EMC optimized design scheme for switching power supply PCB
The interference path of switch type converter noise provides coupling conditions for the interference source and the interfered equipment, and the research on its common mode interference and differential mode interference is particularly important. Mainly analyzed the high-frequency models of the main components of the circuit, as well as the circuit models of common mode and differential mode noise, providing beneficial assistance for the EMC optimization design of switch power supply PCBs.
The common mode interference and differential mode interference of switching power supplies have different effects on the circuit. Usually, differential mode noise dominates at low frequencies and common mode noise dominates at high frequencies. Moreover, the radiation effect of common mode current is usually much greater than that of differential mode current. Therefore, it is necessary to distinguish between differential mode interference and common mode interference in power supplies.
In order to distinguish between differential mode interference and common mode interference, we first need to study the basic coupling mode of the switching power supply. Based on this, we can establish circuit paths for differential mode noise current and common mode noise current. The conduction coupling of switching power supply mainly includes:
Circuit based conductive coupling, capacitive coupling, inductive coupling, and a combination of these coupling methods.
1 Common mode and differential mode noise path models
In switching power supplies, common mode noise and differential mode noise paths are formed due to the coupling capacitance CW between the primary and secondary windings of high-frequency transformers, the stray capacitance CK between power tubes and heat sinks, the parasitic parameters of power tubes themselves, and the mutual inductance, self inductance, mutual capacitance, self capacitance, impedance and other parasitic parameters formed by mutual coupling between printed wires, resulting in common mode and differential mode conducted interference. On the basis of analyzing the parasitic parameter models of resistance, inductance, and capacitance of power switching devices, transformers, and printed wires, a noise current path model of the converter can be obtained.
High frequency models of the main components of the 2 circuit
The internal parasitic inductance and capacitance of the power switch affect the high-frequency performance of the circuit. These capacitors cause high-frequency interference leakage current to flow to the metal substrate, and there is a stray capacitor CK between the power switch and the heat sink. For safety reasons, the heat sink is usually grounded, providing a common mode noise path.
During the operation of PWM converters, common mode noise is also generated along with the operation of switching devices. As shown in Figure 1, for a half bridge converter, the leakage voltage of switch Q1 is always U1, and the source potential varies between 0 and U1/2 with the change of switch state; The source potential of Q2 is always 0, and the drain potential varies between 0 and U1/2. In order to maintain good contact between the switch tube and the radiator, insulation gaskets or silicone with good thermal conductivity are often added between the bottom of the switch tube and the radiator. This results in the presence of a parallel coupling capacitor CK between point A and ground. When the state of switch tubes Q1 and Q2 changes, causing a change in the potential of point A, noise current Ik will be generated on CK, as shown in Figure 2. The current flows from the heat sink to the casing, which has a coupling impedance with the main power line, forming a common mode noise path as shown by the dashed line in Figure 2. Therefore, the common mode noise current generates a voltage drop on the coupling impedance Z between the ground and the main power line, forming common mode noise.
