X Rated Capacitor: How to Use One in a Transformerless Power Supply

What is an X-Rated Capacitor?

An X-rated or X-class capacitor is a capacitor that meets certain safety requirements defined by IEC/EN 60384-14 and UL 60384-14 standards. These standards specify the capacitor’s ability to fail safely under over-voltage conditions to avoid posing a shock or fire hazard.

X-rated capacitors are categorized into three sub-classes based on their peak voltage ratings:

X1 capacitors have the highest voltage rating, followed by X2 and X3. X1 and X2 capacitors may be used between line and neutral, while X2 and X3 are also suitable for line-to-ground applications.

Construction of X-Rated Capacitors

X-rated capacitors are typically metalized film capacitors that use a self-healing dielectric. If a breakdown occurs, the thin metallic layer around the fault evaporates, isolating the defective area and allowing the capacitor to continue operating with negligible loss of capacitance.

The electrodes in an X-rated capacitor are staggered, with the foils offset from each other. This staggered construction prevents an arc from propagating if the dielectric fails. The capacitor leads are also spaced widely apart and insulated to prevent flashover.

X-rated capacitors often use flame-retardant plastic cases and potting compounds to minimize fire risk if the capacitor fails catastrophically. The capacitor may also have an internal thermal fuse that opens if the temperature rises excessively.

Transformerless Power Supplies

Many modern electronic devices use transformerless power supplies to reduce size, weight, and cost compared to transformer-based designs. A transformerless power supply connects directly to the AC line without the isolation provided by a transformer.

While transformerless power supplies offer several benefits, they introduce some safety risks. Without a transformer, the power supply is not isolated from the AC line. Any capacitors connected between line and ground must be rated for the full line voltage and be able to fail safely to avoid creating a shock hazard.

This is where X-rated capacitors come into play. By using an X-rated capacitor for the line-to-ground connection, the power supply designer ensures that the capacitor will fail safely if subjected to over-voltage stress, without causing an electric shock or starting a fire.

Selecting an X-Rated Capacitor

When choosing an X-rated capacitor for a transformerless power supply, consider the following factors:

Voltage Rating

Select a capacitor with a voltage rating appropriate for the application. For a capacitor connected between line and neutral, use an X1 or X2 rated component. For line-to-ground, an X2 or X3 capacitor is acceptable.

The capacitor voltage rating must exceed the peak line voltage, including any anticipated spikes or surges. For example, for a 120 VAC nominal line voltage, the peak is 120 * √2 = 170 V. Adding some margin for surges might dictate a 250 VAC X2 rated capacitor.

Capacitance Value

The capacitance value needed depends on the power supply design and the amount of EMI filtering required. Higher capacitance provides more filtering but increases leakage current. Typical values range from hundreds of picofarads to tens of nanofarads for most applications.

Package and Termination

X-rated capacitors are available in through-hole and surface-mount packages. Through-hole parts often have spade-lug or bolt-down terminals for high-reliability connections. Surface-mount chip capacitors are smaller but must be soldered carefully to avoid damaging the part.

Flammability Rating

Look for a capacitor with a UL94 flammability rating of V-0 or better to minimize fire risk. This ensures that the capacitor will self-extinguish if ignited.

Using X-Rated Capacitors Safely

When using an X-rated capacitor in a transformerless power supply, follow these guidelines to ensure safe operation:

• Always use a capacitor with an appropriate voltage rating for the application. Never exceed the capacitor’s maximum rated voltage.

• Provide sufficient spacing between the capacitor terminals and other conductors to prevent flashover. Follow the manufacturer’s recommended pad layout and clearances.

• Route the traces connecting to the capacitor to minimize EMI and avoid coupling noise into sensitive circuits. Keep the loop area small and use ground planes to provide shielding.

• If the capacitor is dissipating significant power, use wide traces and copper pours to help cool the part. Provide adequate airflow over the capacitor to prevent overheating.

• Consider adding a bleed resistor in parallel with the capacitor to discharge it when power is removed. This prevents the capacitor from holding a dangerous residual charge. Size the resistor to minimize leakage current while providing a reasonable discharge time.

By selecting an appropriate X-rated capacitor and using it properly, designers can create safe and reliable transformerless power supplies for a wide range of applications. Always follow best practices and comply with relevant safety standards to ensure the finished product does not pose a hazard to users.

Frequently Asked Questions

Q1: Can I use a non-X-rated capacitor in a transformerless power supply?

A1: No, it is not safe to use a non-X-rated capacitor in a transformerless power supply. Non-X-rated capacitors are not designed to fail safely under fault conditions and could cause electric shock or start a fire if used on the AC line. Always use an X-rated capacitor with an appropriate voltage rating for the application.

Q2: What happens if an X-rated capacitor fails?

A2: X-rated capacitors are designed to fail safely, without causing an electric shock or starting a fire. If a dielectric breakdown occurs, the thin metallization layer around the fault will vaporize, isolating the defect and allowing the capacitor to continue functioning with only a small loss of capacitance. The capacitor’s staggered electrode construction prevents an arc from propagating, and the flame-retardant case and potting compound minimize fire risk.

Q3: How much capacitance do I need for EMI filtering in a transformerless power supply?

A3: The amount of capacitance needed for EMI filtering depends on the specific power supply design and the amount of noise attenuation required. Higher capacitance values provide more filtering but also increase leakage current. Typical values range from a few hundred picofarads to tens of nanofarads. The best value for a given design must be determined experimentally or through simulation.

Q4: Can I parallel multiple X-rated capacitors to increase the total capacitance?

A4: Yes, it is acceptable to parallel multiple X-rated capacitors to increase the total capacitance. However, each capacitor should be individually rated for the full line voltage. Paralleling capacitors does not increase the voltage rating. Also, be aware that paralleling capacitors increases the total leakage current, which may be a concern in some applications.

Q5: How do I discharge an X-rated capacitor safely?

A5: To discharge an X-rated capacitor safely, use a bleed resistor connected in parallel with the capacitor. The resistor should have a high enough resistance to minimize leakage current during normal operation, but low enough to discharge the capacitor in a reasonable amount of time when power is removed. A typical value might be 100 kΩ to 1 MΩ, depending on the capacitor value and the acceptable discharge time. Always ensure the capacitor is fully discharged before handling it to avoid electric shock.