The 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 current after input rectification 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 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, 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 conducted 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 conducted EMI current is generated that flows between the input end and the loop 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 conduction loop in transformer