FAQ on X- and Y-capacitors

X-capacitors and Y-capacitor placements are needed for performance and mandated for safety in most AC-line systems.

What are X-capacitors and Y-capacitors? No, they are not references to various circuitry-related algebra problems that need to be solved, nor are they the tools of superheroes. Instead, they are AC-line capacitors mandated by good design practice and regulatory standards in AC-line designs.

The designations X-capacitor and Y-capacitor refer to the role and placement of these capacitors in the line-facing input of a circuit. If all your design is at lower AC voltages — typically below 50 volts — or for DC-powered designs, these capacitors are unneeded.

However, if your device plugs into the AC line, you need to know about them and how to select the appropriate type of capacitor. In AC/DC EMC-filter applications, these two special classes of capacitors filter AC power-source noise and are often collectively referred to as “safety capacitors.” The X-capacitors are used for differential-mode EMI filtering, while the Y-capacitors are used for common-mode EMI filtering by bypassing the interference from the wires to the ground.

This FAQ will look at the specifics, similarities, and differences between these capacitor roles, the capacitors used, and the various X- and Y-capacitor classes.

Q: First, the obvious question: why are they called X-capacitors and Y-capacitors (also called “Class-X capacitors and Class-Y capacitors)?
A: Quick answer: it is unclear. I did some research and came up with conflicting, unsupported answers, so the full answer is not known here. However, it may be related to their appearance in a schematic diagram (Figure 1). If you look at such a diagram, perhaps you can roughly see the letters “X” and “Y.” Nonetheless, there’s no point in pursuing this investigation further, so we will move on!

Q: What is an X-capacitor, and where does it get connected?
A: Class-X capacitors are used to minimize EMI/RFI caused by differential mode noise in an AC power supply and are often referred to as “line to line” or “across the line” capacitors. They are placed across the AC “line” (black) and AC “neutral” (white) connections to minimize adverse effects due to conducted interference, overvoltage surges, and voltage transients. Remember that a proper single-phase AC line has three connections: line (also called “hot”), neutral, and ground.

Q: Is any capacitor of the right value suitable?
A: Not at all. Class-X capacitors are subject to all AC line variations and stress in providing a clean AC signal to the circuit, which is their load. This can create a hazardous situation if the voltage or power threshold of the capacitor is exceeded. Because of the risks of this over-stress situation, Class-X capacitors are designed to fail short-circuit to trigger the circuit breaker or fuse to break the supply circuit. More on this later.

Q: What about Class-Y capacitors — what’s their situation?
A: Class-Y Capacitors are commonly called “line to ground” or “line bypass” capacitors. They are placed between the AC supply and ground to handle EMI/RF noise caused by common-mode noise on the AC line.

Q: Is their failure situation the same as that of Class-X capacitors?
A: No. Class-Y capacitors are also subject to AC-line variations via conducted interference, overvoltage surges, and voltage transients. As with Class-X devices, these stresses can lead to hazardous situations if the threshold of the capacitor’s ratings is exceeded and the capacitor fails.

However, there is an important difference: Class-Y capacitors are designed to fail open-circuit, as shorting a Y-capacitor could present a fatal shock hazard for personnel using the equipment. While a failing open circuit exposes the load circuit to an unfiltered AC power source, the fire risk is reduced. Note that the designed-in failure mode of Class-X capacitors is the opposite of the mode for Class-Y capacitors.

While the equipment is shut down by the failure of an X capacitor and the subsequent tripping of an overcurrent protection device when a Y capacitor fails, the equipment could continue operating, but EMI filtering would be significantly reduced.

Q: Do you need Class-X and Class-Y capacitors in a typical application?
A: Usually yes (Figure 2). There are some exceptions, such as when an AC-powered unit is not connected to the ground (a rare situation), and those follow their own special rules.

Q: What are the representative capacitance values for these capacitors?
A: It depends on the application-specific voltage and current levels, as well as other factors. They typically range from as low as 20 picofarads (pF) to 1000 pF but can be larger.

Capacitor standards

Q: Are there other criteria for these capacitors besides their failure mode and value?
A: Absolutely. As with many safety-critical devices, including those connected to the AC line, there are various standards and associated sub-classifications (X1, X2, X3; and Y1, Y2, Y3, Y4) which indicate the capabilities and threshold of safety capacitors. Among these are IEC 60384-14, UL 1414, UL 1283, CAN/CSA C22.2 No.1, and CAN/CSA 384-14, with the first one — IEC 60384-14— most widely used. It defines the safety classification of Class-X and Class-Y according to various levels of “Peak Voltage Pulse” before failure.

