EMC Design Guide for Printed Circuit Boards
Frame 57 of 78
Printed copies are uncontrolled
5.2. Power Systems
20. Power supplies should be located close to power entry point to PCB, and as close as
possible to powered circuitry. Closely routed tracks (to minimize the area between
conductors, and hence the inductance) should be used to connect the power source to
the local power distribution system.
21. Power feeds should always be decoupled at their entry points onto the PCB.
22. Bulk capacitors should always be parallel decoupled with one or more smaller high-
frequency capacitors with low ESL (equivalent series inductance). Place the smallest
value decoupling capacitor closest to a device to be decoupled.
23. Power should be distributed with a 'star', or grid, or power plane configuration but
never with point-to-point wiring (daisy-chaining). Use the positive side of the bulk
capacitor on the output of the voltage regulator as the "star" point (Figure 510).
Figure 510. Power System's Star Point
24. The value of the bulk capacitor should be at least ten (10) times greater than the sum
of all the values of decoupling capacitors.
25. High-frequency, low-inductance ceramic capacitors should be used for integrated
circuit (IC) decoupling at each power pin use 0.1 µF for up to 15 MHz, and 0.01 µF
over 15 MHz. The decoupling capacitor should be located as close as physically
possible from the IC's power pin. Figure 57.
Power distribution system must provide sufficient current, in time, for the device to
function properly. This includes high-peak current requirements during output
switching. Local discrete capacitors, when placed next to the device and attached to
power and ground with low inductance connections, will provide this current.
26. Printed circuit board traces which carry high switching current with fast rise/fall times
(5 10 ns) should maintain at least 3 mm spacing from other signal traces which run
parallel to them, and/or a ground guard traces should be placed between them.