A common mode choke (CM choke) is an electrical component used to suppress common

mode electromagnetic interference (EMI) in electronic circuits. It consists of a magnetic core

with two or more windings of wire (typically bifilar or trifilar wound) that pass through it.
Common mode chokes are primarily used in circuits where differential signals (signal and
return path) are present, and they help filter out unwanted common mode noise while
allowing differential signals to pass through unaffected.

Key Characteristics and Functions:

1. Suppression of Common Mode Noise:

o Common mode chokes attenuate common mode noise, which is noise that
appears simultaneously and identically on both conductors (signal and ground)
of a transmission line.
o They are effective in reducing interference from sources like electromagnetic
fields, power line noise, and radio frequency interference (RFI).

2. Operating Principle:
o The magnetic core of the common mode choke creates inductance around the
conductors passing through it.
o Differential signals (equal and opposite currents in the conductors) see equal
and opposite inductances, thus cancelling each other out and allowing the
signal to pass unaffected.
o Common mode signals (equal currents in both conductors) see the same
inductance in both windings, which adds impedance to the noise, attenuating
it.

3. Construction:
o Typically, common mode chokes consist of two windings wound on a
magnetic core. The windings may be wound in parallel (bifilar) or separately
(trifilar) depending on the design requirements.
o The number of turns and the core material are chosen based on the frequency
range of the interference to be filtered.

4. Applications:
o Power Supplies: Used to reduce EMI in power supply lines, both at the input
(from mains) and output (to load).
o Communication Lines: Applied in data transmission lines (Ethernet, USB,
etc.) to improve signal integrity by reducing noise.
o Electronic Circuits: Integrated into PCB designs to minimize electromagnetic
interference that could affect sensitive analog or digital circuits.

5. Advantages:
o Effectively reduces common mode noise without affecting differential signals.
o Compact size and relatively simple to integrate into circuit designs.
o Provides improved electromagnetic compatibility (EMC) and reliability in
electronic systems.

6. Disadvantages:
 o Can introduce additional resistance and parasitic capacitance, affecting signal
integrity at high frequencies if not properly selected.
o Requires careful consideration of placement and integration to maximize
effectiveness.

In summary, common mode chokes are essential components for suppressing common mode
noise in electronic circuits, ensuring reliable operation and compliance with electromagnetic
compatibility standards. They play a critical role in reducing interference and improving
signal quality in various applications ranging from power supplies to communication
systems.

Selecting a common mode choke (CM choke) for a DC power line involves considering
several key parameters to ensure effective suppression of common mode noise while
maintaining the integrity of the DC power supply. Here are the steps and considerations for
selecting a CM choke:

Steps to Select a Common Mode Choke for DC Power Line:

1. Identify Operating Conditions:

o Voltage Rating: Determine the maximum DC voltage (V_DC) that the choke
will need to withstand. Choose a choke with a voltage rating comfortably
above the maximum DC voltage in the circuit to ensure reliability and safety.
o Current Rating: Determine the maximum DC current (I_DC) that will flow
through the choke. Select a choke rated for at least this current to prevent
overheating and ensure continuous operation.

2. Frequency of Operation:
o Switching Frequency: If the DC power line includes switching converters or
other high-frequency components, identify the frequency range over which
common mode noise needs to be suppressed. This helps in choosing a choke
with appropriate impedance characteristics across the relevant frequency
range.

3. Impedance Characteristics:
o Impedance vs. Frequency: Review the impedance vs. frequency
characteristics of potential CM chokes. Ensure that the chosen choke offers
significant impedance at the frequencies where common mode noise is likely
to occur.
o Frequency Range: Select a choke with a frequency range that encompasses
the switching frequency of the power converters or other noise sources in the
circuit.