For example, the IEC 60384-14 classifications encompass peak voltage as the primary factor (Figure 3) but also define many other criteria.

Q: What are the testing criteria?
A: As expected, they are detailed and complicated, especially as this is a safety-related scenario. As a shorted Y capacitor could lead to the danger of an electric shock, Y capacitors are held to a higher operating standard compared with X capacitors.

Q: What is the nature of the tests?
A: During the certification process, the two key tests performed are the impulse test (Figure 4) and the endurance test (Figure 5). These are done to verify that the X/Y capacitor can withstand ten impulses of alternating polarity, followed by a 1000-hour endurance AC life test. After completing these two tests, the capacitors must perform reliably in the circuit under AC voltage conditions. These tests are part of the IEC 384-14 certification requirements.

Figure 4. For the impulse test in telecom applications, input T1= 10 seconds, T2=700 seconds (per IEC 60950); for mains-power applications, T1= 1.2 seconds, T2=500 seconds (per IEC 60384-14). (Image: Tecate Group)

Q: What capacitor technologies are used to meet these requirements?
A: Ceramic and film capacitors can be used for either Class-X or Class-Y applications. Still, their form factors and individual characteristics may make one type a better choice than others in certain applications. Ceramics can achieve higher capacitance values in a smaller volume, while the film has a self-healing feature inherent in the technology. Ceramic safety capacitors are usually preferred for lower-power applications due to their reduced size, but higher-capacitance value safety capacitors are only found in film technology.

Related impact of X-capacitor

Q: Are there negatives associated with using these capacitors in an actual installation?
A: Yes, there always are unrelated consequences. In addition to the cost of the component itself, the X-capacitor used in EMC filters requires a way to be discharged when the AC line is disconnected (such as unplugging the line cord). This is needed to ensure excessive voltage does not remain on the main cord for a long time, presenting an unexpected risk to the user who sees an unplugged unit.

Industry standards such as IEC60950 and IEC60065 govern the maximum allowable discharge time. This discharge requirement ensures that any high-voltage level present at the pins of the AC plug does not present an electric shock hazard to a user.

Q: What’s the problem here?
A: The standards require that the voltage across the X-capacitor decay with a maximum time constant of one second. Typically, this requirement is achieved by including a resistor as a discharge element parallel to the X-capacitor (sometimes called a “bleeder resistor”). However, this resistance results in continuous power dissipation, impacting the standby power performance.

The power dissipation in the discharge resistors depends on the X-capacitor value. At 230 VAC, assuming that the discharge resistor meets the time-constant requirement, that resistor results in a dissipation of 5.3 milliwatts (mW) for every 100 nF of X-capacitance. Thus, for a typical 470-nF X-capacitor value, 25 mW will be lost in the discharge resistors (1 nanofarad (nF) = 1000 picofard (pF)).

Q: What can be done about this unwanted dissipation?
A: Several suppliers offer integrated circuits that can be placed in series with discharge (bleed) resistors. These resistors automatically discharge the energy in the X-capacitor when the mains voltage is disconnected, thus diverting the energy away from the exposed AC plug and protecting equipment users.

Q: Can you give an example?
A: Texas Instruments offers the UCC2863x family of high-power flyback controllers with primary-side regulation and peak-power mode. Two members of this family, the UCC28630 and UCC28633, include a feature called “Active X-Capacitor Discharge.” This circuit periodically monitors the voltage across the X-capacitor to detect any possible DC condition (which would indicate that AC mains disconnection has occurred) and then discharges the voltage across the X-capacitor using the internal high-voltage current source.

Other vendors offer their approaches. Power Integrations has the CAPZero-3 energy-saving X-capacitor discharge IC, formally titled “Zero Loss Automatic X Capacitor Rapid Discharge IC with Optional Lossless Zero Crossing Signal Generator.” The two-terminal CAPZero-3 ICs enable designers to meet IEC60335 safety approvals for major appliances easily,

Conclusion

“Extra” components such as the X-capacitor and Y-capacitor play important roles in ensuring system performance and user safety. Their functions, siting, and physical construction are governed by detailed standards and tests. Designers need to be fully familiar with the appropriate mandates to get their products approved by the various safety-related regulatory agencies.