Parasitic inductance（2）The effects of parasitic inductance

The root cause of many EMC problems is the parasitic inductance, which is as ubiquitous as parasitic capacitance, and very hidden, and most of them cannot be measured by instruments and equipment, so the influence of parasitic inductance is easy to be ignored by engineers, and the control cost of parasitic inductance is a hurdle that EMC design cannot cross.

01 Parasitic inductance reduces capacitance filtering

The parasitic inductance of the capacitor itself determines the resonance point of the capacitor and the high-frequency filtering characteristics of the capacitor.

Figure 1: Frequency curve of capacitance

In addition to the parasitic inductance of the capacitor itself, the parasitic inductance of the PCB wiring of the filter capacitor is also an important factor affecting the filtering characteristics of the capacitor, and the parasitic inductance of the capacitor installed and routed on the PCB has a much greater impact on the high frequency than the parasitic inductance of the device itself.

02 Parasitic inductance causes parasitic oscillation

Parasitic oscillations often occur between the parasitic capacitance between the pins of power devices such as diodes, MOS transistors, and semiconductor chips, and the parasitic inductance of inductive devices, transformers, beads, and PCB layout wiring in the line. Parasitic oscillation in switching power supply products is one of the important reasons for EMI test failure, which is easy to be ignored by design engineers.

03 Parasitic inductance causes ground potential difference

There is sometimes a coground loop coupling between different module circuits, and the solution is usually to divide the ground plane, after the ground splitting, the integrity of the ground plane is destroyed, and the parasitic inductance also increases, and the highfrequency noise current flows through the junction point of the segmentation, which will produce a ground potential difference between the two different circuits, causing the signal to change the reference ground potential at the source and receiver terminals asynchronously, resulting in signal transmission errors and abnormal phenomena.

04 Special form of parasitic inductance (leakage inductance)

Leakage inductance is that the magnetic flux of the primary (secondary) is not coupled to the secondary (primary) through the core, but returns to the primary (secondary) through air closure, simply put, the magnetic flux is not completely closed through the core. Theoretically speaking, as long as there is magnetic flux, magnetic devices that cannot be completely closed through the core will have magnetic flux leakage, such as U-shaped common mode inductors, transformers, I-shaped inductors, rod inductors, etc.

The leakage inductance of the transformer will produce voltage spikes, which may damage the semiconductor device in severe cases; Leakage inductance and semiconductor parasitic capacitance are prone to parasitic oscillation; The leakage magnetic field is easy to generate induced electromotive force in the nearby signal loop, and it is also easy to have magnetic field coupling with nearby magnetic devices.