Any noise induced on the power supply interconnections (power/ground structure) can cause malfunction. If transients are imposed on the PWR or GND lines, then internal circuits such as flip-flops and inverters can change their state and result in malfunction. Equally, if large, fast-switching currents are present, then higher emissions will occur. As a general rule, all unwanted high-frequency signals must be attenuated by ensuring low impedance paths to ground. Since all signals have currents that flow through PWR or GND, signal lines should be kept as close as possible to the PWR and GND to reduce the current loop area.

 

Using a multi-layer PCB with power planes connected to PWR and/or GND will always give better results, since this always reduces the loop area of all current paths to the minimum and decreases mutual inductance between signal lines. In low-cost applications, normal multi-layer PCB is the first cost reduction, with a two-or one-sided PCB generally used. For this reason, to help combat EMC, more attention is needed on other parts of the PCB design.

 

Ensuring that the PWR and GND are as wide as possible and the decoupling is as close to the integrated circuits as possible will reduce susceptibility. Thin tracks are high impedance with high-frequency currents meaning potential differences will develop across them. The most popular way to generate DC power from the AC power mains is illustrated in figure2. This shows that the AC power mains are stepped down to a smaller AC signal and fed through a bridge rectifier to produce a DC voltage, then either filtered or smoothed, and followed by a voltage regulator. In this case, any metal enclosures are connected to the ground line.