2022-03-01

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Author：pcb

In the switching power supply **PCB board** of the modeling and analysis of the electromagnetic compatibility, the interference path of the switching 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. The high-frequency model of the main components of the circuit and the circuit model of common mode and differential mode noise are mainly analyzed, which provides useful help for the EMC optimization design of the switching power supply PCB board. The common mode interference and differential mode interference of the switching power supply have different effects on the circuit. Usually, the differential mode noise dominates at low frequencies, and the common mode noise dominates at high frequencies, and the radiation effect of the common mode current is usually higher than that of the differential mode current. The radiation effect is much larger, so it is necessary to distinguish between differential mode interference and common mode interference in the power supply. In order to distinguish the differential mode interference from the common mode interference, we first need to study the basic coupling mode of the switching power supply, and on this basis, we can establish the circuit paths of the differential mode noise current and the common mode noise current. The conduction coupling of switching power supply mainly includes: circuit conduction coupling, capacitive coupling, inductive coupling and a mixture of these coupling methods.

1. Common-mode and differential-mode noise path models

In the switching power supply, the coupling capacitance CW existing between the primary and secondary windings of the high-frequency transformer, the stray capacitance CK existing between the power tube and the radiator, the parasitic parameters of the power tube itself, and the mutual coupling between the printed wires are formed. The mutual inductance, self-inductance, mutual capacitance, self-capacitance, impedance and other parasitic parameters constitute common mode noise and differential mode noise paths, thereby forming 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.

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 capacitors make the high-frequency interference leakage current 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. When the PWM converter works, along with the work of the switching device, the common mode noise is also generated accordingly. For the half-bridge converter, the drain voltage of the switch Q1 is always U1, and the source potential varies between 0 and U1/2 with the change of the switch state; the source potential of Q2 is always 0, and the drain potential is 0 and U1/2. In order to keep the switch tube and the radiator in good contact, 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 have a parallel coupling capacitor CK between point A and ground. When the states of the switches Q1 and Q2 change, so that the potential of point A changes, the noise current Ick will be generated on CK, as shown in Figure 2. The current reaches the chassis from the heat sink, and the chassis, that is, the ground, has a coupling impedance with the main power line, forming a common-mode noise path shown by the dotted line in Figure 2. Therefore, the common mode noise current produces a voltage drop across the coupling impedance Z between the ground and the main power line, forming common mode noise. Isolation transformers are a widely used power line interference suppression measure. Its basic function is to achieve electrical isolation between circuits and to solve the mutual interference between devices caused by ground loops. For an ideal transformer, it can only carry differential mode current and not common mode current, this is because for common mode current it is at the same potential between the two terminals of an ideal transformer, so it cannot create a magnetic field on the windings, also There can be no common mode current path, thus playing a role in suppressing common mode noise. The actual isolation transformer has a coupling capacitor CW between the primary side and the secondary side. This coupling capacitor is generated by the existence of non-dielectric and physical gaps between the windings of the transformer, which provides a path for the common mode current.

