Prevent Radiated Emissions

Let’s say you are designing a switching power supply. Alternatively, maybe you are purchasing an off-the-shelf power supply module.

You need to make sure it will pass EMC testing.

Designing or selecting an EMI filter that ensures compliance with conducted emissions requirements is challenging enough. Designing to meet radiated emissions limits is even tougher. How do you make sure cable radiation from the power lines is adequately controlled, without over-designing?

Cable radiation is the biggest EMI problem

Radiated emissions cause more EMC testing failures than any other test.

Why?

1. Most input power cables are unshielded.
2. It takes a tiny amount of little radio frequency energy on the power cables to cause them to radiate at levels greater than specified limits.

The math is hard

Without good software, calculating cable radiation is not practical.

You can use simplified models to attempt to calculated field strength by hand, but the results can be wildly inaccurate. Hand calculations may be off by orders of magnitude. If you use far-field equations, you are going to be very disappointed when your test results are not even close to your predictions.

Many factors affect radiation

There are so many design elements that affect cable radiation, the only meaningful way to predict field levels is with good software, like EMI Analyst.

At a minimum, the radiated emissions analysis model must account for:

– Cable length and twisting
– Height above ground
– Circuitry at both ends of the cable
– Measurement distance
– Field polarity
– Active circuit noise spectrum
– EMI Filter Insertion Loss

A Quick Example

The figure below plots radiated emissions for a switching power supply. Analysis properties are summarized below.

Using EMI Analyst, model creation took about an hour, and the calculations took about 10 minutes.

For more information about predicting radiated emissions, visit the EMI Software website and check out RE Analyst, the radiated emissions application of EMI Analyst™.