Why more than 70% of HV products fail EMC the first time

High-voltage (HV) designs come with difficult constraints to manage, and in some ways they are more difficult from the design perspective than high-density PCBs. Although circuitry may be simple, there are safety risks and EMC risks in HV PCBs, and it can be difficult to balance these to achieve a safe and reliable product that is also EMC compliant. When it comes to EMC, however, the results speak for themselves: a very high percentage of products fail their first round of EMC testing.

Because these products can be so challenging, we developed this article to identify some of the common EMC failure modes in HV PCBs. In particular, we identified five common failure modes related to PCB layout, component selection, and circuit design.

Common HV EMC failures

Common-mode currents from high-dV/dt switch nodes

In high-voltage switch-mode DC/DC converters, the switching stage generates strongly changing voltages with high dV/dt values, both in MOSFET-based and SiC/GaN-based converters. The strong dV/dt at the switching nodes can capacitively couple current into the chassis, earth, heatsinks, cables, and isolated circuitry.

This is more than just an instance of unintended noise coupling; where that coupled noise propagates or dissipates determines how an EMC failure can occur. For example, in the case of a dV/dt switching pulse appearing on a nearby heatsink, the energy is generally radiated and would be noticed as elevated radiated emissions. In the case of chassis coupling, we would expect common-mode currents to begin circulating around the system.

The best solutions focus on reducing the capacitive coupling from the switch node and providing a controlled return path for any common-mode current that remains. This typically requires changes at both the switching stage and the PCB layout.

• Reduce switch-node copper area and keep high-dV/dt conductors away from chassis, heatsinks, cables, and isolated circuitry.
• Reduce the switching edge rate where switching losses and device operation allow.
• Minimize parasitic capacitance between the switch node and grounded or chassis-connected structures.
• Use appropriate Y-capacitor paths to provide a controlled return path for common-mode current across isolation barriers.
• Add common-mode filtering at cable or power interfaces where noise is leaving the enclosure.

Transformer and isolation barrier parasitic capacitance

HV PCBs often use galvanic isolation to separate high and low voltage regions, especially when stepping down the voltage between stages. Any time there is an isolation barrier between two regions of a design, there is some small parasitic capacitance. Specifically, the parasitic capacitance rises in two locations:

• As inter-winding capacitance between transformer coils
• As terminal or barrier capacitance in optically isolated components

At high voltage, power can easily bleed through the capacitance defining the isolation barrier, even at low frequency. The most common instance is at transformers with a high potential difference between isolated coils, which is most easily addressed with an appropriate safety capacitor or by redesigning the transformer. In digitally or optically isolated components, high-voltage transfer across the barrier may indicate a fault condition, as these components are not typically placed in a high-voltage circuit.

This power bleed-through can cause input power, such as an AC input, to appear as a common-mode signal on conductors. In switch-mode DC/DC converters, driving pulses create broadband noise, which may also be common-mode, just as was the case with high-dV/dt switch nodes.

Switching-node ringing

Even if the switching action does not lead to capacitive coupling and common-mode noise, the switch node can still ring strongly, which also produces broadband emissions. In this case, the problem is due to excess inductance along the current path rather than some excess capacitance. The source of the inductance depends on the topology, but in multiple topologies this comes from leakage inductance in the transformer being driven on the primary side of a high-voltage DC/DC converter.

The solutions to issues with switch-node ringing are well known:

• Identify and reduce the source of excess inductance
• Slow down the switch waveform edge rate
• Place [snubbers/clamps]([link: snubber blog post]) to absorb the ringing energy
• Modify the current loop to reduce inductance

Of these options, the third one is most effective but contains the most risk. The reason is that the components must be selected to withstand the ringing waveform and resulting power dissipation. This creates a risk of components failing if they are not selected to handle the intended power level.

Insufficient surge and overvoltage protection

We most often associate voltage or power surges with mains-connected devices, but in reality, they can appear in many scenarios that do not involve AC mains. In high-voltage devices, a "small" surge could reach 100 V or more, a level that would cripple a normal device and can only be addressed with appropriate overvoltage protection. In fact, the ability to handle voltage surges is specified as an EMC requirement in some industry standards and regulations.

A subset of industry standards demanding equipment obey voltage surge requirements is given in the table below. Note that a simple fuse and NTC may not always be enough to handle the surges listed in these or other standards.

Insufficient common-mode filtering

Common-mode filtering in a high-voltage design can fail to provide the necessary level of noise reduction or bandwidth for two possible reasons:

• Core saturation in inductive components
• Bypassing coils via inter-winding capacitance

Both are symptoms of inductive components and reflect their limited ability to handle high power and high frequency. At high frequency, the second item is more important in that it reflects a filter's inability to continue filtering above some resonant frequency; the result is a steady decrease in the impedance of a common-mode filter circuit due to inter-winding capacitance in inductor coils.

This impedance graph illustrates where a common-mode choke in an EMI filter circuit starts to lose its attenuation (above ~50 MHz).

Core saturation is more likely to be the issue in high-voltage systems whenever the common-mode filtering is placed in a high-current path. This is more common on the input of a boost DC/DC converter, where the input is at low voltage/high current. The magnetic saturation at high current also reduces the equivalent inductance of the coil, and thus the filtering capability beyond some power level.

Whether the problem is inter-winding capacitance or core saturation, the problem is normally solved by selecting different magnetic components. This may require custom magnetics in order to hit the design goals without excessively large form factors.

Find the problem areas before the lab

All the factors listed above contribute to high-voltage designs exhibiting over 70% failure rate during EMC testing. Although the risk is high, it's possible to narrow down to the problematic areas in a PCB layout before pre-compliance testing or formal testing. It's best to identify these at the design stage, not after a prototype is found to be at risk of non-compliance.

If you want expert eyes on your high-voltage design before pre-compliance or formal EMC testing, DENPAFLUX  can help. Our EMC experts review your layout, components, and wider system, then hand back a clear report that flags what to change and how urgent each issue is. You choose how much of the EMC work we take on, from an advisory review through to owning the full path to a passing test.

See how we work or contact an expert to get started.

Frequently asked questions