Electric vehicle chargers: new requirements for safety capacitors

Capacitors used to suppress electromagnetic interference, also known as safety capacitors, are permanent fixtures in battery packs and power supplies. As switching power supplies are widely used in many applications such as computers, printers, TVs, and mobile phone chargers, safety capacitors have been used in every family. Due to the increasing electrification of automobiles and the increasing power supply voltage, safety capacitor products are increasingly being used in the automotive industry, and new requirements have been put forward for this.

To protect circuits and personnel, electric vehicle/hybrid vehicle (EV/HEV) on-board chargers impose stricter requirements on safety capacitors.

Authors: Dr. Florian Weyland, Wolfgang Rettlinger and Thomas Wächter from Vishay

Capacitors used to suppress electromagnetic interference, also known as safety capacitors, are permanent fixtures in battery packs and power supplies. As switching power supplies are widely used in many applications such as computers, printers, TVs, and mobile phone chargers, safety capacitors have been used in every family. Due to the increasing electrification of automobiles and the increasing power supply voltage, safety capacitor products are increasingly being used in the automotive industry, and new requirements have been put forward for this.

In the global market, automakers are facing increasing pressure from the electrification of automobiles. This not only means the need to install new assisted driving systems, but also the need to introduce electric vehicles (EV) on the market, whether hybrid electric vehicles (HEV) or all-electric vehicles.

The legislature is promoting this trend and creating conditions for reducing carbon dioxide emissions. Replacing traditional gasoline or diesel fuel systems with electrochemical energy storage systems is a prerequisite for the use of electric vehicles. As we all know, the battery of an electric car is not charged with oil and gas, but charged with a DC voltage on a power socket or charging pile. Therefore, vehicles need Electronic charging devices with corresponding AC/DC conversion functions, so-called on-board chargers (OBC).

In this regard, this type of charger can be compared to the relationship between a mobile phone and its power supply, but it has higher power and can work in a harsher environment, because the car may be in cold Siberia, hot desert, humid tropical, rugged Driving on the road. Therefore, certain requirements for safety capacitors in the automotive industry are different, which is why OBC must use special safety capacitors that meet automotive standards.

General requirements

Safety capacitors transmit high-frequency interference signals-electromagnetic interference (EMI) and radio frequency interference (RFI)-to the chassis ground or neutral conductor, thereby short-circuiting the interference. Reducing EMI can ensure the electromagnetic compatibility (EMC) of automotive Electronic components. In addition, safety capacitors must be able to intercept excessively high pulse voltages to prevent coupling into the power supply system. Safety capacitors are divided into two types-X-type capacitors and Y-type capacitors (Figure 1).

Electric vehicle chargers: new requirements for safety capacitors
Figure 1: Connection of Type X (left picture) and Type Y (right picture) safety capacitors (picture source: Vishay Intertechnology)

The X-type capacitor is connected across the live wire and the neutral wire. In this case, capacitor failure will not cause the risk of electric shock. Class X devices can also be subdivided into X1 and X2 capacitors. According to regulations, X1 capacitors must be able to withstand 4KV voltage pulses, and X2 capacitors must be able to withstand 2.5 kV pulses.

Class Y safety capacitors are connected across the live wire and the equipment casing or between the neutral wire and the equipment casing. In this case, if the basic insulation is missing and the safety capacitor fails, people will be in danger. Therefore, Class Y safety capacitors must have high electrical safety. Class Y devices can also be subdivided into Y1 and Y2 safety capacitors. Y1 safety capacitors must be able to withstand 8 kV voltage pulses, and Y2 safety capacitors must be able to withstand 5 kV pulses.

In addition to complying with the IEC 60384-14 standard, safety capacitors must also be tested and certified by official agencies, such as ENEC in Europe, Underwriter Laboratories, Inc. (UL) in the United States, CSA Group in Canada, and CQC in China (China Quality Certification Center).

There are various types of safety capacitors, including film capacitors, chip ceramic capacitors and multilayer ceramic capacitors. Film capacitors have the advantages of high capacitance, stable capacitance, and stable loss factor within the usable temperature range. In addition, metallized film capacitors have the advantage of self-healing. In the case of a breakdown of the diaphragm, a small arc will be generated if a voltage is applied, causing oxidation around the damaged point of the thin-film aluminum coating. This can isolate the damage point and prevent more serious damage.

Due to its self-healing properties, film capacitors are the first choice for Class X capacitors. On the other hand, chip ceramic capacitors have the highest voltage resistance and are the preferred solution for Y1 applications.

