Background: the AC-DC / DC-DC Supply and Application

The customer in this case is an equipment maker that builds industrial test racks. Each rack includes several AC-DC power supplies that generate intermediate DC rails, plus a group of DC-DC power supplies that create lower voltages for control boards, communication modules and sensors. In the field, these racks sit in noisy industrial environments and are often connected to mains via long cables routed through shared distribution panels.

A new rack generation used higher power AC-DC power supplies and denser DC-DC converter boards to save space. The design team already followed good layout practice and used manufacturer-recommended filters, but they had limited access to EMC test equipment. Previous experience told them that changes in switching frequency, topology and mechanical layout could easily create new EMC risks—even when datasheets promised “compliant filters”.

Because the project timeline was tight, they decided not to rely only on datasheet claims or a single full-day visit to a certification house. Instead, they contacted TPS to set up EMC testing for their power supplies as a focused pre-compliance project, with realistic loading and cabling and enough time for debug.

EMC Challenge: Failed Emissions and Immunity at a Third-Party Lab

Before working with TPS, the customer had already attempted a formal EMC test for an earlier prototype of the rack. The third-party lab report highlighted two main problems:

• Conducted emissions on the AC input, with narrow-band peaks at the switching frequency of the AC-DC power supplies and their harmonics, exceeding limits by several dB.
• Radiated emissions around mid-band frequencies, likely caused by a combination of DC-DC converter layouts and cabling in the rack.

In addition, some functional disturbances occurred during fast transient immunity tests on the AC input. The behaviour was not catastrophic, but a few resets and communication dropouts were enough for the lab to mark the result as a fail for the intended industrial environment.

The customer knew they needed more than a single re-test. They required a way to iterate on filters, grounding and PCB layouts without paying for multiple full test days. That is where EMC testing for AC-DC and DC-DC power supplies in a pre-compliance lab became the obvious next step.

TPS EMC Test Plan for AC-DC and DC-DC Power Supplies

TPS started by reviewing the failed report and the schematics for the switching stages. Based on this, we created a compact EMC test plan that focused on the behaviour of the power supplies themselves while still reflecting the real application in the rack:

• Representative setup. The AC-DC power supplies were tested with realistic mains conditions and neutral / earth connections. The DC-DC converters were connected to representative loads and control electronics to mimic how they operate in the rack.
• Targeted measurements. We prioritised key measurements for emc testing for ac dc power supplies and DC-DC converters: conducted emissions on the mains side, conducted noise on the main DC bus and near-field scans around critical layouts.
• Simple stress scenarios. For immunity, we applied fast transients and surges to the AC input and monitored both the DC rails and basic functional behaviour of the connected electronics.

The goal of this plan was not to replicate every clause of the full standard but to give the design team fast feedback on which parts of the power stage and mechanical design drove the EMC behaviour.

Debug: Filters, Layout and Grounding Improvements

With the test plan in place, the design team and TPS engineers worked side-by-side in the lab. Each EMC measurement was followed by a focused change, so the team could see which ideas really moved the emissions and immunity margins.

• Input filter tuning. Different combinations of common-mode chokes, X and Y capacitors and line routing were tested on the AC-DC input. Small changes in component values and orientation produced measurable differences in conducted emissions.
• DC-DC layout refinements. Near-field probing around the DC-DC converters identified hot spots at switching nodes and loop areas. Adjusting copper shapes and improving return paths reduced local radiation and tightened the spectrum.
• Grounding strategy. The team tried several bonding approaches between the supply PCBs, chassis and rack rails. A clear, consistent grounding concept improved both noise behaviour and immunity to fast transients.

Because the EMC pre-compliance session was organised around fast iterations, the customer left the lab with a validated set of filter values and layout rules, not just a list of theoretical suggestions.

Results: Clean Test Reports and a Reusable EMC Design Flow

After implementing the agreed changes, the customer returned to the third-party EMC lab with updated AC-DC and DC-DC boards. The formal tests showed:

• Conducted emissions on the mains input comfortably below limits in the problem frequency bands.
• Radiated emissions from the rack within limits, with no new peaks emerging in other ranges.
• Stable operation of the rack during fast transient and surge immunity tests, with no resets or communication loss.

Just as important, the project team documented the steps as a simple “EMC design flow” for future power supplies. Instead of treating emc testing for power supplies as a one-off activity at the end of the project, they now plan EMC pre-compliance sessions as a standard part of developing new AC-DC and DC-DC designs.

How to Prepare Your Next Power Supply for EMC Testing

If you are working on AC-DC or DC-DC power supplies for industrial equipment, you can speed up your EMC project by preparing a few items before you contact TPS:

• A block diagram of your system showing mains input, AC-DC stages, DC-DC converters and typical loads.
• Key ratings such as input range, output voltages, power levels and switching frequency families.
• Information about expected cable lengths and how the power supplies will be mounted in the final equipment or rack.
• Any previous EMC results or concerns, especially if you have already visited a third-party lab.

With this information, TPS can prepare a focused plan for emc testing for power electronics in your project, run pre-compliance measurements in our lab and help you arrive at formal EMC testing with far fewer surprises.

Talk to an EMC Engineer About Your Power Supply   Back to “Typical Devices We Test”

Key questions this case answers

This case study answers the core search questions behind EMC testing for AC-DC and DC-DC power supplies:

• What typically goes wrong when switching power supplies are tested for EMC, and why filters alone are not enough.
• How a pre-compliance lab can focus EMC testing for AC-DC and DC-DC power supplies on the most important emissions and immunity risks.
• Which design changes—filters, layout and grounding—usually bring the biggest EMC improvements for power supplies.
• How to prepare information and hardware so an EMC pre-compliance session with TPS is efficient and actionable.