Introduction to EMC optimization design scheme for switching power supply PCB
The interference path of switch mode converter noise provides coupling conditions for the interference source and the interfered equipment, and the study of common mode interference and differential mode interference is particularly important. The high-frequency models of the main components in the circuit and the circuit models of common mode and differential mode noise were mainly analyzed, providing useful assistance for the EMC optimization design of switch power PCB.
The impact of common mode interference and differential mode interference on circuits in switch mode power supplies is different. Usually, low-frequency differential mode noise dominates, while high-frequency common mode noise dominates. 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 switch mode power supply mainly includes:
Circuit based conductive coupling, capacitive coupling, inductive coupling, and a mixture of these coupling methods.
1. Common mode and differential mode noise path models
In switch mode 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 the high-frequency transformer, the stray capacitance CK between the power transistor and the heat sink, the parasitic parameters of the power transistor itself, and the parasitic parameters such as mutual inductance, self inductance, mutual capacitance, self capacitance, and impedance formed by the mutual coupling between printed conductors, 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 conductors, the noise current path model of the converter can be obtained.
High frequency model of the main components of the two circuits
The parasitic inductance and capacitance inside the power switch tube affect the high-frequency performance of the circuit. These capacitances cause high-frequency interference leakage current to flow to the metal substrate, and there is a stray capacitance CK between the power tube and the heat sink. For safety reasons, the heat sink is usually grounded, which provides 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 drain 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 insulating silicone with good thermal conductivity are often added between the bottom of the switch tube and the radiator. This means that there is a parallel coupling capacitor CK between point A and ground. When the state of the switching tubes Q1 and Q2 changes, causing a change in the potential at point A, a noise current Ick will be generated on CK, as shown in Figure 2. The current flows from the radiator to the casing, and there is a coupling impedance between the casing, i.e. the ground, and the main power line, forming a common mode noise path as shown by the dashed line in Figure 2. So, 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.
