By Dr. Florian Weyland, Wolfgang Rettlinger, Thomas Wächter, Vishay Intertechnology
Capacitors for electromagnetic interference suppression, also known as safety capacitors, are permanent fixtures in power packs and power supplies. With the broad use of switching power supplies in computers, printers, televisions, and chargers for mobile phones, among other applications, safety capacitors are used in every household. They 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.
Globally, automobile manufacturers are under pressure to advance the electrification of their vehicles. This does not just mean installing new driver-assistance systems, but bringing battery-operated electric vehicles (EVs), whether hybrid (HEV) or completely electric, to market.
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. Therefore, vehicles need charging electronics, socalled on-board chargers (OBCs), with appropriate AC/DC conversion.
In this respect, these chargers can be compared with mobile phones and their power supplies, just at a higher power level and operated in much harsher environments, as cars may see Siberian cold, desert heat, tropical moisture, bumpy roads, and so forth. Therefore, some of the requirements for safety capacitors in the automotive industry are different, which is why special, automotive-qualified safety capacitors must be used for OBCs.
Safety capacitors conduct high frequency interference signals — electromagnetic interference (EMI) and radiofrequency 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 voltage spikes 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)
X capacitors are connected between a phase and the neutral conductor. In this case, a capacitor failure won’t result in a dangerous electric shock. Class X devices are subdivided into X1 and X2 capacitors. According to regulations, Class X1 capacitors must withstand pulses up to 4 kV and X2 capacitors must withstand pulses of 2.5 kV.
Class Y safety capacitors are used between the phase or neutral conductor and the accessible device housing. In this case, the base insulation is bypassed and, in the event of a safety capacitor failure, people may be put in danger. Therefore, Class Y safety capacitors must have elevated electrical safety. Class Y devices are subdivided into Y1 and Y2 safety capacitors. Y1 safety capacitors must withstand voltage pulses of 8 kV and Y2 safety capacitors must withstand pulses of 5 kV.
Safety capacitors are subject to the IEC 60384-14 standard and must be tested and certified by an official agency, such as ENEC in Europe, Underwriter Laboratories, Inc. (UL) in the U.S., CSA Group in Canada, and CQC (China Quality Certification) in China.
Safety capacitors come in various designs, including film capacitors, ceramic disc capacitors, and multilayer ceramic capacitors. Film capacitors provide the advantage of high and stable capacitance, as well as a stable dissipation factor over the usable temperature range. In addition, metalized film capacitors benefit from their self-healing effect. In the event of a breakdown in the film, a small arc occurs if voltage is applied, causing oxidation around the damaged point in the aluminum coating of the film. This process isolates the damaged point, preventing greater damage.
Thanks to their self-healing properties, film capacitors are the first choice as Class X capacitors. Ceramic disc capacitors, on the other hand, provide the highest voltage immunity and are the preferred solution for Class Y1 applications.
With their small footprint and low height, multilayer capacitors are ideal for downsizing in applications with limited board space and offer lower assembly costs. Built with a reliable noble metal electrode (NME) system and wet process, they have good thermal dissipation for working temperatures up to +125 °C, and offer high reliability in harsh environments.
Special requirements in the automotive industry
The capacitor type considered for the charging electronics in an EV / HEV 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, multilayer capacitors, ceramic disc capacitors with capacitance ranges versus alternating voltages (Image: Vishay Intertechnology)
There are automotive-qualified film capacitors, ceramic disc capacitors, and multilayer capacitors that even exceed the automotive requirements. How that works is described below.
The Automotive Electronic Council (AEC) defines and publishes the requirements placed on electronic components in the automotive industry. The AEC-Q200 requirement describes the tests that passive components must pass. Fig. 3 shows an excerpt.
Vishay Intertechnology, for example, provides two automotive qualified series of ceramic capacitors, the AY1 series, certified in accordance with Class Y1, and the AY2 series, certified in accordance with Class Y2. The range of products also includes the MKP3386Y2 and F340Y2 series of Class Y2 film capacitors and the F339X2, MKP339, and F1772 series of Class X2 film capacitors and Class X1 / Y2 and Class X2 multilayer capacitors – VJ Safety Certified capacitors in C0G (NP0) and X7R dielectrics.
