1 Introduction
Compared to linear regulated power supplies, switching power supplies offer advantages such as lower power consumption, higher efficiency, smaller size, lighter weight, and wider voltage regulation range, making them widely used in computers and peripherals, communications, automatic control, and home appliances. However, a significant drawback of switching power supplies is the generation of strong electromagnetic interference (EMI). EMI signals have a wide frequency range and a certain amplitude, and through conduction and radiation, they pollute the electromagnetic environment, interfering with communication equipment and electronic products. If not handled properly, the switching power supply itself can become a source of interference. With the increasing importance placed on electromagnetic compatibility (EMC) of electronic products, suppressing EMI from switching power supplies and improving the quality of electronic products to comply with relevant EMC standards or specifications has become a growing concern for electronic product designers.
2. The principle of EMI generation in switching power supplies
There are many reasons for EMI in switching power supplies, among which the high-order harmonic interference of the current generated by the basic rectifier and the peak voltage interference generated by the transformer-type power conversion circuit are the main reasons.
The rectification process of a basic rectifier is the most common cause of EMI. This is because a sinusoidal power supply becomes a unidirectional pulsating power supply after passing through a rectifier, and is no longer a single-frequency current. This current wave can be decomposed into a DC component and a series of AC components of different frequencies. Experimental results show that harmonics (especially higher harmonics) can generate conducted and radiated interference along transmission lines. On the one hand, they distort the current waveform connected to the power line at its front end; on the other hand, they generate radio frequency interference through the power line.
Transformer-type power conversion circuits are used to achieve voltage transformation, frequency conversion, and output voltage regulation. They are the core of switching power supplies and mainly consist of switching transistors and high-frequency transformers. The voltage spikes they generate are narrow pulses with relatively large amplitudes, wide bandwidths, and rich harmonics. The main reasons for this pulse interference are:
(1) When the inductive load of the switching power transistor is a high-frequency transformer or energy storage inductor, a large inrush current occurs in the primary of the transformer at the moment the switching transistor is turned on, which will cause spike noise. This spike noise is actually a sharp pulse, which may cause interference or even break down the switching transistor.
(2) Interference generated by high-frequency transformer When the originally conducting switch is turned off, the back electromotive force generated by the leakage inductance of the transformer.
E = -Ldi/dt
Its value is proportional to the rate of change of collector current (di/dt) and proportional to the leakage inductance. When superimposed on the turn-off voltage, it forms a turn-off voltage spike, which generates conducted electromagnetic interference. This not only affects the primary winding of the transformer but also conducts to the power distribution system, affecting the safe and economical operation of other electrical equipment.
(3) Interference generated by the output rectifier diode: When the output rectifier diode is turned off, there is a reverse current. The time it takes for the reverse current to recover to zero is related to factors such as junction capacitance. Among them, the diode that can quickly recover the reverse current to zero is called a hard recovery characteristic diode. Under the influence of transformer leakage inductance and other distributed parameters, this type of diode will generate strong high-frequency interference, which can reach tens of MHz.
The main measures taken to suppress EMI generated by the aforementioned switching power supplies include correctly selecting semiconductor devices and transformer core materials, and implementing shielding, grounding, and filtering in the switching power supply circuit. This article only introduces filtering suppression measures.
3. Filtering measures to suppress EMI in switching power supplies
Filtering is an effective measure to suppress interference, especially conducted and radiated EMI signals from switching power supplies. Conducted interference signals on any power line can be represented by differential-mode and common-mode signals. Differential-mode interference propagates between two conductors and is symmetrical interference; common-mode interference propagates between the conductor and ground (chassis) and is asymmetrical interference. Generally, differential-mode interference has a smaller amplitude and lower frequency, causing less interference; common-mode interference has a larger amplitude and higher frequency, and can also radiate through conductors, causing greater interference. Therefore, the most effective way to reduce conducted interference and control EMI signals below the limits specified in relevant EMC standards is to install EMI filters in the input and output circuits of the switching power supply. The operating frequency of switching power supplies is approximately 10kHz to 100kHz. Many EMC standards specify conducted interference level limits starting from 10kHz. For high-frequency EMI signals generated by switching power supplies, satisfactory results can be achieved by selecting appropriate decoupling circuits or EMI filters with relatively simple network structures.
Figure 1 shows the basic network structure of an EMI filter for a switching power supply.
