When AC Meets DC: An EMC Redesign Case Study

Introduction

A field failure investigation revealed a recurring and costly issue: a Wi-Fi module and the main MCU were burning out intermittently in a production device. The failures were not random. They were triggered by external disturbances, particularly electrostatic discharge (ESD) events near a solenoid cable.

Both components were powered from independent low-voltage rails (3.3V domain), yet both were failing under transient stress conditions.

This case study describes how AC/DC layout interaction, insufficient surge strategy, and power-plane weaknesses led to the failures — and how a complete redesign eliminated the issue.

Several classic design mistakes were combined in this product. Below, we have outlined them in detail.

An additional and revealing symptom was observed: when a hand approached the device, it began to emit a faint humming sound, indicating high sensitivity to external electromagnetic disturbances.

PCBA Issues:

1. The AC input/output lines and the AC filter should be implemented as a separate subassembly together with the transformer and connected to the main PCB using short wires. Alternatively, they should be placed on the PCB as an isolated section, ensuring no interaction with DC signal traces or DC power planes.

The AC and DC routing is mixed together.

Power planes beneath the AC filter must be completely removed. The same approach should be applied anywhere AC lines are routed on the PCB. All DC signals should be laid out at a safe distance from AC traces, maintaining a minimum clearance of 2–5 mm.

2. There is significant coupling on the PCB between the VCC2 power line and the AC_BACK line. This creates a high risk that an ESD event occurring near the solenoid cable could couple into and propagate along the VCC2 rail.

As well there are other crosstalk issues between AC 220V and DC lines VBAT and VBUS and other DC signals.

AC and DC lines are routed together.

3. ESD/EMI Event propagation on PCBA via 24VAC line and impact of RV1 use case.
   

   

   Incorrect placement of the varistors.

4. Varistor use case.

   

   Incorrect use of the varistor.

   Action when both wires have the same voltage level: If both wires (AC_IN and AC_BACK) have the same overvoltage level at the same time, the varistor may not operate because the potential difference between the two wires will not be enough to activate it. To avoid such a situation, we need to use varistors or TVSs from both lines to power GND.   

5. VCC2 should be polygon and not strip line, has potential dropping voltage on such line.

   
   

   

   The power rail should be implemented as a solid polygon.

6. GND Field Signal should be polygon and not strip.

   

   The ground should be implemented as a solid polygon.

7. MCU should be surrounded with GND polygon.
   

   

   Multiple ground polygons are required around the MCU and beneath it.

8. MCU, Wi-Fi and other logic circuits power inputs should be filtered with ferrite beads like in the picture. It will reduce or eliminate EMI event and save the power input of components. 

   

   It is recommended to use a Pi filter for the MCU and Wi-Fi power rails. (picture from WWW)

9. AC filter use case issue.

   Theoretically, if we have voltage spike the high voltage will pass through filter will reduce but stay.

The AC filter is improperly installed on the PCB.

Here's what can happen in this situation:

The filter partially attenuates the pulse: The filter suppresses the high-frequency components of the pulse, but does not completely eliminate its amplitude, especially if the pulse has low-frequency components or if the pulse energy is too high for the filter to completely suppress.

Not enough voltage to trigger the varistor: After filtering, the pulse voltage may be below the varistors' trigger threshold, causing them to not turn on and not absorb the pulse energy. In this case, the circuit components after the filter are still vulnerable to damage.

Damage to components: Despite filtering, the attenuated pulse may still be sufficient to damage sensitive circuit components, especially if they are designed to operate at low voltage levels, such as 3.3V or 5V.

10. EMI/ESD event and impact on the device and power planes. Energy of event 0.02–0.15 mJoule. As can be seen from the presented slides, the impact of the event is quite significant and can lead to failure of the processor or Wi-Fi module. (Scope dimensions, 1v/div 25mSec/div3v3 )

Impact of an ESD spike on the MCU power rail.

11. The 3.3V DC/DC converter exhibits a sufficiently high switching spike and requires proper snubber optimization.

Switching spike by 820kHz

Generally, we should avoid layout AC and DC together; The routing of DC and AC circuits on the DC plane of a PCB is undesirable for several reasons, primarily related to electromagnetic interference, noise, and potential disruptions in circuit operation:

·         Inductive Interference: Alternating currents generate changing magnetic fields, which can induce voltages in DC circuits. This can lead to noise and oscillations in the DC circuits, causing instability in the power supply and affecting the performance of sensitive components.

·         Capacitive Coupling: AC lines can transmit high-frequency noise through capacitive coupling between conductors, especially if they are placed close to each other. This can induce interference in DC circuits, leading to voltage instability or disruptions in control circuits.

·         Electromagnetic Compatibility (EMC): Routing DC and AC together can degrade the electromagnetic compatibility of the system. AC lines can emit electromagnetic radiation, which can negatively affect other parts of the circuit, causing issues with meeting EMC requirements.

·         Different Routing Requirements: DC and AC lines have different routing strategies. DC circuits typically require minimizing resistance and providing a stable path for current, while AC lines need to account for reactive components (inductance and capacitance) to minimize losses and interference.

·         Component Disruption: High-frequency interference from AC lines can affect the operation of components, especially digital and analog circuits powered by DC. This may cause malfunctions, false triggers, or degraded system performance.

Solution

After the redesign and separation of the AC and DC lines, the addition of ESD suppressors on the MCU and Wi-Fi module power rails, and the optimization of the power and ground polygons, the problem was eliminated.