4. Inductance Value:
o Inductance: Choose the inductance value of the choke based on the desired
impedance at the operating frequency. Higher inductance values typically
provide higher impedance against common mode noise but may affect signal
integrity or introduce voltage drops if not carefully selected.
o Trade-offs: Balance between achieving sufficient noise suppression and
minimizing any adverse effects on the DC power line's performance, such as
voltage regulation or efficiency.
 5. Physical Size and Mounting:
o Size and Mounting: Consider the physical size and mounting options of the
CM choke to ensure compatibility with the PCB layout and enclosure
constraints. Evaluate whether surface mount (SMD) or through-hole mounting
is more suitable for your application.

6. Temperature and Environmental Considerations:

o Operating Temperature: Ensure the CM choke is rated for the expected
temperature range of the application environment to maintain reliable
performance.
o Environmental Factors: Consider any environmental conditions such as
humidity or vibration that may affect the performance and longevity of the
choke.

7. Compliance and Standards:

o EMI Standards: Ensure that the chosen CM choke complies with relevant
electromagnetic interference (EMI) standards or regulations applicable to your
industry or application. This ensures that the DC power line meets EMC
requirements.

Example Selection Process:

 Example: For a DC power line with 24V DC and a maximum current of 5A, you
might select a CM choke rated for 30V DC and 10A, with an appropriate impedance
at frequencies up to 1 MHz (suitable for switching converter noise suppression).

Summary:

Selecting a common mode choke for a DC power line involves balancing parameters such as
voltage and current ratings, impedance characteristics, frequency response, and physical
considerations. By carefully evaluating these factors, you can ensure effective suppression of
common mode noise and reliable operation of the DC power supply in your application.

Common mode chokes (CM chokes) can be categorized into different types based on their
construction, intended application, and performance characteristics. Here are the main types
of common mode chokes commonly used in electronic circuits:

Types of Common Mode Chokes:

1. Ferrite Core Chokes:

o Description: Ferrite core chokes are the most common type of common mode
chokes. They use a magnetic core made from ferrite material, which provides
high permeability and effective suppression of common mode noise across a
wide frequency range.
o Applications: Used in various electronic circuits, power supplies, and data
communication lines where effective EMI suppression is required.

2. Toroidal Core Chokes:

 o Description: Toroidal core chokes have a ring-shaped magnetic core (toroid),
which allows for efficient winding of multiple turns of wire around the core.
o Advantages: Toroidal cores offer high inductance and are effective in
reducing electromagnetic interference due to their compact size and uniform
magnetic field distribution.
o Applications: Commonly used in audio equipment, power supplies, and other
sensitive electronic devices where space efficiency and high-performance EMI
filtering are critical.

3. Bobbin Wound Chokes:

o Description: Bobbin wound chokes feature a magnetic core (typically ferrite
or iron powder) with windings wound on a bobbin or spool structure.
o Advantages: Provides flexibility in winding configuration and wire
placement, allowing for customization of inductance and impedance
characteristics.
o Applications: Found in power supplies, industrial equipment, and electronic
devices where precise filtering and noise suppression are required.

4. Surface Mount (SMD) Chokes:

o Description: Surface mount common mode chokes are designed for direct
placement and soldering onto PCBs without the need for through-hole
mounting.
o Advantages: Compact size, suitable for high-density PCB designs. They offer
good performance in EMI suppression and are compatible with automated
assembly processes.
o Applications: Widely used in consumer electronics, telecommunications, and
automotive electronics for noise filtering and EMI compliance.

5. Through-Hole Chokes:
o Description: Through-hole common mode chokes have leads that are inserted
through holes in the PCB and soldered on both sides to provide mechanical
stability.
o Advantages: Provide robust mechanical support and high current handling
capability compared to surface mount chokes.
o Applications: Commonly used in industrial equipment, power distribution
systems, and applications requiring high reliability and rugged construction.

6. Balun Transformers:
o Description: Balun (balanced-unbalanced) transformers are specialized
common mode chokes that convert between balanced and unbalanced signal
formats.
o Applications: Used in communication systems, antennas, and network
interfaces to match impedance and suppress common mode noise effectively.

