Study on Suppressing Common Mode EMI by Shielding Layer of Switching Power Supply Transformer

Abstract: Taking the flyback switching power supply as an example, based on the analysis of the common-mode conducted EMI mechanism formed by its high-frequency transformer, the principle of setting a shielding layer in the transformer design to suppress the common-mode conducted EMI is discussed. A specific design method is given and applied to the design of specific products. The test shows that the setting of the shielding layer can effectively suppress the common-mode conducted EMI of the high-frequency switching power supply. The feasibility of applying the shielding layer in other types of switching power supplies is further studied.

0 Introduction

Electromagnetic Compatibility (EMC) refers to the ability of electronic equipment or systems to work normally in an electromagnetic environment without causing unbearable electromagnetic disturbance to anything in the environment. It includes two aspects: electromagnetic interference (EMI) and electromagnetic susceptibility (EMS). Since there are very high di/dt and du/dt in switching power supplies, all topological forms of switching power supplies have the problem of electromagnetic interference. At present, the main technical means to overcome electromagnetic interference are: setting passive or active filters at the input and output ends of the power supply, setting a shielded shell and grounding it, using soft switching technology and variable frequency control technology, etc.

The fundamental reason for EMI in switching power supplies is the high-frequency rapid changes in current and voltage, which are transmitted through wires and coupled with inductance and capacitance to form conducted EMI. At the same time, changes in current and voltage are inevitably accompanied by changes in magnetic and electric fields, thus leading to radiated EMI. This article focuses on the analysis of the mechanism of common-mode conducted EMI in transformers, and based on this, explains the suppression effect of different shielding layer settings in transformers on common-mode conducted EMI.

1. Generation mechanism of conducted EMI in high-frequency transformers

Taking the flyback converter as an example, its main circuit is shown in Figure 1.

After the switch is turned on, the current on the primary side of the transformer gradually increases, and the energy storage in the magnetic core also increases. When the switch is turned off, the secondary rectifier diode is turned on, and the transformer energy storage is coupled to the secondary side to supply power to the load.

Figure 1 Flyback converter

In a switching power supply, the input rectified current is a sharp pulse current. When the switch is turned on and off, the voltage and current change rate in the converter is very high, and these waveforms contain abundant high-frequency harmonics. In addition, during the switching process of the main switch tube and the reverse recovery process of the rectifier diode, the parasitic inductance and capacitance of the circuit will oscillate at high frequencies, all of which are sources of electromagnetic interference. There are a large number of distributed capacitors in the switching power supply, which provide a path for the transmission of electromagnetic interference, as shown in Figure 2. In Figure 2, LISN is a linear impedance stabilization network, which is used to measure line conduction interference. The interference signal is transmitted to the input and output ends of the converter through wires, parasitic capacitance, etc., forming a conducted interference. There are also a large number of parasitic capacitances between the windings of the transformer, as shown in Figure 3. In Figure 3, the 4 points A, B, C, and D correspond to the 4 points marked in Figure 1.

Figure 2 Typical distribution of parasitic capacitance of flyback switching power supply

Figure 3 Distribution of parasitic capacitance in transformer

In the flyback switching power supply shown in Figure 1, when the converter works in continuous mode, after the switch tube VT is turned on, the potential at point B is lower than that at point A, and the inter-turn capacitance of the primary winding is charged, and the charging current flows from A to B; after VT is turned off, the parasitic capacitance is reversely charged, and the charging current flows from B to A. In this way, differential mode conduction EMI is generated in the transformer. At the same time, the potential difference between the power supply components and the earth will also produce high-frequency changes. Due to the distributed capacitance between the components and the earth and the casing, a common mode conduction EMI current is generated that flows between the input end and the circuit formed by the earth and the casing.

Specifically in the transformer, the potential difference between the primary winding and the secondary winding will also produce high-frequency changes, and through the coupling of parasitic capacitance, a common-mode conducted EMI current flowing between the primary side and the secondary side is generated. The AC equivalent circuit and the simplified equivalent circuit are shown in Figure 4. In Figure 4: ZLISN is the equivalent impedance of the linear impedance stabilization network; CP is the parasitic capacitance between the primary winding and the secondary winding of the transformer; ZG is the equivalent impedance between different points on the earth; CSG is the equivalent capacitance between the output circuit and the ground; Z is the equivalent impedance of the circuit outside the transformer.

Figure 4: Common-mode EMI flow loop in transformer

2 Mathematical model of common mode conducted EMI in transformer

Taking the transformer shown in Figure 3 as an example, the parasitic capacitance between the top primary winding and the secondary winding is the largest, which is the main reason for the common-mode conducted EMI. Therefore, the following mainly analyzes the influence of the distributed capacitance between these two layers on the common-mode conducted EMI, ignoring the influence of other windings of the transformer on the common-mode conducted EMI.

