Power Supply Meets EMC: Essential Testing, Design, and Protection

Figure 3 shows a circuit branch with an additional noise signal represented by a resistive value. The noisy current in the circuit generates a noisy voltage on the parasitic resistive trace, which in turn affects the voltage on the resistive load. In this case, an ideal source can become a noisy source due to the added noise on a common branch.

Measures to reduce coupling interference

This type of coupling can be reduced by decreasing the impedance, and thus the common resistive path, between the two circuits. The effect on the voltage of the resistive load is directly proportional to the impedance of the common trace.

If the location and source of the noise are known, the noise current can be reduced. This can be achieved by addressing the noise directly at the source, using a filter to reduce the noise along its path to the jamming sink, or by lowering the frequency of the noise. The impedance of the common trace is directly proportional to the frequency of the noise, as impedance is a frequency-dependent value. The frequency in a system should be as fast as necessary and as slow as possible.

To avoid a common trace, a star-point topology can be used to separate the noisy parts from the load circuit. The ground systems are connected at a single point. To minimize uncontrollable coupling, both the return path and the forward path should be considered during layout design. As the design typically includes several forward and return paths, each path should be prioritized and categorized as aggressive, sensitive, or indifferent. The most aggressive loop, which has the highest di/dt, should be made as small as possible and given the highest priority among aggressive traces.

The parasitic element Ck in Figure 4 exists in circuitry between adjacent parallel wires. Through this capacitance, which is typically very low, high-frequency (HF) signals may couple to other components, potentially causing problems.

Measures to reduce capacitive coupling:

1. Short traces are one countermeasure against capacitive coupling, as the length of the trace is directly proportional to the coupling between conductive parts. Traces running parallel over long distances are more susceptible to capacitive coupling, which can become problematic (for example, in an analog measurement line running parallel to a clock line).
2. Additional capacitors can create symmetry for highly sensitive signals, allowing the noise to be coupled back early before it causes issues.
3. Capacitive coupling can be reduced by shielding with conductive materials.
4. Capacitive coupling can also be minimized by lowering the frequency.
5. When cables are run over long distances, twisted pairs (possibly shielded) are particularly effective at preventing unwanted coupling.

Live wires can be inductively coupled through magnetic fields. These fields can couple into nearby metal parts, including sensitive traces, and induce current.

Measures to reduce inductive coupling:

1. As with capacitive coupling, traces carrying different signals should not be designed in parallel, as live wires in parallel can induce current in one another.
2. Large loops in noisy circuits generate significant magnetic fields, so the most critical loops with the highest di/dt should be kept as small as possible. Additionally, highly sensitive loops should also be minimized, as circuits with larger loops are more prone to disturbances.
3. To design small loops, the designer must always consider the return path.
4. Traces are less sensitive to frequencies when the loops (and thus the impedances) are small.
5. As the frequency decreases, inductive coupling also reduces. A design should use a frequency that is as slow as necessary but as fast as possible.
6. Magnetic fields can also be shielded using non-conductive materials such as permalloy and µ-metal, commonly found in RFID and NFC applications.
7. Ferrite plates can also be used on controllers to shield magnetic fields.