Introduction
Last month, we described how to measure differential mode versus common mode currents. This will become important when designing power line filters. This month, we’ll turn our attention towards performing our own conducted emissions pre-compliance and troubleshooting testing for products with DC inputs, such as automotive, medical, and military. These measurements are easy to perform in-house and will save much time and money in the long run.
At a minimum, you’ll need a spectrum analyzer that can go as low as 9 kHz (military) or 150 kHz (medical or automotive). My article on pre-compliance software [2] would be a good reference for any of these instruments.
When measuring DC input products for automotive to the CISPR 25 standard, we use a 5µH line impedance stabilization network (LISN). These are available in different current ratings from many manufacturers. This also applies for some MIL-STD-461 tests. However, some standards for military or aerospace products may still specify the 50µH LISN as we used in the January 2025 article, even though the product uses a DC input. Be sure to use the LISN as specified in the appropriate EMC standard applicable to your product.
The typical test setup is shown in Figures 1 and 2. This is easy to set up on your workbench and allows quick filter mitigation experiments.
Figures 1 and 2 show the typical test setup I use when either troubleshooting or performing pre-compliance tests. It’s simple to set up and requires only one LISN. The conducted emissions test requires a ground plane for the return noise currents. I either use heavy-duty aluminum foil or an equivalent flexible ground plane.
Be sure the LISN is well-bonded to the ground plane. I use heavy brass weights or copper tape to bond the LISN to the ground plane. According to most standards, the spectrum analyzer need not be bonded to the plane. Depending on the type of device being tested, it may or may not require bonding to the ground plane. Generally, DUTs bolted to the vehicle chassis should be bonded to the plane.
Tests with Various Filters
I’ll be testing to CISPR 25 Class 5 (for automotive modules) using a 1 MHz GaN-based DC-DC converter board, which has no on-board filtering. I’ll add a custom filter board with the most popular topology for filtering differential and common mode noise voltages. This filter board was really designed for off-line power conversion (150 kHz to 30 MHz), but we’ll see how well it works for DC applications up through 108 MHz. I tied the neutral and ground connections together so the “Y” capacitors would be effective for a straight DC power input.
Test 1 – No Filter
Figure 3 shows the overall test setup with DC power connected through the LISN and powering the DC-DC converter board. This board has a fixed resistor as a load. Figure 4 shows the results. As expected, there were many 1 MHz harmonics over the limit.
Test 2 – With Additional Filter Board
Figure 5 shows the detail of the added filter board. I shorted the neutral and ground connections to connect the two “Y” capacitors to the power return side. Figure 6 shows the results with an average 10 dB reduction in conducted emissions.
Test 3 – With Series 100µH Chokes
I’ve found that when designing filters for military applications, an additional pair of inductors in series with the power input does wonders. Figure 7 shows the test setup with the two series 100 µH inductors indicated. Figure 8 shows the result, which was rather dramatic, with an average 30 dB reduction in emissions throughout the measured spectrum.
Summary
Measuring conducted emissions is easy to do right on your workbench. Once set up, it becomes easy to try different filter topologies and component values to confirm the desired results. This really beats performing these experiments at a third-party test lab!
References
- Wyatt, Kenneth, “EMC Bench Notes: Pre-Compliance Testing for Conducted Emissions – Equipment Needs (AC Mains),” In Compliance Magazine, January 10, 2025.
- Wyatt, Kenneth, “EMC Bench Notes: Pre-Compliance Testing for Radiated Emissions, Part 4 – Measurement Software,” In Compliance Magazine, November 7, 2024.
