chargers can charge a battery in under 15 minutes. Although the time is acceptable, the main challenge for design engineers is to protect the users from 300 kW Free EV charging stations map.

Second, designers must maximize efficiency to minimize power consumption and temperature rise. In addition, structures must function reliably despite exposure to a wide range of environmental conditions.

Figure 1 shows several types of charging stations. North America classifies charging stations into ”Levels”, while the European Union separates charging stations with ”Modes”. Organizations such as the Society of Automotive Engineers, the European Union Automotive Standards Organization and Asian countries such as Japan and China are working to standardize charging stations Free EV charging stations map.

Figure 1Free EV charging stations map . EV charging station types and their current capacities

A DC fast charger (Figure 2) can charge an EV battery to 80% capacity in less than 15 minutes. These chargers have an output of up to 350 kW and can deliver up to 500 A to a battery. To develop these chargers, designers must overcome the challenges of safety, efficiency and reliability.

Charger and user protection methods Free EV charging stations map

Figures 3, 4 and 5 illustrate the circuits and components of a DC charger. A 350 kW charger requires an AC input with high voltage and high current capacity. To protect against overload, use fast-acting, high-current fuses. Make sure the fuses have an interruption rating large enough to exceed the available energy from the AC line. Fuses can have an output of up to 200 kA.

Consider using a surge protector to absorb voltage transients. Surge protectors can absorb as much as 50 kA of transient current. They clamp the voltage transient to a safe level that prevents energy from propagating into the circuit and damaging components. Also consider adding a surge protection fuse to protect the surge protector.

With high mains current, designers should protect against ground current faults. Use a current transformer sensor capable of detecting ground currents below 30 mA. A ground fault protection relay can measure the current from the sensor and disconnect the current if a high ground current is detected.

The DC charger power converter circuitry also interacts with the AC line and requires input fuse and transient overload suppression. Use fast-acting semiconductor fuses to protect downstream semiconductor components. Consider either metal oxide varistors (MOVs) and gas discharge tubes or MOVs and protection thyristors to protect against transients. Lower power charger sub-assemblies only need MOV or a MOSFET for secondary level transient protection.

Protect the communication, user interface and access panel sensor circuits from electrostatic discharge (ESD) damage. Investigate transient voltage suppressor (TVS) diode arrays.

Protect the DC output circuits from load current short circuits with a fast-acting, high-current, high-voltage fuse. The CHAdeMO (CHArge de MOve) Association promotes a fast charging infrastructure for DC charging that requires a reverse flow protection diode in the secondary rectifier. This diode acts as redundant short-circuit protection.

For the loading gun user interfaces, use temperature sensors to prevent overheating and use proximity sensors to ensure the gun is properly plugged in before power is applied.

Circuit topologies and component selection to maximize efficiency

With chargers capable of delivering up to 350 kW, minimizing power losses can significantly impact reduced power consumption. Consider the following recommendations to maximize effectiveness:

• Use of a Wien rectifier stage, a bridgeless power factor correction circuit, which both minimizes current draw from the power line and uses a minimal number of transistors and diodes
• Use of Schottky diodes with low forward voltage drop and low leakage current in rectifier circuits
• Use SiC MOSFETs in the DC-DC converter circuits for fast switching speeds to enable a more efficient, switch-mode DC-DC converter. SiC MOSFETs also have low R _DS(On) which reduces on-state current losses and low R _thJC which allows for simplified thermal design.
• Use efficient gate drivers to simplify control of the MOSFETs and minimize power consumption.

Developing a bi-directional design to return power to the grid can significantly save energy costs. See the MOSFET-based design shown in Figure 5.

Required standards

Because DC charging stations consume and deliver high power, they must meet several safety certifications. See table 1.

Achieve safety and efficiency goals with a manufacturer’s guidance

Designers must ensure that battery charging circuits are protected from environmental hazards and safe for user interaction. Manufacturers of protective products can help designers save development time by offering design recommendations. Applications engineers can provide useful advice on the trade-offs between power, space, efficiency and component cost. Some manufacturers will also assist with standards compliance and may offer pre-compliance testing services. Manufacturers can cost-effectively help designers achieve goals for charger safety and efficiency.

References: