Dr. Florian Weyland, of Vishay Intertechnology, discussed the need for stricter requirements for safety capacitors used in electric vehicle (EV) charging, outlining some of the methods used to avoid issues.

“Safety capacitors, also known as interference suppression capacitors, are permanent fixtures in power packs and power supplies… (and) are also seeing increased use in the automotive industry due to the electrification of vehicles and their increasingly high power-supply voltages, which make new requirements necessary,” Weyland noted.

The demand for electric vehicles has increased over the past few years, both hybrid and complete electric vehicles. “Legislators are pushing this trend with conditions for reducing carbon dioxide emissions. The use of electric motors presupposes exchanging classic fuels such as gasoline or diesel for electrochemical energy storage systems such as batteries. As is known, batteries are not charged at the gas pump but, instead, at a power outlet or a charging station using direct voltage. To do this, the vehicle needs charging electronics, also known as on-board chargers (OBCs), with appropriate AC/DC conversion,” he stated.

Similar to mobile phone chargers and power supplies, for EVs the requirements for safety capacitors are different, which is why special, automotive-qualified safety capacitors must be used for OBCs.

Safety capacitors must be tested and certified by an official agency and are subject to the IEC 60384-14 standard.

“Safety capacitors conduct high-frequency interference signals — electromagnetic interference (EMI) and radio-frequency interference (RFI) — to chassis ground or the neutral conductor, which short-circuits this interference. Reducing EMI ensures electromagnetic compatibility (EMC). In addition, safety capacitors must intercept excessive voltages on the power supply side, preventing coupling in the electrical supply system. These safety capacitors are divided into two classes — X capacitors and Y capacitors (Fig. 1 )”. 

Fig. 1: Connection of Class X (left) and Class Y (right) safety capacitors (Image: Vishay Intertechnology)

Film capacitors are the first choice as Class X capacitors thanks to their self-healing properties and provide the advantages of high achievable capacitance, stable capacitance, and a stable dissipation factor over the usable temperature range. Ceramic disc capacitors, on the other hand, provide the highest voltage immunity and are the preferred solution for Class Y1 applications.

“The capacitor type considered for the charging electronics in an EV/HEV vehicle depends on its circuitry, the expected voltage pulses, and the alternating voltages applied across the safety capacitor. Fig. 2  shows the application area for various safety capacitor solutions as a function of the capacitance and withstand alternating voltage.

Fig. 2: Classification of film capacitors, MLCCs, and ceramic disc capacitors with capacitance ranges versus withstand alternating voltages (Image: Vishay Intertechnology)

There are also automotive-qualified film and ceramic capacitors that even exceed the automotive requirements”.

The Automotive Electronic Council (AEC) defines and publishes the requirements placed on electronic components in the automotive industry.