Ordinary isolation transformers have a certain suppression effect on common mode noise, but the effect of suppressing common mode interference decreases with the increase of frequency due to the distributed capacitance between windings. The suppression of common mode interference by ordinary isolation transformers can be estimated by the ratio of the distributed capacitance between the primary and secondary stages and the distributed capacitance of the equipment to the ground. Usually, the distributed capacitance between the primary and secondary stages is several hundred pF, and the distributed capacitance to the ground is several to several tens of nF, so the attenuation value of common mode interference is about 10 to 20 times, that is, 20 to 30 dB. In order to improve the isolation transformer's ability to suppress common mode noise, the key is to have a small coupling capacitance. For this reason, a shielding layer can be added between the primary and secondary stages of the transformer. The shield has no adverse effect on the energy transfer of the transformer, but affects the coupling capacitance between the windings. In addition to suppressing common mode interference, the isolation transformer with shielding layer can also suppress differential mode interference by using the shielding layer. The specific method is to connect the shielding layer of the transformer to the neutral end of the primary. For the 50Hz power frequency signal, due to the high capacitive reactance formed by the primary and the shielding layer, it can still be transmitted to the secondary through the transformer effect without being attenuated. For differential mode interference with higher frequency, since the capacitive reactance between the primary and the shielding layer becomes smaller, this part of the interference is directly returned to the power grid through the distributed capacitance and the connection between the shielding layer and the primary neutral end without entering the secondary circuit. Therefore, it is very important to model the high frequency of the transformer, especially many parasitic parameters of the transformer, such as: leakage inductance, distributed capacitance between the primary and secondary sides, etc., which have a significant impact on the level of common mode EMI, must be considered. In practice, impedance measurement equipment can be used to measure the main parameters of the transformer, so as to obtain these parameters and carry out simulation analysis. The DC electrolytic capacitor Cin in the half-bridge circuit has corresponding series equivalent inductance ESL and series equivalent resistance. These two parameters also affect the high-frequency performance of the circuit. Generally, the ESL is about tens of nH. In practical analysis, the high-frequency equivalent parasitic parameters of passive components such as resistors, inductors and capacitors can be measured with a high-frequency impedance analyzer, and the high-frequency models of power devices can be obtained from the model library of circuit simulation software. Another factor that has a greater impact on the high-frequency noise of a circuit is the mutual coupling of the printed conductors (striplines) on the printed board. When a high-amplitude transient current or a rapidly rising voltage appears near the conductor, there will be interference problems. The coupling of printed wires is usually characterized by the mutual capacitance and mutual inductance of circuits and wires. Capacitive coupling causes coupling current, and inductive coupling causes coupling voltage. The parameters of the PCB layer, the traces of the signal lines and the spacing between them all affect these parameters. The main difficulty in establishing a high-frequency model of printed circuit board traces and extracting parasitic parameters between traces is to determine the capacitance per unit length of the printed circuit board trace and the inductance per unit length. There are generally three methods that can be used to determine the inductance and capacitance matrix components: (1) Finite Difference Method (FDM); (2) Finite Element Method (FEM); (3) Momentum Method (MOM). After the unit length matrix is determined, a high frequency simulation model of the printed circuit board traces can be obtained through multi-conductor transmission line or partial element equivalent circuit (PEEC) theory. Cadence software is a powerful EDA software. Its SpecctraQuest tool can perform signal integrity and electromagnetic compatibility analysis on PCB boards. It can also be used for high-frequency modeling of printed circuit board traces to achieve a given structure. The parameters are extracted from the PCB board, and the parasitic parameter matrix of the inductance, capacitance, and resistance of the printed wire traces of any shape is generated, and then the EMC simulation analysis can be carried out by using the PEEC theory.

3. Circuit model of common mode and differential mode noise

Usually the common mode interference and differential mode interference in the circuit exist at the same time, the common mode interference exists between any phase line of the power supply and the ground, and the differential mode interference exists between the phase line and the phase line. Differential mode interference dominates at low frequency; common mode interference dominates at high frequency, which shows that differential mode interference and common mode interference of switching power supply have different influences on the circuit; on the other hand, line parasitic parameters have different effects on differential mode The effects of interference and common mode interference are also different. Because the impedance between lines is different from the line-to-ground impedance, after the interference is transmitted over a long distance, the attenuation of the differential mode component is greater than that of the common mode. Therefore, in order to solve the conduction noise problem of switching power supply, it is necessary to distinguish common mode and differential mode interference first, which requires establishing common mode and differential mode noise paths, and then simulate and analyze them respectively. This method is convenient for us to find electromagnetic interference. The root cause of the problem is easy to solve. In engineering, a current probe can be used to determine whether the power supply is common mode or differential mode. The probe first surrounds each wire individually to obtain the inductive value of a single wire; The induction value is increased, the interference current in the line is common mode, otherwise it is differential mode. In the theoretical analysis, for different systems, it is necessary to establish their common mode and differential mode noise current models respectively. On the basis of our above analysis, considering the high frequency model of the power device and the mutual coupling relationship of the printed wires, we get The common-mode and differential-mode interference circuit models of the half-bridge QRC converter are presented. LISN (Line ImpedenceStabilizing network) is a linear impedance fixed network specified for EMC detection. Because the inductance of the LISN is low impedance for the 50Hz power frequency signal, and the capacitor is high impedance, the power frequency signal LISN is basically not attenuated, and the power can be sent to the half-bridge converter through the LISN. For high-frequency noise, the inductance of LISN shows a large impedance, and the capacitor can be regarded as a short circuit, so LISN prevents the transmission of high-frequency noise between the device under test and the power grid. Therefore, LISN acts as a common mode and differential mode interference. The current provides a fixed impedance (50ohm) in the frequency band to be measured (typical value is 100KHz ~ 30MHz) on **PCB board**.