Back in May, DENPAFLUX (previously Mitai) hosted a live EMC webinar with well-known industry expert Francesco Poderico. Entitled “EMC 101: What You Need to Know about Conducted Emissions,” the webinar generated so much interest that one hour was not nearly enough time to answer all the questions that participants submitted.
In this follow-up Q&A post, Francesco shares his experience and expertise on a wide variety of topics related to conducted emissions and EMC testing. The discussion touches on test-setup configurations, testing equipment, filter components, ESD protection, grounding, and the relationship between conducted emissions and radiated emissions. This series of questions and answers is a great opportunity to learn from a leading expert’s insights and advice on real-life EMC issues that engineers are dealing with.
1. What would be the scenario if there are multiple units?
It depends on how you're selling.
The CE test must only be done on one unit if you are selling one at a time.
If all of them need to be used simultaneously by your customer, the testing lab may recommend that you do the CE on all of them simultaneously (“in parallel”).
2. Why is the distance 40 cm? Is this based on a standard?
Standards like CISPR 22/EN55022 and CISPR 32/EN55032 require 40 cm.
Other standards may require different setups.
This 40 cm distance is half the distance between the equipment under test (EUT) and the floor (80 cm).
Therefore, the vertical plate will receive most of the radiated emission coupling.
3. In the CE setup, which other elements should be checked to ensure proper testing conditions, beyond the 40 cm and 80 cm distances?
In order to maximize the radiated emission from the EUT's cable, any cables need to be exposed to the vertical plane.
Support equipment should also be checked against standards in terms of its location and distance from other equipment.
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4. What would be a good way to replicate a test setup (e.g., from an official EMC test lab) in our own “homemade” environment? What parameters should be respected in order to get results representative of those from an EMC lab?
In addition to a spectrum analyzer, you need a line impedance stabilization network (LISN), a vertical and horizontal ground plane, and an insulation transformer.
On YouTube, I have seen a video of an “EMC expert” performing this kind of test at home without a vertical plane. This is absolutely incorrect.
5. Does the LISN maintain the same impedance for the EUT across different testing laboratories?
In short, yes.
Every EMC laboratory should recalibrate their equipment once a year. LISNs can't be used after their calibrations expire.
The calibration company must verify that the LISN is within specification.
6. The filter from slide 18 (Input/Output DC Power Filter), is it usually placed at the input of the EUT? Also, it is recommended to put ESD protection wherever you have a connection to the “outside world.” How would that ESD protection influence the filter, or would it influence it at all?
Figure 18 illustrates a typical filter to mitigate CE.
What is not shown in the drawing is that other types of protection may also be required.
You may need a metal oxide varistor (MOV) transient voltage suppression (TVS) component to protect against surge, for example.
You may also need discrete TVS components to protect against fast transients.
You may need "fast" TVS if you are just dealing with ESD.
7. In case 4 signal data, why don't you consider the capacitance case?
A large capacitor in parallel with the data line may cause the signal to deteriorate at low frequencies.
When selecting a ferrite bead, what is the best value? And according to which equation should we select the impedance and frequency?
A ferrite bead tends to work well at higher frequencies.
Today, however, you can also use material T31, which works at lower frequencies.
Generally, to pass CE, you should avoid ferrite, and instead use another approach (discussed in the podcast).
8. In slide 19, I observe that the negative terminals are different before and after the bead/inductor/resistor. How is this achieved? What’s the impedance path between ground and 0 V?
Well spotted!
Sometimes I create a “local” ground for bypassing ESD on data interfaces. The “local” ground is a pad connected to GND only at one point.
It is a double-edged sword, since if done incorrectly, other problems may arise.
However, I do this to prevent high voltages (e.g., ± 8 kV) from entering the PCB and damaging it.
9. In isolated power supplies, we generally place a Y capacitor between the grounds of primary and secondary. Is it also advisable to place one between the positive terminals of primary and secondary?
It is also advisable to add an X capacitor, as it will reduce differential-mode noise, for example.
10. As troubleshooters in industrial systems, we're usually concerned with disturbances from mains supply or coming from the earthing system. Do you have specific experience with common-mode currents and general EMC problems with large variable-frequency drives for pump or compressor motors in industrial environments or other large kW-sized inverters for UPS systems, or the solar industry? Are there any general aspects for these kinds of systems, or will this always be specific to the individual design?
In the webinar, we discussed the most common topologies.
11. How can we test conducted emissions for a device with a permanent or detachable antenna operating at or below 30 MHz? For example, NFC operating at 13.56 MHz.
CE should be carried out on all power lines and on (some) data lines (to external equipment).
Every RF source below 80 MHz contributes.
12. Can the LISN be on another table next to the EUT table? Or does it have to be on the ground?
When it comes to AC, it is usually placed on the ground table, but sometimes it is placed on the vertical panel.
The LISN can also be on the table on a ground plane for DC.
However, the setup depends on the standard.
13. When you design a filter, how much do you simulate it? What software tool would you recommend for filter simulation (SPICE-based or something else)?
In my younger days, I used LTspice to simulate every time.
In most cases I do not need any more attenuation for common-mode noise, because 30 dB attenuation is enough to pass EMC for the designs I usually do.
14. What should I do when the PCB size is so small that I can't incorporate more bulk capacitors?
You need bulk capacitors!
15. For EV chargers, do we need to consider a charger as floor-standing equipment or table-top equipment during CE testing, so that the results will be similar to vehicle EMC test results?
I have helped more than one EV charger company pass EMC. Usually, the lab uses a resistive load during the test. In my test of the EV charger, I placed it on an 80 cm table.
16. Do you provide paid training sessions?
Yes.
17. First, I want to thank you for your powerful presentation. My question is about the relationship between conducted emission and radiated emission. Does CE result in RE?
Conducted emission should be viewed as low-frequency radiated emission.
18. I have designed the differential-mode and common-mode filter for an isolated DC/DC converter which was operating at a switching frequency of 100 kHz. In the CE test, the emission was declining up to 3 MHz, but after 3 MHz it started increasing and crossed the margin. It again started to decline and came back below the margin at 7 MHz. What might be the reason behind this?
Did you add a capacitor between the Vin– and Vout– terminals?
While we've covered a lot of ground in this Q&A, there's even more valuable insights in the original webinar. If you haven't had the chance to watch it yet, or if you'd like a refresher, we've got you covered!
🎥 Watch the Full Webinar: "EMC 101: What You Need to Know about Conducted Emissions"