Common-Mode Conducted Interference in Flyback Switching Power Supplies
The conducted noise interference of electronic devices refers to the electromagnetic interference that is conducted through power lines to the public power grid environment in the form of noise current when the device is connected to the power grid for operation. Conducted interference is divided into two types: common mode interference and differential mode interference. The phase of common mode interference current on the neutral and phase lines is equal; The differential interference current has opposite phases on the neutral and phase lines. The contribution of differential mode interference to overall conducted interference is relatively small and mainly concentrated in the low-frequency end of the noise spectrum, making it easier to suppress; Common mode interference contributes significantly to conducted interference and is mainly located in the mid frequency and high frequency bands of the noise spectrum. The suppression of common mode conducted interference is a difficult and primary task in the EMC design of electronic devices.
There are some nodes with voltage fluctuations in the circuit of a flyback switching power supply. Unlike other nodes in the circuit where the potential is relatively stable, the voltage of these nodes contains high-intensity high-frequency components [2]. These nodes with highly active voltage changes are called noise active nodes. The noise active node is a common mode conducted interference source in the switching power supply circuit, which generates common mode noise current M when it acts on the stray capacitance to ground in the circuit. The stray capacitance to ground that has a greater impact on EMI in the circuit includes: the parasitic capacitance from the drain of the power switch tube to ground C, the parasitic capacitance from the main winding to the secondary winding of the transformer Cp; The parasitic capacitance C of the secondary circuit of the transformer to ground, the parasitic capacitance C of the main and secondary windings of the transformer to the magnetic core, and the parasitic capacitance C of the transformer magnetic core to ground? The distribution of these parasitic capacitances in the circuit is shown in Figure 1.
The common mode current in Figure L has three main coupling paths in the circuit: from the noise source - the d-pole of the power switch transistor - to ground through C coupling; Coupling from the noise source through c. to the secondary circuit of the transformer, and then coupling to ground through c; The front and secondary coils of the transformer are coupled to the transformer core through C-C, and then coupled to ground through C. These three types of currents are the main factors that contribute to the common mode noise current (indicated by the black arrows in Figure 1). The common mode current flows back through the ground wire at the input end of the power line, and is sampled and measured by LISN.
