One application issue that comes across my desk on a regular
basis is where a customer has gone to an outside lab to certify their equipment
for EMC, and they have failed conducted or radiated noise.
Usually the power supply in question is an open frame type,
which does not have the shielding that a metal enclosed power supply has. There are two areas that are worth checking;
grounding points and wire harnesses.
1. Grounding Points
Power supplies utilize decoupling capacitors; two are
typically connected from input to earth ground (see below). Likewise, two are
connected from the output to earth ground. This keeps the noise currents
circulating close to the power supply, rather than allowing them to radiate
around the end user’s system.
In an enclosed power supply these capacitors will be
grounded through the metal case, but with an open frame type, it is up to the
user to connect these points to ground.
With the power density of products today, there often is more than one
point on the power supply printed circuit board that needs to be
connected. A common mistake is to only
connect one, which can cause excessive radiated and conducted noise.
The installation manual will show which mounting holes /
points need to be grounded. In the
product below, three mounts should be connected (A, B & C).
The photo below shows the same power supply undergoing EMC
testing, and it can be noted that the unit is connected to a metal plate with
metal standoffs.
A quick glance of the underside of the printed circuit board
will show which mounting holes have traces that need to be grounded. This
smaller model has only one grounding point at the bottom right hand side of the
board.
2. Wiring harnesses
In the test photo, it can be seen that the cable harnesses
are neatly dressed and are kept away from the power supply. Wiring that is positioned above or below the
unit will pick up radiated noise, thus defeating the purpose of having the EMI
filter components.
If I am assisting customers on site, I always pack some
tie-wraps in my tool kit to re-route any offending harnesses.
An earlier blog covers the standards http://power-topics.blogspot.com/2007/11/guide-to-emc-standards-for-power.html