Assume that the primary winding has 3 layers, each with m turns, and the secondary winding has only one layer, which is n turns. When the magnetic flux in the transformer core changes, an induced electromotive force will be generated on the primary side and the secondary side at the same time. According to the superposition theorem, this can be considered as the superposition of two situations: only the primary winding has an induced electromotive force and the secondary winding electromotive force is zero, and only the secondary winding has an induced electromotive force and the primary winding electromotive force is zero. The situation where only the primary winding has an induced electromotive force and the secondary winding electromotive force is zero is shown in Figure 5. In Figure 5: e1 is the induced electromotive force of each turn of the primary winding; C1x is the parasitic capacitance between the outermost primary winding and the secondary winding of one turn.

Figure 5: The case where only the primary winding has induced electromotive force

In this case, the common-mode current flowing from the primary to the secondary is:

The situation where only the secondary winding has induced electromotive force and the primary winding electromotive force is zero is shown in Figure 6. In Figure 6: e2 is the induced electromotive force of each turn of the secondary winding; C2x is the parasitic capacitance between one turn of the secondary winding and the outermost layer of the primary winding.

Figure 6: The case where only the secondary winding has induced electromotive force

In this case, the common-mode current flowing from the secondary to the primary is:

According to the superposition principle, the common mode current flowing between the outermost winding on the primary side and the secondary can be obtained:

3 Principle of shielded winding to suppress common mode conducted EMI

According to the structure shown in Figure 3, a transformer is wound, and a 13 mH differential mode filter inductor and a 6.8 differential mode filter capacitor are added after the AC rectifier filter. The switching power supply is tested for conducted EMI, and the results are shown in Figure 6. As can be seen from Figure 6, the conducted EMI is very serious and cannot pass the electromagnetic interference test. A 35 mH common mode filter inductor is added before the AC rectifier, and the conducted EMI test results are shown in Figure 7, and the product can pass the test. Comparing the test results, it can be concluded that in the circuit shown in Figure 3, it is mainly due to a large amount of common mode conducted EMI that the power supply cannot pass the electromagnetic interference test.

Figure 7 Conducted EMI test results without shielding inside the transformer

Remove the common-mode filter inductor, add a primary shield winding to the transformer as shown in Figure 8, and connect E to point A (the positive pole of capacitor Cin). At this time, the primary shield winding replaces the outermost layer of the original primary winding. Assuming that the parasitic capacitance between the primary shield winding and the secondary winding is the same as the parasitic capacitance between the outermost winding of the primary side of the original transformer and the secondary winding, then:

Figure 8: Conducted EMI test results when no shielding is set inside the transformer and a common mode filter inductor is added to the circuit

From formula (4), we can know that: when the circuit working condition remains unchanged, the first term of the common mode current i1 is reduced to the original 1/(2m+1), so the conducted EMI is reduced. The test results are shown in Figure 9.

Since the input filter capacitor Cin is short-circuited in the common-mode conducted EMI model, if E is connected to the negative pole of capacitor Cin, the shield winding's suppression effect on conducted EMI is consistent with the case where point E and point A are connected. The test results are shown in Figures 10 and 11.

Figure 9 Adding a primary shield winding inside the transformer

Figure 10 Conducted EMI test results of a transformer with a primary shielded winding inside and the output line connected to the positive electrode of the input filter capacitor

Figure 11 Conducted EMI test results of a transformer with a primary shielded winding inside and the output line connected to the negative electrode of the input filter capacitor

A secondary shielding winding is added inside the transformer as shown in Figure 12, and point E is connected to point A, and point F is connected to point C. At this time, the distribution of the induced electromotive force and parasitic capacitance of the primary shielding winding and the secondary shielding winding is basically the same, which is approximately:

In formula (5), Cx is the parasitic capacitance between one shield winding and the other shield winding. Combined with formula (3), it can be seen that the common-mode current coupled through the two shield windings is approximately zero, but the primary and secondary shield windings cannot be completely consistent. Therefore, there will still be common-mode interference current between the shield windings, but it has been greatly attenuated. The test results are shown in Figure 13.

Figure 12: Primary shield winding and secondary shield winding are set inside the transformer

Figure 13 Conducted EMI test results of a transformer with two layers of shielded windings inside

If the two-layer shielded winding is replaced with two-layer shielded copper foil, since the distribution of the induced electromotive force and parasitic capacitance of the two layers of shielded copper foil is more similar, the common-mode conduction current can be better suppressed. The test results are shown in Figure 14.

Figure 14 Conducted EMI test results with two layers of shielding copper foil inside the transformer

Theoretical and experimental results show that adding a shielding layer to the transformer can suppress common-mode conducted EMI, and the shielding effect of two layers of copper foil is the best. In the specific design, the corresponding shielding measures can be selected according to the severity of the common-mode conducted EMI of the power supply.

Since the mechanism of common-mode conducted EMI in various converters is the same, the above common-mode conducted interference model and shielding layer design method are also applicable to other topologies.

4 Conclusion

The high-frequency change of the potential difference between the input and output sides of the switching power supply and the earth is the root cause of common-mode EMI. Theoretical analysis and experimental results show that setting a shielded winding or shielded copper foil between the primary and secondary windings can suppress the common-mode current between the primary and secondary sides and reduce common-mode conducted EMI.