Where SMD Y Capacitors Are Used and Why
A Y capacitor is a safety component. It connects a live circuit to earth, or across an isolation barrier, and it is designed so that if it fails it fails open rather than shorting. Because it sits across the barrier that protects the user, its construction, its rating and its approval are all regulated rather than chosen freely.
In practice most Y capacitors are used to divert common mode noise to earth, and the value that can be used is limited by the leakage current that the safety standard allows. That limit, rather than the filtering requirement, is what usually decides the value. Understanding where the parts are actually used explains why the same few values appear across very different products.
What Makes a Capacitor a Y Capacitor
Y capacitors are classified by their safety category, which defines the impulse voltage they must withstand and the situation in which they may be used. The two common classes cover different working voltages and different applications, from a line to earth connection in an appliance to an unearthed connection in a high voltage system. A part that is not certified to the correct class should not be used across an isolation barrier, whatever its capacitance and voltage rating.
Physically the parts are built for reliability rather than for capacitance density. Ceramic discs and multilayer chips are the common forms, and the dielectric is chosen for stability under voltage and temperature rather than for a high dielectric constant. The approved types are also tested for a defined number of pulses at a defined voltage, because the failure mode of concern is one that develops over time rather than at first use.
Where They Are Used
The classic application is the input filter of an offline power supply, where two Y capacitors from live and neutral to earth form a common mode filter with a common mode choke. The values are typically in the range of one to four point seven nanofarads, chosen to keep the earth leakage current below the limit while providing enough impedance at the switching frequency to reduce the emission.
They appear again across the isolation barrier of a switched mode supply, where they provide a return path for common mode current that would otherwise flow through the parasitic capacitance of the transformer and appear as emission. In a power supply that meets a tight emission limit, this is often the single most effective component, and it is the reason a supply with a Y capacitor can pass a test that an otherwise identical design fails.
Leakage Current and Its Limit
The current that flows through a Y capacitor at the mains frequency is proportional to its capacitance and to the voltage across it. A limit of a fraction of a milliamp is typical for a portable appliance, and the calculation shows that this permits only a few nanofarads at a nominal line voltage. The limit becomes tighter as the requirement for patient or user safety rises, which is why medical equipment often uses much smaller values or a different topology.
The capacitance is also affected by the voltage and the temperature, and by the ageing of the dielectric. A part that measures two point two nanofarads at zero volts may measure significantly less at the working voltage, and the filter that was designed around the nominal value may not achieve the attenuation that was calculated. Using the value at the working voltage, and keeping margin for ageing, is part of designing a filter that continues to pass the emission test throughout production.
Physical Layout and Creepage
Because the capacitor bridges the isolation barrier, its placement on the board defines where that barrier runs. The creepage and clearance distances around the part must satisfy the standard for the working voltage, and the pad geometry must not reduce them. Slots cut into the board are sometimes used to increase the creepage along the surface, and the capacitor is placed so that the slot passes beneath it rather than around it.
The layout also affects the effectiveness of the filter. A Y capacitor that is connected to earth through a long thin trace has a high impedance at high frequency and does not divert the noise it was placed to divert. The earth connection should be short and wide, and it should return to the same reference point as the other filtering components. Where several Y capacitors are used, their earth connections should meet at a common point rather than being daisy chained, so that the noise current from one does not flow through the earth path of another.
Selection and Verification
Selection starts with the safety class rather than with the capacitance. The class determines which parts are permitted, and the permitted parts determine the values and the package sizes that are available. The value then follows from the leakage current limit and from the attenuation required at the frequency of interest, and the voltage rating is set by the standard with a margin rather than by the steady state voltage alone.
Verification combines a measurement of the leakage current on the finished product with an emission test at the frequencies the filter is intended to suppress. The leakage current is normally measured with the product in its worst case configuration, which may be at the high line voltage and with the earth connection at its maximum permitted impedance. Because the capacitor is a safety component, any change of supplier or of value should be treated as a change to the safety case, and the EMI suppression design and the approval documentation should both be updated before the change is released.
Process Control and Verification
On a design of this kind, common mode noise is the item that decides how the rest of the board is arranged. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.
Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.
Related reading: our fabrication notes, board quality and design release notes cover the same ground.
Process Control and Verification
On a design of this kind, common mode noise is the item that decides how the rest of the board is arranged. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.
FAQ
Can an ordinary ceramic capacitor be used as a Y capacitor? No, even if its voltage rating is adequate. The safety class defines the construction and the testing, and an uncertified part does not provide the failure mode that the barrier relies on.
Why are the values so small? Because the leakage current through the capacitor at the mains frequency is limited by the safety standard. The value is set by that limit rather than by the filtering requirement.
Does a Y capacitor always reduce emission? It reduces common mode emission when the earth connection has a low impedance. With a poor earth path it can make the emission worse by moving the noise to another part of the circuit.
