The conduction coupling of the switching power supply mainly includes:
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.
