If you are evaluating a design (i.e. power supply) from the EMI/EMC point of view, avoid replacing the real load with some kind of “equivalent resistor.” Differences can be really impressive.
When designing an electronic system, it is not uncommon to replace the real load (antenna, motor, fan, etc.) with some resistor with consumption in power equal to the real load.
But, when evaluating the EMI profile of your design, be careful with this replacement because the behavior of the system can be very different especially if the real load has some kind of non-linear behavior.
For example, consider you are designing a fly-back switching mode power supply where the load for one of the outputs is a fan. The product is failing in radiated emissions (VHF band) in the semi-anechoic chamber at three meters.
First, note how different is the current for the fan if using a linear power supply versus using the switching mode power supply (we measure the differential current in the fan with current probes CP1 and CP2 (Figure 1).
The high frequency ripple in the current waveform for the second probe, CP2, is related with the switching activity of the power supply (SMPS noise).
The radiated emissions at VHF frequencies are related with common mode currents in mains cable (this cable is the antenna).
A setup (Figure 2) was prepared to evaluate the radiated emissions from mains cable when using the switching mode power supply.
Our interest is in VHF radiation (30-80MHz) from the main cable. Common mode current is the origin of the radiation so a common mode probe (CM probe) was used at the input of the power supply. The radiated field is proportional to the injected currents.
Two situations are analyzed: a) the real load (the fan); and b) replacing the fan with a resistor that is “equivalent” from the power consumption point of view.
The measurements results are included in Figure 3.
The yellow trace is the noise floor when the SMPS is OFF. The blue trace is the common mode emissions when the real load (the fan) is used. Note MAX HOLD mode in the spectrum analyzer is used to plot the peak of the emissions. Finally, the pink trace is the measurement of emissions when the equivalent resistor is replacing the fan.
Up to 20dB of difference in emissions in the low frequency range (section I) and the medium frequencies range (section II). So, when estimating the radiated field at three meters, a 20dB error will be included in that estimation only because the equivalent resistor is used.
So, as usual, my final advice: do not replace real loads with “equivalent” resistors when considering the EMI behavior of your circuit.
Arturo Mediano received his M.Sc. (1990) and his Ph. D. (1997) in Electrical Engineering from University of Zaragoza (Spain), where he has held a teaching professorship in EMI/EMC/RF/SI from 1992. From 1990, he has been involved in R&D projects in EMI/EMC/SI/RF fields for communications, industry and scientific/medical applications with a solid experience in training, consultancy and troubleshooting for companies in Spain, USA, Switzerland, France, UK, Italy, Belgium, Germany, Canada, The Netherlands, Portugal, and Singapore. He is the founder of The HF-Magic Lab®, a specialized laboratory for design, diagnostic, troubleshooting, and training in the EMI/EMC/SI and RF fields at I3A (University of Zaragoza), and from 2011, he is instructor for Besser Associates (CA, USA) offering public and on site courses in EMI/EMC/SI/RF subjects through the USA, especially in Silicon Valley/San Francisco Bay Area. He is Senior Member of the IEEE, active member from 1999 (Chair 2013-2016) of the MTT-17 (HF/VHF/UHF) Technical Committee of the Microwave Theory and Techniques Society and member of the Electromagnetic Compatibility Society. Arturo can be reached at firstname.lastname@example.org. Web: www.cartoontronics.com