Selection Considerations:

 Frequency Range: Choose a common mode choke with impedance characteristics

suitable for the frequency range of the noise to be suppressed.
  Inductance and Impedance: Select appropriate values of inductance and impedance
to achieve effective EMI suppression without affecting signal integrity.
 Current Rating: Ensure the choke can handle the maximum current expected in the
application without saturation or excessive heating.
 Mounting Type: Consider whether surface mount or through-hole mounting is more
suitable based on PCB design and assembly requirements.

In summary, the selection of common mode chokes depends on the specific application
requirements, including frequency range, current handling capacity, space constraints, and
environmental factors. Choosing the right type and specifications ensures optimal
performance in reducing electromagnetic interference and improving the overall reliability of
electronic systems.

3.5
what is common mode noise
ChatGPT

Common mode noise refers to unwanted electrical interference or disturbances that appear
simultaneously and identically on both conductors (usually signal and ground) of a
transmission line or power line. This type of noise is characterized by having equal
magnitudes and phases with respect to ground or a reference point, affecting both conductors
in the same manner.

Characteristics of Common Mode Noise:

1. Source:
o External Sources: Common mode noise often originates from external
electromagnetic fields (EMI) generated by nearby electronic devices, power
lines, radio frequency interference (RFI), or electrical motors.
o Internal Sources: It can also be generated internally within circuits due to
improper grounding, switching operations, or signal reflections.

2. Nature:
o Symmetrical: Common mode noise appears as symmetric signals
superimposed on the desired signals, affecting both conductors in the same
direction (either positive or negative).
o Low Frequency to High Frequency: It can occur across a wide frequency
spectrum, from low frequencies (50/60 Hz mains hum) to high frequencies
(MHz and GHz ranges).

3. Effects:
o Interference: Common mode noise interferes with the integrity and accuracy
of signals in electronic circuits, potentially leading to errors in data
transmission, degraded signal quality, or malfunction of sensitive equipment.
o EMI/RFI Compliance: Excessive common mode noise can cause non-
compliance with electromagnetic interference (EMI) regulations or standards,
affecting the reliability and safety of electronic devices.

4. Mitigation:
 o Common Mode Chokes: Effective in mitigating common mode noise by
providing high impedance paths to ground for common mode currents while
allowing differential signals to pass through.
o Shielding and Grounding: Proper shielding techniques and grounding
practices help minimize the coupling of external EMI/RFI sources into
sensitive circuits.
o Filtering: Use of capacitors, ferrite beads, and other filtering components to
suppress common mode noise at specific frequencies.

Examples:

 Power Lines: Common mode noise on AC power lines can result from switching
power supplies, electronic ballasts, or electrical appliances.
 Data Lines: In data communication, common mode noise can be induced by nearby
electrical equipment or environmental sources, affecting signal integrity in Ethernet,
USB, or HDMI cables.

Importance:

Managing common mode noise is critical in electronic design to ensure reliable operation and
compliance with EMI/RFI regulations. Techniques such as proper grounding, shielding, and
the use of common mode chokes are essential to minimize interference and maintain the
performance of electronic systems.

A common mode choke is primarily required at the input voltage path of electronic circuits or
systems for several important reasons related to electromagnetic interference (EMI)
suppression and overall system reliability:

1. Suppression of Common Mode Noise:

o The primary function of a common mode choke is to attenuate common mode
noise that may be present on the input voltage lines. Common mode noise
often originates from external sources such as electromagnetic fields (EMI) or
radio frequency interference (RFI).
o Placing a common mode choke at the input helps to filter out this unwanted
noise, which if left unchecked, can interfere with the proper operation of
sensitive electronic components and circuits.

2. Protection of Sensitive Components:

o Electronic circuits often have sensitive components such as microcontrollers,
analog-to-digital converters (ADCs), or communication interfaces that can be
adversely affected by common mode noise.
o By placing a common mode choke at the input voltage path, these components
are shielded from common mode noise, ensuring their reliable operation and
prolonging their lifespan.

