PCB News - Parasitic components generated by PCB design

In PCB design, it is easy to form two basic parasitic elements that may cause problems: parasitic capacitance and parasitic inductance. When designing a circuit board, placing two traces close to each other will generate parasitic capacitance. It can be done as follows: on two different layers, place one wire on top of the other wire; or on the same layer, place one wire next to the other wire, as shown in Figure 5. In these two trace configurations, the change in voltage over time (dV/dt) on one trace may cause current on the other trace. If the other trace is high impedance, the current generated by the electric field will be converted into voltage.

Fast voltage transients often occur on the digital side of the analog signal design. If the trace with fast voltage transients is close to the high-impedance analog trace, this error will seriously affect the accuracy of the analog circuit. In this environment, analog circuits have two disadvantages: their noise tolerance is much lower than that of digital circuits; and high-impedance traces are more common.

Using one of the following two techniques can reduce this phenomenon. The commonly used technique is to change the size between the traces according to the capacitance equation. The effective size to be changed is the distance between the two traces. It should be noted that the variable d is in the denominator of the capacitance equation. As d increases, the capacitive reactance will decrease. Another variable that can be changed is the length of the two traces. In this case, the length L decreases, and the capacitive reactance between the two traces will also decrease.

Another technique is to lay a ground wire between these two traces. The ground wire is low-impedance, and adding another trace like this will weaken the interfering electric field, as shown in Figure 5.

The principle of parasitic inductance in the circuit board is similar to that of parasitic capacitance. It is also to arrange two traces. On two different layers, place one trace on top of the other trace; or on the same layer, place one trace next to the other, as shown in Figure 6. In these two trace configurations, the current on a trace changes with time (dI/dt). Due to the inductance of this trace, voltage will be generated on the same trace; and due to the existence of mutual inductance, it will A proportional current is generated on the other trace. If the voltage change on a trace is large enough, interference may reduce the voltage tolerance of the digital circuit and cause errors. This phenomenon does not only occur in digital circuits, but it is more common in digital circuits because of the large instantaneous switching currents in digital circuits.

The above is an introduction to the parasitic elements generated by PCB design. Ipcb is also provided to PCB manufacturers and PCB manufacturing technology.