EMI troubleshooting for a portable medical device

Client Context

A UK-based electronics design company, specialising in medical, scientific, consumer, and transport applications, developed a portable medical device. Surprisingly, the product failed radiated emissions testing above 600 MHz, showing broadband noise.

The client sought a quick, practical solution, ideally using simple mitigations such as cable ferrite cores, to avoid a full PCB redesign or product re-spin.

Scope of Work

• Confirm radiated emission failures in our lab

• Identify the root cause of the broadband noise

• Develop the simplest effective fix

Technical Approach

We first used a log periodic antenna to measure the noise in the far-field. 

Due to the compact size of the product, we were able to place it inside a large TEM cell for controlled testing. Near-field probes were then used to locate the dominant noise source.

While broadband noise from switched-mode power supplies (SMPS) is typically observed between 30 MHz and 300 MHz, in this case the emissions extended beyond 600 MHz. This unusual behavior prompted a detailed investigation. 

Troubleshooting & Mitigation

Analysis revealed the presence of a strong resonant structure at the offending frequency. The resonance was traced to a PCB inductor (L) forming a parasitic LC circuit with the intrinsic capacitance of a Schottky diode placed in parallel with a MOSFET.

Semiconductor intrinsic capacitance is often overlooked in design and varies with operating voltage. The Schottky diode was typically included in a design to mitigate reverse-recovery issues associated with the MOSFET body diode. However, in this circuit, which does not drive an inductive load, the diode was unnecessary.

The client confirmed that the diode was intended to improve efficiency and could be safely removed if it caused EMC issues.

Action Taken:

• Using time-domain measurements, we verified that removing the Schottky diode would eliminate the resonance.

• The diode was removed, and the product was retested in the TEM cell.

Results & Validation

• Radiated emissions above 600 MHz were successfully eliminated

• The client achieved compliance without adding new components or performing a PCB re-spin

• The issue was resolved within a single day, allowing the product to proceed toward launch without delay

Conclusion

Parasitic capacitances in semiconductor devices are often overlooked during circuit design but can form resonant structures leading to unexpected EMI. Understanding these parasitic effects and the resulting resonances is critical for rapid and effective EMI troubleshooting.

This case demonstrates that sometimes the simplest solution—removing a problematic component—can be more effective than adding mitigation hardware, saving both time and cost for the client.