3. Compliance with EMI/RFI Standards:

o Many electronic devices and systems need to comply with electromagnetic
interference (EMI) standards and regulations to ensure electromagnetic
 compatibility (EMC) with other devices and to prevent interference with radio
and communication signals.
o Common mode chokes help to mitigate common mode noise, thereby aiding in
compliance with these standards and ensuring that the device operates within
permissible limits of interference.

4. Improvement of Signal Integrity:

o In applications where analog signals or high-speed digital signals are involved,
maintaining signal integrity is crucial. Common mode noise can degrade
signal quality, leading to errors or distortion in data transmission.
o By filtering out common mode noise at the input, common mode chokes
contribute to preserving signal integrity and ensuring accurate data
transmission.

5. Reduction of System Vulnerability:

o Electronic systems are vulnerable to external noise and interference,
particularly at the input where signals are most susceptible.
o Common mode chokes provide a first line of defense against these external
influences, reducing the likelihood of system malfunctions or failures caused
by noise-induced disturbances.

Summary:

Common mode chokes are strategically placed at the input voltage path of electronic circuits
to effectively suppress common mode noise originating from external sources. This
placement not only protects sensitive components from interference but also enhances overall
system reliability and compliance with EMI/RFI standards. By filtering out common mode
noise, common mode chokes play a critical role in ensuring the proper operation and
longevity of electronic devices and systems.

Common mode chokes (CM chokes) play a crucial role in reducing electromagnetic
interference (EMI) and ensuring electromagnetic compatibility (EMC) in electronic circuits
and systems. Here’s how CM chokes help in these aspects:

1. Filtering Common Mode Noise:

 Attenuation of Common Mode Noise: CM chokes are designed to suppress common

mode noise, which is noise that appears simultaneously and identically on both
conductors (signal and ground) of a transmission line.
 Mechanism: They achieve this by presenting a high impedance path to common
mode currents, effectively blocking or attenuating unwanted noise signals.
 Benefits: By reducing common mode noise, CM chokes prevent interference with
sensitive electronic components and circuits, thereby improving the overall
performance and reliability of the system.

2. Compliance with EMI/EMC Standards:

  Regulatory Compliance: Many electronic devices and systems must meet stringent
EMI/EMC standards to ensure they do not interfere with other devices and can
operate without being affected by external electromagnetic disturbances.
 Role of CM Chokes: CM chokes aid in achieving compliance by mitigating common
mode noise, which is a common source of electromagnetic interference. They help
maintain the electromagnetic compatibility of the device or system within acceptable
limits.

3. Protection of Signal Integrity:

 Preservation of Signal Quality: In applications where analog signals, high-speed

digital signals, or communication signals are involved, maintaining signal integrity is
critical.
 Interference Reduction: CM chokes reduce interference that can degrade signal
quality, ensuring accurate data transmission and reliable operation of sensitive
circuits.

4. Applications Across Various Systems:

 Power Lines: CM chokes are commonly used on power supply lines to filter out
noise from switching power supplies or external EMI sources.
 Communication Lines: They are also employed in data communication lines
(Ethernet, USB, HDMI, etc.) to prevent noise from affecting signal transmission.
 Industrial and Automotive: Found in industrial equipment, automotive electronics,
and other applications where robust EMI/EMC performance is essential.

5. Practical Considerations:

 Design Integration: CM chokes are integrated into circuit designs either as discrete
components (through-hole or surface mount) or as part of integrated solutions.
 Selection Criteria: Choosing the right CM choke involves considerations such as
frequency range, impedance characteristics, current handling capacity, and
environmental conditions to ensure optimal performance in reducing EMI/EMC.

In summary, common mode chokes are critical components for mitigating common mode
noise and ensuring EMI/EMC compliance in electronic systems. By effectively filtering out
unwanted noise, CM chokes help maintain signal integrity, protect sensitive components, and
enhance the overall reliability of electronic devices and equipment.