Parasitic Inductance of Through Hole in PCB Design

Via hole exists parasitic capacitance also parasitic inductance, in the design of high-speed digital circuit, the damage of via hole's parasitic inductance is often greater than the influence of the parasitic capacitance. The parasitic series inductance can weaken the contribution of bypass capacitors, weakened the whole power supply system of filtering effect. We can use the following formula to calculate a simply via approximation of parasitic inductance: L=5.08h[ln(4h/d)1],where L refers to the inductance of through hole, h is the length of the hole, and d is the diameter of the central borehole.

As can be seen from the formula, the diameter of the through-hole has little influence on the inductance, while the length of the through-hole has the greatest influence on the inductance. Still using the above example, hole on the inductance can be calculated as follows: L= 5.08 x0.050 [ln (4x0.050/0.010)1]= 1.015nH. If the rise time of a signal is 1 ns, then the equivalent impedance of size: XL = = 3.19 Ω PI L/T10-90.

The impedance in a high frequency current through already could not be ignored, it is important to note that the bypass capacitor needs when connect the power supply layer and formation through two holes, so via the parasitic inductance will be multiplied.