What are the main conductive couplings of switching power supplies?
1 Common mode and differential mode noise path model
In the switching power supply, it is 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 tube and the radiator, the parasitic parameters of the power tube itself, and the mutual coupling between the printed wires. Parasitic parameters such as mutual inductance, self-inductance, mutual capacitance, self-capacitance, and impedance form common-mode noise and differential-mode noise paths, thereby forming common-mode and differential-mode conducted interference. Based on the analysis of the parasitic parameter models of the resistance, inductance, and capacitance of the power switching device, transformer, and printed wires, the noise current path model of the converter can be obtained.
2 High-frequency model of the main components of the circuit
The internal parasitic inductance and capacitance of 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 radiator for safety reasons. , the heat sink is usually connected to ground, which provides a path for common mode noise.
When the PWM converter is working, along with the operation of the switching device, common mode noise is also generated accordingly. As shown in Figure 1, for a half-bridge converter, the drain voltage of switch Q1 is always U1, and the source potential changes between 0 and U1/2 with the change of the switching state; the source potential of Q2 is always 0 , the drain potential changes between 0 and U1/2. In order to maintain good contact between the switch tube and the radiator, an insulating gasket or insulating silica gel with good thermal conductivity is often added between the bottom of the switch tube and the radiator. This makes it equivalent to a parallel coupling capacitor CK between point A and ground. When the status of switching tubes Q1 and Q2 changes, causing the potential of point A to change, a noise current Ick will be generated on CK, as shown in Figure 2. This current reaches the chassis from the radiator, and there is a coupling impedance between the chassis, the ground, and the main power line, forming a common-mode noise path as shown by the dotted line in Figure 2. As a result, 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.
