Common forms of conduction include thermal conduction and electrical conduction. Thermal conduction refers to the transfer of heat or electricity from one part of an object to another. Heat travels from a warmer part of an object to a cooler part along the same path; this is called thermal conduction. Conduction is one of the three modes of heat transfer (conduction, convection, and radiation). Thermal conduction is the primary mode of heat transfer in solids. In gases or liquids, thermal conduction often occurs simultaneously with convection. All substances can conduct heat, but different substances have different heat transfer capabilities.
Heat transfer occurs from a high-temperature region to a low-temperature region without the transfer of matter. Heat conduction originates from the transfer of kinetic energy generated by collisions between atoms and molecules in gases, liquids, and non-metallic solids. Metals are excellent thermal and electrical conductors; energy within metals is transferred through collisions between free electrons passing through the crystal lattice and ions within the lattice. Also known as thermal conductivity, it is one of the three fundamental modes of heat transfer. Heat transfer in solids is the process by which heat is transferred from a higher-temperature region to a lower-temperature region or to another lower-temperature object in contact with it through collisions between molecules, atoms, or electrons. This is the primary mode of heat transfer. In liquids and gases, it often occurs simultaneously with convective heat transfer. All objects, regardless of the relative motion between particles within them, exhibit heat conduction as long as a temperature difference exists. Many industrial processes rely primarily on heat conduction, such as the heating and vulcanization of rubber products and the heat treatment of steel forgings. The laws of heat conduction are essential in the design and calculation of kilns, heat transfer equipment, and thermal insulation, as well as in the design of high-temperature and high-pressure equipment (such as waste heat boilers in ammonia synthesis towers).
The conduction of electricity occurs through the migration of current through matter.
In physics, electrical conduction refers to the process of electric current moving through matter. In good conductors such as metals, electrical conduction mainly originates from the movement of free electrons in one direction under the influence of an electric field. In liquid conductors, electrical conduction is due to the migration of positive ions in one direction and negative ions in the opposite direction. In gases, electrical conduction is due to the flow of positive ions in one direction and electrons in another. Electrical conduction in semiconductors originates from the migration of electrons in one direction and positive electrons in another.
In specific applications of power adapters, conducted emissions are generated by differential-mode current noise and common-mode current noise. Differential-mode current flows in opposite directions between the two input power lines, forming a current loop; common-mode current flows in the same direction on the two input power lines, forming a current loop with ground. This current noise and interference are conducted through high-frequency interference within the power supply (typically 150kHz to 30MHz), and these conducted signals are received and analyzed using specific measurement methods.
Solving power supply conduction problems requires identifying the pathways to reduce interference received at the ports, which involves research and practice in electromagnetic compatibility (EMC). In switching power supply design, EMC mechanisms include pulse width modulation (PWM), pulse frequency modulation (PFM), and hybrid modulation, all of which affect the power supply's conduction characteristics.
question
How is power conduction formed? What are the conduction paths? What are the common methods? What factors affect power supply radiation? How to conduct EMC for high-power circuits?
The power conduction measurement method involves receiving high-frequency interference (typically 150K to 30M) from inside the power supply through the input ports L, N, and PE.
To resolve conduction issues, it is essential to determine the pathways through which to reduce the interference received at the port.
As shown in the figure: there are generally two modes: L and N differential mode components, and common mode components through the PE ground loop. Some frequencies have both differential and common modes.
Filtering methods typically involve a two-stage common-mode circuit with a Y capacitor. The choice of inductor and its placement significantly impacts the filtering performance. A low-inductance (U) inductor, preferably nickel-zinc alloy specifically designed for high frequencies, is usually placed near the input ports. It should be wound with two parallel wires to minimize differential-mode components. The output stage typically uses a larger inductance, around 4MHz to 10MHz, but this is based on experience and requires specific pairing with the Y capacitor. The X capacitor, used for differential-mode filtering, also needs to be placed near the input ports, usually between the two common-mode stages. When placing the Y capacitor, the traces need to be thickened during PCB layout; external mounting is not recommended as it will significantly reduce effectiveness. (These techniques only address input filtering network considerations.)
Of course, one can also address the issue at its source. Conduction is a result of radiation coupling into the circuit, so reducing switching radiation can also benefit conduction. Several factors typically affect radiation, including the turn-on speed of MOSFETs, the turn-on and turn-off of rectifier diodes, transformers, and PFC inductors, among others. The design of these circuits requires compromises with other aspects and will not be detailed here.
Some tips and tricks: For high-power EMC, adding shielding usually provides immediate results. There are generally several options for shielding locations:
First: Shielding between the input EMI circuit and the switching transistor. This has a great effect on EMC, and this method is generally very effective for many problems that are ineffective with filters.
Second: Shielding of the primary and secondary windings of the transformer. If there is space, it is best to add shielding when designing the transformer.
Third: The location of the heat sink can serve as a good shield, and proper layout and selection of the heat sink grounding are also very important.
Fourth: Determining the location of the radiation source generally involves several simple methods, though they may not be entirely accurate. These can be used as a reference. If adding a ferrite core to the input line is beneficial for EMC, it's usually the primary-side MOSFET. If adding a ferrite core to the output line is effective for EMC, it's usually the secondary-side output rectifier, especially at high frequencies above 100MHz. Consider adding a capacitor or common-mode inductor to the output.
Of course, there are many other details and techniques, especially regarding the layout of the circuit board.
