A Common Mode Choke, also known as a common mode inductor, is commonly used in the switch power supply of computers to filter out common mode electromagnetic interference signals. In board card design, the common mode choke also serves the function of EMI filtering, used to suppress the emission of electromagnetic waves generated by high-speed signal lines.
What is Common Mode Choke?
A common mode choke, also known as a common mode filter or common mode inductor, is an electronic component that is used to filter out common mode noise in a system. Common mode noise is a type of interference that is common to both lines in a pair of lines (for example, the positive and negative lines in a power supply or the two lines in a differential signal pair).
The common mode choke consists of two coils of wire wound around a magnetic core. The two lines to be filtered pass through these two coils. The choke works by presenting a high impedance to common mode signals (i.e., noise), which reduces the amplitude of these signals. At the same time, it presents a low impedance to differential mode signals (i.e., the desired signals), which allows these signals to pass through with minimal attenuation.
Common mode chokes are commonly used in power supply circuits to reduce electromagnetic interference (EMI), and in data transmission lines to reduce noise and improve signal integrity. They are also used in audio circuits to reduce hum and noise.
What is Electromagnetic Interference(EMI)?
The motherboard inside a computer is a mix of various high-frequency circuits, digital circuits, and analog circuits. When these circuits operate, they generate a large amount of high-frequency electromagnetic waves that interfere with each other, which is known as EMI (Electromagnetic Interference). EMI can also be emitted externally through motherboard wiring or external cables, causing electromagnetic radiation pollution that affects the normal operation of other electronic devices.
The chips on the PC board are both targets and sources of electromagnetic interference during their operation. Overall, we can divide these electromagnetic interferences into two categories: series mode interference (differential mode interference) and common mode interference (ground interference). Taking the two PCB traces (wires connecting various components on the motherboard) as an example, series mode interference refers to the interference between the two traces, while common mode interference is the interference caused by the potential difference between the two traces and the PCB ground. The series mode interference current acts between the two signal lines, and its conduction direction and waveform are consistent with the signal current; the common mode interference current acts between the signal line and the ground line, and the interference current flows through half of the two signal lines in the same direction, with the ground line as the common return path.
The Role of Common Mode Chokes
If the common mode current generated by the board card is not attenuated and filtered (especially the common mode current on high-speed interface wiring like USB and IEEE 1394), then the common mode interference current can easily generate electromagnetic radiation through the interface data line—common mode radiation caused by the common mode current in the cable. Standards such as the US FCC, the International Special Committee on Radio Interference's CISPR22, and China's GB9254 all have relevant restrictions on the common mode conduction interference and radiation emission of communication ports of information technology equipment. In order to eliminate the interference signals input on the signal line and the induced interference, we must arrange the filter circuit reasonably to filter the common mode and series mode interference. The common mode choke is a component of the filter circuit.
In essence, a common mode choke is a bidirectional filter: on one hand, it filters out the common mode electromagnetic interference on the signal line, and on the other hand, it suppresses itself from emitting electromagnetic interference externally, to avoid affecting the normal operation of other electronic devices in the same electromagnetic environment.
How Common Mode Choke Works
Here's how a common mode choke works:
Basic Structure: A common mode choke consists of two coils (windings) that are wound on a single magnetic core. The two lines of the signal (for example, the power and ground lines in a power supply) are passed through these two windings.
Functioning: In normal operation, the currents in these two lines are equal and opposite (they are 180 degrees out of phase with each other). This means that the magnetic fields generated by these currents in the choke's core cancel each other out. As a result, the choke presents very little impedance to the normal (differential mode) signal.
Filtering Common Mode Noise: However, if a common mode noise signal (which is in-phase on both lines) is present, the currents in the two windings add together. This creates a magnetic field in the choke's core, which causes the choke to present a high impedance to the noise signal, thereby reducing or eliminating it.
Suppressing EMI Radiation: By filtering out the common mode noise, the common mode choke can also help to suppress EMI radiation that could interfere with other devices.
In summary, a common mode choke works by presenting low impedance to normal signals and high impedance to common mode noise, thereby allowing the normal signals to pass while filtering out the noise. This makes it an important component in many types of electronic equipment, where it helps to ensure reliable operation by reducing EMI.
Types of Common Mode Chokes (CMCs)
Common Mode Chokes (CMCs) come in various types to suit different applications and requirements. They can be categorized based on their construction, mounting type, and the nature of the application they are intended for. Here are some common types:
Through-Hole CMCs: These chokes are designed to be mounted on a printed circuit board (PCB) by inserting their leads through holes in the board and then soldering them in place.
Surface Mount CMCs: These chokes are designed for surface-mount technology (SMT), where components are mounted directly onto the surface of a PCB. They are typically smaller and more compact than through-hole components.
Wire-Wound CMCs: These are made by winding wire around a magnetic core. They are typically used in applications requiring high inductance and current handling.
Toroidal CMCs: These use a toroidal (doughnut-shaped) core, which can provide better magnetic field containment and thus lower electromagnetic interference (EMI) than other shapes.
Rod CMCs: These use a rod-shaped core and are typically used in lower-frequency applications.
PCB Integrated CMCs: These are integrated directly into the layout of a PCB. They are used in applications where space is at a premium and a discrete component would be too large.
Power Line CMCs: These are designed specifically for power line applications, where they can help to reduce EMI on AC or DC power lines.
Data Line CMCs: These are designed for use on data lines, such as USB, Ethernet, or HDMI cables, where they can help to reduce EMI without affecting data transmission.
Each type of common mode choke has its own advantages and disadvantages, and the choice between them depends on the specific requirements of the application, including factors like the frequency of the noise to be filtered, the required current handling capability, the available space for the choke, and the method of mounting.
How to Choose Common Mode Chokes (CMCs)
Selecting an appropriate Common Mode Choke (CMC) primarily hinges on the specific demands of your application. Crucial parameters to consider include the choke's impedance at the interference frequency, which should be high for effective noise suppression, and the rated current it can sustain without thermal or magnetic saturation. The CMC should also demonstrate effective noise suppression across the entire frequency range of the interference.
Physical attributes such as size and mounting style should align with your circuit board design. The choke's DC Resistance (DCR) should ideally be low to reduce power loss, and for safety-critical applications, compliance with relevant safety standards is essential. While cost is invariably a factor, it's crucial to strike a balance between budget constraints and the choke's performance and features.
FAQ
1. What is Common Mode Noise?
Common mode noise is a type of electrical noise in which the same electrical potential is induced on both lines of a two-wire system. It can be caused by various factors such as electromagnetic interference (EMI) from other devices, power supply switching noise, or even the design of the circuit itself. This noise can interfere with the proper operation of electronic equipment and cause performance issues.
2. What is a Common Mode Signal?
A common mode signal is a type of signal in which the same voltage is applied to both lines of a two-wire system. In other words, the signal appears equally on both lines and in the same direction. This is in contrast to a differential mode signal, where the signal is applied to one line and the opposite signal to the other. While common mode signals can sometimes be a form of unwanted noise, they can also be used intentionally in certain types of communication systems, such as balanced line technology.