Reducing Switching Power Supply Radiated & Conducted EMI

Posted on
One application challenge that comes throughout my desk on an everyday foundation is the place a buyer has gone to an out of doors lab to certify their tools for EMC, and so they have failed performed or radiated noise.

Often the power provide in query is an open body kind,which doesn't have the shielding steel enclosed power provide has.  There are two areas which might be price checking; grounding factors and wire harnesses.

1. Grounding Factors

Power provides make the most of decoupling capacitors; two are sometimes linked from enter to earth floor (see beneath). Likewise, two are linked from the output to earth floor. This retains the noise currents circulating near the power provide, somewhat than permitting them to radiate across the finish person’s system.
 
In an enclosed power provide these capacitors can be grounded by way of the steel case, however with an open body kind, it's as much as the person to attach these factors to floor.  With the power density of merchandise at this time, there usually is a couple of level on the power provide printed circuit board that must be linked.  A typical mistake is to solely join one, which may trigger extreme radiated and performed noise.

The set up guide will present which mounting holes / factors must be grounded.  Within the product beneath, three mounts must be linked (A, B & C).

 
 
The photo beneath exhibits the identical power provide present process EMC testing, and it may be famous that the unit is linked to a steel plate with steel standoffs.
 
 
A fast look of the underside of the printed circuit board will present which mounting holes have traces that must be grounded. This smaller mannequin has just one grounding level on the backside proper hand aspect of the board.

2. Wiring harnesses

Within the check photo, it may be seen that the cable harnesses are neatly dressed and are avoided the power provide.  Wiring that's positioned above or beneath the unit will decide up radiated noise, thus defeating the aim of getting the EMI filter parts.

If I'm aiding clients on website, I all the time pack some tie-wraps in my software package to re-route any offending harnesses.