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How to Select X and Y Capacitors in Filter Design:?


Designing an effective filter for electromagnetic interference (EMI) suppression often involves the use of X and Y capacitors. These capacitors play a crucial role in ensuring safety and performance in power supply circuits, particularly in compliance with international standards. In this blog, we will delve into the types of X and Y capacitors, their roles, and how to select the right ones for your filter design.

Understanding X and Y Capacitors

X Capacitors are used between the line (live) and neutral wires of a power supply. They help in filtering differential mode noise, which is the noise present between these two lines.

Y Capacitors are connected between the line (or neutral) and earth (ground). They filter common mode noise, which is the noise that is common to both the line and neutral with respect to the ground.

Key Considerations for Selecting X and Y Capacitors

  1. Voltage Rating

    • X Capacitors: These must handle the full mains voltage and any transient spikes. Common ratings include 250V AC for domestic applications and higher for industrial applications.
    • Y Capacitors: These must withstand the voltage between the line/neutral and ground, including any surge. Typical ratings are 250V AC or 300V AC.
  2. Safety Standards

    • X Capacitors: Must comply with standards like UL, IEC, and CSA. The common class is X1, X2, or X3, depending on the maximum transient voltage they can withstand (X1 for up to 4 kV, X2 for up to 2.5 kV, and X3 for up to 1.2 kV).
    • Y Capacitors: Classified as Y1, Y2, Y3, or Y4 based on their voltage withstand capabilities (Y1 for up to 8 kV, Y2 for up to 5 kV, Y3 for up to 2.5 kV, and Y4 for up to 1.2 kV).
  3. Capacitance Value

    • X Capacitors: Typically range from 0.1 µF to 4.7 µF. Higher capacitance values are chosen for better noise suppression but must not affect the line impedance significantly.
    • Y Capacitors: Usually range from 4.7 nF to 0.1 µF. The capacitance must be small enough to prevent significant current leakage to the ground, ensuring user safety.
  4. Temperature Range

    • Ensure the capacitors can operate within the temperature range of your application. Common ranges are from -40°C to +105°C or higher for industrial applications.
  5. Physical Size and Lead Spacing

    • The capacitor size should fit within your PCB layout. Lead spacing (e.g., 5 mm, 10 mm) must match your board design for ease of assembly.

Steps to Select X and Y Capacitors

  1. Identify the Application Requirements

    • Determine the mains voltage and the type of noise (differential or common mode) you need to filter.
  2. Check Safety and Compliance Standards

    • Refer to the relevant safety standards (IEC, UL) for your application and region. Ensure the chosen capacitors meet these standards.
  3. Choose the Capacitance Value

    • For X Capacitors: Start with a typical value like 0.1 µF and simulate or test the filter performance. Adjust the value based on the EMI suppression effectiveness.
    • For Y Capacitors: Use values in the range of 4.7 nF to 0.1 µF. Smaller values are safer but may be less effective at filtering.
  4. Select the Voltage Rating

    • Ensure the voltage rating is higher than the maximum voltage the capacitor will experience, including surges.
  5. Evaluate Environmental Conditions

    • Choose capacitors that can withstand the operating temperature range and any harsh environmental conditions.
  6. Physical Fit

    • Verify that the capacitors physically fit on your PCB with appropriate lead spacing.

Practical Example

Let’s say you are designing a filter for a 230V AC mains power supply.

  1. Application Requirements:

    • Mains voltage: 230V AC.
    • Noise types: Both differential and common mode.
  2. Safety Standards:

    • X Capacitors: Compliance with IEC 60384-14.
    • Y Capacitors: Compliance with IEC 60384-14, typically Y2 class for up to 250V AC.
  3. Capacitance Values:

    • Start with 0.1 µF for the X capacitor.
    • Use 10 nF for the Y capacitors.
  4. Voltage Rating:

    • X Capacitor: Rated for 250V AC or higher.
    • Y Capacitor: Rated for 300V AC or higher.
  5. Environmental Conditions:

    • Capacitors rated for -40°C to +105°C.
  6. Physical Fit:

    • Ensure the chosen capacitors fit within your PCB layout, e.g., 10 mm lead spacing.


Selecting the right X and Y capacitors is crucial for the safety and performance of your filter design. By understanding the application requirements, safety standards, and key characteristics of these capacitors, you can ensure effective EMI suppression and compliance with international standards. Always refer to manufacturer datasheets and safety certifications to make informed decisions.

Feel free to reach out with any questions or comments about your specific design needs. Happy designing!

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