By conductive coupling is meant that the energy of electromagnetic noise is coupled to the victim device by metal wires or other components in the form of voltage or current in the circuit. Conduction coupling can be further divided into direct conduction coupling and common impedance conduction coupling. Direct conduction coupling means that noise is directly coupled to the victim device through actual components such as wires, metal bodies, resistors, capacitors, inductors, and transformers. Common impedance conduction coupling refers to the common ground impedance coupling of noise through the printed circuit board and the chassis ground wire, the common safety ground wire of the device, and the common ground impedance in the ground network; the noise is common through the AC power supply and the DC power supply. A common source impedance coupling occurs when the source impedance is applied.
The switching operation state of the power switching device causes a complex mutual coupling between components in the system to form conducted interference. The highest frequency considered for conducted interference is 30mhz, and the corresponding electromagnetic wave wavelength λ is 10m in vacuum. Therefore, for power electronic devices with size less than λ/2π, it belongs to the near-field range, and the lumped parameter circuit can be used for electromagnetic interference analysis. The disturbance on the input/output conductor of the system can be divided into two parts: common mode interference and differential mode interference according to the conduction interference propagation coupling channel. It is generally considered that the common mode interference is mainly due to the switching action of the power semiconductor switching device in the system converter. The induced dv/dt propagates through the system's stray capacitance coupling to ground, and the voltage change of one pole generates displacement current by capacitive coupling to the other pole. The current generated by the parasitic capacitance does not require a direct electrical connection, or even ground. Its size can be expressed as: i = cdu / dt, where c is the equivalent coupling capacitance between the battery interference source and the sensitive device.
Differential mode interference is mainly due to the conduction of di/dt conductors between the input and output lines caused by the switching of the power semiconductor switching device. Of course, these are only the most essential causes of conducted interference, and different motor systems have different specific causes of conducted interference. In addition, common mode interference and differential interference are mutually transformable, and are not absolutely separate.
In the pwm inverter, in order to ensure that the switch tube does not fail due to overheating, the heat sink must be installed, and in order to prevent short circuit, the metal shell of the switch tube and the heat sink are separated by a thermally conductive insulating medium. At the same time, the heat sink is grounded through the chassis, so a large parasitic capacitance is formed between the inverter and the heat sink. When the inverter is working normally, as the turns of the upper and lower switching tubes of each phase bridge are turned on, the midpoint potential of the bridge arm will change stepwise.
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