Safety certified capacitors are used to filter AC line voltages in most power applications. The capacitors are certified to international safety standards and can safely filter out high frequency noise and electrical surges from sources like lightning strikes. For a long time, certified ceramic capacitors have been limited to leaded disk types, which are large and bulky compared to most modern, miniaturized electronics. But now, KEMET’s CAS series safety capacitors are available in surface mount form factors, allowing customers to design more compact solutions without sacrificing safety.
Capacitors are used in many different applications to filter or suppress noise. When that EMI/RFI suppression is used on AC line voltages, special safety considerations are required. Because these capacitors are directly connected to AC lines, they are particularly susceptible to failures from overvoltages or transients on the line. Depending on the circuit and the type of failure, AC line capacitor failures could cause circuit breakers to trip, fires, or even fatal electric shocks. The solution to this problem is specially constructed capacitors that are designed to withstand overvoltages and surges. And if, despite that, they do fail, they are designed to fail in a safe manner.
Various regulatory and certification agencies have developed ways to test and qualify safety capacitors for this specific application. Safety certified capacitors are those that are designed for safe operation in AC line voltage applications and have been certified by safety agencies for that use.
Safety agencies test capacitors submitted for certification by applying voltage surges, endurance cycles, and other extreme conditions to them, and closely monitoring their performance. Customers can therefore be confident that capacitors certified for safety will perform reliably in AC line applications. KEMET’s CAS series of safety certified capacitors are certified to the international standard IEC 60384-1, and its United States companion, UL 60384-14 (which is almost identical).
EMI/RFI capacitors that are certified as safety caps need to do a couple of things. First, they need to be highly reliable to operate for a long time under extreme conditions. Operational ratings for the CAS series allow their use in line voltage (250 VAC) applications up to 125°C. Second, they need to have higher insulation ratings, internally and externally, so as to safely withstand impulse voltages. KEMET’s CAS safety standard recognized SMD capacitors can withstand up to 5kV impulse voltages. And third, they need to be constructed in such a way that they fail open, to prevent dangerous conditions if they do fail.
The figure above shows a common AC line filter in a home clothes dryer. Safety capacitors are classified for use in either line-to-line/neutral applications (Class X) or line-to-ground applications (Class Y). At AC line frequencies, 50 Hz or 60 Hz depending on where you are in the world, these capacitors appear as high impedance. But at high frequencies, these capacitors appear as low impedance, and are able to shunt EMI noise and transients, stopping them before they can cause damage to the appliance.
Safety capacitor classifications are further sub-classified as follows:
Specific capacitor products can then be certified to an X and/or Y subclass. For example, KEMET’s CAS series surface mount ceramic safety capacitors are rated X1/Y2 250 VAC commercial grade, and X2 250 VAC industrial grade. In other words, customers can use commercial grade CAS capacitors in 250 VAC applications and safely withstand more than 2.5 kV impulse voltages, up to 4.0 kV, if installed line-to-line (class X1). Or, if installed line-to-ground, they can be safely used in home appliances. Industrial grade CAS capacitors can only be used in line-to-line 250 VAC applications and then can only withstand up to 2.5 kV impulse voltages.
Historically, safety capacitors have been limited by technology to large, bulky through-hole package sizes. Design engineers had to work around these large capacitors and use larger product housings to hold them. They also could not use standard reflow solder processes, instead requiring specialty, expensive manufacturing techniques. KEMET’s new CAS series of surface mount safety capacitors offers a new solution to this process. Available in case sizes from 1808, 1812, 2211, and 2225, they enable a great deal of circuit miniaturization not possible with the leaded disk capacitors.
It took a while for surface mount safety capacitors to be developed and certified because the smaller spacings between the electrodes presented an insulation challenge and a safety concern. If a capacitor shorts when it fails, it can cause circuit breaker trips, fires, or shock hazards. KEMET solved this problem with a multi-serial MLCC design that will fail open, instead of short.
KEMET’s CAS series safety certified SMD capacitors offer customers the chance to miniaturize AC line filters for a whole host of applications, while keeping the same certifications and meeting the same safety standards as previous devices. The CAS series is offered in capacitances ranging from 3.0 pF up to 22 nF. They are also available in C0G and X7R dielectrics, and are ROHS compliant.
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