Fig. 3: Overview of the automotive industry’s requirements for safety capacitors (Image: AEC)
An appropriate design for low and high temperatures is required for safety capacitors. Consequently, ceramic and multilayer capacitors are typically designed to operate over the temperature range of –55 °C to 125 °C and film capacitors over the range of –55 °C to 85 °C or 105 °C. As the probability of component failure increases with increasing temperature, the components are tested at their maximum allowed temperatures. This means the capacitors must be tested for at least 1,000 hours at the maximum design voltage.
Vishay exceeds these requirements with the AY2 ceramic capacitor line, which survives 2,000 hours reliably under these conditions, and with the multilayer Safety Capacitor Series, which is qualified to 2,000 hours at 150 °C with 1.7 x AC rated voltage applied.
In addition, the components must survive 1,000 temperature cycles between –55 °C and 125 °C for ceramic capacitors as well as for multilayer capacitors, and between –55 °C and 85 °C or 105 °C for film capacitors. In the event of large temperature cycles with fast temperature changes (minimum to maximum design temperature in less than one minute), the thermal expansion or contraction may result in cracks or delamination inside the capacitor or the cover may separate from the capacitor.
Automotive-qualified capacitors are designed to survive these conditions. It also makes them suitable for non-automotive applications that need increased safety with respect to temperature fluctuations. Solar panel converters that are exposed to freezing temperature as well as to long sunshine hours are a good example.
Moisture resistance is just as necessary. For this, ceramic capacitors must survive exposure to 85 °C and 85% relative humidity for 1,000 hours at maximum rated voltage. At increased temperature and humidity, water can penetrate the cover by diffusion through the coating material or, primarily, at the transition between the coated and the uncoated wire. The penetrating moisture can cause short circuits in the capacitor. To prevent this and to ensure moisture resistance, the coating material and the covering process are adapted to the automotive application. The standard requirements for film capacitors are 40 °C & a relative humidity of 93%, with the rated voltage applied for 1,000 hours. However, Vishay offers series that are designed for extreme conditions exceeding the AEC-Q200 requirements and passing tests at 85 °C and 85% relative humidity at rated voltage, such as the F340Y2 for 1,000 hours and the F339X2 for 500 hours.
The mechanical requirements put on electronic components in the automotive market, such as robustness against vibrations and mechanical impacts, are also higher than in other applications. Vishay Intertechnology allows for this with its automotive-qualified safety capacitors.
In summary, safety capacitors, segmented into Class X and Class Y types, filter out high frequency interference signals (EMC) and protect circuits and users from high voltage surges. Automotive-qualified capacitors are designed to survive extreme environmental conditions. The necessity of certification by authorized agencies goes hand in hand with the safety-critical nature. The AEC publishes regulations for tests that must be passed for automotive applications.
About the authors
Florian Weyland currently serves as manager of product marketing, ceramic capacitors, at Vishay Intertechnology. He is a member of the German Ceramic Society, winner of its Hans-Walter-Hennicke Award, and recipient of its grant for the 2017 American Winter Workshop.
Weyland holds a Dr.Ing. from the TU Darmstadt. Wolfgang Rettlinger currently serves as senior manager of product marketing, film capacitors, at Vishay Intertechnology. He holds a Dipl.Ing. (FH) from Georg- Simon-Ohm FH Nürnberg.
Thomas Wächter joined Vishay Intertechnology’s tantalum capacitors division in 1996, before transitioning into marketing for the company’s line of surfacemount MLCCs in 2002. He holds the position of Senior Manager of Business Marketing, MLCCs, Europe, and serves as Director of Product Marketing, MLCCs, Automotive and Commodities. He holds a Dipl.-Ing. (FH) from Ostbayerische Technische Hochschule in Regensburg, Germany.