EMI Filter – Proper Integration

Proper integration of an EMI filter is a critical component in the design of modern electrical systems, especially in light of the increasing integration of switched electronic systems into the power grid. As a result, electromagnetic requirements (EMC/EMI) are becoming increasingly strict.
EMI filters are essential for preventing interference between devices, ensuring proper functioning in noisy electromagnetic environments, and complying with regulatory standards. In the military field, this is also critical for preventing information leakage.

In this article, we will review the main considerations for the proper integration of an EMI filter.

1. Basic Concepts:

• EMI and EMC interference:
  Electromagnetic interference (EMI) refers to unwanted noise generated in electrical systems, especially by high-frequency switching converters. This noise can propagate in two main ways:
• Conducted EMI:
  Noise currents that spread through the system’s power cables. In the general industry, the frequency range is typically measured between 150 kHz and 30 MHz, and in the military field between 14 kHz and 18 or 40 GHz.
• Radiated EMI:
  Electromagnetic energy emitted from the source into open air.
• Conducted EMI is divided into two types:
  • Differential Mode Noise (DM):
    Noise that flows in opposite directions along the power lines. It is directly related to the switching current of the noise source. DM noise appears mainly at low frequencies (up to a few hundred kHz).
  • Common Mode Noise (CM):
    Noise current flows in the same direction on both power lines, through parasitic capacitance between high dv/dt points and ground.

2. Passive vs. Active Filter:

• Passive Filters:
  A common solution consisting of inductors and capacitors. These filters can provide high attenuation but tend to be bulky and expensive, and may introduce parasitic components into the overall system. In addition, they may cause instability due to resonance if not properly calculated.
• Active EMI Filters (AEF):
  An innovative technology designed to reduce the filter’s size and decrease the cost of passive components. These filters use feedback control to detect noise and inject a canceling current or voltage. These filters offer good EMI attenuation performance and are more effective at low frequencies.

3. Structure of a Passive EMI Filter:

A passive EMI filter usually consists of inductors, capacitors, and resistors.

• X Capacitors (Cx):
  Connected between phases or between phase and neutral. These capacitors attenuate symmetrical (DM) interference at low frequencies.
• Y Capacitors (Cy):
  Connected between the phase or neutral conductor and ground. These capacitors primarily attenuate asymmetrical (CM) interference at high frequencies. Y capacitors are a main source of leakage currents.
• Inductors:
  Used to suppress CM noise by creating high inductance for CM interference and low inductance for differential-mode operating current.
• Cascading Filter Sections:
  By connecting several LC filter units in series, higher attenuation at high frequencies can be achieved in smaller volume and weight.

4. Influence of Operational Factors on Filter Performance:

• Input Voltage:
  A high input voltage increases the peak level of harmonics in passive filters, mainly due to the DC bias effect in ceramic capacitors, which changes their capacitance.
• Load Characteristics:
  The most difficult differential interference usually occurs at maximum load current. Also, filter performance depends on the input impedance (equivalent resistance) of the load. A load with varying input impedance (dynamic or changing load) will result in varying filtering performance.
• Switching Frequency:
  High-frequency switched-mode power converters generate EMI noise around their switching frequency. The switching frequency affects the filter’s efficiency in this frequency range.

5. Leakage Currents:

• Definition:
  Leakage current is the current that flows to ground under normal operation and during a fault condition such as a short between one of the supply conductors and ground. Leakage current is mainly caused by the Y capacitor in filters.
• Types of Leakage Currents:
  • Stationary Leakage Currents:
    These currents result from grid frequency harmonics such as 150 Hz, 450 Hz, 750 Hz.
  • Variable Leakage Currents:
    These currents depend on the switching frequencies of the noise source (such as a voltage converter) or on the capacitance of cables and electric motors.
• Impact on Leakage Currents:
  The main factors that reduce leakage current are:
  • Supply voltage stability
  • Current balance between phases in a three-phase system
  • Proper matching of Y capacitors
• Dangers of High Leakage Currents:
  It is well known that leakage current above 30 mA passing through the human body may cause cardiac arrest or severe damage.
  There are standards that limit maximum leakage current – for example, IEC60950 limits it to 3.5 mA.
  In filters, the leakage current during a first fault (a short between a supply conductor and ground) is just as important as the natural leakage current.
• Solutions for Low Leakage Current:
  There are filters with low leakage, which typically have lower attenuation performance than high-leakage filters.
  Some companies, such as EMIS, have developed filters with very low leakage and high attenuation, even capable of fully complying with red-black standards.
• Floating / IT Systems:
  Floating systems are those with a complete disconnection between neutral and ground. In such systems, it is important to consider the leakage current during a first fault (when one of the supply conductors touches ground).
  In this condition, the filter becomes unbalanced, and the leakage current can reach up to 10 times higher than the natural leakage current.
  As noted, there are advanced passive filter technologies that offer high filtering performance with very low leakage currents.

6. Proper Installation:

• Grounding:
  Proper grounding is critical and requires a system with very low impedance for high-frequency signals.
  Use grounding connections with the lowest possible resistance and shortest possible length.
  The ground conductor should have the largest possible cross-section.
  Ground directly to the grounding bus bar.
• Wiring and Connections:
  Separate power and control wiring. Route conductors close to the device body and ensure that the input and output conductors of the filter are separated and distanced.
  If necessary, use shielded cables grounded at both ends.
• Filter Mounting:
  Install the filter as close as possible to the EMI noise source or to the power entry point.
• Current Carrying Capability:
  The current rating of the filter must be equal to or higher than the circuit breaker or fuse protection rating.
  Identical filters can be connected in parallel to achieve higher current capacity, provided that the connection allows and ensures equal current distribution between the filters.

7. Success:

Successful EMI design is a growing challenge in the development of equipment such as power supplies, converters, and complex complete systems such as server rooms or military vehicle compartments.
To meet these challenges, filters must be designed and selected from day one of the design process, with the appropriate attenuation, frequency range, and desired leakage current.

Article Author: Tamir Dahari, TECT T.D. LTD