What are X-Rated and Y-Rated capacitors?

EMI/RFI suppression is one of the major challenge in the power supply section. Power supply section causes major damage from the EMI/EMC point of view if proper care not taken from the design and layout point of view. It is the design that come first and this is the stage of the design where designer has to make appropriate choices. Also, sometimes the noise may couple back to external systems through the main lines causing the entire devices in the chain to misbehave. This also causes conducted emissions to occur and your system mail in conducted emissions tests. Capacitors help in power line filtering and reduce the EMI. The capacitors specifically used for this purpose are called X-Rated and Y-Rated capacitors. These are metallized polypropylene film capacitors. As these capacitors reduce the EMI/RFI suppression, they are also called EMI/RFI suppression capacitors. The capacitors usage become more critical in AC sections of the AC-Dc supply and hence these are also known as AC Line filter Safety capacitors. When we say Line filtering here, this could be noise filtering that could get impressed on the power lines due to external means. The peak voltages that these capacitors can withstand are in kilo volts. Now, if we summarize the whole scenario, the input EMI filter used comprises common mode choke, X-Rated and Y-Rated capacitors. X-Rated and Y-Rated capacitors are safety certified capacitors.

X-Rated capacitors are connected in series with the AC line. They are also connected Line to neutral in some circuits. This helps block the electrical noise from entering the circuit. The X-rated capacitor helps to eliminate the symmetrical interference (differential noise). When we say differential noise here, the noise is only on the power line and not on the earth ground. The X-Rated capacitors are only 2-pin type. X-Capacitors are  made of polyester, polypropylene, ceramic and paper dielectric materials. 

Y-Rated capacitors are connected between neutral to ground and also between line to ground. The Y-rated capacitor helps to eliminate the symmetrical interference (common mode noise). When we say common mode noise here, the noise is only on the power line as well as on the earth ground. The Y-Rated capacitors are 3-pin type and this helps connect the ground of Y-Rated capacitor to metal chassis. Y-Capacitors are  made of ceramic and paper dielectric materials.

Specifications of X-Rated Capacitors:

• Capacitance value
• Rated voltage
• Peak Impulse voltage
• Class : X1 to X3 (As per EN 60384-14 standard)
• Class : Y1 to Y4 (As per EN 60384-14 standard)
• Compliance Standards (IEC 60384-14, IEC 60065, UL 1414)

Application areas of X-Rated and Y-Rated capacitors:

• Motors
• Inverters
• X1 Rated capacitors are used for high pulse applications
• X2,X3 Rated capacitors are used for general-purpose applications. In most of the circuits we find X2 rated capacitors.
• Y1 Rated capacitors are used up to 500 Vac, with a peak voltage of 8kV
• Y2 Rated capacitors are used up to 300 Vac, with a peak voltage of 5kV
• Y3 Rated capacitors are used up to 250 Vac
• Y2 Rated capacitors are used up to 150 Vac, with a peak voltage of 2.5kV

Why does X-Rated and Y-Rated capacitors requires that much peak voltage rating?

As there is a direct connection to the AC voltage, the capacitors may be subjected to over voltages, voltage transients because of lightning strikes, power surges. with this scenario, these capacitors must be able to handle very high voltages at the input. 

Design guidelines:

1. The voltage rating of the capacitor must be double that of the working voltage

2. Take for example the R.46 series from KEMET which has the approval as per IEC 60384-14,  IEC 60065. This is X2 rated capacitor. While selecting the capacitor, check this approvals for the component. This capacitor also has UL 1414 (used across the line applications) and UL 1283 (used for EMI filters) filters.

3. X1 Rated capacitors are used for high pulse applications

4. Metallized film capacitors are to be used instead of Ceramic as the damage is lesser.

5. X-Rated capacitors are connected in series.

6. Y-Rated capacitor are connected between Line and Earth Ground.

7. While using Y-Capacitors, checking for the leakage current specification of the capacitor is important.

8. The x-Rated capacitor and Y-Rated capacitor shall have a resistor in parallel which helps to provide discharge path for the capacitor. The resistor wattage is the most critical specification. Select the capacitor that can dissipate enough wattage.

The major disadvantage or a safety concern for the engineer is that these capacitors may burst if there is a short circuit across them. Especially, the X-Rated capacitors may cause fire.