Multilayer capacitors have small footprint and low height, which are ideal for applications that save limited substrate space and reduce assembly costs. Multilayer capacitors adopt reliable precious metal electrode (NME) design and wet process manufacturing, and have good heat dissipation performance at operating temperatures up to +125 °C, improving reliability in harsh environments.

Special requirements of the automotive industry

The type of capacitor used in EV/HEV charging equipment depends on the circuit, the expected voltage pulse, and the AC voltage applied across the safety capacitor. Figure 2 shows the relationship between capacitance and withstand voltage in the application areas of various safety capacitor solutions.

Electric vehicle chargers: new requirements for safety capacitors
Figure 2: The relationship between the capacitance and withstand voltage of film capacitors, multilayer capacitors and chip ceramic capacitors (Image source: Vishay Intertechnology)

Automotive grade film capacitors, chip ceramic capacitors and multilayer capacitors even exceed the requirements of the automotive industry. The working principle is as follows.

Automotive Industry Standard

The Automotive Electronics Council (AEC) defines and publishes requirements for electronic components in the automotive industry. The AEC-Q200 requirements describe the tests that passive components must pass, as shown in the cut-out content in Figure 3.

For example, Vishay provides two series of ceramic capacitors that meet the automotive industry standards, namely the AY1 series certified in accordance with Y1 category and the AY2 series certified in accordance with Y2 certification. Other products also include the Y2 category MKP3386Y2 and F340Y2 series film capacitors that meet the requirements of the automotive industry. , X2 class F339X2, MKP339 and F1772 series film capacitors, and X1 / Y2 and X2 class multilayer capacitors C C0G (NP0) and X7R dielectric VJ series safety certified capacitors.

Electric vehicle chargers: new requirements for safety capacitors
Figure 3: Overview of requirements for safety capacitors in the automotive industry (Image source: AEC)

Safety capacitors need to be suitable for different low-temperature and high-temperature design requirements. Generally, the operating temperature of ceramic and multilayer capacitors is C55 °C to 125 °C, and the operating temperature of film capacitors is C55 °C to 85 °C or 105 °C.

As the temperature increases, the probability of device failure increases, and the device needs to be tested at the maximum allowable temperature. In other words, the capacitor must be tested for at least 1,000 hours at the maximum design voltage. Vishay products exceed these requirements. The AY2 ceramic capacitor product line can work normally for 3,000 hours under the same conditions, and the multilayer safety capacitors can work normally for 2,000 hours at 150 °C and 1.7 x AC rated voltage.

In addition, ceramic capacitor components must be C55 °C to 125 °C, and film capacitor components must be C55 °C to 85 °C or 105 °C, and withstand 1,000 temperature cycles. When the temperature cycle is large and changes quickly (from the lowest to the highest design temperature in less than one minute), the internal thermal expansion or thermal contraction of the capacitor may cause cracks or delamination, or the case and the capacitor may separate.

Automotive-grade capacitors can work normally under these conditions. At the same time, it is suitable for non-automotive applications that need to improve the safety of temperature fluctuations. Solar panel converters that are exposed to sub-zero temperatures and long periods of sunlight are a good example of this application.

Capacitors need to be moisture-proof. Therefore, ceramic capacitors must work normally for 1,000 hours at 85 °C, 85% relative humidity, and maximum rated voltage. When temperature and humidity increase, moisture penetrates the shell by diffusion through the coating material, or mainly between the coating and the uncoated. Penetrating moisture can cause capacitors to short-circuit. To prevent this and ensure moisture resistance, coating materials and coating processes need to be suitable for automotive applications. The standard requirements for film capacitors are 40 °C, 93% relative humidity, and 1,000 hours at rated voltage. However, Vishay series capacitors are suitable for extreme conditions, higher than the requirements of AEC-Q200, and passed 85 °C and 85% relative humidity, rated voltage test, such as F340Y2 for 1,000 hours, F339X2 for 500 hours.

The automotive market also has higher mechanical requirements for electronic components than other applications, such as robustness against vibration and mechanical shock. Vishay automotive-grade safety capacitors meet this requirement.

In summary, safety capacitors are divided into X and Y categories, which can filter out high-frequency interference signals (EMC) and protect circuits and users from high-voltage surges. Capacitors that meet automotive standards can be used under extreme environmental conditions. Must be certified by an authorized agency to ensure critical security. AEC released the test specifications that must be passed for automotive applications.

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