Advantages of switching power adapters include low power consumption, high conversion efficiency, compact size, and light weight. However, because switching power supplies always operate at high frequencies, they generate many high-frequency harmonic components. These harmonic components can interfere with the normal operation of other devices through circuitry and space radiation, causing issues such as interference patterns on AV displays or audible current noise in audio, which can affect the normal usage of products.
The interference caused by switching power adapters can be classified into electromagnetic radiation interference (EMI) and radio frequency interference (RFI).
Now let’s analyze the causes of interference generated by power adapters：
1. Interference caused by switching MOSFETs
The power MOSFETs in switching power adapters work in a switching state, which generates large pulse voltages and currents due to the on/off switching. These pulse currents and voltages contain high-order harmonics. When MOSFETs are turned on, electromagnetic oscillation is formed due to the leakage inductance of the transformer and the recovery characteristics of the output rectifier diode. This generates surge voltages on the diode. When the switch is turned off, the sudden change in current caused by the leakage inductance of the high-frequency transformer windings also produces surge voltages. All of these become sources of noise interference.
2. Interference generated by high-frequency transformers
Most of the magnetic flux generated by current in the windings of high-frequency transformers flows through the high-permeability magnetic core. However, a small portion of it radiates outside through the windings and gaps, forming leakage magnetic flux, which results in electromagnetic induction interference.
3. Interference generated by rectification filter capacitors Switching power adapters typically have a high-voltage 220VAC rectification and filtering circuit at the front end of the mains input. The conduction angle of the rectifying diode is small, and the peak value of the rectification current is large. This pulsed rectification current also produces interference.
4. Interference generated by the recovery characteristics of Schottky diodes
During high-frequency rectification by diodes, the existence of junction capacitance causes high-frequency attenuated oscillations during the reverse recovery period, resulting in interference. The peak interference and harmonic interference energy generated by switching power adapters, propagating through the input and output circuits of the power supply, are known as conducted interference. Harmonic and parasitic oscillation energy propagating through the input and output circuits also generate electric and magnetic fields in space. This type of interference generated through electromagnetic radiation is called radiated interference. This interference can be eliminated by using absorbing circuits or resonant switch technology.
Interference suppression in switching power adapters:
Suppressing interference has always been a concern for switching power supply designers. With the increasing requirements of international standards for switching power supply products, interference suppression techniques have become one of the hot research topics in the power supply industry.
a. Employing shielding measures
Using shielding technology can effectively suppress the electromagnetic radiation interference of switching power adapters. To suppress radiation from switching power adapters, shielding covers or heat sinks can be processed according to magnetic field shielding methods. Then, the entire shielding cover can be connected to the system’s chassis and ground, providing effective shielding against electromagnetic radiation.
b. Suppressing interference sources’ voltage levels
Interference is caused by rapid changes in voltage and current, which are caused by power switches, rectifier diodes, and surrounding circuits. To minimize interference, it is important to reduce the rate of change of voltage and current waveforms. Absorption circuits can be used to reduce surge voltage and minimize the leakage inductance of switching power transformers.
c. Using RC absorption circuits to suppress interference
RC and RCV absorption circuits can reduce the effects of pulse currents and voltage variations, thereby improving the working conditions of switching power supply circuits and reducing interference.
d. Employing grounding measures
In circuit design, the “single point ground” principle should be followed. If multiple grounding points are formed, a closed grounding loop may occur, resulting in magnetic induction noise when magnetic lines pass through the loop. In circuit systems where low-frequency and high-frequency circuits coexist, the grounding wires of low-frequency, high-frequency, and power circuits should be connected separately before being connected to a common reference point.
e. Using LC circuits to suppress interference
By utilizing LC suppression circuits, both the input side and the output side of the power supply can be bidirectionally suppressed for interference signals. After the common-mode suppression inductance is connected in parallel with the suppression capacitor, an LC suppression circuit is formed.
There are various methods to suppress interference in power adapters. The power adapters that have passed CE-EMC and FCC certifications can generally solve electromagnetic interference problems encountered in device compatibility. Only a very small number of devices may require special interference suppression measures.
Post time: Aug-08-2023