The steeper the signal edges, the broader the frequency spectrum of the noise. This is no news for communications engineers – but nevertheless, noise emissions from electronic circuits are sometimes damn difficult to get rid of because of the different physical effects at work:
These occur primarily in the frequency range from 150 kHz to 30 MHz. At higher frequencies, the ever-present parasitic capacities between the circuitry and the usually grounded chassis ensure that the conducted emissions are strongly attenuated. In the low-frequency range – up to approx. 1 MHz – they are in-phase with the power supply und thus only flow within the small loop formed by the forward and return conductor. Usually simple RC or LC filters are sufficient to solve the problem.
In the frequency range above 30 MHz, these are the major challenge – as they may easily reach 1 GHz and above. These radiated emissions travel in phase (in common mode) with the forward and return conductor, but flow back via parasitic capacitors and via grounding. Thus, the current flow forms large loops, which tend to cause trouble during EMC tests.
How to suppress common-mode noise
Inductors are always a good idea against high-frequency noise. For common-mode interference, current compensated chokes are first choice. These are components with two identical windings in opposite direction. The current flowing into and out of the choke – and the resulting magnetic fields – compensate each other – hence the name. Current compensated chokes are therefore relatively small and hardly interfere with the circuit design. For the high-frequency noise, however, this choke acts like a strong filter.
Common mode filters work in both directions
A filter circuit for common-mode noise, travelling the same direction on the forward and return conductors, is also known as a common-mode filter (CMF). In principle, it works in both directions, i.e. it attenuates both the interference coming from the power supply and the interference flowing back from the electronics.
That the inductance of a current-compensated choke can also serve as L of the LC filter against conducted emissions, is only to be mentioned briefly here.
Interference suppression is sometimes “tricky”
As you can see: EMC or the correct interference suppression of switching power supplies, converters and other electronic components is not always trivial. Frequently, design errors or problems are only detected in the qualified test laboratory, requiring rework and further expensive measurements. The faster and cheaper way for circuit designers is to use specialized service providers, who support the design-in, have the necessary know-how and can perform pre-qualifications in their own EMC laboratory.
Do you have questions regarding this matter?
Your contact person: Herr Dieter Bauernfeind
Company: Elec-Con technology GmbH