What is an X-Rated Capacitor?

An X-rated capacitor, also known as a line-filter capacitor or across-the-line capacitor, is a type of capacitor specifically designed and safety rated for use directly across AC power line connections or in series with the AC line. The “X” in the name refers to the fact that these capacitors are used to attenuate (block) high frequency electromagnetic interference (EMI) “across” the AC line.

X-rated capacitors are essential components in EMI filters, switch-mode power supplies, and other applications that require filtering high frequency noise on AC power lines. They are designed to fail safely without causing fires or releasing hazardous materials if subjected to overvoltage or other extreme conditions.

X-Rated Capacitor Safety Classes

X-rated capacitors are categorized into different safety classes based on their peak voltage ratings and failure modes. The main safety classes are:

Safety Class	Peak Voltage Rating	Failure Mode
X1	≤ 2.5 kV	Open circuit, no flammable or hazardous materials
X2	≤ 2.5 kV	Open or short circuit, no hazardous materials
X3	≤ 1.2 kV	Open or short circuit, no flammable materials

X1 rated capacitors are the safest and most commonly used in direct across-the-line applications. X2 and X3 capacitors can be used in some scenarios but with more precautions.

How X-Rated Capacitors Work in EMI Filtering

The main function of X-rated capacitors in EMI filtering is to short high frequency noise to ground while passing the AC line voltage and frequency. This is achieved by the capacitor’s frequency-dependent impedance characteristics.

A capacitor’s impedance decreases with increasing frequency according to the formula:

Xc = 1 / (2πfC)

Where:
– Xc = capacitive reactance (impedance) in ohms
– f = frequency in hertz
– C = capacitance in farads

So at the 50/60 Hz line frequency, a typical X-capacitor value like 100 nF will have a very high impedance in the megaohm range and pass negligible current. But at EMI frequencies in the kilohertz to megahertz range, the impedance drops to a few ohms or less, effectively shorting the noise.

X-rated capacitors are connected between line and neutral and/or line/neutral to ground. In a typical single-phase EMI filter configuration:

• Cx = X-capacitor line-to-neutral
• Cy = Y-capacitor line-to-ground
• L = common mode choke inductors

The X-capacitor mainly filters differential mode noise, while the Y-capacitors in combination with the inductors filter common mode noise.

Using X-Rated Capacitors in a Transformerless Power Supply

In a transformerless power supply, the AC mains are directly rectified and filtered to generate a high voltage DC bus. X-rated capacitors play a critical role in such designs for both EMI filtering and high voltage energy storage.

Transformerless Power Supply Block Diagram

A basic transformerless power supply consists of the following stages:

1. EMI filter with X and Y capacitors
2. Bridge rectifier to convert AC to pulsating DC
3. Filter capacitor(s) to smooth the rectified DC
4. DC-DC converter to step down and regulate the high voltage DC to low voltage output(s)

The X-rated capacitors serve multiple purposes here:
– Cx1 and Cx2 form a line filter to block conducted EMI
– Cx2 also functions as a high voltage reservoir capacitor to smooth the rectified DC

Selecting X-Rated Capacitors for Transformerless Supplies

Choosing the right X-rated capacitors for a transformerless power supply requires considering several factors:

1. Voltage rating: The capacitors must be rated for the peak AC line voltage including transients. For universal mains input (85-265 VAC), use 250 VAC X1 or X2 types.

2. Capacitance: Higher capacitance means lower impedance and better filtering but also higher cost and inrush current. Typical values range from 100 nF to 1 uF.

3. Ripple current rating: The capacitors must handle the ripple current imposed by rectification without overheating. Select types with sufficient ripple current specs.

4. Inrush current: High capacitance and low ESR increase turn-on inrush current, which can trip protections. Use NTC inrush limiters or active inrush control.

5. Safety approvals: The capacitors should be safety agency approved (UL, IEC, etc.) for the intended application and environment (humidity, altitude, pollution degree).

Example selection for 100-240 VAC input, 100 watt power supply:

Position	Part Number	Type	Capacitance	Voltage	Safety	Features
Cx1	MKP1848C31050JK2	X2	100 nF	275 VAC	IEC/UL	Metallized film
Cx2	B32924C3155M	X1	1.5 uF	250 VAC	IEC/UL	Metallized film, high ripple current

X-Rated Capacitor Failure Modes and Protection

Although X-rated capacitors are designed to fail safely, precautions must be taken to prevent hazards and protect other components from capacitor failures:

• Always use X1 or X2 class for across-the-line. Never use Y-rated or general purpose types which can fail short and cause fires.

• Include overcurrent protection. Use fuses or circuit breakers to disconnect power if an X-cap fails short. Fuse both line and neutral close to the cap.

• Use series resistors or inductors. Inserting impedance in series with X-caps limits short circuit current and dissipates energy if a cap fails.

• Provide enough creepage and clearance. Follow safety standards for spacing between X-cap terminals and other conductors to prevent arc-over.

• Monitor and indicate failures where possible. Use X-cap monitoring circuits to detect and notify failures before complete breakdowns occur.

X-Rated Capacitor FAQs

Q1: Can I use regular ceramic or electrolytic capacitors instead of X-rated types for EMI filtering?

A1: No, regular capacitor types are not designed or safety certified for connection across hazardous AC line voltages. They can fail catastrophically and cause fires or electric shock. Always use X-rated capacitors for safety and EMI compliance.

Q2: What is the difference between X and Y capacitors?

A2: X-capacitors are used in series or across line-to-line or line-to-neutral connections, where the full line voltage is applied. Y-capacitors are used line-to-ground and see lower voltage stress. Y-caps have stricter leakage current specs but can fail short safely.

Q3: How do I test X-rated capacitors?

A3: Measure the capacitance and dissipation factor with an LCR meter at the rated frequency and voltage. Also perform an insulation resistance or leakage current test at the rated voltage using a megohmmeter or hipot tester. Replace the capacitor if out of spec.

Q4: What causes X-capacitors to fail?

A4: X-capacitors can fail due to overvoltage, overcurrent, overtemperature, aging, or manufacturing defects. Excessive inrush current, line transients, and high ripple current are common causes. Using capacitors with insufficient ratings or poor quality increases the risk of failure.

Q5: How often do X-capacitors need to be replaced?

A5: X-capacitors have no fixed service life and do not require scheduled replacement if operated within specifications. However, their performance can degrade over time, especially in high temperature or humidity environments. It is recommended to periodically inspect and test X-caps for signs of degradation and replace as needed based on the application criticality and risk factors.

Conclusion

X-rated capacitors are critical safety components in AC line filters and transformerless power supplies. They provide essential EMI filtering and high voltage energy storage functions while being designed to fail safely under fault conditions.

Proper selection and use of X-rated capacitors based on voltage ratings, capacitance values, ripple current, and safety approvals is crucial for reliable operation and compliance with safety standards.

By understanding the characteristics, applications, and failure modes of X-rated capacitors, engineers can design robust and safe power systems that meet EMI and safety requirements.