Our fundamental passive and inherently analog components – resistors, capacitors, and inductors – are so conceptually simple and easy to describe that we often don’t fully articulate or appreciate the many roles each can play in a system or circuit.
Consider the humble capacitor with its many functions such as power-rail bypassing, DC blocking, bulk-power filtering, RC filtering, and more. Each of these often demands a capacitor with not only the right value but also the suitable materials, design, and fabrication tailored to the unique requirements of the application.
In AC/DC EMC-filter applications, there are two special classes of capacitors called X-capacitors and Y-capacitors used to filter AC power-source EMI; they are often collectively referred to safety capacitors. These capacitors are mandated by good design practices as well as regulatory standards. Note that this EMI “noise” is very different and potentially more harmful than basic signal noise which just degrades SNR; we’re talking about line power here.
X- and Y-capacitor basics
The X-capacitors are used for differential-mode EMI filtering, while Y-capacitors are used for common-mode EMI filtering by bypassing the interference from the wires to ground (Figure 1).
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| Figure 1. | The proper placement of the X- and Y-capacitors with respect to the AC line is well-defined and unambiguous. |
Class-X capacitors are used to minimize EMI/RFI that may be 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 negative 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.
Not just any capacitor of the right value is suitable for either X or Y roles. Class-X capacitors endure all the AC line variations and stress in their role of 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. Due to the risks of this over-stress situation, Class-X capacitors are designed to short-circuit when they fail, which will trigger the circuit breaker or fuse to break the supply circuit.
In contrast, class-Y capacitors are often referred to as “line to ground” or “line bypass” capacitors. Class-Y capacitors are placed in between the AC supply and ground to handle EMI/RF noise caused by common-mode noise on the AC line.
Their failure-mode situation is not the same as it is for Class-X capacitors. Class-Y capacitors are designed to fail open-circuit, as shorting of a Y-capacitor could present a fatal shock hazard for personnel using the equipment. While failing as an open circuit exposes the load circuit to an unfiltered AC power source, the risk of fire is reduced. Note that the failure mode designed into Class-X capacitors is the opposite of the mode for Class-Y capacitors. Both Class-X and Class-Y capacitors are needed in a typical AC power application (Figure 2).
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| Figure 2. | This schematic shows how to use both capacitors in an AC-line input circuit. |
Representative capacitance values for these capacitors are a function of the application specifics, current voltage and current levels, and other factors. They typically range from as low as 20 picofarads (pF) to 1,000 pF but can be larger.
There are many mandates on these capacitors, in addition to their failure mode and value. There are also various standards and associated sub-classifications (X1, X2, X3; and Y1, Y2, Y3, Y4), which indicate the capabilities and threshold of safety capacitors, with IEC 60384-14 the most widely used. It defines the safety classification of Class-X and Class-Y capacitors according to various levels of “peak voltage pulse” before failure.
These capacitors undergo strictly defined tests, and different technologies are used to meet these requirements. Ceramic capacitors and film capacitors can both be used for either Class-X or Class-Y applications, but their form factors and individual characteristics may make one type a better choice than the other in certain applications. Ceramics can achieve higher capacitance values in a smaller volume, while the film boasts an inherent self-healing feature.
Implications of these capacitors
There are additional implications when using these capacitors in an actual installation. In addition to the cost of the component itself, the X-capacitor used in EMC filters needs to be discharged when the AC line is disconnected (such as unplugging of the line cord). This ensures that excessive voltage doesn’t “linger” on the main cord, which presents an unexpected risk to the user who touches the exposed plug prongs.
The maximum allowable discharge time is governed by industry standards such as IEC60950 and IEC60065. This discharge requirement ensures that any high-voltage level present at the pins of the AC plug doesn’t present an electric shock hazard to a user. The standards require that the voltage across the X-capacitor decay with a maximum time constant of one second.
Typically, this requirement is met by including a resistor as a discharge element in parallel with the X-capacitor (sometimes called a “bleeder resistor”). However, this resistance results in continuous power dissipation that impacts the standby-power use.
The power dissipation in the discharge resistor depends on the X-capacitor value. At 230-V AC, with a discharge resistor that meets the time-constant requirement, that resistor results in dissipation of 5.3 milliwatt (mW) for every 100 nanofarad (nF) of X-capacitance. About 25 mW will be lost in the discharge resistor for a 470-nF X-capacitor value.
Supporting power ICs
Of course, vendors of power-related ICs see this as an opportunity. They offer ICs that can be placed in series with discharge (bleed) resistors that will 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.
For example, Texas Instruments offers the UCC28630 and UCC28633 ICs that 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.
Another example is from Power Integrations. When AC voltage is applied to its CAP300DG CAPZero-3 X-capacitor-discharge IC – formally called “zero loss automatic X capacitor rapid discharge IC with optional lossless zero crossing signal generator” – it blocks current flow in the X-capacitor safety-discharge resistors, thereby reducing the power loss to less than 5 mW (close to zero) at 230-V AC. When AC voltage is disconnected, CAP300DG automatically discharges the X-capacitor by connecting the series-discharge resistors.
The function and need for these X- and Y-capacitors are clear, at least once you know about them. But what have you done when you encounter a routine-looking passive component which you can identify, yet provides a function you cannot determine?