Optimised EMC design solution for switching power supply PCBs
The interference path of switching converter noise provides coupling conditions for the interference source and the interfered equipment, and the study of its common mode interference and differential mode interference is particularly important. The high-frequency model of the main components of the circuit as well as the circuit model of common mode and differential mode noise are mainly analysed to provide useful help for the EMC optimization design of switching power supply PCB.
The effects of common mode interference and differential mode interference on the circuit of switching power supply are different. Usually, the differential mode noise dominates at low frequency and the common mode noise dominates at high frequency, and the radiation effect of the common mode current is usually much larger than that of the differential mode current, so it is necessary to distinguish between the differential mode interference and the common mode interference in the power supply.
In order to distinguish between differential-mode interference and common-mode interference, we first need to study the basic coupling mode of switching power supplies, on the basis of which we can establish the circuit paths of differential-mode noise currents and common-mode noise currents. The conduction coupling of switching power supply is mainly:
Circuit-based conduction coupling, capacitive coupling, inductive coupling, and a mixture of these coupling methods.
1 Common mode and differential mode noise path model
Switching power supply due to high-frequency transformer coupling capacitance CW between the primary and secondary windings, power tubes and heat sinks between the presence of stray capacitance CK, the power tube's own parasitic parameters as well as printed wires due to the formation of mutual inductance, self-inductance, mutual capacitance, self-capacitance, impedance, and other parasitic parameters due to the formation of mutual coupling, common mode noise and poor-mode noise path, thus forming a common mode and poor-mode conduction interference. The noise current path model of the converter can be obtained on the basis of the analysis of the parasitic parameter models of the power switching devices, transformers, and the resistance, inductance, and capacitance of the printed wires.
2 High-frequency model of the main components of the circuit
The internal parasitic inductance and capacitance of the power switching tubes affect the high frequency performance of the circuit. These capacitances allow high frequency interference leakage currents to flow to the metal substrate, and there is a stray capacitance CK between the power tubes and the heat sink, which is usually grounded for safety reasons, which provides a common mode noise path.
The operation of the PWM converter is accompanied by the operation of the switching devices and the corresponding common mode noise. 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 as the switching state changes; 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 switching tube and the heat sink, an insulating spacer is often added between the bottom of the switching tube and the heat sink or insulating silicone with good thermal conductivity is smeared on the bottom of the switching tube and the heat sink. This makes the point A to the ground is equivalent to the existence of a parallel coupling capacitance CK, when the state of the switching tube Q1, Q2 changes, so that the point A potential changes, it will produce noise current Ick in CK, as shown in Figure 2. The current from the heat sink to the chassis, and the chassis, that is, the earth and the main power line there is a coupling impedance, the formation of common mode noise path shown in Figure 2 dotted line. As a result, the common mode noise current generates a voltage drop across the coupling impedance Z between the ground and the main power line, forming common mode noise.
