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In EMI/EMC, Never Say “Nothing is different from previous version”

During one of my courses, one of the attendees asked me about a problem with the power supply of a product. The power supply was a design in production without problems for many months. Suddenly, the production was running into problems.

In that product the ripple of the output voltage in power supply changed to almost 100% in peak value. Nothing had been modified. All was exactly as in previous versions of the product.

Note the sentences: “Nothing had been modified.” and “… exactly as in previous versions.”

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How often this way of thinking creates problems! Be sure, if you get a different result, that you have included (intentional or unintentional) changes in your design or measurement process.

In our case, the system under study was a fly-back power supply with two outputs: 12V/175mA and 5V/450mA (see Figure 1).

Figure 1: A general view of the power supply with two outputs.
Figure 1: A general view of the power supply with two outputs.

We measured the old and “new but unchanged “ versions of the product. The 5V output offered the ripple as you can see in Figure 2.

Figure 2: Output of the power supply (AC mode) to compare  the ripple in outputs between the original and the new versions of the product.
Figure 2: Output of the power supply (AC mode) to compare
the ripple in outputs between the original and the new versions of the product.

I asked about the output filter of the power supply and any change in the last weeks. I found that the 470uF electrolytic capacitors had been changed to “equivalent” but “low cost” 470uF capacitors (Figure 3).

Figure 3: Two “equal” 470uF capacitors.
Figure 3: Two “equal” 470uF capacitors.

One of the very important subjects for electronic designers to understand is the concept of the “hidden schematic” as introduced (I think) by Mark J. Nave some years ago.

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Components may not always be what you think. And because of this, your design can fail. Many times the secret for troubleshooting is related to your capabilities to see the “hidden effects”, “hidden parasites”, “hidden antennas”, etc.

In my lab, two units of capacitors were characterized using an Agilent 4294A impedance analyzer. Note in Figure 4 how both capacitors look like a series resonant RLC circuit in the range from 40Hz to 10MHz. A model for capacitors is included in Figure 4.

Figure 4: Impedance (module and phase) for both capacitors: GOOD (left) and BAD (right).
Figure 4: Impedance (module and phase) for both capacitors: GOOD (left) and BAD (right).

GOOD CAPACITOR:

C=425.88uF; L=7nH; and R=83mΩ.

BAD CAPACITOR:

C=401.47uF; L=9.2nH; and R=161.8mΩ.

Note that the component is not an ideal capacitor in the full frequency range. It goes from capacitive to resistive and inductive behavior as frequency increases.

In Figure 5 the measured impedances are plotted in the same graph including the measurement for the 4.7uH inductor and an ideal capacitor of 470uF.

Figure 5: Comparing impedance both capacitors and the inductor.
Figure 5: Comparing impedance both capacitors and the inductor.

For the 35.7kHz frequency of the ripple, the impedance in both capacitors is resistive (double in value for the bad capacitor). That is because the ripple is bigger for the same transient in currents.

Because the PI filter included in the power supply is a combination of two capacitors and the inductor, we can see the effect of the non ideal behavior of the components in Figure 6 where the insertion losses are plotted as a function of frequency.

Figure 6: Response of filter: ideal, with good and bad capacitors.
Figure 6: Response of filter: ideal, with good and bad capacitors.

At 35.7kHz we can expect -37.7dB from a filter with ideal components. If we use real components including the GOOD capacitor, the response is degraded to -20.1dB. The situation is worse for the BAD capacitor, with -14.4dB, a difference of 23dB with the ideal design.

Note in Figure 6 how the resistive effects in components avoid the underdamping effect (peak in response typical from ideal circuits or very high quality components) around 4kHz.

So, as a final recommendation: be careful with any change (hardware or software) in your designs.

Take time to check for EMI, efficiency, temperature, safety, immunity, or any other requirement in your product.

From EMI perspective do not forget the hidden schematic!

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author mediano-arturoArturo 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 and The Netherlands. 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!, 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 (now Chair) 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 a.mediano@ ieee.org. Web: www.cartoontronics.com.

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