Interference and Suppression of Switching Power Supply
The interference generated by the switching power supply itself directly endangers the normal operation of electronic devices. Suppressing the electromagnetic noise of the switching power supply itself is an important topic in the development and design of switching power supplies. This article briefly introduces the mechanism of electromagnetic interference generation and propagation in switching power supplies, and summarizes several main methods for suppressing the generation and propagation of electromagnetic interference in switching power supplies.
Switching power supply, as a power supply device for electronic devices, has the advantages of small size, light weight, and high efficiency, and has been widely used in digital circuits. However, due to its high frequency switching state, it is a strong interference source, and the interference generated by it directly endangers the normal operation of electronic devices. Therefore, suppressing the electromagnetic noise of the switching power supply itself and improving its immunity to electromagnetic interference to ensure the long-term safe and reliable operation of electronic devices is an important topic in the development and design of switching power supplies.
Generation of Switching Power Supply Interference
The interference of switching power supplies is generally divided into two categories: firstly, the interference formed by internal components of switching power supplies; The second is the interference caused by external factors in the switching power supply. Both involve both human and natural factors.
Internal interference of switching power supply
The EMI generated by switching power supplies is mainly caused by the high order harmonic current interference generated by the basic rectifier and the peak voltage interference generated by the power conversion circuit.
Basic rectifier
The rectification process of a basic rectifier is the most common cause of EMI. This is because the power frequency AC sine wave is no longer a single frequency current after rectification, but becomes a DC component and a series of harmonic components with different frequencies. Harmonics (especially higher-order harmonics) will generate conduction interference and radiation interference along the transmission line, causing distortion of the front-end current. On the one hand, it will cause distortion of the current waveform connected to its front-end power line, and on the other hand, it will generate radio frequency interference through the power line.
Power conversion circuit
The power conversion circuit is the core of the switch regulated power supply, which produces a wide bandwidth and rich harmonics. The main components that generate this pulse interference are
1) There is a distributed capacitance between the switch tube and its heat sink, the shell, and the leads inside the power supply. When the switch tube flows through a large pulse current (generally a rectangular wave), the waveform contains many high-frequency components; At the same time, the device parameters used to turn off the power supply, such as the storage time of the switch power transistor, the large current of the output stage, and the reverse recovery time of the switch rectifier diode, can cause an instantaneous short circuit in the circuit, generating a large short-circuit current. In addition, the load of the switch tube is a high-frequency transformer or energy storage inductor. At the moment of the switch tube conduction, there is a large inrush current in the transformer primary, causing peak noise.
2) The transformer in a high-frequency transformer switching power supply is used for isolation and voltage transformation, but due to leakage inductance, electromagnetic induction noise can be generated; At the same time, under high-frequency conditions, the distributed capacitance between the layers of the transformer will transmit high-order harmonic noise from the primary side to the secondary side, while the distributed capacitance of the transformer to the shell forms another high-frequency path, making it easier for the electromagnetic field generated around the transformer to couple and form noise on other leads.
3) When the secondary side rectifier diode of the rectifier diode is used for high-frequency rectification, due to the factor of reverse recovery time, the charge accumulated in the forward current cannot be immediately eliminated when the reverse voltage is applied (due to the presence of carriers and current flow). Once the slope of the reverse current recovery is too large, the inductance flowing through the coil generates a peak voltage, which will generate strong high-frequency interference under the influence of transformer leakage inductance and other distribution parameters, with a frequency of up to tens of MHz.
4) Capacitors, inductors, and wire switching power supplies can cause changes in the characteristics of low-frequency components due to their operation at higher frequencies, resulting in noise.
