Parasitic inductance refers to the inductance that is parasitic on the PCB traces or other components of a circuit board. Where there are wires, there will be parasitic inductance, such as copper wires on PCB, vias, bonding wires inside the chip, component pins, cables, etc., and the inductance of these parasitic inductors is generally nH level.
01. Via parasitic inductance
Via parasitic inductance refers to the parasitic inductance that exists on the via, and the harm caused by the via parasitic inductance is often greater than the effect of the parasitic capacitance. Via parasitic inductance causes impedance discontinuity, especially on high-speed differential signals, and should be controlled during design.
Figure 1: Via parasitic inductance
02. Parasitic inductance of device pins
The parasitic inductance of the plug-in device pins has attracted attention, and the parasitic inductance of the capacitor pins is the main factor leading to the reduction of the high-frequency filtering performance of the capacitor. The parasitic inductance and parasitic capacitance of the pins of power devices are one of the main causes of high-frequency oscillation noise.
Figure 2: Parasitic inductance at the device pins
03. Parasitic inductance of PCB wiring
Figure 3: Parasitic inductance of PCB wiring
According to the definition of inductance, there is parasitic inductance in the PCB layout itself, and the size of the inductance is affected by the length and width of the wire. The parasitic inductance of PCB routing is one of the important factors that lead to the degradation of filter capacitor performance and parasitic oscillation. In addition, the parasitic inductance on the reference plane is the root cause of the ground potential difference, which is one of the important factors leading to common mode noise interference.
04. The parasitic inductance of the cable itself
Figure 4: Parasitic inductance of a cable
Cable parasitic inductors are similar to wiring parasitic inductors, but they are easy to ignore. In particular, the parasitic inductance of the cable ground wire is the main factor causing the signal reference potential difference between the two ends, and has little influence on the ground potential difference of the strong signal. The impact on weak signals and small signals is very obvious, which is an important reason for the failure of the immunity test and an important reason for the excessive radiated emissions. (Ground potential difference generates common-mode noise current)
05. Parasitic inductance at the metal lap joint
Figure 5: Parasitic inductance at the metal lap
06. Parasitic inductance of the reference plane
Figure 6: Parasitic inductance of the PCB reference layer
Many engineers will feel that the immunity performance of multi-layer board PCB design is better than that of two-layer board, because the multilayer board has a complete reference plane and the two-layer board does not have a complete reference plane, the inductance of the complete reference plane parasitic high-frequency state is very small, and the ground potential disturbance generated when the high-frequency noise current flows is also very small, which is not easy to interfere with sensitive signals, and the two-layer board has a large parasitic inductance because of the integrity of the reference plane, and the disturbance caused by the high-frequency noise current flow is also large, which is easy to interfere with sensitive signals.