This filter is a passive low-pass network composed of lumped-parameter elements. L1 and L2 are two independent coils wound on the same magnetic ring, called common-mode inductors (LCMs), while L3 and L4 are independent differential-mode rejection inductors. If one end of the filter is connected to the interference source and the load end is connected to the device being interfered with, then L1 and Cy, and L2 and Cy, respectively form two pairs of independent L-E and N-E low-pass filters, used to suppress common-mode EMI signals present on the power line, attenuating them and controlling them to a very low level.
The equivalent circuit of the common-mode filter network is shown in Figure 2, which consists of LCM and Cy. The right side of the figure shows the equivalent circuit of the common-mode noise of the switching power supply. The parallel capacitor Cp includes the distributed capacitance between the collector of the switching transistor and ground, and the distributed capacitance between the primary and secondary windings of the high-frequency transformer; Rp is the parallel resistance of the current source. The source internal resistance ZSMPS of the equivalent circuit of the common-mode noise of the switching power supply is high impedance capacitive.
In Figure 1, the two coils L1 and L2 have the same number of turns but are wound in opposite directions. When the filter is connected to the circuit, the magnetic flux generated by the current in the two coils cancels each other out in the magnetic ring, preventing the magnetic ring from reaching magnetic saturation and thus keeping the inductance of the two coils constant. However, due to various reasons, such as the magnetic ring material not being perfectly uniform and the winding of the two coils not being perfectly symmetrical, the inductances of L1 and L2 are not equal. Therefore, (L1-L2) forms a differential-mode inductor LDM. This, together with the independent differential-mode suppression inductors L3 and L4 and the Cx capacitor, forms a low-pass filter between the L-N independent ports to suppress differential-mode EMI signals on the power line.
The equivalent circuit for differential-mode interference is shown in Figure 3. It consists of two parts: a high-impedance interference equivalent circuit and a low-impedance interference equivalent circuit. In Figure 3, switch S indicates whether the bridge rectifier diode is conducting or not; therefore, the high and low equivalent circuits cannot exist simultaneously. Rs is the distributed resistance, and Ls is the distributed inductance, both of which are very small. To distinguish them from the common-mode case, Rp and Cp are represented by Rp′ and Cp′, respectively.
The equivalent circuit of the differential-mode EMI signal filtering network is shown in Figure 4. LDM is the differential-mode inductor, which includes the differential-mode inductor formed by the common-mode coil and the independent differential-mode suppression inductor; CLL is the parallel capacitor selected for the filtering network. Compared with Figure 4(a), Figure 4(b) adds a CLL2, and the value of the CLL2 is chosen to make the filtering network mismatched with the load.
Since the circuit in Figure 1 is a passive network, it is reciprocal. When installed in a system, it can effectively suppress EMI signals from outside the electronic device from entering the device, and also greatly attenuate the EMI signals generated by the device itself and transmitted to the power grid, thus simultaneously attenuating two sets of common-mode EMI signals and one set of differential-mode EMI signals.
The four capacitors in the switching power supply EMI filter use two different subscripts, "X" and "Y," which not only indicate their role in the filter network but also their safety class. Whether selecting or designing an EMI filter, the safety classes of CX and CY must be carefully considered. In practical applications, the CX capacitor is connected between the L and N lines of a single-phase power supply. In addition to the rated power supply voltage, it also receives the peak voltage of the EMI signal existing between L and N. Therefore, the appropriate safety class of the CX capacitor must be selected based on the application of the EMI filter and the potential peak EMI signal. The CY capacitor is connected between the power supply lines L and N and the metal casing (E). For a 220V, 50Hz power supply, in addition to meeting the withstand voltage requirement of 250V peak voltage, this capacitor must also have sufficient safety margin in terms of electrical and mechanical performance to avoid potential breakdown and short circuits.
EMI filters are reciprocal, meaning the load can be connected to either the power supply or the load terminal. In practical applications, to effectively suppress EMI signals, the filter's network structure and parameters must be selected based on the impedances of the EMI signal source and the load to be connected to the filter. When the impedances at both ends of the EMI filter are mismatched (i.e., Zs≠Zin, ZL≠Zout in Figure 5), the EMI signal will be reflected at both its input and output terminals, increasing attenuation. The relationship between the signal attenuation A and the reflection Γ is:
A = -10lg(1 - |Γ|2)
The purpose of electromagnetic compatibility design is to select component parameters as reasonably as possible while ensuring that the network structure conforms to the principle of maximum mismatch, so as to maximize the attenuation of EMI signals.
4. Conclusion
When using a switching power supply filter, pay attention to the power supply frequency of the filter at its rated current. When installing the filter, pay special attention to the spacing between the input and output wires; do not bundle them together, otherwise EMI signals can easily couple from the input line to the output line, which will greatly reduce the filter's suppression effect.
