[NEW] RFI Capacitor Class X2 Type KNB1580 for Harsh Environment Conditions

Tuesday, 14 May 2019

The RFI/EMI metalized film capacitors (MKP) class X2 are designed to suppress radio frequency interference. They are mostly positioned in parallel with the supply mains, directly at the input or output of a device. Their main purpose is to suppress interferences which enter the device from the outside and protect the device. They also suppress any interferences produced by the device itself. This is necessary to prevent the spread of interference over the electrical grid. Typical applications are audio and video devices, chargers, measurement instruments, electrical hand tools, electrical home appliances, and white goods. 

Figure 1: Construction of a metalized film capacitor.

The suppressor capacitor class X2 can withstand peak pulse voltage in service up to 2.5 kV. They are made of a self-extinguished plastic case and resin. One of its best features is also self-healing property. Self-healing occurs when dielectric breaks down, due to overvoltage. In this case, the energy released by the arc discharge in the breakdown channel is sufficient to totally evaporate the thin metal coating close to the channel. This results in insulation restoring and a small capacitance drop.

Figure 2: Self- healing.

Because of all safety features, the X2 capacitor is a popular choice to be used as a capacitive divider in low-power consuming appliances. The capacitive reactance of a capacitor is used to reduce the mains voltage to a lower voltage. Such an application is called capacitive power supply or a capacitive dropper. This type of power supply is used in power meter, emergency lighting, thermostat, digital stove clock, ironing station, etc.

Figure 3: Capacitor and equipment in series connection with the supply main

Figure 4: Typical applications.

The problem with the standard X2 capacitor is that it is not optimized to work under continuously voltage 230V/50Hz for a long time. During operation, the capacitor is losing its capacitance due to the corona effect. If a capacitor in serial connection with the supply mains loses a certain amount of capacitance, the appliance stops working. This problem, is not so noticeable, where the capacitor is in parallel connection with the supply mains because appliances could still run, even with a small percent of initial capacitance. Also in many cases, these appliances (RFI filter) were not continuously under the voltage.

Figure 5: Standard X2 capacitor. Electrodes demetallized due to corona effect at continuous voltage.

Iskra had in the year 2005 initially introduced X2 capacitor type KNB1530 275 VAC in custom design (CD) version, that could withstand all tests according to standard IEC 60384-14 (Fixed capacitors for electromagnetic interference suppression and connection to the supply mains) and also had very stable capacitance over an operational lifetime. We later developed capacitor type KNB1560 275VAC/300VAC CD with similar electrical characteristics but in smaller dimensions than predecessor type.

In last few years there was a trend to install in applications X2 capacitor with a rated voltage higher than 300VAC. At the same time request for a capacitor which can withstand harsh climatic conditions appeared. The most recognized accelerated life test conditions were at climatic conditions: 85°C/85% Relative humidity/240 VAC/1000 hours. This testing method is called THB (temperature, humidity, bias) test. We also made a lot of investment in laboratory equipment so we can now make extensive testing procedures according to automotive standard AEC-Q200.

Iskra is proudly introducing the new type KNB1580 with rated voltage 310VAC which includes all advantages of the previous CD version, has very stable capacitance and can withstand severe climatic conditions (THB test).